Refrigeration and air conditioning provide a broad range of benefits to society, including the preservation of food, comfort conditioning of living spaces and workplaces, temperature and humidity control of different industrial processes. Refrigerants are the basic working fluids in refrigeration, air conditioning and heat pumping systems. They absorb heat from one area such as an air-conditioned space and reject it into another system such as outdoors usually through evaporation and condensation processes respectively.
The development of refrigeration and air conditioning industry depends to a large extent on the development of refrigerants to suit various applications – and the development of various system components. At present the industry is dominated by the vapour compression refrigeration systems, even though the vapour absorption refrigeration systems have also been developed commercially. The success of vapour compression refrigeration systems owes a lot to the development of suitable refrigerants. The theoretical thermodynamic efficiency of a vapour compression system depends mainly on the operating temperatures. However, important practical issues such as the system design, size, initial cost and operating cost, safety, reliability, and serviceability depend very much on the type of refrigerant and compressor selected for a particular application.
Throughout the history of air conditioning and refrigeration, numerous substances have been tried and used as refrigerants. However, choosing a refrigerant has become more complex in recent years. Earlier generation of refrigerants chlorofluorocarbons (CFCs) and hydro-chlorofluorocarbons (HCFCs) contributed to the depletion of stratospheric ozone and are being phased out under international treaty. CFCs and HCFCs largely have been replaced with hydrofluorocarbons (HFCs). Some of these HFCs have high global warming potentials (GWP) and are becoming subject to use restrictions in some countries as the world deals with global climate change. Recently, lower GWP HFCs (referred to as hydrofluoroolefins or HFOs) have been introduced. They have zero ozone depleting potential (ODP) and very low GWP, but some of them are mildly flammable and vast research is going on to overcome this problem as they are anticipated as next generation refrigerants.
The history and trend of refrigerant use for betterment of mankind so far can easily be understood from Fig.1.
Recently, global warming has been one of the most important issues facing mankind and in 1997, Kyoto protocol was proposed to control greenhouse gases including HFCs. MAC (Mobile Air Conditioning) directive specifically prohibits the use of fluorinated greenhouse gases whose GWP is greater than 150.
Fig.1 – Trends of refrigerants…
Refrigerant Development
Historically the development of refrigerants can be divided into three distinct phases, namely:
- Refrigerants prior to the development of CFCs
- The synthetic fluorocarbon (FC) based refrigerants
- Refrigerants considering ODP and GWP
Refrigerants prior to the development of CFCs
Water is one of the earliest substances to be used as a refrigerant, though not in a closed system. However, those natural refrigeration systems working with water have many limitations – and hence were confined to a small number of applications. William Cullen observed the production of low temperatures by evaporation of ethyl ether in 1748. Oliver Evans described a practical system that uses ethyl ether as the refrigerant in a closed cycle to produce ice from water. After these developments, ethyl ether was used as refrigerant for several years for ice making in breweries. Ethyl ether appeared to be a good refrigerant in the beginning, as it was easier to handle since it exists as a liquid at ordinary temperatures and atmospheric pressure. Ethyl ether has a normal boiling point (NBP) of 34.5 degree C, due to this, to have low temperatures than system will be operating under vacuum and operational vacuum can lead to air leakage into the system which may form explosive mixtures with ether. In addition to that ether is also toxic. So, as a result people have tried to develop new refrigerants which do not have these shortcomings.
In 1866, the American T.S.C. Lowe (1832-1913) introduced carbon dioxide as refrigerant. Carbon dioxide has excellent thermodynamic and thermo physical properties, however it has a low critical temperature (31.7 degree C) and very high operating pressures. Also, it is non-flammable and non-toxic. It was completely replaced by CFCs after sixty years of application. One of the landmark events in the history of refrigerants is the introduction of ammonia in 1872. Ammonia with a NBP of -33.3 degree C has excellent thermodynamic and thermo physical properties. However, ammonia is toxic and has a strong smell and slight flammability. In addition, it is not compatible with some of the common materials of construction such as copper. At present, ammonia is used in large refrigeration systems (both vapour compression and vapour absorption) and also in small absorption refrigerators.
In 1874, Raoul Pictet (1846-1929) introduced sulphur dioxide (NBP = -10.0 degree C). Sulphur dioxide was an important refrigerant and was widely used in small refrigeration systems such as domestic refrigerators due to its small refrigerating effect. Sulphur dioxide has the advantage of being an auto-lubricant. However, sulphur dioxide was subsequently replaced by CFCs.
The synthetic fluorocarbon (FC) based refrigerants
Midgely and his colleagues developed a whole range of new refrigerants which are obtained by partial replacement of hydrogen atoms in hydrocarbons by fluorine and chlorine. They showed how fluorination and chlorination of hydrocarbons can be varied to obtain desired boiling points (volatility) and also how properties such as toxicity, flammability are influenced by the composition. The first commercial refrigerant to come out of Midgley’s study was Freon-12(CCl2F2) in 1931.Freon-12 has a normal boiling point of -29 degree C and is one of the most popular synthetic refrigerants. It was exclusively used in small domestic refrigerators, air conditioners, water coolers etc for almost sixty years. Freon-11 (CCl3F) used in large centrifugal air conditioning systems was introduced in 1932. This is followed by Freon-22 (CHClF2) and a whole series of synthetic refrigerants to suit a wide variety of applications.
The introduction of CFCs and related compounds has revolutionized the field of refrigeration and air conditioning. Most of the problems associated with early refrigerants such as toxicity, flammability, and material incompatibility were eliminated completely. Freons enjoyed complete domination for about fifty years until the Ozone Layer Depletion issue was raised by Rowland and Molina in 1974. Rowland and Molina argued that the highly stable chlorofluorocarbons cause the depletion of stratospheric ozone layer. In view of the seriousness of the problem on global scale, several countries have agreed to ban the harmful Ozone Depleting Substances (CFCs and others) in a phase-wise manner under Montreal Protocol.
It is found that not all chlorofluorocarbons are equally capable of destroying the ozone layer. Figure 2 shows the ODP of different CFC and HCFC refrigerants. CFC -11 has been assigned ODP of value 1 and all other refrigerants have been rated against this reference. CFC and HCFC based refrigerants were also found to contribute significantly to global warming. Carbon dioxide has been given a global warming potential of 1. On this scale, it is observed that CFCs have global warming potential of several thousand as shown figures 3, 4, and 5. The refrigerants should be chosen keeping in mind the immediate personal safety issues such as flammability and toxicity as well as the long-term environmental issues such as ozone layer depletion and global warming.
Fig. 2 – Ozone depletion potential of pure CFC and HCFC refrigerants…
Fig. 3 –Global warming of pure CFC and HCFC refrigerants…
Refrigerants considering ODP and GWP
The first step taken to encounter the ozone depletion was to introduce alternative refrigerants like hydrofluorocarbons (HFCs) and their mixtures. The reintroduction of natural refrigerants such as carbon dioxide (in a supercritical cycle), water, various hydrocarbons and their mixtures is expected to be a better solution. HFC-134a (synthetic substance) and hydrocarbons have emerged as alternatives to Freon-12. No clear pure fluid alternative has been found as yet for the other popular refrigerant HCFC-22. However several mixtures consisting of synthetic and natural refrigerants are being used and suggested for future use. In general, the non-ODP synthetic refrigerants such as HFC-134a have high global warming potential (GWP), hence they face an uncertain future.
Fig. 4 – Global warming of pure HFC refrigerants…
Fig. 5 – Global warming of HFC mixtures refrigerant…
Most desirable properties of refrigerants
Careful selection of refrigerant has significant impacts on the safety, reliability and energy consumption of the system. A refrigerant must satisfy a number of requirements related to safety, chemical stability, environmental properties, thermodynamic characteristics and compatibility among materials.
Thermodynamics properties: The thermodynamic characteristics most importantly normal boiling point, critical temperature and heat capacity must match the application for the system to operate efficiently. The thermodynamic properties of some of the common refrigerants are listed in table 1.
Chemical stability: A refrigeration system is expected to operate many years, and all other properties would be meaningless if the refrigerant decomposes or reacts to form something else.
Health and safety: The ideal refrigerant should have low toxicity and be non-flammable at the same time should have zero ODP and lowest GWP.
Thermo-physical properties: Favourable transport properties like low viscosity and high thermal conductivity have an impact on the size of the heat exchangers, and thus cost of the overall system.
A final set of practical criteria relate to materials and impact the long-term reliability of a system.
The refrigerant must be compatible with common materials of construction, including metals and seals.
Possible and favourable next generation refrigerants
The alternative refrigerants have been categorized as transitional refrigerant or HCFC/HFC partly chlorinated refrigerants and into medium and long term refrigerants (Fig.6).
HCFC/HFC (partly chlorinated refrigerants) such as R22 and R134a are on the way to phase-out due to environmental concern.
Under medium and long term refrigerants like HFC chlorine free and their blend, low global warming potential (GWP) refrigerant (R1234yf,1234ze) and halogen free refrigerant (natural refrigerant) are at present looking as the viable options for future refrigerant.
Low global warming refrigerant
HFO (Hydrofluoroolefin)
Hydrofluoroolefins (HFOs) are a family of unique compounds compared to the compounds previously developed for commercial use for heat transfer, blowing agent, solvent and propellant applications. After extensive research and development some members of this family have been found to offer comparable performance to today’s most widely used refrigerants.
Fig. 7. Life cycle of refrigerant…
Fig. 8. GWP of refrigerant R1234yf…
These products have short atmospheric lifetimes and have very low global warming potential with respect to R134a as shown in Fig.7 and Fig.8 respectively.
Recently, R1234yf (2,3,3,3-Tetrafluoropropene) having chemical formula CH2=CFCF3 has been proposed as a possible alternative refrigerant for HFC134a . R1234yf has zero ODP and excellent life cycle climate performance (LCCP) as compared to HFC134a. HFO-1234yf has the lowest switching cost for automakers among the currently proposed alternatives, although the initial cost of the product is much higher than that of R-134a.
Another HFO based refrigerant HFO 1234ze (trans-1,3,3,3-Tetrafluoroprop-1-ene, CF3CH=CHF) is an energy-efficient alternative to traditional refrigerants in air-cooled and water-cooled chillers for supermarkets and commercial buildings, as well as in other medium temperature applications such as heat pumps, refrigerators and CO2 cascade systems in commercial refrigeration. Refrigerant HFO-1234ze is the best medium pressure, zero ODP and low GWP refrigerant on the market when considering the balance of all properties. A unique characteristic of this refrigerant is the absence of flammable mixture with air under 300C of ambience.
R-449A is a non-ODP, lower GWP hydrofluoro-olefin (HFO) based refrigerant replacement for R-404A/R-507, R-407A/F and R-22. R449A is designed for use in positive displacement direct expansion low and medium temperature commercial and industrial applications. It has a GWP of 1397, which is a 65% reduction and provides energy consumption 8-12% lower than R-404A/R-507. R-449A is presently the best choice to replace 404A for stationary refrigeration systems.
Refrigerants R-513A, DR-55, R-452A are non-ODP, low GWP HFO based refrigerants. R-513A is developed to replace R-134a in positive displacement, direct expansion, medium-temperature commercial and industrial, including centrifugal chillers. DR-55 will be the leading low GWP replacement for R-410A as it is easily convertible from R-410A designs while offering the optimal balance of energy performance.
HFC/HCFC blend
R-407F is a non-ODP replacement for R-22 and lower GWP (1825) replacement for R404A in various air-conditioning applications particularly in low-temperature applications. Since it is a close match to R-22, it also serves as a retrofit fluid in applications where R-22 is used. R-407F contains HFC-32, HFC-125, HFC-134a. R-409A (a HCFC blend) containing HCFC-22 (60%), HCFC-124 (25%), HCFC-142b (15%) is an interim replacement for R-12 in stationary positive displacement air-conditioning and refrigeration systems such as walk-in coolers, beverage dispensers and supermarket systems. Its ODP is 0.046 and GWP is 1909. R-401B may act as an interim replacement for R-12 in low-temperature commercial refrigeration systems, for transport refrigeration, low temperature retrofits, retrofits including air conditioners and dehumidifiers. It is made up of HCFC-22 (61%), HFC-152a (11%), HCFC-124 (28%). The ODP and GWP values for this mixed refrigerant are 0.036 and 1288 respectively. R-422D is a non-ozone depleting replacement for R-22 in low- and medium-temperature commercial refrigeration systems suitable for direct expansion evaporators. It contains HFC-125 (65.1%), HFC-134a (31.5%), HC-600a (3.4%) and has a GWP of 2729.
It is now clear that HCFC/HCFC blends are good refrigerants considering ODP, but at same time they are having somewhat more GWP compared to what expected from future refrigerants (GWP < 150). Obviously, they are not viable option for future and already the phase down has been started in several developed countries (termed as non-article country in Fig. 9) under Kyoto protocol.
Fig. 9 – HCFC phase down schedule in (%)…
Natural refrigerant
Natural refrigerants are now reconsidered for future refrigerants for different applications (which have been shown in Fig. 10) after the restriction imposed on the ODP and GWP values of refrigerants through international protocol. Ammonia (R717) has been used in industrial applications since the 1930s and is generally acknowledged as being the most efficient refrigerant. It has minimal environmental impact having zero ODP and zero GWP. It is anticipated that use of ammonia for large-scale air conditioning systems will continue to increase. Ammonia installations, however, tend to be relatively expensive due to the requirement for steel tubing, semi hermetic compressors and the installation of a number of safety devices, such as gas detectors. Carbon dioxide (R744) has been used as a refrigerant since 1862 despite the high pressures associated with it. Carbon dioxide is continuing as refrigerant in marine refrigeration as a non-toxic alternative to ammonia and methyl chloride. As a result of modern methods and developments, carbon dioxide is coming back into use as a refrigerant in systems, which have efficiencies at least as great as the efficiencies of halocarbon and ammonia systems. Carbon dioxide can be used as the low temperature refrigerant in carbon dioxide/ammonia cascade systems, as a volatile secondary refrigerant and also as a direct refrigerant. From an environmental perspective CO2 is a very attractive refrigerant with zero ODP and a GWP of 1. Some researchers believe that CO2 will be the main refrigerant in multipack commercial refrigeration systems.
Fig. 10 – Different roots of refrigerant development…
Hydrocarbons
Hydrocarbons are excellent refrigerants in practice provided precautions are taken against their high flammability. They are miscible with mineral oils and have relatively high critical temperatures. Propane (R290) and propylene (R1270) have normal boiling points below –40oC and are therefore suitable for general refrigeration applications. Butane (R600) and isobutene (R600a) have much higher boiling points but they also have high critical temperatures, which tends to make them very efficient in operation. Isobutene (R600a) was introduced in household appliances in some parts of the world from the start of CFC phase out. Over 50 million such refrigerators have been produced. R170 (Ethane) is a natural refrigerant suitable for use in very low temperature refrigeration applications. It is non-toxic with zero ODP and very low GWP of 6. Propane (R290) was introduced later and replaced R134a, R22 or R404A in some appliances. The use of R290 is increasing due to its low environmental impact (zero ODP and GWP of 3) and excellent thermodynamic performance.
Conclusions
From last few decades vast research on new environment friendly alternative refrigerant is going on. Initially refrigerants based on HCFC/HFC blends were tried as short term solution. More recently, natural refrigerants are being reconsidered. However, the most promising finding is the use of HFOs based refrigerant like R1234yf, R1234ze. Theses refrigerants have the potential to replace R134a and R22 respectively. Industries are preferring HFO refrigerants over natural refrigerant as the most efficient refrigerant. On the other hand natural refrigerant has greater chances to come on limelight in near future as best possible alternative.
Use of ammonia will continue, especially for larger industrial systems and for larger air conditioning systems. After an absence of 50 years, carbon dioxide has returned to the field of refrigeration and its use is increasing rapidly. Hydrocarbons would not appear attractive for large-scale air conditioning applications but they will certainly appear as a refrigerant for window air conditioners of low charge. The use of low environmental impact refrigerants like the natural refrigerants (R290, R1270 and R744) and HC/HFC refrigerants in air conditioning and heat pump applications play a vital role in the developing countries like India for reducing the environmental impact of halogenated refrigerants.
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