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Results from a new Department of Energy (DOE) Building Technologies Office (BTO) funded study in the prestigious journal Nature Communications suggest that further research and development of alternative air conditioning refrigerants is needed to meet international safety standards. With the international community focused on a planned phase-out of hydrofluorocarbons resulting from the recent Kigali Accord, this research is needed more than ever to determine the safest and most efficient refrigerants. Hydrofluorocarbons (HFCs) are greenhouse gas refrigerants that are more harmful than alternative refrigerant options.

Research funded by BTO at the National Institute of Standards and Technology (NIST) and The Catholic University of America used an exhaustive, multistaged screening process to identify the safest, greenest, and most effective refrigerants “allowed by chemistry,” ultimately concluding that of the 184,000 thermodynamically appropriate chemicals they identified, none currently meet the flammability standards under U.S. ANSI/ASHRAE 15-2013 Safety Standard for Refrigeration Systems.

NIST’s study, entitled “Limited options for low-global-warming-potential refrigerants,” focused on residential and small-commercial single-package and split air conditioning systems, but the authors said their conclusions also generally apply to room air conditioning units and other refrigeration and heat-pump systems currently using common, ozone-depleting refrigerants such as R-410A or R-22, which are still used in some developing countries today. “The takeaway is there is no perfect, easy replacement for current refrigerants,” NIST chemical engineer Mark McLinden said.

NIST’s research is the culmination of a four-year comprehensive search for the best single-component replacement for common refrigerants that will not deplete the ozone. McLinden and his colleagues started by screening through a database of more than 60 million chemical structures to search for molecules that might be able to replace current refrigerants, which yielded 138 fluids. The team then simulated each compound’s performance, screening out unstable or very toxic compounds and those with low energy efficiencies. Of the final list of 27 low-GWP fluids remaining that met NIST’s performance criteria, all were at least slightly flammable. Accordingly, this study has demonstrated to industry and policymakers that in order to meet the challenges of phasing out high-GWP refrigerants, researchers must look beyond nonflammable chemicals and work to address issues that accompany slightly flammable and flammable compounds.

This work was overseen by BTO’s HVAC, Water Heater, and Appliances Technology Manager Antonio Bouza and Technology Manager Bahhman Habibzadeh. BTO is very involved with industry and academic partners to advance innovations toward the marketplace through a diverse portfolio of HVAC, water heater, and appliance technologies.

While recent studies have found that alternative refrigerants can perform as well as conventional refrigerants in single-package and split air conditioning units across building types and climate zones, including some of the most punishing ones, the flammability of these options remained a concern. “We’ve always been well-aware of the flammability challenges of most known low-GWP replacement candidates,” Mr. Bouza said. “We are continuing to work with innovators and industry to develop alternative refrigerants that meet domestic and international safety standards.”

Mr. Bouza is optimistic, though, that planned and future research efforts will eventually break through the challenges made clear in NIST’s study. “Good research does not always result in finding a single perfect solution, it’s also building the knowledge base that guides us to sound solutions. NIST’s study helped close the door on one avenue of research, but importantly helps us to identify the research needed to see widespread usage of low-GWP refrigerants.” It is critical to know where best to put time and resources for future projects, and this NIST study is helping to narrow down the next steps in HFC alternative refrigerants.

DOE recently announced a jointly funded, $5.2 million partnership with the Air Conditioning, Heating, and Refrigeration Institute (AHRI), the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE), the Alliance for Responsible Atmospheric Policy, and other governments and international organizations to research how to safely use mildly flammable (A2L) and flammable (A3) alternative hydrochlorofluorocarbons (HCFC) and HFCs refrigerants. DOE anticipates that their research will establish the knowledge base necessary to evaluate whether the International Mechanic Code and International Residential Code could be modified to accept these low-GWP alternatives.