Of the many comforts that technology has brought to the modern world, refrigeration is located near the top of the list of conveniences. From the pleasant rush of coolness experienced upon entering an air-conditioned building after a trek through a hot and humid city to the critical temperature-regulating functions that chillers perform for many industrial processes, refrigeration has become an integral part of contemporary civilization.

A 30 lb cylinder of R-22 refrigerant. Introduced in the 1950s, R-22 became one of the most widely-used refrigerants for residential cooling. Unfortunately, R-22 released into the atmosphere depletes the ozone layer and traps 1,810 times more heat than CO2. As a result, a phase-out of the refrigerant in the U.S. began in 2010. All production and import of R-22 will be banned in the U.S. starting January 1, 2020. The only source of the refrigerant in the country after that date will be from stockpiles or reclaimed fluid from existing units. Source: Refrigerant DepotThe power to control the temperature of our surroundings, however, comes with a price. According to a 2018 study in the International Journal of Refrigeration, refrigeration consumes 20% of global energy consumption, contributing significantly to the carbon dioxide (CO₂) emissions that are driving climate change.

Furthermore, the special chemicals known as refrigerants that serve as the lifeblood of refrigeration systems – absorbing heat from areas to be cooled and releasing it in different areas in cycles of evaporation and condensation – tend to be far more potent greenhouse gases than CO₂. The Global Warming Potential (GWP) of R-410A, a common refrigerant used in air conditioning applications, is 2,088. GWP is a measure of how much heat a gas absorbs in the atmosphere relative to CO₂ over a period of time (typically 100 years). This means that a single kilogram of R-410A leaked from an air conditioning system is equivalent to the release of 2,088 kg of CO₂ emitted from a power plant. Many older refrigerants are also highly destructive to Earth’s ozone layer, a portion of the atmosphere that protects life from harmful ultraviolet rays from the sun.

[Learn more about refrigerants and search for refrigerant suppliers by specification on Engineering360.]

Regulatory pressure

Phase-down schedule for HFC’s under the EU’s F-gas regulations. The graph shows the maximum quantity of HFCs allowed to be sold on the EU market (expressed as a percentage of average levels of tonnes of CO2 equivalent from 2009 to 2012). Source: European Parliament Regulation 517/2014. (Click image to enlarge.)

As a result, global pressure is mounting to reduce the negative environmental consequences of refrigerants. The landmark 1987 Montreal Protocol, the highly successful international treaty designed to protect the ozone layer by banning substances with high ozone depletion potential (ODP) – including many legacy refrigerants – was amended for the ninth time in 2016 with the Kigali Amendment. According to the Kigali Amendment, countries will phase down high-GWP refrigerants known as hydrofluorocarbons (HFCs) by more than 80% over the next three decades. The amendment went into effect on Jan. 1, 2019, and has been ratified by 73 countries so far.

Europe’s F-gas regulations are squarely aimed at reducing the use of HFCs. The regulations introduce increasingly strict restrictions that prohibit the sale of new equipment using refrigerants with high GWP. They also phase down HFC use by limiting sales on the EU market of HFCs to a maximum quantity expressed in tonnes of CO₂ equivalent. In 2015, the maximum quantity was frozen at the average level of tonnes of CO₂ equivalent sold from 2009 to 2012. Since then and through 2030, the maximum quantity will step down over time according to a schedule.

List of prohibited products with high-GWP refrigerants according to the EU’s F-gas regulations. Source: European Parliament Regulation 517/2014, Annex III. (Click image to enlarge.)The U.S., however, is trailing other countries on the issue of limiting high-GWP refrigerants. Although the U.S. ratified the Montreal Protocol in 1988, it has not yet ratified the Kigali Amendment. Moreover, new EPA regulations limiting HFCs under the Significant New Alternatives Policy (SNAP) were blocked by the U.S. Court of Appeal for the District of Columbia Circuit in recent years and the U.S. Supreme Court has declined to hear the cases.

Although U.S. action at the federal level is lagging, the global trend to limit high-GWP refrigerants and action by U.S. states like California to implement strict HFC rules is putting pressure on manufacturers of refrigeration equipment to develop solutions that employ low-GWP refrigerants.

No magic bullet

The push for refrigerants with reduced negative environmental impact is also driving industry and academia to search for better chemicals. But developing the perfect fluid with all of the desired properties of an ideal refrigerant is no easy task. Chemical engineers are encountering fundamental tradeoffs dictated by chemical properties and thermodynamics.

An example of the tradeoffs involved in choosing the ideal refrigerant. Source: ACEEE Summer Study on Energy Efficiency in Buildings (2010). (Click image to enlarge.)Finding the ideal refrigerant is a balancing act that seeks to maximize a number of competing refrigerant characteristics. These tradeoffs include environmental attributes like ODP and GWP, safety factors like flammability and toxicity, as well as thermodynamic properties that affect system efficiency such as heat capacity, isentropic efficiency, temperature glide and pressure drop.

Improving one of these attributes tends to result in the degradation of other attributes. For example, refrigerants with very low GWP tend to have less ideal thermodynamic properties that result in inferior cooling efficiency.

The difficulty in developing the ideal refrigerant can be grasped by considering a list of its desired qualities. According to a study by Ko Matsunaga at Columbia University, the ideal refrigerant should be:

• Low cost
• Easy to handle
• Easy to detect
• Easy to manufacture
• Self-lubricating (or compatible with lubricants)
• Compatible with other materials used in the fabrication and servicing of refrigeration systems
• Fully stable inside a refrigeration system
• Low ODP
• Low GWP
• Non-toxic
• Non-flammable
• Easy to recycle or destroy including environmentally benign decomposition products.

In addition, it should:

• Have a boiling point within an acceptable range
• Not require extreme pressures of operation
• Have a low required flow rate per unit of cooling to minimize charging quantity and compressor size

Furthermore, the best refrigerant can vary based upon the specific application for which it is employed. A refrigerant’s thermophysical properties and other characteristics should be matched to the conditions encountered in the application. For example, ammonia might be an acceptable refrigerant for large-scale industrial refrigeration applications where the extra cost to mitigate its toxicity and flammability risks is justified. But it is unlikely to be accepted as a solution for residential and small commercial air conditioning systems due to those hazards.

[Discover refrigeration compressors on Engineering360 and learn more about how they work.]

A graph comparing refrigerants based on several properties, including flammability, required compressor displacement volume, GWP and coefficient of performance (COP). COP for a refrigeration system is the ratio of the heat removed from the cooled space to the work performed to remove that heat. Source: Optimized Thermal Systems, Inc. (Click image to enlarge.)

The search for a flawless fluid

The list of potential candidates for the perfect refrigerant is growing slimmer. Scientists have been steadily sifting through the periodic table in the search for a refrigerant with the ideal balance of environmental and thermodynamic properties but have so far failed to discover a flawless fluid.

Early synthetic refrigerants include chlorofluorocarbons (CFCs) like R-11, R-12, R-502 and R-13. Although thermodynamically efficient, CFCs have high ODP due to their chlorine content as well as high GWP. CFCs have been banned worldwide due to their destructiveness to the ozone layer.

Hydrochlorofluorocarbons (HCFCs) are chemically similar to CFCs but contain less chlorine. As a result, they generally have lower ODP than CFCs but they still have high GWP. The common HCFC R-22 is banned in the EU and is being phased out in the U.S.

A comparison of the properties of several refrigerants. Source: Optimized Thermal Systems, Inc. (Click image to enlarge.)

Hydrofluorocarbons (HFCs) contain no chlorine and pose little threat to the ozone layer, with very low or zero ODP. HFCs, however, have high GWP, presenting a risk of warming the environment if leaked into the atmosphere. Common HFCs include R-134a, R-404A, R-407C and R-410A.

Synthetic refrigerants called hydrofluoroolefins (HFOs) are available with both zero ODP and very low GWP such as R-1234yf and R-1234ze. Their cost, however, is high and there are concerns about chemical stability limiting their long-term operation.

Natural refrigerants with no ODP and very low GWP include ammonia (R-717) and CO₂ (R-744). These refrigerants were used prior to the 1930s before the introduction of R-12, the first member of the Freon family of products. Ammonia, however, is toxic and CO₂ systems have low efficiency and require very high operating pressures.

Other natural refrigerants include hydrocarbons like methane (R-50), propane (R-290), propene (R-1270, also known as propylene) and butane (R-600). Hydrocarbons tend to have very low GWP, no ODP and very good thermodynamic properties. For example, propane has nearly double the latent heat of vaporization of R-22, resulting in a greater cooling effect for the same mass of refrigerant. On the other hand, hydrocarbons are highly flammable, earning them an ASHRAE 34 safety classification of A3, and they pose an explosion risk.

Promising candidates

A perfect refrigerant with ideal properties for every application does not exist. Some fluids, however, are better than others, as evidenced by the bans placed on ozone-depleting refrigerants. Although the perfect refrigerant with an ideal mix of environmental and physical properties has not yet been developed, some promising candidates exist.

R-32’s GWP is 68% lower than the mainstream refrigerant R-410A. Source: DaikinR-32, which comprises 50% of the common refrigerant R-410A (the other 50% is R-125), has a number of advantages. Compared to legacy refrigerants, R-32 has lower GWP and zero ODP. R-32’s GWP of 675 is 68% lower than R-410A’s GWP of 2,088. In addition, R-32 has favorable thermodynamic properties that improve cooling efficiency. Furthermore, R-32 uses up to 30% less charging volume compared to R-410A.

R-32 is not, however, perfect. The refrigerant significantly exceeds the EU’s GWP target of 150 for low-GWP refrigerants. R-32 is also slightly flammable, with an ASHRAE 34 rating of A2L.

Perhaps the best use of R-32 is as a near-term bridge to zero-GWP solutions. Daikin, a Japanese manufacturer of air conditioners and the creator of R-32, itself announced this year that it is working on a low-GWP successor to R-32, recognizing the continual push to reduce refrigerant GWP. Results from this research are expected in March 2023 but the actual introduction of the refrigerant to the market will take several more years for completion of safety approvals and new product development.

R-152a is another refrigerant with future potential. It has zero ODP and meets low-GWP requirements, with a GWP of around 140. The major drawback of R-152a is its ASHRAE 34 rating of A2, indicating flammability. As a result, its use has generally been limited so far, functioning as a fractional component of refrigerant blends.

[Read about a recent study by the National Institute of Standards and Technology (NIST) that identified several alternative low-GWP refrigerant blends to replace R-134a in an air-conditioning application.]

GWP fails to provide a complete picture

Chemical engineers have not yet found the perfect refrigerant, but as the search continues, all aspects of an ideal solution should be considered.

Importantly, looking only at a refrigerant’s GWP provides an incomplete picture of its climate impact. Indeed, GWP is only important if the refrigerant is released to the atmosphere, for example by a system leak or a purge of old refrigerant to the atmosphere by a maintenance technician. For many systems, however, refrigerant release is minor.

According to a study by the American Council for an Energy-Efficient Economy (ACEEE), although mobile air conditioners and refrigeration systems for large supermarkets have been known to leak up to 30% of their refrigerant each year, stationary air conditioning and refrigeration systems with factory-sealed equipment leak only 2% of their refrigerant annually.

[Discover refrigerant leak detectors on Engineering360.]

The major climate impact for low-leakage systems is not the GWP rating of the refrigerant, but the amount of energy consumed to run the system. If the source of this electricity is carbon-hungry power plants burning coal or natural gas, the thermodynamic efficiency of the refrigerant is much more important than its GWP. For low-leakage systems, up to 98% of the global warming impact is indirect (energy consumption) instead of direct (refrigerant leakage).

Total Environmental Warming Impact (TEWI) measures the total warming effect of a refrigerant, both indirect (energy consumption) and direct (refrigerant leakage). The TEWI metric indicates that efficiency is the most important parameter for evaluating refrigerants in low-leakage systems. Source: ACEEE Summer Study on Energy Efficiency in Buildings (2010). (Click image to enlarge.)In light of this, a singular focus on low-GWP refrigerants may be misguided. Instead, a better measure of refrigerants’ effect on the environment is Total Environmental Warming Impact (TEWI). TEWI takes into account both the direct and indirect warming effects of refrigerants, considering both the warming potential of leaked refrigerants and the warming potential of carbon emitted in the generation of electricity that powers refrigeration systems.

By this measure, high-GWP refrigerants can actually be superior to low-GWP refrigerants, producing less warming overall. Since the vast majority of the global warming impact for low-leakage systems is due to energy consumption, the overall warming impact of a high-GWP refrigerant with high cooling efficiency (and thus low TEWI) can be lower than a low-GWP refrigerant with low cooling efficiency (and high TEWI). Efficiency is the key metric by which to evaluate a refrigerant’s impact on the environment in typical circumstances in which most of the electricity that powers the refrigeration system is generated by fossil fuel-burning power plants.

The refrigeration industry has a long road of change ahead of it. As the sector attempts to adapt to changing regulations, manufacturers will continue to advance products that reduce environmental impact while minimizing energy use and cost. Solutions will come in many forms, including hybrid systems that use different refrigerants for primary and secondary cooling loops, cascade configurations and other innovative designs that employ both known and yet-to-be-developed working fluids. The search for the ideal refrigerant is not yet concluded.

Resources

Matsunaga, K. O. "Comparison of environmental impacts and physical properties of refrigerants." [PDF] Earth Engineering Center, Columbia University (2002).

Pham, H., and H. Sachs. "Next Generation Refrigerants: Standards and Climate Policy Implications of Engineering Constraints." [PDF] American Council for an Energy-Efficient Economy (ACEEE) Summer Study on Energy Efficiency in Buildings (2010): 282-294.

Ciconkov, Risto. "Refrigerants: There is still no vision for sustainable solutions." International Journal of Refrigeration 86 (2018): 441-448.

Bellos, Evangelos, and Christos Tzivanidis. "Investigation of the Environmentally-Friendly Refrigerant R152a for Air Conditioning Purposes." Applied Sciences 9.1 (2019): 119.

Nasuta, D. & Radermacher, R. An Evaluation of R32 for the US HVAC&R Market. [PDF] Optimized Thermal Systems, Inc. (2016).