Hydrofluorocarbon (HFC) refrigerants are potent greenhouse gases (GHG) that are being phased down by the U.S. and 196 other countries under the Montreal Protocol of 2016. NIST helps the air conditioning industry find new refrigerants to replace HFCs that will have a much lesser impact on the climate.
NIST researchers conducted a comprehensive search for refrigerant molecules that could replace hydrofluorocarbon (HFC) refrigerants used in modern air conditioning systems. In partnership with the U.S. Department of Energy, NIST applied its extensive expertise in refrigerant fluid thermodynamic property research to identify the best candidate replacement refrigerants. Starting with a NIST-developed computational method, researchers looked at the molecular structures of millions of chemicals in a public database, making estimates about which molecules would best match fluid property characteristics of an ideal refrigerant.
Of the 60 million screened, just 138 fluids met the criteria. Next, NIST simulated the performance of those 138 fluids in air conditioning equipment and assessed the fluids’ safety, yielding a set of just 27 candidates. While the exhaustive search of millions of chemicals did not yield a single ideal replacement, NIST’s list offers potential new options because the 27 refrigerants are not yet included in the Environmental Protection Agency’s list of acceptable substitutes.
NIST’s findings will help the air conditioning industry and policy makers better understand options, limits and tradeoffs as they transition away from HFC refrigerants. For example, most of the best candidate materials were found to be slightly flammable. Safety codes might be changed to allow refrigerants that are slightly flammable, or there may be ways to compromise between performance, flammability and environmental properties by blending different refrigerants.