Ejector-enhanced air-source heat pump systems using ultra-low-GWP zeotropic mixtures in cold climates

This study investigates the thermodynamic performance and environmental benefits of dual-temperature ejector-enhanced air-source heat pump systems (DTEHP) using various zeotropic refrigerant mixtures for space heating and domestic hot water in buildings. The objective is to assess efficiency improvements, energy savings, and sustainability compared to a dual-temperature air-source heat pump (DTHP) and standard heat pump systems, particularly in cold climate applications. A validated thermodynamic model was developed to analyse two proposed DTEHP configurations using four zeotropic mixtures: R1234ze(E)/R600, R1234yf/R1233zd(E), R600/R290, and R600a/R290. The zeotropic mixtures exhibit ultra-low-global warming potential and ozone depletion potential, making them environmentally sustainable alternatives to traditional refrigerants. The performance of these systems was evaluated in terms of effects of high-temperature condenser inlet saturated temperature, low-temperature condenser outlet temperature, ambient air temperature, load ratio and zeotropic mixture mass fraction on system heating coefficient of performance (COP_h), volumetric heating capacity, exergy efficiency, and entrainment ratio. The results demonstrated that zeotropic mixture composition significantly influenced system efficiency. The analysis revealed that the DTEHP2 system consistently outperformed the other configurations across all zeotropic refrigerant mixtures, delivering the highest values for COP_h, volumetric heating capacity, and exergy efficiency. The DTEHP1 system also demonstrated significant performance improvements over the DTHP system, validating the efficacy of ejector-based enhancements in improving system performance. More specifically, the DTEHP2 system achieved the highest COP_h ranging from 4.912 to 5.334; demonstrated the highest volumetric heating capacity among all systems, with values peaking at 2695.68 kJ/m³ for R600a/R290, and delivered superior exergy efficiency, with a maximum of 56.37 % using R1234ze(E)/R600. The DTEHP1 consistently displayed higher performance than DTHP, with COP_h values ranging from 3.665 to 3.957; the volumetric heating capacity and exergy efficiency followed similar trends, reinforcing the impact of the ejector in improving system efficiency. In DTEHP configurations, R1234ze(E)/R600 and R600a/R290 offered the best overall performance. The integration of an ejector in the system led to efficiency improvements ranging from 10 % to 120 % over a standard DTHP system. The DTEHP2 system, in particular, achieved COP_h enhancements of up to 86 %, confirming the effectiveness of ejector-assisted cycles. The results suggest that R1234ze(E)/R600 and R600/R290 offer a balanced trade-off between performance and environmental impact, making them viable alternatives to conventional high-GWP refrigerants. This research highlights the potential of ejector-enhanced heat pump systems using zeotropic mixtures to optimise energy use and reduce environmental impact. These findings contribute to the development of high-performance, eco-friendly heating solutions for cold climates.