The laws of thermodynamics place very general limitations on conversion of energy from one form to another. In this time of increasing energy demand and diminishing resources, these matters are of the utmost practical importance. We conclude this chapter with a brief discussion of a few energy-conversion systems, present and proposed.
Over half of the electric power generated in the United States is obtained from coal fired steam-turbine generating plants. Modem boilers can transfer about SOto 90 percent of the heat of combustion of coal into steam. The maximum Cannot efficiency of a steam turbine is given by Eq. (18–14); TH is the temperature of the boiler, and Tc is the aperture of the condenser. The highest practical value of TH is about SOOK, and condenser temperature is determined by the temperature of its surroundings, 300 K. Hence I – (300 K)/(SOOK) = 0.63, or 63%. In practice, the turbine leniency is less than this ideal maximum, about 50%. The efficiency of large e generators in converting mechanical power to electrical is very high, typi Since efficiency is the ratio of the useful energy output to the energy input. thermal efficiency of such a plant is the product of the individual roughly (0.S5)(0.50)(0.99), or about 42%.