SOLAR ENERGY

radiation from the Sun capable of producing heat, causing chemical reactions, or generating electricity. The Sun is an extremely powerful energy source, and solar radiation is by far the largest source of energy received by the Earth, but its intensity at the Earth's surface is actually quite low. This is partly because the Earth's atmosphere and its clouds absorb or scatter as much as 54 percent of all incoming sunlight. Despite this, in the 20th century solar energy became increasingly attractive as an energy source owing to its inexhaustible supply and its nonpolluting character, which are in stark contrast to such fossil-fuel sources as coal, oil, and natural gas. The sunlight that reaches the ground consists of nearly 50 percent visible light, 45 percent infrared radiation, and smaller amounts of ultraviolet light and other forms of electromagnetic radiation. This radiation can be converted either into thermal energy (heat) or into electrical energy, though the former is easier to accomplish. Two main types of devices are used to capture solar energy and convert it to thermal energy: flat-plate collectors and concentrating collectors. Because the intensity of solar radiation at the Earth's surface is so low, both types of collectors must be large in area. Even in sunny parts of the world's temperate regions, for instance, a collector must have a surface area of about 430 square feet (40 square m) to gather enough energy to serve one person for one day. The most widely used flat-plate collectors consist of a blackened metal plate, covered with one or two sheets of glass, that is heated by the sunlight falling on it. This heat is then transferred to air or to water, called carrier fluids, that flows past the back of the plate. The heat may be used directly or it may be transferred to another medium for storage. Flat-plate collectors are commonly used for hot-water heating and house heating. The storage of heat for use at night or during cloudy days is commonly accomplished by using insulated tanks to store the water heated during sunny periods. Such a system can supply a home with hot water drawn from the storage tank or, with the warmed water flowing through tubes in floors and ceilings, it can provide space heating. Flat-plate collectors typically heat carrier fluids to temperatures ranging from 66 to 93 C (150 to 200 F). The efficiency of such collectors ( i.e., the proportion of the energy received that they convert into usable energy) ranges from 20 to 80 percent, depending on the design of the collector. (See also solar heating.) When higher temperatures are needed, a concentrating, or focusing, collector is used. These devices reflect sunlight from a wide area and concentrate it onto a small blackened receiver, thereby considerably increasing the light's intensity in order to produce high temperatures. The arrays of carefully aligned mirrors used in these so-called solar furnaces can focus enough sunlight to heat a target to temperatures of 2,000 C (3,600 F) or more. This heat can be used to study the properties of materials at high temperatures, or it can be used to operate a boiler, which in turn generates steam for a steam-turbineelectric-generator power plant. The solar furnace has become an important tool in high-temperature research. For producing steam, the movable mirrors are so arranged as to concentrate large amounts of solar radiation upon blackened pipes through which water is circulated and thereby heated. Solar radiation may be converted directly into electricity by photovoltaic cells. In such cells, a small electrical voltage is generated when light strikes the junction between a metal and a semiconductor (such as silicon) or a junction between two different semiconductors. (See photovoltaic effect.) The voltage generated from a single photovoltaic cell is typically only a fraction of a volt. By connecting large numbers of individual cells together, however, as in modern solar batteries, more than one kilowatt of electric power can be generated. The energy efficiency of most present-day photovoltaic cells is only about 7 to 11 percent; i.e., only that fraction of the radiant energy received is converted to electrical energy. And since the intensity of solar radiation is low to begin with, huge and costly assemblies of such cells are required to produce even moderate amounts of power. Consequently, photovoltaic cells that operate on solar light have so far been used mainly for low-power applicationsas power sources for calculators and watches, for example. Larger units have been used to provide power for weather and communications satellites. Solar energy is also used on a small scale for other purposes besides those described heretofore. In some countries, for instance, specially designed solar ovens are employed for cooking, and solar energy is used to produce salt from seawater by evaporation. The potential for solar energy is enormous, since each day the Earth receives in the form of solar energy about 200,000 times the total world electrical-generating capacity. Unfortunately, though solar energy itself is free, the high cost of its collection, conversion, and storage has limited its exploitation.