In this lesson, we'll go over solar and anaerobic digestion in a little more detail. We will see some of this in our travels, and I want you to have a better understanding of some of the basics.

Learning Outcomes

By the end of this lesson, you should be able to:

• identify different types of solar technologies;
• explain the basic process by which solar panels generate electricity;
• explain the difference between irradiance and irradiation;
• calculate energy use/generation given power and time;
• evaluate the impact of tilt and orientation on solar PV output;
• calculate the output of a solar array;
• explain how anaerobic digestion converts organic material to biogas; and
• identify some of the benefits of anaerobic digestion.

Lesson Roadmap

Questions?

If you have any general course questions, please post them to our HAVE A QUESTION discussion forum located under the Discussions tab in Canvas. I will check that discussion forum regularly to respond as appropriate. While you are there, feel free to post your own responses and comments if you are able to help out a classmate. If you have a question but would like to remain anonymous to the other students, email me through Canvas.

If you have something related to the material that you'd like to share, feel free to post to the Coffee Shop forum, also under the Discussions tab in Canvas.

As was detailed in an earlier lesson, solar energy is electromagnetic (aka radiant) energy that is generated by the (nuclear) fusion of hydrogen atoms into helium atoms in the sun. The amount of radiant energy that is released by an object is related to its temperature, and since the sun is so hot (~10,000º F!) [1], it is able to reach the ~94,000,000 miles (the distance depends on the time of year) to the earth. It is a massive amount of energy! A commonly cited statistic is that enough solar energy reaches the earth each hour to provide all of humanity's energy for an entire year. There is no shortage of solar energy.

Types of Solar Energy Technologies

Without the sun, life on earth would not be possible. It provides energy for vegetation to grow and provides sufficient heat to allow water to exist in liquid form, among other things. But there are many ways that humans can use this radiant energy more deliberately. The following is an overview of the major types of solar technologies. We could spend weeks analyzing each of these - keep in mind that this is just an overview.

• Solar photovoltaics (solar PV): Certain materials have the natural property of converting energy from the sun into electricity. When the sun hits these materials, electrons start to flow, creating a direct current (DC). This is the photovoltaic effect. This is described in more detail below.
• Solar thermal: This is a broad term for systems that use energy from the sun to heat water (or other material) for a variety of purposes. One common application is heating water for domestic hot water or swimming pools.
• Concentrated solar: There are a variety of ways to concentrate solar power for use. All of them gather solar energy over a wide area (usually by using mirrors) and concentrating it into a smaller location. This very high level of power is then often used to generate electricity. The DOE has a good explanation of some technologies here [2].
• Passive solar: Passive solar is a type of solar thermal that uses passive system design (e.g. south-facing windows, strategically-placed overhangs) to passively heat interiors of buildings. This is a great low-tech way to use solar energy! (Click here for more information about passive solar from the DOE.) [3]

Solar Photovoltaics and Availability of Solar

The rest of the solar lesson will focus on solar photovoltaics or solar PV. As noted above, photovoltaic technology (aka the photovoltaic effect) converts radiant solar energy into electricity. View the short video below from the U.S. Department of Energy for a brief explanation. Note that the narrator of the video indicates that photons provide the energy that is converted into electricity. NASA describes [4] the relationship between photons and electromagnetic energy thusly: "Electromagnetic radiation can be described in terms of a stream of mass-less particles, called photons [5], each traveling in a wave-like pattern at the speed of light [6]." So photons are generally considered to be what carries the energy that is emitted in waves.

Okay, so a solar panel converts radiant to electrical energy by using the unique properties of a semiconductor, usually, silicon combined (doped) with other elements (usually boron and phosphorous). But how much energy and power does a panel generate? As you might guess, it depends on a lot of factors. The following is an overview of some of these factors.

• First, a quick primer on power vs. energy.
  • Energy is the ability to do work. It is a discrete amount of "something," and that "something" makes things happen (makes things move, generates sound, generates heat, etc.). If one thing has more energy than another thing, then it is hypothetically capable of doing more "stuff." In the U.S., energy is usually measured in Btus, or if it's electrical energy, kilowatt hours (kWh). One kWh is 1,000 Wh, which is also a unit of energy. The international unit of energy is the Joule (J).
  • Power is the rate at which energy is converted. Practically speaking, it indicates how quickly you are "using" energy or the rate at which energy is being provided. Power is usually measured in watts (W) or horsepower (HP). A light bulb that uses 100 W of power is converting 100 Joules of electricity into heat and light each second. A 200 W light bulb is converting energy twice as quickly. A 1,000,000 W power plant (1 MW) is providing energy twice as quickly as a 500,000 W (500 kW) power plant. (Note that the light bulbs and power plants are all just converting one form to another, but from our perspective, one is "using" energy and one is "generating" energy. It's a matter of perspective.)
  • Energy = power x time. For example:
    • If you use a 100 W light bulb for 1 hour, you use (100 W x 1 h = ) 100 Wh.
    • If you use a 100 W light bulb for 1 hour each day for a year, you use (100 W x 1 hr/day x 365 days/yr = ) 36,500 Wh = 36.5 kWh.
    • If you use 10 light bulbs, each 100 W, for 1 hour you use (10 bulbs x 100 W/bulb x 1 hr = ) 1,000 Wh or 1 kWh.
    • If a 5,000 W (5 kW) solar array operates at full capacity for 1 hr, it generates (5,000 W x 1 hr = ) 5,000 Wh or 5 kWh of electricity. This is the energy that is generated by the panels, but some of that energy will be lost by the time it is used.
    • If a 1,000,000,000 W (1 GW) power plant operates at full capacity for 24 hours, it would generate (1 GW x 24 hr = ) 24 GWh of electricity.
    • If a 100,000 Btu/hr furnace operates at full capacity for 2 hours, it would use (100,000 Btu/hr x 2 hr = ) 200,000 Btu of energy.
• Irradiance is the amount of solar power (not energy) incident on a given surface area at any given time. This is usually measured in Watts per square meter (W/m²). All else being equal, more irradiance results in more output from a panel. Irradiance levels change throughout the day, as you can see in the image below. These charts illustrate the average hourly irradiance in State College, PA in July and December. As you can see, the peak irradiance in July averages about 750 W/m². This is the irradiance on a horizontal surface. If you could tilt the surface (say, a solar panel) so it is perpendicular to the sun, the noon irradiance would be more than 1,000 W/m². So if you want to imagine 1,000 W/m², think of how your skin feels on a really hot summer day at Penn State. Hopefully, this gives you a "feel" for irradiance. 

Figure 6.1: Average hourly irradiance in State College, PA in July (top image) and December (bottom image). This is "global horizontal irradiance," which is the solar power that hits a flat, horizontal surface. The irradiance levels would be much higher if the surface (e.g. a solar panel) were directly facing the sun. 

Credit: National Renewable Energy Laboratory, System Advisor Model (available for download here [7]).

• Irradiation is the amount of solar energy that hits a surface over a given period of time. This is usually measured in kWh/day/m² or kWh/yr/m². See the chart below for a map of irradiation in the U.S., which illustrates the average daily irradiation levels throughout the year in the U.S. Of course the irradiation will change throughout the year (more in the summer and less in the winter), but this chart provides a clear idea of the overall amount of solar energy that is available all year. (Note that you could easily find the average annual irradiation by multiplying the daily irradiation by 365.) Keep in mind that this is the average daily irradiation on a surface that is "latitude tilt," which will be explained in more detail below. The chart provides an indication of solar potential. (Note that due to panel inefficiency and a few other factors, a solar panel will only convert a fraction of this solar energy into electricity.)

Figure 6.2: Average daily irradiation in kWh/m²/day in the U.S.  You can see clear patterns here that correlate with U.S. climate zones - the desert Southwest gets around 6.5 kWh/m²/day on average, while most areas in Alaska receive much less. Note that this image indicates the output at a perfect tilt, ideal azimuth, and without derating. Larger map image available here [8]. 

Credit: National Renewable Energy Laboratory Dynamic Maps, GIS Data, & Analysis Tools [9].

We will experience some solar PV installations and technology while traveling, so I provide some more details about it below.

• In order to maximize panel output, the panel should be as close to perpendicular to the sun's rays as possible, which allows it to capture the most solar energy possible at a given time. (Here is a really good interactive animation [10] of why a perpendicular panel captures the most sunlight.) Most solar panels are fixed, i.e., they don't move. This means that you need to choose your location carefully. For fixed panels, there are two factors to consider in this regard:
  • Azimuth/orientation is the compass direction that the panel faces. Typically, 0º is due north, 90º is east, 180º is south, and 270º is west.
  • Tilt is the angle above the horizon to which the panel is tilted.
• Shading is a very important consideration as well. All else being equal, more shading means less output.
• In the Northern Hemisphere, the rule of thumb to maximize the output of a fixed panel is that the panel should be faced due south (180º orientation) and at "latitude tilt." You may recall from a Geography class that latitude is how far north or south a location is from the equator. The equator is 0º, the tropics are at 23.5º, and the North and South Poles are at 90º north and south, respectively.
  • For example, State College, PA is at just over 40º north, so the ideal array tilt is about 40º, and the ideal azimuth is 180º (south).
  • Anchorage, Alaska is at about 61º north, so the ideal tilt and azimuth are about 61º and 180º (south), respectively.

Figure 6.3: Rooftop solar array. There are 270 panels in this array. You can see some of them in the left image. The right image shows the side view of a panel row. The panels are tilted at about 13º. You cannot see it in the image, but the panels have an orientation of about 222º, which means they are faced southwest.

Credit: D. Kasper.

A few more terms that are important to know:

• A solar cell is the smallest current-generating part of an array. They can be any size but are normally around 6 inches by 6 inches. It's difficult to see, but in the image above there are 72 cells in each panel (12 rows, each with 6 cells).
• A solar panel (aka module) is a number of cells wired together in a single panel. They can be any size but are usually about 5 feet by 3.25 feet [11].
• A solar array is a number of panels wired together. The image above shows part of a solar array, which as I noted has 270 total panels.
• Capacity refers to the rated maximum output of a panel or array. Under optimal conditions, a 250 W panel will output about 250 W of electricity. Note that this is only the immediate output. By the time the electricity goes to the building or the grid, some of it is lost due to a variety of factors (usually 10% - 15%).
  • The array above has 270 panels, and each panel has a capacity of 305 W. Thus, the capacity of the array is (305 W x 270 = ) 82,350 W, which is 82.35 kW.
• A solar panel outputs DC (direct current) electricity. DC means that the electricity flows in one direction. Household outlets (and the electric grid) use AC (alternating current) electricity, which rapidly alternates direction of flow (60 times per second in the U.S., in case you are interested), so electricity from an array must be converted to AC if it is to be used in a building or distributed to the grid. An inverter is a piece of equipment that converts DC to AC and is a standard part of most solar arrays. 
• A grid-tied array is one that is connected to the grid. A standalone or off-grid system is not tied to the grid.

The Impact of Tilt and Orientation 

Recall that the rule of thumb is that the optimal tilt is "latitude tilt," and the ideal orientation in the Northern Hemisphere is due south (180º). This begs the question: what happens if the tilt and orientation are not optimal? The answer, as you might guess, is "it depends." This impact can be quantified by something that is called tilt and orientation factor (TOF). The tilt and orientation factor is a decimal that indicates what percent of the maximum solar output you would receive throughout the year at said tilt and orientation. So if you install an array and it has a TOF of 0.85, that means that it will only be able to output about 85% of the energy it would output if it were at the ideal tilt and orientation. 

Wilmington, Delaware is at about 40º north. As it turns out, the ideal tilt is closer to 35º (rules of thumb are only rules of thumb, after all!). The tables below show the TOF at different tilts and azimuths. The first table illustrates the TOFs of different tilts, all with an orientation of 180º. The second table shows the TOFs at different azimuths, all at a tilt of 35º (the ideal tilt). (You can investigate the TOF for locations throughout the U.S. by going to the Solmetric website [12].)

Calculating Solar Output

Okay, now we're ready to calculate the solar output. There are a number of software programs and a formula or two that can do this, but the National Renewable Energy Laboratory (NREL) provides a free one that is well-regarded in the energy industry called PVWatts [13]. In the video below, I demonstrate how to calculate the annual output of the array in the images above, which has the following specs:

• address: 400 Stanton Christiana Rd, Newark, DE
• 82.35 kW capacity
• 13º tilt
• 222º azimuth

Figure 6.4: Chart depicting the average monthly output in a typical year for the system specs indicated above. This chart was generated using the output from the PVWatts calculation detailed in the video above.

Credit: D. Kasper.

A Few More Notes

Hopefully, by now you have a relatively good grasp on some of the considerations that go into designing and calculating the output of solar PV. Solar PV really took off in the early- to mid-2000s, led by residential array installations, which generally had capacities of a few kW. The solar industry in the U.S. is not dominated by utility-scale solar, which is much cheaper per W to construct because of economies of scale. Utility-scale arrays can be thousands of watts (multiple MWs) in capacity!

Finally, there are a few ways that people can use and pay for solar PV:

• Systems can be purchased and owned by individuals. This can be done out-of-pocket or using loans. Out-of-pocket purchases usually generate the highest return.
• Power purchase agreements allow people or businesses to pay a third party to build an installation on their own roof. The individual or business then pays the third party for the electricity generated at a contractually agreed-upon rate. This is the model that SolarCity (now Tesla Energy) used to rise to prominence. This is a popular model because it requires no up-front cost (among other reasons).
• Community-owned solar or just community solar is used increasingly in the U.S. and elsewhere. Community solar allows individuals to "buy into" a solar array that is installed somewhere else, or perhaps on a shared roof or field space. This provides access to individuals and businesses that would not otherwise be able to use solar due to factors such as not having a suitable site (e.g., a heavily shaded roof) or living in rented space. (The SEIA provides a good overview of community solar here [14], and NREL provides a more robust explanation here [15].) 
• Most utility-scale installations effectively act as power plants, and the electricity is sold to the grid.

You may recall from EM SC 240N that bioenergy is energy that comes from living or recently living things. Common examples include wood from trees used for heating and ethanol from corn used as a gasoline additive. Another form - and one that we will see while traveling - is called anaerobic digestion. "Anaerobic" refers to "without air" and the "digestion" part refers to the microorganisms that digest organic material. Putting it together, anaerobic digestion refers to microorganisms breaking down organic material when no oxygen is present. The following descriptions of anaerobic digestion are from the EPA's Anaerobic Digestion website. All points of emphasis (bold letters) are mine:

Anaerobic Digestion

Anaerobic digestion is the natural process in which microorganisms break down organic materials.  In this instance, “organic” means coming from or made of plants or animals.  Anaerobic digestion happens in closed spaces where there is no air (or oxygen).  The initials “AD” may refer to the process of anaerobic digestion or the built system where anaerobic digestion takes place, also known as a digester.

The following materials are generally considered “organic.”  These materials can be processed in a digester:

• Animal manures;
• Food scraps;
• Fats, oils, and greases;
• Industrial organic residuals; and
• Sewage sludge (biosolids).

All anaerobic digestion systems adhere to the same basic principles whether the feedstock is food waste, animal manures or wastewater sludge.  The systems may have some differences in design but the process is basically the same

Byproducts of Anaerobic Digestion

Biogas is generated during anaerobic digestion when microorganisms break down (eat) organic materials in the absence of air (or oxygen).  Biogas is mostly methane (CH₄) and carbon dioxide (CO₂), with very small amounts of water vapor and other gases. The carbon dioxide and other gases can be removed, leaving only the methane. Methane is the primary component of natural gas.

The material that is left after anaerobic digestion happens is called “digestate.” Digestate is a wet mixture that is usually separated into a solid and a liquid. Digestate is rich in nutrients and can be used as fertilizer for crops

Uses of Anaerobic Digestion Byproducts

Biogas is produced throughout the anaerobic digestion process.  Biogas is a renewable energy source that can be used in a variety of ways. Communities and businesses across the country use biogas to:

• Power engines, produce mechanical power, heat and/or electricity (including combined heat and power systems);
• Fuel boilers and furnaces, heating digesters and other spaces;
• Run alternative-fuel vehicles; and
• Supply homes and business through the natural gas pipeline

How biogas is used and how efficiently it’s used depends on its quality.  Biogas is often cleaned to remove carbon dioxide, water vapor and other trace contaminants.  Removing these compounds from biogas increases the energy value of the biogas...Biogas treated to meet pipeline quality standards can be distributed through the natural gas pipeline and used in homes and businesses. Biogas can also be cleaned and upgraded to produce compressed natural gas (CNG) or liquefied natural gas (LNG).  CNG and LNG can be used to fuel cars and trucks.

Digestate is the material that is left over following the anaerobic digestion process.  Digestate can be made into products like:

• Bedding for livestock;
• Flower pots;
• Soil amendments; and
• Fertilizers.

When properly processed, dewatered digestate can be used as livestock bedding or to produce products like flower pots.

Digestate can be directly land applied and incorporated into soils to improve soil characteristics and facilitate plant growth. Digestate can also be further processed into products that are bagged and sold in stores. Some emerging technologies can be employed post-digestion to recover the nitrogen and phosphorus in digestate and create concentrated nutrient products, such as struvite (magnesium-ammonium-phosphate) and ammonium sulfate fertilizers. 

The video below from Michigan State University does a great job of explaining how they use anaerobic digestion to convert organic cafeteria and farm waste into useful energy and fertilizer. To view the transcript (and the video on YouTube, click this link [16].)

Thermodynamically speaking, the energy conversion process is:

• Sunlight is converted to biomass by plants.
• Plants are either used in the digester and converted to methane and CO₂ or...
• Biomass is eaten by animals and converted to another form of biomass.
• Organic waste from animals is used in the digester and converted to methane and CO₂.
• The methane can then be used for electricity and/or heat.

It is extremely important to keep in mind that this is a natural process, and thus will occur any time organic material is subjected to low- or no-oxygen conditions. One important implication of this is that organic material that ends up buried in a landfill will convert partially to methane because there is very little oxygen underneath all of that "junk." As I'm sure you recall, methane is about 30 times more potent than CO₂ in terms of its global warming impact. If you took the same organic material and let it biodegrade in the open air (i.e., with access to oxygen) it would release mostly CO₂. The sad irony of this is that well-meaning people and companies can actually make the (climate change) problem worse if their biodegradable containers end up in the landfill. This methane can be captured, and in many places in the U.S. and throughout the world is. This is also why impoundment hydroelectric facilities (big dams) can cause methane emissions - organic material collects upstream of the dam, and low-oxygen conditions often occur near the bottom of the reservoirs, causing methane to be released. Systems thinking, everyone!

Digesters can be pretty much any size. I've seen one as small as a car inner tube that was used to power a gas grill and heat a small greenhouse. Some of them can be larger, as you'll see below.

The pictures below are from a cooperatively-owned anaerobic digester in Lemvig, Denmark. I'm particularly fond of this because the entire setup is owned equally by about 25 farmers, and is a non-profit operation. All of the organic waste from the farms is transported to the digester, including leftover vegetation and various types of manure. The biogas is used to generate electricity in a turbine which is either sold to the grid or used in the digester, and the "waste" heat is used to run the anaerobic digester. The remaining heat is used for district heating for the town - it heats up water, which is then run through underground pipes to be used to heat homes. This type of generator is considered cogeneration, which means it is used to generate electricity and useful heat. Recall that most power plants are about 35% efficient because so much energy is wasted as heat. Believe it or not, this cogeneration system is over 90% efficient when you include all of the "waste" heat that is captured and used! All digestate is then returned to the local farms and used as organic fertilizers. It is truly a closed-loop system!

Figure 6.5. Digestion tanks in Lemvig, Denmark.

Credit: D. Kasper

Figure 6.6. Methane storage tank, Lemvig, Denmark. This is like a giant bladder that can expand as it fills up with methane.

Credit: D. Kasper

Figure 6.7. Gas generator, Lemvig, Denmark. This generator uses cogeneration. It generates electricity and captures most of the waste heat to be used for other purposes.

Credit: D. Kasper

Figure 6.8. This digestate is used as high-quality organic fertilizer for the local farms.

Credit: D. Kasper

The images below show details of a smaller installation in Kussnacht, Switzerland. This installation is run by a single farmer (Seppi), who collects organic waste from his farm, other local farms, and area restaurants. Like the one above, Seppi collects the biogas and uses it in a cogeneration system that is about 90% efficient (50% heat, 40% electricity, and 10% is wasted). He runs a 100 kW generator and uses the electricity on his farm and sells the leftover to the grid. The heat is used to run the digester, and to provide space and water heating to his farm. He uses some digestate on his farm and gives the rest back to local farmers for free.

It just so happened that at the time of our visit (I brought students there for a study abroad experience), his previous digester had burnt down due to a generator fire. The upshot of this is that we were able to see inside the digester he was building, which you will see below.

Figure 6.9. Digestion tank in Kussnacht, Switzerland. The tank is under construction and is the green building in the middle.

Credit: D. Kasper

Figure 6.10. Inside the digestion tank. Note the mixing blades that are used to keep the organic material from settling. This was mentioned in the video previously.

Credit: D. Kasper

Figure 6.11. Feedstock for anaerobic digestion. Seppi uses organic material from his farm and local farms, as well as material like this from local restaurants. The plastic is removed before it is put into the tank.

Credit: D. Kasper

Figure 6.12. Natural gas cogeneration system in Kussnacht, Switzerland. This system is about 90% efficient!

Credit: D. Kasper

Figure 6.13. Digestate that remains after anaerobic digestion. Both (solid on the left and liquid on the right) can be used as high-quality organic fertilizers.

Credit: D. Kasper

Figure 6.14. Just for good measure, I've included a photo of Seppi charging up the electric bus that we were riding around Switzerland on. Many places in Switzerland have car charging stations. We went from place to place, charging almost exclusively using renewable energy.

Credit: D. Kasper