Heat pumps: an approach to select the highest performing refrigerant mixture

An article recently published by Danish researchers in the International Journal of Refrigeration performs a critical analysis of known approaches for the selection of working fluids for the design of high-performance heat pump cycles based on numerical modeling.

This article is one of the most downloaded IJR articles in 2019.

The authors have compared different approaches for the heat exchanger design and showed that fixing the pinch point temperature differences yielded the results that were closest to the values for an economically optimized solution. This means that an increased investment must be accepted in order to allow the mixtures exploiting their full thermodynamic and economic potential.

The screening method was demonstrated for two case studies focusing on the integration of a heat pump to utilize the excess heat from data centers for supplying district heating. Both cases assumed a temperature of 25 °C at the data center inlet. Case I assumed an outlet temperature of 50 °C, while a lower allowable temperature increase in the server rooms was assumed for case II, resulting in an outlet temperature of 40 °C. The required cooling load was assumed to be 500 kW in both cases.

The two analyzed cases differed by the heat source temperature glide. Several studies already described the benefits obtainable by selecting the working fluids among zeotropic mixtures with the aim of matching the temperature profiles with the heat source and heat sink.

In the first case analyzed, the authors found that a zeotropic mixture of 30% propylene and 70% R-1234ze(Z) is expected to improve the thermodynamic performance by > 35% and decrease the levelized specific cost of heat by 10% compared to ammonia. In the second case, a mixture of 60% propylene and 40% butane was found to show the best economic performance with a cost reduction of 8% and an improvement of 30% in COP, compared to the best performing pure fluid.

Based on these findings, it was concluded that zeotropic mixtures have the potential to significantly improve both the thermodynamic and economic performance of heat pumps in suitable applications, but require an appropriate cycle design. Suitable applications are applications in which there is a potential performance increase expected when using zeotropic mixtures and in which the economic boundary conditions remunerate solutions with a high thermodynamic performance.

An appropriate design requires designing the components based on the specific refrigerants in order to fully exploit the potential benefits as demonstrated by the suggested screening procedure.

Zühlsdorf B., Kjær Jensen J., Elmegaard B., Heat pump working fluid selection—economic and thermodynamic comparison of criteria and boundary conditions, International Journal of Refrigeration 98 (2019) 500–513. Available following this link.