Thermodynamic performance analyses and optimization of subcritical and transcritical organic Rankine cycles using R1234ze(E) for 100-200 °C heat sources

The working fluid has a crucial effect on the thermodynamic performance of the organic Rankine cycle (ORC) system. Trans-1,3,3,3-tetrafluoropropene, R1234ze(E), is a new-typed environmental friendly and safe organic fluid that presents a great potential to be used in ORC systems. This study focuses on subcritical and transcritical ORCs using R1234ze(E) driven by the 100-200 °C hot water without the outlet temperature limit. For various heat source temperatures, the optimal cycle type (subcritical or transcritical), optimized cycle parameters (turbine inlet temperature and turbine inlet pressure), and system performance were studied in the view of the maximum system net power output for the per mass flow rate heat source fluid. Results show that the transcritical ORC has a higher system efficiency, whereas its system heat absorption capacity is lower than that of the subcritical ORC for R1234ze(E). The subcritical ORC is more suitable for heat source temperatures below 160 °C with the transcritical ORC for higher temperatures. For subcritical and transcritical ORCs using R1234ze(E), the optimized turbine inlet temperature and turbine inlet pressure for various heat source temperatures among 100-200 °C were also provided. Compared to R245fa and R600a, the maximized system net power output of R1234ze(E) is the largest for the approximately 100-167 °C heat sources without the outlet temperature limit, and it is 31.4% larger than that of R245fa at most and 25.8% larger than that of R600a at most.