Raising cycle efficiency by intercooling in air-cooled gas turbines

Abstract

The working temperature of a gas turbine, necessary to achieve high efficiency, makes cooling of the first turbine stages unavoidable. Air and steam can be used for cooling. A model for an air-cooled gas turbine based on the work of Young and Wilcock [J.B. Young, R.C. Wilcock, ASME J. Turbomachinery 124 (2002) 207–221] is implemented in Aspen™. Simple cycle calculations with realistic parameters of current machines are made and confirm the results of Wilcock et al. [R.C. Wilcock, J.B. Young, J.H. Horlock, ASME J. Eng. Gas Turb. Power 127 (2005) 109–120] that increasing the turbine inlet temperature no longer means an increase in gas turbine cycle efficiency. This conclusion has important consequences for gas turbines because it breaks with the general accepted trend of increasing the TIT. An intercooled gas turbine cycle is intensively investigated, taking the turbine cooling into account. Intercooling not only lowers the work of compression, but also lowers cooling air temperatures. The major influences of the intercooling on the gas turbine cycle are mapped and explained. Optimum intercooling pressure for maximum gas turbine cycle efficiency is much lower than halfway compression. A simulation of the LMS100, the most recent gas turbine on the market from GE Energy, is made to verify the simulation methodology. The claimed intercooled cycle efficiency of 46% is confirmed. Further increasing the pressure ratio and TIT can still improve the performance of the intercooled gas turbine cycle.