The search for low-global-warming refrigerants

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2017

But finding compounds with the desired properties is proving challenging for researchers.

 

KEY CONCEPTS
• A new effort is underway to identify refrigerants that exhibit low ozone depletion and low global-warming-potential.
• In a four-step process, researchers reduced the number of potential candidates from 60 million to 27.
• None of the 27 candidates meet the low reactivity, low-global-warming-potential needed to replace the refrigerant R-410A.

DURING THE PAST 30 YEARS, regulatory drivers have changed the types of refrigerants used in stationary and mobile applications. This process started in 1987 with the ratification of the Montreal protocol that is leading to the phasing out of chlorofluorocarbon and hydrochlorofluorocarbon refrigerants that have been very effective because of their stability, non-flammability and low toxicity but were found to deplete ozone in the Earth’s stratosphere.

The need for refrigerants that did not deplete ozone prompted the refrigeration industry to move to hydrofluorocarbons. Dr. Mark McLinden, chemical engineer in the Applied Chemicals and Materials Division of the National Institute of Standards and Technology in Boulder, Colo., says, “One of the key hydrofluorocarbons used today is R-410A, a 50:50 blend of difluoromethane (R-32) and pentafluoroethane (R-125). This refrigerant is widely used in small, self-contained air conditioning systems” (see Figure 1).


Figure 1. Replacing a refrigerant known as R-410A, widely used in small, self-contained air conditioning systems, with a low reactive, low global-warming-potential alternative, is proving to be challenging. (Figure courtesy of the National Institute of Standards and Technology.)

While the move to hydrofluorocarbons helped stabilize the ozone layer, another problem, global warming, has become apparent. The key parameter measured is global-warming-potential (GWP), which is defined as the ability of a substance to heat the atmosphere to carbon dioxide.

Hydrofluorocarbons have high GWP values, meaning that the refrigeration industry is now looking for new alternatives that combine low ozone depletion and low GWP values. McLinden and his fellow researchers conducted a first screening for refrigerants with low GWP five years ago. As described in a previous TLT article (1), the researchers used a systematic approach by searching the PubChem database, which lists more than 60 million chemical structures. 

The researchers reduced the number of possible chemical structures down to 56,000 candidates. Further screening reduced the number of compounds to 1,200. Among these, the largest class of compounds were fluorinated olefins (HFOs), which are now one of the leading class of refrigerants under evaluation.

As part of an ongoing project to identify new refrigerants, McLinden and his research team have just completed another study by again searching the PubChem database (2).

FOUR-STEP SCREENING PROCESS
McLinden and his colleagues used a four-step process in screening for specific compounds that exhibit low GWP values yet function effectively as refrigerants. He says, “We evaluated potential candidates through the use of thermodynamic and environmental screens and then simulated refrigerant performance in small air-conditioning systems. Our objective was to not only include known low-GWP refrigerants in our study but to also search for new candidates.”

The first step focused on determining the ideal properties for a refrigerant. The researchers examined the effect of thermodynamic parameters (such as critical temperature) on performance parameters such as coefficient of performance (removed heat divided by work input to the compressor) and volumetric capacity (refrigeration effect per unit volume of refrigerant entering the compressor). 

McLinden says, “Refrigeration is a thermodynamic cycle, so it was important to evaluate the thermodynamic properties of the candidates. By applying the results of the first step, we were able to reduce the number of candidates from over 60 million to about 184,000.”

This was accomplished by restricting the candidates to molecules with fewer than 18 atoms and only considering specific elements (carbon, hydrogen, fluorine, chlorine, bromine, oxygen, nitrogen and sulfur). McLinden says, “We needed to restrict the size because large molecules have boiling points that are too high and heats of vaporization that are too low to be refrigerants.”

In the second step, the researchers further reduced the number of candidates to 138 compounds by evaluating the critical temperature and GWP of each compound. McLinden says, “The critical temperature is the temperature above which there is no difference between the liquid and vapor phases of a compound. Once above the critical temperature, the compound cannot boil or be condensed.”

The researchers established a critical temperature range between 320 K and 420 K (47 C and 147 C), the optimum temperature range for refrigerants that must operate in current refrigeration equipment.

A third screening was to evaluate the chemical stability and toxicity of the 138 compounds. Candidates that contained reactive groups such as peroxides, ketenes and allenes were eliminated in this phase of the study.

The final screening was simulating each of the remaining compounds in a typical air conditioning system. McLinden says, “We used a process describing the equipment that is typically found in an air conditioner in a generic fashion, but it was important that we did not consider the system to be operating in an ideal manner where there is no pressure drop or where the compressor is perfectly efficient.”

In screening the compounds, the researchers reduced the number of viable candidates to 27. McLinden says, “While 27 may seem to be a generous list to choose from, there are not many candidates that can operate at the high pressure typical of equipment currently using R-410A.”

The researchers conclude that none of the 27 compounds meet the low-GWP, low-reactivity requirements needed to directly replace R-410A. McLinden says, “All of them are either flammable or mildly flammable. Our next step is to evaluate blends of compounds where we may be able to combine low-GWP, flammable compounds with non-flammable, high-GWP compounds to find an acceptable compromise of the desired properties.”

The blend study will start in the near future according to McLinden. Additional information on this work can be found in a recent article (2) or by contacting McLinden at markm@boulder.nist.gov.

REFERENCES
1. Canter, N. (2012), “Systematic identification of new refrigerants,” TLT, 68 (12), pp. 10-11.
2. McLinden, M., Brown, J., Brignoli, R., Kazakov, A. and Domanski, P. (2017), “Limited options for low-global-warming-potential refrigerants,” Nature Communications, 8, Article number 14476.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.