Welcome to Lesson 1! Here we start talking about solar energy and the value that societies derive from solar energy options, both past and present. Many of us who are extremely interested in solar energy have yet to learn about the deep, deep roots that the solar field has established over and over again in societies across the world. Additionally, we hear about amazing new developments in solar in the media, events that seem to be happening weekly (and sometimes daily)! We would like to generate a sense for why solar energy applications are growing now, why they did grow and sometimes bust in the past, and what we might expect in the future.
We will use examples from reading, images, and your own experience to explore the differences between:
This lesson will also explore some historical aspects of the solar field, where societies have found fuels (geofuels such as coal, petroleum products, natural gas, and the biofuels such as wood and manure) more challenging to access due to various constraints. We will see that, in fact, an inability to access fuels is often the driving force for solar development. In contrast, when access to fuels is unconstrained, we find that solar development tends to slow or cease in society. In this lesson, we will also see how emerging solar industries correlate with global shifts in perspective regarding anthropogenic global warming, sustainability, and energy security. Frameworks will be explored for policy and entrepreneurial responses to these new perspectives.
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Canvas Calendar for specific timeframes and due dates - those can vary from semester to semester. Specific directions for the assignments below can be found further within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 2 - "Context and Philosophy of Design" J. Perlin, "Let It Shine: The 6000-Year Story of Solar Energy", Chapters 2, 3, and 6. (These books can be accessed online via Library Resources tab in Canvas or by search of Penn State's Library. You must be a Penn State student to access this text via the E-Reserves). |
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To Do: |
Learning Activity: Identifying the Components of SECSs Yellowdig Discussion: Historical Development of Energy Resources Download NREL's System Advisor Model (SAM) Engage in all Try-This and Self-check activities (not graded) |
Topic(s): | Energy constraint, value of solar energy, history of developing solar energy |
If you have any questions, please post them to the Lesson 1 General Questions and Comments discussion in Yellowdig. I will check that forum regularly to respond. Feel free to go through the comments and post your own responses if you are able to help out a classmate.
Please review the Energy Explained [1] portion of the USA Dept. of Energy's Energy Information Administration [2] website.
When you are reading, I want you to focus on Forms of energy, Sources of energy, and Supply (production) vs. Demand (consumption) of energy. These four terms are simple but very specific. One thing that you can use to remember them: energy is neither created nor destroyed, but can be transformed from one form to another, and we call our sources of energy resources. The economics of energy is also directly discussed in terms of supply and demand.
We believe the global demand for energy in its various forms will keep rising, spurred on by an expected increase in population and industrialization of many developing countries. Policy makers, entrepreneurs, and scientists will be faced with serious questions on how to produce and deliver required energy to consumers. But focusing in on the USA, how does a country use energy where local population growth is smaller and energy use has been outsourced to other developing nations?
1880 to 1920:
1883:
1950-1980 (Post WWII):
2010:
2010-2020:
2019:
2020:
Year | Fossil fuels | Wood biomass | nuclear | renewables |
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2000 | 77.3% | 10.2% | 5.9% | 6.6% |
2020 | 78.0% | 6.7% | 4.0% | 11.2% |
Source: World Economic Forum [5], 2022
Click here [6] to see the larger version of this chart.
This annual energy flow chart shows the total energy generated by different sources and consumed by different sectors. The units used here are quadrillions of Btu’s (“quads” for short) indicate massive amounts of energy used at the national scale.
The total estimated energy consumption in the US in 2022 is around 100 Quads, and this number is on the upper end of the typical consumption bracket:
The contribution of solar energy has not yet approached the magnitude of traditional fossil fuel sources in the US, however its contribution to the renewable share of the electricity generation is actually substantial.
Let’s crunch some numbers:
National Electricity Mix
Total renewable energy share (including solar, wind, hydro, biomass, and geothermal) in the electricity sector sums up to 7.9 Quads (21%), which is comparable to 8 Quads (22%) for nuclear energy, 8.9 Quads (24%) for coal, and 12.5 Quads (33%) for natural gas. That is about 1/5 of the entire national electricity mix. If we look back at similar data for the year of 2012, renewables only accounted for 12% at that time.
What sources are fastest growing?
Comparing the data from the historical energy data from a decade ago (2012) and the most recent (2022), the fastest growing generation capacities are solar, wind, and natural gas. Nuclear remained steady, hydropower and geothermal showed small decline, biomass – small increase, and coal was on significant decline over these ten years.
Summarizing the trends:
Sector | Growth from 2012 to 2020 |
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Solar | + 698% |
Wind | +182% |
Natural Gas | +28% |
Biomass | +13% |
Nuclear | 0% |
Geothermal | - 7.5% |
Hydropower | - 14% |
Coal | - 43% |
Look up the historical versions of energy flow charts here [7].
By a big margin, solar has been the fastest growing source (almost 7-times growth!) in the national energy industry. The impacts were primarily seen in the residential and commercial sectors, which in addition to the grid may benefit from the distributed generation options (small-scale and off-grid installations).
Projections shown in this plot are based on the analysis of energy markets by EIA: “we project that renewable energy will be the fastest-growing U.S. energy source through 2050. Policies at the state and federal levels continue to provide incentives for significant investment in renewable resources for electricity generation and transportation fuels. New technologies continue to lower the cost to install wind and solar generation, further increasing their competitiveness in the electricity market, even as the policy effects we assume level out over time.”
“We project that consumption of natural gas will keep growing as well, maintaining the second-largest market share overall. The expected growth in natural gas consumption is driven by expectations that natural gas prices will remain low compared with historical levels.” (EIA, AEO2022 [8])
In the past century, society has been dependent on combustible products such as coal, natural gas, and petroleum products as the fuels of choice. While these energy sources are relatively cheap, they are not always available or located where we most need them, and they are non-renewable. In addition to this, there are real concerns about the effects burning these products could have on human health and safety as well as large impacts on the environment in general. The following are geofuels (resources from the Earth that are non-renewable).
Nuclear energy is an additional geofuel that does not have a major CO2 impact and is a major resource in countries like France. However, it has a strong "yuck factor" for the majority of society in Germany and the USA. It has the additional challenge of undesirable proliferation of fissile material for arms use. Again, there are numerous countries including the USA that make use of nuclear power for low-CO2 energy, but infrequent, high-visibility events such as Three-Mile Island, Chernobyl, and Fukashima Daiichi continue to influence popular will to invest and develop the resource.
Renewable energy sources provide a suitable alternative to using fossil fuel combustion (which generates CO2) to meet our energy needs. The well-planned use of renewable energy sources such as solar energy must form a part of the portfolio of energy sources. There are numerous real challenges for renewables like solar, such as intermittency and diurnal cycles (night-day), as well as the ability to identify economic opportunities, which is why we are putting a lot of effort into understanding the solar resource and the related economics in this course.
The following link uses Gapminder World to show the increases in cumulative CO2 production through time associated with population growth. Click on the link and press "Play" in the bottom left of the diagram:
You can explore this tool later and create your own plots with respect to time. For example, if you were to plot energy production (Supply) or use (Demand) you would see the same trend, or if you were to plot cumulative CO2 (log) vs. total energy production (log), they would show a rough linear correlation. But, for now, I want you to see where there are links between population, energy production, and CO2 production. Why is the USA more or less stable in its CO2 production?
This semester we are adopting a new platform for class discussions – Yellowdig! If you have only participated in the Canvas discussions so far, this may feel a little different. My hope is that this tool will help us make the discussions more engaging while maintaining the breadth and depth of learning we hope for. Please refer to the course orientation page that explains the steps to establish your Yellowdig account and set yourself up for participation.
Already have the Yellowdig account? – Then go to Canvas course menu, and click on “Yellowdig” link on the left to enter the conversation space.
For this first week of the class, I would like you to engage in conversations about the historical development of energy resources in your area – solar and beyond! Here are some guiding questions that will set you up and help you create good posts and initiate productive discussions:
Some of these questions will require you to dig in a bit and research outside of this class content. Expect that it will take some time to find good, resourced information.
Yellowdig Tip: When you create a post in the Yellowdig discussion space, you are required to choose a topic tag. For Lesson 1 discussions, use either (or both) of these tags:
You can create a single post or split it into two different topics – whatever feels best for you!
When it comes to Yellowdig, your posts, comments, replies, questions and answers – all these may add points to your yellowdig score, so don’t hold back! In order for these activities to work best for your learning, you need to participate frequently, keep up with conversations going and take opportunities to interact with your peers. Sometimes spontaneous conversations sparkled over a single fact mentioned bring more value than initial content.
Yellowdig discussions will account for 15% of the total grade in the course. Your posting and interactions will contribute to your weekly participation score (1000 pts.). After each week, all the Yellowdig points you earned will be transferred to Canvas and added to the your cumulative participation grade. Check the Orientation Yellowdig page for more details on the points earning rules – how many you get for starting a conversation, posting a comment, or attracting traffic on your post.
Be sure to set notification preferences in Yellowdig if you’d like to receive emails about someone posting or replying to your post. That will allow you not to miss important action as you go through the week.
I encourage you to create your posts in the middle of the study week (Sunday) and not to wait until the end of the lesson. That will allow others to respond in a timely manner. Each conversation will stay open after the lesson is done, but it will be harder to earn a high score with your work once everyone moved on to the next lesson. Note that point earning window for each lesson is limited to one week.
(Electronic course reserves, or "e-Reserves," are articles and book chapters that are available online through the University Libraries. Access this lesson's reading by going to Library Resources in Canvas and selecting E-Reserves for EME 810)
These two readings will provide our introductory perspective on solar energy in society. I want you to think about the impact of perception in society. Some of the "constraints" in society are real, physical limits to fuel access, while others are much more subtle--but both have a similar response to induce people to adopt alternative energy strategies.We can observe that popular perception of solar energy is strongly influenced by access to inexpensive fuels. In periods when fuels were effectively accessible, inexpensive, and unconstrained, a light-induced energy transfer (for sensible or latent heat change or for electricity generation) is usually perceived as diffuse and insufficient for performing work. However, for periods in history where fuels have become constrained (e.g., inaccessible due to high cost or high risk), innovation has turned to solar technology solutions. In such cases, the use of light-induced energy transfer is perceived as ubiquitous, and ample for performing work.
Evidence of such fuel constraints is observed as far back as the fifth century BC. During this period, the Greeks faced severe shortages of wood fuel. Archeological remains demonstrate that home designs evolved such that all houses could draw maximal utility from the Sun's warmth in winter months. It is interesting that famous Greek individuals commented on this solar design of homes. Aristotle has commented that home builders would shelter the north side of a home to keep out the cold winter winds. Socrates also lived in a home heated by the sun, and observed, "In houses that look toward the south, the sun penetrates the portico in winter" keeping the space warm (Note: a portico is just an older term for a porch). The Greek playwright Aeschylus further noted that only primitive cultures "lacked knowledge of houses turned to face the winter sun, dwelling beneath the ground like swarming ants in sunless caves." How primitive have we been these years without solar designed homes?
A renewable resource has a rate of withdrawal (a flow) from the stock that does not exceed the rate of resource replenishment.
Sun: pretty big stock for wind and solar.
The sun provides energy in the form of radiation. Solar radiation is the most important natural energy resource available to us because it drives all environmental processes acting at the earth's surface. It drives the earth's rain cycle, which powers modern hydroelectric generators, and large-scale atmospheric circulations which provide the winds that have powered windmills for centuries. The sun doesn't warm the air, because the sky is largely transparent. The sun warms the ground which then warms the air. We will show how solar energy conversion devices often take advantage of the solar-thermal connection. In fact, we can often take advantage of air masses contained by walls, trees, or hills to trap and store thermal energy from the sun.
Solar energy technologies convert solar irradiance (sunlight) into forms of energy found useful to society. Historical and current developments in solar technology often coincide with specific economic or fuel constraints. As we observe from our reading assignment, technologies using solar energy conversion have been developing for a long time. Solar and wind power are among the options providing a low or non-CO2 associated energy source for electricity production. Solar energy is also a natural part of heat production in buildings or solar thermal plants. This section will review the value of solar energy from a historical context while also discussing the origins of using solar energy and the present-day development of the solar energy industry.
Below is a link to a video of the Ryoan-ji Zen temple in Japan. Look to the left of the marker at the white field just to the south of the brown roof (you will need to zoom in a few times first). This is the famous rock garden of the temple. The white field reflects light into the adjoining room to the north, and the walls surrounding the field keeps cool air inside the area. As a side note, the icon you see on the building is a map symbol for a Buddhist temple.
If solar energy is this important, and the Greeks in the fifth century B.C already recognized this, you may ask why solar energy applications are not more prevalent than they are at the moment? While there is no simple answer to this, there are still many obstacles that lie in the way of its widespread adoption. The future of solar power development will depend on how we deal with constraints such as scientific and technological problems, marketing and financial limitations, and political actions and agreements that favor other energy sources.
For more information on solar home design (also termed "passive solar"), see the California Solar Center: Passive Solar [12].
Horace de Saussure was a Swiss naturalist of note to the history of solar development in the 18th century. He began to explore the role of a glass cover on a confined space like a box or a room. In the 1760s, de Saussure observed the following: "It is a known fact and a fact that has probably been known for a long time, that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass." To find out how effectively a glass cover works to trap solar gains, de Saussure constructed a large flat pine box that was insulated inside, with a glass cover on top. Inside the box, he placed smaller boxes. When the flat cavity-cover absorber was exposed to the sun (no concentration), the internal box heated to 109 °C (228 °F), which is 9 degrees higher than the boiling point of water.
Functionally, the clear glass allows shortwave irradiance to transmit through and be absorbed by the dark interior of the box. In addition, the glass cover prevents the warm air from escaping. This is a classic flat plate cover-absorber system. The hot box that de Saussure began with later became a prototype for flat plate solar hot water panels, providing hot water to millions since 1892. The hot box also led to the development of modern solar cookers. For more information on solar thermal design, visit the following links:
While photovoltaic materials were explored in the 1860s, tied to research in transatlantic telegraph cables (using selenium; by electrician Willoughby Smith), they did not emerge into the larger market until nearly 100 years later. Photovoltaics using silicon material were introduced to the commercial world during the early 1950s by three lead scientists at Bell Laboratories: Calvin Fuller (a chemist), Gerald Pearson (a physicist and materials experimentalist), and Darryl Chapin (a device engineer). The development of a silicon-based PV cell led to applications in space and telecommunications first, followed by applications for the petroleum industry (for anti-corrosion in wells).
PV has proved itself as a standard technology for decades. All of today's satellite communications are powered by photovoltaics, starting with the Vanguard 1 [19] (which had no "off" switch, and so continued to transmit long after it needed to). So, be thankful, we would not even have a modern society without the advent of PV!
Constraints have included wood shortages in Greece and Rome in early centuries BCE, shortages of trees and fuel in the Chaco civilizations of the Anasazi peoples of North America, coal reserve constraints in 19th century France, and fuel access constraints in rural California before the 1920s. We have recently (the past decades) entered into a new period of constrained fuel consumption due to climate forcing effects from anthropogenic greenhouse gases (associated with fuel combustion, agriculture, and high energy demand), as well as increased risk in supply chains for fuels (the quest for energy independence).
We are jumping far, far ahead to see where we can take the concept of a Solar Energy Conversion System in society. The text in Chapter 12 complements your reading of Ch 2 and goes in depth to identify the different technological patterns in solar energy conversion systems. I think you will find the content relevant in the broad sense of designing systems, and that it will engage your creative processes for the future project at the end of the course.
Think about the way in which so many of our technologies and biologies are in fact "solar energy conversion systems," with some or all components of a functional aperture, receiver, distribution mechanism, storage, and control mechanism. What may be even more interesting is when you start to identify collections of different SECSs in the same space.
An important context of the approach to solar energy is understanding what a system is. We define a system as a collection of elements that are connected together via weak or strong network relations and that have a pattern or structure that yields an emergent set of behaviors. We are concerned, in this course, with environmental systems, each with boundaries that describe the system and its surroundings.
A Solar Energy Conversion System (SECS), as the name implies, is a system that converts the energy from the solar resource into work found useful by society. This system has the potential to be deployed as an ecosystems technology or an environmental technology, meaning the energy system interacts in a constructive way with the patterns of nature. As a process, solar energy conversion calls upon designers and engineers to include all the elements essential for the proper functioning of a conversion system. These include the Sun, the Earth and the applied technological system (for example solar thermal or solar photovoltaic) in question. These systems call upon researchers to simultaneously assess scales of solar resource supply and use, systems design, distribution needs, predictive economic models for the fluctuating solar resource, and storage plans to address transient cycles.
We have reviewed the basic system concept that can be used to design solar energy conversion applications, and more detailed and thorough information will be presented in a future lesson. At this point, we move on to the nature and composition of SECS on the Earth side (incident surfaces).
A Solar Energy Conversion System consists of the following elements:
The next section of this lesson provides you with an exercise to identify these elements in several different solar energy conversion systems functioning in diverse settings.
Based on the reading in the previous page of the lesson, we understand that almost everything exposed on the Earth's surface can be described in terms of a solar energy conversion system. Some systems are capable of producing more useful work than others, and there are both technologically designed and natural systems. We will use this activity to explore this concept and to identify various SECSs embedded in society. Some will be obvious in the context of this class, while others will require a more nuanced view or a creative perspective.
Take a look at each of the following five images, and thoughtfully locate as many SECSs as you can in each one. Now, try to identify and list the functional components (aperture, receiver, storage, distribution, mechanism, and control mechanism) for each of these SECSs and think about how they work. Please write a short paragraph for each image that identifies all of the SECS’s as well as an explanation of how each one serves as a component. It may even be possible that some of the systems are themselves components of a larger system, in which case you will need to dial out your perspective to a larger system.
Do your best to explore and be creative rather than looking for all the "right" answers.
Please fill in the "matrix" (Excel spreadsheet template provided) based on the results of your analysis. You are also welcome to provide any additional discussion on the systems you observe in the images. Submit your work into “Lesson 1 Learning Activity Dropbox” in Canvas.
This activity is graded out of 20 points. Each image is worth 4 points, 2 points for identification of the components and 2 points for your explanation.
See the Canvas calendar for specific due dates.
Come back to review this Chapter specifically focusing on the Frameworks sections.
The last activity emphasized the way that SECS can "work" as a functional system. Now, I would like you to read and reflect on the broader concept of "design" as pattern with a purpose in society and the environment. As you are reading, look into the ways that society has established frameworks for integrating solar goods and services into local solutions (which we will expand upon as "locale"). The following points are highlighted in the text.
In this section, we are going to be looking at some of the same pictures we saw in the Learning Activity, but from a different perspective. We will call the following case studies "frameworks," and investigate the value of solar energy conversion systems in different contexts. Consider the following frameworks as interpretations of valuing the solar resource and the resulting solar goods and services by society (in economic and sustainability terms): the same resource can be applied to different client needs, the way that we have set up legal ramifications to protect access and rights to the resource, the entrepreneurial/ecopreneurial spirit that is a part of SECS design and implementation, and the ecosystems services that surround our SECSs.
Many of us will come into the field of solar energy with a slightly biased view that the solar resource is useful for making electricity (a preferred solar good by many stakeholders) using photovoltaic technologies (the Solar Energy Conversion System of note in current society) or for making hot air or water (like a solar hot water panel, another SECS technology). However, we must be also aware of the use for daylighting. Daylight is as essential to human health as clean water and air. The variable intensity of daylight has been found to increase alertness within the office space (as opposed to constant light conditions with artificial lighting).
In the case of a Liter of Light (see link to NPR article, above), the ecopreneurial venture by this non-profit means using appropriate technology to deliver a high solar utility (again, meaning a preference to the set of goods and services from solar energy conversion systems) to their clients at accessible costs. The bottles are discarded 1 L plastic soda bottles, filled with water and a drop of bleach (to minimize algal growth or other microbes). The technology is the same as a "light pipe," or a fiber optic, using different indices of refraction between air, water, and plastic to create a phenomenon called total internal reflection. On boats, this type of light direction uses a centuries-old SECS technology called a deck prism [30].
Not only do these warm climate homes benefit from better lighting, they also avoid fuel costs for electricity (if available) and combustible fuels such as kerosene. Additionally, the solar bottle light pipes will improve indoor air quality by reducing fuel combustion inside.
While the solar resource from the sun is available at no cost to us, there are laws that may restrict the way we intend to use the sun for purposeful work. Designers will often call the accessible area for solar implementation the "solar envelope," but how do we maintain the legal rights to make use of or access our own solar envelope, and can we make sure that solar technologies can even be installed in our locale? Legal structures for solar energy is not a new concept. In ancient Rome and Greece, legal structures were set up in the form of easements, allocated government lands, and sometimes strict urban planning for orientation and elevation limitations on entire communities.
Solar rights define access to solar energy and hold significant economic consequences. They dictate whether a property owner can grow crops, illuminate his space without electricity, dry wet clothes, reap the health benefits of natural light, and perhaps most significantly in our modern era, operate solar collectors.
In the USA, we distinguish between solar rights and solar access.
Entrepreneurs are generally great contributors to the commercialization of interesting and useful technology. The field of solar energy is no different. An important character in the development of SECSs is Frank Shuman, an eclectic inventor in the late 1800s. Shuman formed the Sun Power Company in 1910 and successfully harnessed solar power physics to generate steam pump power in Egypt in 1911.
New entrepreneurial ventures in solar are often also humanitarian and ecological in nature. SolarFire.org and Liter of Light are two examples.
The Prometheus 100 solar concentration systems for steam generation can be seen in the image above. Mirrors (the aperture) are focusing shortwave light onto an upper central receiver (glowing bucket), where steam is produced. The steam is running a small steam generator pictured in the lower left of the image. The supply water is pumped in from the tube seen in the upper right. The entire system plans are available as open-source information, and can be machined with accessible local technologies and inexpensive materials. This system was installed in Rajkot, India.
Any solar technology will have an impact on the ecosystem in which it is deployed. In addition, it could add ecosystems services to the area if designed with an awareness for landscape architecture and ecology. Presently, design teams discuss manners in which photovoltaic arrays can add desirable shading in addition to power generation (desirable shading would be considered a preferred solar service). One common method is to design and install a solar panel to also serve as a shading structure for cars in a parking lot.
You may wish to read the short descriptions of the various services from The Economics of Ecosystems and Biodiversity (TEEB [35]) website. They have created a fairly concise list with descriptive sections that clearly identify valued processes that emerge from a resilient ecosystem. Remember: our technologies and our societies are always a functioning part of our ecosystems in the locale that we operate, even if it doesn't quite seem that way in our daily lives.
NREL's System Advisor Model (SAM) [37]will be a useful simulation tool for this class. You will be required to use it for some of lesson assignments and your project. So I would like you to take the time to download and try launching SAM on your computer this week.
The NREL website linked above also has a webinar video, which walks you through the key functions of SAM. Please feel free to watch it at your leisure.
There is no deliverable for this activity, but I strongly encourage you to take initial steps for installation and creating a project in SAM this week. Lesson 2 will give you some work to complete in SAM, so it will be useful to have it up and running.
Feel free to share your thoughts, questions, and tips on the SAM software in the Yellowdig space. Use the topic "Starting with SAM" for your posts. This is not a required activity, but it can still boost your participation score for this week.
We have just finished looking at the historical and modern contexts for valuing solar energy (and SECSs) in society. I wanted to put you in the frame of mind that solar energy as a valued resource is a flexible concept. The value expands with energy constraints and with positive health and safety implications; the value shrinks with the reduced costs of alternatives such as fuels. There are many different ways that society values the solar resource, however, and as a part of a design or engineering team (or part of a policy team), we should remain creative and persistent in trying to develop and expand SECS deployment into society.
You have made your first step toward the solar assessment project that we will present to our peers in Lesson 12 at the end of the course. I would like you to make sure that you have found the SAM (System Advisor Model) [39] website and downloaded the software onto your systems. Your future clients/stakeholders will be informed by your creative ability to present them with a valuable solar resource in their locale, one which also yields financial benefit, social benefit, and/or greater ecosystems services. SAM is a useful complementary tool in that communication.
You have reached the end of Lesson 1! Double-check the to-do list on the Lesson 1 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 2.
This is the first of three fairly intense lessons covering the fundamentals of solar energy. You will need to distill a lot of information and work hard to internalize the key topics. Make sure that you use the discussion forums to communicate with your peers, and be sure to ask questions. Most of us have learned bits and pieces of the following materials, but never together in one setting. And for some of us, this is the very first time we are learning to juggle all the balls that tie together solar energy. You can do it!
Time and Space are related! This lesson will discuss the tools needed to evaluate spatial and temporal relationships:
Just like with navigation in a ship or an airplane, time and space relations are linked together in Solar Energy and can be represented and communicated as geographic information. We input that geographic information in terms of angles and use key relations from spherical trigonometry to make time and space relations easy to calculate with a computer. For our purposes:
angles = coordinates in space and time.
The tools we develop are going to explain the sun’s position relative to any point on the surface of the Earth. Once developed, our useful equations can also be applied to estimate the time and location of shadows that block SECSs or tracking technologies for SECSs.
You will observe substantial mathematical relations in this lesson, and you will be expected to demonstrate your skill at applying them to solar problems in shading assessment. These equations are at the core of software like SAM, and a student completing this course should be very familiar with their application. Stick with it!
By the end of this lesson, you should be able to:
This lesson is loaded with material on Sun-Earth geometry and will take us two weeks to complete. Please refer to the Canvas Calendar for specific timeframes and due dates - those can vary from semester to semester. Specific directions for the assignments below can be found further within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 1 - Introduction, Communication of Units and a Standard Solar Language J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 6 - Sun-Earth Geometry J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 7 - Applying the Angles to Shadows and Tracking W.A. Beckman, J.W. Bugler, P.I. Cooper, J.A. Duffie, R.V. Dunkle, P.E. Glaser, T. Horigome, E. D. Howe, T.A. Lawand, P.L. van der Mersch, J.K. Page, N.R. Sheridan, S.V. Szokolay, G.T. Ward (1978). Units and symbols in solar energy [40]. Solar Energy 21, 65–68. |
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To Do: |
Quiz (see Canvas) - due by the end of first week Yellowdig Discussion: Reflection on time conversions / Daylight savings Learning Activity: Shadow diagrams with Sun Charts - due by the end of the second week Engage in all Try-This and Self-check activities (not graded). |
Topics: | Directionality of light: source to sink Solar Time vs. Watch Time Sun-Earth relations Sun-Observer relations Collector orientation and shading effects |
If you have any questions, please post them to the Lesson 2 General Questions and Comments Discussion Forum in Yellowdig. I will check these forums regularly to respond. Feel free to go through the comments and post your own responses if you are able to help out a classmate.
We start by reviewing these small sections on language. There are things to measure and symbols for those metrics that we need to agree upon throughout the class.
While reading, consider the following points:
Units and symbols in solar energy [40] (Beckman et al., 1978). You can also access this article through Penn State's Electronic Course Reserves.
Solar Energy Journal [41] was established for the International Solar Energy Society (ISES) [42] and has been around for some time now. Solar Energy Journal stands as an important forum for peer to peer sharing of solar research for energy conversion and human applications of solar energy. What I want to establish here is that there is precedent for the complex system of notation used in the solar energy world that has been in use for decades. The original authors have established the following observations:
"Many disciplines are contributing to the literature on solar energy with the result that variations in definitions, symbols and units are appearing for the same terms. These conflicts cause difficulties in understanding which may be reduced by a systematic approach such as is attempted in this paper.
It is recognized that any list of preferred symbols and units will not be permanent nor can it be made mandatory, as new terms will emerge and old ones become less used with the development of the subject. But in the meantime, a list would be appreciated by the many workers who are entering this multi-disciplined field...
...Energy: The S.I. (Systèm International d'Unités) unit is the joule ( ). The calorie and derivatives, such as the langley (cal cm-2), are not acceptable.
No distinction between the different forms of energy is made in the S.I. system so that mechanical, electrical and heat energy are all measured in joules. However, the watt-hour (Wh) will be used in many countries for commercial metering of electrical energy...
Power: The S.I. unit is the watt ( ). The watt will be used to measure power or energy-rate for all forms of energy and should be used wherever instantaneous values of energy flow are involved. Thus, energy flux density will be expressed as W/m2 or specific thermal conductance as . Energy-rate should not be expressed as $J/h$.
When energy-rate is integrated for a time period, the result is energy which should be expressed in joules, e.g. an energy-rate of 1.2kW would if maintained for one hour produce 4.3 MJ."
W. A. Beckman, et al.
It is preferable to say | Rather Than |
---|---|
In summary: received energy flux density (or power density, called irradiance) can be expressed in units of W/m2. We also note that the received radiative energy density (called irradiation) can be expressed in units of J/m2, or in units of Wh/m2. Notice that we did not use radiation, which is an expression of light glowing outward (emitted light, different direction than what we want).
In today's maps of the solar resource, you will often see the units expressed in kWh/m2. You should be aware that these are still only representations of solar light energy density, and not the hourly/daily/annual quantity of potential electricity that could be produced. To find that value, we need a simulation tool like SAM (System Advisor Model, which you should have downloaded at the end of Lesson 1), which takes irradiation data and converts it into power data.
I would like you to now take a short self-quiz to see if you recall the common uses of the notation and descriptions for solar energy (used in particular in this class.).
When we want a shorthand to describe spatial relationships on continuous surfaces that are sphere-like, as with the Earth and the surrounding sky and stars, we choose to use Greek letters. In contrast, when we are trying to communicate things like linear distances, lengths, time, or simple Cartesian coordinates, then we will tend to use Roman letters for our shorthand.
You may notice in your reading of older textbooks that several systems of sign convention for the angles have emerged for practical use. Also, the various systems can have different approaches to azimuth that we should be aware of. For instance, the software SAM (System Advisor Model) will use both the $360^\circ$ clockwise standard (from Meteorology) as well as the $\pm180^\circ$ standard used extensively in the component-based models of TRNSYS and SAM. We will be sure to become familiar with both.
Below are four tables showing the Angular Symbols for Standard Solar Relations.
Angular Measure | Symbol | Range and Sign Convention |
---|---|---|
altitude angle | (alpha) | 0o to + 90o; horizontal is zero |
azimuth angle | (gamma) | 0o to + 360o; clockwise from North origin |
azimuth (alternate) | (gamma) | 0o to ±180o; zero (origin) faces the equator, East is + ive, West is - ive |
Angular Measure | Symbol | Range and Sign Convention |
---|---|---|
latitude | (phi) | 0o to ± 90o; Northern Hemisphere is +ive |
longitude | (lambda) | 0o to ± 180o; Prime Meridian is zero, West is -ive |
declination | (delta) | 0o to ± 23.45o; Northern Hemisphere is +ive |
hour angle | (omega) | 0o to ± 180o; solar noon is zero, afternoon is +ive, morning is -ive |
Angular Measure | Symbol | Range and Sign Convention |
---|---|---|
solar altitude angle (complement) | (alphas is the complement of thetaz) |
0o to + 90o |
solar azimuth angle | (gammas) |
0o to + 360o; clockwise from North origin |
zenith angle | (thetaz) |
0o to + 90o; vertical is zero |
Angular Measure | Symbol | Range and Sign Convention |
---|---|---|
surface altitude angle | (alpha) | 0o to + 90o |
slope or tilt (of collector surface) | (beta) | 0o to ±90o; facing equator is +ive |
surface azimuth angle | (gamma) | 0o to + 360o; clockwise from North origin |
angle of incidence | (theta) | 0o to + 90o |
glancing angle (complement) | (alpha) |
0o to + 90o |
Please scan all of Chapter 6 right away, to get an initial overview of the role of angles and time together with the relative positions of the Sun, Earth, and the SECS that your client would like to install. We use several angles throughout this chapter (check back to the Table of Angular Symbols [43] anytime, also found in the textbook Ch. 1). We also use a whole lot of dense equations. Don't be intimidated by the equations; they are all based on the trigonometry for a spherical surface, and we will break them down in chunks in this lesson. Just take note of them and keep reading. Take a few notes in the margins as you go!
In this first assignment, we are going to get familiar with the angular relations between the Earth and the Sun, and the relation of those angles to things like Seasons! You are all familiar with the concept that winters are cold, and summers are hot, but why??
Keep an eye out for the cosine projection effect. This is something that we often wish to minimize by tilting our solar energy conversion systems up toward the predominant diurnal (daily) arc of the Sun averaged over the year.
As we have seen in our reading, the Earth rotates with a roughly constant speed, so that every hour the direct beam (a ray pointing from the surface of the Sun to a spot on Earth) will traverse across a single standard meridian (standard meridians are spaced 15° apart). The implications are that the unit of one hour is equivalent to the rotation of Earth 15 degrees. When Earth rotates such that the beam of the sun shifts +1° of longitude from East to West: it takes 4 minutes of time.
Wild fact: a time zone change of one hour is really just 15 degrees of separation between standard meridians.
The axis of rotation of the Earth is tilted at an angle of 23.5 degrees away from vertical, perpendicular to the plane of our planet's orbit around the sun.
The tilt of the earth's axis is important, in that it governs the warming strength of the Sun's energy. The tilt of the surface of the Earth causes light to be spread across a greater area of land, called the cosine projection effect.
When you tilt a surface away from a beam of light, you spread the same density of light across a larger area. Recall that irradiance is in units of W/m2, so a larger denominator means a smaller value of irradiance, right?
Explore the concept of the cosine projection effect in the following experiment.
This links directly with Chapter 6: Experiment with a Laser of SECS.
Watch the video of the virtual flashlight below and then answer the questions.
The sun is about 93 million miles away from the Earth (equivalent to ~150 million km). That is so far away that the photons from solar irradiation effectively travels in parallel rays. So, unlike the flashlight experiment, the tilt of the sun has no bearing on the intensity of the radiation reaching the Earth's surface. Instead, we find that the Earth's tilt controls the intensity of irradiation and the seasons.
Keep in mind that the Earth's axis points to the same position in space (toward the North Star, Polaris). As the Earth travels in a near spherical (a very small eccentricity into an ellipse) orbit around the sun, the Northern Hemisphere can be tilted toward or away from the sun, depending on its orbital position.
Click on "Summer" in the above animation. When the Northern Hemisphere tilts toward the sun, the irradiation has a lower angle of incidence, meaning more photons strike a smaller area during the daytime. Answer the following questions for yourself. If you have any questions, please post to the Lesson 2 Discussion Forum.
Now, answer the same questions for autumn, spring, and winter.
Forecasters and meteorologists use different criteria to determine the "meteorological seasons." For example, meteorological winter in PA runs from December 1 to Feb 28/29, a period that statistically includes the three coldest months of the year. This is also centered on a time about 25 days after the Winter Solstice.
Meteorological summer runs from June 1 to August 31, a period that includes the warmest three months of the year. Again, this is a period centered about 25 days from the Summer Solstice.
As one more example, review Pittsburgh's plot of annual average high temperatures. The maximum daily temperature occurs in late July, long after the summer solstice.
In this continued reading of Chapter 6, you will be focusing on the way that we account for time in solar energy and the relation of time to more spherical angles. We will use those angles to later calculate the estimated irradiance and irradiation conditions just outside of the Earth's atmosphere (called Air Mass Zero: AM0).
Pay attention to the use of solar time vs. watch time (which will be expressed as standard time vs. daylight savings time). In solar simulation tools like SAM (System Advisor Model), we will only be using solar time, which is the industry norm. Watch time is just a convenient way to get everyone to work at the same time, and to coordinate conference calls. Your job is to get out of watch time and into thinking in terms of solar time, and the angles those times represent.
As we see in the reading, time is a critical parameter in solar energy resource assessment, and we use a "different" time from your mobile phone (or if you have one, a watch). The time that we use in solar energy is the apparent time and path of the sun relative to the aperture or collection device, called Solar Time.
The time that you are used to using on your laptops, phones, and (if you still have them) watches is called Standard Time, and is referenced back to the Coordinated Universal Time (UTC), the primary time standard by which the world regulates clocks and time.
Our notion of time is also tightly coupled with our system of longitude (the longitude symbol for us is ). We have used lines of longitude, or meridians, as a reference for time and position E-W. Because time and angles are all linked together, we cannot escape the sexagesimal (base 60 math) system for geographical locations. As we demonstrated earlier in the lesson, each standard meridian (a major line of longitude) is spaced 15° apart, beginning with the Prime Meridian () in Greenwich, England, and continuing for 360 degrees, or 24 hours.
I live someplace other than Greenwich. How do I account for Longitude ()?
Your standard time zone will tell you the standard meridian (). For example, the EST is -5h from UTC, while Central European Time (CET, like Paris) is +1h from UTC.
Answer the following questions for yourself. If you have any questions, please post them to the lesson 2 discussion forum.
Where you live, or where your future solar site assessment will occur, will likely be well within the edge of a time zone (meaning ). We already learned that every 1° of angular rotation on Earth is equal to 4 minutes of time. Standing in one spot on the surface, this means 4 minutes of relative time correction locally per degree of deviation from a Standard Meridian ( ). So, locales will have a local longitudinal refinement to account for, in order to account for not living directly on a 15-degree incremental Standard Meridian on Earth.
Standard Meridians define the beginning of a time zone, and not the end of a time zone. So, you are always going to look to the start of a time zone to find the Standard Meridian.
There are a few other cities that actually are well seated for solar time zone correction (close enough for our calculations):
My client lives someplace other than a Standard Meridian, how do I account for that?
First, go to Google and type "<insert city name> longitude". You should get a quick response of both the Longitude (lambda) and the latitude (our symbol for latitude is lowercase Greek "phi": phi), represented in decimal form (more useful to us for trigonometry and angles).
Have you noticed that real time zones are more often political boundaries that zigzag around, rather than following an actual Standard Meridian? So, actually, there can be locales for clients that are East of their own time zone Standard Meridian, instead of the normal relative locations West of the time zone Standard Meridian. This is why, in the reading, you will see minutes per degree of local longitudinal shift away from the time zone's Standard Meridian.
Even in Greenwich, where no longitudinal correction is necessary, "noon" UTC will generally not be the time when the sun is directly overhead. We can see in the plot below that watch time and solar time are the same in Greenwich for only 4 days in the year.
As you will have read, our interpretation of watch time assumes an even progression for Earth's planetary rotation, with no weebles or wobbles or precession of the polar axis. However, you will now know that wobbling occurs, and there is great variability in the rotation of the earth throughout the months of the year. This is why we add leap years and leap seconds to our calendars. So, we create a "mean time" based on the length of an average day to keep things simple. Solar time has to correct for this mean time approximation. Equation of time correction versus day of the year is shown in Figure 2.7. As an exercise, you can try to calculate these curves based on the empirical equations (6.10) and (6.11) in Brownson textbook.
The following picture was a composite of images, taken at the same watch time every few days for an entire year, to record the position of the sun. We call the shape an analemma. Notice how there is a big loop and a little loop, and compare the same big waves and little waves in the first image of the Equation of Time correction above. If you were to draw a line down the center, you would have removed the error from watch time, and you would be one step closer to solar time.
For views of amazing solar analemma photography by Anthony Ayiomamitis, also, please visit the Stanford SOLAR Center [49].
These images were taken at the same time and location every day for one year. You will see the curve described by the Sun over that year. An analemma is a beautiful way to capture both the range of declination $\delta$ (along the length of the analemma) and the Equation of Time Et (the expansion or width of the analemma) in a graphical format.
As we have seen in the reading, we have a method to correct local watch time to solar time, to correct for the true time of day according to the sun relative to the meantime.
The time correction factor is a function of the equation of time, the meridian of the local standard time zone (Lst), and the longitude of the collector/observer (Lloc), and the Equation of Time (Et). The equation of time correction accounts for the wobble and is the first step. Then we need to correct for the change in longitude leading to time zones (standard meridians occur every 15 degrees) and the change of time for locales that are not located along standard meridians.
In these conversions, each year is assumed to begin just after midnight, Dec. 31, and time counts up from there. The time corrections here are in terms of minutes, not hours. The Equation of Time corrects the time to mean solar time (by adding or subtracting up to 16 minutes of correction). The Time Correction Factor corrects time for your shift in Latitude from each Standard Meridian (which occurs every 15 degrees away from the Prime Meridian). Daylight Savings Time is an optional correction, as there is a +60 minute difference from Standard Time between March and November in the USA (and some other regions of the world).
In reviewing these sections, you should notice that three common angular symbols keep popping up: the declination (), the local latitude (), and the hour angle (). As we shall again see in the next section, these are three of our key Earth-Sun angles.
We additionally include the use of longitude () in our calculation of time, and in particular, converting time to an angle: the hour angle.
When we convert time to an angular value, we can no longer use a 24 hr format. We need to convert hourly time into a useful angle based on the properties of a sphere, again using spherical trigonometry.
Finishing Step for Time Conversions! The Hour Angle () in decimal degrees.
We represent the apparent displacement of the sun away from solar noon, either as a negative or positive angle. An of zero indicates that the sun is at its highest point for that given day.
Another way of looking at it is the angular difference between the local meridian of the observer/collector and the meridian that the beam of the sun is intersecting at a given moment.
Now, the day won't really begin at -180 degrees, anywhere between the arctic circles. However, I wanted to emphasize that the day only begins at -90° on two days a year. Those days occur during the Equinox moments in the orbit of the Earth about the Sun, and the length of those days is actually 12 hours long. All other days are either shorter than 12 hours, or longer than 12 hours. As such, they end either less than 90° or greater than 90°.
The images below demonstrate singular arcs of the Sun for each hour in solar time, at five different latitudes on Earth (no analemma correction necessary). The peak hour (or the hour with the highest solar altitude angle) is defined as solar noon. You will notice that the solar equinox has twelve sun spots for latitudes below the arctic circle, and that the sun rises due East, while setting due West. During the solar solstices, you see multiple arcs: one for winter and one for summer. Notice that there are more hours of the day in the summer, and the sun rises farther from the equator in the Summer (sun rise in the northeast for the Northern Hemisphere).
What else do you notice in comparing the four critical times of year at different latitudes?
Now, we are ready to use hour angle to find out positions in our solar design projects!
Remember that the Earth rotates at 15 degrees per hour, and 0.25 degrees per minute. It can get confusing when you're comparing spatial seconds with temporal seconds, right? That's why we will stick with decimal notation in all of our references to latitude ($\phi$), longitude ( ), and angles (degrees).
Step 1: Correcting for Standard Time (Standard Meridian) in Hours: Find the time zone of your client. The hourly longitude correction is plus or minus X hours from the Prime Meridian, where UTC = 0h. Multiply the positive or negative hour value by 15°, and you will know your standard meridian for the Time Zone.
Key: Your standard meridian for your time zone begins on the East side of the time zone. (Sun rises in the East, right?)
Step 2: Correcting for Time from the Local Longitude Relative to Standard Meridian in Minutes:
You need to know the longitude for the standard meridian of the client ( ) from their local time zone, and the local longitude of the client's location in question (). We calculate the longitudinal correction, from these two.
Keep in mind: Time zone borders are political boundaries, and can be constructed on both sides of a Standard Meridian. A locale to the east of the Standard Meridian would still be input as a positive value (the resulting minutes would be positive).
Note: we use the sign convention that longitudes are positive valued for to the East of the Prime Meridian, and negative valued to the West of the Prime Meridian. This equation is valid for both sides of the Prime Meridian. The exterior negative sign is there to make the time correction algorithm work correctly.
Step 3: Calculating Equation of Time (Analemma) Correction in Minutes (Et): We begin with the two simple coefficients of n (the day of the year, from 1-365), and B (see the first equation). Our assumption is that the year begins at midnight of the new year, and the trigonometric portions of the equation of time will take an argument of "B" in degrees.
You have seen that the Equation of Time has a graphical representation, the analemma. Once we determine a correction in the scale of minutes, we can use it in the Time Correction Factor, TC.
Step 4: Completing the Time Correction Factor ($TC$): The time correction factor is a time shift in minutes.
Recall that 0.25° of longitudinal rotation (of the planet) will consume 1 minute of time. That would make each degree of change equivalent to four minutes. Hence, we need to multiply our longitudinal correction by a factor of four to yield a consistent unit of minutes in time. This is shown as the value of the longitudinal correction (Lc) in units of minutes (temporal, not geospatial).
Step 5: Accounting for Daylight Savings Time (DST) in 60 minutes.
The equations do not tell you when DST occurs from country to country. There is a +60 minute difference between March and November in the USA. So, if we had to correct for DST, then we would need to subtract that 60 minute addition back out.
Again, make sure that the data is presented in minutes, rather than hours.
I hope you are getting the hang of Yellowdig conversation platform and are ready to pick up the pace! You can access the discussion platform through the Canvas menu.
For this week, I would like you to question why we do all of these time conversions for solar energy. Why do we need to know these time and angle relations as solar energy specialists? There are many terms and concepts to reflect on – some are well-known, and some may be new to you. In this discussion you will have a chance to reflect on the meaning and your understanding of those and perhaps some of your peers’ interpretations of those concepts will be helpful to your comprehension of this lesson. Here are some guiding questions you may use as starting points for your posts:
This is an open-ended discussion, and you may ask questions or raise your concerns with things that might still remain confusing. Feel free to post and comment on any of the above-listed topics and whatever you want to share after working through the lesson material.
Yellowdig Tip: When you create a post in the Yellowdig discussion space, you are required to choose a topic tag. For Lesson 2 discussions, use any of the following:
Know that you can tag your post with one or several of these topics depending on its content. It gives you flexibility to discuss these things cumulatively or, if you prefer, to break your writing into several smaller posts.
Once again: the more you participate, the more opportunities for your discussion score to grow through the week. Comment, ask questions, react, throw in additional thoughts – it all is in your benefit!
Yellowdig points you earn over the weekly point earning period (from Saturday to next Friday) will count towards 1000 pts. weekly target. But you can go above it (to 1350 pts. max) if you are feeling active. That may help you to make up for some low-activity weeks. Yellowdig discussions will account for 15% of the total grade in the course. Check back the Orientation Yellowdig page in Canvas for more details on the points earning rules.
There is no hard deadline for participating in these discussions, but I encourage you to create your posts in the middle of the study week (Sunday) to allow others to engage and respond while we are learning specific topics in the lesson.
The following sections are closely coupled to the reading assignment, but I have enlarged the images for you. Our Solar Energy Conversion Systems are going to be fixed at some location on the surface of Earth, right? So, we will place an "Observer" at the future location and describe the relative position of the Sun and Earth and SECS. However, there are a few underlying spherical relations that do not depend on the location of the SECS or the observer.
This section discusses those geometric relations that are effectively "Independent of the Observer." Keep an eye out for the declination (delta, ), the latitude and longitude ( and ), and , the hour angle.
The Greek notation used below is now fairly standard for the field, but you may see different approaches in informal settings or unusual textbooks. I follow the established nomenclature defined by Beckman, et al., in the research journal Solar Energy in 1978, reprinted in Solar Energy in 1997. As noted earlier, the document is available to you from the Library E-Reserves.
Again, this is the way we represent the apparent displacement of the sun away from solar noon. An hour angle of zero degrees indicates that the sun is directly above, and the sign of the hour angle is determined by occurring either before noon (negative) or after noon (positive). Another way of looking at it is the difference in angle between the local meridian of the observer/collector and the meridian that the beam of the sun is intersecting at a given moment.
This is not an observed angle relative to the surface of the Earth and the Collector. Declination is the observed angle (due to the polar tilt) between the plane of the Earth’s equator and the plane of the ecliptic (the plane in which the Earth's orbit about the Sun lies). Declination has a maximum angle of 23.45° at either solstice. In this case of coordinates, the sun observed in the North is positive, and in the South is negative. One can imagine this as a series of sun paths over the course of a year. In contrast, the declination reaches a midpoint at either of the equinoxes. The range of declination is limited to Earth’s tilt: −23.45° (winter solstice) (summer solstice).
Declination is independent of location on the planet!
The declination can be calculated by a simple approximation (first equation) or a Fourier series (second equation).
where:
The complement of longitude () in geospatial coordinates is the latitude. When combined together, we have identified a singular point on the surface of Earth. However, separately, they refer to arcs or circles spanning huge distances. We can look up the latitude and longitude of a site on a search engine like Google.
Key latitude angles of interest are:
Once we have identified our specific location on the surface of Earth, we use the spherical relations between the Sun and the fixed observer at the locale to describe the relative motion of the Sun during the day and over the year. Once again, this text is closely complementing the text in the book.
Pay attention to the fact that this particular set of relations is only for the special case of a horizontal surface. We are only describing angles for an imaginary flat surface on Earth, like a table top. In real SECSs, we will often tilt the receiving surface up (tilt has a symbol of ), specifically to minimize the cosine projection effect that occurs at a given latitude. Bear in mind that the angles for a non-horizontal surface (tilted, with some accompanying azimuth orientation ), or for a tracking system, are an entirely different set of general equations.
Also, as these relations refer to the apparent motion of the Sun, they have the subscript of "s" for the two coordinate angles of solar altitude angle and solar azimuth. The complement of the solar altitude is the zenith angle, which has a special subscript of "z".
The Solar Altitude () measures the angle between the central ray from the Sun (beam radiation), and a horizontal plane containing the observer. Note that the subscript "$_s$'' is there to indicate that the altitude relative to the observer of the Sun. This will become important in evaluating the altitude angles of other objects projected onto the sky dome, like buildings, overhangs, wing walls, and arrays of solar receivers.
The Zenith Angle () is the geometric complement of the solar altitude angle. We direct your attention to the use of here, as the general concept of the angular deviation of the Sun's ray from the normal projection of a surface is called the angle of incidence, . In effect, the zenith angle represents the angle of incidence for a horizontal surface.
Recall: "normal" means perpendicular to a surface.
The Solar Azimuth () measures the angle on the horizontal plane between the meridian of the 0 degrees axis (South, for the Northern Hemisphere, opposite Down Under) and the meridional projection of the Sun's central beam (the Sun's meridian). The convention we will use is also used in the advanced design tools for solar energy, TRNSYS (UW-Madison: Transient eNergy Simulation Software), PVSyst, and SAM (NREL: System Advisor Model). The angle varies from 0 degrees at the South-pointing coordinate axis to degrees. East (earlier than noon) is negative and west (later than noon) is positive in this basis. This is a well-used standard taken from the original text of M. Iqbal.\cite{iqbal83}
Note: the function "sign()" is specifically defined as a cases form of positive and negative notation (meaning there are two alternate cases to choose from).
There are several texts and software available that use a solar azimuth, measuring clockwise on the horizontal plane from a North-pointing (0 degrees) coordinate axis. This is also the convention for the field of meteorology and for the wind industry. This azimuth convention uses 360 degrees for the meridional projection of the Sun's central beam.
However, the majority of professional software does not use this convention yet, so you must be aware of the difference, as the math changes with the two methods. It is relevant to be aware of the difference, in that the North basis is used in the basic educations tools for simulation called PVWATTs, for the sun path tool at the University of Oregon, and in the interactive software PVEducation.org [21].
Once we have identified our specific location on the surface of Earth, and used those coordinates to identify the location of the Sun in the skydome relative to a fixed observer in a given locale, we can enter the angles for the orientation of our SECS. Again, this text closely complements the text in the book.
What is different here?
This new set of relations is for the general cases of any surface (horizontal or tilted, fixed or tracking). We are now describing angles for real oriented surfaces on Earth, like tilted PV panels and walls of a building. As we noted in the last section, for real SECSs, the receiving surface is tilted (beta, , with an azimuth orientation γ) to minimize the cosine projection effect that occurs at a given latitude.
Also, pay particular attention to the angle of incidence (theta with no subscript, ). As part of a solar energy design team, one of our primary mechanisms to increase the solar utility for the client in a given locale is to minimize the angle of incidence at a given time during the year or day. This is an extension or refinement of reducing the cosine projection effect.
Finally, I will give you a heads-up for the next page: after reviewing these sections and completing this page, you will need to take the Lesson 2 Quiz. Note that this is a quiz with calculations, and you will likely want to have a numerical software open to work through the numerical problems.
The following angles describe the orientation of the collector surface and the relation of the collector surface to the Sun. As these three angles describe our primary surface of interest, the SECS, they do not have a subscript for the two coordinate angles of collector tilt and collector azimuth.
For general cases, where the collector has a non-horizontal orientation (), the angle of incidence is not the same as the zenith angle (θz). In fact, the zenith angle is a special case of an angle of incidence for horizontal surfaces, where the zenith is referenced to the aperture as a normal projection.
The first and second angles of tilt and surface azimuth (and γ) are typically known for fixed surfaces. The third key angle is the angle of incidence (θ), which uses the following rather lengthy equation:
In order to generate an actual value for theta, we will need to also take the arccosine of the long equation. For your calculators and math programs, all of the arguments are in terms of degrees, not radians. You will need to convert degrees to radians in most programs.
However, this is a really long equation that can actually be broken down into parts. We will break up the equation for the angle of incidence (theta, ) into three lines. Take a look at them, and look for common arguments to the sine and cosine functions.
In the introduction of the textbook, there is a reference to the goal of solar energy design: to maximize the solar utility for a client in a given locale.
There are three main design mechanisms that will increase the solar utility of a SECS for a client or group of stakeholders.
This is a brief recap to set the stage for orthographic projections and polar projections used in the shading analysis project to come.
In a spherical coordinate system, the angles are the coordinates. So, if I were standing in a field in North Dakota, looking at something tall like an enormous wind turbine, I could define the position of the top of the nacelle relative to me by stating the general azimuth angle (, the rotation across the horizon from due South) and the general altitude angle (, the rotation up from the horizon). Effectively, this is my x () and y () coordinates on an orthographic projection of the sky dome on a flat surface.
Many of you will be familiar with Cartesian coordinates in space (x, y, z). However, when dealing with spatial relations of spherical objects like the Earth and the Celestial Sphere, we find that working with basic spherical coordinate systems makes trigonometry available to us to solve for space and time equations. For spherical coordinates, we need information of radial distance, zenith angle, and azimuth. However, in solar positioning studies, a radius of one (unit radius) is all we need to establish a unit vector, and we are left with equations for only the zenith angle and azimuth (and the complement of the zenith angle, the altitude angle). Note how the zenith angle in Figure 2.15, above (the generic angle), is congruent with the solar zenith () of the Sun, and the generic azimuth angle () is congruent with the solar azimuth ().
The following equations describe the Cartesian coordinates (x, y, z) for unit vectors, followed by the equivalent functionals using the complementary altitude angle.
As seen in Figure 2.16, we need to pull back to our old trigonometry mnemonics! Recall "Soh Cah Toa" when looking at the figure:
The emphasis of this lesson is the Sun Chart tool (or Sun Path). These flat diagrams are found in many solar design tools, but may look completely foreign to the new student in solar energy. How do we interpret the arcs and points plotted on a sun chart? Why do we have two different types of plots (one looks like a rectangle, and one looks like a circle)? Why do some plots go from 0-360°, while others go from -180° to +180°?
If we want to visually convert our observations of the sky-dome onto a two-dimensional medium, we can either use an orthographic projection or a spherical projection on a polar chart. These projections are useful for calculating established times of solar availability or shadowing for a given point of solar collection.
The Sun Path describes the arc of the sun across the sky in relation to an earth-bound observer at a given latitude and time.
All light incident upon Earth's surface must pass through the atmosphere and be attenuated (lost from absorption or back scattering). In order to simplify the many points of origin of light, we divide the sky and the Earth's surface into components, or spatial blocks of an imaginary hemispherical projection on the sky. The Sky Dome refers to the sum of the components for the entire sky from horizon to zenith, and in all azimuthal directions. In our following sections, a collecting surface is assumed to be horizontal first, as a pyranometer measuring device is mounted horizontally and facing the sky to measure the Global Irradiance/Irradiation in the shortwave band for the sky dome. Most of our solar collectors will be tilted up from horizontal in some way (PV, solar hot water, windows, walls, even your eyes). Those surfaces oriented otherwise are termed a Plane of Array measurement (POA), requiring specific tilt and azimuth information in the description. For those solar collecting surfaces that are not horizontal, the reflectance of the ground is an additional source of light, through the albedo effect. The beam, sky diffuse, and ground diffuse light sources incident upon the tilted collector are estimated using models of light source components.
The sky dome can be projected onto flat surfaces for analysis of shading and sky component behavior.
Go to the University of Oregon Solar Radiation Monitoring Laboratory [64] website. The scientists at the site have provided an excellent tool for plotting sun paths onto orthographic projections or polar/spherical projections. The default page is for creating an orthographic projection of your site of interest. The alternate page for polar projections [65] will use the same data you can input, but will output the alternate form. Note that both use the meteorological standard for azimuth angles, where North is set at 0°, increasing clockwise to 360°.
When designing a solar energy conversion system for any application, we must pay special attention to the occurrence of shadows throughout the year. We discuss a method to assess the shading using 2-D projections.
The next page gives you an opportunity to print and analyze your own sun chart.
Go to the University of Oregon Solar Radiation Monitoring Laboratory [66] [64]web page and generate a PDF SunCharts for two locations: (1) State College, PA, and (2) your own location.
1. Download the PDF chart for the orthographic (Cartesian) projection
2. Download the PDF chart for the polar projection
3. Reflect on the charts:
Now, based on your understanding of the SunCharts, work through the self-check questions below. While you work through these steps, try to think about all the calculations that went into each plotted point on the curves. You should reflect on the fact that the SunChart tool is simply plotting points and lines based on solar calculations of time and longitude. The same calculations that you are mastering now in class. Pretty exciting!
Answer the same questions for your own locale. Are the answers different or the same compared to State College, PA?
Here, we are going to work through a problem of plotting critical points of shading onto an existing sun path diagram. Keep in mind that we will use the plots that you developed from the University of Oregon Solar Radiation Monitoring Laboratory [66] on the previous page of this lesson.
Here, I am using a program called Skitch [67] to draw over the top of my PDF files. There is a 30-day free trial of the editing software if you would like to use it to digitally mark up your documents. Otherwise, I recommend that you print out your work, draw on the print directly with a pen, and then take a snapshot of the edited image with your phone or scanner to upload.
First, we will go over the key features of the orthographic plots with the arcs of six days plotted out for the first half of the year.
Second, we can compare the key features of the orthographic plots to the polar plots, again with the arcs of six days plotted out for the first half of the year. You should notice the similarities (East is on the left, solar time is represented, the same location is plotted), as well as the distinct differences (the June and December arcs are "flipped").
Now, we need to add an additional layer of information to the plot. I'm going to talk through the addition of critical points on the sun chart, followed by connecting those points and shading the areas correctly. In each of the following examples, we will use orthographic projections, but there is no reason why you couldn't use polar plots instead.
The first example will come from the textbook. We will be plotting shading relative to a single receiving point: X, with three critical points of shading (A, B, and C).
In the next example, we will look at setting up the problem to assess a PV array shading problem. Our intent is to plot shading relative to multiple receiving points: A, B, and C, with three critical points of shading (1, 2, and 3). The result is nine values for altitude coordinates ( , no subscript) and nine values for azimuth coordinates ( , no subscript).
You will notice how the horizontal surfaces of the building that create shadows are transposed to the projection as an arc, not as a horizontal line, while the vertical surfaces remain vertical. This has to do with the manner in which spherical data is distorted in an orthographic projection. Hence, the plot of a building shading a point on a window will look a lot like a slice of bread, flat on the sides with a soft curve across the top. The same will be found for this example, where the receiving points A, B, and C are shaded at different times by critical points 1, 2, and 3.
Next, we will be interpreting our results, and inputting the shadow data into SAM.
In your homework assignment, you will be asked to do a similar procedure for the solar array, but with the use of the polar projection of the SunChart, which provides somewhat better accuracy for determining the shading factors. While the solar coordinates are positioned differently in the polar projection, the principle of plotting critical points is the same - you find the positions for the points and connect them to define the shade/no-shade boundary. See an additional video demo on this in Canvas.
I hope you were successful installing SAM software and getting it ready to use in the last lesson. Now you will be applying your interpretation of the shading diagram to determine shading factors and input them into SAM for system energy performance simulations where shading interferes with the solar resource.
We have come a long way in interpreting sun path diagrams and plotting shaded areas as overlays onto the sun path charts. How can we input those shade/no-shade conditions to the PV performance model in SAM? The video below provides a demo of how shading conditions can be applied in SAM simulations.
This video is made using the 2020 version of SAM. In newer versions, some tab specifications may differ.
So, the important take-aways are that we can use SunCharts or other geometric shading data to get us to a shade-based performance adjustment of PV performance simulations, and that shading matters in solar energy. How significant can it be? Quite significant, but you will get a more accurate answer after performing your homework assignment in this lesson! We use these SAM simulations in solar project design, and this will be the way to assess the shading losses for any specific scenario (and in your final course project, eventually).
When working with SAM shading table, we can input partial values for hours that are on the shade/no-shade borderline (e.g. 50% etc.). SAM conveniently grades the color scheme from white (full sun) to red (full shade) in the process. Also, note that the locale matters! Do not forget to set your location in the Location and Resource tab first: the default for SAM is often in the Southwest USA, and you will need a correct solar resource file before running your simulations.
This activity allows you to apply the concepts of time and space coordinates you learned in this lesson to a specific example of a solar energy conversion system. In the process, you will work with both Sun Chart and SAM software to model the performance of a PV array. You will be required to demonstrate your learned skills for Sun Chart interpretation, shadow tracking, and including shading information into SAM simulations.
Model the effect of shading on the PV array performance (kWh of energy generated) over a year. So, in the end, you will need to produce the energy output data from the array for the full-sun and shaded scenarios and observe the difference.
Apply shading. Study the “Array Packing” example in Chapter 7 of the J. Brownson’s SECS Book, understanding how shading coordinates can be obtained for a specific system geometry. Plot all the critical points by shading coordinates given in Table 7.3 on your Sun Chart. Connect the points and show the shaded zone on your diagram (see lesson videos and textbook for more guidance on how to do it). You can use a graphics software for this purpose or do it by hand if that is easier.
NOTE: the solar azimuth coordinates used by SunChart are different from those Brownson's book. The former considers 180o as true south, while the latter considers 0o as true south. So, you would need to convert the solar azimuths from the book by adding 180 in order to correctly plot the points on the SunChart.SAM Simulaton A (you have already downloaded SAM as instructed in Lesson 1):
Discussion. Provide a brief discussion comparing your shaded and non-shaded results and what you learned from this activity.
Prepare a written report including the following results:
Upload your report file to the Lesson 2 Learning Activity DropBox in PDF or docx format.
See the Canvas Calendar for specific due dates.
So far, we have delved deep into the world of angles, time, and shading. Why have we done this? The solar resource is all about timing and placement of the SECS relative to the Sun, and the use of angles as coordinates in time and place are fundamental building blocks to the solar expert. We now have computational tools that make the work of calculating details trivial, but one must always know what the foundations are underneath those tools.
Who knows, maybe you will be the next open software developer to create a simple solar tool for resource assessment. You now have the keys to access many of those computational tools. More to the point, these fundamentals are not shrouded in mystery, and you don't necessarily need to pay for software to get access to them!
Looking ahead, I can tell you that students in residence at Penn State have already found a solar systems design plugin for Trimble SketchUp [68] called Skelion [69], which does shading analysis just like you performed, but on the fly inside of the SketchUp design software. Both are free to use, and I'd recommend you take a look at each in preparation for your end of semester design projects! This is an untapped resource for you to explore, now that you know "how" the shading calculations work.
Knowing these principles of angles and time also exposes us to the strengths and potential weaknesses of purely geometric relations in the solar resource. As we shall see in the next lesson, the role of the atmosphere and meteorology will tend to muck up our ideal angles of beams of light, and produce anisotropic (uneven) intensities of irradiance over the day, which will of course influence our solar resource estimations.
On to the next lesson--let's learn about the role of weather and the sky dome in estimating the solar resource!
The Goal of Solar Design is to:
Given the goal of solar design, we have learned of three mechanisms to leverage during the design process that will increase the solar utility for our client in their locale of interest:
In order to leverage these three tools, we have demonstrated that one needs to know where and when the Sun will be relative to our collector system.
You have reached the end of Lesson 2! Double-check the to-do list on the Lesson 2 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 3.
This is the second of three very dense lessons on the fundamentals of solar energy. Keep it up! The lessons will lighten up a bit as we transition past these core materials.
The Sun provides us with shortwave band light at the Earth's surface, and the Earth emits longwave band light up toward the sky; but what is the role of the Sky/Atmosphere in the energy balance on Earth, and how does the atmosphere affect the bands of light that we collect and find useful at the Earth's surface? In solar energy conversion systems, engineers and design specialists apply meteorological information, particularly shortwave band irradiation (from the Sun) to inform project development and design, and also to inform daily performance of installed systems. To a lesser extent (for some radiative thermal estimations), longwave band irradiation from the Earth and atmosphere is used, but the main driver for our SECSs is the shortwave band.
Applied solar energy workers need to use the same science as those working in the field of Meteorology. The shortwave band of light can be collected via ground detectors (pyranometers/pyrheliometers [70]) or from satellites (GOES East [71], GOES West [72]) to probe and diagnose the behavior of light interacting with the atmosphere and the surfaces of Earth over time. Satellites in orbit have instruments called radiometers [73] to detect and measure the intensity (brightness) of the light scattered back to the satellite. Satellite images of visible light display the shading from clouds, the cloudiness of the Earth's surface, and features such as smoke emerging from large forest fires or volcanoes spewing ash. The signal is a measurement of electromagnetic radiation, both the intensity of the light from the Sun, and the ability of the particles or surfaces to reflect that light back up to the radiometer. Think about what determines the brightness of the light that reflects (is "back-scattered") up to the satellite.
We will start with the concept of electromagnetic radiation, including some important bands of electromagnetic radiation invisible to the human eye. We will also point out ways that a meteorologist and climatologist collect and interpret satellite or radar imagery derived from electromagnetic radiation.
As a preview to Lesson 3, please review the learning objectives on the next page to get an initial sense of the overarching goals that encompass this stage in your forecasting apprenticeship.
At the end of this lesson, you should be able to:
This lesson has two learning activities and will there will take use two weeks to complete. Please refer to the Canvas Calendar for specific timeframes and due dates - those can vary from semester to semester. Specific directions for the assignments below can be found further within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 3 - Laws of Light (review) C. A. Gueymard (2012) Clear-sky irradiance predictions for solar resource mapping and large-scale applications: Improved validation methodology and detailed performance analysis of 18 broadband radiative models [74] Solar Energy 86, 2145-2169. |
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Optional Reading (not required): | Bird and Hulstrom (1981): Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces, Technical Report No. SERI/TR-642-761, Golden, CO: Solar Energy Research Institute Meteorology for Scientists and Engineers, 2nd ed. by Roland B. Stull (Chapter 7). J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 8 - Measure and Estimation of the Solar Resource |
To Do | Learning Activities
Discussion: Climate Regimes Quiz Assignment (See Canvas) - due by the end of the first week Engage in all Try-This and Self-check activities included in the lesson (not graded). |
Topic(s) | Principles of Light behavior Engineering Air Mass Meteorological Air Masses Clear Sky Models Anisotropy from clear and non-clear sky (and ground) Clouds and solar intermittency |
If you have any questions, please post them to the Lesson 3 General Questions and Comments Discussion Forum in Canvas. I will check these forums regularly to respond. Feel free to go through the comments and post your own responses if you are able to help out a classmate.
While reading, pay attention to the basic rules of light measurement. Our historical affinity for burning fuels has been paired with an affinity and strong awareness of temperature, but we lack an awareness for measures of light, found through radiometry. Think of how you “know” what 80 °F versus 65 °F means. Why not measure the irradiance/irradiation along with the temperature? What if we, as a solar culture, were to learn what 1200 W/m2 meant versus 500 W/m2 for an average hourly or minute irradiation upon an exposed surface (vertical wall or sloped rooftop)?
For solar resource assessment, radiometry (the measure of electromagnetic or radiant energy) is more valuable than thermometry (the measure of temperature). Solar energy is comprised of the shortwave band of light found between 250-2500 nanometer wavelengths of light ( , or one billionth of a meter). Within the solar field, we measure the shortwave band in terms of irradiance, as W/m2 (Watts per square meter, a flux of light per receiving area). If we group irradiance over a block of time, say an hour, we call the measure irradiation, in units of Wh/m2 (Watt-hours per area, similar units to electrical energy measure).
To convert energy values as electron volts from nanometer wavelengths: divide 1239.8 nm•eV by the wavelengths that you found in nanometers to find their respective energies in electron volts (eV). Because the error is negligible for us in the field, we can even use 1234.5 nm•eV (just count to five and use the right decimal point).
For more information on how we get to this equation, see the Key Equations [76] page in the Resources box.
Traditional silicon photovoltaics collect energy for electricity production from wavelengths <1100 nm. Other systems like solar hot water panels will collect almost all of the spectrum from the Sun. In fact, almost all of the opaque surfaces about our environment (buildings, roads, water) tend to absorb strongly over the entire shortwave range (leading to an increase in temperature!).
And yet, our wonderfully adaptive human eyes only capture the tiny visible band from 380-780 nm. Thanks to the complex feedback systems from our irises, lenses, and eyelids, eyes can adapt well to extreme conditions of bright or dim lighting but cannot be used to quantitatively gauge irradiance conditions that energetically drive our buildings and photovoltaic systems. Our eyes impart important information, but because they are highly sensitive and adaptive systems, our eyes cannot be used to reliably distinguish and evaluate the solar resource (bare skin may actually be a better receptor for the shortwave band on days that are not windy).
Notice that the "visibility" of a photon is really limited only by the type of detector being used. Let's review the distribution of photons across the electromagnetic spectrum, and then we will discuss types of photon detectors beyond the rods and cones in our eyes. As a reminder, 1 micrometer wavelengths (also called 1 micron wavelengths) are equivalent to 1000 nanometer wavelengths (by unit conversion). It is often easier to use units of nanometers (nm) for the shortwave band, while using units of micrometers (um) for the longwave band. Either will work, but it is helpful to be aware of the relationship.
Now, let's zoom in to the shortwave and longwave bands important to SECSs. Wavelengths are expressed either in nanometers (nm) or micrometers ($\mu m$, also called microns). For wavelengths greater than 380 nm, electromagnetic radiation transitions from the ultraviolet (UV) to the violet visible range for the human eye. As we will observe in the Granqvist figure below, the red visible range ends for wavelengths greater than 780 nm (to the human eye). Notice how the "infrared" crosses over both the shortwave and longwave bands of light.
All objects glow with energy in proportion to temperature. As temperatures increase, the wavelength oscillations become shorter and the density of photons measured is effectively increased. So, the light becomes more intense (more photons), and each photon is packed with more energy.
Most photons are emitted from a thermal surface, where the constituent atoms are vibrating in the material (this means: temperature). Every material has a thermal property, and hence "everything glows." This concept works well for something called "blackbody emission" (zero emittance) or "greybody emission" (non-zero emittance) but not so well to describe your standard laser pointer or microwave oven. So, just to clarify all cases, there are also photons emitted from the Sun and from solid state devices that are "stimulated emission" (lasers), which can allow for photon emission from surfaces that are high in energy, but not high in thermal temperature.
This is our first principle of seven for the behavior of light. So, let's begin with the simple yet important statement: Light is directional. Light, as a photon, is born (emitted) from a surface in many directions and then impinges upon other surfaces, where it is either absorbed, reflected, or transmitted (and refracted). We use that directionality to describe the resource. In the text chapter, we have defined a visual shorthand for light that is emitted, transmitted, reflected (scattered), and/or absorbed by various surfaces.
Notice that the "Ir-" instructs us that the light is incident upon a surface. For example, we measure "irradiance" on a pyranometer, because light is absorbed by the device. Also, notice that my choice of "-iance" or "-iation" determines if the light is a measure of rate (as in power: energy per time) or energy (rate integrated over a span of time).
All objects glow (even the gases in the sky); it is just the intensity and peak wavelength that shifts with respect to their thermal temperature. This brings us to the second of the seven principles of light behavior. Light (e.g., derived from the sun) has a broad spectrum of energies, which are discussed as wavelengths. We tend to group similar wavelengths in the spectrum according to the properties of their emitting surface or receiving surface.
This is a "group" of high-energy wavelengths of light that are emitted from the Sun (See Light is Directional), comprising the majority of the total energy collected by the Earth's surface or our designed SECS, given the decrease in the light power density with distance to Earth (93 million miles, 150 million km). This bundle of wavelengths of light emitted from the sun includes the ultraviolet (UV), the visible, and the near infrared (IR) sub-bands.
The Sun is effectively the only surface from our surrounding environment that regularly and naturally emits shortwave radiance. We can deliver shortwave band energy to a new surface using a reflector, though, as is the purpose of light concentration onto a central receiver.
Long wave band consists of low energy light. The wavelengths of light that are emitted from our skin, from pavement, the sky, ice cubes, and even hot ovens are coming from surfaces that are quite cool relative to the surface of the Sun, right? We are comparing 250-500K surfaces to a 6000K surface. Planck's Law (discussed in the next page) suggests that the distribution of wavelengths will be very different from cool bodies than from extraterrestrial bodies with internal fusion reactors.
The group of wavelengths from cool bodies on Earth is termed the longwave band. This is >2500nm at Earth's surface, but also can be >3000nm if we were to group the wavelengths from space. The longwave band photons are of lower energy, but there is a very wide band of wavelengths included in the group. The longwave band contributes to the greenhouse effect, and keeps our atmosphere comfortably warm. With the addition of CO2 and water vapor to the atmosphere, we make the sky into a better reflector for keeping longwave energy instead of allowing it to escape into space.
Now, we're going to take a moment to explore the following multiple figure graphic from Smith and Granqvist. There is a lot of information in here, so take your time and revisit it as the class progresses.
Light intensity decreases in proportion to the square of the distance between emitter and receiver. This is called the Inverse Square Law, which applies to solar energy as well as to any other light source. In case of the Sun, the emitted energy flux is 6.33x107 W/m2 at the surface of the Sun drops down to 9126 W/m2 at the surface of Mercury (d=58 million km), and down to 1361 W/m2 at the exterior of Earth's atmosphere (d=150 million km).
As shown in the next figure, the reason for the inverse square law is geometric in nature. As light is emitted from a point, like the Sun at a very large distance, the same quantity of photons is spread out over an increasingly larger area with distance. Area is a spatial term in units of distance squared. This same principle works for ordinary surfaces, as photographers well know.Mathematically, this process of light “dilution” can be represented by the following equation (which is another form of Equation 3.2 in the SECS textbook)
\[G_2^{} = {G_1}\left( {\frac{{d_1^2}}{{d_2^2}}} \right)\]where G1 is the light intensity (or flux) at distance d1, and G2 is the light intensity at distance d2.
If we take G1 as the radiative energy flux at the surface of the Sun, we can then estimate the radiative energy flux at the Earth’s orbit (G2) using this equation simply based on the planetary distances. Try to make this calculation with d1 being the Sun’s radius, and d2 being the distance from the Sun to the Earth and see what you get. You will have a chance to share your result in the Lesson 3 Activity further in this lesson.
We continue our review of the seven basic rules, or "laws," of light. These four are a bit closer to physical laws than what's on the prior page, but we need to understand the basics of light if we are to move on to the physics of light interacting with the atmosphere.
You may want to use the math/science site Wolfram Alpha [79] to solve several of these Self-Test questions quickly, or to check some of the notes in Wolfram Alpha against our own work here. Wolfram Alpha is the Google of math and physics. If you want to do calculations on Wien's Law, you type "wien's law" and you will get a calculator for Wien's Law with all sorts of other information.
This radiative transfer law is very important when considering energy balance. It states that at thermal equilibrium, the emissivity () of a body or surface equals its absorptivity ().
Mathematically, we can conceptualize Kirchoff's law as
The radiant energy emitted from a real surface is represented as E (W/m2), while that of a blackbody (a theoretical condition) is given by EB (W/m2).
Simply put, a surface at steady state temperature will absorb light equally, as well as it emits light. Though light is directional, surfaces exchange photons in both directions.
This law is generalized to mean that all objects have some internal temperature, and given that temperature, they all glow. Max Planck was able to establish the dependence of the spectral emissive energy of a blackbody for all wavelengths of light (), given a known equilibrium temperature of the blackbody.
The Wien's displacement law provides us with expected values for the most probable wavelengths in the Bose-Einstein distribution of blackbody radiation. The law implies that the distribution of photons emitted from a surface at any temperature will have the same form or shape as a distribution of photons emitted from a surface at any other temperature. Mathematically,
The Stefan-Boltzmann Law states that the radiative energy emitted by a surface is proportional to the fourth power of the surface's absolute temperature. The Stefan-Boltzmann Law shows that if you were to integrate all the energies from the wavelengths in Planck's Law, you have an analytical solution of the form below:
where = Stefan-Boltzmann constant
Keep in mind that this is for the surface of the emitter. A surface like the Sun will have a very high value for the energy density on the surface, which then decreases in proportion with the Inverse Square Law from the last page (over the 93 million miles distance to Earth's surface).
Wolfram Alpha website [80] is the Google of math and physics. If you want to do calculations on Wien's Law, you type "wien's law" and you will get a calculator for Wien's Law with all sorts of other information. The purpose of this activity is to practice some basic calculations of physical properties of sunlight.
This video below (6 min) presents a short demo on how to use the Wolfram Alpha site. The Stefan-Boltzmann Law is used as an example here, so it provides you with a tip for solving the second problem.
Prepare a written report with your solutions and upload it to Canvas (Lesson 3 Learning Activity Dropbox) in PDF or docx format. You can use Wolfram Alpha screen shots to present your results for the first two problems (insert the screenshots into your report - don't submit them as separate files). Make sure to present the equations you are using and provide comments to the steps in your solutions.
View the Rubric in Canvas by which this assignment will be graded.
See the Calendar tab in Canvas for specific due dates.
Remember how we stated that light is directional? Light also has a "life" including the birth and death of a photon. Electromagnetic radiation is emitted from a surface and will encounter objects in its many paths. The fate of electromagnetic radiation (as a photon) depends on wavelength of the photon at hand and the physical composition of the objects along its path. Again, the photon is a packet of radiation and can interact with molecules or atoms in its path, being transmitted, absorbed, or reflected.
The action of "reflection" is more appropriately described as "back-scattering" by atmospheric physics, which is why you see that term in the figure of possible paths of light. We also have seen a shorthand diagramming technique from the textbook to describe the life events of ensembles (really big groups) of shortwave and longwave photons.
I would like to pull the concept of photon lifetimes in the following video.
Each of these phenomena is happening simultaneously, and the next video opens up that discussion.
Emission ($\epsilon$), absorption ($\alpha$), reflection ($\rho$), and transmission ($\tau$) can all occur at the same time (in fact it always does). We also know that there is a relationship between light energy balances and temperature. When a receiving surface or material absorbs more energy than it emits, the internal temperature of the material will increase (effectively pumping up the system's energy density). Consider a bright summer day, where your rooftop absorbs much more solar radiation than it emits (or conducts away). The roof surface temperature will rise from just after sunrise to the late afternoon.
Recapping: Electromagnetic radiation (as light) is first emitted, and the emitted photons have an opportunity to be transmitted, absorbed, or reflected by an intervening surface. If that surface is a particle or an assemblage of particles, there will be a tendency for the reflection to be more accurately described as "back-scattering."
In solar energy resource assessment, clouds are the most important atmospheric factor influencing device performance, followed by aerosols (dispersed atmospheric particles).
Case Goal: Develop a working concept of the atmosphere as a case study of a selective surface, interacting in concert with the Earth surface to contribute to the global energy budget. Then, move on to more specific reflective surfaces that we might use as a secondary shortwave resource (diffuse reflectors and specular reflectors) in designing SECS.
The Earth is a vast solar energy conversion system! The atmosphere encasing Earth's land and water mass is a collection of gases and particles that vary in pressure, temperature, and chemistry continuously. If we collectively imagine for just a moment that the atmosphere is a simple cover on top of our main absorber (the Earth), we can begin forming a concept of the atmosphere interacting with electromagnetic radiation across broad bands of wavelengths.
Now, let us consider the optical properties of a single material that reflects light for some bands and transmits or absorbs light for alternate bands, just like the atmosphere represented above. We call the surface of such a material a selective surface, because the light interaction occurs at the surface. A selective surface is non-reflective to some bands of light, while being reflective to other bands of light. The atmosphere is a case study for a selective covering surface between shortwave and longwave bands.
For any given wavelength or band of similar wavelengths, the following three simple phenomena will occur when light interacts with a material surface (where light is either being absorbed or emitted):
This means that each of the simple optical phenomena can be represented by fractions from 0 to 1, with the sum of these equating to 1.
For opaque materials, there is a relation between reflectivity and emissivity (the glow of an object) and between reflectivity and absorptivity.
We now know the relation for surfaces in optics called Kirchoff's Law of Radiation. When a surface is in thermal equilibrium with the surroundings, the emissivity is equal to its absorptivity at each wavelength ($\epsilon = \alpha$). This allows us to make the same relations among reflectivities and absorptivities.
We can show the ways in which the Earth-Atmosphere is selective in the relative properties of each to absorb, reflect, and transmit different bands of light.
We have already described shortwave (280-2500 nm at ground level) and longwave (>2500 nm) bands of irradiation incident upon a surface. The study of optics is that of light-matter interactions, regardless of wavelength. We know that materials like glass are semi-transparent in most of the shortwave band, and materials like pure aluminum are reflective in the shortwave band. We are not so familiar with the way that materials behave in the longwave band, and we often have trouble with materials that are opaque vs. semi-transparent.
Let's review the transmittance graphic that I introduced earlier. This plot has a logarithmic x-axis, so, we can count from 0.2 micrometers (200 nm) up to 1 micrometer wavelengths in increments of 0.1 micrometer. Then, we count from 1 micrometer to 10 micrometers in increments of 1 micrometer, and so on...
We see that from about 0.3 micrometers to about 2.5 micrometers, there is a significant amount of white showing on the plot of the Total Absorption and Scattering, meaning that the absorption and reflection of visible light by the atmosphere is relatively small. In other words, the atmosphere transmits 70-75% of the sun's shortwave light from the top down to the Earth's surface. Along the way, clouds can back-scatter [82] (reflect) some of the visible light into space. In times and places where transmitted sunlight reaches the Earth's surface, then the land, oceans, deserts, grasses, and trees, etc., reflect some of the surface-incident shortwave light back into space (again, with limited absorption along the way).
We also see that from 8-13 micrometers, there is an atmospheric window where the longwave band light is not absorbed or scattered. This is the way that the Earth and the Atmosphere can "leak" energy back into space. If you like, go back to take a look at a similar figure from Granqvist. From which side of the sky window is water vapor absorbing the longwave light, and from which side is CO2?
Using this example, when incident light (irradiance) is not absorbed or transmitted through the surface and bulk of a material, it is reflected by the surface (the fraction of reflection is called the albedo).
I would like you to review Figures 5.8 and 5.9 in SECS Chapter 5 relating to the spectral reflectances of the following substances across a broad range of wavelengths:
Material properties that interact with light from the Sun (terrestrial shortwave band is around 250-2500 nm) extend over a far greater spectrum than what our eyes can perceive (visible sub-band 380-780 nm). Consider the shortwave range of light, 250-2500 nm. Anything with an albedo (reflectance) >0.2 could be a good contributor to a diffuse ground reflectance that increases light incident on a tilted collector. Even if the reflectance only cover a portion of the full spectrum from the Sun. Natural surfaces and built surfaces can be used to promote or inhibit solar reflectance, coupled to the performance of a Solar Energy Conversion System. Keep that in mind in the future!
Question: Which of these objects would be effective reflecting surfaces for shortwave irradiance? Which surface surprises you in its behavior?
There is a common phrase used in the solar world, which holds two distinct meanings to two user bases. First, I want you to understand the difference between an air mass coefficient used to qualify the testing conditions of a PV cell, and then I want you to understand the air mass (relative to source regions) that we will use to help in our assessment of the dynamic solar resource for clients in their multiple climate regimes.
For the good of the engineering community, air masses were "fixed" in 1976 as the U.S. Standard Atmosphere [83] for the ideal clear sky atmosphere in the mid-latitudes of the USA. Yep, and nothing has changed since 1976, right? (cough)
Actually, the American Society for Testing and Materials (ASTM) developed the "Terrestrial Reference Spectra for Photovoltaic Performance Evaluation" so that we could compare and evaluate the performance of our solar technologies, like PV cells and modules. The full documentation of the reference standard is held by the National Renewable Energy Laboratory (NREL AM Standard [84]).
The specified atmospheric conditions are:
Actually, we are speaking of an air mass coefficient: a relative measure of the optical path length passing through the Earth's atmosphere [86], as described for a fixed moment in time and space. We express the air mass coefficient as a ratio of the direct path of the global shortwave irradiance incident upon a specially tilted surface, relative to the path length for a horizontal surface (optical path oriented vertically upwards as the normal).
The first thing we need to estimate is what the intensity of the global horizontal irradiance would be just "outside" of our thin covering atmosphere. So, how do we determine the solar energy incident upon the extraterrestrial surface for the Earth-Atmosphere system?
First, we begin with the annual average solar constant, in units of irradiance (recently updated as of 2012! No longer 1366 or 1367 W/m2 if you knew that value). The average annual Solar intensity will actually vary according to an oscillation linked to 11-year cycles of sunspots. Given a large number of sunspots, the solar constant will be higher (~ 1362 W/m2) while the value will drop to ~ 1360 W/m2 when there are not many sunspots (changes about 0.01%). You can see NASA's Solar Radiation Climate Research Area [87] (accessed Feb 13, 2013).
Here, G is for Global irradiance, and "sc" stands for solar constant.
Second, we estimate the irradiance of the Sun for the intensity collected on a plane perpendicular to the beam shooting out from the Sun that is also perpendicular to the surface of the Sun (the normal radiant exitance). We would term that "normal global irradiance at AM0" (G0,n). This would be the equivalent of an estimate at zero cosine projection error, or the angle of incidence is zero. The cosine function in the following Equation delivers the change from zero to one, reflecting the cyclic orbit of the Earth relative to the Sun. The parameter n in this equation is the day number in the year.
Here, G is for Global, subscript "0" is for Air Mass Zero, and subscript "n" stands for radiant exitance normal to the surface of the Sun (don't confuse it with the day number).
During the Australian summer, the actual value exceeds 1416 W/m2, and then drops down to around 1326 W/m2 during the summer in the Northern Hemisphere.
Why would a Canadian summer not be as bright as an Australian summer (in outer space, of course)?
Third and finally, we need to estimate the AM0 irradiance for any given horizontal surface on Earth's surface (G0). Think about it, a horizontal surface on Earth (or just outside the atmosphere) will be tangent to the curvature of that locale on Earth. We should notice in the following equation that the Earth-Sun relations of declination ( ), latitude (), and hour angle () are all included in this irradiance equation for AMO.
By comparison with our work from Lesson 2, I can point out that the equation is actually multiplying the Equation for G0,n with the Equation for the solar altitude angle: Another way to write this would be Wow! Lesson 2 really comes in handy!
As a reminder, the relation to find is:
Consider that the sine of the altitude angle ( ) is the same as the cosine of the zenith angle ( ).
Other equations of use: The notation for irradiance is G, but the notation for irradiation (energy density, in J/m2 or Wh/m2) is broken down into hourly values and daily values. In the standard literature, hourly values use the coefficient I, while daily values use the coefficient H (no, I don't know why--it's just a quirk).
where the hour angle for sunset has been defined in Lesson 2 as:
The "standard" for testing. The air mass coefficient is defined in proportion to the cosine of the zenith angle (): the angle between the beam from the Sun and the normal vector pointing directly up to the zenith of the sky (normal to the horizontal surface).
One needs
By equivalence, the cosine of the zenith angle is the sine of the altitude angle , as we just demonstrated above.
Anecdotally, we noticed that the "ideal, engineered" clear sky conditions for AM1.5 or better (<AM1.5) are related to a window of time in a given locale. As a rule of thumb in the industry, we often mark off approximately 6 hours (centered on solar noon) for "prime" solar resource times during the day.
If one were to invert the problem and instead calculate the hour angle (and thus the time) from the air mass coefficient and zenith angles, we would find a mirror image of two times when that AM1.5 condition occurs on an ideal clear day in the summer (near the solstice). In the mid-latitudes of the continental USA or Europe, those instances of AM1.5 would occur sometime before 9am and after 3pm solar time. Any time within that window, the air mass coefficients would be less than 1.5.
Voila, the six-hour rule of thumb emerges from the Air Mass calculations.
Figure 3.9 illustrates the concept of airmass with respect to a zenith angle of (AM1.5), or an altitude angle of The hypothetical receiving surface would be tilted at ß = 37º for global irradiance Gt and an angle of incidence of
Gueymard has since developed a modified air mass model that will also apply for zenith angles >80°, as the former model tends toward infinity at .
Source: Matthew J. Reno, Clifford W. Hansen, and Joshua S. Stein. Global horizontal irradiance clear sky models: Implementation and analysis. Technical Report SAND2012-2389, Sandia National Laboratories, Albuquerque, New Mexico 87185 and Livermore, California 94550, March 2012.
Why is meteorology so important to solar energy conversion systems? Think about meteorology and the scientists that explore the field. In effect, they can help us in the analogous manner that a geologist helps to define and quantify mineral resources in the ground. It is up to us to learn the language of meteorology to communicate effectively with our peers and increase our success in project development and planning.
As noted in the text book, we need to use the Air Mass phrase suitable for the systems science perspective, reflecting changing parcels of air shifting across the surface of Earth. Those parcels are generating clouds and dust storms, tornadoes, hurricanes, and sunny blue skies. This is the meteorological concept of the Air Mass that we will use to help in our assessment of the dynamic solar resource for a client in their multiple climate regimes.
The Air Masses used in engineering standardization for SECS technologies are not nearly as useful as the real, dynamic air masses that we will use for solar resource assessment.
The sky can be divided up into large volumes with common properties. Air masses are large pancakes of gases and particles (we call them parcels) with common properties of temperature, chemistry, and pressure. Aside from the strong role of clouds, those common physical properties each affect the way in which solar energy intensity will be decreased along the particular path length through the sky. Air mass will affect the way that light is absorbed and scattered, and hence will affect the characteristic behavior of light incident upon an aperture. The changes between air masses are weather fronts, and the study of the regional dynamic dance of air masses and accompanying fronts on a sub-annual basis is called meteorology. When we expand our studies of air mass behaviors to multiregional scales (larger spatial scales) and time scales in the range of decades to millennia, this is called climatology.
I want you to think of an air mass from meteorology as a turbulent pancake that interacts with both shortwave and longwave light, resulting in highly variable irradiance conditions at the ground level, and also for satellite remote sensing in orbit. Irradiance conditions are constantly shifting over a given locale, and we will be in need of effective forecasts of irradiation (over different time intervals) for many SECS technologies.
It is always important to solar energy conversion that we figure time and scale into our understanding. We observe that each location under study is influenced by multiple air masses during different times of the year. In the midlatitudes of North America, we call this periodicity seasons (deep thought, yes). Hence, our solar energy conversion systems are effectively in different locales for each period of air mass dominance.
One locale for your client will be divided up into seasons. For us, each season (a block of time) is like an independent region, different from the next season--and we call these regions climate regimes. In the mid-latitudes we observe four climate regimes, while regions affected by monsoonal swings may have two or three climate regimes.
Consider: there is not one State College, but four! One State College for each season, or specifically, one statistically different solar resource fingerprint for each synoptic climate regime.
Regime changes are often noticed by us in terms of the air temperature and humidity, but the sky regimes will change as well, with respect to wind speed, the size of weather cells, and emergent cloud behaviors.
From the perspective of design in solar energy conversion systems, the skies allowing sunlight through to Philadelphia from December 1 through February 28 exist in a different climate regime from the Philadelphia skies of June 1 through August 31.
Recall that the change of irradiation over seasons is due to the tilt of Earth (which leads to our measure of declination). When the Earth is unevenly energized by the Sun, air masses are mobile. Any locale will observe changes in residential air mass behavior over time. Air masses are created in source regions, where the mass acquires its dominant characteristics of temperature and humidity. Then air masses move from the region of origin into new regions. Source regions have been mapped out in Figure 3.11, below, and they will tend to have lighter winds, allowing the air mass to accumulate the temperature and humidity conditions of the accompanying portion of the Earth's surface (the big solar energy conversion device). Hence, areas affected by jet streams will not be regions to create new air masses.
The Bergeron classification system is accepted and used by atmospheric science and indicates the origin locale. This is important information, affecting the quality of the solar resource. Properties of thermal behavior and humidity are conveyed in the Bergeron classification, and the classification has a relatively simple two or three letter coding.
First, air masses are labeled according to their origin above a land mass, continental (c) or above an ocean, maritime (m). Maritime air masses will contain more humidity derived from the underlying body of water. Next, air masses can be grouped into main thermal categories of the site origin: (A, P, T, E). This grouping is largely separated by jet streams and changes in latitude, which of course are each derived by the tilt of Earth relative to the projection of irradiance from the Sun. By coupling the two letters together, we can then map out the source regions for meteorological behaviors across the planet and through the seasonal shifts.
The relation of air mass to monsoon cycles is less used in continental meteorology of North America, but is highly important to solar development in Asia.
Again, air masses are labeled according to the origin of the parcel above a land mass,
Air mass source regions are also grouped according to their thermal characteristics, the thermal generator that is charging up the parcel:
Clouds have the ability to scatter and even focus light, which means that sometimes a cloud will remove energy from the solar resource on the ground, and other times clouds will collect and focus light that's even more intense than light on the clearest day in your locale. Yet, we know that clouds are ephemeral and dynamic systems in and of themselves.
The albedo is the reflective fraction of light from a given surface, a fractional value from 0-1.
Let's consider a jar of clean water (upper left in the photograph), into which one may add a small amount of milk (upper right). Milk is a "colloidal suspension" of solid particles supported/floating in liquid. As such, the solid particles of fat reflect light, scattering the light in many directions like a cloud would do.
In the lower left and lower right corners, the milk concentration has been steadily increased. We can see through the clear glass of water, suggesting that the transmission of visible light from behind the glass to an observer standing behind the camera is high. For the low concentration mixture of milk and water in the upper right, the small amount of added milk has slightly decreased the transmission of light. We can still see the louvers of the window blinds behind the jar, however.
With more milk added to the mixture in the glass on the lower left, the transmission of light was further reduced, and one's ability to perceive the louvers on the other side of the glass was diminished. Finally, adding more milk to the glass in the lower right cut the transmission of visible light to very low levels, and we can't see through the glass.
What is not being transmitted or absorbed is being reflected. Consider this experiment in the context of albedo. The albedo of clean water in the visible sub-band is much lower than the increasing albedos of all three mixtures of water and milk. Of the three mixtures, the glass with the greatest concentration of milk has the highest albedo.
Recap: we have seen how reflection and albedo is a bit more complicated when light is interacting with millions of surfaces in a suspension, like a cloud. We can also see why a more effective term is backscattered radiation, which is used to describe the collective phenomena. And yet, we can use the backscattered data from clouds to infer useful information about the solar resource at the ground. How do you think we could use that data?
Time to explore the concept of climate regimes and the influence of meteorological fingerprints in solar resource assessment.
One locale for your client will be divided up into seasons. For us, each season (a block of time) is like an independent region, different from the next season--regions that we will call climate regimes. In the mid-latitudes, we observe four climate regimes, while regions affected by monsoonal swings may have two or three climate regimes.
Recall: there is not one St. Louis, there are four! One for each season, or one statistically different solar resource fingerprint for each synoptic climate regime of St. Louis, Missouri.
Climate regime changes are often noticed by us in terms of the air temperature and humidity, but the sky regimes will change as well, with respect to wind speed, the size of weather cells, and emergent cloud behaviors.
I want you to think about the way that seasonal (or synoptic) variations in a locale will generate multiple fingerprints. While each fingerprint is different from another in a given locale (translated: winter solar is not like summer solar), additionally, each set of fingerprints is different from another regional set (translated Mumbai monsoons are not like humid summers in Missouri). Every one of our locales for solar design is going to have "cloudy conditions" at some time, and everyone will have "sunny days" at other times. How can we better understand the variety of intermittencies and trends for our locale and synoptic climate regime (our fingerprint)?
This is just a play on words to emphasize something that you probably can guess: these three locations are not like each other!
Please identify the contributions or physical parameters that relate to the solar resource (other than the sun). First look at the questions and write down your answers in your notes. Then scroll over the answers to see how your thoughts match up.
1. What is the role of the sky dome in the solar resource?
Click for answer.
2. What is the role of clouds in the solar resource?
Click for answer.
3. What is the role of aerosols in the solar resource?
Click for answer.
We just introduced a totally foreign (yet intuitive) concept of climate regime to our lexicon in solar design. This is a new concept, but an essential one for advanced project development. You can start the discussion by posting what you perceive the meaning of climate regime to be from your personal reading and thought process. You can also comment on any uncertainty you initially found regarding the meaning, the intent, or application of climate regime to a project design, or to long-term project management. Creative interpretations of potential implications are welcome!
Here are some guiding questions:
When you create a post in the Yellowdig discussion space, you are required to choose a topic tag. For Lesson 3 discussions, feel free to use any of the following:
You can tag your post with one or several topics at the same time. All posts and contributions you create are added up to one score at the end of the week.
Yellowdig tip: check in the Yellowdig site at least once per day. Commit to making at least one contribution daily – read new posts, ask questions, give your peers reactions and accolades. If reading generates some thoughts, share them, don’t postpone until later. This is a team learning space, so you are also helping others learn by being more active.
Yellowdig points you earn over the weekly point earning period (from Saturday to next Friday) will count towards 1000 pts. weekly target. But you can go above it (to 1350 pts. max). Yellowdig discussions will account for 15% of the total grade in the course. Check back the Orientation Yellowdig page in Canvas for more details on the points earning rules.
There is no hard deadline for participating in these discussions, but I encourage you to create your posts in the middle of the study week (Sunday) to allow others to engage and respond while we are learning specific topics in the lesson. Also, remember that each weekly point earning cycle ends Friday night, so all points accumulated by then will represent your weekly grade, and then you start all over again (it’s like playing a set in tennis..)
In the section discussing components of light, we reviewed the effects of the atmosphere on irradiance. Now, we discuss how to model the transparency of a hypothetical sky that is "clear" of the effects of clouds. Clouds are a major contributor to the reduction of terrestrial irradiance; however, particles in the atmosphere also play a significant role. There are physical properties of the sky that we cannot see with our eyes, but which strongly affect the solar resource on a clear day. As we will see in the reading, aerosols and water vapor present in the atmosphere play an important role in scattering light, and they may be present on "clear sky" days when visible clouds are absent.
Daily or monthly irradiance data is required for proper design of any solar energy collection system. However, this data is not always available. This requires the use of well-designed models to estimate irradiance. Hence, the need for clear sky models.
Clear sky models are used to estimate what is called a clearness index. For a location, a clear-sky model must be properly calibrated to provide an accurate measure for the clearness index. We will be looking at two modeling approaches, the Bird Clear Sky Model (which we can download and use as a spreadsheet) and the REST2 model by Gueymard (which can also be downloaded and run as an executable file--we will not do so here).
The Bird Clear Sky Model was developed by Richard Bird and a number of other scientists at what is now the Department of Energy National Renewable Energy Laboratory. The model requires the following input data
Many openly available codes incorporate this model. The output is a "clear sky" estimate for the total or global horizontal irradiance (GHI), direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI, or DIF) across wavelengths from 305nm to 4000nm. The model calculates these conditions for a single point in solar time, given the latitude (), longitude () and the Time Zone.
Download RReDC: Bird Clear Sky Model [96]
Note: You will be asked to use this tool in this week homework assignment.The REST2 model has been found to be most accurate, as we shall observe in our reading. We will only need to explore one method (Bird) for this class, but it is important that a resource professional is also aware of the modern application of clear sky modeling. REST2 accepts atmospheric inputs of:
The REST2 model will then output estimations of diffuse horizontal irradiance (DHI), direct normal irradiance (DNI), and global plane of array (POA) irradiance.
Clouds are the big challenge in solar resource assessment. Think about how we just established a "clear sky" model, but most of our skies are filled with clouds that are dynamic and diverse in character. What is it about clouds that makes resource assessment challenging? Take a moment now to familiarize yourself with cloud phenomena and types.
Clouds are emergent phenomena within the atmosphere, and strongly perturb the behavior of the solar resource in a given locale. When we do not have a clear sky day in our locale (typical in many locations on Earth), the effects of clouds will strongly draw down the beam component of our solar resource and increase the net fraction of diffuse sky irradiance.
Clouds can develop in environments that are termed active and passive, as well as from updrafts near the Earth’s surface. As we have seen in the reading, clouds have the ability to either scatter and reduce incident light, or to act as a lens at the perimeter, focusing light well above clear sky or even AM0 irradiance conditions.
From the perspective of an observer standing on the Earth's surface, clouds can be classified by their physical appearance. Accordingly, there are essentially three basic cloud types:
Let's review some basic types of clouds. Please do comparative reading between the figure below (with images) and your list of cumuliform, stratocumulus, and stratiform clouds in Ch. 5 of SECS.
Clouds are listed here with images that pop up when you click on the red dots. We see variants of cumuliform clouds, stratocumulus clouds, and stratiform clouds, as noted in our reading.
There are four general classifications of clouds: high, middle, and low clouds as well as clouds of vertical development. The table below summarizes each of these classifications while giving you a sense for the typical altitudes at which their cloud ceilings (the height of their bases) are observed, and the basic chemistry of the clouds (ice and water interact with light in different ways).
Recapping: You should begin to think about how clouds can both block the beam component of light from the Sun (making the skydome more diffuse in nature), and can refract and scatter light like a lens, increasing the irradiance on a locale far higher than predicted from a clear sky model.
Researchers are now trying to predict cloud behavior with respect to GHI and DNI variation in a given locale. We are still a few years off, but soon we may see solar forecasts that include cloud interference for solar resource assessment.
Acknowledgment: Content of this page comes from Meteo 101: Understanding Weather Forecasting; author: Lee Grenci and David Babb.
SECS, Chapter 5: Meteorology, "Robot Monkey Does Space-Time" section
When we look at clouds, think of how they float by us without really changing in form all that much. Kind of like train cars passing by on the railroad, only much slower. We can use this to connect space and time (or frequency) in a useful way for solar resource assessment.Clouds (and weather fronts) occur on multiple scales of space and time. These scales contribute to the concept of solar fingerprints or climate regimes. In our reading, we learn about Sir Geoffrey I. Taylor and his work on turbulence theory. Clouds lead to intermittent solar conditions (POA and DNI irradiance) that, in turn, affect our solar energy conversion technologies, such as PV and building systems.
The intermittent variation of clouds as they affect the solar resource is crucial when solar is deployed as a large-scale farm, or as a large community of decentralized PV arrays on buildings. Variable peaks and valleys of electrical power can be detrimental (costly) to utilities if there are no other ways to store or shave variation on the grid. Now, storage solutions are emerging slowly in research, but right now we need to address the intermittency problem by better understanding the drivers and the basic science underneath those drivers.
A series of changes in time at a fixed place is due to the passage of an unchanging spatial pattern over that locale. That is, when observing a cloud or thunderstorm passing overhead, the clouds of the thunderstorm floating by are effectively "unchanging spatial patterns" (big blocks of cloud). Put another way, the lateral change in the cloud conditions across regions of the meteorological event (e.g., cloud-sky-cloud-sky-cloud) can be directly connected locally with a variation in irradiance measurement over time (e.g., a periodic measure of dark-bright-dark-bright-dark).
Taylor’s hypothesis states that events that change in time for a fixed place are due to the flow or passage of unchanging spatial patterns over a locale. This is like observing a thunderstorm passing by directly overhead and making the connection to the rate of the clouds advecting with the thunderstorm. Taylor's hypothesis from the 1930s allows us to flip from the Eulerian frame of reference (time sequences) to the Lagrangian frame of reference (spatial changes).
The Eulerian frame of reference: When an observer is stuck in one place, only watching the changing phenomena as it passes by.
The Langrangian frame of reference: When the observer moves with the meteorological phenomena instead of remaining fixed. Imagine a flying carpet floating along with a cumulus cloud moving from one county to the next.
Hence, all scales of time (or frequency) are also spatial scales! Taylor's Hypothesis holds so long as the advective wind speed is much greater than the timescale of the evolving meteorological event being investigated, as is often the case. When you have a lazy cloud day, where the clouds are changing form faster than they move, the time-space connection doesn't hold anymore.
By using spatio-temporal scales of cloud features established by Ted Fujita, we can estimate an average translation (advection) speed of 17 m/s for meteorological phenomena. We can then convert the spatial scales of variability into a relevant timescale. Spatial scales associated with power transmission congestion are distances of 25-1000 km. These distances are relevant for meteorological phenomena within time scales on the order of 25 seconds to 16 hours.
Cumulus 2-5 km, which means 10-100 minutes.
Cumulonimbus (including anvil) 10-200 km, which means 1-5 hours.
Cumulonimbus cluster (including merged anvils) 50-1000 km, which means 3-36 hours.
Synoptic (including cyclone waves, short and long Rossby waves) 1000-40000 km, which means 2-15+ days.
If we plot all of these phenomena and draw a line down the middle, we have a really rough estimate of the average advection rate of meteorological phenomena on Earth. Here, we have represented that rate in three different unit scales:
As such, we can connect Synoptic, Mesoscale, and Microscale meteorological phenomena between spatial and temporal scales!
Synoptic Weather: This is weather on the scale of 1000+ km in distance, and (given advection of ~17 m/s) seasonal timescales of 2-15+ days.
Mesoscale Weather: This is weather phenomena, including solar condition, on the scale of 100-1000 km distances, and time scales less than one day (hours).
Microscale Weather: This is variable weather phenomena on the scale of 10-100 km distances, and time scales of minutes to seconds.
Taylor’s Hypothesis “works” when the air in the sky moves significantly faster (advection: wind speeds) than the evolution time of a cloud or a thunderstorm of clouds. So, this approximation breaks down for those days when the cumulus clouds just "hang" in the sky all day.
The following readings will be helpful for completing this lesson activity.
Especially:
The goal of this activity is to use the BIRD Clear Sky Model to predict the solar irradiance at a specific time at a specific locale and further compare those predictions to actual irradiance measurements.
So there are two main tasks in this activity:
These two tasks will be performed for a specific date - July 31, 2007 (in this assignment) for the specific locale - SURFRAD Meteorological Station at Rock Springs, located just outside State College, PA.
In the end, you will need to assess how the modeled (predicted) and measured data fit and provide some discussion on it.
The Bird model is an old but advanced clear sky model used by solar professionals to estimate clear sky conditions at a locale. The clear sky model for a horizontal surface is the basis for almost all modern comparisons of "ideal skies" to actual skies (with clouds and dynamic light interference effects). You will see what the global, beam horizontal, and diffuse components of solar light should look like when you set the meteorological parameters affecting the sky clearness. The model could also provide DNI (Direct Normal Irradiance), which would show irradiation values higher than the global horizontal values for the mornings and evenings.
Note: access the AOD data through this graphical page [103]. Take the daily average as input for the Bird model. In this case, 415nm AOD can be used as a proxy for 380nm AOD input and 500nm AOD data can be used as is.
The model plot will look similar to the one below. This is what you need to produce.
The SURFRAD site at Penn State collects real-time data for solar irradiance (using local pyranometers) and a number of other atmospheric parameters at 3-minute time intervals. These data are accessible through the Earth System Research Laboratory website [104]. This site provides you with access to data collected at the Penn State location. For each year, you can open the parent directory with .dat files, which are named by the day number. So if you are looking for July 31st, for example, that will be day #212, so check the file psu07212.dat.
Your goal will be to make sense of these data, extract your irradiance values and plot them versus time of the day.
Here are the data labels in order that correspond to the numbers in the .dat file (omit 0-value columns):
Year, jday, month, day, hour, min, dt (decimal time), zen (degrees), dw_solar (W/m^2), uw_solar (W/m^2), direct_n (W/m^2), diffuse (W/m^2), dw_ir (W/m^2), dw_casetemp (K), dw_dometemp (K), uw_ir (W/m^2), uw_casetemp (K), uw_dometemp (K), uvb (mW/m^2), par (W/m^2), netsolar (W/m^2), netir (W/m^2), totalnet (W/m^2), temp (degC), rh (%), windspd (m/s), "winddir (degrees, clockwise from north)", pressure (mb)
From this list, you will specifically need:
dt (decimal time) - your time coordinate for data plotting (Column G in MS Excel)
dw_solar - Downwelling global solar (W/m^2) = corresponding to Global Hz (GHI) in BIRD model (Column I in MS Excel)
netsolar - Net solar (W/m^2) = corresponding to Direct Hz (DHI) in BIRD model (Column AG in MS Excel)
diffuse - Real downwelling diffuse solar (W/m^2) = corresponding to Dif Hz (DIF) in BIRD model (Column O in MS Excel)
Also, 'pressure (mb)' - the last value in each array of data - daily average should be used for your Bird model input (Column AU in MS Excel).
To treat the data, you can either convert the .dat file to a spreadsheet (see video in Canvas Module 3 how to!) or, if you have programming skills, you can create a script to plot the irradiance data versus time.
Important Note: you may notice that the "0" decimal time in the .dat file does not correspond to the actual local midnight, but rather corresponds to Greenwich 0:00, so you may need to synchronize decimal time with the Bird model to make the calculated and measured curves match.
Prepare a written report including the following:
Save your report as .docx or PDF and submit to the Lesson 3 Activity - Clear Sky Model dropbox in Canvas.
Please see the grading rubric in Canvas
See the Calendar tab in Canvas for specific due dates.
In this lesson, we have learned about the diverse ways in which the atmosphere can interact with shortwave light to affect the solar resource received at the ground by our Solar Energy Conversion Systems. We have observed that the sky has a distinct character derived from air mass source regions, and that the weather has seasonal "fingerprints" that distinguish a single site as multiple unique climate regimes. We further found that a "clear sky" is a fairly complex system of its own, and modeling the clear sky is not trivial. When we finally add clouds to the skies, we see where the real source of solar intermittency stems from. By connecting space and time from Taylor's Hypothesis, we find that we hold additional information on the scales of intermittency to expect in a given locale and for each seasonal climate regime in that same locale.
You have reached the end of Lesson 3! Double-check the to-do list on the Lesson 3 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 4.
This is the last big push to bring everyone in the class up to speed on solar resource topics, so stick with it, folks! We will be learning a lot more about measuring light directly in the field relative to estimated light calculations based on empirical (data-driven) correlations.
Why do our eyes create so much bias in determining solar decision making? Is there something better than vision to compare the solar resource? Is the human eye really designed to be a solar detector of intensity? It's time to come eye to eye with our embedded ethics of measurement in society. More on that soon!
We use the concept of components to break the sky dome and the ground into digestible chunks of surfaces with common emission/absorption/ scattering characteristics (e.g., direct, Gb, diffuse, Gd, circumsolar diffuse, ground reflected diffuse, Gg). Components of global irradiance relate to the sources of light within the sky dome. A component is a term for the groups of physical orientations and scattering of light (e.g., diffuse component, beam component). The characteristic measures are assessed within temporal blocks as statistical summations of irradiation on a horizontal surface (e.g. G / I / H / H-bar / annual) and the degree of light scattering found on a horizontal surface via the clearness indices (e.g., kT, KT, K-barT ).
We shall see that when it is challenging or costly to measure multiple components of light (scattered and unscattered), we have old and somewhat dated tools to attempt broad estimations on the contributions of each component to the total irradiation incident on the aperture of interest. You will see how we often rely on historical observations and empirical correlations by solar scientists and engineers for hourly, daily, and monthly average day data. The main tools used for these older equations are both measured hourly Global Horizontal Irradiation (GHI, or I) gathered from a horizontally mounted pyranometer, and daily extraterrestrial irradiance (Air Mass Zero = AM0, or Top Of Atmosphere = TOA, or just I0), which you learned about in the last chapter. We shall also find that one can infer more than just the components of light from the ratios of measured irradiation to AM0 calculated irradiation--however, there can be significant errors included in the process. We can also describe the fractions of days in a given month where lighting conditions will be clear or overcast/cloudy.
You will also see reference to the Typical Meteorological Year. Keep an eye out for that...it will be a major part of SAM simulation software.
You will observe several equations that are long and complicated. They are empirical relations that will be used to estimate the solar resource on non-horizontal surfaces. As with prior lessons, these equations are at the core of software like SAM, and a student completing this course should be very familiar with their application. Stick with it!
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 8 - Measure and Estimation of the Solar Resource. D. T. Reindl, W. A. Beckman, J. A. Duffie (1990) Diffuse Fraction Correlations [105]. Solar Energy. 45(1) 1-7. |
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Optional Reading (not required): | Liu and Jordan (1960) The Interrelationship and Characteristic Distribution of Direct, Diffuse, and Total Solar Radiation [109]. Solar Energy J. 4(3), 1–19. S. Wilcox and W. Marion (2008) Users Manual for TMY3 Data Sets [110]. NREL/TP-581-43156 S. Wilcox (2012) National solar radiation database 1991- 2010 update: User’s manual [111]. NREL/TP- 5500-54824 Perez, Ineichen, and Seals (1990) Modeling Daylight Availability and Irradiance Components from Direct and Global Irradiance [112]. Solar Energy J. 44(5), 271-289. |
To Do |
Learning Activity: Clearness Index calculation Discussion: Typical Meteorological Year Quiz Assignment: Components of Light (see Canvas) Engage in all Try-This and Self-check activities (not graded). |
Topic(s) | Irradiance/irradiation measurement (ground vs. satellite) Horizontal Surfaces: Day/Month/Hourly Clearness Indices Typical Metrological Year (TMY) Tilted surfaces: Isotropic/Anisotropic sky models and POA Direct Normal Irradiance |
If you have any questions, please post them to the Lesson 4 General Questions and Comments Discussion Forum in Canvas. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
As modern society, we each seem to lack a cultural awareness for measures of light per radiometry is our biological link to vision. Along with your reading, think about why your vision has quite different criteria for performance than a solar hot water panel, or a PV module. I would like you to shift your metrics from clarity and information, linked with photometry, to that of irradiance on a given surface. We present this page to "illuminate" our collective bias that vision brings in to our solar resource estimations.
Read the following description of your eyes, and think about the type of equipment that we will need to assess the solar resource for economic decision-making. There are numerous physical challenges incorporated into vision that strongly bias perception of a solar resource. But solar resource assessment is about metrics (physical measurements), not perception.
Consider: I have personally heard people tell me that solar technology is not viable in Pennsylvania, North Dakota, and Minnesota. The news media have erroneously stated solar is too diffuse for all of the United States (old Fox News report: because Germany was supposed to be brighter?). Additionally, my colleagues have been told stories that solar is not viable in places such as Santa Barbara and San Francisco, as well.
None of these armchair philosophy assessments is correct. First, speculation on the solar resource using visual cues is not appropriate; our eyes do not actually measure the solar resource in a meaningful way for SECSs. Second, speculative resource arguments (even measured data) must ultimately be tied to economic arguments. Here, we lack the financial argument (the economics) associated with the avoided cost of fuels from incorporating solar energy technologies in a given locale for a client of interest. We will cover the financial and economic discussion in the next lessons to come, but let's go back to vision for a moment.
Sight Perception is a funny thing
Sight perception works to our advantage as individuals when we wish to minimize risk, such as avoiding that lion prowling through the forest in the evening. It also adapts with weak or intense signals, trying to feed the brain a useful stream of information. As such, sight also has limitations, in that our sensory systems are combined with a cognitive system to extrapolate small signals into big information or really intense signals into reasonable information. The goal of sight is information about the world around us, not the amount of light delivering that information to us.
The eye has two main conversion molecules: rods and cones, located in the retina. The two systems have adapted for dim lighting (rods) and full daylight. Rods absorb only certain wavelengths of light that are longer and lower energy, while the system of cones (actually multiple kinds of cones) absorbs the wavelengths of light that we interpret as color. In both systems of absorption, the maximum range of wavelengths is limited and does not include the ultraviolet and infrared regions that comprise about 50 percent of the shortwave band of solar irradiance. So, our eyes do not detect a large range of solar wavelengths, and the response factor of those receptors is not linear either. This is a detraction to using the eye as a quantitative solar detector.
Notice in the figure that the rods and cones are distributed across the back of the eye, but the two systems are not distributed in the same fashion. Rods are distributed broadly across the retina, with the exception of the fovea centralis. In complement, cones are distributed tightly within the fovea centralis. The two distributions are linked to the lens system of the eye.
The lens system in the eye conveniently allows us to think about concentrating solar systems ahead of schedule (CSP, concentrating solar power for thermal steam production; and CPV, concentrating photovoltaics for electricity production).
Any lens will focus light onto a focal point (the fovea centralis here), but can only collect light from the direction that the lens is pointing. Meaning, a contracting system has to track the bright light sources for better performance.
The implications of this property of concentrating optics is that cones will only detect light in the direction that the eyes are pointing. So, our color detection system is relatively poor at sensing diffuse or scattered light from areas of the sky or ground at which the eye is not pointing. Mark one more detraction for the eye as a solar detector.
Going back to the rods...they are distributed everywhere except the focal point, and so will detect diffuse light entering the eye from all angles. Recall that rods are not color sensitive; they just detect long wavelength photons, such as those at night or during the twilight. If you want to see more at night while riding a bike or running, you are encouraged to defocus your vision to allow peripheral light to be detected. The optical implications are that your rods are not part of a concentrating system and will detect diffuse light better at the expense of color discrimination, but only at low levels of light. Mark yet another detraction for the eye as a quantitative solar detector.
And now, your additional macroscopic feedback system to control light acceptance, the iris. As the object of your eye is to provide your brain with the best information, not power, the iris is a feedback system meant to open wide when the light is dim, and squeeze up small when the light is intense. No matter what your rods and cones are doing, your iris is constantly adapting to maximize the signal of visual information to the brain. In a power detection system, we do not want an adaptive iris system, because it again detracts from our goal of linear detection of irradiance changes across the day.
We can also add the eyelids and eyebrows to your optical system, as they block or shade much of the bright light to your eyes. We can add behavior to our eye system, in that very few people actually look up to sense the light in the sky, and tend to look to the horizon instead, meaning our lenses are not trained vertically upward, but more along a horizontal plane (we mount solar detectors flat, to point up to receive the entire sky dome of light). All in all, we can come to the conclusion that the eye is not the ideal constant solar detector to inform us quantitatively of the amount and changes in irradiance during the day.
When a device takes one form of energy as an input and transforms it into different new forms of energy as outputs, the process is called energy conversion. The source of that input energy is called a resource. Now, if we were to draw upon the resource of the Sun from the point of a dude on Earth's surface, the energy form is electromagnetic radiation. We will call this light, bearing in mind that light from the Sun can be visible or invisible (ultraviolet and infrared) to the eye.
If one wished to transform the light into an electronic, or electrochemical signal, the resulting device (an energy conversion system) could be tailored to provide lots of power (more energy, less information) or to provide lots of information (less energy, more information). Let's use two examples: a photovoltaic cell (solar-electronic transducer), and the human eye (solar-electrochemical transducer).
The eye is designed to provide you with the information sufficient to avoid bumping into bad things in extreme lighting conditions. This is called information, but it is not the useful information for assessing the solar resource quantitatively. A linear detector like a photovoltaic device is required to accurately measure the power of the solar irradiance.
Measurement is an important aspect of all scientific endeavors. It is especially important in the proper and efficient design of solar energy collection systems. Proper solar assessment involves metrological and climate data, and correct measurement of global (beam and diffuse) radiation is essential to any solar design effort. Without adequate and precise measurement of the solar resources, system designers and engineers would essentially be "flying blind." In this section, we will discuss the equipment used to perform the required measurements.
Pyranometers act as solar energy transducers, in that they collect irradiance signals and transform them into electrical information signals. That information is passed on to a data logger and computer, and then we either present the data in short bursts (1 second) or integrate and average the data over longer periods of 1 minute to 1 hour.
Research grade pyranometers use a film of opaque material to collect thermal energy. The thermal energy diffuses into a thermal transducer called a thermopile (a stack of thermal devices) that produces a small current proportional to the temperature. We should note that metals (in general) are very good reflectors, making them also very poor absorbers. So, how do we get a material that functions on thermal gradients to make use of the radiation from the sun?
The key is in the absorber material: Parson's black is a paint with very low reflectance across shortwave and longwave bands of light (~300-50,000 nm; making it an effective blackbody). However, if covered by glass (a selective surface), the "window" of light acceptance from the Sun is about 300-2800 nm. This system assembly forms a shortwave (band) global (component) pyranometer. Now imagine, if we develop a thermopile with a thin coating of a black absorber, but replace the glass with a material that is transparent in the longwave band (many organopolymers/plastics), we will have created a longwave (band) global (component) pyranometer.
On the other hand, inexpensive pyranometers can use photodiodes. Photodiodes are photovoltaics (just small). They are semiconductor films that directly convert shortwave band radiation into electrical signals (no thermal conversion step necessary). While the cutoff for a silicon photodiode is <1100nm, the integrated power response is fairly comparable to that of a Parson's black-coated thermopile detector. However, they do not perform as well (relative to thermopile detectors) near sunrise and sunset due to a cosine response error.
Remember the cosine projection effect that we discussed in Lesson 2? It matters here for solar measurement. In the morning and evening, at low solar altitude angles (), some of the radiation incident on the detector is reflected, which produces a reading less than it should be. Some correction can be made for this using a black cylinder casing and a small white plastic disk cover (with a low reflectance at low angles to minimize the cosine error).
For standard research, technicians mount pyranometers in a horizontal orientation. Pyranometers produce a voltage in response to incident solar radiation. Provided that a pyranometer uses a thermopile (thermoelectric detector), the device acts as an "integrator" of all components and bands of light. In the case of a glass enclosure, even a thermopile detector will operate only in the shortwave band. Pyranometers based on photodiodes are used only for shortwave global radiation measurements. The following two images are explained in detail at the University of Oregon's Solar Radiation Monitoring Laboratory [70] (maintained by Dr. Frank Vignola). The left image is a LI-COR pyranometer [115], which uses a silicon photodiode to measure irradiance (a little PV cell). The right image, which looks like a flying saucer from the 1950s, is an Eppley Precision Spectral Pyranometer (PSP) [116]. The Eppley is a First Class Radiometer, and uses a thermopile to measure irradiance. The white ring is to reflect stray light away, such that the system does not heat up and so that the influence of the ground reflectance (the albedo) is minimal.
Standard pyranometers are designed to be mounted horizontally in shadow-free areas, with the normal vector relative to the surface of the collector (which is horizontal) pointing vertically. Measurements of downwelling shortwave band irradiance from a horizontal pyranometer collect Global Horizontal Irradiance, or GHI. However, through a simple modification, a pyranometer may also be used to measure diffuse irradiance. By using an occulting disk or band, beam radiation can be blocked from the sensor surface of the pyranometer, leaving only diffuse radiation to be measured.
If we wished to measure only the direct component of downwelling irradiation, we would use a pyrheliometer. The device is a combination of a long tube with a thermopile at the base of the tube and a two-axis tracking system to always point the aperture of the device directly normal to the surface of the Sun. A measure of irradiance from a pyrheliometer is therefore called Direct Normal Irradiance (DNI) (Gb,n) data. An Eppley Normal Incidence Pyrheliometer [117] is displayed below on the left, while an Eppley Solar Tracker [118] is displayed on the right.
Curious side note: The World Meteorological Organization (WMO) has a definition for "sunshine." Sunshine means irradiance conditions of >120 W/m2 from the direct component of solar radiation. Really, sunshine has a definition!
Until now, we have assumed that measurements of GHI or DNI will come from surface-based measurement methods. By reading Ch. 4 of the CSP Best Practices, we also see that satellites can be used to retrieve GHI (not typically DNI). Geostationary Satellites are used to collect GHI data.
In the United States, the National Oceanic and Atmospheric Administration's [119] geostationary satellites go by the name of "GOES," which is an acronym for "Geostationary Operational Environmental Satellite." Two operational geostationary satellites, GOES-13 and GOES-11, currently orbit over the equator at 75 and 135 degrees longitude West, respectively. As an aside, GOES-12 is currently drifting east toward , where it will provide images of South America.
To access images from GOES or geostationary weather satellites operated by other countries visit:
Geostationary satellites are far from perfect. Consider that images of clouds at high latitudes will become highly distorted due to the cosine projection effect, or from viewing the Earth at increasingly oblique angles. For latitudes poleward of approximately 70 degrees, geostationary satellites become essentially useless. But, this is also where the solar resource becomes quite limited. Polar-orbiting satellites can therefore collect at high latitudes where geostationary satellites are not efficient. Each polar orbiter has its cycle effectively fixed in space, completing 14 orbits per day while the Earth rotates.
Please make sure you read all of Ch 8 in SECS for this lesson, and focus on the section "Empirical Correlation for Components" and this page content. In the two additional readings, it is OK to scan the Stoffel Ch 4 and the Gueymard paper for key elements that are parallel with the lesson and the textbook. I included them also so that you could look back to them later as resources for your career development.
Now that we have our measurements, how do we make use of them to estimate irradiance on any given tilted surface? In the following section, we want to sort out the way that we measure light in comparison to the way that we use solar data for simulations of SECS in software like SAM (System Advisor Model). The SAM model will only have hourly Global Horizontal Irradiation metrics to use (I), but we will want to estimate the hourly irradiation for an oriented surface (POA, It).
As you move through this lesson, think about the Plane of Array (POA) for a Solar Energy Conversion System, and think about how we often measure irradiance using a horizontal pyranometer (and ONLY a horizontal pyranometer, unfortunately). What is the value of DNI in estimating the solar resource components for any given tilted (and maybe even moving) surface?
As you have learned from reading Chapter 8, the main way that meteorologists have measured irradiation is from a horizontal surface. However, most of our SECSs are mounted on non-horizontal surfaces. This presents a challenge.
When measuring the solar irradiation incident on a surface of interest, we measure the total or Global solar irradiance, which is a sum of the two components: Beam and Diffuse. Here, we present an equation for components of irradiance, G (we could have shown components of hourly irradiation in the same way) incident upon a horizontal surface.
Solar radiation that reaches the earth from the sun is generally not constant. A number of factors can affect the amount of radiation we receive. These factors include time of day and year, state of the atmosphere, and presence of aerosols. As stated earlier, the total solar radiation incident on a surface comprises of different components, and there is a simple reason for this. Not all the light emitted by the sun reaches the surface of the earth without any interference. As the emitted light passes through the atmosphere, a number of things generally happen. Some of the light may be absorbed, scattered, or reflected by the air molecules, water vapor, and aerosols. This portion eventually reaches the earth but not with the full intensity it had when originally emitted by the sun. We call this diffuse irradiance (Gd).
When a collector is tilted: the diffuse component from the ground tends to increase in contribution.
Air chemistry in the path of the beam will scatter the energy into a small cone of light, called the circumsolar component of the sky dome. Next, the scattering events that occur during the day produce a blue or white hue across the hemispherical surface. This is referred to as the sky diffuse component of irradiance. A horizon diffuse component is observed as the path length increases for scattering in the sky. Finally, the reflectance of the ground surfaces will contribute an extra component as long as the collection system is not mounted horizontally. Some light will be reflected from the ground back to the tilted surface. This component is appropriately called ground reflected component.
There also exist portions of the emitted light that reach the earth directly with no interference from the atmosphere. This is called the direct or beam irradiance (Gb). If we have a measurement of DNI (Direct Normal Irradiance), then we can quickly estimate beam irradiance for the horizontal surface via the cosine relation to the zenith angle (). Atmospheric conditions, however, have a strong influence on the amount of beam radiation we receive. On clear, dry days, atmospheric condition can attenuate beam radiation by around 10% and by nearly 100% on very dark cloudy days.
In the following flow chart of data processing, we see that measured irradiance (shortwave, from the Sun) is measured in four typical manners:
In the figure, we have integrated the time step to 1 hour of irradiation on a horizontal and tilted surface, respectively: I and It.
Measurement 1 (GHI) is the most common for a local site assessment in SECS design. Equipment for measuring DNI and DHI are atypical in an application site where the initial site assessment is beginning, and are absent from our satellite maps of the solar resource. Measurement 4 (POA) is becoming more and more popular, as it can potentially remove several steps of error propagation from empirical correlation on site.
Looking across the top line (Measurement 1), we see that we must perform "empirical correlations" using a metric called an "hourly clearness index" (kT) to arrive at "calculated horizontal components of Ib and Id (beam and diffuse hourly irradiation). Then, we must apply an "anisotropic diffuse sky/ground model" and sum the tilted components of irradiation, to finally arrive at a calculated POA irradiation estimate.
Paths 2 and 3 add to the level of precision by stepping past the clearness index correlations (which is error-prone) before applying an anisotropic diffuse sky/ground model and summing to a new calculated POA estimate.
Path 4 skips all of the empirical models and directly measures and integrates the irradiation on a POA surface. The one additional benefit would be to have a DNI measure along with a POA measure, for component decomposition if necessary (required for windows, for example).
Please make sure you read all of Ch 8 in SECS for this lesson, again maintaining focus on the same section "Empirical Correlation for Components" and this page content. In the two additional readings, it is OK to scan the Reindl et al. paper and the Gueymard paper for key elements that are parallel with the page content.
System designers do not always have the benefit of designing SECS with horizontal surfaces. Many times, these surfaces are tilted at various angles and have various orientations. In such situations, designers and engineers must make estimations for tilted surfaces based on data for horizontal surfaces.
In order to estimate, we first have to break apart the beam horizontal component from the diffuse horizontal components. This has been achieved historically by a methodology established in the 1950s and 60s by Profs. Ben Liu and Richard Jordan (our supplemental reading that is included to add context and the entire line of research that has been applied from then until now).
The availability of solar data is very important when calculating the amount of radiation incident on a collector. Engineers and designers commonly make use of average hourly, daily, and monthly local data. However, the most common measurement available is the Global Horizontal Irradiance (GHI), which is then integrated through a data logger into hourly irradiation, or minute irradiation.
Estimation is an effective tool that involves the use of empirical models that were developed over the last 4-5 decades. The only tools we need are the equations for calculating hourly and daily extraterrestrial irradiance (Air Mass Zero, or AM0) and the integrated energy density (J/m2) gathered from a horizontally mounted pyranometer. These empirical methods to decouple beam and diffuse horizontal components are termed Liu and Jordan transformations, after the initial paper in 1960.
The linkage between the two data for horizontal orientation are the clearness indices (kT, KT, and ). This index is simply a measure of the ratio of measured irradiation in a locale relative to the extraterrestrial irradiation calculated (AMo) at the given locale.
For KT →1: atmosphere is clear. For KT →0: atmosphere is cloudy. However, this measure incorporates both light scattering and light absorption. Keep in mind that a fraction is not a percentage, and in our case for a cumulative distribution, it is a decimal value between 0--1.
There is also an alternate indicator for the way that the atmosphere attenuates light on an hour to hour or day to day basis. This is the "clear sky index" (kc). Mathematically, the clear sky index is defined as
and it has been proposed that 1-kc is a very good indicator of the degree of "cloudiness" in the sky.
So, why do we use either the clearness index or the clear sky index? The answer at the moment is persistence. While it is likely that the clear sky index is more useful than the older clearness index in the long term, all the core research for the empirical calculations used in softwares like TRNSYS, Energy+, and SAM was based on kT.
In the 1960s, Liu and Jordan found that for different US locations with the same value of , the cumulative distribution curves of KT were identical, almost irrespective of latitude and elevation.\marginnote{A cumulative distribution describes the frequency or fraction of occurrence of days in the month below a given daily clearness index, KT}. This work was expanded into equations by Bendt et al.,\cite{Bendt81} using 20 years of real measurements in 90 locations in the USA. However, it was determined that the data sets were not so similar from region to region (e.g., the tropics had different correlations than the temperate USA, India was different from Africa, etc.) This work was followed by Hawas and Muneer for India and Lloyd for the UK, among others.\cite{Hawas85,Lloyd82}
Remember this! KT distributions are not universal---they are regional and empirically derived. For all of our future work, we will only rely on hourly kT values, and the manner in which kT is used to back out a value of Ib, the hourly beam irradiation component on a horizontal surface.
Please make sure you read all of Ch 8 in SECS for this lesson, still related to "Empirical Correlation for Components," but paying attention to the isotropic and anisotropic sky models and this page content. This is the third page for which we have included a review of the Gueymard paper, so you should be familiar with the findings by now. The Perez et al. paper will be useful to you in Learning Activity 4.2 and for your Lesson Quiz.
Earlier, we discussed the different components of light, beam and diffuse on a horizontal surface. Now, we will discuss how these components can be estimated for tilted surfaces through isotropic or anisotropic diffuse sky/ground models of light source components.
We shall see that we do not need to measure every component of light (scattered and unscattered) to make estimations on the contributions of each component to the total irradiation incident on the aperture of interest. We can rely somewhat on decades of historical observation and empirical correlation by solar scientists and engineers for hourly, daily, and monthly average day data.
The main tools we need are the equations for hourly and daily extraterrestrial irradiance (Air Mass Zero, or AM0) and the integrated energy density (irradiation: $MJ/m^2$) gathered from a horizontally mounted pyranometer, which you learned of in the last section. We shall also find that we can infer more than just the components of light from the ratios of measured irradiation to AM0 calculated irradiation--we can describe the fractions of days in a given month where lighting conditions will be clear or overcast/cloudy.
Following our step to break apart the beam horizontal component from the diffuse horizontal components, we then estimate the components on a tilted surface.
For a tilted plane of array,
Total Radiation = beam + diffuse, sky + diffuse, ground
A simple calculation of the beam component can be achieved using
Radiation on a sloped surface can be calculated for the beam component of irradiation by the geometric scaling factor of
In order to estimate the diffuse component, we use alternate models that become increasingly better fits with the empirical data. We can integrate any of these equations over an hour or a day (irradiation). I prefer to offer the irradiance version as a bit easier to read. Note: all of these estimation models use irradiation values that were measured from a pyranometer mounted along the horizontal plane, and then estimated for beam and diffuse components from data correlation or directly measured using a shadow band measurement and energy balance equations.
The isotropic sky model was developed in the 1960s to estimate the diffuse sky on a tilted surface, complemented by an estimate for diffuse light from the ground. This model assumes that the sky is uniform in composition across the sky dome.
The following expression gives the total solar irradiance incident on a tilted surface as
where,
The fraction proportional to the collector tilt is called the diffuse sky irradiance tilt factor for an isotropic sky model, and the reflectance of the ground is called the albedo (a fraction between 0 and 1), and is multiplied by the GHI and the diffuse ground irradiance tilt factor for an isotropic sky model.
Note: "Surface": the aperture. : is the collective reflectivity of the ground (the albedo). reduces the irradiance G by a value between 0--1. On an inclined surface, Gd,ground increases, relative to a horizontal collector.
This model incorporates isotropic diffuse, circumsolar radiation and horizontal brightening. It also employs an anisotropic index A defined mathematically as
The total irradiance on a tilted surface is then calculated by using
Go to Kalogirou (Solar Energy Engineering) Ch 2 (pdf from Library) this will be labeled the "Reindl model"
This is an anisotropic diffuse sky model that takes into consideration the real observations of subcomponents of diffuse light. The Perez model adds the circumsolar diffuse component and the horizon diffuse component to the diffuse$_{sky}$ component of the isotropic model. Notice how the beam component is not mentioned here--it doesn't change.
Sidenote: Richard Perez is a Senior Research Associate in the Atmospheric Sciences Research Center in SUNY Albany. He has a great website [122].
The shape factors (F) in this model can be reviewed in the original article by Perez et. al (1990). However, we can inspect the equations and observe in the equation that $F_{surface-sky}$ is reduced by a proportion of $F_1$ (circumsolar radiance), and $F_2$ can either increase or decrease the contribution of horizon radiance.
You can see that one may select "Irradiance Components used for Calculation": this is specifying the type of horizontal irradiation components that you will use in your tilted model. In a data set called the Typical Meteorological Year, the data for the beam is often not actually a measured value.
You can also see that one may select three diffuse sky/ground transposition models to transform the "Irradiance Components" (horizontal) to tilted values. The default is the Perez model that we describe below (and in your supplemental reading). The isotropic model is not used in practice, but it contains the basis for the other anisotropic models of Hay-Davies-Klutcher-Reindl (HDKR) and Perez et al. 1990.
Liu, B.Y.H., Jordan, R.C., 1960. The interrelationship and characteristic distribution of direct, diffuse, and total solar radiation. Solar Energy 4(3),1-19
Perez, R., Ineichen, P., Seals, R., Michalsky, J., Stewart, R., 1990. Modeling daylight and Irradiance components from direct and global irradiance. Solar Energy 44(5), 271-289
Your reading in Ch 8 of SECS will introduce the concept of Meteorological Years, while the Ch 2 reading in Kalogirou offers a condensed description of how a TMY data set is formed and how the files are formatted. The Supplemental readings address the most recent version of the TMY data sets in the USA (TMY3), and point to the National Solar Radiation Database maintained by NREL.
The data that you will find in your SAM simulation software, and the data that is all over the web in resources like the NREL Dynamic Maps of the USA solar resource [123], come from a single database, called the NSRDB, or the National Solar Radiation Database. There are currently three generations of TMY.
What is a Typical Meteorological Year? Why would we use a synthesized year of data for solar resource simulations?
A typical meteorological year (TMY) data set provides designers and other users with a reasonably sized annual data set that holds hourly meteorological values that typify conditions at a specific location over a longer period of time, such as 30 years. TMY data sets are widely used by building designers and others for modeling renewable energy conversion systems. Although not designed to provide meteorological extremes, TMY data have natural diurnal and seasonal variations and represent a year of typical climatic conditions for a location. The TMY should not be used to predict weather for a particular period of time, nor is it an appropriate basis for evaluating real-time energy production or efficiencies for building design applications or solar conversion systems.
...The TMY data set is composed of 12 typical meteorological months (January through December) that are concatenated essentially without modification to form a single year with a serially complete data record for primary measurements. These monthly data sets contain actual time-series meteorological measurements and modeled solar values, although some hourly records may contain filled or interpolated data for periods when original observations are missing from the data archive.
Wilcox and Marion (2008) NREL/TP-581-43156
Estimation can often be evaluated relative to 30 year averages of weather conditions at specific locations, termed the Typical Meteorological Year (TMY). These data are not reasonable estimates of extreme conditions (e.g., hurricanes, tornadoes) and may also be inaccurate for evaluating site or time-specific data. The most common database is TMY3, now collected from the period of 1991 to 2010.
TMY data was initially developed to aid in building simulation, for modeling the energy demands in counterpoint with the solar/meteorological gains. As in your software SAM (and the source code, TRNSYS), TMY is also used by SECS design teams for initial estimates of energy and financial returns on investment. We can use SAM's TMY data set to evaluate PV, solar hot water, and CSP systems.
The source data for the TMY set in the USA comes from the NSRDB, or the National Solar Radiation Database.
Let's go back to the SAM software and explore the solar resource data in its browser.
Reminder: This information will be used when you create your final project.
We have just read about Typical Meteorological Years (TMY) as simulation inputs in beginning project assessment. I want you to consider the positive and negative attributes of TMY data sets in terms of project design and then in terms of project operation and management. Post your answers to the following questions, and then let's have a discussion about them.
Here are some guiding questions:
When you create a post in the Yellowdig discussion space, you are required to choose a topic tag. For Lesson 4 discussions, please use these tags:
You can tag your post with one or several topics at the same time. All posts and contributions you create are added up to one score at the end of the week.
Yellowdig tip: remember to check in the Yellowdig site often - it is much easier to aborb the posted information in bits rather than reading multiple posts and comments in a bulk. Click to "Home" icon - you will be able to see all the unread posts in one place.
Yellowdig points you earn over the weekly point earning period (from Saturday to next Friday) will count towards 1000 pts. weekly target. But you can go above it (to 1350 pts. max). Yellowdig discussions will account for 15% of the total grade in the course. Check back the Orientation Yellowdig page in Canvas for more details on the points earning rules.
There is no hard deadline for participating in these discussions, but I encourage you to create your posts in the middle of the study week (Sunday) to allow others to engage and respond while we are learning specific topics in the lesson. Also, remember that each weekly point earning cycle ends Friday night, and a new period starts on Saturday.
Review the linked presentation from solar expert Dr. Chris Gueymard. It provides a historical context of where we came from in solar resource assessment, the current pressures associated with bankability, and the new challenges that we expect in the field in the near future. Now, compare the presentation to your earlier reading of Sengupta et al. (2015). In particular, scan the Chapter 6: Applying Solar Resource Data to Concentrating Solar Power Projects (p. 97).
Finally, go back to your Ch 8 SECS reading, and review the section "When Empirical Correlations are not Appropriate." You may begin to realize where the old empirical methods are useful, and the occasions when they are not useful to project implementation for our stakeholders.
I want you to think about the need for estimation in preparing a new project design using a software like SAM. When does the estimation process give way to more detailed measurements in a SECS project?
Think about where estimated data sets like TMY fit in the process of applying solar resource data detailed below.
When do we need to work as a larger design team with solar resource specialists who can monitor a site actively and maintain the equipment? When is the investment in measurement equipment appropriate for the planned project? For solar incorporations on the facade of a building (roof, windows, walls), do we need to measure the solar resource at the site?
As the world looks for low-carbon sources of energy, solar power stands out as the most abundant energy resource. Harnessing this energy is the challenge for this century. Photovoltaics and concentrating solar power (CSP) are two primary forms of electricity generation using sunlight. These use different technologies, collect different fractions of the solar resource, and have different siting and production capabilities. Although PV systems are most often deployed as distributed generation sources, CSP systems favor large, centrally located systems. Accordingly, large CSP systems require a substantial investment, sometimes exceeding $1 billion in construction costs. Before such a project is undertaken, the best possible information about the quality and reliability of the fuel source must be made available. That is, project developers need to have reliable data about the solar resource available at specific locations to predict the daily and annual performance of a proposed CSP plant. Without these data, no financial analysis is possible. This handbook presents detailed information about solar resource data and the resulting data products needed for each stage of the project."
--Tom Stoffel, Dave Renné, Daryl Myers, Steve Wilcox, Manajit Sengupta, Ray George, Craig Turchi; NREL/TP-550-47465
The purpose of this activity is to learn how the clearness index ( ) can be determined for a specific day and time based on collected meteorological data and knowledge of the extraterrestrial solar irradiance. By definition, is essentially the attenuation factor of the atmosphere, showing the ratio between the solar radiation incoming into the Earth atmosphere and that reaching the ground:
where is the energy density measured at the horizontal surface at a locale, and is the energy density just outside the Earth’s atmosphere at AM0. Of course, there are multiple natural phenomena that are responsible for scattering and reflection losses.
The clearness index was developed by the researchers Liu and Jordan at the University of Minnesota in the 1960s, and to this day this metric is still used by various models and empirical correlations for quantifying components of solar light.
In this activity, you will need to calculate the hourly values for two different hours on July 31st, 2007 at Penn State SURFRAD location (Rock Springs). This activity builds upon irradiance data you were treating in Lesson 3, so you will use the same SURFRAD file for the clearness index calculations.
Extract GHI data from the SURFRAD file (Penn State location) for July 31st, 2007 for two different hours: (a) 8-9 am and (b) 1-2 pm.
Convert GHI data (measured in ) to energy density values (in ) by multiplying them by time. Note: SURFRAD data are recorded at 3 min step.
Find the total solar energy density (irradiation) delivered per square meter over each hour period. This is the measured value. Present your results in .
Plug in your and values into the ratio and obtain values for both 8-9 am and 1-2 pm hours. Your result should be a number between 0 and 1.
Provide a brief discussion of results. What are the reasons for clearness index to change during the day?
Your report should include the following: (a) Data tables with GHI for specific hours (8-9 am and 1-2 pm on 7/31/07) and corresponding irradiation values; (b) hourly energy density in ; (c) calculation of extraterrestrial irradiation shown; (d) hourly extraterrestrial energy density in ; (e) values for each of two hours; (f) brief discussion of the obtained values.
Upload your report file to Canvas (Lesson 4 Learning Activity DropBox) in PDF or docx format.
This activity is graded out of 30 points.
Criteria | Available Points |
---|---|
Table of GHI data with corresponding irradiation values
Data for both 8-9 am and 1-2 pm hours |
Up to 5.0 points |
Correct hourly energy density (irradiation) value is reported (in ) |
Up to 5.0 points |
Calculation of the extraterrestrial energy density is shown and steps are explained |
Up to 5.0 points |
Correct extraterrestrial energy density is reported (in ) |
Up to 5.0 points |
Clearness index is calculated for both 8-9 am and 1-2 pm hours |
Up to 5.0 points |
Discussion of the results is provided |
Up to 5.0 points |
See the Calendar tab in Canvas for specific due dates.
This was the last of four lessons wrapping up the arc of core solar resource assessment content. In Lesson 4, you were introduced to the key elements of the solar resource and the ways we measure or estimate the resource in a given locale. We built upon our knowledge from Lessons 2 and 3, and also drew some content in a Lesson 3 activity to be applied in a Lesson 4 activity.
You went through a detailed reading on Best Practices for Solar Collection and Use to start, which provided us with a good amount of information for both CSP and non-CSP practices too. There were elements in the reading that stressed the importance in finding the appropriate data set for a SECS development plan. Solar resources are important to residential and commercial buildings, as well as to PV arrays, as well as to utility scale CSP and solar hot water or steam applications. Your job is to be aware of the type of data you use, to know the transformations that are done to the data to turn it into a useful input (for simulation), and to develop levels of confidence in using various qualities of data as a practitioner.
Now that you have completed the lesson, you should be able to:
You can always go back to these readings and dig into the references. There is now extensive documentation to guide you in your practical development; you just need to reach out and read it!
Double-check the to-do list on the Lesson 4 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 5.
OK, we are now out of the deep end of the class and moving into the frameworks for design and valuation of the solar resource. We will be developing a second major arc through Lessons 5, 6, and 7, working through economic and financial issues. So you should see connectivity among these three lessons.
In Lesson 6, we will discuss ways to meet the Goal of Solar Energy Design and Engineering: to maximize the solar utility for a client or group of stakeholders in a given locale. We will dig into a short statement, and find a nearly infinite variety of options for design. But first, we need to find out about our clients or stakeholders as "utility maximizers" in Lesson 5; what makes people demand solar energy products, and how easily will they change their minds? Are there any losses or risks that people are avoiding by choosing solar energy goods and services? Essentially, what are the driving forces for people to adopt solar energy?
Solar Energy Economics helps us to establish the following argument: just because one perceives the solar resource to be weak in a region does not mean that it cannot be successful as a technology in that society. The solar resource is ubiquitous, and we make use of it whether we decide to or not. What is interesting for solar energy is that our raw "product'' is the photon. We apply technologies and skilled effort to convert photons into a diversity of goods that society is interested in purchasing.
The economics of solar technologies helps us to address why we make decisions to use the Sun. We make use of the Sun throughout our lives, but in solar design, we work to develop compelling arguments to the client to increase their marginal demand for the Sun. There is a sense that energy is somewhere between a product and a good in demand by society, and it must be supplied by non-trivial mechanisms, at some cost for the exchange of goods. In order to make marginally (or incrementally) more use of the Sun, we have to learn about the skills to measure and predict the variable phenomenological behavior of solar irradiance as well as the dependence of the variable irradiance on the location of the client in question.
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 9 - Solar Economics Selected readings from EBF 200 course |
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Optional Reading (not required): | G. Mankiw Principles of Economics [127]. This might be a nice resource for your future study but is not required for this course. |
To Do |
Discussion 1: Light as a mineral resource Discussion 2: Hypothesis of Energy Constraint |
Topic(s) | Supply and Demand Light as a solar reserve Price elasticity of demand A hypothesis of the Energy Constraint Response |
If you have any questions, please post them to the Lesson 5 General Questions and Comments Discussion Forum in Yellowdig. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
We start with the material on solar economics in the Brownson's book:
Then I would like you browse through a few pages from anoter course: EBF 200: Introduction to the Energy and Earth Sciences Economics:
As a refresher on energy terms and definition, please refer to this website (see "What is Energy?" link).
When we deal with goods and services tied to energy systems, things get pretty interesting! When you think about energy and natural resources, the tendency in energy economics is to think mainly of "non-renewable resources" or exhaustible resources like coal/oil/natural gas, etc. I want you to think about how much of our social economic perspective on energy is based on exhaustible resources.
We want to better understand why our clients and stakeholders (or even we) make decisions to adopt technologies that deliver goods and services from the Sun. The form of energy is radiant, from a Solar resource system, and we transform radiant energy into other useful forms to do work.
The readily accessible energy that can be used to "do work" in society is still considered a limited natural resource, or good. In economic terms, we would say that many of our useful energy goods are scarce.
As we read in EBF 200, "What is Economics?" Prof. Gregory Mankiw lists seven microeconomic principles. Recall that microeconomics refers to individual economic actors considered as people and firms and their corresponding interactions in markets.
(Watch the following YouTube video of Thinking at the Margin by Prof. Mario Villarreal-Diaz.)
In solar systems design, we work to Maximize Solar Utility for the client or stakeholders in a given locale. We will describe the methodologies to do so in the next lesson. But our clients are individuals who are in demand of a solar good. The firms developing or deploying SECSs are supplying access to the solar goods.
Across the planet, there are non-uniform, ever-increasing demands for energy as thermal heat and electrical power. Light, as electromagnetic radiation, is another form of energy, used as well for visual comfort and indoor activities. The photon can be harvested via a solar energy conversion device. To be clear, photons are ephemeral (flows); they are not collected like fuel in a tank (not stocks).
Energy can be described in terms of sources (as in energy re-sources) and in terms of forms (as in energy trans-form-ations). Think of it this way, an energy source is a resource system, from which we appropriate useful resource units in a given form. Energy is neither created nor destroyed, so if the energy is in a less useful form, we must use an Energy Conversion Device (ECD, not a very technical term, but useful here) to transform one form into a more useful form.
Energy scarcity is partially related to the loss of energy quality with successive transformations. Light happens to be an incredibly high quality of energy, which is then transformed into chemical energy by plants (photosynthesis), or into thermal energy by opaque materials, or kinetic energy via wind, or electrical energy via photovoltaics. (Nuclear and gravitational energy are not linked so directly to radiant energy here.)
Our society is used to beginning with "concentrated sunshine" (geofuels from stored photosynthesis in coal, oil, and gas), and then transforming the chemical form to the thermal form (hot steam), which is then transformed into the motion form (to spin a turbine-generator) and finally transformed into electrical energy.
The terms Heat and Power have been adopted by several industries to have a specialized trade meaning.
Thus, in the energy industry, we hear about Combined Heat and Power (CHP) for energy conversion systems that provide two useful forms in one system.
Optional: G. Mankiw Principles of Economics [133]. This might be a nice resource for your future study but is not required for this course.
We want to focus on how the resource units like electricity, heat, daylight, and money derived from SECS have an elasticity of demand. How do we value the products of light, or how do we even value the solar resource itself as an energy reserve? In our reading, we find that sunlight can be analyzed similarly to a mineral reserve like copper ore.
Our decision to choose solar technologies often depends on the value that we place on light and the value of the resource units derived from shortwave light. You are going to need to think about light as a commodity, or a good that is interchangeable with other goods/services. This is a bit abstract, so take some time to reflect at the end of the page.
Stocks and flows exist in nature and in society. We see stocks in business. In nature, a lake is a stock of water, with a river flowing into it. And the Sun is a stock of nuclear fusion yielding a flow of radiant energy.
A resource system is considered renewable if the rate of withdrawal from the stock does not exceed the rate of resource replenishment. In the case of shortwave light, the solar resource system has physical conditions that define an upper limit of flow without disturbing or harming the constitution of the stock. We can't really withdraw sunlight at a faster rate than it comes to us. Hence, sunlight is flow-limited.
A resource system is considered non-renewable if the rate of withdrawal from the stock exceeds the rate of resource replenishment. In the case of geofuels, the process to make them takes 10s-100s of millions of years (and heat/pressure underground), yet the rate of withdrawal can be almost as fast as we want. Hence, geofuels are stock-limited.
Compared to what is available on Mars, the quantity of light is abundant on Earth! Even between the Arctic Circles (), there is a great abundance of light available to society to do work. As a society, we are not as skilled at transforming light into useful work as we are at transforming fuel into useful work. We are still struggling to frame light as a valued good, especially as it is all around us every day. So, let's take a look at that value structure.
The value of light from the Sun is variable. There is "less" of an energetic resource from the Sun in the annual irradiation budget for Germany than in the US state of Georgia, yet the value of solar power (as electricity) is much higher in Germany than in Georgia. So is the value of the light to the clients relative to the "quantity" of light, or relative to other parameters.
In the mineral economics of commodity goods, the value of the resource units will vary with respect to two general driving forces:
If the demand for a good goes up, the value of the resource units will go up. If the cost of an alternative good goes up (like the price of geofuels), then the value of the resource units (like solar) will go up.
An increased demand for a mineral commodity will increase the value, and a high cost of alternative goods will increase the value.
Value and quantity are joint properties here. As such, the "quantity" of a mineral reserve can expand or shrink in response to three main pressures. Solar resources follow the same commodity trend. In the case of the solar resource framed as a type of mineral reserve, the solar reserve is available when it is economically feasible, expanding and contracting in response to the following three pressures. That is to say, there are three levels that open up, or expand, the solar reserve in a given locale.
The value of an unconverted photon is a variable quantity, much like the value of a mineral resource in a geologic formation. Once again, the value of any commodity varies with the demand for the good and the costs of alternatives. The three main drivers that affect the valuation of light as quantified mineral reserve are:
Let us compare the way in which light is valued with the way that a metal ore (in this case, zinc) is valued. An ore is an unrefined rock composed of minerals, which contains a raw metal that is valued, but which must be processed to access that metal. In our reading from the USGS Commodity Statistics [136], Appendix C, we see that an entire lexicon has been developed for classifying mineral resources. (This site as a whole is also an excellent public resource for evaluating mineral reserves from the US perspective.) We have since classified geofuels as "minerals" in the commodity perspective. So, why not extend the concept outward to the commodity of light, and the derived goods and services?
The following terms are within the textbook reading, and were developed from the U.S. Geological Survey Circular 831, Principles of a Resource/Reserve Classification for Minerals (1980). Note the difference among a resource, a reserve base, and a reserve.
When we are "thinking on the margin," what do we mean? When an incremental change occurs in the price of a SECS or in the alternative price of electricity from the grid, how do we respond? Do we jump in, or do we wait and see?
In economics, the measured response (in the market) of how the quantity of a product in demand is changed by the incremental change in the price of that product is termed price elasticity of demand. The demand is considered elastic if a small change (like a decrease) in price leads to people demanding more of the product. The demand in considered to be inelastic if a large change (again, a decrease) in price does not lead to people demanding more of the product. The elasticity of demand for solar power will depend on a few general rules, and we will try to contain our examples to solar scenarios for a client or group of stakeholders.
The price of PV just changed. What do you do? Do you go out and invest in a PV system for your roof, or do you wait and see? Clients and consumers (us too!) are influenced by several criteria. The four main factors affecting the price elasticity of demand are:
First, one evaluates the availability of close substitutes for the particular SECS of interest. If the desired useful energy form or technology has many available close substitutes, then it will be easier for clients/stakeholders to switch among goods for the same desired feature, and the demand will tend to be elastic.
Next, we ask, is the energy form a necessity or a luxury? Our electricity from the coal/nuclear power plant is typically a necessity right (and thus inelastic)? Is there anything about residential PV that seems to be a luxury to families? When did mobile phones stop being a luxury and become a necessity in modern society?
What share of income can an individual or firm (as clients) devote to paying off a loan for solar technologies or directly purchasing a SECS? If a SECS consumes a large share of my income, what tradeoffs will I need to consider (what will I have to give up in return)?
Finally, when making decisions for energy systems, we must consider the time horizon, or the period of evaluation. For energy consumers, when the cost of energy (in dollars per kilowatt-hour, $\$/kWh$) goes up briefly (on the order of hours or days, or for one month) there's not much that they can do to respond. As such, the price elasticity of demand is said to be inelastic for shorter time horizons. In contrast, when the period of evaluation is framed in terms of decades, as is done for PV systems that have productive life cycles of 30-50 years, then the client perspective can shift and become more elastic. When you buy a house, you're in it for the long-term, right? Similar thinking with SECSs.
And now for two short perspectives on the Price Elasticity of Demand to complement the reading. Please watch the following two videos: "Episode 16: Elasticity of Demand" by Dr. Mary J. McGlasson, and "Elasticity - Characteristics that determine elasticity" (Dr. McGlasson is an economics faculty at the Chandler-Gilbert Community College.) I want you to think about solar energy and the resource units derived from the conversion of shortwave light.
When fuels (geofuels, biomass) are effectively:
light and the associated Solar Energy Conversion Systems are not perceived as a viable alternative. Light is framed as diffuse and insufficient to do work.
However, when fuels are:
then light and the associated Solar Energy Conversion Systems are counter-interpreted as ubiquitous and vast, and capable as a viable alternative.
Our energy use in society is coupled to the locale and to our comfort expectations. Energy use is also coupled to the availability of inexpensive fuel resources. The four main factors constraining fuels are described below:
In this lesson we have been discussing the value of goods in an economic sense. The tendency in the public is to judge the value of a solar technology in a given locale based on metrics (perceived or measured) of the quantity of light (MWh). But I would like you to consider an alternate valuation system related to the value of a mineral resource.
So, this week discuss the following questions in Yellowdig:
Another topic in this lesson that deserves some discussion is the hypothesis of energy constraint response. You already had a chance to review your locale sunlight resource and perception earlier in this class, so do you see any evidence of this hypothesis being true for the area you live in?
You can review the following information to develop your conclusion on this topic:
Do you think that the above observations support or disprove the hypothesis of the energy constraint response? I would be curious to hear your opinion.
When thinking about the solar resource in an economic framework, try to be objective and describe the conditions that you observe around you, rather than what you think "ought" to be happening. Most of us have not really framed the solar conditions in rational terms. If you have conflicting ideas about light and irradiance from your own background, feel free to discuss those and see what others think.
When you create a post in the Yellowdig discussion space, you are required to choose a topic tag. For Lesson 5 discussions, please use these tags:
You can tag your post with one or several topics at the same time (just be sure to address all those in your post). All posts and contributions you create are added up to one score at the end of the week.
Yellowdig tip: Post early in the study week - that way you have higher chance of generating interest and traffic on your post, which gets you points!
Yellowdig points you earn over the weekly point earning period (from Saturday to next Friday) will count towards 1000 pts. weekly target. But you can go above it (to 1350 pts. max). Yellowdig discussions will account for 15% of the total grade in the course. Check back the Orientation Yellowdig page in Canvas for more details on the points earning rules.
There is no hard deadline for participating in these discussions, but I encourage you to create your posts early in the study week to allow others to engage and respond while we are learning specific topics in the lesson. Also, remember that each weekly point earning cycle ends Friday night, and a new period starts on Saturday.
Good work completing our first lesson dealing with solar economics! We have transitioned from the dense topics of spherical trigonometry, meteorology, and component modeling (Lessons 2, 3, and 4) into the driving forces for our clients to make the decision to adopt a solar energy conversion system. In this lesson, we learned that our clients are situated on the demand side of the energy economic framework, and consumers such as our clients are called utility maximizers.
We saw that there are two general motives to shift the value of any commodity from the perspective of a consumer: demand for a good and the cost of alternatives. Specifically, within the solar field, the three main drivers that affect the valuation of light are:
Each of these should make sense within the framework by Mankiw for microeconomic principles. We also observed how light can be put in the context of a mineral commodity, much like the USGS has done for geofuels. The solar resource as a reserve is a variable quantity depending upon the value of that resource in a given locale. As such, value and quantity are joint properties.
Also, the measured response (in the market) of how the quantity of demand is changed by the incremental change in the price is termed price elasticity of demand. The demand is considered elastic if a small change in price leads to people demanding more of the product. The demand is considered to be inelastic if a large change in price does not lead to people demanding more of the product.
Finally, we tied all of the economic forces and responses together with the Hypothesis of the Energy Constraint Response. There is historical evidence across many locales, in the USA and abroad, for solar adoption tied to fuel constraints. We can even consider the pressure of climate change as a new fuel constraint for society, leading to increased demand for solar energy resource units.
You have reached the end of Lesson 5! Double-check the to-do list on the Lesson 5 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 6.
In Lesson 6 we will progress through the second of three lessons tied to solar economics and finance. Lesson 6 will discuss the criteria for developing a solar project as a professional in an ethically sound manner. We will cover the concept of maximizing the solar utility for the client in a given locale. In Lesson 5, we already discussed consumers as "utility maximizers," and we posed the term utility as a preference among a set of goods and services. Hence, solar utility will be that maximized preference among the set of solar-derived goods and services.
Solar energy design has broad criteria that may be explored to develop a successful resource proposal and project implementation. We design for our clients, or stakeholders, who live in a specific locale, right? Hence, the concept that solar utility has to be constrained by the preferences of our clients and the limitations or opportunities presented within their respective individual locale. We can influence solar utility in a given locale first through physical and engineering considerations, and second through consideration of financial concepts tied to the performance of our project.
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Canvas Calendar for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 16: Project Design J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 6: Solar Geometry (A Comment on Optimal Tilt) |
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Optional Reading (not required): | Greentech Media Article: Solar Balance-of-System: To Track or Not to Track, Part I [141] (Nov. 2012) M. Lave and J. Kleissl. (2011) Optimum fixed orientations and benefits of tracking for capturing solar radiation in the continental United States. Renewable Energy, 36:1145–1152. C. B. Christensen and G. M. Barker (2001) Effects of tilt and azimuth on annual incident solar radiation for United States locations. In: Proceedings of Solar Forum 2001: Solar Energy: The Power to Choose, April 21-25 2001 T. Huld, M. Šúri, T. Cebecauer, E. D. Dunlop (2008) Comparison of electricity yield from fixed and sun-tracking PV systems in Europe. European Commission, Joint Research CentreInstitute for Energy, Renewable Energies Unit, via E. Fermi 2749, TP 450, I-21027 Ispra (VA), Italy (poster, PDF). DSIRE (NC Solar Center) Database of State Incentives for Renewables and Efficiency [142] |
To Do: |
Learning Activity: Pre-Design Charette Plan Discussion 1: Communicating Information to Your Client Discussion 2: Brainstorming Your Solar Proposal (due after Lesson 7) Quiz: See Canvas (Module 6) Engage in all Try-This and Self-check activities (not graded) |
Topic(s): |
Geospatial assessment as utility maximization: supply side
Incentive assessment as utility maximization: demand side
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If you have any questions, please post them to the Lesson 6 General Questions and Comments Discussion Forum. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
We are jumping ahead to the elements of project design, so that you will be able to address Learning Activity 6.1. Chapter 16 will help us to prepare for our tasks ahead. You might consider a quick review of Ch. 2 in light of our new perspective on energy economics, and put it into the context of solar utility and the influence of stakeholders.
I want you to now focus on the connections among the client, the affected and diverse stakeholders, and the concept of solar utility. Your goal is nothing so specific as "highest efficiency" or "most power" or "least expensive system." You may be able to assess your client and address mechanisms to provide them with a high solar utility from a project proposal standing.
A design effort without constraints and boundaries can quickly spiral out of control. Your SECS is dependent on the locale, and the client! How can we efficiently maximize the client's preference for goods derived from the solar resources to help meet client goals in their particular locale? When we speak of solar utility, we are referring to maximizing the preference for solar goods and services in order to provide needed power, heat, light, food, etc.
Solar utility can be met from many technologies, it's not just about PV and solar hot water. Sometimes effective shading for a building or space will reduce demand for electricity, which is completely within the scope of a solar energy design team. Maximizing solar utility is definitely not about "efficiency" or "more power" (or more cow bell) [144], because these are systems variables that are all married into the whole balance of energy supply, demand, fiscal wealth, and broader happiness.
One of the systems approaches to increase solar utility for SECSs is by engineering means (e.g., applying what we learned in Lessons 2-4). You need light to produce electricity and heat, right? These strategies increase the amount of light incident on the SECS through general orientation, tracking, and avoiding shading. Note that they are systems solutions, and not "find a more efficient panel."
The second systems approach that we will cover (to increase/maximize solar utility for the client in their given locale) is economic in nature. This approach is concerned with technology costs relative to metrics of financial payback, levelized costs of energy (LCOE), and net present value. For this particular lesson, we are going to focus on the costs of electricity from the grid (which is mainly coal, nuclear, natural gas, and hydroelectric power), and the incentives that are available to our clients in their locale. Both the grid and the incentives available to our clients are locale-based, as with the solar resource above.
Remember, the design team maximizes solar utility by considering the locale and the client needs, then selects a technology (or suite of approaches) that is appropriate.
The term locale in the SECS context implies more than just site or location – it rather represents a set of key parameters that would have critical impact on the system in question. Locale is defined in both time and space (because meteorology implies both time and space). Here are several parameters we can consider specifying locale for a project:
This cartoon summarizes this concept in the nutshell. Further, in the following lessons, when you are asked to provide some characterization of the locale, be sure to include some information on the following four key elements.
Note that locale does not refer to the design elements of the SECS (when we describe the locale, we do not yet have system in place), neither it considers client. Those two things will come into play next.
Your "client" is a utility maximizer. They may (or may not) make rational decisions to implement a SECS. Your function in the solar design team is to be their informed advisor. However, your client may also be a whole cohort of people, or a group of stakeholders. Stakeholders are all those affected by the decision to design and install a solar energy conversion system (SECS). These may include the client, the engineers and installers, building managers, the local community members, and so on. Again, a stakeholder can be a client or just an invested individual participating in the system as a whole. One of your jobs is to identify stakeholders and asses the role their multiple perspectives may play in the design process.
Common business language states that the customer is always right. More appropriately, the customer is always the one who decides "go" or "no go" in a solar project. The basic logic of that statement guides the design process. Every SECS is designed with the needs and requirements of a client in mind. No particular system can be used by all clients in all locales regardless of how well the said system is designed. Design requires that we have a close understanding and appreciation of what a client needs. A solar designer may start the design process by posing questions to the client, such as: How much power/energy do you need? How many hours of power do you need and what time during the day do you need this power? Is this power needed year round, or only during particular seasons? Answers to these questions will ultimately guide the designer and lead to efficient design.
Your design team holds stakeholders to the concept of the Four Es: Everybody Engaging Everything Early (developed by PA design firm 7Group [147]). We want to engage the stakeholders in the integrative design process, and the pre-design process can involve brainstorming events called charrettes.
The design team also needs to educate the client on the different options available to him/her. For example, if a client decides to install photovoltaic panels to provide electricity, the design team will need to inform the client on the various PV technologies, the advantages and disadvantages of each in regard to price and function in different locales, the different options for funding the project through government grants and loans, etc.
As such, your client (and associated stakeholders) and your locale are the two major super parameters that can guide systems design.
Locale is the space or an address in time and place within which the client occupies and demands energy resources. Recall that our clients are on the demand side of solar goods and services, and as such they seek maximal utility when making decisions.
The goal of solar design is to:
We have already learned that the solar resource can be affected by the locale of the site. The solar resource is determined by the locale, as the climate regime affects the seasonal and daily irradiation patterns and frequencies of intermittence. The character or quality of the solar resource will in turn constrain the design team's options for technological solutions that compete with conventional fuel-based technologies.
According to our review of SECS Chapter 6: given that goal for solar project design, we have three main engineering approaches that we can leverage to affect the solar utility for a client in a given locale:
These are the three main engineering parameters linked to the locale that will constrain your design options (you can look back to the Angular Solar Symbols guide [43] to refresh your memory). They all affect system performance, without necessarily directly influencing the cost of the system (in the beginning). Let's review how they affect system performance.
How does the tilt and azimuth each affect the design in SECS, and how does regionality affect the design decisions in solar energy?
We have seen in our reading of Lave and Kleissl that an annual optimum for tilt and azimuth can be selected, while Christensen and Barker demonstrate that annual optimum is not really "peaky," and fixed-tilt systems can be oriented across a broad range of directions in a given locale without dropping solar gains by more than 10-20%. If we were to adjust the tilt for a seasonal optimum, we would select a lower tilt for the summer season and a higher tilt for the winter season. Effectively, we are working to correct for the cosine projection effect of our particular latitude and climate regime (one climate regime per season, recall the "fingerprints").
On broad scales, sites near the equator will have different design constraints than sites near the Arctic Circle, due to the cosine projection effect driving our solar resource across latitudes and the seasons. In this context, the project locale serves as an effective system constraint. The amount of sunlight available on a daily basis and on a seasonal basis differs with locale. Using and implementing the same system design for a client in State College, Pennsylvania (, ) and another in Lagos, Nigeria (, ), for example, will yield totally different results and lead to unsatisfied clients.
You see two images of a cartoon Sun, drawn from Ch 4 of the SECS text. The top image shows the effect of inverse square law on the Sun-Earth view factor (). The distance of 150 million km reduces the intensity of the Sun from to 1361 |(). This effect is fairly uniform year-round. The bottom image shows the cosine projection effect as it affects the Sun-Earth view factor. Here, the inverse cosine of the zenith angle ( ) reduces the intensity of the Sun's irradiance. Hence, the farther away your client is located from the Equator, the more the designer will need to make collector orientation adjustments to compensate for the losses from the cosine projection effect.
Note also that the tilt of the Earth's axis will drive one to consider summer or winter optimized orientations (away from the Equator).
How does tracking affect the design decisions in solar energy?
Well, a fixed axis SECS is often oriented toward the equator at a tilt ( ) somewhat less than the local latitude (do not fall for the latitude = tilt rule of thumb), per our readings from Christensen and Barker, and Lave and Kleissl. When we track the Sun, then more beam is collected (the angle of incidence tends to be consistently lower than for a fixed tilt). By looking at the poster from Huld et al. (2008), we see that a single-axis tracking system, with an axis inclined at an optimum angle towards South, should offer 12-50% improvement over a fixed axis tilted at the optimum, where a 2-axis tracker will offer a very similar solar gain of 13-55%.
So, a tracking system will minimize the angle of incidence ( ), but there will be a cost in terms of land requirements. Why? Because of shading. There will also be a cost in terms of the balance of systems (e.g., the non-SECS trackers). This is why we could read "Solar Balance-of-Systems: To Track or Not to Track, Part I" for more information.
But the reality of solar development (whether on a rooftop or on a field) is that the systems are often "area constrained." We can make certain tradeoffs in systems choices to deliver a better unit cost to the client, but we may not get all the land that we desire to accomplish an optimal tracking system. As such, we must work with the stakeholders to find the highest solar utility solution given the available area.
Finally, a large group of our SECSs rely on access to the shortwave light from the Sun. If we shade a collector, then we reduce or remove that working energy that we wish to convert to heat or electricity. We performed the shading analysis in Lesson 2 using orthographic and spherical projections specifically to be able to avoid shading of our array over the course of an entire year.
Of course, if we were to design a system to avoid the Sun's rays, that would be different. We have seen examples of solar design for Parasoleil frameworks (shading systems) in the beginning of the textbook (e.g., southern awnings).
In this lesson we continue making connections between the technical information on solar resource and system design with project economics.
Then, we also collect various weather and climate information about the locale. Some supporting information is available from the TMY data, environmental monitoring stations, or direct measurements. Seasons may look different in different climate zones. How do we relate all that information to the SECS performance?
Those characteristics of the locale are not just FYI. We are trying to reveal their economic impact on system performance, payback, and return on investment for our client.
We will revisit some of the previous topics in Yellowdig (maybe at somewhat different angle) and will add some new as well to provide you with the discussion space on the questions above:
You can tag your post with one or several topics at the same time (just be sure to address all those in your post). All posts and contributions you create are added up to one score at the end of the week.
Yellowdig tip: Remember to respond to questions. If your post generated some, it is a good thing! The best way not to miss questions is to set email notifications in Yellowdig - then, whenever someone reacts to your post, you will get instantly notified.
Remember that Yellowdig point earning period ends each Friday. Posting commenting, reacting regularly through the weeek will make the 1000 pts. an easy target and guarantee a high participation grade in the course. Yellowdig discussions will account for 15% of your total grade.
One of the highly visible SECS technologies is photovoltaics, which delivers (generation) electricity to the client, and now pushes excess electricity onto the electricity power grid. The electricity power grid is the physical system that delivers (transmission) electricity from the place where it is generated to the site where it is used (end-use, demand).The electricity leaving the generating station enters a sub-station with a step-up transformer that raises the voltage extremely high for long-distance transmission.
When electricity travels through wires (a conductor), some energy is lost, but less energy is lost when the electricity is transmitted at a higher voltage. At a high voltage, the same amount of power can be transmitted, but using a lower current. The amount of energy lost from the conductor is called line loss, and line losses are directly proportional to the current. By reducing current, we reduce losses for the same power transmitted. Typically, in the U.S., line losses between generation and end-use are in the 6% to 8% range.
The high-voltage electricity is carried over transmission lines to local substations, where a step-down transformer reduces the voltage to levels suitable for customer loads. Distribution lines carry the lower-voltage electricity from the local substations to customer sites.
The Power Grid [150] is a simulation created by the Cyber Resilient Energy Delivery Consortium for education.
Primitive as it may seem, the energy storage technology that is "grid-tied" and having the largest capacity is accessed by simply pumping water up to a higher elevation, and storing it as potential energy. Called pumped storage, or pumped storage hydroelectricity, the energy is recovered when the water from the higher elevation is used to drive turbines for hydroelectric power conversion.
The Energy Storage Association [152] reports, "Pumped storage hydropower can provide energy-balancing, stability, storage capacity, and ancillary grid services such as network frequency control and reserves." While the US has 20 GW of installed capacity, worldwide over 100 GW of capacity exist. The US figure accounts for roughly 2% of the country's generating capacity, while other areas' figures are as high as 10%.
All in all, however, this process uses more electricity than it produces. So, why do it? When a power plant has extra capacity, it generates electricity used to pump water uphill. Then, when the plant is stretched to capacity and electricity is at its highest price, this pumped storage can be used to generate low-cost hydroelectricity.
Reference:
Modified from Vera Cole, Power Grid, EGEE 401 [153]. Accessed October 2013.
The main form of energy that we think of in society is power from electricity. As a society, we typically deliver electric power through a complex distribution system called the power grid. In this reading, Stoft provides a fairly useful background to the pricing of utility scale electricity. I think that exposure to this content will be very helpful in your career development with solar and power systems.
As supplemental reading, you can also review the text on ISO/RTO strategies in the SECS chapter on Solar Economics (also citing Blumsack as below), and we will jump ahead briefly to Ch 14 to show a PV example of capacity factor.
The first reading from Stoft presents the core metrics being evaluated (energy, power, and capacity) and their associated utility scale pricing units of $/MWh.
Capacity is likely the newest term to everyone; it is a measure of the potential for power delivery. The price of power or capacity is metered as monetary units (dollars, euros, yuan, etc) per time unit of an hour, per MW of power that flows. The price of energy is just dollars per MWh (analogous to dollars per MJ), which end up as the same effective unit cost metric, but from different perspectives.
In traditional power systems, we have turbine-generators that yield power from spinning magnets. A generator size is set by the maximum power production it can yield, measured in units of MW. We pose the capacity of a generator in terms of the potential to produce a flow of power in MW, the same units as power.
The capacity factor is the fraction (from 0-1; or a percentage from 0-100%) of flow utilization over the duration of a load. We find this fraction as a ratio of the power generator's true output (evaluated over a period of time, such as a month) relative to the potential power output that would occur ideally when operating full out (nameplate capacity) for an indefinite period of time.
The capacity factor (cf) of a fueled power plant (coal, NG, fission reactor) can have a range depending on the applied technology >>30-40%. However, the capacity factor of PV is highly dependent upon the solar resource of the locale.
For example, the capacity factors for PV in the USA range from 10.5% in Alaska, to 18-19% in most of the USA, up to 26.3% in Arizona, Nevada, and New Mexico. [see Table 14.2 in SECS, Brownson]. The capacity factor for PV in sunny Germany is about 11%, while the cf calculated for the desert regions of Peru is >25%.
The second reading by Stoft links in with our prior reading of Solar Economics in SECS and the role of market supply and demand for electricity. Electricity is not easily or efficiently stored in large amounts--we don't have pumped hydro storage everywhere, and large-scale batteries are not ready for the utility market.
In an electricity grid, power generation and power consumption must be closely matched at all times. These are key concepts in our understanding of electricity. If power generation and power consumption get out of balance, blackouts and other systemic failures occur.
Reference:
S. Blumsack. Measuring the benefits and costs of regional electric grid integration. Energy Law Journal, 28:147–184, 2007
In this reading, we are digging in to the lever of microeconomic incentives. To increase solar utility for a client, or to increase the preference of the client to solar goods and services, there exist incentives for solar that effectively internalize a positive externality. While you read Chapter 9 and scan the paper by Pfund and Healey (look to the summary and the key graphics at first), I want you to think about the following:
In other nations about the world, policy for renewable energy development can emerge at either a national or regional level. In the United States, there are currently no over-arching Federal mandates requiring the development of alternative energy power generation. Thirty-three states have varying standards and mandates for the production of renewable energy for power generation.
To see this diversity of strategies, we can review the Center for Climate and Energy Solutions [157] report of Renewable & Alternative Energy Portfolio Standards.
Try This! Interactive Map of US Electricity Portfolio StandardsClick on the image to access the interactive map showing the electricity portfolio standards for each state. Click on each state to see the updated renewable or alternative energy goals and milestones. |
The legislative process in various states has influenced how these programs have evolved. If we look at the states that have "alternative energy portfolio" standards rather than renewable energy portfolios (Michigan, Ohio, West Virginia, and Pennsylvania), we note that all of those states have significant industries such as auto, coal, steel, etc. One needs to consider the various reasons why these portfolios were outlined as "alternative" as opposed to "renewable." You should also develop an awareness for those states that have no standards whatsoever, and consider what impact this has on solar development. For example, we may consider that Wyoming has a very small population with the highest per capita energy demands, while being abundant in coal.
On December 16, 2004, Governor Edward Rendell signed into law Pennsylvania's Alternative Energy Portfolio Standard, requiring that qualified power sources provide 18.5 percent of Pennsylvania’s electricity by 2020. There are two tiers of qualified sources that may be used to meet the standard.
Unique markets have been created by government stimulus of the renewable energy industry. A key market driver has been the capacity markets that have been formed in several states. In these markets, Energy Distributions Companies (EDC) are required to purchase Solar Renewable Energy Credits (SREC) or face steep fines in order to facilitate the states' requirements for renewable energy. (SREC Trade: SREC Online Auction [161])
Something to look at closely is that in Massachusetts and New Jersey specifically, these credits can't be sold or purchased outside of the state. This has a significant impact on how these markets operate. New Jersey has outpaced all other states in the eastern PJM grid in the production of power from Solar PV. An unexpected development in the REC markets has been the pace at which these markets have met and exceeded the portfolio standards. Please note how this has impacted REC pricing.
Points of reference:
PJM originally stood for Pennsylvania, New Jersey, Maryland. It is simply known now as PJM; however, the grid it operates is now much larger.
PJM is the grid operator for all or part of 13 eastern states. It is the market through which Energy Distribution Companies (EDC) purchase power from Energy Generation Companies (EGC)
PJM is also responsible for charting future requirements of the grid and its users. It makes recommendations to its members, the EDCs and EGCs, regarding how to best meet these requirements. It is the principal link between the state regulated EDCs and the non-regulated EGCs. PJM is a non-profit corporation.
The Generation Attribute Tracking System (GATS) of PJM Environmental Information Services (EIS) has been instrumental in finding out trends including the growth of solar, captured methane, hydro, wind, and other renewable resources of production within PJM. The GATS has shown that based on the number of certificates generated, from 2005-2015, wind production has increased around 4,000%. In 2005, solar energy generation stood at 100 MWh, and increased to 81,000 MWh in 2009, which is a growth of 3,000%. In 2015, it stood at 2,900,000 MWh, a growth of 29,000% in a decade. Source: pjm [163] EIS [163]
In normal markets, supply and demand are the key drivers. A good deal of the growth of the renewable energy industry has been driven by regulation and government subsidy. So, conventional market drivers appear to be misaligned.
Of note is that renewable energy for the foreseeable future will continue to be an incredibly small part of power generation. What role will natural gas play in the future of our energy mix? I think it will have a significant impact on Pennsylvania because of the Marcellus Shale formation. Nuclear power could also grow at a significant rate should much smaller, localized nuclear power plants be developed that would enable cities and industrial sites to have their own sources of energy. Source: MIT Technology Review [164]
From our reading, we have seen that there are market failures in our energy industry, both from the negative externalities of emissions (greenhouse gases, SOx and NOx, and aerosols) and from the positive externalities of using SECS technologies that provide carbon neutral energy.
In the presence of a positive externality, the social value for a good exceeds the private value. Government policies can correct this form of market failure by subsidizing the good. In the presence of a negative externality, the social cost exceeds the private cost. Policies can be implemented to correct this market failure by taxing the emissions of Pigovian tax [165]).
After reading Pfund and Healey (2011), we should see that other energy sources displayed a higher social value in their own time. At the time of the late 1700s, coal was perceived in a similar fashion to our solar energy technologies like PV in the 1970s when it had several detractions, such as its bulk that made coal difficult to transport. States provided tax exemptions and incentives to move coal along, such that it surpassed timber as an energy resource in the late 1800s.
"Nature made coal abundant, policy made it cheap." p. 14
(cited from Sean Patrick Adams, The Journal of Policy History Vol. 18, No. 1, “Promotion, Competition, Captivity: The Political Economy of Coal” (2006)).
Modified from ENGR 312, Sustainable Energy Entrepreneurship, by Wieslaw Grebski, Shaobiao Cai, and Christopher Flynn;
Penn State Hazleton. Accessed May 2013.
We just talked about all these things that affect the cost of an energy system, and now let's take a look to see how the real data can fit into our simulation software for project design. Time to break out SAM again and do some exploration!
The basics:
The first few things to notice is that the Loan Term (and Analysis Period) is 25 years as a default. This is the standard period of covered life for a PV module. Much like your computers, the actual life will be longer than the warranty, but 25 years is the most risk that the manufacturers will currently take on to guarantee their products. In general, all the SAM defaults are going to be conservative, and you can indeed adjust them for your own projects.
You want to enclose your period of loan or mortgage ( ) within the full period of evaluation ( , years of analysis), so that . The loan rates are assumed to be a bit high, but you could change it to a lower rate if appropriate.
Tax, insurance, and property rates can be left at the defaults unless you know better from practical experience. When working with a full team in industry, you will need to be working with an expert knowledgeable in these areas to accurately represent them for the client.
The salvage value will almost never be zero in a real project. Just think, a PV system at the end of 25 years may be operating at 60-80 percent of its original peak performance, but will not catastrophically fail that year. In fact, it will likely keep on truckin' for decades more. Even a 20-year-old operational truck has a resale value that is a significant percentage of the original value. So, change it to something greater than zero, but less than 100, and you can still be conservative.
If you have any questions or comments, please post them to the Lesson 6 General Questions Forum.
Congratulations! Your design team has been hired by Costco Wholesale Corporation [175] to propose solar integration in one of their regions (to showcase one of their commercial retail buildings in each location). Your job is to make a short written survey of the case, suggesting a plan to maximize the solar utility for their regional management in Texas.
Building Location: Austin, TX
I would suggest you develop the outline that addresses three topics that commonly occur in a solar integration discussion: (1) energy efficiency, (2) adding solar technologies on site or off-site, and (3) economic and environmental rationale.
Diverse resources available from the USA Dept. of Energy:
There are also extensive resources available at the 7Group website [147].
Submit your outlines as PDF files in the Canvas Learning Activity 6.1 Dropbox: Pre-Design Summary. Appropriately cite any references used in your report.
You will be graded on your ability to develop a compelling outline that raises new questions and provides scope for the upcoming charrette. All this is based on limited information of the actual site, but extensive access to general information about the type of building and potential stakeholders. The activity assesses your knowledge of investigating the client and stakeholders and the locale when planning to maximize solar utility in the pre-design phase.
This is a 20 point assignment
See the Calendar tab in Canvas for specific due dates.
Consider this activity as a slight detour in preparation for your final project proposals in this course.
While you are still on your way through the course lessons that explore solar design concepts, it is probably about time to start thinking about a potential locale and client you want to direct your efforts towards for your couse project. This final proposal will be the synthesis of your prior work, learned skills and tools in the form of a professional project SECS design, which eventually may become the basis for the real implementation scenario.
Here are some guiding points to start this off:
Create a post in Yellowdig with your brainstormed project ideas using the Course Project Topics tag. Be sure to check what everyone is doing and respond with comments and suggestions and answer any questions to yours.
I would like everyone complete their topic brainstorm by the middle of Lesson 7 week (following Sunday), so you have some extra time to search and choose until then. And this is actually a mandatory activity! I need to see what everyone's plans are for the project, so if nothing is posted, I will get back to you and bug you :)
This was a pretty good lesson to help us to put boundaries around our design projects. We learned that we need to identify the constraining features of our design problem. A design effort without constraints and boundaries can quickly spiral out of control, having too many possibilities to draw from. We address that challenge using the goal of solar energy design and engineering:
We found that locale in this course means a broad range of factors in time and space that affect SECS design. Locale is tied to the meteorology and physical placement of the SECS, and locale is tied to the cost of fuels (here, as electricity) and incentives available to our client.
Which brings us back to our client. We do not design to make the coolest SECS (although, a really cool SECS is pretty fun to admire and brag about), we design to offer the highest solar utility to our client, as an individual, a corporation, a community, or a group of stakeholders with financial shares in the potential development. It is the client who responds to high fuel costs (seeking a solar substitute), and it is the client who responds to incentives in project proposals. We have observed that there are market and government drivers that can strongly affect the financial portion of the solar utility argument. Keep in mind also that our clients will not always behave as rational agents within the market. It is our job to learn about the locale and the client to best serve them in the design and project development arc.
You have reached the end of Lesson 6! Double-check the to-do list on the Lesson 6 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 7. And get ready for Lesson 7, because it will be a bit more intensive than Lessons 5 and 6.
We are now completing the last part of a three-lesson arc in economics and solar project finance. By now, you should observe significant connectivity between the past two lessons. We have discussed the economic drivers in energy systems, the basics of clients as utility maximizers, and then addressed multiple ways in which we as designers/engineers on a team can access the goal of maximizing solar utility for our clients in a given locale.
In Lesson 7, we will discuss ways to deliver useful metrics to our clients from a finance perspective. We will approach SECS through Life Cycle Cost Analysis (LCCA), dealing with concepts of financial paybacks on investment, solar savings, time value of money for long periods of evaluation, and levelized costs of energy.
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 10 - Solar Project Finance W. Short et al. (1995) Manual for the Economic Evaluation of Energy Efficiency and Renewable Energy Technologies [179]. NREL Technical Report TP-462-5173. (selected sections) |
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To Do: | Discussion Activity: Life Cycle Cost Analysis for Solar Hot Water - Financial Spreadsheet
Quiz Assignment (see Canvas, Module 7) Project Topic - Submit your ideas for course project to the forum. |
Topic(s): | Time value of money and financial spreadsheets Life Cycle Costs/Savings Solar Savings Solar Fraction Levelized Cost of Energy |
If you have any questions, please post them to the Lesson 7 General Questions and Comments Discussion Forum. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
We will be considering cash flows - namely, revenues - expenses or savings - costs - in a process called Life Cycle Cost Analysis (LCCA). As you have seen in the reading, cash flows can be developed for systems operations, for investment decisions, and for financing. We will be representing cash flows in a simple, discrete pattern called end-of-period cash flow, where the periodicity is 1 year, and the compounding or discounting uses an annual rate.
Did you see that last bit?
Clients will perceive increased uncertainty and risk without better information available. That's your job! To provide better information and transparent project evaluation, which demonstrates an understanding of both the solar resource and the financials associated with a proposed SECS. Chapter 10 of the textbook discusses how conveying the financial metrics within a project proposal is one way to provide useful information in a transparent manner.
In Life Cycle Cost Analysis, one of the important criteria is the period of analysis, or period of evaluation. The "period" conveys a time horizon for your LCCA. If we recall our microeconomic drivers affecting the elasticity of demand, we know that the time horizon is an important factor. In our case, SECS will tend to have long life-spans.
As such, we are dealing with the concept of "value" at various points in time.
You will notice that the same topics are discussed in detail in the assigned reading of the Manual for Economic Evaluation by Short et al. (1995).
There are two ways to represent discount rates, and you will observe both in the SAM simulation software or similar financial analysis tools. Using these rates, we can produce a discounted cash flow model (DFM) to compare projects.
The Short et al. article shows the Nominal Discount Rate loosely approximated as. But will the fuel inflation rates be the same as labor inflation rates? Or insurance inflation rates? We will have an example in the discussion where we pull apart different inflation rates and use real discount rates in the analysis of a solar hot water system.
We have already seen that the DSIRE website [166] for the states and federal government of the USA is a useful resource for incentives. Part of those incentives are tied in to tax credits, and there is a significant portion of your reading devoted to the concept of depreciation.
One of the things that occurs in an LCCA at the end of the Period of Analysis is the question of how to finish the summation. This is like the Monty Python movie, The Holy Grail [183], where the old fellow says: "I'm not dead!" At the end of your 15-25 year evaluation for LCCA, you will no doubt still have a fully functional SECS! They don't just break down and fall apart, and in fact they will likely last for decades beyond your evaluation period. So how do we assess the value of the system at the end of the period?
We assume that the system has a net salvage value (a resale value) that is a fraction of its initial value, translated into present dollars. In our discussion, we will assume a 20-year-old solar hot water system still has 30% of its initial value, framed in present dollars for year 20.
In this case, if the total system cost is $16,000, its 30% salvage value will be 4,800.
Applying the Present Value formula (see above), with the market discount rate of 8%, we can find:
Salvage value = $4,800 / (1 + 0.08)20 = $1,030
This will be monetary value of the system at the end of its 20-year service life.
When I think about a SECS and the potential solar utility for a client in a given locale, I am familiar with the variable costs (VC) of fuel in a home or a commercial building. I am also familiar that SECSs have a relatively high fixed cost (FC) of the system's initial investment. So, I need a metric that can show me the annualized and cumulative flow of cash as costs and savings (in today's dollars) over the period of analysis.
We see in our reading that earlier solar engineers had developed strategic ways to apply the concepts of Life Cycle Costing Analysis (LCCA) for SECSs. Because solar technologies like PV (photovoltaics) and SHW (solar hot water) tend to substitute for fuels that need to be purchased, the authors recognized a value in specifying SECS financial potential in terms of avoided fuel costs (another FC), otherwise termed fuel savings (FS). The opposite of a "cost" is a "savings" in marginal analysis, right? But saving fuel is only one of at least seven parameters affecting the flows of cash for a system. Annualized cash flows are the sum of costs and savings in a year.
SS = FS - incremental mortgage/loan payment
- incremental maintenance/insurance
- incremental parasitic energy costs
- incremental property taxes
+ tax credit incentives
+ production credit incentives
We know that a local SECS like a Solar Hot Water system will have a certain quantity of demand from a residential family.
We often design a domestic solar hot water (DSHW) system to provide an annual fraction F = 0.4-0.7 (40-70% of the total annual demand), sized for the summer loads, because the heat would be wasted/dumped in the summer. That would mean the client would be buying a bigger system that does not have utility in the summer. Better to have a less sufficient system for hot water in the winter, than for the client to pay for something they cannot use part of the year.
In our reading, we made the distinction between the annual solar fraction (uppercase F) and the monthly solar fraction (lowercase f). We can use the solar fraction as a factor in project finance to estimate an ideal array size for our client in his/her locale. Consider that a large solar fraction will entail more modules or panels, and will increase the cost for the client in the system investment (according to the unit cost). It will also increase the time to payback the investment. Our clients will no doubt have finite cash on hand to put a down payment into a SECS, and to acquire a loan for the rest of the investment. They may also require a fast payback that will influence the sizing of the system.
annual fuel savings (considered before discounting or fuel inflation rates)
We have covered methods to account for the costs and savings for a generic SECS in the previous pages. In those readings, we introduced the time value of money. So, let's think about the "time value of money" using a spreadsheet. The questions below are to be leading topics that will dig into the coupled meanings of Life Cycle Savings, Solar Savings, Fuel Savings, time value of money, systems payback, and paying back a loan. Some of the questions may be easier than others, but there are not necessarily clear answers to all of them. Also some people in class may have more experience with this type of analysis than others, so it would be beneficial to work together as a group through this discussion.
An example spreadsheet for solar hot water systems in a residential home (Domestic Solar Hot Water, or DSHW) is published as a shared Google spreadsheet. The direct link to access the file is in the middle of this page. This spreadsheet is set up in many columns: each column is representing a separate sequence of years for discrete financial analysis. There are accompanying graphs to link with the data, presenting loan payments and annualized Solar Savings increasing each year. Because the spreadsheet is dynamic, it would be better if you download a copy of the file and try changing things like the discount rate, fuel cost, loan size, and systems size (solar fraction) and see what the response will be.
There are two example systems analyzed in the spreadsheet. The first system has a solar fraction F = 0.65, costing \$16k with a 20% Down Payment and the remainder paid through a back loan at 7% interest. The second system has a solar fraction F = 0.85, costing \$26k with a 20% Down Payment, and the remainder paid through a back loan. Both systems have a potential resale value of 30% of initial investment ($16k), framed in Present Value (a different kind of "PV"). This is a detailed spreadsheet presenting you with an example of discrete financial analysis where we consider the time value of money over 20-year span. Half the battle in developing a useful spreadsheet is figuring out where everything is. Later, we will also dig into the financial output in SAM simulations.
NOTE: You must be logged into Google in order to view this spreadsheet.
Link to Google spreadsheet [184]
Study the spreadsheet and then discuss the following questions in the Yellowdig community.
There is no hard deadline for this discussion activity, but it would be good to have some initial relfections posted in the middle of the study week (Sunday), and comments and replies will be due by the end of the point-earning period.
I want you to think about the ways that figures of merit serve as various economic metrics to allow a client to compare alternatives in energy systems selection and design in an "apples to apples" fashion, despite the fact that SECS are coupled to an intermittent solar resource. You may find it easier to read chapter 4 of Short et al., and then jump back to chapter 3 of Short et al. We will focus on the figure of merit below; but really, these pages are chock-full of useful information for future project development!
What are the figures of merit to which our clients will respond?
Now that we've entertained the idea of a Levelized Cost of Energy, let's try out a web tool designed by NREL to estimate LCOE (link directs to the documentation site first). [185]
The OpenEI (Open Energy Information; site home here [186]) has a supplemental resource called the Transparent Cost Database [187]. (Make sure you are looking at "Generation.").
I would value hearing back from you as to whether these tools are useful, or not so much. Please take a moment to post your perspective on whether these government-based online tools seem useful to you for the future on the General Forum for Lesson 7.
Good progress, class! We have now completed our three-lesson arc through Lesson 5: economic analysis, Lesson 6: solar utility for the client and locale, and finally Lesson 7: financial life cycle cost analysis.
In Lesson 7, we read about and discussed ways to deliver metrics to our clients that would be useful for financial assessment and project comparison. We called the overall process Life Cycle Cost Analysis (LCCA), dealing with concepts of financial paybacks on investment, solar savings, time value of money for long periods of evaluation, and levelized costs of energy. We introduced solar-specific terms such as the annual Solar Fraction (F), the Solar Savings (SS), and the Life Cycle Savings (LCS).
We discovered that financial analysis can be as direct as using a spreadsheet and some basic assumptions to assess financial cash flows and energy flows, or it can be a detailed simulation using meteorological data. We used discrete annualized methods of analysis common to project management in industry.
Coming up in the next three lessons, we will add to that strategy, and you will keep developing your arguments by building from sources found on the web (or from clients).
Design is pattern with a purpose.
Whereas art and science provide mechanisms to ultimately open windows into apparent patterns about us, design and engineering are purposeful approaches to establish systems that fit the revealed pattern. [Brownson, SECS, Ch. 16]
You have reached the end of Lesson 7! Double-check the to-do list on the Lesson 7 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 8.
We encounter two really big factors that create a spread of possible outcomes for energy systems when we look into the future: weather, and people demanding energy. In Lesson 8, we will advance our knowledge regarding the role of weather and the electric power grid. We will cover a general idea of risk and uncertainty. Then, we will show again that time and space are related using Taylor's Hypothesis. More important, we will describe how events in the future can be predicted (to certain degrees of confidence) from historical knowledge and from knowledge of present events that are connected spatially to the locale of interest. In particular, we are interested in tools used by meteorologists.
Both weather systems and demand on the grid (called loads) will be presented as dynamic, coupled parameters. Both weather and grid loads will deeply affect our clients in terms of the required decisions for solar technology deployment and management. Both will be shown to have a spread of possible outcomes for a given time horizon; and thus, there will be uncertainty and risk in making decisions.
Now, the point of Lesson 8 is to familiarize yourself with modern elements of time horizons that affect project risk assessment and management due to the dynamic behaviors of the grid and of the local weather systems. We are not going to give you the full chops of a meteorologist, no more than in the last lesson were we going to turn you into a financial analyst. The whole point of transdisciplinary research and practice is for experts from different disciplines to work jointly as a team around a shared goal (e.g., the goal of solar design) to address a common challenge. However, it is hard to work jointly on a team when you don't know the language of your team members. So, onward!
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapters 5, 8, and 9 (sections related to Risk, Time-Space, and Meteorology) W. Short et al. (1995) Manual for the Economic Evaluation of Energy Efficiency and Renewable Energy Technologies [180]. NREL Technical Report TP-462-5173. (read pp. 27-34: Uncertainty and Risk) |
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Optional Reading (not required): | J. Rayl, G. S. Young, and J. R. S. Brownson. Irradiance co-spectrum analysis: Tools for decision support and technological planning. Solar Energy, 2013. doi: http://dx.doi.org/10.1016/j.solener.2013.02.029 [191]. A. C. McMahan, C. N. Grover, and F. E. Vignola. "Evaluation of Resource Risk in Solar-Project Financing", Kleissl, J. ed. Ch 4 in: Solar Energy Forecasting and Resource Assessment (2013) Academic Press. F. E. Vignola, A. C. McMahan, and C. N. Grover. "Bankable Solar-Radiation Datasets", Kleissl, J. ed. Ch 5 in: Solar Energy Forecasting and Resource Assessment (2013) Academic Press. |
To Do: |
Yellowdig Discussion: Space and time relationships in solar forecasting Engage in all Try-This and Self-check activities (not graded). |
Topic(s): | Time Horizons Nowcasting Load Forecasting Numerical Weather Prediction (NWP) Weather Research & Forecasting (WRF) |
If you have any questions, please post them to the Lesson 8 General Questions and Comments Discussion Forum. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
...but you'd better have a transdisciplinary team to really be competitive in the solar market. In this lesson and the next three, we are going to explore content for which you are not necessarily expected to be the expert. But you do need to know about the important roles of your future partners in design, and the core communication topics relevant to engaging them in the integrative design process.
On your dream team for solar design, do you have a meteorologist, an economist skilled in energy business and finance, a systems engineer, a quantitative analyst [192] to assess large data, a communications expert, and a practical electrician/roofer/plumber? Yes? Well, then you're way ahead of most everyone else. OK, does your team share a common framework: such as the goal of solar design? Do they understand and work to synthesize concepts across disciplines? Do they work together to address your problems?
This is just a quick review of the term "transdisciplinary." In exploring a spectrum of techniques for the last three content lessons, we are going further and further from "core" disciplinary topics. You know, we've been saying that this integrative design process is pretty important, and it involves a transdisciplinary approach. But what does that mean, and how is it practical to the core philosophy of a solar design team?
Reference:
Patricia L. Rosenfield (1992) The potential of transdisciplinary research for sustaining and extending linkages between the health and social sciences, Social Science & Medicine, 35(11) 1343-1357. DOI: http://dx.doi.org/10.1016/0277-9536(92)90038-R [193]
If we really want to see solar energy grow in new directions and adapt quickly to the diversity of challenges in society and the environment, then we need to get comfortable with "not knowing" everything, or even not knowing a lot. The patterns are generally just too big for a single person to be able to hold all of the core knowledge and be able to execute the design-build-operate process alone. We also need to get comfortable with building knowledge across disciplines, linked by a common conceptual framework. We need integrative design teams that "get" the value of transdisciplinary work.
In solar project design and project management, we would like to work for our clients to minimize risk. We describe risk as the dispersion of outcomes around an expected value. Something is riskier if the spread of possible outcomes is bigger around an expected value. The more specific terms of variance and standard deviation describe the spread of data (the dispersion) about an expected value. Events that occur in the future will have a spread of possible outcomes, because we cannot know the final value for the future with 100% confidence until it actually occurs.
When we really know what to expect, what we imply is that the dispersion of possible outcomes is clustered tightly about that expectation. From that information, we can adapt or make changes for the future appropriately. However, knowing the spread of possible outcomes about an expected value is deeply important, even if you know that the spread of outcomes is very broad. The greater the dispersion of outcomes, the higher the risk. In our reading, we comment that “riskier” scenarios in solar project development, or systems operation and management, will have a larger dispersion of outcomes around the expected value.
Risk is often framed as the probability of an uncertain event occurring in the future multiplied by the expected loss should the event occur. We call the model of the probability distribution the pdf, or the probability distribution function. Note that there are specific applications for continuous or discrete distributions (cf. probability density function [194], and probability mass function [195]). If we know the probability distribution [196] of all possible outcomes, then we also know the expected value of the outcome, surrounded by the dispersion of outcomes around that expected value. If the pdf is normalized, then the probability of any event can be evaluated by integrating a section under the curve. If we integrate under the entirety of a normalized pdf, then we are integrating across all possible outcomes. The total probability is then equal to 1.
A common measure for the spread of data can be the variance [197] ( ). Given a sample of multiple events, the variance is a measure of the spread of the data about an expected value or outcome. We have also discovered in our reading that a portfolio of renewable energy can potentially be used to reduce the variance of the power generation coupled to the same grid. This is something to bear in mind in the future of large, distributed PV.
In Figure 8.1, below, we present a normal probability distribution function (also called a Gaussian). Now, many distributions in solar energy are not normally distributed, but this is a starting point. The data tends to be strongly skewed toward clear days (more clear days than overcast), or bimodal in nature. Quantitative analytics often use available statistical software such as R (The R Project for Statistical Computing) [198] to estimate density functions based on discrete real data (e.g. a histogram). This is called density estimation [199]. The peak(s) in a pdf represent the highest likelihood of expected values.
Because the solar resource is intermittent (variable), so too is the power production from a technology such as PV. The expected values and the spread for natural data like the weather is not necessarily "normally distributed" or even unimodal (one peak). This is why we often plot histograms of the data to observe the manner in which the data is spread out.
We would like to describe our level of confidence that a certain level of power production (or capacity) will be met, in order to minimize the risk in managing a system. In our "Try This" example, we saw how a data set can be summarized using quartiles (minimum, 25%, 50% or mean, 75%, and maximum). So, in this case, we would like to break the spread of data into bins that are both useful and tied to probabilities.
A value of "P50" or "P90" (or any value from 0-100) describes an annual value of power production from the intermittent resource with a probability of 50% or 90%, respectively. In fact, that quartile summary can be viewed as P25, P50, and P75. For P50, there is a 50% chance that the mean power production will not be reached at any given time. For P90, there is a 10% chance that the P90 level will not be reached.
Banks and investment firms working on wind farm projects often require P50 and P90 values of the wind resource at a location to determine the risk associated with a project’s ability to service its debt obligations and other operating costs.
-Dobos, Gilmanjavascript:submit_mid('sp_ig_all'), Kasberg (2012)
Inside of the SAM software, there is an advanced feature to evaluate P50/P90. There is an accessible database (*.cbwfdb file format, a proprietary format developed for SAM's P50/P90 capability) from the National Climatic Data Center [202] (NCDC). The database is quite large (1.1 GB) but allows us to explore a number of cases for this course.
The long term NCDC/NSRDB dataset includes the impact of large volcanic activity and other phenomena that occur on timescales larger than one year. In particular relevance to solar plants, the eruption of Mt. Pinatubo introduced large quantities of aerosols into the atmosphere that reduced incident irradiance levels between 1991 and 1993. Other variations include the cyclic El Niño and La Niña phenomena, as well as the 11 and 22 year sun spot cycles.
-Dobos, Gilman, Kasberg (2012)
Ok, so after digging into Dr. von Meier's paper on the grid, we should have noticed a few things. First, renewables in CA are continuing to grow as a contributing portion of electricity generation. Second, the power grid linked with CA (the Western Interconnection) will have to accommodate the new sources of power just as the grid has to accommodate new loads (sources of demand). Before renewables, we had baseload power mainly from coal, nuclear, and hydroelectric, and everything was pretty smooth on the generation side of the equation. But now, wind and solar power are pushing in with "intermittent power."
In essence, renewable and distributed resources introduce spatial and temporal constraints on resource availability: we cannot always have the resources where we want them, when we want them.
-A. von Meier (2011)
This is a really neat paper, in that it points out the orders of magnitude of challenges that the markets and grid operators have to deal with on a regular basis, both spatially and temporally! And yes, you might also note that those scales were incorporated into Figure 5.10 of the SECS textbook for the Fujita relation of meteorological phenomena in space and time. You can compare the units in the von Meier paper with the table and graphic below.
We just learned that large portions of the electricity grid can be managed by an Independent System Operator (ISO, or RTO), where demand on the grid is managed through markets. That is great, but the grid also has problems that accumulate anyway, which need to be dynamically adapted to by the utilities or the clients. In our main reading, we see all the different systems elements that need to be coordinated in our power grids. These are amazingly complicated systems that are then perturbed by pesky things like weather.
Temporal Phenomenon | Short | Long | Unit |
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demand response | 4 | 4600 | sec |
hour-ahead scheduling | 1.75 | 7 | hour |
service restoration | 0.5 | 28 | hour |
day-ahead scheduling | 18.5 | 48 | hour |
T&D Planning | 1 | 16 | year |
Carbon emissions goals | 13 | 80 | year |
Spatial Phenomenon | Short | Long | Unit |
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DG interconnection criteria | 50 | 20,000 | m |
voltage regulation | 0.5 | 10 | km |
distribution feeder | 2 | 25 | km |
transmission congestion | 25 | 1,000 | km |
stability problems | 500 | 3,000 | km |
CAISO | 450 | 800 | km |
WECC | 1,600 | 3,000 | km |
The weather systems that we deal with have their own orders of magnitude in time and space. Let's compare the time and space for weather phenomena in the following graphic. We saw this image in Lesson 3; but now, we can see that those time and space scales above actually fit with the scales right here, described by Ted Fujita. Notice the line that trends through the data of clouds (which are the nemeses of solar energy...). From a fit of $17\ m/s$, we have a quick conversion tool to flip between characteristic time scales and characteristic distance scales! Just remember $17\ m/s$ as the FRYB relation (Fujita-Rayl-Young-Brownson), and how to convert time from hours to seconds (3600 seconds in an hour), or kilometers to meters (you don't need that conversion, do you?).
Now that we know a few things about the scales of time and space for the power grid (using CAISO and WECC for scale), we can compare it with the weather scales. Look to the top and right for the von Meier scales, and the core diagram in the center for the Fujita scales. The diagram on the lower portion shows us a rough sketch of the power spectral density for the meteorological spans of synoptic, mesoscale, and microscale weather.
By applying the average meteorological advection speed of , (which we are calling the FRYB relation for the class), we can convert an example spatial scales of variability associated with transmission congestion (red vertical bar on the right) from distances of 25-1000 km into a time horizon. The relevant time scales for meteorological phenomena exist within 25 minutes to 16 hours (involving events from cumulus, cumulonimbus, and cumulonimbus clusters interfering with the Sun's irradiation).
Alternately, we observe that the harmonic effects propagating within the grid along distances of 30-300 meters would be relevant for meteorological phenomena spanning 1.8-18 seconds. This scale of events is too small to be incorporated into the presented meteorological phenomena.
You have been reading about the relations between time and space, and you have read about different time horizons of interest to the solar energy and electric grid fields. Now, let us apply that knowledge to Table 8.2 from the Chapter by Coimbra, Kleissl, and Marquez (2013).
Technique | Sampling Rate | Spatial Resolution | Spatial Extent | Suitable Forecast Horizon | Application |
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Persistence | High | 1 point | 1 point | Minutes | Baseline |
Sky-Imagery | 30 sec | 10-100 m | 2-5 m radius | Tens of minutes | Short-term ramps, regulation |
GOES Satellite Data | 15 min | 1000 m | U.S.A. | 5 hours | Load following |
NAM weather model | 3600 sec | 12 km | U.S.A. | 10 days | Unit commitment |
Questions to discuss:
Please share your reflections on these questions in the Yellowdig community. I wonder if everyone will come to the same conclusions.
When you create a post in the Yellowdig discussion space, you are required to choose a topic tag. For Lesson 8 discussion, please use these tags:
Yellowdig points you earn over the weekly point earning period (from Saturday to next Friday) will count towards 1000 pts. weekly target. But you can go above it (to 1350 pts. max). Yellowdig discussions will account for 15% of the total grade in the course.
There is no hard deadline for participating in these discussions, but beware of the weekly limit. You need to create posts every week to stay on target with your total discussion participation grade. Each weekly point earning cycle ends Friday night, and a new period starts on Saturday.
The first forecasts that we will explore from the reading are related to the modern electrical power grid. You are about to observe us jump from a +20 year time horizon in Lesson 7 (Life Cycle Cost Analysis), right down to spans of days and hours. If you recall from the reading by von Meier, this jump in orders of magnitude is pretty common for analysis related to financial and engineering decisions in energy systems.
Within the technological ecosystem of the grid, one follows the demand for electricity. Recall from Lesson 7 that the term for energy demand (including losses) is called the Load. Supply must be managed to match those dynamic demands. So, why might we forecast for loads in the electricity market? The time horizons of interest to engineers and financial experts working with the grid have been developed within the technical ecosystem of the modern power grid. When managing the grid as a dynamic system, we can think in time spans of seconds to minutes (Intrahour), over a few hours (Intraday), and over the course of a few days (Intraweek).
Recall that California mainly has an Independent System Operator, with the exception of entities like SMUD (Sacramento Municipal Utility District). Also recall that an ISO uses markets (a zonal or nodal market) to manage the grid. In the last super graphic, which tied together weather scales and grid management scales, we observed "Hour Ahead" and "Day Ahead" markets along the top. But in our reading, we see that the time horizon is a bit different from the simple description:
By reviewing the reading on Taylor's Hypothesis in Ch. 5 of the SECS text, we see that periods in time and distances can be related: a series of changes in time for a fixed place is due to the passage of an unchanging spatial pattern over that locale. So, in the following list, we can connect space and time (in fact, we do the same for power systems).
Taylor’s hypothesis permits a time series of irradiance observations over fixed locations to be converted into an equivalent translation across space (at the advective or propagation speed to the corresponding spatial pattern). Hence, all time scales are also spatial scales so long as the advective wind speed is much greater than the time scale of the evolving meteorological event being investigated, as is often the case.
-Brownson, SECS (2013)
Just keep this in mind with forecasting, as units of time actually also imply units of distance, and vice versa.
A quick reminder, summarizing the reading from the previous page.
Events to be evaluated for <2 hours will use statistical approaches such as time series (Autoregressive Integrated Moving Average; ARIMA) or artificial intelligence (e.g., Artificial Neural Networks; ANN).
As a slight correction from the reading assignment, we provide the following standard terminologies in meteorology for forecasting ranges, called lead times. These are not stated clearly in the reading, and they are important enough to have in your vocabulary. In the prior reading assignment, you should notice that the time horizons tied into solar energy models are not yet aligned with the approaches for meteorological forecasting. This is an indication of the relatively new start of forecasting applied to solar energy. We are still learning the common language of meteorology, and hopefully that language will soon converge. Similarly, meteorologists are beginning to adapt to the solar field's language of GHI, DNI, irradiation, etc.
Numerical Weather Prediction [206] uses an assemblage of modeling methods, along with current weather observation data to forecast weather in a future state. Note that the observations tied to the current state of the data are very important to NWP.
Local dedicated NWP models have been developed as a collaboration among NOAA and NCAR. The approach is termed WRF (pronounced "worf") [209]. This is an advanced application of NWP, but the skill with which one can forecast will still decay with increasing lead times due to the chaotic atmospheric behavior.
The Weather Research and Forecasting (WRF) Model is a next-generation mesoscale numerical weather prediction system designed to serve both atmospheric research and operational forecasting needs. It features two dynamical cores, a data assimilation system, and a software architecture allowing for parallel computation and system extensibility. The model serves a wide range of meteorological applications across scales ranging from meters to thousands of kilometers.
-WRF Homepage [209] (Accessed Oct. 20, 2013)
As a recap, in Lesson 8 we established that transdisciplinary research is a core part of integrative design, and while each of us doesn't need to be an expert in every field, we do need to stretch ourselves to understand and integrate our approach towards a common framework. In solar energy, that framework is tied to the goal of solar energy design and the foundations of sustainability systems. We then explored the role of risk and uncertainty in delivering solar resource units to the grid or managing the energy outputs on site, and defined risk as the dispersion of outcomes around an expected value, using the statistical variance of the data as a metric. Another way to explore dispersion from a banking and finance perspective has also been developed for renewables, using exceedance probabilities.
We then described the time-space relationship in both the grid, markets, and meteorological assessment. While there is still a lot to learn about the complex systems of weather, energy markets, and people demanding energy from the grid, we at least established a map to explore the scales of relevant time and distances using a general Earthly atmospheric advection speed of , which we dubbed the FRYB relation.
We explored the early methods to forecast solar energy metrics, and we found that some of the current limitations of solar resource forecasting are tied to the early nature of the field, while others are based in the general diminishing skill to forecast further into the future due to the chaotic nature of the Earth's atmosphere. We discovered that resolution limitations can be found in either spatial or temporal scales, too.
The relevant meteorological metrics for many common SECS technologies are tied into DNI, but we see that the field currently has very few methods to evaluate DNI to a high accuracy without directly measuring the parameter with expensive equipment. Finally, we note that most solar forecasts are linked to GHI only, which has limited value given our prior knowledge of the inherent error bound to Liu and Jordan-style transformations of GHI to and , and later POA for oriented arrays. These transformations also hold very little value to extract a meaningful DNI measure. This should be a reminder that the field of solar forecasting is still young, and we should be on the lookout for new progress that will enable us to minimize risk for our clients, hopefully increasing their solar utility in the respective locale!
You have reached the end of Lesson 8! Double-check the to-do list on the Lesson 8 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 9.
We now continue our development of broader impacts in systems design and management. So far, we have addressed the Goal of Solar Energy Design through engineering applications (Lessons 2, 3, 4) and through financial/economic applications (Lessons 5, 6, 7). In Lessons 8, 9, and 10 we are addressing the Goal of Solar Design by helping the client to manage risk in the given locale.
The Goal of Solar Design is to:
In Lesson 8, we discovered that there are meteorological phenomena that are really out of our control, and our job as a design and management team is to help the client to manage risk. Now, we continue with important design elements tied to managing risk, but within the social realm of community, regional, and federal stakeholders. In Lesson 9, we want to address helping our client to manage risk in the social environment, through our developed awareness of permitting and policy within the given locale. As we shall see, just because you have a good solar resource and a sound financial return (in principle), doesn't mean that the policies of the locale will allow one to actually implement a SECS, or to implement a system in a financially responsible manner.
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found in this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 11 - The Sun as Commons (Intro through "Framework: Emerging Local Policy Strategies") S. Bronin (2009) "Solar Rights" [213] Boston University Law Review. |
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Optional Reading (not required): | R. Knowles, "The Solar Envelope: Its Meaning for Energy and Buildings" [219], Energy and Builduings, 35, 15-25 (2003). |
To Do: | Learning Activity: Community Solar Gardens Discussion: National Targets for Electricity Production No Quiz Assignment this week Engage in all Try-This and Self-check activities (not graded). |
Topic(s): | Energy Policy Permitting Incentives Solar Rights Solar Access Community Solar |
If you have any questions, please post them to the Lesson 9 General Questions and Comments Discussion Forum. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
One of the main limitations to deploying solar energy is just the simple ability to make use of the Sun's energy. Here, we focus on the right to access and use the Sun's energy, and the functional ability to access unobstructed solar energy within the solar envelope. Try to remember that many technologies and objects that are outside serve as SECSs: homes and solar panels, yes, but also clothes lines, patios, and trees.
The DSIRE solar portal describes both solar rights and solar access (easements), along with a map of US states having each or both. Note that PA still has neither solar access laws, nor solar rights laws. The Wikipedia description is also a general framework to think about access to solar energy in society.
Just think, in Germany, you have the right to be exposed to sunlight directly in any office scenario (no windowless cubicles permitted). In many states, you have the right to hang dry your clothes on a line outside. In ancient Rome, an individual had the right to access solar gains or be compensated for an obstruction of that resource. When we talk about policies for solar energy, one of the first is the basic ability to access that sunlight and then make use of it to do work!
The concept of solar rights is still emerging in the USA. I am going to recommend that you take about 10 minutes to review the current state of solar rights from the Solar America Board for Codes and Standards (the Solar ABCs [221]; choose 1 page summary). The coverage is brief but useful when considering the scope of solar legal status in the USA.
The phrase "solar rights" is often used in a general sense and a specific sense. In general, solar rights are the broad class of legal rights to access and make use of the light from the Sun. In the specific sense, solar rights are distinguished from "solar access" by the following description:
Here, solar rights describe the ability to make use of solar energy in your locale.
Solar access is the functional ability of a SECS within a locale to receive or "access" solar gains across property lines without shadowing or other obstruction occurring from buildings/trees/landmarks set in a nearby property. It also has to do with the enacted local policies to manage the commons of the solar resource system, and an individual's ability to be granted compensation if access is blocked in some way. Bronin has described solar access as being managed in three manners: as a solar easement, through covenants, and through lease agreements.
We have already discussed the role of minimizing shading in SECSs that intentionally collect and convert solar gains (rather than shading devices that attempt to control solar gains). Solar access has to do with access to the solar resource within the locale over many hours of the day, and across the months of the year. Recall that we already developed some skill to assess the solar access at a site through the sun path diagrams in Lesson 2. Hey, we just tied together something that was a lot of work earlier with an advanced topic in Lesson 8! Great!
The solar envelope is an extension of solar access, conceived for urban scenarios with a cluster of buildings and obstructions. Professor Emeritus Ralph Knowles of the Dept. of Architecture, the University of Southern California, conceived and developed an extensive exploration of the solar envelope as a concept for policy development and planning. You are welcome to read about Dr. Knowles' solar envelope concept in complement to our assigned reading.
R. Knowles, "The Solar Envelope: Its Meaning for Energy and Buildings" [219], Energy and Buildings, 35, 15-25 (2003).
One of the more recent trends from the past decade of photovoltaic adoption is the ability to connect directly into the power grid. In such cases, policies have been adopted to allow renewable generators to "bank" excess power into the grid and receive credits for the excess energy (in kWh). But each locale will be subject to different policies enabling or prohibiting such practices. Net metering is expanding in creative ways, including aggregation of multiple distributed energy sources for credit, and distributing joint credit from a centralized solar garden [224] among multiple participants in a community.
Permitting is the unseen monster of project development that will slow or stop a project from moving forward. The key rewards of an established permitting process for SECS include reduced costs of installation and faster turnaround from design to deployment. It will be well worth your integrative design team's time to familiarize yourselves with the permitting process for your locale of interest.
Best practices have been itemized by IREC in our reading. We can distill those into a few core features that we would hope for in an integrative process that includes permitting:
So, first, "REN21" is the acronym for the "Renewable Energy Policy Network for the 21st Century". REN21 is a non-profit network of stakeholders established to connect key contributors from governments, international organizations, industry partners/associations, participants from science and academia, and society at large. We are using their centralized information base to expose the diverse approaches that countries can take to develop energy generation and manage energy demand at the Federal level.
After reviewing the REN21's Renewables GSR, you will have a little broader perspective on the various approaches, and will be able to compare solar policies in Germany vs. China vs. Columbia vs. Kenya.
For this lesson discussion, do some online research on national targets for PV solar power. You can use the REN21 Renewables Global Satus Report [216] as a starting point, but certainly feel free to search other available resources. Take a look at different several key players representing different continents that are actively setting PV targets:
After reading and taking down some comparative notes, use these guiding questions to create your post in Yellowdig:
Please feel free to use the following topics for your discussion posts this week:
Remember that Yellowdig point earning period ends each Friday. Posting commenting, reacting regularly through the weeek will make the 1000 pts. an easy target and guarantee a high participation grade in the course. Yellowdig discussions will account for 15% of your total grade.
I want you to consider the perspective where we view the resource units derived from the sun (light, converted electrons, converted heat, etc.) and the resource systems to enable those conversions (the Sun, the grid, our buildings) as coupled, but separate entities. This is an extension of our work on economics, but leads us into thinking about management of solar energy among the community.
While reading these materials, think how we might collectively manage both the resource units, and the resource systems in a given locale. Community solar is a new and exciting space in the solar field, and this sets up the foundation to address it and think of new strategies in management.
In both readings, we are going to see the language developed by Nobel Laureate in Economics Elinor Ostrom, from her seminal book, Governing the Commons: The Evolution of Institutions for Collective Action.
From this, we state that the Sun is an energetic resource system providing the flow of light (as resource units) in the shortwave band (280–2500 nm). Once we have established our language for resource systems and resource units, we can review the Typology of Goods, discussed with examples in the article by Brownson (2013). We summarize those four main goods below:
NREL Report: J. Coughlin, J. Grove, L. Irvine, J. F. Jacobs, S. Johnson Phillips, A. Sawyer, and J. Wiedman (2012) "A Guide to Community Shared Solar: Utility, Private, and Nonprofit Project Development" [218] USA DoE National Renewable Energy Laboratory.
Our final reading puts a few boundaries around the varieties of "community solar" that appear to be emerging in the USA of late. There are good examples in the document, which will be helpful for our Learning Activity in this lesson.
While community solar PV is just now emerging onto the market, note that community solar products have been in existence for millennia as shared fields and gardens for produce development and resale. Based on the prior examples from farming, the management of a community resource is well within the scope of SECS expansion.
As of 2020, Community Solar is now authorized in 19 states and Washington, D.C. There are companies that specialize in community solar that would arrange deals with farmers to lease portions of their land to build solar projects. As this segment of the solar industry expands, farmers (and landowners in general) may take advantage of this new revenue stream. In addition to generating local revenue, solar projects help states make progress toward their clean energy and sustainability goals [Gahl, 2020].
Browse through this report to learn about several representative community solar case studies across the country:
SEIA Report: Gahl, D., How Community Solar Supports American Farmers [225], SEIA, February 2020.
The case studies presented in the report show that typically, farms will lease portions of their land to community solar companies for a fixed term at a fixed price. These solar lease payments tend to be higher than those for traditional agricultural operations and are normally based on the state policies, where the project is located. Land leasing for solar often provides farmers with higher and more stable income than that obtained through producing agricultural products, which creates an incentive.
Imagine you are part of a superior design team based in PA, a state with a restructured electricity market such that community solar is possible. Your integrative design team is already capable at commercial and residential PV installations, but wants to branch out and take advantage of this "community solar" concept. Your job is to make a short survey of best practices available elsewhere and suggest a model for developing community PV in an urban community in the Philadelphia area. Understandably, your report would be based on limited information of the actual site, but you can leverage extensive access to general information about the types of building and potential stakeholders in the locale of interest. Based on your findings, you need to prepare a concise but convincing summary document for your supervisor to review.
Submit your Summary as a PDF document into the Lesson 9 Learning Activity Dropbox: Community Solar in Canvas. Remember to appropriately cite any sources of information used in your report.
You will be graded on your ability to develop a compelling outline that provides scope for applying the community solar concept in a residential Philadelphia neighborhood. The activity assesses your knowledge of investigating the potential client and stakeholders at the locale when planning to maximize solar utility in the pre-design phase. Please see specific grading rubrics in Canvas.
Please see the Canvas Calendar for specific due dates.
In this lesson, we attempted to flesh out some major policy topics tied to solar energy. In doing so, we were able to describe the connections between policy-making and renewable energy adoption at the local, regional, and federal levels. We explored the deeper meanings and implications of solar rights and solar access, which engage multiple stakeholders from a local and regional government. Then, we addressed the policy and permitting barriers/opportunities in solar project development.
Recall that we want to help our client to manage risk in the social environment, through our skill in negotiating permitting and policy barriers within the given locale. A good solar resource and a sound financial return (in principle) alone doesn't guarantee the locale will even allow a SECS, or to implement a system in a financially responsible manner.
The research of Dr. Ostrom suggested to us how local community action for solar energy is not only possible but is actively used today in other similar industries, where the dynamic appropriation and provisioning challenges of a resource system and resource units are addressed in a sustainable fashion. In turn, we used that thought process to describe new and expanding methods for community solar development.
You have reached the end of Lesson 9! Double-check the to-do list on the Lesson 9 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 10.
This is our last lesson of three that are tied to the broader impacts of solar design. In Lessons 8, 9, and 10 we have been addressing the Goal of Solar Design by helping the client to manage risk in the given locale. This lesson deals with risk in terms of uncertainties that are encompassed in the long-term time horizon of projects as they have societal and environmental impacts.
Sustainability plays a major role in focusing our views of solar energy deployment and managing long-term risks (generational time scales). This lesson tries to encompass those broader impacts of developing a renewable energy project and addresses the motivation for sustainability system thinking in project design. We frame this lesson in terms of sustainability ethics and ecosystems services, and we will develop our activities in this lesson around discussions and essays. You will be using the answers from your Learning Activity to inform the broader impacts section of your final projects.
We are often reminded of how energy technologies, when deployed on a large scale (natural gas, oil production, coal combustion, etc), will have significant environmental impacts that are disruptive to the global, regional, and local ecosystems. I want you each to consider how large-scale solar energy deployment can also induce ecosystem change and reduction in ecosystems services, which must be avoided in future project development. This lesson will build upon the Millennium Ecosystem Assessment [226] that was called for in 2000 by the United Nations.
By the end of this lesson, you will be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Required Reading: |
J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 1 - Introduction (Reread, with renewed focus on The Ethics of Sustainability and Ecosystems Services.) Geoffrey Carr (Nov 21, 2012) "Sunny Uplands [227]" The Economist. |
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Optional Reading (not required): |
"Ecosystems and Human Well-being: General Synthesis" (Millennium Ecosystem Assessment site page [231]) (155 pg. report [232]) Ong et al. (2013) "Land-Use Requirements for Solar Power Plants in the United States [233]" [13] NREL Technical Report: NREL/TP-6A20-56290. (supplemental reading) |
To Do: |
Discussion: Ultra-Mega Solar Activity: Sustainability and Ecosystems Services Pitch (video) Engage in all Try-This and Self-check activities (not graded) |
Topic(s): | Energy constraint, value of solar energy, historical cases of developing solar energy |
If you have any questions, please post them to the Lesson 10 General Questions and Comments Discussion Forum. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
Please read about the rate of growth of PV (it's been quite high for decades), tied to the "learning curve" of PV costs from a doubling in cumulative production capacity. There is a positive feedback loop occurring here, and it suggests that PV (and other solar) will become very, very big globally within the next decade.
From our reading, we have seen the:
Now we are going to tie our focus on scale to ecological impact. This reading is complementary to the concept of solar energy. Consider that large scale solar is called a "solar farm" or community scale solar can be termed "solar gardens." Large scale farming leads to large scale land use changes, and may undermine environmental resilience and existing ecosystems services.
We keep hearing about exponential growth of the solar industry, and larger and larger SECS projects are being rolled out each year. Solar consumes a lot of land, so here we especially focus on the impacts that come with major land use changes. Consider how the land use of these solar projects is linked to the observed ecological disruptions.
These very large solar projects (4,000-5,000 acre scale), started around 2012, are now considered "medium scale”, given the doubling time of PV capacity. There are now Gigawatt scale solar farms in development, with one near Tibet at Longyangxia Dam Solar Park at 850 MW scale (see satellite images from NASA [243]). So the expected ecological impacts will be even more dramatic and may result in not only local, but regional and global consequences.
Furthermore, the direct land use impacts (such as clearing or changing the land for solar installation) is only a trigger for a larger scale land impact. The Dust Bowl [244] of the USA in the 1930s was initiated by the land use changes of only about 1000 acres in marginal lands, but eventually affected 100 million acres (400 thousand km2). System causal connection and feedbacks within natural ecosystems propagate the initial disturbance to a greater territory and over a greater period of time.
Recognizing the challenge of cultivating marginal arid land, the United States government expanded on the 160 acres offered under the Homestead Act—granting 640 acres to homesteaders in western Nebraska under the Kinkaid Act [245] (1904) and 320 elsewhere in the Great Plains under the Enlarged Homestead Act [246] (1909).
-Dust Bowl [244] (Wikipedia entry, accessed Nov. 15, 2013)
Significant land areas are being designated for solar projects, including both natural and partially developed. However, the solar development does not have to be detrimental to the health and values of local environment. As well as providing low carbon energy, solar farms can also provide important benefits for biodiversity and ecosystem services. Furthermore, land use change for solar installation presents an opportunity to address the urgent challenges of mitigating ecosystem degradation. In other words, degrading lands can be brought back to health.
However, given the high rate of solar industry development, it is an important time now when we need to determine the best ways to design and manage utility scale solar plants. In fact, many solar projects are built on low-grade or otherwise intensively managed agricultural land and may create an opportunity to enhance biodiversity and return the fields to a natural state.
Let us then review the main types of ecosystem services we are talking about here.
There are sometimes no distinct boundaries between these types, as the same resource may be considered within several categories. For example, trees or soil can be both part of supporting system and provisioning products. Or another example is storing carbon in biomass can be both regulating (carbon sequestration) and resource generating or provisioning (fossil fuel).
At the stage of solar project design, we may want to take a close look at the ecosystem services existing in the target area and assess their environmental significance for the local community. Next, we may want to plan measures and design features that either have minimal impact on those services, enhance them, and add additional value. At the same time, we may want to avoid the actions or design features that are detrimental.
A Group of researchers at the Universities of Lancaster and York (UK) developed an online tool – “Solar Park Impacts on Ecosystem Services" (SPIES) – that helps practitioners make informed decisions on solar design and environmental management. This tool is evidence-based, so all the impacts and strength of the impacts associated with different projects activities on site are researched and referenced. SPIES compiles 457 peer-reviewed academic articles collected via a systematic review of relevant issues.
The interactive interface allows users to arrange scientific evidence by ‘ecosystem service’ and to generate a list of management interventions that will affect the achievement of a desired environmental outcome.
The SPIES tool can be used for planning applications by showing how solar projects can contribute to the environmental and biodiversity targets, if managed properly. The options presented by SPIES can help developers decide which ecosystem enhancements will be the most appropriate for the particular locale. This tool can also be useful to local authorities and policy makers who are required to consider environmental benefits and risk and approving project proposals.
Go to the SPIES website: https://www.lancaster.ac.uk/spies/ [247]
Scroll down to the bottom and click to download the SPIES tool – you will receive the login information for using the database. Feel free to check out other supporting resources.
Hope you find it useful in your own development!
The major work of research on ecosystem services came out of the Millennium Ecosystems Assessment [231] from the United Nations. Even the synthesis report is a heavy read, so we have included it as supplemental reading. Here is an excerpt from the report that I considered to be highly appropriate in the context of developing large scale solar technologies globally:
Relationships between Ecosystem Services and Human Well-being (p. 49)
"Changes in ecosystem services influence all components of human well-being, including the basic material needs for a good life, health, good social relations, security, and freedom of choice and action (CF3). (See Box 3.1.) Humans are fully dependent on Earth’s ecosystems and the services that they provide, such as food, clean water, disease regulation, climate regulation, spiritual fulfillment, and aesthetic enjoyment. The relationship between ecosystem services and human well-being is mediated by access to manufactured, human, and social capital."
This statement is closely linked to the "sustainability ethic" - the term that has been eloquently summarized by Dr. Christian Becker (former faculty in the PSU Department of Philosophy, and expert on sustainability and ethics) as the following:
"Acknowledge and seek solutions that respect a systemic and simultaneous moral obligation to 1) contemporary global communities, 2) future generations of human society, and 3) the natural community or environment supporting life and biodiversity on Earth."
We can see that this invokes a pretty deep perspective, and there is a lot of value encompassed within such a concise statement. I want you to consider several ways of how we might incorporate the sustainability ethic as a motivator into our working lives as professionals.
(Image Credit: Community Energy [248]) |
The folks at Community Energy [249] and EDF Renewables [250] in Pennsylvania have developed just such an approach to project development, in their Keystone Solar [251] project. This 6 MW PV project was developed on Amish farmland, with specific research applied to the soil quality before and after the project installation. No concrete was used in the ground mount installation here. The design included vegetative buffering with native grasses, shrubs, and trees, allowing the solar installation to blend into the natural landscape. The project received a Project of Distinction Award at the 2013 PV America East Conference.
Several universities and other organizations signed up for a share of the renewable energy credits, including Drexel University, Franklin & Marshall College, Eastern University, Clean Air Council, the Philadelphia Phillies, Juniata College, Millersville University, and Marywood University.
Best practices were applied, and at the end of the contract employing the solar farm, the land owner will have the option to remove the entire installation and return the land back to farmland for agricultural crops. This is still not the norm in the industry, it is a best practice by a firm seeking to lead the industry.
“ UNIVERSITY PARK, PA. — On Feb. 5, 2019, Penn State and Lightsource BP announced the development of a large-scale, ground-mounted solar array of over 150,000 solar panels near Penn State’s Mont Alto Campus. This 70-megawatt, off-site solar energy project will support the University’s Strategic Plan, helping implement the plan’s "Stewarding Our Planet’s Resources" key pillar and supplying up to 25 percent of the University system’s electricity.” (Verdi, 2019 [254])
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This project boasts significant environmental benefits due to shifting the campus electricity generation from fossil fuel based to renewable sources. It is estimated that, once implemented, the project will result in 57,000 metric tons of avoided CO2e emissions per year, which is equivalent to 12,102 fuel-burning cars of the road. The project helps Penn State exceed its goal of reducing carbon emissions by 35% by 2020 and also provides the university with $14M in energy cost savings over the 25-year term of PPA agreement. But that is not the end of it. From the very start, the intention was to demonstrate that utility-scale solar can be and should be developed in a sustainable way, with careful consideration of local ecological values.
The bidding process required developers to evaluate the potential impacts on the land and nearby ecosystems by using a mapping tool developed by The Nature Conservancy (TNC) [259], a global environmental non-profit organization that advocates responsible use of land and sets priorities for conservation of sensitive and ecologically critical zones across all continents.
Lightsource BP is recognized for building solar farms that enhance environmental benefits to farm lands, preserving biodiversity and agricultural value of land. They worked together with Penn State’s researchers to come up with “environmentally-conscious” system design, which included elements, such as created wildlife habitats, plant communities that promote pollination and help uphold honey bee population, and sheep grazing (Ludt, 2019 [260]). By design, the solar farm allows for co-location of the traditional agriculture benefits (crop and livestock growing) with additional ecosystem services.
The construction of the plant has been completed in 2020, and Penn State announced in October 2020 that the university had begun purchasing solar electricity:
Penn State: Power by the Sun, Lightsource BP – Penn State Brochure 2020 [263].
Penn State Powers Up with Solar [264], BusinessWire, 10/15/2020.
This web tool provides you with several metrics and interface to evaluate the environmental sensitivity of a region or site. If you consider developing a tract of land or re-purpose a natural area or farmland for solar installation, it is important to access the potential impacts on biodiversity, water ways, soil, species migration routes, and other factors. For example, it may help you to choose an area with the highest resilience or avoid areas that are prioritized for conservation.
First, it is helpful to understand the core concepts of terrestrial resilience. Visit this webpage [265] to study the metrics used in mapping.
Resilient Land Mapping Tool [266]
Zoom in the location of your interest. There are several options on the right hand menu to display different metrics on the map: Resilient Sites, Connectivity and Climate Flow, Recognized Biodiversity Value, and Resilient and Connected Network. You can try to switch between the options and interpret the markings based on the legend at the bottom of the menu.
Then try to “Sketch a Polygon” over a specific site (see button on the upper right). In a minute, the system will generate a profile report for that site, which gives you some quantitative information on ecological sensitivities in this area.
Think how this information can help you make a case for sustainable solar project. Feel free to use this tool for site assessment in your course project.
Portland State University of Oregon, USA, has developed an IGERT (Integrative Graduate Education and Research Traineeship; federally funded graduate research) on "Ecosystem Services for Urbanizing Regions." They have clearly identified our urban ecosystems as essential as well. Looking at the summary of content from our reading, we should note how closely it all aligns with the integrative design approach in our own text book.
Now, how are you going to think about urban ecosystems services in your own solar design projects?
Now that we have seen indicators of the growth of solar globally in both rural and urban regions, let us discuss the ultra-mega solar project that was proposed for India...which was ultimately stopped and shifted to the state of Gujarat due to environmental concerns! Let us set the stage as if we were in 2013:
Guiding questions:
Please post your thoughts and findings on this case on Yellowdig using the following topic tags:
Under the third topic, feel free to share any existing projects in the US or worldwide that provide good examples of ethical project development and ecosystem conservation.
Remember that Yellowdig point earning period ends each Friday. Posting commenting, reacting regularly through the weeek will make the 1000 pts. an easy target and guarantee a high participation grade in the course. Yellowdig discussions will account for 15% of your total grade.
Consider a scenario where you are an entrepreneur in a new solar energy technology startup. Your challenge is to integrate the principles of sustainability and environmental justice with your solar technology and generate a convincing pitch for your stakeholders.
Think about your course project proposal and identify the ecosystem services at your locale that need to be maintained, enhanced, or created and how you would address them in your business plan.
These questions become particularly important in formulating the core business philosophy for a specific firm. However, the context of the assignment here still needs to be couched within a real locale and for a well-defined client. (That is why I encourage you to tie it to your project proposal)
As a deliverable for this learning activity, you will need to create a 3-min video with your pitch to a potential client or stakeholder, specifically outlining the environmental benefits of your project design approach and share it on the Lesson 10 discussion forum in Canvas.
The style of the video is your preference – whatever works best for you personally. You can use a couple of slides, images, your own footage, or even show some data (if it helps) and narrate over it, or you can just speak to the camera.
Because you need to keep it within 3-min limit and still to be informative, writing a script beforehand is highly helpful, but it is not required for submission.
Be sure to check out other videos and provide comments to your peers.
If you are making video with your phone, it will need to be uploaded it to a video-sharing platform (e.g. YouTube or Google). You can designate it “unlisted” if you do not want to make it public. Then share a link or embed the video to your Canvas post.
Please share the link to your video in your post on Lesson 10 Discussion Forum in Canvas. Please do not include your video as an attachment – in that case viewers will have to download the file to their computer to see it (and that is what we try to avoid). Wherever you share the video, be sure to set permissions so that anyone with the link could view it.
You will be graded on your ability to clearly convey the benefits of your ecosystem services plan to a broad audience. This activity is worth 20 points. Please see grading rubric in Canvas.
Lessons 8, 9, and 10 addressed the Goal of Solar Design by helping the client to manage risk in the given locale. We addressed managing risk by looking at uncertainties that are encompassed in the long, generational term time horizons, and by considering societal and environmental impacts of a design. In Lesson 10, you were dealing with broader impacts and motivations driving solar energy project development. By now, you should be familiar with:
Great work on this lesson, which ties the project design to sustainability consideration in a broader sense!
This is the end of our formal lesson content for the course! We will now spend the rest of the semester developing and presenting our design proposals that synthesize the learning over the last few months. Best of luck to you on that important task!
Double-check the to-do list on the Lesson 10 Learning Outcomes page to make sure you have completed all of the activities listed there before you begin Lesson 11.
In this lesson, you will be pulling together all the elements of the course from the past months and composing the first draft of your solar project proposal. The drafts will then go through the peer-review process. This week you will spend some time doing background search and developing your ideas, and also you will have a chance to learn from others by reviewing their work. Comments from your reviewer may provide an additional angle to the problem you are solving and expose some deficiencies that you may not notice. Even if you feel confident in your development, constructive critique will not make it better!
Your proposal will convey the full spectrum of skills that you have been developing to better accomplish the goal of solar design: to maximize solar utility for your client and stakeholders in their specific locale. We have broken down the course into three main blocks, which I will review here:
Be sure that your pre-proposal is balanced and touches upon all of the above areas.
The formal lesson content will be minimial this week to let you focus on the project related tasks.
By the end of this lesson, you should be able to:
This lesson will take us one and a half weeks to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found within this lesson.
Required Reading: | SECS, Chapter 16 - Project Design |
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To Do: | Learning Activities
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Topic(s): | Energy constraint, value of solar energy, historical cases of developing solar energy. |
If you have any questions, please post them to the Lesson 11 General Questions and Comments forum in Yellowdig. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
Design is an extremely important aspect for successfully implementing solar energy systems. To illustrate this point, let us begin with a simple concept of what design is. A proposed theory that helps to provide understanding is that Design is PATTERN with a PURPOSE. Simply put, design is utilizing purposeful approaches to create systems that fit patterns. In Figure 11.1, below, we can see a cartoon illustrating a simplified pattern of relations. At the outside is the influence of the solar resource and ecosystems services, while the three inner rings are related to the coupled influence of the locale, the client, and the actual installed solar energy conversion technology. No useful project can be implemented without context and associated patterns. In fact, these patterns serve as guidelines and parameters for what a system can accomplish.
We can think about the region with respect to the connectivity to additional services (power grid, thermal lines), and think about the region with respect to the diversity of solar systems possible. Does a homeowner in Puerto Rico need large quantities of hot water, or the capacity to cool space?
A well-designed project will make use of an integrative process, an approach that essentially dictates that the whole is greater than the sum of the parts. System design does not merely involve putting together all the required parts and constructing it into a functioning system. It involves careful consideration of how each part fits into the overall system goals set by the stakeholders (clients), while also taking account of the environmental context the system is meant to work in.
By emphasizing the integrated design approach, we identify the solar utility, client and stakeholder, and project locale as the three main components for the goal of solar design. Each of these cannot be addressed without engaging the frameworks of the other two. These components also play a vital role as constraints. Constraints are useful tools for designers since they serve as guidelines for the design decision process.
The design process starts with establishing system goals. The responsible stakeholders generally define the system goals by stating what requirements they have for the system. The solar designer must take this information, coupled with the project locale and solar utility, to determine how much energy can be produced. In the next few sections, we will go into some detail on all of these.
Failure to consider all the important aspects in the design of any project will eventually lead to a system that performs poorly.
Now it's time to pull the prior work together for a proposal to your clients. You are to use both SAM and your knowledge about the solar resource and economic decision making to propose one or more solar solutions for your client.
Your proposal should demonstrate the full spectrum of skills that you have been developing to accomplish this goal of solar design. At the same time, the proposal should not be a heavy technical paper but rather should be crafted as a well-justified pitch to your client. The technical data on project design should be used to strengthen and support your message, not to confuse your potential reader.
In this course, the information covered has been broken down into three distinctive arcs:
The project proposal should present some data and analysis to address all three of these important aspects of project development.
Use any and all available tools to form a creative project proposal that will engage your client and be a compelling first step in project development. Using SAM software will be required, and I recommend that you use the "Create/Duplicate Case" menu to explore various options for a SECS project on behalf of your client. Go back to Lesson 6 for ideas, too. You are welcome to use the financial analysis spreadsheet we worked with in Lesson 7, although that is optional - you can as well use SAM for financial modeling.
Here are some resources from the USA Dept. of Energy:
There are also the extensive resources available at the 7Group website [147]:
When writing, try to look at your narrative through a stakeholder's lens. It is important to relate the analysis you do to your client's goals and clearly explain the presented information to your audience. It is good to finish your proposal with a strong statement summarizing your key findings and benefits your project would bring.
This week, at the pre-proposal stage, you may not be able to complete all of the elements and include all data you want to include. That is fine. If you need more work on a specific part of your proposal - just make space for it, provide a heading, and a description of what is to be completed and to be included in the final document, so that your reviewer sees that you do are not forgetting anything important. Also, the above list can be used as proposal structure, but if you see a need to modify it to better fit your case, please feel creative.
For the first draft (pre-proposal), develop a 5-7 page document (Doc or PDF format) that would be a hybrid of an outline and written report (you can expand on finer details later in the final submission). This work is somewhat similar to Lesson 6, but this time you are specifying your own client and locale. Please submit your pre-proposal draft and *.sam file to "Project Proposal Draft" dropbox in Canvas. Further, you will be able to see someone else’s proposal assigned to you for review in the same dropbox. The peer review assignments will be made by the instructor.
Once all pre-proposals are in, you will have several days for the peer-review task. You will need to send your peer-review and annotated author's proposal directly to the author and will submit it to a separate dropbox in Canvas for instructor's assessment. More detailed peer review instructions are given on the next page of this lesson and in Canvas. Please provide constructive feedback to your peers. You may also get new ideas for your own project from your peers during this review process, and your peer will provide you with ways to strengthen your own proposal. So, the benefits should be multifold, and if you each strive for excellence and creativity, your final proposal due in the final week will be strong and compelling and will have a high likelihood of receiving a high grade.
Please see the grading rubrics for the pre-proposal, peer-review, and final proposal tasks in Canvas.
Check the Canvas Calendar for specific due dates.
In Canvas you will be provided with access to one of the submitted pre-proposals to make your review. While you should feel creative about reviewing your peers' work, I also want to provide you with some guidelines to make sure none of the key points are missed.
You can use the following 8 criteria elements to structure your reviewer summary. Try to compose your comments in the third person, and back up your constructive criticism with examples found in the document (or the absence of content). Most reviews of compelling proposals will be 1-2 pages. If extensive errors or omissions are found, the itemized listing of corrections may extend the review length. You will be evaluated on your ability to provide constructive feedback to your peer that will strengthen their final proposal.
Peer Review should include (feel free to use these items as subheadings in your review document):
Please upload your (i) marked-up PDF and (ii) Reviewer Summary to the "Peer Review of Project Proposal" dropbox in Canvas, and, please, also send a copy of each directly to your peer through Canvas Inbox by the assigned deadline.
Your peer review will be evaluated out of 15 pts.
Thank you for your great work pulling your first drafts together, and reviewing the content from your peers. By this time, you should be finding that the goal of solar design and engineering provide us with a framework to flesh out compelling proposals for future solar energy conversion systems development. You may also have noticed the benefit of working with peers to improve your approach and strengthen or focus your proposals. This is again a part of the integrative design process.
Think about the larger teams that you will need to be a part of in future projects. Is there an ideal transdisciplinary team that you might recruit to develop and deploy future solar projects?
You are ready to move on to the final phase of EME 810.
See you in Lesson 12!
This lesson is the culmination of our work over the semester. You will be finishing your Solar Design project proposals on behalf of your clients in their given locale. By this time, you should have received feedback from your peers on your draft outline. This week, we will refine that document into a formal proposal for the client, synthesizing the many aspects that have made up the goal of solar design: to maximize the solar utility for the clients/stakeholders in their given locale.
Good luck with your work this week, and please take advantage of the forum or email correspondence to help to refine your design concepts. Project design does not occur in isolation!
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found in this lesson.
Required Reading: | None |
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To Do: | Finish composition of your final proposals and submit your work in Canvas. Contribute to the final discussion forum. |
Topic(s): | Goal of Solar Design, Integrative Design Team. |
If you have any questions, please post them to the Lesson 12 General Questions and Comments Discussion Forum. I will check the forum regularly to respond. While you are in a discussion, feel free to post your own responses if you, too, are able to help out a classmate.
So far, you have been working individually, or with the feedback from a peer, to design a project proposal. This proposal has elements of solar resource assessment, system engineering, financial assessment, social and policy awareness, ecosystem services - hollistically combined in a sustainable approach to energy systems. If that seems like a lot for one individual to wrap their mind around, you're right!
In our last discussion activity of the semester, I would like you to post your dream team of integrative design. Imagine that you just started a small solar design firm. You now need to hire a team to help you build your company name by reputation of being highly reliable, economically competitive, and producing high-quality solar projects.
What members would you want in your team, and what skills would they need to bring to the table?
Post your thoughts on the Yellowdig Discussion this week. Do not forget to read everyone's posts and provide your feedback too.
We have a couple of new topics added to Yellowdig menu to facilitate discussion over the final weeks of the semester. Also, feel free to revisit any previous topics, especially if your project building research provided you with some new insights or resources you'd like to share:
These last two weeks of the semester are your last chance to contribute to the EME 810 Yellowdig Community and boost your participation grade. Each weekly point earning period ends on Friday. You can't go back and make things up - it is a live forum! Any activity you generate contributes to the current week grade only. Maximizing your score now (max 1350 per week) does help you offset missed weeks in the total semester score. Remember Yellowdig discussions will account for 15% of your final grade.
Thank you for your active participation in the community discussions this semester!
We have finally come to the conclusion of EME 810! Great work! In this semester, we focused on the major topics of project proposal development for solar energy conversion systems. We addressed engineering tasks, financial tasks, and broader social, ecological, and policy aspects in proposal development.
You have put in a lot of effort, and hopefully the study material and practical assignments in this course brought your understanding of solar energy to a new level. Congratulations!
Please check the proposal submission date in Canvas Calendar and your to-do list in Canvas for any outstanding tasks.
Links
[1] http://www.eia.gov/energyexplained/
[2] https://www.eia.gov/
[3] https://www.eia.gov/about/copyrights_reuse.php
[4] https://cleanchoiceenergy.com/news/6cities
[5] https://www.weforum.org/agenda/2022/04/visualizing-the-history-of-energy-transitions/
[6] https://flowcharts.llnl.gov/sites/flowcharts/files/2023-10/US%20Energy%202022.png
[7] https://flowcharts.llnl.gov/commodities/energy
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[11] https://creativecommons.org/licenses/by-nc-sa/4.0/
[12] http://www.californiasolarcenter.org/history_passive.html
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[27] https://www.e-education.psu.edu/eme810/sites/www.e-education.psu.edu.eme810/files/images/Lesson_01/Shade_of_Tree_panoramio_lg.gif
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