Performance investigations on nonflammable CO2/HFOs mixtures for an air-source heat pump water heater during wintertime

Abstract

With heating demand in buildings on the rise, air-source heat pumps (ASHPs) offer a vital, low-emission alternative to traditional methods. In cold regions, ASHP faces application resistances like combustion and system complexity. These can be mitigated using zeotropic mixtures with hydrofluoroolefins (HFOs). However, the role of ternary mixtures in enhancing performance and safety remains underexplored. This study proposes novel mixtures of CO₂ and HFOs in a simple recuperative configuration. Within the nonflammable range, the optimal ternary mixture CO₂/R1234ze(E)/R1336mzz(E), shows up to 35.8 % improvement in the coefficient of performance (COP) compared to binary mixtures. Up to 14.3 % enhancement in COP versus a traditional ASHP system is achieved, resulting in an annual performance boost of 7.0 % in cold climates. Exergy analysis indicates the low irreversible losses in throttling and condensing are the main contributors to the improvement. A life cycle assessment reveals both economic and environmental advantages, with reductions in costs and emissions of up to 10.7 % and 25.5 %, respectively. Sensitivity analysis indicates that easily adjustable operating pressures significantly impact COP, but the optimal mixture still maintains high efficiency despite variations. Ternary mixtures also show comparable COP to ASHP using CO₂, with significant discharge pressure reductions. The proposed mixtures enhance safety, energy efficiency, and system simplicity, supporting the global adoption of ASHP for energy savings and emission reductions, thus advancing carbon neutrality efforts.