Thermodynamic analysis and performance optimization of transcritical power cycles using CO2-based binary zeotropic mixtures as working fluids for geothermal power plants

Transcritical power cycles (TPCs) using CO2-based binary zeotropic mixtures for the conversion of low-grade geothermal water at 100-150 °C into power are presented and analyzed when cooling water temperature (Tcw) is 10-30 °C in this paper. 6 refrigerants are selected as the additives into CO2 according to 2 criteria. Performances of TPCs with 6 selected CO2-based mixtures are optimized by pattern search algorithm (PSA). Thermodynamic and economic analyses are conducted. The results reveal that R161/CO2 is the best fluid for a TPC in terms of both thermal performance and economic performance while R290/CO2 might be an unsuitable working fluid in TPC due to its lower thermal performance. When Tcw = 10-20 °C, a TPC using R161/CO2 at XR161 = 0.7 can increase net power output by 14.43-50.46% and decrease cost per net power by 27.96-48.72% compared with a TPC using CO2. The selected CO2-based mixtures can reduce CPP of a TPC compared with CO2 although more heat transfer area is required. Meanwhile, the CO2-based mixtures can decrease the high operating pressure, and enlarge the condensing temperature range compared with CO2. In addition, the CO2-based mixtures except R161/CO2 and R152a/CO2 are not recommended for a TPC when Tcw is low.