Many modern-day refrigerants are hydrofluorocarbons (HFCs), which have attracted attention recently as being a potent greenhouse gas. HFC’s are widely used in air conditioning systems and their usage is anticipated to rise, with their negative impact on global warming also being predicted to increase.
There are plans to phase-out HFCs in air-conditioning systems and new replacement fluids are currently being investigated. Unfortunately, after recent studies, it appears that the options are slightly limited.
HFCs were developed in the 1990’s as a replacement to the ozone-damaging chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). They are now the most dominant cooling agent in new refrigeration, air-conditioning (AC) and heat-pump equipment. HFCs are widely used in small, and self-contained, AC systems.
HFCs replaced HCFCs in many AC systems, but even now HCFCs are still used in developing countries. Even though the replacement of CFCs and HCFCs by HFCs reduced the risk to the ozone layer, they still possess enough high global-warming-potential (GWP) to warrant research efforts into finding an alternative solution.
Refrigerants are the essential working fluids in a vapour compression refrigeration cycle. They absorb heat at low temperatures in the evaporator and release it at high temperatures through a condenser.
A team of US researchers has got together and scoured the PubChem database (a database with more than 60 million chemicals) to find potential fluids with chemical, environmental, thermodynamic, flammability and safety properties necessary for a refrigerant- of which their findings yielded a few potential options.
The researchers screened a comprehensive chemical database searching for critical temperature and global warming potential screens and simulated the potential chemicals performance in small air conditioning systems. By using evolutionary algorithms, the researchers could examine the performance of potential fluids in an idealised system.
However, the researchers found that the efficiency-versus-capacity trade-off in an ideal analysis is not present when a real system is considered. They found that the maximum efficiency occurs at a high volumetric refrigeration capacity, but unfortunately, there are not many available fluids in this range.
A low-GWP fluid must also possess many other properties. These can include zero (or very low) ozone-depletion potential, chemical stability, thermodynamic properties relative to the application, low toxicity, non-flammable (although currently being reconsidered) and materialistic compatibility with the unit construction.
After exhausting the list of potential candidates, the researchers only found a few pure liquids that have the potential to be used as refrigerants. There are also many refrigerant blends currently in use and the research included several of these, but was not a complete list as the focus was on single-component pure fluids.
The type of fluid that yields a great potential does vary depending on the intended refrigeration application and high-pressure fluids do not apply to all systems. Central plant chillers focus around low-pressure fluids but the focus was the application of small AC systems- where medium-low pressure refrigerants work best.
Most of the fluids that seemed to have potential at first turned out to be classed as flammable or mildly flammable. Six of these slightly flammable, novel molecules were identified, but present unknown risks, are tetrafluorodioxole, trifluoromethanethiol, trifluoropropyne, difluoromethanethiol, (E)-1,2-difluoroethene (R-1132(E)) and tetrafluoromethaneamine.
However, this prompted the question- if these molecules are still flammable, why not use a known flammable molecule, with known risks, such as propane? But obviously, this can’t be considered under current legislations.
Whilst the regulations are being considered to allow for flammable fluids, they currently are not being considered as a replacement for HFCs. But, they could be a potential consideration for further investigation in the future if the laws do change.
The other potential is refrigerant blends. 1,1,2,2-Tetrafluoroethane (commercially known as R-134) is a molecule that has been previously considered but was never used commercially. It is now been stipulated that it could be a good candidate for refrigerant blends.
Refrigerant blends are currently being developed and tested, but they don’t offer as low of a GWP as pure fluids do, but they minimise the flammability and in some cases, can remove it completely. It is a compromise that is being considered and developed, but it does not offer the ‘ideal’ refrigerant that many people are searching for.
The problems in finding a new pure refrigerant not only lies in its flammability. There are also limitations on size, as the thermodynamics only allows for small molecules, of which there are a finite amount. The unknown risks of some molecules promote the apprehensiveness to use such molecules as years of R&D could yield a useable molecule, so a number of people willing to take a chance on these molecules will be low- even if there is potential.
This screening has found what is to be considered the best, currently available, potential molecules to replace HFCs. Barring the discovery of a fantastic refrigerant blend, or the flammability laws changing, it begs the question- If these are the best choices, what chance do we have of efficiently replacing HFCs?
Such is the challenge of replacing HFCs- and it is going to be a long process.
McLinden M. O., Brown J. S., Brignoli R., Kazakov A. F., Domanksi P. A., Limited options for low-global-warming-potential refrigerants, Nature Communications, 8, 14476
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