Solar Energy, the energy generated by the sun. This energy is in the form of electromagnetic radiation and travels to the earth in waves of various lengths. Some of the radiation becomes evident as heat, some as visible light. All life on earth depends ultimately on the sun's radiation. It warms the earth and provides the energy that green plants use to make their food. (Without plants, there would be no animals, since all animals must feed on plants or on plant-eating organisms.)

Since ancient times attempts have been made—with varying success—to put the energy from the sun to practical use. In the third century B.C., the Greek mathematician and physicist Archimedes is said to have used the sun's rays reflected from mirrors to set fire to an invading Roman fleet. In the 19th century, John Ericsson, designer of the ironclad warship Monitor, built an engine that was powered by the sun's energy.

Solar Heat

Solar heat supplies energy for a variety of uses. The preservation of fruits, vegetables, meat, and fish by sun drying has been practiced for centuries. Some industrial products are also dried by the heat of the sun. In some warm, arid regions, the heat of the sun is used to evaporate seawater or brines to recover salt and other minerals.

Water for domestic use can be heated by solar energy by the use of roof-mounted devices consisting of heat collectors through which water pipes pass. As the water is heated it flows into storage tanks. Heat collectors can also be used to heat homes and other buildings. The sun's heat is transferred to a fluid—usually water or air—which then heats the interior of the building. For heating at night and on cloudy days, some form of heat storage is necessary. A common storage system consists of an insulated tank to hold solar-heated water. In many regions, additional heat from a conventional heating system is required for extended cloudy or cold periods.

Industrial installations that use large arrays of mirrors to produce intense solar heating have been developed in a number of countries. A large solar furnace at Odeillo, in the French Pyrenees, uses an array of thousands of movable mirrors to direct sunlight on a parabolic mirror. This mirror focuses the sunlight on an oven, yielding temperatures of more than 6,000° F. (3,300° C.). The furnace is used to study the effects of high temperatures on certain substances and for various industrial processes.

In the southwestern United States, a few experimental installations have been built that use a large array of computer-controlled mirrors to concentrate sunlight onto a boiler atop a high tower. Steam produced in the boiler powers a turbine that generates electricity.

Photovoltaic, or Solar, Cells

Photovoltaic cells convert sunlight directly into electricity. The cells are made of a semiconductor material, usually silicon. A solar battery consists of an array of solar cells connected together to generate electric power.

Solar batteries are the source of power on most artificial satellites. Solar batteries are used in remote locations as a source of power for navigational buoys, irrigation pumps, and other equipment. Small solar batteries are used in some calculators and wrist watches.

To a very limited extent solar batteries have been used to supply electric power to businesses and residences. However, photovoltaic cells are relatively costly to manufacture and are thus not practical for generating large amounts of electricity commercially. Research in the use of photovoltaic cells for solar energy is directed toward finding ways of increasing the efficiency of the cells and of reducing their cost.