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 ). 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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