Welcome to the first lesson of the EME 812. In this lesson, we will overview the main types and principles of solar energy conversion to usable outputs, such as electricity, heat, and fuel. There are quite a few technologies that help to do that. Some of those technologies are quite old and well-known, and some are still subject to current research. We will read a couple of recent review papers to learn about those technologies and their impact. Also, we will spend some time reviewing the concept of efficiency, which is a key metric of any process of energy conversion. Also, at the end of this lesson, I will ask you to refresh your knowledge of units and main terms used in the solar energy studies. Let's get started!
By the end of this lesson, you should be able to:
Journal article: Crabtree, G.W. and Lewis, N.S., Solar Energy Conversion, Physics Today, 60(3), 37 (2007) [1].
Journal article: Hernandez, R.R. et al., Environmental Impacts of Utility Scale Solar Energy, Renewable and Sustainable Energy Reviews, 29, 766 (2014) [2].
L. Radovic, Efficiency of Energy Conversion [3]
J. R. Brownson, EME 810 Solar Resource Assessment and Economics. 2.2. Basic Solar Jargon for Energy and Power [4]
The energy that is naturally available from the Sun is quite enormous. The Sun delivers 1.2 x 105 TW of radiative power onto the Earth, the amount that surpasses any other energy resource by capacity and availability. That would convert to 3.78 x 1012 TJ of energy per year. For comparison, according to Crabtree and Lewis (2007), all recoverable Earth's oil reserves (~3 trillion barrels) account for 1.7 x 1010 TJ of energy. Thus, the sun supplies this amount of energy to the Earth in only ~1.6 days!
According to reviews of University of Oxford [5], the current global energy utilization is close to 1.6 x 105 TWh per year (i.e. 5.76 x 108 TJ/year). If we again compare this amount to the global solar energy flux, the Sun is able to cover this demand in only 1 hour and 20 min! This is sort of mind blowing..
However, to be utilized, the solar radiation needs to be converted into other forms of energy, such as electricity or usable heat. The question is: can we effectively do that at the scale of our demands?
Evidently, the solar resource contains enough energy to cover those demands. However, the critical limitations in solar energy conversion will be the efficiency of existing technologies and availability of earth materials to scale up those conversion devices.
Before considering various types of conversion of solar energy, let us briefly review what solar radiation actually is. Here are a few main things we know from physics:
This mix of various types of electromagnetic radiation allows the sunlight to be converted through a variety of physical mechanisms, which are:
Now we are going to take a closer look at various technologies that are able to convert solar radiation and learn what the main objectives and challenges are there.
Read the following article to overview the main types of solar energy conversion, and try to find the answers to the self-check questions below.
Journal article: Crabtree, G.W. and Lewis, N.S., Solar Energy Conversion, Physics Today, 60(3), 37 (2007) [1] - 6 pages
This article reviews the multiple possibilities to convert solar radiation into usable forms of energy. It discusses various ideas and recent advances in scientific research directed towards raising the conversion efficiency through better understanding the physicochemical phenomena.
As we perceive from this reading, numerous technologies and areas of research and innovation in solar energy conversion target the overarching objective to raise the device efficiency, thus making it more economically viable for implementation. This is especially true in the light of quite high capital costs for solar energy systems. This challenge is related to both initial materials and manufacturing.
We will talk more about efficiency on the next page of this lesson.
Efficiency is a very important metric in energy conversion. It is most commonly used for evaluating and comparing various methods and devices in terms of technical performance, which is, in turn, related to cost of the technology. The efficiency concept is frequently used in cost estimates and commercial decision making. So, we should spend some time refreshing our basic understanding of the efficiency as a universal metric of conversion systems.
Please refer to this Efficiency of Energy Conversion book chapter [3], and refresh your basic knowledge of the efficiency definition and use. This text uses a number of simple efficiency calculation examples related to traditional fuel systems. I encourage you to learn from those, and then we will see how the same approach may apply to solar energy systems and devices.
We see that efficiency of conversion,η, is a key metric of system performance. When applied to solar energy conversion systems, efficiency of solar energy conversion would be defined as the ratio of the useful output power (delivered by the conversion device) to the incident power (of the solar radiation):
When the efficiency is compared for different types of photovoltaic (PV) cells, we need to make sure that conditions under which the cells are operating are standardized, so that any difference in cell performance is due to the properties of materials and design and not due to the variability of external factors. The nominal efficiency of PV devices is measured at standard conditions [ASTM G173 guide]:
When the external conditions are kept constant, measured efficiency is solely a device characteristic. To determine efficiency experimentally, we need to measure both the solar irradiance and the power of the cell.
Generally, to estimate the efficiency of solar energy conversion, you would need:
Consider the example below, which shows estimation of the standard efficiency of a PV module.
Standard solar input (irradiance) at the module surface: S = 1000 W/m2
Identifying power input to the PV cell:
Identifying power output from the PV cell:
(Note: from physics, power is equal voltage times current)
Then, for efficiency, we can write:
Conclusion: only 11.25% of energy flowing to this panel is converted to electricity.
The reason that energy conversion systems have less than 100% efficiency is that there are losses. The origin of those losses can be a complex issue, which could be better understood based on the physics and design of a particular conversion device – PV cell, concentrator, or thermal collector. We will get back to those considerations when talking about specific conversion technologies in detail in respective lessons of this course.
There is an important distinction between the total power (measured in Watts) and power density or flux (measured in W/m2). When we talk about the performance of a particular solar energy conversion device (for example, a solar cell), power density characterizes the "quality" of the energy conversion - how much power is generated by each square foot or square meter of the PV cell area. That may depend on properties of the cell material, design, and physical principles behind the conversion process. In contrast, the total power reflects the overall output - the "quantity" of usable energy generated by the whole device per unit of time. In applications of solar energy (say, if we want to power a building), we often look at the total wattage of the system and ways to maximize that total "quantity" of energy supply.
For example, imagine a solar module. At a particular moment of operation, the output power of the device can be expressed as
Logically, to increase the total output from that module, we need to either increase the efficiency or increase the total input power.
The avenue of raising cell efficiency leads us to the physics of the conversion process, materials properties, and cell design. The main research and development question here is how to make a better working cell.
The avenue of increasing the total input power leads us to three issues: (i) concentration of light, (ii) sun tracking, and (iii) system scale-up. Concentrating the ambient incident light would indeed increase the amount of energy supplied to the module per unit of time via increasing the S parameter in the above equation. Tracking - i.e., the orientation of the solar panel perpendicular to the sunlight beam - is another way to maximize the amount of absorbable radiation and also contributes to increasing the S parameter. Finally, increasing the size of the module by adding more cells to the system, increasing cell area, or multiplying modules (scale-up) would increase the total active area of conversion (A).
The technology scale-up is the way to match the solar power to commercial applications and consumer's needs. The utility-scale solar power, which is the primary focus of this course, is discussed in the next section.
There are two main solar technologies that are being considered for large scale power generation: (1) Photovoltacs (PV) and (2) Concentrating Solar Power (CSP). Another type - concentrating photovoltaic (CPV) is currently not a major player, but there are a few large facilities that use CPV technology. PV and CSP are principally different in the type of energy conversion and type of solar resource they rely on. We are going to review the basics of those technologies and their current state in the energy market in this lesson before considering more technical details further on.
So, what do we mean by the Utility-Scale Solar Power?
Please read the introduction on the website of the Solar Energy Industries Association (SEIA) [8] and watch the video below to get the basic idea about utility-scale photovoltaic systems.
Understanding the limitations in efficiency of solar energy conversion and taking into account the demands of centralized power generation, the technology scale-up is one of the important issues being developed by the government agencies in order to build sustainable energy future.
Obviously, there is a strong push for large-scale systems from the government and industry. But, along with the promise, the scale-up process brings new challenges to the energy conversion system design. Some of those challenges are:
All these issues deserve more attention and will be covered in more detail in further lessons of this course. In this lesson, we are not yet digging into any technical details of the considered technologies but, rather, taking a plunge into the context.
The following materials will give you an idea of the current state of utility scale solar market in the US.
Industry Report: U.S. SOLAR MARKET INSIGHT [10], 2022 year in review, Executive summary, SEIA, Wood Mackenzie Power and Renewables, Published March 9, 2023.
The SEIA 2022 Market Report provides a general outlook of the role of PV solar technology at the scale of national energy development. In the year of 2022, solar accounts for 50% of all energy added to the national grid. However, unlike previous years, 2022 was dominated by the growth of residential solar (40%), while the utility sector had a slower progression due to some global market uncertainties and supply chain disruptions. Nevertheless, 11.8 GW(DC) of new capacity was installed for the year, further increasing the contribution of solar energy conversion into the US energy industry.
In more detail, photovoltaic technologies will be studied in Lessons 4-6.
The other prominent technology developed on the utility scale in the US and worldwide is Concentrating Solar Power (CSP). While CSP is currently outpaced by PV on the global and domestic market, this technology may be advantageous in the areas with high annual insolation.
Watch this 2-min video to overview the utility-scale Concentrating Solar Power (CSP) systems:
Web article: 2018: The Year Sees Explosive Expansion of Concentrated Solar Power Capacity Globally [12], HELI SCSP, Accessed: April 2019.
While PV system significantly outpaced CSP in growth over the past decade, there is still a significant economic potential for converting solar thermal energy into power in a number of locations around the globe.
I hope these materials give you a clear idea what kind of systems will be the subject for learning in this course. The following self-check questions allow you to iterate the basics once more before we move ahead.
Utility-scale solar power installations are on the rise worldwide - the tendency fostered by advances in technology, new energy policies, and markets. Because of this growth, there has been an increased interest among stakeholders to understand the broader impacts of such systems on society and environment. In spite of the often idealistic public perception of solar technology as "green" panacea, an objective examination of the solar technology lifecycle reveals both positive and negative impacts. Careful impact assessment of large solar projects is important in order to steer the energy infrastructure development towards the optimal solutions that would take into account economic, environmental, and social values. Understanding the sensitivities and existing ecosystem services at the locale at the utility project planning stage is becoming a key step in responsible solar development.
Please read the following review article, which nicely covers the multiple effects of utility solar power.
Journal review article: Hernandez, R.R. et al., Environmental Impacts of Utility-Scale Solar Energy [2], Renewable and Sustainable Energy Reviews, 29, 766 (2014). - 11 pages
This article will be the background of the Lesson 1 forum discussion, and you will get a few questions on this material in the reading quiz (see the Summary and Activities page of this lesson for more details).
At the conclusion of this lesson, I want to refer you to some resources on basic energy units, conversions, and terminology that specifically applies to solar energy systems. If you have just completed the EME 810 course, you will find many of these things familiar. For example, can you clearly answer these questions:
To get a better sense of solar language we are using here, feel free to check this unit from the EME 810 Course (Solar Resource Assessment and Economics) that explains the basic jargon and units used to measure solar energy and power.
Basic Solar Jargon for Energy and Power [4]
Also, it would be useful to look through the original technical paper by Beckman et al. (1978), too, and use it in the future if any notation questions arise. The main purpose of this material is for everyone to be on the same page when analyzing the solar energy conversion technologies further in this course.
These presentations provide additional explanations and illustrations to the concepts of energy conversion and efficiency. These resources are optional, but can be helpful if you need to revisit the basics.
Readings and activities in this lesson give you a general perspective of this course and set the context without yet addressing the specific science of the solar energy conversion technologies. Here, we try to figure out what aspects and what impacts would be important when the conversion technologies are scaled up to the utility level. Hopefully the materials of this lesson also provided you with a good refresher of such basic concepts and terms as energy conversion, efficiency, power, power density.
Type | Description / Instructions | |
---|---|---|
Reading | Complete all necessary reading assigned in this lesson. | |
Discussion | Discussion Forum "Environmental Impact of Solar Power"
|
|
Reading Quiz | Complete the Lesson 1 Quiz. |
Brownson, J.R., EME 810 Solar Resource Assessment and Economics. 2.2. Basic Solar Jargon for Energy and Power [4]
Crabtree, G.W. and Lewis, N.S., Solar Energy Conversion, Physics Today, 60(3), 37 (2007).
Duffie, J.A. and Beckman, W.A., Solar Engineering of Thermal Processes, John Wiley & Sons 2013.
Radovic, L. Efficiency of Energy Conversion [3]
Links
[1] https://physicstoday.scitation.org/doi/10.1063/1.2718755
[2] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/1-s2.0-S1364032113005819-main.pdf
[3] http://www.ems.psu.edu/~radovic/Chapter4.pdf
[4] https://www.e-education.psu.edu/eme810/node/483
[5] https://ourworldindata.org/energy#all-charts-preview
[6] http://commons.wikimedia.org/wiki/File:Electromagnetic-Spectrum.svg
[7] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Slide_Efficiency_of_conversion.pdf
[8] http://www.seia.org/policy/power-plant-development/utility-scale-solar-power
[9] https://www.youtube.com/watch?v=0elhIcPVtKE
[10] https://www.seia.org/research-resources/solar-market-insight-report-2022-year-review
[11] https://www.youtube.com/watch?v=rO5rUqeCFY4
[12] http://helioscsp.com/2018-the-year-sees-explosive-expansion-of-concentrated-solar-power-capacity-globally/
[13] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Energy%20and%20Power%20Basics_0.pdf
[14] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Energy%20Efficiency%20Basics%20SU20.pdf