International. In a paper published in August in the International Journal of Refrigeration (IJR), the National Institute of Standards and Technologies (NIST) identifies 22 refrigerants that could theoretically function as low-GWP alternatives to R134a, in air conditioning applications.
Most of the identified mixtures combine R134a with one or two other pure refrigerants.
In 2017, NIST published a first study indicating that no low-GWP alternative single-component refrigerant was simultaneously non-flammable and worked well in an air conditioning system.
With this new research, NIST suggests that blends could offer better solutions, particularly when it comes to flammability.
In fact, the study was conducted at the request of the U.S. military, which wants to resolutely resort to non-flammable refrigerants. "Home applications are moving toward mixtures that are at least slightly flammable," according to Ian Bell, an engineer at NIST.
Researchers participating in the August 2019 IJR study selected 13 fluids capable of producing a mixture with the desired characteristics in terms of flammability, GWP, COP and volumetric capacity. Among these fluids were 8 HFO, 4 HFC of low GWP and CO2. The projection was based on a simplified cycle model, but with the inclusion of pressure drops in the evaporator and condenser.
But no mixture of these 13 fluids met all the selected criteria.
The study identified 14 non-flammable mixtures that offered a reduction in GWP of up to 51 percent compared to the GWP of R-134a of 1300. Eight additional mixtures that were marginally flammable were identified with GWP reductions of up to 99 percent. The researchers simulated the performance of these 22 mixtures in a detailed cooling cycle model. The study was conducted with computational tools; the researchers plan to conduct laboratory experiments to verify the results.
The study revealed several trends. The most promising non-flammable mixtures have a slightly lower efficiency compared to R-134a. These non-flammable mixtures have a GWP lower limit of 640; this is due to the need for a large amount of R-134a in the mixture to suppress the flammability of low GWP fluids. Other mixtures containing a significant amount of carbon dioxide were also not flammable, but had a very low efficiency compared to R-134a and were not considered viable alternatives.
