Thermodynamic modelling and performance analysis of a closed endoreversible indirectly-fired gas turbine cycle

Abstract

SYNOPSIS Indirectly or externally-fired gas-turbines (IFGT or EFGT) are interesting technologies under development for small and medium scale combined heat and power (CHP) supplies in combination with micro-gas-turbine technologies. The emphasis is primarily on the utilisation of the waste heat from the turbine in a recuperative process and the possibility of burning biomass (even “dirty” fuel) by employing a high temperature heat exchanger (HTHE) to avoid the combustion gases passing through the turbine. In this paper, the theory of finite time thermodynamics is used in the performance analysis of a class of endoreversible closed IFGT cycles coupled to variable temperature heat reservoirs. The analytical formulae for the dimensionless power output and efficiency, as functions of the total pressure ratio, component (HTHE, hot- and cold-side heat exchangers) effectivenesses, compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficient and the heat reservoir inlet temperature ratio, are derived and analysed based on illustrations. IFGT cycles are most efficient under low compression ratio ranges (2.0–5.0) and fit for low power output circumstances for integrating with micro-gas-turbine technology. Optimal total pressure ratio πc under maximum power output is always higher than that under maximum cycle thermal efficiency. When either of the heat transfer effectivenesses of the hot or the cold-side heat exchanger, the pressure recovery coefficient and the heat reservoir inlet temperature ratio increases, the dimensionless power output and efficiency and their corresponding optimal total pressure ratios increase. It must be noted that the optimal total pressure ratio πc under the maximum cycle thermal efficiency decreases with the increase of heat transfer effectiveness of the HTHE. The model derived can be further used to optimise the operational parameters and forecast performance of practical IFGT configurations and choices.