Assessment of Blends of CO₂ with Hydrocarbons as a Zeotropic Refrigerant for HTHPs

The increasing need for energy-efficient and sustainable high-temperature heat pump systems (HTHPs) has led to extensive research into working fluids with optimal thermodynamic performance. Among various refrigerants, carbon dioxide is a promising candidate due to its favourable effects on the environment, good thermophysical properties, and economic feasibility. However, its low critical temperature and high operating pressure pose significant efficiency challenges. One of the methods to explore the use of CO₂ as a refrigerant is to mix it with other hydrocarbons and form zeotropic refrigerant mixtures. Thus, the present study investigates the performance of CO₂-based zeotropic refrigerant mixtures with six hydrocarbons. Key performance indicators, including coefficient of performance (COP), sink outlet temperature, Lorenz efficiency, and condensation heat transfer coefficient, were evaluated for the selected mixtures at different source inlet temperatures. The results indicated that at a source inlet temperature of 90°C, a CO₂/butane mixture delivers a sink outlet temperature of 147.3°C with a COP of 5.73, making it a strong candidate for moderate temperature applications. Additionally, CO₂/butane exhibits the lowest pressure ratio and highest Lorenz efficiency of 77.7% at 90°C source inlet temperature, exhibits reduced compressor workload and improving overall efficiency. For high-temperature applications exceeding 150°C, CO₂/acetone emerges as the most suitable mixture. At a source inlet temperature of 130°C, it achieves a sink outlet temperature of 188.76°C with a highest COP of 6.41 among all tested mixtures. Additionally, CO₂/acetone exhibits a high condensation heat transfer coefficient (1706.9 W/m²K at the same source inlet temperature) leading to lower exergy destruction and enhanced thermal efficiency. Those findings highlight the potential of CO₂-based zeotropic mixtures to enhance HTHP performance by reducing exergy destruction and improving heat exchanger thermal matching without the need for complex system modifications.