Comparative analysis of natural and conventional working fluids for use in transcritical Rankine cycle using low-temperature geothermal source

Abstract

SUMMARYTheoretical analyses of natural and conventional working fluids-based transcritical Rankine power cycles driven by low-temperature geothermal sources have been carried out with the methodology of pinch point analysis using computer models. The regenerator has been introduced and analyzed with a modified methodology considering the considerable variation of specific heat with temperature near the critical state. The evaluations of transcritical Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature and optimized gas heater pressures at various geothermal source temperature levels ranging from 80 to 1201C. The performances of CO 2 , a natural working fluid most commonly used in a transcritical power cycle, have been indicated as baselines. The results obtained show: optimum thermodynamic mean heat injection temperatures of transcritical Rankine cycles are distributed in the range of 60 to 70% of given geothermal source temperature level; optimum gas heater pressures of working fluids considered are lower than baselines; thermal efficiencies and expansion ratios (Exp r ) are higher than baselines while net power output, volume flow rate at turbine inlet (V 1 ) and heat transfer capacity curves are distributed at both sides of baselines. From thermodynamic and techno-economic point of view, R125 presents the best performances. It shows 10% higher net power output, 3% lower V 1 , 1.0 time higher Exp r , and 22% reduction of total heat transfer areas compared with baselines given geothermal source temperature of 901C. With the geothermal source temperature above 1001C, R32 and R143a also show better performances. R170 shows nearly the same performances with baselines except for the higher V 1 value. It also shows that better temperature gliding match between fluids in the gas heater can lead to more net power output.

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