The optimization of energy conversion systems is of great significance in the utilization of low-grade heat. This paper presents an analysis of 6 working fluids in 12 thermodynamic cycles to optimize the energy conversion systems. The optimal exergy efficiency of the system is dependent on the type of the thermodynamic cycle, the choice of appropriate working fluid, and the working conditions. A zeotropic mixture of R134a and R245fa shows advantages in energy conversion process, as well as its heat exchange with the heat source and heat sink. The exergy efficiency of a 0.5R134a/0.5R245fa-based supercritical Rankine cycle system is 0.643–0.689 for a turbine inlet temperature of 415–445K, which is about 30% improvement over the exergy efficiency of 0.491–0.521 for a pure R32-based organic Rankine cycle under the same temperature limits. Furthermore, the 0.5R134a/0.5R245fa mixture saves more than 60% of the cooling water during the condensation process than the pure R32, R134a and R245fa.

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