WEEK 03: GENERATION: ALTERNATIVE SOURCES: PHOTOVOLTAIC CONVERTERS
Photovoltaic Conversion. the direct conversion of sunlight into electric power was primarily developed for use in space missions and was rarely used elsewhere until the late 1970s. Photovoltaic cells (PV) and modules have found many commercial applications as the prices have dropped and the efficiencies have increased. These applications range from a few small cells in calculatos and watches to arrays making a few megawatts of peak power for utility generation. Many of these applications are found in sites remote from exisiting lectric power grids such as meteorological data collection stations, radio repeater towers, and navigational bouys. One of the fastest-growing applications is battery charging for sailboats and small motor craft. There are more than 10,000 small systems in use in Africa and Asia for powering lights, refrigirators, and highway lighting; most of these installations have battery storage. The output of solar arrays is direct current; as a result, it must converted and conditioned if it is intended to feed power synchronously into exisiting alternating-current networks.
There are several commercially available and competing PV technologies:
1. Single-crystal silicon, which uses a uniform chemical structure. Conversion efficiencies is usually about 11.5%
2. Polycrystalline silicon, which uses a different manufacturing method to create a chemical structure that us a series of crystalline structures within one photovoltaic cell.Conversion efficiencies is usually about 7.0%
3. Amorphous silicon, which uses a another manufacturing method to give an almost random chemical structure.Conversion efficiencies is usually about 5.0%. Laboratory efficiencies for this technology have already exceeded 11% and is expected to be at 25% in 1995.
In general, as the atomic structure becomes more random, less energy input and manufacturing complexitiy is required. However, more uniform structure means increased current collection and increased efficiency.
The typical solar cell consisted of one of these types of silicon treated with a thin layer of cadmium sulfide, gallium arsenide, indium phosphide or similar compound. This produces a semiconductor junction between n-type and p-type materials. Sunlight impinging on the cell creates electron-hole pairs, the electrons being attacted to the positively charged n-type material and holes to the p-type.
At present the performance of the crystalline and semicrystalline forms of silicon PV cells is greater than that of the amorphous silicon cells. Althouh the predominant type of collector has been the flat-plate system, PV systems which concentrated the sun's rays on the PV cells withe fresnel lens or parabolic reflectors are also available. These concentrator systems can increase the efficiency of the overall system and can use less-photosensitive material to achieve the same power output. Most of these systems are used with a tracking support system that effectively follows the sun's movement, keeping the photovoltaic array aligned for maximum power production.
Most PV power uses flat-plate modules of cut and polished waferlike cells of crystallinesilicon in converting sunlight to electricity. AN up-to-date plant, including dc-to-ac conversion and conditioning, can produce energy at 30 to 50cents per kWh.
Bulk PV power will become feasible in the bright, clear areas where incident solar energy is about 2,600 kWh/m³ annually. There are two main reaons why the thin-film technologies offer the promise of significantly reduced costs. First, the thin-film cells use only a few microns of direct material, instead of tens of mils of slicon used in the crystalline, polycrystalline, or ribbon silicon modules. Cadmium telluride can absorb 99% of the sun's energy in less than 0.5 mm thickness, as opposed to the 8-mil requirements for crystalline silicon.
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