Power enhancement potential of a mixture transcritical cycle for a low-temperature geothermal power generation

The power enhancement potential of a mixture transcritical cycle was investigated by comparison between the power of an R134a subcritical cycle and that of binary HFC (hydrofluorocarbon) mixture transcritical cycles for a low-temperature geothermal heat source of about 100°C. To fairly compare the power of the cycles by using different working fluids, each cycle was optimized from the view point of the power by three (or four for mixture cycles) design parameters. The first two are the turbine inlet temperature and pressure and the third is the ratio of the length of the vapor generator to the combined length of the vapor generator and the condenser. For the mixture cycles, the mixture composition was also optimized. In contrast to previous studies, to achieve a more realistic comparison, the characteristics of the working fluid's heat transfer and pressure drop were considered. Results show that the optimized R125-R245fa mixture transcritical cycle yields 11% more power than does the optimized R134a subcritical cycle under the simulation conditions considered in the present study.