International. An article recently published by Danish researchers in the International Journal of Refrigeration makes a critical analysis of the known approaches for the selection of working fluids for the design of high performance heat pump cycles based on numerical models.
The authors have compared different approaches to the design of the heat exchanger and have shown that by correcting the temperature differences of the compression point, the results closest to the values for an economically optimized solution were obtained. This means that a greater investment must be accepted to allow the mixtures to exploit their thermodynamic and economic potential.
The detection method was demonstrated for two case studies that focus on the integration of a heat pump to use excess heat from the data centers to supply district heating. Both cases assumed a temperature of 25 ° C at the entrance of the data center. Case I assumed an exit temperature of 50 ° C, while a lower permissible temperature increase was assumed in the server rooms for case II, which resulted in an exit temperature of 40 ° C. It was assumed that the required cooling load was 500 kW in both cases.
The two cases analyzed differed according to the temperature of the heat source. Several studies have already described the benefits that can be obtained by selecting the working fluids between the zeotropic mixtures in order to match the temperature profiles with the heat source and the heat sink.
In the first case analyzed, the authors found that a zeotropic mixture of 30% propylene and 70% of R-1234ze (Z) is expected to improve the thermodynamic performance at> 35% and reduce the specific heat cost leveled at an 10 % compared to ammonia. In the second case, it was found that a mixture of 60% propylene and 40% butane shows the best economic performance with a cost reduction of 8% and an improvement of 30% in COP, compared to the pure fluid of better performance.
Based on these findings, it was concluded that zeotropic mixtures have the potential to significantly improve the thermodynamic and economic performance of heat pumps in suitable applications, but require an adequate cycle design. Suitable applications are applications in which a potential increase in performance is expected when zeotropic mixtures are used and in which the conditions of economic limits pay for solutions with high thermodynamic performance.
Proper design requires designing components based on specific refrigerants to maximize the potential benefits, as demonstrated by the suggested selection procedure.