Performance and economic advantages of partial admission turbo-expanders in geothermal reservoir–binary cycle systems

Geothermal binary cycle systems experience long-term declines in reservoir temperature and mass flow rate, reducing thermo-economic performance and causing deviations from design conditions, motivating improved off-design strategies. In this study, full admission was represented by 25 stator blades, while partial admission ratios of 0.92 and 0.75 were achieved with 22 and 19 blades, respectively. The present study coupled the reservoir, binary cycle, and turbo-expander with partial admission economic models. The artificial neural network was trained using turbo-expander data from ANSYS-CFX under off-design conditions and used to predict the mass flow rate of the working fluid, the shaft work, and isentropic efficiency. Relative to the full admission turbo-expander, the partial admission expander achieves a 24 kW increase in average net output power, a $0.22 million improvement in net present value, and a reduction in the levelized cost of electricity to 0.00185 $/kWh. Six scenarios involving the decay rate and starting year of annual geofluid mass flow rate reduction have been examined. The use of partial admission significantly increases net power output by 3.2 %–8.17 % compared to the baseline full admission configuration. Simultaneously, the levelized cost of electricity is reduced by 3.08 %–7.45 %, and net present value increases by 5.13 %–14.38 %. The partial admission with optimized rotational speed increases the net power output by 0.53–0.77 %, reduces the levelized cost of electricity by 0.31–0.53 %, and enhances the net present value by 0.72–1.29 %. Comparing the two strategies, the partial admission design yields more substantial improvements.