On optimum disposal of waste heat

Abstract

The irreversibility associated with waste heat disposal across an exchanger represents an inherent trade-off point in the design of power plants. On the one hand, a large degree of irreversibility from a large temperature drop across the heat-transfer surface is detrimental to the thermodynamic efficiency of a cycle that produces waste heat. On the other hand, reduction of irreversibility can be achieved only by employing a larger heat transfer area. Therefore, one expects that there must be an optimum design that maximizes the work output per unit of heat transfer area for a given cycle. The efficiency at this optimum condition should be much closer to efficiencies in comparable cases of “good” design, and it must serve as a much more reasonable yardstick in assesing engineering merit. To exemplify this type of optimization, a Carnot engine cycle with a finite heat transfer coefficient during its heat rejecting process is analyzed in detail. An optimizing temperature drop accompanying the transfer of waste heat is displayed, and the maximum efficiency is calcuated. Influences of the heat transfer at the high-temperature side and the entropy produced during (real) adiabatic processes are also assessed to show the realistic potential of this simple design technique.