Demonstration of an Inverse Relationship Between Thermal Efficiency and Specific Entropy Generation for Combustion Power Systems

Abstract

Maximum thermal efficiency is commonly assumed to correspond to minimum entropy generation. However, previous work has disproven this assumption for various power generation systems. In order to reconcile these two optimization approaches, second law analysis is performed here in terms of specific entropy generation (SEG), defined as the total entropy generation per mole of fuel. An inverse relationship between thermal efficiency and SEG is derived here, and it is shown that maximum thermal efficiency always corresponds to minimum SEG for lean fuel/air mixtures. Furthermore, the maximum efficiency limit of conventional power plants is shown to differ from the Carnot efficiency. Finally, a modified second law efficiency is introduced, and it is shown that the exhaust combustion products are bounded by a theoretical minimum temperature.