Air conditioning faces a flammable future

USA: In the search for more than 60 million chemicals to replace R410A in air conditioning systems, only 27 liquids were found with reasonable efficiency – but all of them are at least highly flammable.

The multi-year study was conducted by researchers from the US National Institute of Standards and Technology (NIST) to identify the best candidates for future use as air conditioning refrigerants that have the least impact on the climate.

The study did not find an ideal refrigerant that combines a low GWP with other desirable performance and safety features such as non-flammable and non-toxic. All 27 NIST fluids identified as the best from a performance standpoint are highly flammable at best, which US safety codes do not allow for most end-use applications. And several liquids on the list of refrigerants are highly flammable.

The authors of the report published in Nature Communications claim that the 27 liquids are the “best” low GWP liquids allowed by chemistry.

“It is highly unlikely that better performing fluids will be found, and unknown risks associated with the lesser-known fluids may further narrow the list,” say the authors.

“There is no perfect and easy takeaway substitute for current refrigerants,” said Mark McLinden, chemical engineer at NIST. “When we went into the study, we thought that there must surely be something else. It turns out not so much. So it was a bit surprising, a bit disappointing, ”he said.

The recent global decision to phase out HFCs under the Montreal Protocol, which was added to the already existing European phase out of F-gas, has resulted in regulations removing many of the highest GWP refrigerants from certain applications. These include common refrigerants such as R404A and R134a, for which suitable alternatives are known.

“It is highly unlikely that better performing fluids will be found, and unknown risks associated with the lesser-known fluids can further narrow the list.”

R410A, a mixture of R32 and R125, and currently the dominant refrigerant in small air conditioning systems, is somewhat exposed with its relatively high GWP of around 2000 – 50% higher than R134a. Many believe that a replacement for the R410A must be found if the global phase-down goals are to be met.

R32 was introduced by Daikin and others for use in small fractions, and propane is also being considered in similar applications in some Far Eastern markets. However, their flammability precludes their use under current national and international safety standards in all but the smaller systems.

“The way forward will involve compromise,” said Mark McLinden. “The security codes could be revised to allow the use of highly flammable refrigerants. Mixing two or more liquids could result in a non-flammable refrigerant, but with a higher GWP. Carbon dioxide is not flammable but would require a complete redesign of the AC devices. “

Since 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 screening resulted in 184,000 molecules being further considered.

Screening for energy properties corresponding to liquids that can be used in small AC systems and a GWP of less than 1,000 revealed 138 liquids. This included the new low GWP HFOs R1234yf and 1234ze out of an incredible 45 HFOs.

The researchers then simulated the performance of these 138 compounds in air conditioning systems. Another screening to exclude chemically unstable or very toxic compounds or those with low energy efficiency resulted in the final list of 27 liquids with low GWP.

An abbreviated list of the 27 fluids identified. Refrigerants such as ethane, fluoromethane and CO2 were not simulated because they would be almost critical or supercritical in the condenser

The report focuses on single component refrigerants (pure liquids) but recognizes that refrigerant blends offer additional possibilities, although the tradeoff to reduce flammability will be higher GWP.

“We do not consider mixtures explicitly, but for the sake of completeness we include several liquids that, on their own, would not be suitable low GWP liquids, but which could be useful as a mixture component,” the report says.

“Looking ahead, the conclusions of the NIST study indicate that compromises in planning for the future need to be identified and addressed,” said McLinden.

“For example, how should the security codes be changed to ensure that flammable refrigerants can be used safely? Mixtures of different refrigerants can offer a compromise between safety and GWP. For example, a low GWP but combustible fluid mixed with a non-combustible but high GWP could result in a non-combustible fluid with a moderate GWP, noted McLinden.

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