Tom Christen
Refrigeration and Air Conditioning
January 15, 2013
Ultra-Low Global Warming Potential (GWP) Refrigerants
Traditional refrigerants used for refrigeration and air conditioning are hydrofluorocarbons
(HFCs) and hydrochlorofluorocarbons (HCFCs). Both HFCs and HCFCs have serious ozone depleting
effects along with a high GWP, which has caused governments around the world to promote phasing out
their use.
GWP is a measure of how much heat a greenhouse gas traps in the atmosphere. It compares the
total heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar
mass of CO2. The baseline greenhouse gas used in these calculations is Carbon Dioxide (CO2) which has a
GWP of 1. If a specify gas has a GWP of 100, it would mean that this gas traps 100 times more heat in
the atmosphere than CO2. An ultra-low GWP refrigerant traditionally has a GWP under 50 and also does
not have any harmful effects on the ozone layer.
ASHRAE Standard 34-2007 breaks down refrigerants into different safety groups based on
flammability and toxicity. Chart 1 below shows how these safety groups are broken down, along with
how they are named3:
Safety Group
High Flammability
A3
B3
A2
B2
A2L*
B2L*
A1
B1
Low Toxicity
High Toxicity
Low Flammability
No Flame Propagation
*A2L and B2L refrigerants have a max burning velocity of 10 cm/s
Chart 1 – Safety Group Classifications
There are 4 main ultra-low GWP refrigerants: Carbon Dioxide, Ammonia, Hydrocarbons (e.g. R290, R-22a), and A2L fluids (e.g. R-1234yf, R-1234ze). CO2 has low toxicity and flammability, so is
therefore put into safety group A1. Ammonia has high toxicity and moderate flammability which puts it
into safety group B2. Hydrocarbons generally have low toxicity but high flammability, so it is put into
safety group A3. A2L fluids generally have low toxicity and very low flammability which puts them into
safety group A2L. This is summarized into chart 2 below3:
Refrigerant
Safety Group
Refrigerant Designation
GWP
Carbon Dioxide
A1
R-744
1
Ammonia
B2
R-717
0
Hydrocarbons
A3
R-290, R-600, etc.
<5
A2L Fluids
A2L
R-1234yf
4
Chart 2 – Refrigerant Safety Groups
Using CO2 as a refrigerant has many advantages and disadvantages associated with it. One
advantage is that it has zero ozone depletion potential (ODP). ODP is a measure of the amount of
degradation that a chemical compound can cause to the ozone layer, with the maximum ODP being 1.0
for a molecule containing 3 chlorine atoms. Another advantage is that the GWP of CO2 is just 1, which is
very low compared to HFCs and HCFCs2,3. Additionally, CO2 has a high volumetric cooling capacity which
makes it more efficient in refrigeration than a compound with low volumetric cooling capacity. CO2 is
also able to work well in various refrigeration cycles, such as cascade, CO2 only, and as a secondary fluid.
Lastly, CO2 is considered non-toxic and non-flammable. One disadvantage of CO2 is that it is lethal to
humans at high doses, which is any concentration of over 10% (100,000ppm). It also requires very high
operating pressures compared to other refrigerants, which requires more robust design of piping and
also high safety factors when designing a refrigeration system.
Carbon Dioxide currently has a variety of applications which it is well suited for, such as: Motor
Vehicle Air Conditioning (MVAC), Small Stand-Alone Refrigeration, Supermarket Systems, Industrial
Systems, and Heat Pumps3.
CO2 refrigeration systems face a variety of regulations in the USA, Europe, and Japan. In the
USA, CO2 is approved by the Environmental Protection Agency (EPA) for use in MVAC systems in cars
and trucks, retail food refrigeration, cold storage warehouses, and industrial refrigeration. It is not
currently EPA approved for any air-conditioning applications. Additionally, vending machine
manufacturers are currently being allowed to test new vending machines using CO2 as a refrigerant3.
With all the advantages to using CO2 as a refrigerant and minimal disadvantages, you may find yourself
wondering why it is not used more in other applications. The answer to this is simple. CO2 does not
currently have the EPA’s Significant New Alternatives Policy (SNAP) approval to replace typical
refrigerants in other applications. SNAP is the EPA's program to evaluate and regulate substitutes for
HFCs and HCFCs that are being phased out under the ozone protection provisions of the Clean Air Act
(CAA). In Europe, equipment using CO2 as a refrigerant falls within the scope of the Pressure Equipment
Directive (PED) which “categorizes equipment based on pressure and volume.” Based on the PED, CO2
systems are subject to extra reporting and safety assessments than traditionally refrigerants which
causes companies to be hesitant to invest in these systems. In Japan, CO2 is regulated under the High
Pressure Gas Safety Act (HPGSA). One problem with the HPGSA is that it does not cover mobile
applications. The HPGSA applies to stationary applications like air-conditioning and refrigeration, and
does not impose any barriers that would cause companies in Japan to turn away from using CO23.
Overall, using CO2 as a refrigerant does not face any major regulations in the USA, Europe, or Japan.
Using Ammonia as a refrigerant has many advantages and disadvantages connected with it. One
advantage is that it has zero ODP and a very low GWP, similar to carbon dioxide3. Another advantage of
ammonia is that it has relatively high energy efficiency. Ammonia has the highest coefficient of
performance (COP) among all refrigerants in 2009 at 3-10%. Due to this relatively high COP, less power is
consumed when using ammonia as refrigerant which helps indirectly to lower CO2 emissions that are
direct products of power plants. Another advantage of using ammonia as a refrigerant is its excellent
thermodynamic properties. For example, ammonia has a high critical temperature (132oC) which
permits a higher COP. Additionally, the specific heat of ammonia is 4 times that of R-22, and the latent
heat is 6 times that of R-22. This allows for ammonia systems to use smaller piping compared to R-22
systems for the same application6. One very important advantage to using ammonia is the fact that it is
very easy to detect a leak. Ammonia has a very pungent smell, which allows for leaks to be instantly
identified before people are harmed. Ammonia is also very relatively cheap compared to traditional
refrigerants. For example, one kg of ammonia is 6-8 times cheaper than a kg of R-22. This price
difference is widened when comparing to other HFCs, especially in countries where there are high ecotaxes. Ammonia does however have some fairly significant disadvantages. One disadvantage would be
that it is highly toxic. If a leak is not detected in time, fatalities could easily occur. Ammonia is also
moderately flammable, which puts it into safety class B2 according to ASHRAE Standard 34-20073.
Ammonia is currently used in a range of applications, such as large industrial systems,
supermarket systems, indirect air conditioning, and chillers2,3.
Ammonia refrigeration systems face many regulations for use in the USA, Europe, and Japan,
mainly due to its toxicity. Once ammonia is officially switched to safety class B2L from class B2, some of
these regulations may change to benefit using ammonia as an alternative refrigerant. In the US,
ammonia is currently EPA SNAP approved in most refrigeration and air-conditioning applications,
however certain restrictions do apply. ASHRAE Standard 15-2007 establishes requirements for safely
applying ammonia in refrigeration systems. The EPA and Occupational Safety and Health Association
(OSHA) also impose requirements on companies when using large amounts of ammonia. One specific
requirement is that having ammonia on company premises requires a risk management and project
safety program to be implemented2. These requirements all impose a large burden on manufacturers
and facilities and may turn them away from using ammonia as a refrigerant. In Europe, PED limits
pressure and volume limits of flammable and toxic refrigerants, such as ammonia. International
Electrotechnical Commission (IEC) Standard60355-2-40 also regulates ammonia, as it places charge
limits on flammable air-conditioning equipment3. This presents a significant barrier for companies
looking to use ammonia in large commercial equipment. European Norm (EN) 378 additionally requires
machine rooms using ammonia to install a catchment system, install emergency washing facilities, and
not allow ammonia to come in contact with zinc. When ammonia and materials such as zinc come in
contact, an explosive chemical reaction occurs. Although ammonia is considered moderately flammable,
equipment using ammonia is exempt from sealing potential ignition sources under EN378, since its
flammability is on the low end of the flammability spectrum. In Japan, HPGSA only applies to stationary
applications like air-conditioning and refrigeration, and it does not cover mobile applications. Ammonia
applications are required to meet certain safety measures under this act, such as the explosion-proof
structure requirement. There also exists an uncertainty in how to regulate flammable refrigerants such
as ammonia based on the HPGSA, which provides an unnecessary burden on manufacturers. However,
this burden is not likely to be prohibitive3. Overall, ammonia is likely to remain restricted to industrial
applications in these 3 regions due to its high toxicity.
Using hydrocarbons (HCs) as refrigerants have many advantages and disadvantages connected
with them. Some examples of HC refrigerants are R-290 (propane), R-600a (isobutene), and R-600. One
advantage is that they have a low GWP (usually less than 5 for most HCs) and they have zero ODP,
similar to CO2 and ammonia7. Another plus for HCs is that their atmospheric lifetime is usually less than
one year. They also have a higher efficiency than many HFCs and HCFCs, as well as CO2 and ammonia, in
small scale systems such as a household refrigerator5. Another advantage is that HCs are considered
non-toxic7. One big knock against using HC refrigerants is that they have a very high flammability. This
leads to stringent requirements for manufacturers to meet regarding leakage. Even a small leak could be
end up being catastrophic due to this high flammability2,3. Another disadvantage to using HCs as
refrigerants is that their efficiency decreases as you increase the size of the application you are using
HCs for. This results in HCs having a lower efficiency than CO2 and ammonia in large scale applications.
Hydrocarbons are currently used in a variety of applications such as small commercial
refrigeration equipment, small commercial and residential air-conditioning systems, industrial
applications, and large chillers3,4. One specific application which has been used recently is shown in
Figure 1 below. It shows a hydrocarbon refrigeration display cabinet for use in supermarkets. The
specific cabinet was used in a zero-carbon Tesco store recently opened in Thailand1.
Figure 1 – Hydrocarbon Refrigeration Display Cabinet
Hydrocarbon refrigeration systems face fairly stringent regulations in the USA, Europe, and
Japan due to their high flammability. In the US, hydrocarbons are not approved by EPA SNAP as an
acceptable substitute for any air-conditioning or refrigeration system except for industrial applications.
However, the EPA has proposed SNAP approval for several HC refrigerants to be used in household
refrigerators and freezers, and for propane to be used in retail food refrigerators and freezers3.
Additionally, ASHRAE Standard 15-2007 limits non-A1 refrigerants in residential and commercial
residences. Also, several states have recently modified their State Regulations to allow for the use of
HCs in mobile air-conditioning applications, which is a good step forwards for HC refrigerants. In Europe,
regulations are generally less restrictive than the US regarding HCs. Safety is addressed through “the use
of charge limits and safety requirements for application, which permit the use of hydrocarbons in nearly
all applications3.” EN 378 contains charge limits based on height of equipment which make floor
standing air-conditioning units impractical. Ceiling and elevated systems are favored. The IEC charge
limits restrict the use of HCs in large commercial systems, similar to SNAP in the US. Additionally, the
PED limits pressure and volume limits of all flammable refrigerants, such as HCs. In Japan, additional
safety requirements such as the explosion-proof structures are required for the use of HCs due to their
high flammability. There is currently uncertainty on how to regulate mobile air-conditioning systems
based on a lack of information in the HPGSA3. Overall, HC refrigerants are likely to remain limited to
small applications due to flammability concerns.
A2L fluids are very similar to A2 fluids, with one difference. A2L fluids have a burning velocity
lower than 10 cm/s, putting them in a lower flammability class than A2 fluids. Using A2L fluids as
refrigerants has advantages and disadvantages associated with it. Some examples of A2L fluids are R1234yf, R-1234ze, and R-32. One advantage to using A2L fluids is that they have a lower GWP than HFC
and HCFC refrigerants. For example, R-1234yf has a GWP of 4 and R-32 has a GWP of 675, compared to
R-22 with a GWP of 1810. Additionally, the ODP is very low compared to HFCs and HCFCs. One
disadvantage for using R-1234yf specifically is that it has a low heat capacity which requires large heat
exchange coils. This adds to the cost of the overall system. Another disadvantage is that A2L fluids are
mildly flammable, which limits their use3.
A2L fluids are currently used in a few applications, such as residential and commercial airconditioning, commercial refrigeration, and also MVAC3.
A2L fluids face a good amount of regulations in the USA, Europe, and Japan. In the USA, ASHRAE
Standard 34-2007 currently recognizes 2L fluids, but Standard 15-2007 currently does not. For this
standard, revisions regarding the 2L subclass are still being negotiated. Until this standard is updated,
other standards (such as UL) are unlikely to be modified. ISO Standard 817 and 5149 also already contain
the 2L subclass for equipment using 2L refrigerants. One A2L refrigerant which has already received EPA
SNAP approval is R-1234yf, and it is currently used in mobile air-conditioning systems3. In Europe, EN
378 has not considered changes to incorporate A2L charge limits. Additionally, IEC standards with strict
charge limits present a big barrier to large refrigeration applications. Large A2L refrigeration systems
also fall under the PED due to their flammability, which requires extra reporting and safety assessments
as well as limits pressure and volume levels of the equipment. In Japan, there exists an uncertainty
regarding regulations of flammable refrigerants in mobile air-conditioning systems. Japanese
automakers have recommended that the HPGSA be updated in regards to A2L fluid to clear up this
confusion3. Overall, A2L fluids are gaining increasing acceptance in these 3 regions, but require more
changes to safety standards5.
In conclusion, these 4 ultra-low GWP refrigerants face some obstacles before they can be used
extensively. CO2 faces no major regulatory barriers in the USA, Europe, or Japan. Ammonia is likely to
remain restricted to industrial activities due to its toxicity. Hydrocarbons are likely to remain limited to
small applications due to flammability concerns. Lastly, A2l fluids are gaining increased acceptance, but
still face regulatory barriers before they can be widely used. When comparing these low GWP
refrigerants to HFC and HCFC refrigerants it is easy to see how much better the low GWP refrigerants
are for the environment. Chart 3 below shows ODP and GWP of various traditional and alternative
refrigerants:
Refrigerant Name
R-12
R-134a
R-22
R-410a
R-22a
CO2
ODP (max=1)
1
0
0.05
0
0
0
GWP
10,890
1,300
1,700
2,100
3
1
Atmospheric Lifetime
100
14
12
Not
<1
5
(years)
available
Chart 3 – ODP, GWP and Atmospheric Lifetime of Various Refrigerants.
Ammonia
0
<1
<1
As shown above, ammonia, CO2, HCs, and A2L fluids all have much lower ODP, GWP, and atmospheric
lifetime than typical HFC and HCFC refrigerants. Hopefully, these 4 low GWP refrigerants will be more
used in the future.
Works Cited
1. Carter win award for zero-carbon Tesco store in Thailand. n.d. 8 January 2013. <http://www.creltd.co.uk/news.php?id=253>.
2. Goetzler, William. Review of Regulations. 30 January 2011. 6 January 2013.
<http://tc31.ashraetcs.org/pdf/Regs%20Stds%20for%20Low%20GWP%20Refrigerants.pdf>.
3. —. "Ultra-Low GWP Refrigerants." ASHRAE Journal (2010): 10. 21 December 2012.
4. Hydrocarbon Refrigerants. n.d. 7 January 2013.
<http://collegeofclimatechange.wordpress.com/2011/12/20/hydrocarbon-refrigeranthydrocarbon-refrigerant-manufacturer/>.
5. Low GWP Refrigerants. June 2011. 7 January 2013. <http://www.areaeur.be/_Rainbow/Documents/AREA%20%20PP%20Low%20GWP%20refrigerants%20(110629).pdf>.
6. Scott, Jodi. Alternative Low-GWP Refrigerants To Be Presented At ASHRAE/NIST Refrigerants
Conference. n.d. 3 January 2013. <https://www.ashrae.org/news/2012/alternative-low-gwprefrigerants-to-be-presented-at--ashrae-nist-refrigerants-conference>.
7. Why Hydrocarbons. n.d. 6 January 2013. <http://www.oz-chill.com/about-hydrocarbonrefrigerants/why-hydrocarbons/>.