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Which HVAC Refrigerants Are the Greenest?

May 24, 2017

Multi-year study reveals mixed results on climate-friendly replacement.

There is not yet a clear replacement for hydrofluorocarbons (HFCs), which can contribute to global warming, according to a multi-year study recently completed by the National Institute of Standards and Technology (NIST). However, the research yielded valuable data that will help consumers determine what kinds of tradeoffs they should make when it comes to greener refrigerant replacements.

The study focused on possible replacements for a blend of HFCs called R-410A, a common refrigerant choice for AC systems in smaller businesses. The units are sealed, but any leaks can release HFCs into the atmosphere and refrigerant can also escape when the unit is serviced or discarded. All HFCs are greenhouse gases with high global warming potential (GWP), a measure of how much heat a gas will trap if it’s released into the atmosphere, so the study aimed to find replacement fluids with significantly lower GWPs than R-410A’s value of 1,924.

Analysis of 184,000 molecules yielded 27 low-GWP fluids that are relatively energy-efficient and chemically stable. Unfortunately, the best candidates are all at least slightly flammable, which is not allowed for most end users under U.S. safety codes. For instance, propane has a GWP of just 3, but is highly flammable. Ammonia is commonly used in large industrial refrigeration systems but is toxic and slightly flammable. Carbon dioxide has a GWP of 1 and is nonflammable, but would require high pressure and a different type of refrigeration cycle – meaning that substituting it necessitates a thorough redesign of AC equipment.

“The path forward will involve tradeoffs,” explains Mark McLinden, a chemical engineer for NIST. “Safety codes could be revised to allow the use of slightly flammable refrigerants. Blends of two or more fluids could yield a nonflammable refrigerant, but at a higher GWP.”

The study was supported by the DOE. It appears in the journal Nature Communications.

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