Theoretical Investigation on the Working Fluids of Transcritical Power Cycle Using Low-temperature Heat Sources

Owing to the potential feasibility in recovering low-grade heat (solar energy, geothermal energy and the waste heat in the industry) for power generation, Organic Rankine Cycle (ORC) attracts extensive attention recently. The ORC system in subcritical cycle is studied by many researchers and R123 is recommended as the working fluid with satisfactory performance. Transcritical power cycle was investigated in this study and a thermodynamic screening of 8 working fluids for this cycle was conducted. The thermal efficiency, volumetric expansion power and expansion ratio are used as criterion in the assessment of the economy of the ORC system, combined with environmental, physical and chemical performance. The result shows that R125 exhibits the best cycle performance comprehensively. The comparison of the performance between the R125 transcritical power cycle and the R123 ORC subcritical cycle was conducted, using the equivalent mean temperature as a reference temperature. The result reveals that R125 transcritical power cycle gives the optimal economical performance.