An Investigation of Energy versus Exergy Based Optimization for Power Cycles

Abstract

A comparison of optimization studies based on energy and exergy methods and for the ideal and actual working models of common power cycles are performed. The cycles considered include the simple gas-turbine cycle, the regenerative gas-turbine cycle, the regenerative gas turbine cycle with reheating and intercooling, the simple steam cycle, the reheat steam cycle, and the combined gas-vapor cycle. The optimization is performed to determine the optimum pressure ratios in gas-turbine cycles and the optimum boiler pressures in steam cycles that maximize the thermal efficiency of the cycle in energy method and the exergy efficiency in exergy method. The optimum points are also searched for maximizing the net work of the cycle in both energy and exergy methods. The results show that the optimum boiler pressures that maximize the network are identical based on both energy and exergy approaches and for both ideal and actual operating models in simple steam and reheat steam cycles. The optimum boiler pressures that maximize the cycle efficiency are about the same based on both energy and exergy approaches when ideal operations are considered in simple and reheat steam cycles. The optimum points differ when actual operations are considered for these cycles. The optimum pressure ratios that maximize the network are identical based on both energy and exergy approaches but different depending on the selection of an ideal or actual model in simple and regenerative gas-turbine cycles, and in combined cycle. No agreement with respect to the optimum pressure ratios that maximize the cycle efficiency is observed for all types of gas-turbine cycles including the combined cycle.