Solar Cell

Energy is measured in a number of ways, including the clories,the Btu(british thermal unit),the quad, the foot-pound and the kilowatt hour. For the benefit of those who may not have memorized the appendices of their freshman physics books, we repeat the definitions of these quantities for an earth-based system at or about a temperature of 27'C 1 calorie is the heat needed to raise the temperature of 1 ml of water 1'C. 1 Btu is the heat needed to raise the temperature of 1 pound of water 1'F. 1 quad is 1 quadrillion (10^15)Btus. 1 foot-pound is energy expended in raising one pound through a distance of one foot. 1 kilowatt hour is the energy expended by a kilowatt oprating for one hour. With these definitions, the following equivalencies can be determined. 1 Btu = 252 calories 1 kWh = 3413 Btu = 2655000 ft-lb 1 ft-lb = 0.001285 Btu 1 quad = 2.930*10^11 kWh Since the emphasis of this text will be on electrical generation, and since the kWh is the common unit for electrical energy, the equivalence between kWh and ft-lb is especially noteworthy. For example, suppose a 150 pound person wished to generate 1 kWh, assuming a system with 100% efficiency. One way would be to climb to the top of a 17700 foot mountain to create 1 kWh of potential energy. Then,by returning to sea level by way of a chair, connected via a pulley system to a generator, the person's potential energy could be converted to electrical energy. this kwh could then be sold at wholesale for about 3 cents. An-other somewhat simplar method is to burn approximately 11 fluid ounces of petroleum to produce steam to turn a steam turbine. Still another method is to deploy about 2 m^2 of photovoltaic (PV) cells. This system will produce about one kWh per day for 20 years or more with no stops for refueling, no noise, minimal maintenance and no release of CO2(�G��ƺ�)�BSO2(�G��Ʋ�)�BNO2(�G��ƴ�) while the electricity is being produced.