On the Thermodynamic Optimization of Power Plants With Heat Transfer and Fluid Flow Irreversibilities

This note addresses the current debate on the correctness of power plant models and analyses of the type published by Curzon and Ahlborn (1975) among others. Such models are based on the highly questionable assumption that the heat input is freely available, i.e., a degree-of-freedom for steady-state operation. This modeling assumption is wrong when the heat input (e.g., fuel) is in limited supply. On the other hand, it is shown that a model with freely varying heat input is possible if the roles of heat source and heat sink are played by two streams pumped from fluid reservoirs of different temperatures, as in geothermal and ocean thermal energy conversion systems, for example. The simplified model has both heat transfer and fluid flow irreversibilities, however, it neglects other possible sources. Several new results are developed. There exist optimal flow rates of hot fluid and cold fluid such that the net power output is maximized. As an alternative to power maximization, the model can be optimized for maximum efficiencies (net, first law, or second law). The note illustrates the importance of separating the questioned modeling assumption (e.g., Curzon and Ahlborn, 1975) from the generally applicable method of modeling and optimization (entropy generation minimization, EGM).