Effects of two-phase expander on the thermoeconomics of organic double-flash cycles for geothermal power generation

Abstract

The use of a two-phase expander in place of the throttling valve can improve the organic double-flash cycle (ODFC) thermal efficiencies. This work compares the thermoeconomics of a regenerative ODFC (RODFC), an expander-ODFC (EODFC) and a double expander-ODFC (DEODFC). The evaporation and flash temperatures of these cycles are optimized to maximize the net power outputs for geofluid temperatures of 120–180 °C. The results show that the two-phase expander in EODFC leads to the lowest optimal first-stage flash temperature with the highest vapor mass flow rate for the high-pressure turbine. The power generated by the two-phase expanders in DEODFC is less than that generated by the expander in EODFC but the power generated by the turbines is 5.9–8.4% higher. DEODFC produces the maximum net power, followed by EODFC and RODFC for each geofluid temperature. The exergy losses during evaporation are reduced by up to 41% by regeneration in RODFC, EODFC and DEODFC. The two-phase expanders increase the exergy losses during power generation but decrease the losses during throttling and condensation; thus, the exergy efficiency of DEODFC increases to 32.8–46.8%. The levelized cost of electricity for DEODFC is 15–33% lower than for ODFC due to the great increase in the net power output.