Finite time thermodynamic modelling and analysis for irreversible IFGT cycles

Abstract

<title/>Indirectly or externally fired gas turbines (IFGTs or EFGTs) are interesting technologies under development for small and medium scale combined heat and power 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. The possibility also exists for 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 irreversible 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) effectiveness, compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficient, the heat reservoir inlet temperature ratio, are derived. Analyses are carried out based on the formulation. Indirectly fired gas turbine cycles are most efficient under low compression ratio ranges (2·0-5·0) and fit for low power output circumstances for integration with micro gas turbine technology. Optimal total pressure ratio π c under maximum power output is always higher than that under maximum cycle thermal efficiency. When either of the heat transfer effectiveness of the hot or the cold side heat exchanger, the pressure recovery coefficient, isentropic efficiencies of the gas turbine and the compressor and the heat reservoir inlet temperature ratio increase, the dimensionless power output, cycle efficiency and their corresponding optimal total pressure ratios increase. It must be noted that the optimal total pressure ratio π c under the maximum cycle thermal efficiency decreases with increase in 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.