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All life on Earth depends on energy from the sun. Solar energy is the source of energy for photosynthesis. It provides the warmth necessary for plants and animals to survive. The heat from the sun causes water on the Earth\'s surface to evaporate and form clouds that eventually provide fresh rainwater.
Solar energy is the result of thermonuclear fusion reactions deep within the sun. These reactions produce so much energy that they keep the surface temperature of the sun at about 10,300B0F. Even though solar energy is the largest source of energy received by the Earth, its intensity at the Earth\'s surface is actually very low due to the large distance between the Earth and the sun and the fact that the Earth\'s atmosphere absorbs and scatters some of the radiation. Even on a clear day with the sun directly overhead, the energy that reaches the Earth\'s surface is reduced about 30 percent by the atmosphere. When the sun is near the horizon and the sky is overcast, the solar energy at ground level can be negligible. It also varies from one point to another on the Earth\'s surface.
Nevertheless, in the 20th century, the sun\'s energy has become an increasingly attractive source for small amounts of direct power to meet human needs. A number of devices for collecting solar energy and converting it into electricity have been developed, and solar energy is used in a variety of ways. Solar energy is used to heat houses, and in many countries specially designed solar ovens are used for cooking. The sun also supplies energy to electric generators that provide power for weather and communications satellites and for radio and television equipment.
Because the intensity of the sun\'s radiation at the surface of the
Earth is so low, collectors designed to capture solar energy must be large.
In the sunniest parts of the continental United States, for example, in order for a collector to gather enough energy to serve one person for one day, the area of the collector\'s surface must be about 430 square feet. The actual energy that can be used depends on the efficiency of the collector and of the device that converts the radiation into usable energy.
Flat-plate collectors. The most common flat-plate collectors consist of a dark metal plate, covered with one or two sheets of glass, that absorbs heat. The heat is transferred to air or water, called carrier fluids, that flows past the back of the plate. This heat may be used directly or it may be transferred to another medium. Flat-plate collectors are used for home and hot-water heating. Flat-plate collectors typically heat carrier fluids to temperatures ranging from 150B0 to 200B0F. The efficiency of such collectors varies from 20 to 80 percent.
Concentrating collectors. When higher temperatures are required, a concentrating collector is used. These collectors reflect and concentrate sunlight from a wide area. One such device, called a solar furnace, was installed in the Pyrenees in France and has several acres of mirrors focused on a single target. The energy concentrated at the target is 3,000 times that received by any single mirror, and the unit produces temperatures of up to 3,630B0F (2,000B0C). Another structure, the so-called "power tower" plant near Barstow, California, generates 10,000 kilowatts of electricity. Here, the furnace acts as a boiler and generates steam for a steam turbine-electric generator power plant.
In sophisticated concentrating collectors such as the California tower, each mirror is rotated by a heliostat that directs the sun\'s rays from the mirror to the target. Positioning motors, drives, and controllers make such systems expensive. Less costly collectors can produce temperatures lower than those of more advanced concentrating collectors but higher than those of flat-plate collectors. For example, parabolic reflectors that concentrate sunlight on black pipes can produce fluid temperatures of about 400B0 to 55 0B0F and can concentrate the solar energy up to 50 times its original strength.
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