Modelling,analysis,and irreversibility reduction of the triple-pressure reheat H-system combined cycle

Abstract

The triple-pressure reheat H-system combined cycle is considered the most efficient combined cycle that is commercially available. The gas turbine of the H-system combined cycle is cooled using a closed loop of steam and an open loop of air. Compared with the regular triple-pressure reheat combined cycle, the heat recovery steam generator (HRSG) of the H-system has a different configuration and a reduced load since superheating of steam is partially performed by the gas turbine. In this paper, the triple-pressure reheat H-system combined cycle was modelled, including detailed modelling of the HRSG and the expansion and cooling processes of the gas turbine. The performance of the H-system combined cycle was analysed and a feasible technique to reduce the irreversibility of the HRSG was introduced. The reduced-irreversibility triple-pressure reheat steam-air gas turbine-cooled (H-system) combined cycle was compared with the regularly designed triple-pressure reheat H-system combined cycle, which is the typical design for a commercial combined cycle. The effects of varying the turbine inlet temperature (TIT) on the performance of all cycles were presented and discussed. The results indicate that the reduced-irreversibility H-system combined cycle is 1.45–1.65 per cent higher in efficiency and 2.5 per cent higher in the total specific work than the regularly designed H-system combined cycle when all compared at the same values of TIT and minimum temperature difference for pinch points (PP_m)- The reduced-irreversibility H-system combined cycle was 1.35 per cent higher in efficiency than the most efficient commercially available H-system combined cycle when compared at the same value of TIT. Economical analysis of the reduced-irreversibility cycle was performed and showed that applying the introduced technique could result in an annual saving of ten million US dollars for a 440 MW power plant.