Effect of Critical Temperature on the Energy and Exergy of Transcritical Organic Rankine Cycles with Zeotropic Mixture Working Fluids in Low-grade Heat

A thermodynamic analysis model of the transcritical organic Rankine cycle associated with zeotropic mixtures has been developed. This model was employed to investigate the effects of critical mixture temperature on the first and second law efficiencies of thermodynamics, specific power, net-power-to-cost ratio, irreversibility of the components, and exergy loss of the heat source at expander inlet temperatures of 150-190 degrees C. The results indicate that evaporator irreversibility and exergy loss of the heat source significantly decreased with an increase in the expander inlet temperature and decrease in the mixture critical temperature, which resulted in improved specific power and second law efficiency. However, the condenser irreversibility was increased with a decrease in the mixture critical temperature owing to the effects of condensation pressure for low critical temperature fluid. Based on the analysis, a universal criterion for the temperature difference between the critical temperature and expander inlet temperature on the maximal second law efficiency has been proposed. Finally, the high critical temperature of the mixed fluid is accompanied by a low condensation pressure, resulting in high first law efficiency, low condenser irreversibility and excellent netpower-to-cost ratio.

Automated summary

A thermodynamic analysis model of the transcritical organic Rankine cycle associated with zeotropic mixtures has been developed. The effects of critical mixture temperature on the system performance were investigated. The results show that increasing the expander inlet temperature and decreasing the mixture critical temperature can improve the energy conversion efficiency and performance of the system.