Investigation on working fluids selection for organic rankine cycles with low-temperature heat sources

The Influence of mixed and pure working fluids on the performance of organic Rankine cycles (ORCs) is discussed. Specifically, the performance of mixed and pure working fluids is analyzed based on certain characteristics of low-temperature heat source and heat sink. A method of selecting binary zeotropic mixed working fluids that match with different heat sinks is introduced. Thermodynamic processes of ORCs for various heat sources are simulated in Matlab. The performance characteristics of pure and mixed working fluids are compared under different inlet temperatures and temperature gradients of sensible heat sources. The results demonstrate that when the initial temperature of a heat source is lower and its temperature gradient is higher, and the temperature gradient of the heat sink is higher, mixed working fluids have better performance than pure working fluids. However, for the opposite heat source and heat sink situations, pure working fluids perform better. Mixtures with low critical temperature components exhibit the best performance among all working fluids when the temperature gradient of the heat source is large. The analysis also shows that introduction of a recuperator may reduce the cycle efficiency when the heat source temperature is low and the temperature gradient of the heat source is large.