A Modified, High-Efficiency, Recuperated Gas Turbine Cycle

The thermal efficiency of an intercooled/recuperated cycle may be increased by: (a) evaporatively aftercooling the compressor discharge; and (b) injecting and evaporating an additional amount of water in the recuperator. Comparative computations of such a modified cycle and intercooled/recuperated cycles carried out over a wide range of pressure ratios and turbine inlet temperatures and at two different levels of component technologies show an advantage of over five percentage points in efficiency for the modified cycle. About 60 percent of this improvement results from modification (a) and 40 percent from modification (b). The modified intercooled/recuperated cycle is compared with nonintercooled steam-injected gas turbine systems at each component technology level. The present cycle is found to be superior by about 2.75 percentage points in efficiency and to require a substantially smaller water flow rate. To assist in interpreting those differences, the method of available-work analysis is introduced and applied. This is identical to exergy analysis for systems with a pure-substance working fluid, but differs from the latter for systems using a mixture of pure substances insofar as the thermodynamic dead state is defined for the chemical and phase composition realized at the exhaust conditions of practical engineering devices and systems. This analysis is applied to the heat-recovery processes in each of the three systems considered. It shows that the substantial, fundamental available-work loss incurred by mixing steam and gases in the steam-injected system is the main reason for the superior efficiency of the precent cycle.