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Solar Energy

Solar Energy

BIBLIOGRAPHY

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 parabolas 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 longrun 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 Energys National Renewable Energy Laboratory. On the economics of solar and other energy sources, see Economics of the Energy Industries, by William Spangar Peirce (1996).

SEE ALSO Energy; Energy Sector

BIBLIOGRAPHY

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 T113:2004. 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/.

Steven E. Henson

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Solar Energy

Solar energy

Earth's surface receives energy from processes in Earth's interior and from the Sun . Heat from the interior comes from radioactive elements in the mantle and core, tidal kneading by the Moon and Sun, and residual heat from the earth's formation. This interior heat is radiated through the surface at a global rate of 3 × 1013 watts (W)about .07 W per square yard (.06 W/m2). The Sun, in contrast, provides 1.73 × 1017 W, 5,700 times more power than Earth radiates from within and about 30,000 times more than is released by all human activity. Clouds , air, land, and sea absorb 69% of the energy arriving from the Sun and reflect the rest back into space . The ocean, which covers about 70% of the earth's surface, does about 70% of the absorbing of solar energy.

Between its absorption as heat and its final return to space as infrared radiation, solar energy takes many forms, including kinetic energy in flowing air and water or latent heat in evaporated water. Solar energy keeps the oceans and atmosphere from freezing and drives all winds and currents. A small fraction of Earth's solar energy income is intercepted by green plants, providing the flow of food energy that sustains most

earthly life. Only a few organisms, including thermophilic bacteria infiltrating the crust and organisms specialized to live in the vicinity of hydothermal deep-sea vents, derive their energy from Earth's interior rather than from the Sun.

Regional variations in solar input contribute to weather patterns and seasonal changes. On average Earth's surface is more nearly at a right angle to the Sun's rays near the equator, so the tropics absorb more solar energy than the higher latitudes. This creates an energy imbalance between the equator and the poles, an imbalance that the circulation of the atmosphere and oceans redress by transporting energy away from the equator. During each half of the year the daylight side of each hemisphere is tilted at a steeper angle to the sun than during the other half, and so intercepts less solar energy; this results in seasonal climatic changes.

Solar energy is also of technological importance. Utilization of the Sun as an energy source has been routine on spacecraft for decades and is becoming more frequent on the ground. Electromagnetic radiation from the Sun, unlike the major conventional power sources, produces no smokestack emissions, greenhouse gases , or radioactive wastes; and its production cannot be manipulated for profit or political leverage. On the down side, sunlight is a diffuse or spread-out energy source compared to any fuel and is directly available only during the day. Yet, even at high latitudes in Europe and North America , where most of the world's energy is consumed, the ground receives from the Sun a long-term average of 83.6 W per square yard (100 W/m2). This average is inclusive of "dark" hours. Both indirect and direct harvesting of this energy income is possible. Indirect solar schemes, including wind power, wood heat, and the burning of alcohol, methane, or hydrogen, run on energy derived at second hand from sunlight. Direct schemes use sunlight as such to heat buildings or water, generate electricity , or supply high-temperature process heat to industrial systems.

Because conventional electricity generation is expensive and polluting, much effort has been devoted to solar electricity generation. Electricity can be generated from sunlight either thermally or photovoltaically. Thermal methods focus the Sun's rays on looped pipes through which molten salt, hot air, or steam flows. This hot fluid is then used either at first or second hand to run generators, much as heat from coal or nuclear fuel is used in conventional power plants. Photovoltaic electrical generation depends on flat, specially designed transistors (solar cells) that convert incident light to electricity. At 83.6 W/yard2 (100 W/m2) average solar input, 38 square yards (32 m2) of 33% efficient solar cellsa square 18 feet (5.5 m) on a sidecould supply 800 kilowatt-hours of electricity per

month, the approximate usage of the average U.S. household. An efficiency of 32.3% has been demonstrated in the laboratory, but most commercial photovoltaic cells are only about 10% efficient. Unlike the unused heat from a ton of coal or uranium, however, the sunlight not converted to electricity by a solar cell entails neither monetary cost nor pollution, and so cannot be viewed as waste.

Despite its obvious advantages, photovoltaic electricity generation has long been limited to specialized off-grid applications by the high cost of solar cells. However, cell prices have fallen steadily, and several large-scale photovoltaic electricity projects are now under way in the U.S. and elsewhere.

See also Atmospheric circulation; Coronal ejections and magnetic storms; Energy transformations; Global warming; Insolation and total solar irradiation; Meteorology; Ocean circulation and currents; Seasonal winds; Solar illumination: Seasonal and diurnal patterns; Solar sunspot cycles; Sun; Ultraviolet rays and radiation

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solar energy

solar energy, any form of energy radiated by the sun, including light, radio waves, and X rays, although the term usually refers to the visible light of the sun. Solar energy is needed by green plants for the process of photosynthesis, which is the ultimate source of all food. The energy in fossil fuels (e.g., coal and oil) and other organic fuels (e.g., wood) is derived from solar energy. Difficulties with these fuels have led to the invention of devices that directly convert solar energy into usable forms of energy, such as electricity.

Solar batteries, which operate on the principle that light falling on photosensitive substances causes a flow of electricity, play an important part in space satellites and, as they become more efficient, are finding increasing use on the earth (see solar cell). Thermoelectric generators convert the heat generated by solar energy directly into electricity (see thermoelectricity). Several projects have produced electricity on a large scale by using the solar energy available in desert areas. In one system, large numbers of solar batteries generate electricity for Coconut Island, off the coast of Australia. In another, oil flows through pipes that are set in reflecting parabolic troughs that can trap the heat from sunlight falling on them. The heat from the oil is then converted into electricity (see power, electric) using a steam turbine. Another system uses mirrors to focus solar radiation on a tower where water or salts are heated to high temperatures; in both cases electricity is ultimately produced using a steam turbine.

Heat from the sun is used in air-drying a variety of materials and in producing salt by the evaporation of seawater. Solar heating systems can supply heat and hot water for domestic use; heat collected in special plates on the roof of a house is stored in rocks or water held in a large container. Such systems, however, usually require a conventional heater to supplement them. Solar stoves, which focus the sun's heat directly, are employed in regions where there is much perennial sunlight. See also energy, sources of.

See F. Daniels, Direct Use of the Sun's Energy (1964, repr. 1974).

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solar energy

solar energy Heat and light from the Sun consisting of electromagnetic radiation, including heat (infrared rays), light and radio waves. About 35% of the energy reaching the Earth is absorbed; most is spent evaporating moisture into clouds, and some is converted into organic chemical energy by photosynthesis in plants. All forms of energy (except nuclear energy) on Earth come ultimately from the Sun. Solar cells are used to power instruments on spacecraft, and experiments are being conducted to store solar energy in liquids from which electricity can be generated.

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