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In the simplest solar thermal application, a solar thermal collector gathers solar radiation to heat air or water for domestic, commercial or industrial use.

The solar panel is usually a flat plate collector that consists of a metal box with a glass or plastic cover and a black absorber plate at the bottom or a series of evacuated tubes. Evacuated tube collectors have absorber plates that are metal strips running down the center of each tube. Absorber plates are usually painted with selective coatings that absorb and retain heat better than ordinary black paint. They are normally made of metal, typically copper or aluminium, because it is a good conductor of heat. Copper is more expensive, but it is a better conductor and is less prone to corrosion than aluminium. The sides and bottom of the flat plate collectors are usually insulated to minimize heat loss. With evacuated tube systems convective heat losses are reduced by virtue of the vacuum in the tube.The solar collector is usually mounted on the roof and is connected to a circuit containing water with propylene glycol anti-freeze added, if necessary. The heated liquid flows around the circuit, either under the action of a pump to warm the main hot water tank, or by a thermo-syphoning action to warm a solar water storage tank that then feeds the hot water tank.

In locations with average available solar energy, flat plate collectors are sized at approximately 0.5 to 1 square foot per gallon of daily hot water use.

Source: http://www.solarbuzz.com/Consumer/Solarthermal.htm