Environmental Design + Construction - October 2008 - (Page 32) Cool Roofing Shown is an example of a crystalline silicon rack-mounted system. > by brad burdic 32 Cool Roofing Photovoltaic Cells Selecting the right solar technology for your roof. J > Fall 2008 ust like roofing systems, solar solutions come in a variety of types, sizes, efficiencies and price points. Before investing in a rooftop solar system, it is important to understand the types of photovoltaic (PV) cells available and which technology might be best for a particular application. Solar energy is generated by exposing PV cells to light that is absorbed and converted into electricity. Built of razor-thin layers of semiconducting material, every cell is a power generator. When light falls on a PV module, the radiant energy is converted into direct current (DC), which is then converted into household-compatible alternating current (AC) by means of an inverter. The most commonly used PV cells are made from crystalline silicon and thin-film amorphous silicon. However, other technologies such as photochemical and concentrating photovoltaic are being developed. CRYSTALLINE SILICON PV CELLS The majority of PV cells produced today use crystalline silicon (c-Si) as its light-absorbing semiconductor. The c-Si technology originally was developed for the semi-conductor industry to produce PV cells for integrated circuits and microchips. These PV cells have an energy conversion efficiency between 11 percent and 16 percent. The energy conversion efficiency of a solar cell is the percentage of sunlight converted by the cell into electricity. While the efficiency of c-Si is high, it absorbs light poorly and requires many layers to perform efficiently in solar applications. The two types of crystalline silicon technology used to produce PV cells are mono- and multi-crystalline. Mono-crystalline technology uses thin wafers sliced from a single, pure crystal silicon ingot. With multi-crystalline or polycrystalline technology, silicon crystals are cast into a block and then sliced into wafers. Multi-crystalline silicon is not as pure as mono-crystalline and therefore produces lower-quality wafers. However, it is significantly less expensive. THIN-FILM PV CELLS The high demand for c-SI has outstripped production causing the price of c-Si to rise. With the high cost of c-Si, a number of PV cell manufacturers have begun using lessexpensive semiconductor materials
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