by Ginny Carlton & Liese Dart
Banner photo: jurvetson, "Solar Harvest," flickr.com, c. December 12, 2008, http://www.flickr.com/photos/jurvetson/3730751907/, accessed June 14, 2010.
The sun has been with us through time immortal. Life on Earth would be drastically different without solar energy. Indeed most life-forms wouldn’t exist at all. Although there are exceptions, almost all on Earth depends, either directly or indirectly on solar energy to sustain its very existence. Man has harnessed this energy source in various ways and continues to develop new technologies to put this vast resource to use.
The sun, the source of all solar energy, is a giant nuclear fusion reactor running on hydrogen fuel. Temperatures and pressures in its inner core are so high that hydrogen nuclei fuse to form helium nuclei, releasing enormous amounts of energy. “Every second it converts about 11 billion pounds (5 billion kg) of matter into energy.”1
Figure 1: Wavelength, Frequency and Photon Energy of Solar Radiation
Source: US DOE, 2008
The sun gives off three types of radiation (Figure 1) ultraviolet radiation (9%), visible light (49%) and infrared radiation (42%).2 Wavelength and frequency are inversely related—the shorter the wavelength, the higher the frequency and the greater the energy.
At the speed of light, radiation makes the 93 million mile trip between the sun and the earth in slightly more than eight minutes. “Earth a tiny target in the vastness of space, receives only about one-billionth of this output.”3 Even though much of this energy is lost to space, the solar radiation intensity (the average amount of solar energy at the Earth’s outer atmosphere) remains fairly constant at an average of 1367 watts (W) per square meter (m2).4
“About 34% of the solar energy reaching the earth’s troposphere is reflected right back to space by clouds, chemicals, and dust and by the earth’s surface of land and water.”5 Most of the un-reflected solar radiation is degraded into infrared radiation (which we experience as heat) as it interacts with the earth. The rate at which heat flows through the atmosphere and back into space is affected by heat-trapping greenhouse gases. Without this atmospheric thermal “blanket,” known as the greenhouse effect, Earth would be nearly as cold as Mars, and life as we know it could not exist.
Most of the remaining 64% of solar energy warms the troposphere and land, evaporates and cycles water through the ecosphere, and generates wind. A tiny fraction (0.023%) of this solar energy fuels the photosynthetic process that creates the organic compounds needed by life forms to survive.6
Figure 2: Although the quantity of solar radiation striking the Earth varies by region, season, time of day, climate, and air pollution, the yearly amount of energy striking almost any part of the Earth is vast. Shown is the average radiation received on a horizontal surface across the continental United States in the month of June. Units are in kWh/m2/day. Source: U.S. Department of Energy, Energy Efficiency and Renewable Energy, Solar Technologies Program (2008) Light and the Solar Cell Retrieved from http://www1.eere.energy.gov/solar/pv_cell_light.html on March 26, 2010.
Unlike many other energy resources, solar energy can be found distributed widely across the Earth. The amount of solar radiation available in a particular location at a particular time is called insolation, incident solar radiation, or solar energy potential. Insolation varies with: latitude, seasonal change, time of day, climate, air pollution, and topography (see Figure 2).
Some estimates suggest that the total amount of sunlight falling on the United States each year is upwards of 40,000 quads (quadrillion British thermal units, or BTUs)7 which is an interesting comparison with 2007 estimates of total U.S. energy consumption of 100 quads.8 Even at its northern latitude, the solar energy falling on Wisconsin each year is roughly equal to 844 quadrillion BTUs of energy.9 According to the Wisconsin Office of Energy Independence the total resource energy consumption in 2008 was 1743 trillion BTUs.10
Plants and some types of bacteria, use the sun’s energy in a process known as photosynthesis—the conversion of solar radiation to biomass —which has provided the foundation upon which humans have powered their civilizations. Even though it is the most prevalent use of solar energy, photosynthesis is rarely mentioned in solar energy discussions. Obviously it is foundational to our use of bio-fuels.
Solar radiation also plays a major role in the heating and cooling of air and water. Without solar energy to create differentially heated air masses and evaporate water, wind and hydro power would be impossible.
In addition to these natural solar technologies, technologies are being developed to concentrate the solar energy that strikes the Earth, creating a higher quality energy source. Below is a list of solar technologies ranging from simple to complex.
Figure 3: Trombe Wall with Vents. Source: MIT OpenCourseWare. Architecture Department, Introduction to Integrated Design, Ann Watson Instructor
“Passive solar” refers to a type of building that is designed to trap and retain solar energy. Typically, passive solar buildings located in the northern hemisphere have large expanses of glass on the south side through which sunlight passes to the interior of the building. Thick walls and floors are designed to store heat during the day for direct solar gain and release it at night, termed indirect solar gain.
Some passive solar buildings have a specialized wall called a Trombe wall. Often this is a concrete wall located just a few inches from the glass surface and painted black to maximize heat gain— it can reach temperatures up to 130°F.11 The heat is transferred to the interior of the wall and ultimately into the interior space of the building by the process of conduction. Vents at the top and bottom of the wall aid air circulation and convection.
Perhaps the most famous passive solar building was London’s Crystal Palace, home of the Great Exhibition of 1851. The building created an artificial Mediterranean microclimate in Hyde Park for 18 months. It was dismantled and reassembled in south London where it housed tropical plants, educational displays, and concerts until it was destroyed by fire in 1936.
Solar cookers are used widely throughout the world in areas that lack electrification and have a short supply of biofuels such as wood. The first known person to build a solar box cooker was Horase de Saussure, a Swiss naturalist who built a solar cooker in 1767 much like a modern day greenhouse constructed from heat trapping glass. However, it is entirely possible that other cultures also invented this relatively simple solar cooking technique prior to de Saussure.
Solar cookers provide an alternative to wood and propane when such fuels are limited or expensive. They can range from the truly uncomplicated (a glass bowl inverted over the food with a cardboard box covered in tinfoil to provide a sunlight reflecting surface) to more elaborate multi-reflector solar cookers. Both complex and simple solar cookers can heat food to temperatures between 250-400°F,12 which are adequate to kill food and water-borne diseases.
Figure 4: Flat Plate Collector Solar Hot Water Heater
Source: National Renewable Energy Laboratory
Solar hot water heaters are similarly designed to be solar collectors—converting solar energy into heat energy and using the processes of convection and conduction to manage the heat flow. The basic physics principal involved in solar hot water heaters is stratification—the hottest water will rise to the top of the tank, because when a fluid is heated its molecules expand causing it to be less dense than the surrounding cooler molecules.
Flat plate collectors are the most widely used type of collector in a solar hot water system.13 A typical flat plate collector solar hot water system consist of a grid of closely spaced copper pipes welded to a thin copper metal plate called an absorber plate. The pipes and plate are usually painted black to maximize absorption of solar radiation. The tubes can be filled with water, or, in areas where freezing is a concern, a glycol-water mixture.
The pipes and plate are enclosed in a rigid frame. As a group these three parts are called a collector. A solar hot water heating system is compromised of a collector, a thermostatically controlled pump which controls the rate of water flow through the tubes, and a heat exchange tank (which warms the water that is actually used—e.g., in a swimming pool or shower).
Passive solar buildings and solar hot water heating energy systems put solar radiation to effective use. However they don’t produce electricity. For that an active solar energy system consisting of photovoltaic cells is needed.
The photovoltaic (PV) cell is a solar technology which produces electricity on site. The process utilizes materials such as silicon, cadmium sulfide, germanium or gallium arsenide.14 A unique property of these materials is the ability to absorb solar energy, causing a small energy difference, known as voltage, at the atomic level,
Figure 5: Solar cell. Source: U.S. Department of Energy, Energy Efficiency and Renewable Energy, Solar Technologies Program (2006)
“A PV cell consists of two layers of somewhat differing semiconductor materials placed in contact with one another. One layer is an "n-type" semiconductor with an abundance of electrons, which have a negative electrical charge. The other layer is a "p-type" semiconductor with an abundance of "holes," which have a positive electrical charge.”15
Where the two layers meet a p/n junction is formed. Excess electrons (originally acquired through the intentional addition of another element—typically phosphorous—to the silicon) shift from the n-layer side toward the p-layer side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.
When light hits a silicon atom in the p-layer, it knocks an electron out of the crystal lattice. “Impurities (typically introduced boron) in the p-silicon prevent the electron from falling back into place. Instead it follows the path of least resistance traveling through the connecting wire to the n-layer”16 thereby creating an electric field.
The typical silicon solar cell will produce 0.45 volts of electricity.17 It will produce this voltage output even at very low light levels. The five major factors that affect the performance output are: load resistance, sunlight intensity, cell temperature, shading, and crystalline structure of the material (e.g., silicon).
The current generated by a silicon solar cell varies considerably with the wavelength of the solar radiation (light intensity) and the surface area of the cell. In 2007, scientists from Spectrolab, Inc., a Boeing subsidiary, achieved the 40% efficiency milestone.18
Solar cells grouped together are called a photovoltaic module and are typically combined with glazing material, hardware, a frame, and electrical connections to create a photovoltaic panel. The panels can then be inter-connected to form a solar array. The ARCO Solar, Inc. plant in Hesperia, California consists of 108 arrays each containing 256 photovoltaic cells. In 1983, it became the first solar “plant” to generate 1 megawatt (one million watts) of electrical energy.19
Since solar cells generate DC (direct current) electricity, an inverter is needed to change the output to AC (alternating current) in order to be compatible with most appliances. In home applications, a storage battery system is also typically used to provide back-up power when sunlight is limited.
Figure 6: A solar collector assembly. Source: the National Renewable Energy Laboratory: http://www.nrel.gov/csp/troughnet/solar_field.html
The solar technology with perhaps the most potential for generating significant quantities of electricity is known as concentrating solar power or CSP, which consists of a series of “focused collectors”. As the sun follows its azimuth path from East to West during the day, the concentrating solar panels collect the sun’s rays focusing the energy on a receiver. Unlike PV, only direct rays, not diffuse sunlight, are employed. CSP also differs from PV in that it does not generate electricity directly from solar panels. The heat generated from the solar energy is the desired “commodity.” Like a traditional fossil fuel combustion power plant, heat obtained from a CSP system is used to boil water to produce steam that turns a generator. The CSP plant is comparable to a coal or nuclear plant, but the original energy source is the sun instead of coal, natural gas, or a nuclear fission reaction.
There are several different CSP designs available on the market. Parabolic troughs. built out of U shaped mirrors focus the sunlight on a receiver tube suspended in the air above the trough (Figure 6). The tubes are filled with oil or water and can reach temperatures of up to 750 degrees Fahrenheit.20
Currently, these are considered to be the most successful design for large scale solar. Another design, called a “central receiver” involves mirrors, known as heliostats, which concentrate the sun’s energy onto a receiver attached to a high tower. The heat collected by the receiver can be used directly to heat industrial or commercial facilities, or can be used as the heat source in a steam generator.21
Solar energy is a low density fuel. Sunlight dispersed across the landscape must be concentrated in order to substitute for traditional fossil fuels whose high energy density has allowed humans to work efficiently for centuries. As early as the 2nd and 3rd centuries B.C. humans began to concentrate solar energy through the use of mirrors to light fires.
Architects throughout history have incorporated the concept of solar insolation into their designs. The megaron—a rectangular room with axial colonnades that led out onto a shaded porch— was the foundational design. “Greek temples like the Parthenon are basically shaded colonnades for worshipers to stroll through on their way to the statutes of the gods.”22 The need for shade in summer and sunshine in winter led the Ancient Greeks, Romans and North Americans to design shelters using passive solar principles. The influential American architect Frank Lloyd Wright, incorporated these same principles in his project known as the Robie House, constructed 1908-09 in Chicago..23
After World War I, architectural style, especially that of sanatoriums, reflected the goal of getting as much daylight into the building as possible. The twentieth century Swiss-French architect Le Corbusier developed the brise-soleils, an external building surface feature designed to allow light in, but prevent summer heat gain. Today’s passive solar building technologies carry many of these ideas forward.
In 1839, Edmond Becquerel, a nineteen year old (March 24, 1820 - May 11, 1891) French Scientist, discovered the “photovoltaic effect” or process of using sunlight to produce an electric current in a solid material.24 British scientist Willoughby Smith, in 1873, observed that selenium was light sensitive.25 Smith concluded that selenium’s ability to conduct electricity increased in direct proportion to the degree of light intensity. “In 1880, Charles Fitts developed the first selenium-based solar electric cell.”26
After World War II, Bell Laboratories scientists (Daryl Chapin, Calvin Fuller, and Gerald Pearson), in their quest for a dependable way to power remote communication systems, discovered that silicon is sensitive to light. When silicon, the second most abundant element on earth, was treated with certain impurities, it generated a substantial voltage. “By 1954, Bell developed a silicon-based cell that achieved six percent efficiency.”27 and they later achieved efficiencies of 11%.28 “The first non-laboratory use of photovoltaic technology was to power a telephone repeater station in rural Georgia in the late 1950’s.”29
Another early use of photovoltaic cells was by the National Aeronautics and Space Administration (NASA). “Scientists, seeking a lightweight, rugged and reliable energy source suitable for outer space, installed a PV system consisting of 108 cells on the United States’ first satellite, Vanguard 1.”30 The ability to generate electricity from sunlight continued to be closely linked to the development of our space and aeronautical programs. “Photovoltaic technology was spectacularly demonstrated in 1990, when a solar-powered aircraft, the Sun Seeker, flew 2,500 miles (4,060km) across the US, setting a record for fuel-less flight.”31
Solar technology development in the United States was originally the task of the Department of Interior.32 As early as the 1950’s, solar research was seen as controversial. In 1951, the Truman Administration created the Paley Commission, also known as the Materials Policy Committee, set to study “material shortages” resulting from the Korean War. The Paley Commission’s 1952 report advised that the US should reduce its dependence on foreign oil by “direct utilization of solar energy… [is perhaps the most important contribution technology can make to the solution of the materials shortage.”33 Ignoring the recommendations from the previous administration, President Eisenhower limited solar research funding to $100,000 per year despite calls from members of the Senate to increase it to $1 million per year in the face of further US dependence on foreign oil.34
Energy development and the creation of jobs have always gone hand in hand. In 1979, the Joint Economic Committee of the U.S. Congress commissioned a report called Employment Impact of the Solar Transition. The purpose of this report was to investigate whether policies that encouraged energy conservation and solar technology would employ more Americans than large centrally located fossil fuel generators. The report’s authors found that “an investment of $65.6 billion per year in energy efficiency and solar technologies would cut U.S. annual consumption of fossil-fuel and nuclear energy by 18 percent in twelve years and save $118.8 billion in avoided expenditures on fossil and nuclear fuels by the year 1990.”35 Despite these findings, “between 1980 (the last year of the Carter administration) and 1990, DOE outlays for R&D of all renewable-energy sources crashed from $557 million to $81 million, a near 90-percent reduction in terms of constant dollars.”36 Historically, calls have been made by advocacy groups for the creation of American jobs in the renewable energy sector. However, secure funding for solar research has continued to face an uphill battle until recently. President Barack Obama introduced a 2011 budget request of $2.36 billion for the Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE), including a 22% increase for solar energy funding over the previous fiscal year.37
No technology is free of impacts to the environment and to human health. Although the life cycle analysis of solar panels suggests a much lower environmental footprint than conventional fossil fuels, the processes used to manufacture photovoltaic panels and parabolic trough collectors involve the use of flammable substances, create occupational health hazards, and require toxic chemicals which must be appropriately managed to avoid contamination of the environment.
Many chemicals are used in the production of solar cells and panels. Crystalline silicon photovoltaic cell production involves chemicals such as trichloromethrthane (TCA) and methylene chloride, which have contaminated the groundwater supplies of neighborhoods near photovoltaic plants in Silicon Valley California and are suspected carcinogenic substances.38 According to Science Daily, toxic chemicals such as mercury, lead, and cadmium are also used to manufacture photovoltaic panels.39
The National Photovoltaics Environmental Research Center (NPERC), a part of the Brookhaven National Laboratory in New York was created to help address toxicity issues in solar panel production. According to NPERC’s website, the organization works “to identify and examine potential health and safety barriers, and hazard control strategies for new photovoltaic material, process or application options, before their large-scale commercialization.”40
Solar energy, like all types of energy, is regulated at the federal level by the Federal Energy Regulatory Commission and by state regulatory agencies. For solar systems to be widely deployed financial incentives to encourage electricity customers to install solar photovoltaic paneling and solar hot water heaters may be required. In order to build solar thermal plants, state public service commissions must approve the project. As of March 2010, twenty-four state legislatures and the District of Columbia had passed renewable portfolio standards (RPS) and non-binding renewable energy goals.41 Despite geographic constraints for renewable resources, politics has played perhaps a more important role in state solar energy goals than geography. For example, sunny California has yet to pass a solar provision. Just to the southeast, Arizona has set an aggressive target of producing 4.5% of their electricity needs through distributed solar generation by 2025. Nearby Colorado has set a requirement of producing 0.8% of its energy from solar by 2020 while Nevada has set a goal of producing 1.5% of its electricity from solar by 2025.42
Government and private research and development dollars greatly influence the types of solar technologies available on the market today. There are three main types of solar technology that has been pursued by the United States’ Department of Energy: photovoltaic panels (PV), solar hot water heaters, and concentrating solar power (CSP), also known as solar thermal. All three technologies hold promise; however CSP is the most cost competitive in the current U.S. energy market. When the United States Congress passes a comprehensive bill to price greenhouse gas emissions, CSP and other solar technologies may become one of the 21st century’s fuels of choice, replacing some of our need for coal, oil, and natural gas.
The National Renewable Energy Laboratory (NREL) located near Golden, Colorado is the main research enterprise funded through the Department of Energy to research renewable energy technologies. Scientists at NREL conduct research on CSP and photovoltaic technology and conduct studies on solar radiation and energy policy. In addition to R&D, state and federal financial incentives for solar energy projects have encouraged solar development at the residential level (rooftop paneling) and the utility scale (concentrating solar power plants). The federal government has a tax credit program for both solar hot water heaters and photovoltaic paneling that is available to all Americans through 2016.43 The program offsets 30% of the capital cost of installing a solar hot water heater or PV panels. This federal program is complimented by rebate programs in individual states that have passed laws to increase the use of solar technology, such as California44 and Texas. Used as a peak electricity supply, solar hot water heaters and photovoltaic panels can mitigate the need for investments in natural gas plants which are expensive to operate. Other policy options for financing distributed solar installations include what is known as feed-in tariffs, a financing tool which is widely used in Germany to encourage residential scale renewable energy installations.
Private investment in solar electricity is becoming cost competitive with traditional fossil fuel generators. Companies such as First Solar located outside of Toledo, OH are designing cheaper ways to manufacture photovoltaic panels through the advanced thin film semiconductor process. Utility-scale developments in solar thermal electricity are underway in Arizona and Nevada. Until recently, the Nevada Solar One project near Boulder City was the largest solar thermal project in North America at 64 megawatts. However, the Solana Generating Station, a plant that will soon be built near Gila Bend, Arizona, about 70 miles southwest of Phoenix is larger. This plant will consist of a three square mile “solar field” containing 2,700 parabolic trough collectors.
The Solana plant is designed to produce 280 MW of electricity, which is approximately half of the generation of a typical coal fired plant. According to the Solana website, it will produce enough electricity to power 70,000 homes in Arizona.
The economic potential of deploying solar energy on a large scale remains uncertain due to doubt about the nature of technological advances. Improvements in solar technology to reduce investment payback times and decrease the carbon lifecycle analysis of solar technologies should make solar energy more cost effective and less carbon intensive over time. In addition to questions of “how well it works,” advocates both for and against solar energy are concerned with a number of economic and environmental issues. These include whether demand for energy could be met through energy conservation and efficiency, the impacts of solar installations on forest fragmentation, the storage and reliability of solar power, and the regulatory environment for carbon dioxide emissions.
Although solar energy is expensive to produce today relative to low cost/fossil fuel sources of electricity, if the United States passes legislation to include the external costs of carbon in the price of electricity, electricity produced from solar energy would be very competitive. Once installed, solar thermal plants do not emit greenhouse gases or other harmful pollutants such as nitrous oxide, sulfur dioxide, mercury, and particulate matter. However, legislators, regulators, utilities and advocacy organizations argue about how to cost effectively meet carbon reduction goals and what those goals should be.
Since it takes energy to make energy, policy makers must take into consideration that solar panel production is not carbon free. When considering the carbon footprint of solar technologies, comparative studies look at the “energy payback time” which is dependent on both the type of solar as well as the location of the installation. Recent studies have shown this to be between 1 and 4 years dependent on the location’s average daily solar radiation.45
Storage is one of the biggest issues with both solar and wind development. The main types of storage that are under consideration are solar thermal and batteries. Solar thermal is the process of “harvesting” the sun’s heat through hundreds of mirrors and then storing that heat in molten salt for up to several days. The molten salt can then be used as a heat source for a traditional steam generator. According to a 2008 New York Times article interviewing John S. O’Donnell of the Ausra solar company, “a coffee thermos and a laptop computer’s battery store about the same amount of energy… the thermos costs about $5 and the laptop battery $150… that’s why solar thermal is going to be the dominant form.”46 The Solana Generating Station which is scheduled to open in 2012 will have containers on site that store molten salt in “large thermos-like buildings.”47
Figure 7: Concentrating Solar Power Resources in the United States. Map produced by the National Renewable Energy Laboratory in Golden, Colorado.
In the US, the best locations for photovoltaic and solar thermal installations are in southern California and Arizona as shown in maps produced by NREL. Although CSP plants use less water per kilowatt hour to produce electricity, they do require more land area than traditional power plants.48 In addition to the land requirements for solar thermal plants, these installations require a vast network of new transmission lines to bring these renewable resources to market. Limited populations make parts of Arizona and California attractive for solar development. However, due to the land requirements, many environmental groups have expressed concerns about the extent to which solar development might encroach on sensitive habitats, for example--the desert tortoise which is the Arizona state reptile and is protected under the federal Endangered Species Act.49
Figure 8: Solar radiation resources of the United States. May produced by the National Renewable Energy Laboratory in Golden, Colorado.
See these pages for more information on crystalline silicon solar cells (USDOE, 2005):
See these pages for more information on early architecture, passive solar design, and the concepts of direct and indirect gain:
See these pages for more information on state and federal policies and research and development of solar technologies:
APS website, Solana: “Frequently Asked Questions.” http://www.aps.com/main/green/Solana/FAQ.html (accessed on April 5, 2010).
Baker, David R., “Rebates for solar water heaters signed by governor,” The San Francisco Chronicle, October 13, 2007.
Behling, Stefan and Behling, Sophia. Solar Power: The evolution of sustainable architecture. Munich, Germany: Prestel, 2000.
Berman, Daniel M. & John T. O’Connor. Who Owns the Sun? People, Politics and the Struggle for a Solar Economy. White River Junction, Vermont: Chelsea Green Publishing Company, 1996.
Borowitz, Sidney. Farewell Fossil Fuels, Reviewing America’s Energy Policy. New York: Plenum Press, 1999.
Carless, Jennifer. Renewable Energy. New York: Walker Publishing Company, Inc., 1993.
Kaufman, Allan. Exploring Solar Energy: principles & projects. Ann Arbor, Michigan: Prakken Publications, Inc, 1992.
Mieszkowski, Katharine. “The tortoise and the sun.” Salon.com. http://www.salon.com/env/feature/2009/01/22/desert_tortoises/ (accessed on April 30,2009).
Miller, G. Tyler Jr. Environmental Science: Working with the Earth Fifth Edition. Belmont, California: Wadsworth Publishing Company, 1995.
National Photovoltaics Environmental Research Center. “Mission Statement.” http://www.bnl.gov/pv/mission.asp (accessed on April 26, 2010).
National Renewable Energy Laboratory. “Solar Heated Swimming Pool: Image # 09320.” http://www.nrel.gov/data/pix/searchpix.php?getrec=09320&display (accessed on March 22, 2010).
PhysOrg.com. “40% efficient solar cells to be used for solar electricity.” http://www.physorg.com/news99904887.html (accessed on April 9, 2010).
Radabaugh, Joseph. Heaven’s Flame: A Guide to Solar Cookers. Ashland, Oregon: Home Power Publishing, 1998.
Ramlow, Bob. Solar Water Heating: A comprehensive guide to solar water and space heating systems. Gabriola Island, British Columbia: New Society Publishers and Bob Ramlow and Benjamin Nusz, 2006.
Renewable Energy World. “US Government Budget Proposals Increase Clean Energy Funding.” http://www.renewableenergyworld.com/rea/news/article/2010/02/us-government-budget-proposals-increase-clean-energy-funding (accessed on April 26, 2010).
Science Daily. “Easing Concerns About Pollution From Manufacture Of Solar Cells.” http://www.sciencedaily.com/releases/2008/02/080225090826.htm (accessed on April 26, 2010).
Solar Energy International. Photovoltaics Design and Installation Manual: Renewable Energy for a Sustainable Future. (Gabriola Island, British Columbia: New Society Publishers), 2004.
Strum, Harvey. “Eisenhower’s Solar Energy Policy.” The Public Historian, Vol. 6 No. 2 (Spring 1984): 37-55.
The Energy Information Agency. Official Energy Statistics from the United States Government. http://www.eia.doe.gov/
The Pew Center on Global Climate Change. http://www.pewclimate.org/
The Solar Cooking Archive. “Horace de Saussure and his Hot Boxes of the 1700's.” http://solarcooking.org/saussure.htm (accessed on March 27, 2010).
Upson, Sanda. “How Free is Solar Energy?.” IEEE Spectrum, February 2008, http://spectrum.ieee.org/green-tech/solar/how-free-is-solar-energy (accessed on April 5, 2010)
US Department of Energy. “Database of State Incentives for Renewable Energy.” http://www.dsireusa.org/userfiles/image/summarymaps/solardgrpsmap.gif (accessed April 26, 2010).
U.S. Department of Energy, Energy Efficiency and Renewable Energy Solar Technologies Program. “The History of Solar.” 2002 http://www1.eere.energy.gov/solar/pdfs/solar_timeline.pdf (accessed on March 26, 2010).
U.S. Department of Energy, Energy Efficiency and Renewable Energy, Solar Technologies Program. “The Crystalline Silicon Solar Cell.” 2005 http://www1.eere.energy.gov/solar/crystalline_silicon_cell.html (accessed on March 26, 2010).
U.S. Department of Energy, Energy Efficiency and Renewable Energy, Solar Technologies Program. “The Photoelectric Effect.” 2006 http://www1.eere.energy.gov/solar/photoelectric_effect.html (accessed on March 26, 2010).
U.S. Department of Energy, Energy Efficiency and Renewable Energy, Solar Technologies Program. “Light and the Solar Cell.” 2008 http://www1.eere.energy.gov/solar/pv_cell_light.html (accessed on March 26, 2010).
Wald, Matthew L. “New Ways to Store Solar Energy for Nighttime and Cloudy Days.” New York Times, April 15, 2008.
Walisiewicz, M. Alternative Energy: A beginner's guide to the future of energy technology. New York: DK Publishing, Inc, 2002.
Watson, Ann. “Trombe Wall with Vents.” http://www.flickr.com/photos/mitopencourseware/3359628993/ (accessed on March 22, 2010).
Wisconsin K-12 Energy Education Program.Wisconsin K-12 Energy Education Program Student Book. Stevens Point, Wisconsin: Focus on Energy and the Wisconsin Center for Environmental Education, 2005.
Wisconsin Office of Energy Independence. “2009 Energy Statistics Book.” http://energyindependence.wi.gov/subcategory.asp?linksubcatid=3368&linkcatid=2847&linkid=1451&locid=160 (accessed on April 12, 2010).
Concentrating Solar Power (solar thermal) A technology that uses mirrors to concentrate the sun’s energy on a small surface (such as a tube filled with liquid) which transfers heat to a conventional power plant.
Direct solar gain The area to be heated actually receives the solar energy.
Feed-in tariffs A policy used to encourage small scale renewable energy development in which the host is paid a renewable energy compensation rate for putting electricity on the grid. Feed-in tariffs have been used to spur developments such as rooftop solar
Indirect solar gain A device, such as a masonary wall, is positioned between a source of sunlight and an area to be heated. The device collects the solar energy, stores it, and then later transmits the stored heat energy into the adjoining cooler area.
Insolation or incident solar radiation or solar energy potential The amount of solar radiation available in a particular location at a particular time. Insolation varies with latitude, cloud cover, time of year (which impacts the distance the Earth is from the sun), time of day, and topography of the land surface.
Passive solar energy system A system that uses direct gain devices (e.g., windows, walls and floors) and the principles of physics (e.g., gravity, convection, conduction, evaporation, etc.) to collect solar energy, convert it into thermal energy, store it and then distribute it as needed, without the use of other energy inputs.
Renewable Portfolio Standard (RPS) A piece of legislation that requires a state to obtain a certain percentage of its electric power from renewable resources
Solar radiation intensity The average amount of solar energy reaching a surface normal to the sun's rays at the Earth’s outer atmosphere at the average earth-sun distance of 92,955,888 miles; also known as the solar constant; equivalent to 436.5 Btu/ft.2/hr or 1377 watts/m2. [Based on definition in: Kaufman, Allan. (1992) Exploring Solar Energy: principles & projects. Ann Arbor, Michigan: Prakken Publications, Inc.
1 M. Walisiewicz, Alternative Energy: A beginner's guide to the future of energy technology (New York: DK Publishing, Inc, 2002), 46.
2 Allan Kaufman, Exploring Solar Energy: principles & projects (Ann Arbor, Michigan: Prakken Publications, Inc, 1992), 3.
3 G. Tyler Miller, Environmental Science: Working with the Earth Fifth Edition (Belmont, California: Wadsworth Publishing Company, 1995), 59.
4 US Department of Energy, 2008.
5 G. Tyler Miller, Environmental Science: Working with the Earth Fifth Edition (Belmont, California: Wadsworth Publishing Company, 1995), 59.
6 G. Tyler Miller, Environmental Science: Working with the Earth Fifth Edition (Belmont, California: Wadsworth Publishing Company, 1995), 59.
7 Jennifer Carless, Renewable Energy.(New York: Walker Publishing Company, Inc.,1993),11.
8 The Energy Information Agency. http://www.eia.doe.gov/oiaf/aeo/pdf/execsummary.pdf
9 Wisconsin K-12 Energy Education Program, Wisconsin K-12 Energy Education Program Student Book (Stevens Point, Wisconsin: Focus on Energy and the Wisconsin Center for Environmental Education, 2005), 324.
10 Wisconsin Office of Energy Independence, “2009 Energy Statistics Book,” http://energyindependence.wi.gov/subcategory.asp?linksubcatid=3368&linkcatid=2847&linkid=1451&locid=160.
11 Allan Kaufman, Exploring Solar Energy: principles & projects (Ann Arbor, Michigan: Prakken Publications, Inc, 1992), 18.
12 Joseph Radabought, Flame: A Guide to Solar Cookers (Ashland, Oregon: Home Power Publishing, 1998), 24.
13 Bob Ramlow, Solar Water Heating: A comprehensive guide to solar water and space heating systems (Gabriola Island, British Columbia: New Society Publishers and Bob Ramlow and Benjamin Nusz, 2006), 23.
14 Allan Kaufman, Exploring Solar Energy: principles & projects (Ann Arbor, Michigan: Prakken Publications, Inc, 1992), 11.
15 U.S. Department of Energy, Energy Efficiency and Renewable Energy, Solar Technologies Program The Photoelectric Effect 2006, http://www1.eere.energy.gov/solar/photoelectric_effect.html Emphasis added
16 M Walisiewicz, Alternative Energy: A beginner's guide to the future of energy technology (New York: DK Publishing, Inc, 2002), 50.
17 Allan Kaufman, Exploring Solar Energy: principles & projects (Ann Arbor, Michigan: Prakken Publications, Inc, 1992), 43.
19 Allan Kaufman, Exploring Solar Energy: principles & projects (Ann Arbor, Michigan: Prakken Publications, Inc, 1992), 13.
20 Jennifer Carless, Renewable Energy (New York: Walker Publishing Company, Inc.,1993), 20. However, in Farewell Fossil Fuels, author Sidney Borowitz notes that it is possible to 1000 degrees Fahrenheit but that to make electricity, temperatures lower than 1000 degrees will suffice. Page 110.
21 Jennifer Carless, Renewable Energy (New York: Walker Publishing Company, Inc.,1993), 21-22.
22 Stephan Behling and Sophia Behling, Solar Power: The evolution of sustainable architecture (Munich, Germany: Prestel, 2000), 93.
23 U.S. Department of Energy Energy Efficiency and Renewable Energy, Solar Technologies Program “The History of Solar,” http://www1.eere.energy.gov/solar/pdfs/solar_timeline.pdf.
24 U.S. Department of Energy, Energy Efficiency and Renewable Energy, Solar Technologies Program, Light and the Solar Cell, 2008, http://www1.eere.energy.gov/solar/pv_cell_light.html.
25 Solar Energy International. Photovoltaics Design and Installation Manual: Renewable Energy for a Sustainable Future (Gabriola Island, British Columbia: New Society Publishers, 2004), 2.
26 Solar Energy International. Photovoltaics Design and Installation Manual: Renewable Energy for a Sustainable Future (Gabriola Island, British Columbia: New Society Publishers, 2004), 2.
27 Solar Energy International. Photovoltaics Design and Installation Manual: Renewable Energy for a Sustainable Future (Gabriola Island, British Columbia: New Society Publishers, 2004), 2.
28 U.S. Department of Energy, Energy Efficiency and Renewable Energy Solar Technologies Program, The History of Solar, http://www1.eere.energy.gov/solar/pdfs/solar_timeline.pdf
29 Solar Energy International. Photovoltaics Design and Installation Manual: Renewable Energy for a Sustainable Future (Gabriola Island, British Columbia: New Society Publishers, 2004), 2.
30 Solar Energy International. Photovoltaics Design and Installation Manual: Renewable Energy for a Sustainable Future (Gabriola Island, British Columbia: New Society Publishers, 2004), 2.
31 M Walisiewicz, Alternative Energy: A beginner's guide to the future of energy technology (New York: DK Publishing, Inc, 2002), 51.
32 Harvey Strum. “Eisenhower’s Solar Energy Policy.” The Public Historian, Vol. 6 No. 2 (Spring 1984): 37-55.
33 Harvey Strum. “Eisenhower’s Solar Energy Policy.” The Public Historian, Vol. 6 No. 2 (Spring 1984): 37-55.
34 Harvey Strum. “Eisenhower’s Solar Energy Policy.” The Public Historian, Vol. 6 No. 2 (Spring 1984): 37-55.
35 Daniel M. Berman & John T. O’Connor. Who Owns the Sun? People, Politics and the Struggle for a Solar Economy (White River Junction, Vermont: Chelsea Green Publishing Company, 1996), 137.
36 Daniel M. Berman & John T. O’Connor. Who Owns the Sun? People, Politics and the Struggle for a Solar Economy (White River Junction, Vermont: Chelsea Green Publishing Company, 1996), 194.
37 Renewable Energy World, “US Government Budget Proposals Increase Clean Energy Funding,” http://www.renewableenergyworld.com/rea/news/article/2010/02/us-government-budget-proposals-increase-clean-energy-funding.
38 Daniel M. Berman & John T. O’Connor. Who Owns the Sun? People, Politics and the Struggle for a Solar Economy (White River Junction, Vermont: Chelsea Green Publishing Company, 1996), 191.
39 Science Daily, “Easing Concerns About Pollution From Manufacture Of Solar Cells,” http://www.sciencedaily.com/releases/2008/02/080225090826.htm.
41 US Department of Energy, “Energy Efficiency and Renewable Energy statistics,” http://apps1.eere.energy.gov/states/maps/renewable_portfolio_states.cfm.
42 US Department of Energy DSIRE, http://www.dsireusa.org/userfiles/image/summarymaps/solardgrpsmap.gif.
43 United States Department of Energy, Energy Star Program. http://www.energystar.gov/index.cfm?c=products.pr_tax_credits.
44 David R. Baker, “Rebates for solar water heaters signed by governor,” The San Francisco Chronicle, October 13, 2007, Business section,.http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/10/13/BUDCSP9QV.DTL
45 Sandra Upson, “How Free is Solar Energy?” (IEEE Spectrum, February 2008), http://spectrum.ieee.org/green-tech/solar/how-free-is-solar-energy. (This statistic refers to a Brookhaven National Laboratory study.)
46 Matthew L. Wald “New Ways to Store Solar Energy for Nighttime and Cloudy Days,” The New York Times, April 15, 2008 http://www.nytimes.com/2008/04/15/science/earth/15sola.html.
48 CSP plants require land resources at the site of the plant but do not require additional land for mining coal and natural gas.
49 Katharine Mieszkowski. “The tortoise and the sun,” Salon.com,http://www.salon.com/env/feature/2009/01/22/desert_tortoises/.