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In a broad sense, most energy that individuals use is some form of solar energy. Other renewable energy sources (such as wind, hydropower, and wood) indirectly harness solar energy by using the atmosphere, oceans, and forests as solar collectors. Even exhaustible fossil fuels (oil, coal, and natural gas) are solar energy that was originally captured by plants and concentrated by geological processes into forms with high energy densities per unit of weight and volume.
In more common usage, solar energy refers to the two primary ways in which people harness and directly use solar energy using manufactured collectors: heating, and generating electricity.
Space and water heating systems for buildings can be either passive or active. Both approaches use glass to trap heat, as in a greenhouse. Passive design uses no moving parts or fluids; rather, it involves incorporating features into the siting and design of a building to take advantage of the natural solar radiation available. Such features include large windows facing south, heat-absorbent material such as brick or tile in floors and walls, and orienting a building on its site so as to maximize sun exposure.
Active heating systems use water or another liquid piped through collector units. The most common type of collector is a roof-mounted flat-plate design, consisting of an insulated glass-covered box painted black to maximize heat absorption. Water circulates in a loop between the collectors, where it is heated, and a tank, where it is stored until needed for either domestic uses or space heating.
There are two technologies for converting solar energy to electricity. Solar thermal-electric power plants (also called concentrating-solar-power, or CSP, power plants) use mirrors to gather solar radiation and focus it on a small area to produce high temperatures. The concentrating collectors may be parabolic troughs or dishes, or a system of mirrors that are spread over a wide area and that focus sunlight on a receiver at the top of a tower in what is called a power tower or central receiver system. A fluid circulates through a receiver unit at the parabola’s focal point, where it is boiled. The resulting steam drives a generator as in a conventional power plant. Unlike solar-heating systems, which are installed at the point of energy consumption, CSP plants are typically large, central-station generating facilities.
The other solar-electric technology is photovoltaic cells. Photovoltaic cells are made of a semiconducting material, such as silicon, that releases electrons when struck by light. Cells are typically combined into modules, which in turn are assembled into larger arrays. Arrays can be sized for residential, industrial, or electric-utility use. The most commonly used material is crystalline silicon, but research since the 1970s has produced advances in such newer designs as thin-film cells using noncrystalline (amorphous) silicon, cadmium telluride, and other materials.
Interest in solar energy was stimulated in the 1970s by high oil prices and has been further stimulated by government policies, such as tax credits. Enthusiasm diminished in the 1980s and 1990s as the prices of oil and natural gas fell and many government subsidies lapsed. After the late 1990s interest was renewed by rising energy prices, but the use of solar energy remains limited. In Renewable Energy (2002), the International Energy Agency estimates that in the year 2000, solar heating made up 0.3 percent of world energy consumption and photovoltaic cells contributed less than 0.05 percent.
The major impediment to solar energy is cost. Though solar radiation is abundant and nonpolluting, the equipment required to gather and utilize it is expensive. Solar heating systems have found some commercial adoption in sunny locations for certain applications, especially for heating swimming pools. CSP technologies, though technologically proven, are not yet competitive with other sources of electricity. Perhaps the most promising technology is photovoltaics. By 2002, photovoltaic costs had fallen to about 20 to 30 percent of their 1980 levels. They have become cost-effective in some specialized applications, particularly in remote locations far from existing power lines. From 1992 to 2003, installed photovoltaic capacity worldwide grew by about 30 percent annually.
Economic theory predicts that as exhaustible energy resources are depleted, their prices will tend to rise, making renewable sources more attractive over time. The long-run prospects for solar energy will depend on how its cost compares with other energy sources.
For more information on solar technologies and research, see the Web sites for the International Energy Agency and the U.S. Department of Energy’s National Renewable Energy Laboratory. On the economics of solar and other energy sources, see Economics of the Energy Industries, by William Spangar Peirce (1996).
- International Energy Agency. 2002. Renewable Energy. http://www.iea.org/.
- International Energy Agency. 2004. Trends in Photovoltaic Applications: Survey Report of Selected IEA Countries between 1992 and 2003. Report IEA-PVPS T1-13:2004. http://www.iea.org/.
- National Renewable Energy Laboratory. Solar Research. U.S. Department of Energy. http://www.nrel.gov/solar/.
- Peirce, William Spangar. 1996. Economics of the Energy Industries.2nd ed. Westport, CT: Praeger Publishers.
- Renewable Energy Working Party. 2002. Renewable Energy … into the Mainstream. International Energy Agency. http://www.iea.org/.
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