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.
The global warming potential of many working fluids used nowadays for vapor compression refrigeration systems and heat pumps is very high. Many of such fluids, which are used in currently operating refrigerators and heat pumps, will have to be replaced. In order to avoid a redesign of the system, it would be very helpful if efficient and ecological alternative working fluids for a given plant could be found. With modern process simulation tools such a selection procedure seems possible. However, it remains unclear how detailed such a model of a concrete plant design has to be to obtain a reliable working fluid ranking. A vapor compression heat pump test-rig is used as an example and simulated by thermodynamic models with different levels of complexity to investigate this question. Experimental results for numerous working fluids are compared with models of different complexity. Simple cycle calculations, as often used in the literature, lead to incorrect results regarding the efficiency and are not recommended to find replacement fluids for existing plants. Adding a compressor model improves the simulations significantly and leads to reliable fluid rankings but this is not sufficient to judge the adequacy of the heat exchanger sizes and whether a given cooling or heating task can be fulfilled with a certain fluid. With a model of highest complexity, including an extensive model for the heat exchangers, this question can also be answered.
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