High-temperature heat pumps and zeotropic refrigerant mixtures: A perfect match?

Abstract

Refrigerant mixtures can improve the efficiency of high-temperature vapor compression heat pumps by matching their non-isothermal phase change to temperature changes in the heat source and sink. Previous studies suggest that the advantage of mixtures compared to pure refrigerants increases for larger temperature changes of heat sources and sinks. However, the potential efficiency increase has not yet been systematically studied. Thus, the conditions are still unclear under which zeotropic mixtures are particularly beneficial. In this work, we therefore screen a comprehensive set of pure refrigerants and all their binary mixtures for a broad range of sink and source temperature changes. The refrigerant set includes 33 natural and halogenated refrigerants. To assess the mixture performance, we model the heat pump process. Here, the compressor efficiencies depend upon the refrigerants. The study confirms the basic theory that mixtures can substantially improve the coefficient of performance (COP) and the benefits increase with the temperature changes of sink and source. However, the temperature change is shown to be more relevant at the heat source: mixtures are already beneficial at small temperature changes of the heat source, while a minimal temperature change of 10 K is needed for the heat sink. The greatest COP increase of a mixture, at 27%, is identified at maximum temperature changes of 40 K in the source and in the sink. The optimal mixture is highly specific to the temperature changes of heat sink and source. However, binary mixtures are identified that show good performance over broad ranges of source and sink temperature changes when their composition is tailored, e.g., pentene/cyclopropane. Such refrigerant mixtures could have the potential for universal use in heat pumps.