Exergy Analysis of Combined Cycles: Part 2—Analysis and Optimization of Two-Pressure Steam Bottoming Cycles

Results of a study for selecting the optimum parameters of a dual-pressure bottoming cycle as a function of the gas turbine exhaust temperature are presented. Realistic constraints reflecting current technological practice are assumed. Exergy analysis is applied to quantify all loss sources in each cycle. Compared to a single pressure at typical exhaust gas temperatures the optimized dual-pressure configuration is found to increase steam cycle work output on the order of 3 percent, principally through the reduction of the heat transfer irreversibility from about 15 to 8 percent of the exhaust gas energy. Measures to further reduce the heat transfer irreversibility such as three-pressure systems or use of multicomponent mixtures can therefore only result in modest additional gains. The results for the efficiency of optimized dual-pressure bottoming cycles are correlated against turbine exit temperature by simple polynomial fits. Sensitivity of the results to variations in the constraint envelope are presented.