Natural refrigerant mixtures in low-charge heat pumps: An analysis of the potential for performance enhancements.

• Screening non-fluorinated refrigerants with low GWP and reduced flammability. • Performance criterion characterizing heating capacity for low-charge heat pumps. • The most attractive refrigerants were identified through decision-making techniques. • DME-CO 2 mixtures resulted in improvements in efficiency and levelized cost of heat. • Further investigations were recommended on mixtures with DME, Propylene and CO 2. The study investigated the characteristics of different natural refrigerants and their mixtures in a residential heat pump with low refrigerant charge. Three main evaluation criteria were utilized for comparing different mixtures: Coefficient of Performance (COP), Volumetric Heating Capacity (VHC), and a newly proposed indicator, the heating capacity at charge limit. Propane was used as a reference refrigerant. It was found that some mixtures significantly improved both COP and the heating capacity at charge limit while maintaining similar volumetric heating capacity and operating conditions. Alternative multi-criteria decision-making techniques were adopted to rank the best refrigerant mixtures. Mixtures rich in Dimethyl Ether (DME), such as DME-CO 2 [0.96-0.04] and DME-Propylene [0.75-0.25] were found consistently among the best options. Those were followed by Propylene-rich mixtures such as Propylene-CO 2 , Propylene-Isobutane and Propylene-Butane. The levelized cost of heat (LCOH) could be improved by up to 12 %. To accelerate the transition to natural refrigerants, without compromises on efficiency and costs, further research and certification activities on non-fluorinated refrigerants based on Dimethyl Ether (R-E170), CO 2 (R-744) and Propylene (R-1270) are recommended. [ABSTRACT FROM AUTHOR]