Solar Energy. Solar radiation is an energy resource many times larger than humanity's energy needs.

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1 Solar Energy Solar Energy. Solar radiation is an energy resource many times larger than humanity's energy needs. Humankind has been able to make use of this resource only on a limited scale, mainly for drying crops and producing salt from brines. However, environmental pollution, increasing energy demands, and rising costs of fuels have led to considerable interest in solar energy for additional uses, not only in highly industrialized but also in developing countries. The sun is a sphere of intensely hot gaseous matter. In its interior a continuous fusion process occurs in which hydrogen nuclei combine to form helium nuclei. The mass of the helium is less than that of the hydrogen, so some mass is always being converted to energy. This energy is radiated out from the surface of the sun, and an extremely small fraction of it is intercepted by the earth. The average intensity of this radiation just outside the earth's atmosphere 93 million miles ( million km) from the sun is 1.36 kilowatts per square meter (436 Btu/hr.ft. 2 ). This value is called the solar constant. Solar energy is absorbed, scattered, and reflected by the atmosphere. It reaches the ground as diffuse radiation, which comes from many directions, or as beam radiation, which comes from the direction of the sun. On a very clear day the atmosphere may transmit 90% of the solar radiation. Most of this transmitted radiation is beam radiation. On a very cloudy day the atmosphere may transmit less than 10% of the solar radiation. All of this transmitted radiation is diffuse radiation. The solar energy reaching the ground has a maximum intensity of about 1.2 kilowatts per square meter, or about 1.3 horsepower per square yard. Almost half this solar energy is visible light, about half is near infrared radiation, and a very small percentage is in the form of ultraviolet radiation. Chemical and Biological Conversion of Solar Energy Solar radiation can be converted to chemical energy by photochemical processes. An ideal photochemical reaction would absorb as much as possible of the solar energy spectrum in bringing about an energy-storing (endothermic) chemical reaction. For practical applications, it must also be possible to reverse the reaction to recover the energy and save the chemicals for reuse. The reaction might produce a single compound that could be stored and reacted at a later time, or the reaction might yield two or more products that could be separated and later recombined to recover energy. The chemicals in the reaction must be inexpensive and easy to handle. Photosynthesis

2 Photosynthesis in plants meets these requirements as closely as any photochemical process now known. Solar energy is absorbed by plants containing chlorophyll. The plants use this energy to convert carbon dioxide and water to carbohydrates and oxygen. The carbohydrates are stored as fuel or food. When they are oxidized later in a furnace or an animal, energy previously absorbed from the sun is recovered, and carbon dioxide and water are produced. Photosynthesis is the natural means by which all conventional fuels have been produced, with the products modified and stored in nature as oil, coal, natural gas, wood, or other materials derived from plant residues. However, photosynthesis is inefficient. The conversion of a year's solar radiation to plant material usually occurs with an efficiency much less than 1%. Photosynthesis by algae can provide much better efficiency in the use of sunlight. Algae are tiny plants that grow in suspension in water. They contain carbohydrates, proteins, and fats useful in nutrition. The algae can be cultured in ponds or plastic bags and harvested. However, there are problems in separating the tiny plants from the water, in controlling the temperature of covered ponds to avoid overheating, and in avoiding biological contamination. Algal culture is practiced in Japan and a few other countries where the dried algae are used for human food. Conversion of Solar Energy to Thermal Energy Solar radiation of all wavelengths can readily be converted to thermal energy by using a black surface to absorb the radiant energy. Two kinds of devices are used to intercept solar radiation and convert it to thermal energy: a flat-plate collector and a focusing collector. Flat-Plate Collectors A flat-plate collector uses a blackened plate and tubes or fins arranged so that a fluid can pass by the plate and pick up heat from it. The front side of the plate has one or more transparent covers that admit solar energy and also provide thermal insulation. The back and sides of the plate also are insulated to reduce heat losses from the plate. A flat-plate collector is mounted in a fixed position, and it absorbs both beam and diffuse radiation. It can operate at temperatures up to about 125 F (70 C) above the daytime air temperature at the site. A flat-plate collector is a practical device. Its heated surface can be used for water heating, house heating, air conditioning, evaporation of salt water, and distillation of salt or brackish water to produce drinking water.

3 Solar Water Heaters. In its most common form, a solar water heater uses a flat-plate collector with tubes fastened to the blackened flat plate. Water flows from a storage tank, passes through the tubes where it is heated, and returns to the tank. Hot water accumulates in the upper part of the tank, which is insulated to reduce heat losses. Hot water is then withdrawn from the tank when needed. In household systems, the storage tank usually is located above the collector, and the upward water circulation depends on natural convection. A home installation might include a collector with an area of 25 to 50 square feet (2.5 to 5 sq meters) and an insulated storage tank with a capacity of 50 to 100 gallons (200 to 400 liters). Larger heaters for schools, hospitals, and similar buildings use pumps to circulate water. Solar water heaters of these and other designs are widely used in Australia, Japan, and Israel. Some are used in Florida and other states where the weather is good and conventional energy costs are high. Solar House Heating. A solar house heating system uses large arrays of flat-plate collectors and some kind of energy storage unit. The collectors can heat water or air, and the heat can be stored in insulated water tanks or in pebble beds. Energy is withdrawn from the storage unit when needed in the house, as, for example, at night, when solar energy is not available. In climates with long and uncertain sequences of cold and cloudy days, it is impractical to provide all the annual heating needs by means of solar energy. Consequently, most solar houses also have a conventional furnace to provide heat at times when solar energy is not available and the energy storage unit is depleted. However, a solar house heating system can provide most of the annual heating needs. Although solar house heating is still experimental, about 20 solar houses have been built and operated. Solar heating may become important in the United States, particularly for new houses in moderate climates. Air Conditioning. Solar cooling uses a flat-plate collector to supply heat to operate an absorption airconditioner. (See also Refrigeration.) The air-conditioner can then cool a building in the conventional way. Only a few experiments with solar cooling have been made. Solar Evaporation. Solar energy is used to produce salt from salt water. In solar evaporation, brines are placed in large open shallow ponds or pans from which water evaporates. The brines become concentrated, and crystallization occurs. After all the water is driven off, the salt is harvested mechanically. Solar evaporation, an ancient art, now is a large-scale industrial process. In the United States, the annual production of "solar salt" is about 2 million tons, about 4.5% of the total U.S. salt production.

4 Solar Distillation. Solar energy also is used to produce drinkable water from salt water. In the most common solar still, brines are put in black pans that have transparent sloping covers made of glass or plastic. Solar energy enters through the cover and is absorbed in the basin. The absorbed solar energy evaporates water from the brine, leaving salt water in the basin. The water vapor rises inside the still and comes in contact with the cover, where it condenses. The fresh water runs down the sloping underside of the cover to collection troughs at the edges of the cover and then goes to a storage tank, ready for use. On a good day a solar still can produce about one pound of water per square foot (5 kgm/sq meter) of basin area. The output of a still depends on the intensity of the radiation, which in turn depends on the time of year. Solar stills have been built in Spain, Greece, Australia, and Latin America for small community water supplies. They range in area from a few hundred to about 100,000 square feet (9,300 sq meters). Focusing Collectors A focusing collector uses a concave reflector or lens to concentrate solar beam radiation, thereby raising its intensity by from 2 to 10,000 times. The focused radiation is absorbed on a blackened small receiver, which may be insulated to reduce thermal losses. The higher energy flux at the receiver allows energy collection at temperatures ranging from about 200 F (100 C) to about 5000 F (2760 C). A focusing collector must be movable so that it can be oriented to gather sunlight. No long-time practical application of focusing systems have yet been made, partly because it is difficult to maintain a highly reflective surface in the atmosphere over a long period of time. However, focusing collectors have been used in short-term applications involving very high temperatures. In such cases, they are called solar furnaces. Solar Furnaces. A solar furnace is a precise optical instrument for concentrating beam radiation on very small targets to produce high-energy fluxes and high temperatures. A solar furnace usually consists of an accurate parabolic reflector to focus the radiation, a tracking system to align the axis of the paraboloid and the incoming beam radiation, a shutter mechanism to control the radiation, and a receiver or target-holding device. Solar furnaces are useful in studying the high-temperature properties of materials such as metal oxides and in exposing materials to intense thermal shock. Many small solar furnaces have been built from five-foot-diameter searchlight reflectors. The largest solar furnace is at Odeillo in the Pyrenees Mountains of France. It is made up of 9,600 curved glass reflectors and has

5 an area of about 20,000 square feet (1,860 sq meters). Solar radiation is reflected into it from 63 large movable flat mirrors arrayed on a hillside. The system can deliver 1,000 kilowatts to the material being heated. Conversion of Solar Energy to Electrical Energy Much of the world's energy needs must be met by using energy in its electrical or mechanical form. However, conversion of solar energy to electrical or mechanical energy is much more difficult to do economically than is the conversion of solar energy to thermal energy. Solar Cells A solar cell is a solid-state device that directly converts the energy of solar radiation to electrical energy. (See also Photoelectric Cell.) Solar cells have been developed primarily for providing electric power for spacecraft. They have no moving parts and thus are free from the mechanical problems that arise in converters using engines. Solar cells work as well in small sizes as in large, and they operate for very long times if protected from damage. Most solar cells have been made of silicon. They can convert solar energy to electrical energy at efficiencies of 12% to 14%. Solar cells made of cadmium sulfide can be made thinner and lighter than silicon cells, but they have a maximum efficiency of about 6%. Applications of solar cells on earth have been limited because the cost of electricity from solar cells is more than 100 times greater than the cost of electricity from conventional sources. Nevertheless, solar cells have been used for special applications such as small electric power supplies for radios, telephones, and other communications equipment. They also have been used to provide electric power for lighthouses and buoys. In most of these applications, the power needs are small (less than a kilowatt), and mobility or isolation makes it impractical to draw power from central stations. Further research may bring costs down, as, for instance, by developing new processes for the manufacture of silicon cells or by developing new solar cell materials such as cadmium sulfide. Another possibility is the use of solar cells in space, where their power output is higher. In one plan, a solar-cell power plant in a synchronous orbit would transmit power to earth by using a microwave beam. Heat Engines A heat engine can be used to convert solar energy to mechanical energy, which can then be used directly or converted to electrical energy by conventional techniques. Collectors are used to heat a working fluid such as

6 steam, which then passes through an engine where part of its energy is converted to mechanical energy. The higher the temperature of the fluid supplied to the engine from the collector, the more efficient is the engine. On the other hand, collectors work best at lower temperatures. Flat-plate collectors are practical, but they are low-temperature devices. Thus, supplying energy from them to engines means low engine efficiencies and very large collectors to achieve useful power outputs. Focusing collectors can heat fluids to higher temperatures and provide better engine efficiencies, but practical and economical focusing collectors are yet to be developed. The successful development of solar power systems could open many new applications and meet many energy needs. On a very large scale, solar energy might contribute to regional or national electric power generating capacity. On a smaller scale, important needs such as pumping irrigation water might be met. Historical Background The ancient Chinese, Egyptians, Phoenicians, Greeks, and Romans used solar energy to evaporate salt water to produce salt. Solar crop drying is also an ancient art. The first major solar still was built at Las Salinas, Chile, in It operated for about 30 years, had a total area of more than an acre (about 4,000 sq meters) and was used to produce drinking water for animals used in nitrate mining. In 1913 in Egypt, a 13,269-square-foot (1,233-sq-meter) parabolic collector provided steam to run an engine for pumping irrigation water from the Nile. One of the first solar-heated houses was built by Massachusetts Institute of Technology at Cambridge, Mass., in It was the first in a series of experimental solar houses. Studies of the performance of solar collectors by the American scientists H. C. Hottel and B. B. Woertz in 1942 led to the design of practical flat-plate collectors. Successful silicon solar cells were announced by the American scientists D. M. Chapin, C. S. Fuller, and G. L. Pearson in John A. Duffie University of Wisconsin Further Reading Artz, Robert J., and J. Robert Walker, Solar Energy (Reston 1984). Balcomb, J. Douglas, ed., Passive Solar Buildings (MIT Press 1992).

7 Daniels, Farrington, Direct Use of the Sun's Energy (Yale Univ. Press 1983). Greenwald, Martin L., and Thomas K. McHugh, Practical Solar Energy Technology (Prentice-Hall 1985). Zweibel, Ken, Harnessing Solar Power: The Photovoltaics Challenge (Plenum Press 1990). How to cite this article: MLA (Modern Language Association) style: Duffie, John A. "Solar Energy." Encyclopedia Americana Grolier Online. 22 Apr < APA (American Psychological Association) style: Duffie, J. A. (2009). Solar Energy. Encyclopedia Americana. Retrieved April 22, 2009, from Grolier Online Chicago Manual of Style: Duffie, John A. "Solar Energy." Encyclopedia Americana. Grolier Online (accessed April 22, 2009). Source: Encyclopedia Americana