From parameter coupling to constraint-aware optimization: Geothermal organic flash cycles with reinjection and pinch-point limits

Organic flash cycles (OFCs) improve temperature matching between the geothermal brine and the working fluids, but their practical design is constrained by site-dependent reinjection temperature and heat exchanger pinch limits. This study compares a basic OFC (BOFC) and a modified OFC (MOFC) to determine how the primary heat exchanger (PHE) outlet temperature, flash temperature, brine reinjection temperature, and PHE pinch-point temperature difference (PPTD) govern the feasible regions and thermo-economic performance. A segmented PHE model enforces the pinch constraints, a Sobol analysis quantifies the parameter importance and interactions, and NSGA-II–TOPSIS identifies compromise solutions. Results show that, in the BOFC, the PHE outlet temperature and reinjection temperature govern the pinch-point location and temperature matching. In the MOFC, the flash temperature controls the PHE inlet condition and internal heat recovery, and consequently determines the reinjection temperature with the active pinch constraint. Increasing the PHE PPTD shifts the reinjection temperature corresponding to the maximum exergy efficiency upward in both cycles. The levelized electricity cost (LEC) of the BOFC exhibits a minimum with increasing reinjection temperature, while that of the MOFC decreases markedly and then levels off at higher reinjection temperatures. TOPSIS selects reinjection temperatures of 52.4–59.9 °C for the BOFC with geothermal source temperatures of 120–150 °C and 85.4–89.0 °C for the MOFC with source temperatures of 140–170 °C. Therefore, the BOFC favors greater geothermal heat utilization and closer PHE temperature matching, while the MOFC is better suited for higher reinjection temperatures, larger PPTDs and higher source temperatures.