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Solar energy

Radiation from the Sun can produce heat, generate electricity, or cause chemical reactions. Solar energy is inexhaustible and nonpolluting, but converting solar radiation to electricity is not yet commercially competitive. Solar batteries play an important part in space satellites. 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.

Wind power

Like waterwheels, windmills were among the original prime movers that replaced human beings as a source of power. Their most important traditional use was for grinding grain, though in certain areas their use in land drainage and water pumping was equally important. Windmill use became increasingly widespread in Europe (particularly the Netherlands) from the 12th century to the early 19th century, but thereafter slowly declined. Interest in windmills for generating electric power revived in the 1970s. Though wind is irregular and spread out, it contains tremendous amounts of energy. Sophisticated wind turbines have been developed to convert this energy to electric power.

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