Beyond Temperature Glide: The Compressor is Key to Realizing Benefits of Zeotropic Mixtures in Heat Pumps

Abstract

Zeotropic mixtures are widely discussed as alternative refrigerants for vapor‐compression cooling appliances and heat pumps. Mixtures can increase efficiency due to their nonisothermal phase change. In theoretical studies, zeotropic mixtures show significant benefits for efficiency if the temperature glide of the mixture matches the temperature change in the heat transfer fluids. Such large benefits have never been observed in experiments. First, this article clarifies the gap between simulations and experiments. Second, it is shown how zeotropic mixtures could increase efficiency in real plants. The analysis is based on experimental results from a heat pump with three zeotropic mixtures and on theoretical studies that also include a physical compressor model. The compressor performance is shown to depend strongly on composition. Therefore, the compressor efficiency is the key parameter for large benefits of zeotropic mixtures beyond well‐matching temperature glides. Based on these findings, a fluid database is screened for fluids with well‐matching temperature glides and high compressor efficiencies, utilizing a physical compressor model. As a result of the screening, the zeotropic mixture R152a/R32 is identified. The corresponding simulations show that zeotropic mixtures can achieve large benefits in heat pump efficiency if the pure components have similar and high compressor efficiencies.

Refrigerant Selection for Heat Pumps: The Compressor Makes the Difference

“…For the full set of model equations, the reader is referred to the Section S2, Supporting Information; for the validation results to the original literature. [ 26,30 ] To account for varying compressor geometry, Roskosch et al [ 30 ] propose to scale the effective flow areas and by the piston area, which is proportional to the square of the diameter for valve i (either suction or discharge) …”

“…Nevertheless, literature indicates that the isentropic efficiency indeed strongly depends on refrigerant properties: Some of the present authors and coworkers analyzed if these refrigerant‐induced differences in isentropic efficiency can be overcome by tailoring the compressor design to the refrigerant. [ 26 ] For reciprocating‐piston compressors, the authors simulate tailoring the compressor design to the refrigerant by scaling the compressor geometry and by reducing prevalent losses for each refrigerant. The isentropic efficiency is shown to depend strongly on the refrigerant even after tailoring the compressor design to the refrigerant.…”

“…So far, the model has been used for pure refrigerants [ 30,31 ] and for refrigerant mixtures. [ 26 ] However, as a (semi‐)physical compressor model, it solves mass and energy balances and is therefore computationally expensive.…”

“…To conclude, current refrigerant selection methods often model the compressor as refrigerant‐independent, assuming one fixed isentropic efficiency identical for all refrigerants. While this assumption does not seem to hold in practice, [ 26 ] it is yet unclear how the assumption influences the results of refrigerant selection. To date, to the best of our knowledge, no studies exist that systematically assess the influence of the compressor model on systematic refrigerant selection.…”

Investigation of the Interactions between Refrigerant, Flowsheet, and Compressor in Residential Heat Pumps

“…[22][23][24]36] Beyond the standard approach of constant compressor efficiencies, sometimes simple empirical compressor models were used. [37][38][39][40][41] For example, Xu et al [42] used a polynomial correlating the compressor efficiency with the pressure ratio. However, these models are hardly enable to fully cover the refrigerant and operation point dependency of compressor efficiency.…”