Performance Levels Obtainable from Steam-Gas Turbine Combined Cycles

This study aims to the evaluation of the best performance obtainable from a steam-gas turbine combined plants both in a new plant design and in improving existing steam plants by adding a topping gas turbine system. A method of comparison is presented here, based on the choice of a steam-gas reference cycle which has shown to be particularly suitable for a general study. A thermodynamic analysis has been carried out showing the influence on the combined plant overall efficiency of the parameters characterizing both the gas and steam cycles. The reference cycle as well as those derivable from it by modifying the gas portion cycle only has been studied. The analysis was also extended to evuation of the gas to steam units output power ratio and of the efficiency increase when repowering a steam unit. It has been shown that the combined cycle plants maximum overall-efficiency is achieved, whatever the steam cycle, when the gas turbine cycle operates at maximum specific work. The result is that the best performance in combined cycle plants is achieved by using multiple expansion with reheating in the gas cycle, when designing a new plant. When steam plants are to be repowered by means of existing gas turbine units, afterburning may be useful to improve performance.