Optimizing Synergistic Performance of Binary Mixtures and Vapor Injection in CO 2 Heat Pump: Comparative Analysis

The widespread adoption of CO ₂ heat pump systems is still hindered by their inherently high operating pressures and limited thermodynamic efficiency under transcritical conditions. Existing enhancement strategies typically focus either on refrigerant modification or on cycle optimization, while the combined potential of binary mixtures and vapor-injection technology has not been fully explored. This study develops a comprehensive thermodynamic model to investigate four CO ₂ -based mixtures—CO ₂ /R32, CO ₂ /R161, CO ₂ /R1270, and CO ₂ /R41—operating under a vapor-injection heat-pump configuration. The findings reveal a strong synergistic effect between mixture thermophysical properties and the vapor-injection process. Specifically, CO ₂ /R161 increases the heating efficiency by 19.87% while lowering the discharge pressure to 3.53 MPa, thereby substantially reducing the mechanical load on the compressor and improving cycle reliability. The results highlight that mixture-based vapor-injection cycles provide a promising approach for enhancing the performance and operational robustness of CO ₂ heat pump systems, offering useful insights for advancing their practical application and broader engineering deployment.