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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.

Nevertheless, in the 20th century, the sun's energy has become an increasingly attractive source for small amounts of 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 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.

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...