Multi-objective optimization of ultra-low temperature Joule-Thomson cycle using CO₂-based eco-friendly and non-flammable zeotropic mixtures as sustainable refrigerant alternatives.

This study investigates a Joule–Thomson refrigeration system targeting temperatures below − 60 ◦C using ecofriendly and nonflammable zeotropic refrigerant mixtures. Since CO₂ cannot exist as a liquid below its triple point (-56.3 ◦C), it was blended with other components to lower its freezing point. The system performance was evaluated in terms of coefficient of performance (COP), volumetric cooling capacity, global warming potential (GWP), flammability, and freezing point. Among the tested mixtures, R1234ze(E)/CO₂/R125/R14 achieved the highest COP of 0.956, representing a 4.8% improvement and a 35.5% reduction in GWP compared to the baseline R134a/R23/R14 mixture. To simultaneously enhance COP and reduce GWP, multi-objective optimization was conducted. The resulting optimized mixture, R1234ze(E)/CO₂/R32/R14, achieved a GWP below 2000 while retaining over 94% of the baseline COP. All proposed mixtures were verified to be nonflammable and exhibited freezing points below − 90 ◦C, enabling stable operation at an evaporator-inlet temperature of − 70 ◦C. These results highlight the potential of CO₂-based refrigerant mixtures as sustainable alternatives to high-GWP refrigerants in ultra-low-temperature applications.