A Comparative Study of the Transcritical CO2 Cycle and the Organic Rankine Cycles Using R245fa and Low GWP Refrigerants in Low Temperature Geothermal Utilization

Low temperature geothermal energy has high potential in clean and sustainable energy applications though it is difficult to be efficiently applied to conventional power units due to the low enthalpy. This study compares transcritical CO2 (T-CO2) cycle and organic Rankine cycles (ORCs) in terms of heat recovery efficiency ( \({\eta }_{HR}\) ) and system efficiency ( \({\eta }_{sys}\) ), using 90 °C–150 °C ( \({T}_{h,in}\) ) of hot water as the heat source. The analysis was conducted via MATLAB and NIST REFPROP database. The working fluid selected for ORCs are R245fa and low global warming potential refrigerants, i.e. R152a, R600a, R1234ze(E) and R1234yf. The results show that the T-CO2 cycle with lower \({T}_{h,in}\) has dominant advantage in terms of \({\eta }_{HR}\) , and its \({\eta }_{sys}\) is comparable with the ORCs. Along with \({T}_{h,in}\) increases, the \({\eta }_{HR}\) of the ORCs increase more rapidly; hence, the thermodynamic advantages of the T-CO2 cycle would be diminished. However, the floor-layout costs of the T-CO2 cycle is lower than the ORCs due to its small volumetric flow rate at the expander outlet, especially with high \({T}_{h,in}\) . In comparisons between the ORCs, R1234yf and R1234ze(E) can provide better system performance than the others, while R152a performs relatively inferior.