subject: What are Solar Heat and Photovoltaic Collectors [print this page] Solar collectors are used to convert energy from the sun into easily storable energy. The converted energy can be used to heat water (the energy is converted into radiation) or can be converted into electricity (referred to as photovoltaic energy). The solar energy striking the earth's surface varies by location and season. Overall, it averages about 1,000 watts per square meter under clear skies.
Solar collectors can be mounted on the roof but need to face the sun. If the roof is flat, it is necessary to make changes to receive the sun's rays. To face the sun, it is ideal to have a south-facing roof in the Northern Hemisphere and a north-facing roof in the Southern Hemisphere. Where sunshine is readily available, a 2- to 10-square-meter array will provide plenty of energy for an average home.
Solar collectors that store heat are referred to as a "batch" type system. Other types of solar collectors do not store energy, but use fluid circulation to transfer heat for use or storage. The direct radiation is captured using a dark-colored surface that absorbs the radiation as heat. The selective surface of the collector reduces heat-loss caused by infrared radiation.
As the solar collector heats up, thermal losses from the collector reduce efficiency, which increases radiation. This can be countered by placing a glass plate around the collector. Another way to counteract this loss is to cool the absorber plate, thus ensuring the coldest available heat is transferred through the absorber.
Photovoltaic collectors are referred to as photovoltaic modules and use light to generate electricity. Photovoltaic modules use crystalline silicon cells or thin-film cells. They are a packed assembly of photovoltaic cells that require protection from the environment. The modules are fastened together with wire and covered with glass, as well as framed. These are connected electrically to one another and the rest of the system.
All photovoltaic cells require protection, but crystalline wafer cells can become particularly brittle without protection. These typical film-cells have power that ranges from several watts up to 200 watts per module. Some producers create preassembled panels with several 100 watts. Although most modules are rigid, more recently flexible modules are becoming available. These can be used on boats and can be walked upon without causing any damage to the collector itself.
Most photovoltaic collectors consist of 36 or 72 cells. Solar cells are interconnected with thin contacts on the upper side of the semiconductor material, which can be seen as metal net on solar cells. The net is thin so that it does not disturb the photon stream. Module efficiency is usually 1% to 3% lower than solar efficiency due to glass reflection as well as frame shadowing.
