Experimental Evaluation of Binary and Combined Cycle Geothermal Power Plants Operating with Optimised Working Fluids

Abstract

This study presents a comprehensive experimental and analytical assessment of geothermal power plant configurations utilizing binary and combined-cycle systems for the efficient generation of electricity from low- and medium-temperature geothermal resources. The research used a modular geothermal energy block equipped with an ORMAT Model 3G-250 unit and various working fluids, including R245fa, isobutane, R134a, and R227ea. Experimental scenarios simulated geothermal water temperatures ranging from 95 to 175°C under different condensation and load conditions. The results demonstrated that the combined cycle, integrating a dry saturated steam turbine with an organic Rankine cycle, achieved the highest performance, with a peak electrical output of 305 kW and a thermal efficiency of 15.9%. Binary configurations operating on R245fa–isobutane mixtures exhibited a 5–7% higher specific electricity yield than pure R134a. Additionally, lowering the ambient air temperature for condenser cooling enhanced output power by up to 15.7%. The findings highlight the significant advantages of optimised working fluid compositions and regenerative heating in improving efficiency.  These findings validate the thermodynamic feasibility of binary and combined-cycle configurations for sustainable power generation, demonstrating superior conversion efficiencies particularly when valorizing low-to-medium enthalpy geothermal reservoirs.