Performance Optimization And Economic Analysis Of Geothermal Power Generation By Subcritical And Supercritical Organic Rankine Cycles

In a sustainability context, using renewable energy sources to hedge against increasing consumption of fossil fuels and reduce greenhouse gas emissions becomes increasingly important. The geothermal resource has a great application prospect due to its rich reserves and convenient utilization, and Organic Rankine Cycle (ORC) is a effective method to convert the low-grade geothermal to electricity. To improve the performance of geothermal ORC system, working fluid selection, system parameter optimization and the cycle design are the main approaches. Zeotropic mixtures may show superiority as ORC working fluids due to the temperature glides during the phase transitions, which leads to better temperature matches between the working fluid and the heat source/sink. Moreover, owing to the changing temperature during the transition from liquid to vapor in the vapor generator, supercritical ORC provides a great potential in geothermal utilization and irreversibility reduction.This paper displays an investigation on the performance optimization and economic analysis of various working fluids under subcritical and supercritical conditions. To avoid the silica oversaturation, the geothermal water reinjection temperature should not be less than 70 degrees C : Turbine inlet temperature, condenser outlet temperature as well as turbine inlet pressure (for supercritical ORC) are optimized to maximize the net power output. Moreover, economic analysis is conducted by taking heat exchanger area per unit power output (APR) and the specific investment cost (SIC) as indicators under the optimal net power output condition. The results shows that working fluid with a medium critical temperature yields greater net power output in supercritical ORC and mixture produces larger net power output compared with its pure components in subcritical ORC. Compared with isobutane (R600a) under subcritical condition, isobutane/isopentane (R600a/R601a) and isobutane/pentane (R600a/R601) under subcritical condition, R134a and R1234ze(E) under supercritical condition yield 3.9%, 3.8%, 8.5% and 8.8% more net power outputs, respectively. In addition, R600a/R60 1 a and R600a/R601 under subcritical condition own higher APR and SIC while R134a and R1234ze(E) under supercritical condition possess lower APR and SIC.