A comparative study of power optimization in low-temperature geothermal heat source driven R125 transcritical cycle and HFC organic Rankine cycles

In order to compare the power output of an R125 transcritical cycle with that of HFC subcritical ORCs for a low-temperature geothermal heat source of about 100 °C, an R125 transcritical cycle and subcritical ORCs using R134a, R245fa and R152a were optimized for power output by a simulation method. In contrast to previous studies, to fairly compare the power of different cycles, power optimizations were carried out for given heat source and sink inlet temperatures, and given flow rates based on the typical power plant thermal-capacitance-rate ratio. The total overall conductance (TOC) which implies heat exchanger cost was fixed, whereas the allocation of the overall conductance between the vapor generator and the condenser was optimized in the simulation. Results show that under simulation conditions considered in the present study, the power output of an R125 transcritical cycle was greater than that of subcritical ORCs when a TOC was higher than 35 kW/K. When a TOC was lower than that, transcritical cycle's power output was slightly less than that of subcritical cycles.