On Thermodynamics of Gas-Turbine Cycles: Part 1—Second Law Analysis of Combined Cycles

Abstract

The energy-balance approach to cycle analysis has inherent limitations. These arise from the fact that the first law of thermodynamics contains no distinction between heat and work and no provision for quantifying the “quality” of heat. Thus, while producing the correct final result, energy-balance analysis is incapable on its own of locating sources of losses. Identifying and quantifying those sources can be a useful design tool, especially in developing new, more complex systems. The second law of thermodynamics, applied in the form of entropy and availability balances for components and processes, can locate and quantify the irreversibilities which cause loss of work and efficiency. Perhaps one reason that such analysis has not gained widespread engineering use may be the additional complication of having to deal with the combustion irreversibility, which introduces an added dimension to the analysis. A method of cycle analysis, believed to circumvent this added difficulty for combustion cycles, is proposed and applied to complex combined cycles with intercooling and reheat. The fuel is treated as a source of heat, which supplies potentially available work to the cycle depending on the peak temperature constraint on work extraction. The availability is then traced as it cascades through the cycle, portions of it being wasted by the various components and processes, and the balance emerging as shaft work. Linkage with the traditional first-law efficiency is thus preserved, while establishing the location, cause, and magnitude of losses. Analysis and results for combined cycles with component irreversibilities are presented. The air-standard approximation with constant properties is used for simplicity. The turbine is treated as adiabatic since the cooling losses depend on the type of technology applied, particularly at higher temperatures. A model for quantifying those losses is presented in Part 2 of this paper.