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.
By the end of this lesson, you should be able to:
To Read | Lesson 6 Online Content | You're here! |
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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.
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.
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.
We will experience some solar PV installations and technology while traveling, so I provide some more details about it below.
A few more terms that are important to know:
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).
Tilt (º) | Azimuth (º) | TOF |
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0 | 180 | 0.87 |
5 | 180 | 0.905 |
10 | 180 | 0.935 |
15 | 180 | 0.959 |
20 | 180 | 0.978 |
25 | 180 | 0.991 |
30 | 180 | 0.999 |
35 | 180 | 1.0 |
40 | 180 | 0.996 |
45 | 180 | 0.985 |
50 | 180 | 0.969 |
55 | 180 | 0.947 |
60 | 180 | 0.92 |
65 | 180 | 0.888 |
70 | 180 | 0.851 |
75 | 180 | 0.81 |
80 | 180 | 0.764 |
85 | 180 | 0.715 |
90 | 180 | 0.662 |
Tilt (º) | Azimuth (º) | TOF |
---|---|---|
35 | 90 | 0.797 |
35 | 100 | 0.833 |
35 | 110 | 0.897 |
35 | 120 | 0.898 |
35 | 130 | 0.926 |
35 | 140 | 0.951 |
35 | 150 | 0.972 |
35 | 160 | 0.986 |
35 | 170 | 0.996 |
35 | 180 | 1.0 |
35 | 190 | 0.997 |
35 | 200 | 0.989 |
35 | 210 | 0.976 |
35 | 220 | 0.956 |
35 | 230 | 0.932 |
35 | 240 | 0.905 |
35 | 250 | 0.874 |
35 | 260 | 0.839 |
35 | 270 | 0.803 |
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 [12]. In the video below, I demonstrate how to calculate the annual output of the array in the images above, which has the following specs:
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 now 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!
There are a few ways that people can use and pay for solar PV:
Incentives have played a very important role in the growth of the solar industry, and renewable energy in general. The following provides a description of some incentives.
States and countries have implemented a variety of policies meant to incentivize or encourage private investment in clean, renewable energies. The most common of these policies are tax credits, grants/rebates, and performance-based incentives (PBI), including feed-in tariffs (FIT) and renewable portfolio standards (RPS).
A tax credit is just that, a credit. When an individual or business investor earns a tax credit it means that the amount of the credit will be subtracted from a future tax bill. For example, in the United States, we have a Federal Residential Renewable Energy Tax Credit [15] available to the residential (not commercial or industrial) sector which provides a tax credit covering 26% of the cost of an installation. A typical residential system costs about $3/Watt to install. If you put a 7 kW photovoltaic system on your roof, it should cost around $21,000 before incentives (7,000 W x $3/W). If so, you would earn a $5,460 tax credit. The government doesn’t mail you a check for this amount. It means you get to deduct that amount from your next tax payment. To realize this money, you will need to have paid at least $5,460 in taxes, but excess credits can "generally" be carried over to future tax years. Note that even if you were owed a refund, this tax credit can be used to increase your refund, as long as you paid at least $5,460 in federal income tax throughout the year. Other renewable energy technologies qualify for this credit. See the link above for details.
A rebate means that a government agency or other group (sometimes utility companies) will refund some of the investment. These are usually based on the size/capacity of the system. For example, Pennsylvania used to provide a solar rebate program that provided rebates to investors of $1.75/Watt. Rebates are checks mailed directly to the investor (or their designate). Many states still have such programs such this solar PV rebate program in Oregon [16](description from DSIRE of course!). Different states often have different program specifics. See DSIRE for more examples of programs. Specifics vary within states as well. The program in Oregon, for example, has different rebate levels for different utilities and for different sectors (residential, agricultural, industrial, non-profit, government). For example, if you are a residential customer of the PGE utility in Oregon, you would receive $0.25/W up to $1,750. So for a 7 kW system you would recieve a $1,750 (7,000 W x $0.25/W) rebate.
Performance-based incentives (PBIs), also known as production incentives, provide cash payments based on the actual output of the system. For wind and solar electric, this is the number of kilowatt-hours (kWh) generated. This is usually only applied to utility-scale systems.
Rnewable portfolio standard (RPS) are very imnportant policies. Breifly, an RPS requires utilities to use renewable energy credits (RECs) to account for a certain percentage of their retail electricity sales. (RPSs are established by state legislatures, and not all states have an RPS policy.) A REC is earned by a qualified grid-tied facility for every 1,000 kWh (i.e., 1 MWh) of electricity that is generated using a renewable energy resource. The RECs are then bought and sold through a market. The settlement price varies depending on REC supply and demand at any point in time, though special auctions with guaranteed pricing and incentives are sometimes used. If you live in a state that has an RPS policy, you can "sell" your credits on an annual basis and receive a payment. For example, if you generate 7,000 kWh in a years and you can sell your credits for $40/MWh, you would earn $280 (7,000 kWh = 7 MWh x $40/MWh). You would receiv a check for this at the end of the year. See below for states that have RPS policies.
Another type of production-based incentive, a feed-in-tariff (FIT) pays grid-tied renewable energy generators a specified price for the electricity they generate and guarantees them this price for a specified amount of time. This type of policy is widely used in Europe, most notably in Germany, but less so in the USA. This is also usually used on utility-scale systems.
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 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
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
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 [19].)
Thermodynamically speaking, the energy conversion process is:
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!
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.
By now you should be able to:
You have reached the end of the lesson! Double-check the to-do list on the Lesson Overview page [20] to make sure you have completed all of the activities listed there before you begin the next lesson..
Links
[1] http://coolcosmos.ipac.caltech.edu/ask/7-How-hot-is-the-Sun-
[2] http://www.energy.gov/eere/solar/concentrating-solar-power
[3] http://www.energy.gov/energysaver/energy-efficient-home-design/passive-solar-home-design
[4] https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html
[5] https://imagine.gsfc.nasa.gov/resources/dict_jp.html#photon
[6] https://imagine.gsfc.nasa.gov/resources/dict_qz.html#speed_of_light
[7] https://sam.nrel.gov/download
[8] https://www.nrel.gov/gis/solar-resource-maps.html
[9] http://www.nrel.gov/gis/solar.html
[10] http://www.pveducation.org/pvcdrom/properties-of-sunlight/motion-of-the-sun
[11] http://news.energysage.com/average-solar-panel-size-weight/
[12] https://pvwatts.nrel.gov/
[13] http://www.seia.org/initiatives/community-solar
[14] http://www.nrel.gov/docs/fy15osti/63892.pdf
[15] http://programs.dsireusa.org/system/program/detail/1235
[16] http://programs.dsireusa.org/system/program/detail/936
[17] https://ncsolarcen-prod.s3.amazonaws.com/wp-content/uploads/2020/09/RPS-CES-Sept2020.pdf
[18] https://www.dsireusa.org/resources/detailed-summary-maps/
[19] http://www.youtube.com/watch?v=aULRryCVMyY
[20] https://www.e-education.psu.edu/emsc297/813