Identification of zeotropic mixtures of natural refrigerants for enhanced performance in geothermal district heating heat pumps
Decarbonizing district heating supply is essential for achieving climate targets, and heat pumps utilizing renewable geothermal energy represent a viable technology option for this. Conventional heat pumps typically employ pure or azeotropic refrigerants, which, under the large sink and source temperature variations characteristic of geothermal district heating, can result in poor temperature matching and increased irreversibilities, thereby limiting system efficiency. These irreversibilities can be mitigated by operating several heat pumps in series, subsequently complicating the system design; thus, a solution using a single heat pump is preferable. Using zeotropic mixtures in a single-stage heat pump is an option for achieving high efficiency with a simple system design. This study aims to numerically investigate possible performance improvements of heat pumps in geothermal district heating applications by systematically optimizing the mixture components and the composition of the applied zeotropic mixture. All investigated mixtures were blends of hydrocarbons, and the optimization objective was to maximize the yearly average efficiency across five representative operating conditions. A detailed numerical model, implemented in Python and using the REFPROP for thermophysical property calculations, was applied to simulate a single-stage heat pump cycle with an internal heat exchanger. Results indicate that the optimized zeotropic mixtures provide improved alignment with varying heat source and sink temperatures, yielding an increase in annual average coefficient of performance (COP) compared to the best-performing pure fluid. These findings underscore the potential of natural zeotropic refrigerant mixtures to significantly improve the efficiency of geothermal-based district heating systems across different operating conditions.