Researchers involved in a multi-year study to find climate friendly alternatives for use in the air conditioning industry have described the results as disappointing.
The US-based National Institute of Standards and Technology (NIST) has just completed the study to identify the "best" candidates for HVAC use that will have the lowest impact on the climate.
Unfortunately, all 27 fluids NIST identified as the best from a performance viewpoint are at least slightly flammable, which is not allowed under US safety codes for most end uses.
Several fluids among the list of refrigerants are highly flammable, including propane.
Basically, the NIST study failed to find an ideal refrigerant that combined low Global Warming Potential (GWP) with other desirable performance and safety features like being nonflammable and nontoxic. The results are published in Nature Communications and features an interview with NIST chemical engineer, Mark McLinden.
"The takeaway is there is no perfect, easy replacement for current refrigerants," he said.
"Going into the study we thought surely there has to be something else. Turns out, not so much. So it was a bit surprising, a bit disappointing.
"The path forward will involve tradeoffs. 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. Carbon dioxide is nonflammable, but would require a complete redesign of AC equipment."
The four-year search by NIST for low-GWP replacement fluids was initiated to help US policymakers with a global phasedown under the Montreal Protocal. The NIST study focused on possible replacement fluids for small AC systems typical for homes and small businesses. Currently R-410A is the most popular refrigerant for these systems.
The study screened a database of more than 60 million chemicals, estimating the properties based solely on their molecular structure. Because all current refrigerants are small molecules, the NIST search was limited to molecules with 18 or fewer atoms and only eight elements that form compounds volatile enough to serve as refrigerants. This initial screen resulted in 184,000 molecules to be considered further.
Screening for energy properties corresponding to fluids usable in small AC systems and GWP of less than 1,000 yielded 138 fluids. The researchers then simulated the performance of these 138 compounds in air conditioners.
Further screening to rule out chemically unstable or very toxic compounds or those with low energy efficiency resulted in the final list of 27 low-GWP fluid.
Propane has a GWP of 3, much lower than R-410A's value of 1,924. Potential refrigerants with the lowest GWPs include ammonia, commonly used in large industrial refrigeration systems but toxic and slightly flammable, and dimethylether, a propellant and potential fuel that is only slightly less flammable than propane.
Carbon dioxide has a GWP of 1 and is nonflammable, but it would require a different type of refrigeration cycle operating at high pressures, the researchers said.
Other low GWP compounds include hydrofluoroolefins, a current focus of industry research but slightly flammable. NIST's list may offer new ideas because it includes refrigerants currently not included on the EPA list of acceptable substitutes.
The end result? McLinden said blends of different refrigerants may offer a compromise between safety and GWP.