Most of the small and medium capacity RACHP (refrigeration, air conditioning and heat pump) systems are working on compression refrigeration cycles. The halogenated refrigerants have been widely used in RACHP systems. The environmental impacts due to indirect green house gas emissions because of combustion of fossil fuel for power generation and the direct green house gas emissions due to leakage of refrigerants from vapor compression based RACHP systems, contribute significantly to global warming. Hence, it is essential to identify a long term sustainable replacement to meet the system performance requirements, chemical compatibility, safety and service requirements. The developed countries have already reduced the production and consumption of halogenated refrigerants thus inducing demands for sustainable alternatives. The natural refrigerants with zero ozone depletion potential and negligible global warming potential are considered as long term alternatives. The hydrocarbon refrigerants have flammability issues, which restrict the usage in existing systems. Moreover, the carbon-dioxide is having high operating pressure and low critical temperature, and so has limitations for use in existing RACHP systems. In addition, the refrigerant mixtures provide flexibility in searching environment friendly alternatives to match the required thermodynamic, thermo-physical and chemical properties with existing halogenated refrigerants.
The historical transition of refrigerants during last 100 years is shown in Figure 1. During the year 1930, the chlorine based refrigerants such as CFC and HCFC refrigerants were introduced for the use in refrigeration, air conditioning and heat pump applications due to their good thermodynamic and thermo-physical properties. In 1974, Rowland and Molina have identified that the presence of chlorine in CFC and HCFC refrigerants are responsible for ozone depletion and global warming. Montreal protocol 1987 restricts the use of chlorine based refrigerant in RACHP systems. During the year 1990, the hydro-fluro-carbon (HFC) refrigerants such as, R134a, R152a, R404A, R407C, R410A, R507 have been introduced for the use in RACHP systems. The Kyoto protocol 1997 identified six green house gases, which includes HFCs being used as refrigerants. During the year 2010, the hydro-fluoro-olefins (HFO refrigerants) (unsaturated HFC refrigerants) have been introduced, which has very low global warming potential. The HFO refrigerant has low global warming potential due to its short atmospheric life time. The HFO refrigerant gets decomposed and forms tri-fluro-acid, which is more harmful to the aquatic systems. Paris protocol 2016 restricts the use of refrigerants with significant global warming potential. The natural refrigerants are identified as the good alternative to phase-out the halogenated refrigerants in existing RACHP systems.
Properties of refrigerants
Thermodynamic properties such as, critical temperature, critical pressure, freezing point, boiling point and molecular weight of hydrocarbon refrigerants are found to be good compared to the existing halogenated refrigerants. Hence, the replacement of existing halogenated refrigerants with hydrocarbon refrigerants does not require major changes. Further, the thermo physical properties such as, saturation pressure, latent heat, liquid density, thermal conductivity and specific heat in both liquid and vapour phase of hydrocarbon refrigerants are superior than existing halogenated refrigerants. Hence, better performance will be expected with halogenated refrigerants.
The refrigerant compatibility with all the components of the system and with lubricant is essential during refrigerant retrofitting. The hydrocarbon refrigerants are compatible with all the components of the system and also with lubricant using compressors. The hydrocarbon refrigerants are flammable and it is essential to adopt required safety measures in the system while retrofitting. The toxicity of refrigerants are grouped into two levels as A and B, which are lower and higher levels. Similarly, flammability of refrigerants is categorized by numerical value in the range between 1 and 3. The flammable index of hydrocarbon refrigerants is categorized as A3. The flammable risks of hydrocarbon refrigerants are reduced by blending with HFC refrigerants. The refrigerant should contain extremely low level of impurities. Presence of impurities leads to formation of acids, which will damage the system. The moisture content in the system will freeze and clog the expansion device.
The major environmental impact of halogenated refrigerants are ozone depletion potential and global warming potential. The chlorine atoms released from CFC and HCFC refrigerants act as a catalyst to destroy the stratospheric ozone layer which protects the earth from direct UV rays. More than 90% of the ozone exists in the stratosphere between 10 and 50 km above the earth surface. The RACHP system manufacturers have restricted the use of CFC refrigerants from the year 2000. However, the HCFC refrigerants are still being used in RACHP systems, which were banned from the year 2017. The global warming potential is caused due to release of halogenated refrigerants. The presence of halogenated substance in the atmosphere is capable of absorbing the reflected radiations from the earth surface, which heats up the earth and behaves like a black body radiating energy, which results in enhancement of global temperature.
Limited pure refrigerants have suitable properties and provide alternative to halogenated refrigerants. The refrigerant mixtures provide solutions to this problem. Three types of refrigerant mixtures were used as working fluids: azeotropes, near azeotropes and zeotropes. Azeotropic mixture of the substances is one, which cannot be separated into its components by simple distillation. An azeotrope evaporates and condenses as single substance with properties that are different from those of other constituents. Near azeotropes may alter their composition and properties under leakage conditions. Zetropic mixture does not behave like a single substance when it changes its state. Instead, it evaporates and condenses between two temperatures (temperature glide). Hydrocarbon blends are the zeotropic substances which have greater potential for improvements in energy efficiency and capacity modulation.
The future refrigerant options in major RACHP systems are presented in this section. The thermodynamic properties of various refrigerants are shown in Table 1.
Presently, the domestic refrigerator manufacturers have shifted their production using R600a as the working fluid. The refrigerant R600a is having good thermodynamic and thermo-physical properties for the requirements in domestic refrigerators. However, the pure R600a is not possible as a drop-in substitute to R134a due to mismatch in operating pressure and volumetric cooling capacity. The hydrocarbon refrigerant mixtures composed of R290 and R600a is a probability to use as a drop-in substitute to replace R134a in small capacity refrigeration systems during its servicing to extend its life time.
The commercial refrigeration units are using R134a and R404A for their good thermodynamic and thermo-physical requirements. The high global warming potential of R134a and R404A has restricted its usage in commercial refrigeration units. The commercial refrigeration system manufacturers have shifted their manufacturing to R290 considering its good thermodynamic and thermo-physical requirements to meet the low temperature refrigeration system requirements. The small capacity refrigeration units using R134a can be retrofitted with R290/R600a mixture as a drop-in replacement to extend its life.
The ammonia is the most widely used refrigerant in industrial refrigeration systems because of its good thermodynamic and thermo-physical requirements with zero ozone depletion potential and negligible global warming potential. However, the ammonia is more toxic and highly reactive with copper tubes. Hence, steel tubes are recommended for the use of ammonia. Moreover, the use of ammonia requires secondary refrigerant circuit to tackle the toxic nature of ammonia.
The refrigerant R290 is a good option for transport refrigeration.
Domestic Air Conditioning
Presently, R22, R32 and R410A are the three halogenated refrigerants dominating the residential air conditioning sector. The refrigerants R22 and R410A need to be phased out soon because of high global warming potential. The sustainable option for residential air conditioner is R290 due to its good thermodynamic and thermo-physical properties. The R290 can be used as a drop-in substitute to replace R22 in small capacity residential air conditioners. The R290 is not being taking-up in India due to lack of flammable standards for Indian climatic conditions. The R410A has high global warming potential of around 2000. The refrigerant R32 can be used as an alternative to phase-out R410A in residential air conditioners to reduce its environmental impact. The replacement of R32 with R744/R290 mixture is possible, to reduce the environmental impact of residential air conditioners using R410A. However, due to lack of commercial availability of R744/R290 mixture, it is not possible to use it as an alternative.
Commercial and Industrial Chillers
The halogenated refrigerants such as, R22, R134a, R32, R410A are being used in industrial chillers. These refrigerants are having high global warming potential, that should be phase-out. The refrigerant R290 is recommended for industrial chillers. Plate heat exchangers are recommended in the chillers to reduce the refrigerant inventory.
Automobile Air Conditioners
The refrigerant R134a is the most commonly used refrigerant in automobile air conditioners during last three decades. Paris protocol 2016 restricts the use of R134a in automobile air conditioners due to its high global warming potential of 1430. Presently, the automobile air conditioners have shifted to R1234yf due to its lower global warming potential. However, R1234yf is not a sustainable refrigerant option due to its short atmospheric life. The R1234yf gets decomposed and forms tri-fluoro-acidic acid, which are highly harmful to the aquatic systems. The refrigerant R152a is also possible to use as a possible alternative to R134a due to its similar performance. However, the R152a has higher compressor discharge temperature, which may affect compressor life. The R290 was successfully used in automobile air conditioners for both cooling and heating applications. The refrigerant R744 (caron-dioxide) is being used in automobile heat pump system operating under low ambient conditions. The use of R290 in automobile air conditioner requires secondary loop system to reduce the flammable risk.
Heat Pumps Systems
The refrigerants such as, R22, R32, R134a, R407C and R410A are being commonly used in heat pump systems due to good thermodynamic and thermo-physical requirements. These refrigerants should be phased out soon as carrying high global warming potential. Presently, the R1234yf is recommended due to its low global warming potential. The hydrocarbon refrigerants such as R290 and R1270 are recommended for heat pump applications due to its good thermodynamic and thermo-physical properties to meet the requirements in heat pumps.
A lot of research and developments have been carried out identifying environment friendly alternatives to phase out the halogenated refrigerants. The outcome of the research confirmed that, the hydrocarbon refrigerants are found to be good option for replacing the halogenated refrigerants in refrigeration, air conditioning and heat pump systems. The use of natural refrigerants in RACHP applications plays a vital role in reducing the environmental impacts.
Dr. M. Mohanraj,
Consultant for Refrigerant,
HVAC, Heat pump and Energy,
is M.E. and Ph.D. in Heat pumps.