Solar energy is a term that encompasses a broad range of energy forms. The Sun's energy profoundly affects the world's wind patterns, causes ocean water to evaporate as part of the hydrologic cycle, and is essential for plant growth. The winds may be used to turn windmills (see \Twindmills and windpower\t); the hydrologic cycle makes \Thydroelectric power\t possible; and vegetation, which grows in the presence of sunlight, may be burned directly--for instance, as wood in a stove or fireplace--or processed into fuels, such as the fermentation of grain to produce ethyl alcohol, which can be used alone or mixed with gasoline (see \Tgasohol\t) to power an internal-combustion engine. Solar energy also makes it possible to harness \Tocean thermal energy\t, which uses the temperature difference between Sun-warmed surface water and cold water from the ocean depths to produce power. Although all these indirect means of using energy from the Sun are considered solar technologies, the term solar energy most commonly refers to direct use of the Sun's energy: by means of photovoltaic \Lsolar cell\ls, which convert sunlight into electricity (see \Tphotoelectric effect\t), and by the use of various devices and techniques to convert solar radiation to heat that may be used for heating and cooling buildings; providing hot water for homes, businesses, and industry; and generating electricity by thermal means. POWER GENERATION Besides the direct conversion of sunlight into electricity by means of photovoltaic cells, sunlight may be used to generate steam, which can then be used to power a turbine for producing electricity. One solar-thermal design, called a power tower, consists of a field of movable mirrors that surround a tower, at the top of which is a boiler. The mirrors, called heliostats, track the Sun so as to constantly focus its light on the boiler. At the bottom of the tower is a building that houses a turbine-generator. A prototype 10-megawatt power tower, Solar One, constructed in Barstow, Calif., as part of a federal program to develop the concept, began commercial operation in 1982. Some scientists have suggested building huge photovoltaic arrays in Earth orbit for producing large amounts of power. Such arrays, called solar space power satellites (SSPSs), would beam collected energy to Earth by way of microwaves. INDUSTRIAL APPLICATIONS About 40% of the energy consumed by U.S. industry is used to provide temperatures of less than 315 deg C (600 deg F); such temperatures can be attained by using concentrating solar collectors that focus sunlight onto a pipe (parabolic-trough collector), onto a single point (parabolic-dish collector), or through a Fresnel lens. Such collectors must track the Sun as it moves across the sky in order to maintain proper focus of the light. Solar furnaces use large arrays of mirrors to achieve temperatures high enough (thousands of degrees) for metallurgical work; sunlight, unlike fuels, does not introduce impurities into the crucible in which a metal is formed. Solar energy is also being used in agriculture to power irrigation systems and to produce the heat needed to dry crops. RESIDENTIAL APPLICATIONS The greatest public interest in solar energy has been directed toward its use in heating and cooling buildings. Residential and commercial solar applications may be divided into two types: active systems, which rely on \Tsolar\t \Tcollector\t panels; and passive design, by which a building is designed, situated, and oriented so as to receive and store heat from the Sun during the winter, but also to keep sunlight out--and thereby keep the interior cool--during the summer. Active Systems Active solar heating systems commonly consist of several hundred square meters of solar collector panels, plus a storage medium to hold the heat collected during the day, and a set of automatic controls that monitor and regulate both heat collection and delivery between the storage medium and the living space. Active systems use either a liquid (of which the most popular is a mixture of water and an antifreeze, such as propylene glycol) or air as the heat-transfer medium. Insulated pipes or ducts carry the heat-transfer medium, called the working fluid, to the collector panels--where the fluid absorbs heat--and then back to the storage, which in liquid-based (hydronic) systems is an insulated tank or in air systems is an insulated bin of fist-sized rocks. (Alternatively, phase-change materials may be used to store heat.) The absorbed heat is transferred to the storage medium, and the cooled working fluid is then returned to the collectors to pick up more heat. Heat is removed from storage and delivered to the living space as needed. Most types of active systems require an auxiliary heating system to provide extra heat during extended periods of cloudiness or extreme cold. A typical active heating system might cost between $2,000 and $5,000 per thousand square feet of living space in the northeastern United States--depending on the type of system, its efficiency, and so forth. A large portion of a building's annual domestic hot water (DHW) needs can be supplied by a relatively inexpensive (between $2,000 and $2,500) active hydronic system using about 9 sq m (100 sq ft) of collectors for a typical residence. A heat exchanger, usually in the hot-water tank, keeps the working fluid separate from the potable water supply. Such systems, although they require a backup energy source, may pay for themselves in energy savings in less than 10 years. High-temperature solar collector panels may be used to power absorption-chiller \Tair conditioning\t. Such systems are relatively expensive but may be cost-effective in climates where plentiful sunshine and a substantial need for air conditioning exist. Also, \Lheat pump\ls may be used in conjunction with solar panels; solar heat boosts the heat pump's source during the winter, and during the summer the heat pump can discharge heat to the outdoors at night through the collectors. Passive Solar Many architects are now designing new houses and retrofitting older houses to passively use the Sun and other environmental factors to reduce energy costs. Passive systems are characterized by having few or no moving parts; usually, the south side of the building has extensive areas of insulating glass (or even a greenhouse); the east and west sides, less glass; and the north side, which receives no sun and is exposed to winter winds, little or no glass. (The orientation is reversed in the Southern Hemisphere.) Roof overhangs jut out over the south-facing glass; their function is to admit sunlight to the building in the winter, when the Sun is low in the sky and heating loads are high, and to keep sunlight out of the building's interior in the summer, when the Sun's path is higher. Effective insulation is considered an essential element of passive design. Aside from passive adaptations of more-or-less conventional houses, two other passive approaches have received considerable attention: the underground house and the thermal-envelope house. Underground houses make use of the fact that below the frost line--typically a foot or two below the ground's surface--the Earth's temperature remains nearly constant (usually about 13.3 deg C/56 deg F) throughout the year. The walls and roof are insulated and waterproofed, and the roof is covered with soil and vegetation, which provide additional insulation. Usually, an underground house will follow conventional passive-design practice in the incorporation of south-facing glass and roof overhangs. Such houses require only small amounts of auxiliary heating and cooling because the constant temperature of the earth is close to the desired interior temperature. The thermal envelope house, conceived by California architect Lee Porter Butler, is actually a house within a house--it has two south walls, two roofs, and two north walls. The south side of the house incorporates a greenhouse area between the two walls; as the Sun warms the air in the greenhouse, that air rises into the double roof area, while the coolness of the north wall causes air to fall from there into an insulated basement or crawl space that contains a thermal mass (rocks or dirt). Heat produced in the greenhouse is carried over the house and deposited in the thermal mass; spaces between the greenhouse's floorboards allow the air to rise again into the greenhouse to maintain a convective loop. At night the cool greenhouse glass causes a loop to operate in the reverse direction, and heat from the thermal mass warms the space between the building's two skins, both of which are insulated. During the summer, windows at the top of the greenhouse are opened, and a 30-m (100-ft) culvert pipe buried beneath the frost line is opened to the basement or crawl space. Warmed air in the greenhouse rises and flows out the windows, and replacement air is sucked through the culvert, where it is cooled, and into the envelope. East and west windows, as well as windows and doors opening into the greenhouse, may be opened or closed to provide temperature adjustments at any time of the year. The thermal envelope house requires little auxiliary energy, and its cost is comparable to that of a nonsolar house. HISTORY Humanity's use of the Sun's energy extends far back into prehistory. The Indian pueblos of the American Southwest, for example, seem to have been designed to use solar energy. Many ancient and primitive cultures have exhibited a sophisticated knowledge of how to use solar energy to maintain comfortable temperatures inside dwellings. Adaptive architecture probably played an important role in the survival and success of human groups in prehistoric times. Coupled with the use of fire, solar shelter orientation probably made it possible for some human communities to remain in one place year-round rather than migrating with the changing of the seasons. The ancient Greeks were also aware of how to build and orient structures so as to take advantage of the Sun's heat in the winter and to keep sunlight out--and living spaces cool--in the summer. \TXenophon\t wrote (c.400 BC) of how the Sun should penetrate and warm south porticoes in the winter but would be overhead in the summer, leaving the roofed porticoes in shade and relatively cool and comfortable. He also wrote that the south side of a house should be tall, to let more sunlight in, and the north side lower, to keep out the winter winds. The Roman architect \TVitruvius\t wrote in detail about solar-oriented architecture in Ten Books on Architecture (before 30 BC). The ancient Inca city of \TMachu Picchu\t, the ruins of which lie 2,340 m (7,675 ft) above sea level in the Andes, appears to have been built so that solar energy was used to maintain comfortable indoor temperatures day and night. Thick rock walls were built facing east, to be sun-warmed in the morning, when heat would be needed most. The rock retained the heat all day and into the night. The city itself was built on an east-facing hillside, and the west sides of the buildings were therefore backed with earth, which held additional heat from the daytime into the night and reduced the rate of heat loss from the buildings' interiors. As the use of fire became more technologically advanced, however, adaptive architecture became less and less necessary until, with the rather large heat-to-weight ratios of coal and oil, architecture became freed from the constraints of energy-efficient design. Energy became so cheap during the 20th century that it became customary to maintain constant temperatures of 22 deg C (72 deg F) indoors year-round by artificial means: heating in the winter and air conditioning in the summer. Although civilization by this time had grown accustomed to the "comfort zone" of building temperatures, and styles of dress had changed accordingly (no more heavy woolens for indoor winter comfort), vast quantities of "cheap" energy--derived, mostly, from fossil fuels (coal, oil, and natural gas)--were being used for space conditioning. Also, by the middle of the 20th century, industries and electricity-generating plants throughout the developed world were consuming massive amounts of these same fossil fuels. A few individuals in recent history experimented with the use of solar energy to provide mechanical power. Among them were the Frenchman Augustin Mouchot (1825-1911), who built several solar-powered steam engines, one of which operated a printing press in Paris in 1882; the Swedish-American inventor John \TEricsson\t, who built efficient solar-powered hot-air engines, which were later converted to run on coal and gas because the apparatus required for collecting the solar energy was costly; and Dr. Charles Greeley Abbot (1872-1973), an American, who investigated solar energy and many of its possible uses from the late 19th century until his death. Dr. Abbot is considered by many to have been the father of modern solar-energy use. One of his friends and fellow scientists, Robert \TGoddard\t, also did a great deal of pioneering work in solar power during the 1920s. During the 1970s solar energy emerged from relative obscurity and became a promising alternative energy source. At the beginning of that decade the industrialized world was still enjoying the age of cheap and plentiful petroleum. In 1973, however, the Organization of Petroleum Exporting Countries (\TOPEC\t) declared an oil embargo against the industrialized world in a bid for sharp price increases, and the price of oil shot up to its real value--some 1,000% above the 1973 price. At the same time, it became apparent that world petroleum reserves were finite and would not necessarily be freely available in the future. Major oil-consuming countries embarked on extensive oil exploration projects. A decade after oil had been discovered on Alaska's North Slopes, a consortium of oil companies finally won congressional approval to build the huge Trans-Alaska Pipeline, a project that had long been opposed because of the fear of its possibly negative impact on the environment. During the same decade public demands for a cleaner environment further increased the cost of using fossil fuels (oil, gas, and coal) and nuclear energy, and environmental hazards, widely spoken about, were the impetus for the public's cautious attitude toward these energy sources. Solar energy, on the other hand, was perceived by many to be a clean and safe energy source, besides being one that could not suddenly be cut off or made more costly. The U.S. government responded to public and private interest in solar energy by providing increased funding to encourage rapid development of this resource. By 1984, however, oil consumption in Western countries had dropped by 14%, the result of careful conservation practices, more fuel-efficient automobiles, and new furnace technologies that squeezed more energy out of fuel. The price of oil plummeted, \TOPEC\t could no longer enforce its price or production schedules, and in the United States the administration of Ronald Reagan, seeing only prosperity ahead, severely reduced funds for research into solar energy. Despite the cutbacks in government support, organizations in the field have made some progress in solar-energy advance, particularly in the area of solar cells. GEORGE ELLIS Bibliography: Anderson, B., and Riordan, M., The New Solar Home Book (1987); Boer, Karl W., ed., Advances in Solar Energy, vols. 1-3 (1985-86); Brown, Dan, Alternative Home Heating (1980); Butti, Ken, and Perlin, John, A Golden Thread: 2500 Years of Solar Architecture and Technology (1980); Davidson, J., The New Solar Electric Home (1987); Fisk, M. J., and Anderson, H. C. W., Introduction to Solar Technology (1982); Garg, H. P., Advances in Solar Energy Technology, 3 vols. (1987); Howell, Yvonne, and Miller, Harry, Everyone's Guide to Passive Solar Design (1983); Krieder, J. F., and Kreith, Frank, Solar Heating and Cooling, 2d ed. (1982); Myers, J. D., Solar Applications in Industry and Science (1984); Swan, C. C., Suncell: Energy, Economy, and Photovoltaics (1986); Watson, Donald, Designing and Building a Solar House (1977); Yanda, Bill, and Fisher, Rick, The Food and Heat Producing Solar Greenhouse, rev. ed. (1980); Yuncu, H., and Paykoc, E., eds., Solar Energy Utilization (1987).