Possibilities, Limits, and Tradeoffs for Refrigerants in the Vapor Compression Cycle

We explore the possibilities for refrigerants having low global warming potential (GWP) by use of two distinct, but complementary, approaches. In the first approach, we evaluate the effect of a refrigerant's fundamental thermodynamic parameters on its performance in the simple vapor compression cycle and several variations on the basic cycle; this defines the limits of what is thermodynamically possible for a refrigerant. The analysis employs evolutionary algorithms, and it identifies the critical temperature, critical pressure, and ideal-gas heat capacity as the most significant fluid parameters. There is a fundamental tradeoff between high efficiency and high volumetric capacity for the vapor compression cycle. Performance differences between refrigerants in the simple cycle can be reduced by proper cycle modifications. In the second approach, we examine more than 56 000 chemical compounds from a public-domain database of chemical structures. A subset of about 1200 candidate fluids is identified by applying screening criteria to estimates for GWP, flammability, stability, toxicity, and critical temperature. The fluids with critical temperatures below 400 K (i.e., those that could be used in current equipment with minor modifications), are dominated by halogenated olefins. Additional chemical families, including ethers and cyclic compounds, are represented among the fluids having critical temperatures above 400 K.