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With the help of thin, smooth, glasslike wafers, clean and inexhaustible sunshine can be put to work making electricity— with neither noise nor motion nor effluent. Individually, the wafers, known as photovoltaic cells, produce very small amounts of electricity; grouping cells into panels, and then joining the panels into larger assemblages called arrays, is necessary to yield enough electricity to run house lights and appliances. Although there are some new experimental materials for cells, most now are made of silicon, the common element of sand. Despite the abundance of the raw material, however, the laborious manufacturing process renders the cells—and hence the solar-produced electricity—extremely expensive. Amortizing the cost of a solar array over its projected 20-year life span yields electricity for about five times the cost of utility-produced power and about twice that of wind-generated kilo watts. The cost decreased tenfold between 1970 and 1980 and is likely to continue to decline. But for now, this cleanest, quietest method of producing electricity at home remains as uneconomical as it’s appealing. The workings of a solar cell. The energy in sun light, penetrating a solar cell, dislodges electrons from billions of silicon atoms. Because the cell— usually about 3 inches in diameter—is divided into two layers, each treated with a different chemical additive to induce opposite electrical charges in either side of the cell, the freed electrons rise to the top. There, the electrons are routed by scores of tiny metallic bus bars to larger bus bars reaching to the edge of the cell. A wire connected to the largest bus bar allows electrons to flow to a metal base plate on the bottom of the cell, where they can rejoin the electron-poor atoms. This moving file of electrons is electric current. As long as the sun shines and the plastic coating that protects the cells holds up—usually about 20 years— the cells will route electrons up, around the circuit, and back down in a never-ending stream. A solar array. Five solar panels, each com posed of 24 disklike photovoltaic cells, are joined within a sturdy frame to form a solar array. The output of a solar array is equal to the yields of allot the individual cells added together; the array shown at right, For example, will produce about 54 volts and 1.3 amperes—or about 70 watts—enough to power a large black-and-white television set. If more power is required, sever al arrays can be wired together through junction boxes on the backs of the panels. To exploit all available sunlight, arrays are mounted on south-facing racks angled to within 15° of the local latitude. Like wind- arid water-powered generating systems, photovoltaic systems also may be interconnected with utility lines or designed to stand alone, relying on a bank of batteries to store power for use during the night and on cloudy days. BUS BARS; LAYER TOP LAYER BASE PLATE [123] |