Automotive air-conditioning system technology: a review

162 Progress in Industrial Ecology – An International Journal, Vol. 14, No. 2, 2020 Automotive air-conditioning system technology: a review S. Sharmas Vali Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, 575025, India Email: sharmasvali.nitk@gmail.com S. Saboor* and S. Prithivi Rajan School of Mechanical Engineering, Vellore Institute of Technology, Vellore, 632014, India Email: saboor.nitk@gmail.com Email: sprithivirajan12@gmail.com T.P. Ashok Babu Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, 575025, India Email: tpashok@rediffmail.com Abstract: Air conditioning in the automobile has become an important area of research. The performance of an air conditioning system in an automobile depends upon three basic important factors such as compressor speed, evaporator load, and condensing temperature. How these factors when varied affects the COP of the system have been detailed in this review paper. Several performance studies on various refrigerants (R134a, R152a, CO2 and R1234yf) used in the automotive air conditioning system operating with various conditions revealed the better COP for R152a in comparison with R134a, whereas COP of R1234yf and CO2 was observed to be slightly lower than R134a. However, safety measures must be followed while using R152a due to its slightly flammable nature (ASHRAE A2 group). In this work, various alternative air conditioning systems used for automobiles have been presented in detail. Keywords: alternative refrigerants; GWP; ODP; automotive air-conditioning. Reference to this paper should be made as follows: Vali, S.S., Saboor, S., Rajan, S.P. and Babu, T.P.A. (2020) ‘Automotive air-conditioning system technology: a review, Progress in Industrial Ecology – An International Journal, Vol. 14, No. 2, pp.162–184. Biographical notes: S. Sharmas Vali is a PhD Research Scholar of Mechanical Engineering Department at National Institute of Technology Karnataka, Surathkal, Mangalore, India. His research interest areas are green refrigerants for air conditioning, flammability study of fluids, design of VCR system Copyright © 2020 Inderscience Enterprises Ltd. Automotive air-conditioning system technology 163 components, energy engineering and heat transfer. He earned his MTech in Refrigeration and Air Conditioning from JNTU Ananthapur, India. He obtained his BTech in Mechanical Engineering from MITS Madanapalle (affiliated to JNTU Hyderabad). He has published several research articles in various reputed international journals and international and national conferences. S. Saboor is currently working as Senior Assistant Professor in the Department of Thermal and Energy Engineering, School of Mechanical Engineering, Vellore Institute of Technology, Vellore-632014, Tamil Nadu, India. He pursued his Doctoral degree in Mechanical Engineering from National Institute of Technology Karnataka, Surathkal, Mangalore, India in 2016. He published and reviewed many research articles in various international journals of high repute. He presented many papers and received many awards for his papers presented at national and international conferences. His areas of interest are heat transfer in buildings, solar passive buildings, energy-efficient buildings, built environment, building materials, refrigeration, and air-conditioning. S. Prithivi Rajan is an undergraduate student in School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India. T.P. Ashok Babu is currently working as a Professor in Mechanical Engineering Department of National Institute of Technology Karnataka, Surathkal, Mangalore, India. His research interest is in refrigeration and air-conditioning, renewable energy and heat transfer. He earned his PhD in Mechanical Engineering from Indian Institute of Technology Delhi, India in 1997. He has published several research articles in various reputed international and national journals and also in international and national conferences. This paper is a revised and expanded version of a paper entitled ‘A review on automotive air-conditioning’ presented at International Conference on Progress in Automotive Technologies held at VIT, Vellore, Tamil Nadu, India, 7–9 March 2019. 1 Introduction Automobiles consume an extensive amount of energy for air-conditioning (Omi, 2009; McEvoy, 2015). Compressor, condenser, condenser fan, expansion valve, receiver drier/air conditioner accumulator, evaporator, and evaporator fan are the components that makeup the air conditioning system in automobiles (Mohamed et al., 2017). If orifice tube is used in the air conditioning system then air conditioner accumulator would be used in the low-pressure side of the air conditioning system or if expansion valve is used then receiver drier would be used in the high-pressure side of the air conditioning system, but the purpose of using drier or air conditioner accumulator is same. To begin with the process, the low-pressure gaseous refrigerant enters the compressor where the compressor compresses the gaseous refrigerant entering it and gives out high pressure and high temperature gaseous and superheated refrigerant. The reason for the refrigerant to get high in temperature is as the collision rate of the molecules increases inside the compressor kinetic energy of the molecule increases which ultimately results in the increase of internal energy of the refrigerant and thereby increasing its temperature and pressure. Now this high pressure and temperature gaseous refrigerant passes through 164 S.S. Vali et al. the condenser where the heat in the refrigerant is blown off it to the outside atmosphere with the help of the condenser fan and hence ultimately the gaseous refrigerant gets converted into liquid state losing its temperature to the atmosphere whereas the pressure remains constant that is it is still in high-pressure state. Now, this liquid refrigerant which is high in pressure and moderately low in temperature passes through the receiver drier in case of the expansion valve. As mentioned above both air conditioner accumulator and receiver drier performs the same function that is to remove the contaminants and the moisture from the refrigerant so that other parts of the air conditioning system could work efficiently. If by chance any moisture gets remained that could go and freeze in the evaporator and could damage the part. In case of any repair, this part of the air conditioning system is examined and replaced. After passing through the receiver drier the liquid refrigerant high in pressure and moderately low in temperature passes through the expansion valve or orifice tube and its pressure drops considerably. As a result of pressure drop temperature will also drop. Now the liquid refrigerant low in temperature and pressure passes through the evaporator. Here the evaporator fan blows the ambient air over the evaporator fins and, since refrigerants have a very low boiling point it will eventually boil and turn into gas after absorbing heat from the ambient air that flows over it. As it turns into gas rapidly absorbing heat from the air it’s going to cool down the coil which when used a fan blower sends cool air to the cabin area where passengers sit. After looking at the basic principle on how an air conditioning system in an automobile works we are going to look on how important factors such as compressor speed, evaporator load, condensing temperature, and refrigerant charge will affect the coefficient of performance (COP), cooling capacity and compression work of the air conditioning system in an automobile. Factors and their effects on cooling capacity, compression work and COP: It is noticed that condenser surface temperature increases with the increase of compressor speed. Better heat transfer can be achieved by a higher mass flow rate of the refrigerant with the increase of compressor speed. An increase in the mass flow rate increases the cooling capacity of the air conditioning system (Mohamed et al., 2018). It is found that the increase of refrigerant charge will increase the cooling capacity but the further increment of refrigerant charging decreases the cooling capacity of the system. One of the important factors noticed is that sub-cooling of the condenser temperature will approximately lead to the achievement of optimal conditions of cooling capacity. Also, refrigerant overcharge can lead to overheating of the suction temperature and thus decreasing effective cooling capacity that will cause a considerable drop in performance. With the increase in compressor speed and refrigerant charge compression work is increased. This is due to the fact that the mass flow rate and the pressure ratio of the compressor increase with the increase of speed and refrigerant charge level. Such factors also affect the COP of the air conditioning system in a wide range as COP is the ratio of cooling capacity to compression work. COP for the system decreases with increasing compressor speed whereas the rate of total energy per unit evaporator load increases with it. The air conditioning system in the automobile can be operated more efficiently at lower compressor speeds and evaporator load. After briefly studying the important factors that affect the cooling capacity, compression work and COP of the air conditioning system in automobiles and a detailed investigation had been done on the type of refrigerants that should be used in the air conditioning system of automobiles. Over the years many refrigerants have been used, out of which some of them got banned due to their adverse effects on the environment. The others were stopped being used because of their inefficiencies and also because there were better Automotive air-conditioning system technology 165 alternatives available. This paper presents the effect of the automotive air-conditioning system on environment. This work provides essential information about the selection of suitable alternative air conditioning systems with minimal environmental impact to be used in automobile industries. This paper also presented various alternative refrigerants to be used in automobiles in order to reduce the global warming effect of an atmospheric environment. The significant contribution of the present study is that, it can provide a road map towards the selection of an alternative air conditioning systems to be used in automobile industries and the present assessment is essential because it gives the useful information to the investigators to carry further research work on alternative automotive air conditioning systems operating with various alternative low GWP refrigerants. This work also discusses in detail several factors that will affect the performance of the automotive air conditioning system and hence, this study can be taken as a reference to carry out further research work with various alternative refrigerants in order to study the performance of the system. 2 Alternative air conditioning systems for automotive air conditioning The various alternative air conditioning systems which can be used for automobiles have been presented below (Mohamed et al., 2018). 2.1 Vapour compression refrigeration air conditioning system Vapour compression refrigeration (VCR) air conditioning system is the most common and widely used system for automobiles. The basic representation of the VCR system is shown in Figure 1. The VCR system is mainly composed of four basic components such as compressor, condenser, expansion device and evaporator. The change in phase of refrigerant in addition to the variation of its pressure and temperature along the system are responsible for producing the desired cooling effect. Figure 1 Schematic diagram of VCR air conditioning system (see online version for colours) 166 S.S. Vali et al. 2.1.1 VCR air conditioning system driven by solar energy This type of air conditioning system is eco-friendly in nature but still utilizes the hydrofluorocarbon (HFC) refrigerants which may have considerable adverse environmental impacts like high global warming potential (GWP). The performance of the VCR air conditioning system driven by solar energy is very closer to that of the conventional VCR air conditioning system which is driven by internal combustion (IC) engine power. However VCR air conditioning system driven by solar energy has several drawbacks like low conversion energy efficiency, energy is intermittent, heaviness in the energy storage system, a larger size of the system and high initial investment cost. Therefore more studies must be needed in order to overcome the above limitations and to make this system commercialization. 2.1.2 VCR air conditioning system driven by electrical energy This type of air conditioning system is essentially related to electric vehicles and it is viable only for the electric type of vehicles. This system is better eco-friendly in nature as compared to that of conventional VCR air conditioning systems which are driven by IC engine power since it consumes electrical energy instead of consuming the fuel energy coming from the combustion of fuel in IC engines. However, this system still utilizes the HFC refrigerants and may have considerable adverse environmental impacts like high GWP. Many studies have to be conducted in order to improve the air conditioning for electrical vehicles by using variable-speed or variable-capacity compressor and by utilization of waste heat which is coming from electrical devices as an added heat source. 2.1.3 VCR air conditioning system driven by fuel cell energy This type of air conditioning system is technically developed and it is viable only for fuel cell vehicles. The air conditioning system for fuel cell vehicles is similar to that of electric automobiles. This system is also better eco-friendly in nature as compared to that of conventional VCR air conditioning system which is driven by IC engine power since it consumes the fuel cell energy instead of consuming the fuel energy coming from the combustion of fuel in IC engines. However, this system still utilizes the HFC refrigerants and has considerable adverse environmental impacts like high GWP. The demerits of this system are larger size and cost. 2.2 Air cycle air conditioning system The air cycle air conditioning system is an alternative system used for automobiles in which the system is operated by the mechanical compressor. Air cycle air conditioning system is very similar to the conventional VCR air conditioning system and it essentially varies from the VCR system in working substance to be used. Air cycle air conditioning system uses air as a working substance (refrigerant) which passes through the cycle without changing its phase. The representation of an air cycle air conditioning system is shown in Figure 2. Automotive air-conditioning system technology Figure 2 167 Schematic diagram of the air cycle air conditioning system (see online version for colours) 2.2.1 Air cycle air conditioning system driven by recovered engine exhaust gas energy From the past many years’ air cycle refrigeration system is used in aircraft air conditioning applications. The air cycle system is used in an automobile air conditioning system due to its eco-friendly nature. This system uses eco-friendly refrigerant such as air and utilizes engine exhaust gas heat as driving energy. However, this type of air conditioning system is in the testing phase due to various practical limitations. Demerits of air cycle air conditioning system are poor COP compared to vapour compression system, bulky in system size and need for costly components like compressor and turbine. Figure 3 Schematic diagram of the thermoelectric air conditioning system (see online version for colours) 168 S.S. Vali et al. 2.3 Thermoelectric air conditioning system The thermoelectric air conditioning system works on the principle of the Peltier effect. The schematic diagram of this type of system is shown in Figure 3. In this system, the cooling response is fast however its efficiency predominantly depends on temperature difference across the thermoelectric device. 2.3.1 Thermoelectric air conditioning system driven by recovered engine exhaust gas energy Due to energy and environmental issues, thermoelectric air conditioning got more attention in automobiles since thermoelectric systems are eco-friendly in nature. The thermoelectric air conditioning system driven by recovered engine exhaust waste gas energy is used for cooling effect as well as the heating effect for seat occupant applications. However, the commercialization of the thermoelectric air conditioning system still requires more studies on developments to enhance thermoelectric material performance and also on cost. Demerits of this system are poor COP compared to the VCR system and requirement of a larger electrical generation system which cannot be fixed in the compartment of the engine. 2.4 Ejector air conditioning system The representation of the ejector air conditioning system is shown in Figure 4. The ejector air conditioning system utilizes two streams of refrigerants having different pressures. Streams of refrigerants are passing through a convergent-divergent nozzle, mixing chamber and a diffuser. The system design delivers compression to the refrigerant at the exit of the diffuser. In this system, an increase in refrigerant pressure is only because of heat powering not by a compressor. Figure 4 Schematic diagram of the ejector air conditioning system (see online version for colours) Automotive air-conditioning system technology 169 2.4.1 Ejector air conditioning system driven by recovered engine exhaust gas energy Ejector based air conditioning system can be powered by utilizing the engine exhaust gas heat and this system is also eco-friendly in nature. Demerits of the ejector are low COP, lack of sufficient engine exhaust waste gas heat to run ejector and difficulty of auxiliary system required for the ejector. 2.5 Absorption based air conditioning system The basic line diagram of an absorption air conditioning system is shown in Figure 5. In this system, two working substances are used. One is absorption material (ammonia) which performs as a chemical compressor, whereas the other substance (water) works as a refrigerant. This system utilizes the waste heat as an operating mechanism in ordered to produce the refrigeration effect. 2.5.1 Absorption air conditioning system driven by recovered engine exhaust gas energy Absorption based air conditioning system is highly eco-friendly in nature. Generally, this type of system is driven by recovered engine exhaust waste heat energy. Commonly ammonia-water can be used as a refrigerant-absorbent pair in this system. In the absorption system, energy wasted from engine exhaust is sufficient to produce the cooling effect in automobiles. But the COP of this system is very low compared to the VCR air conditioning system. Demerits of absorption air conditioning system are the toxicity of working fluid (ammonia), more time is required in order to obtain effective cooling, larger size, and complexity of the system and poor COP. This type of system is still in the prototype phase. Figure 5 Schematic diagram of the absorption air conditioning system (see online version for colours) 170 S.S. Vali et al. 2.6 Adsorption based air conditioning system The line diagram of the adsorption air conditioning system is shown in Figure 6. In this system, a pair of solid and fluid materials are used. Solid materials like solid desiccants perform like a sponge in order to desorb or absorb huge amounts of refrigerant (water). This system is technically moderate and it is in the prototype phase. Figure 6 Schematic diagram of the adsorption air conditioning system (see online version for colours) 2.6.1 Adsorption air conditioning system driven by recovered engine exhaust gas energy This air conditioning system is driven by recovered engine exhaust waste gas energy since heat is the driving force to produce the refrigeration effect. Activated carbon-ammonia can also be used as adsorbent-refrigerant pair in this system. Adsorption based system is more useful for automobiles. This system is also highly eco-friendly in nature. However, the drawbacks of the adsorption based air conditioning system are very similar to that of the absorption based system. Demerits of adsorption air conditioning system are poor COP compared to VCR system, larger system size, high cost of adsorbent beds and toxicity of working substances like ammonia. 2.7 Metal hydride air conditioning system The metal hydride air conditioning system utilizes the heat energy obtained from chemical reactions to produce the refrigeration (cooling) effect without using a compressor. In order to achieve the cooling effect of the metal hydride system, a regeneration process run by solar energy or engine exhaust waste gas heat energy is required. The schematic layout of this system is shown in Figure 7. Automotive air-conditioning system technology Figure 7 171 Schematic diagram of metal hydride air conditioning system (see online version for colours) 2.7.1 Metal hydride air conditioning system driven by recovered engine exhaust gas energy The metal hydride air conditioning system has got great attention in automobiles as an alternative air conditioning system because of the accessibility of engine exhaust waste gas heat energy. This system is highly eco-friendly in nature. Demerits of this system are very low COP, system size is bulky for cars and it is not meeting the requirements of automotive air conditioning. This system is still under the prototype phase. The most popular and the most widely used air conditioning system for automobiles is a VCR system. In the VCR system, refrigerant plays a major role in the performance of the system. 3 Evolution of automotive vapour-compression air conditioning systems 3.1 Desirable properties of refrigerants The boiling point and freezing point of the refrigerant should be low so that refrigerant vaporizes in the ambient room temperature and low freezing point because it should not freeze during the process or application which may result in damaging the working parts of the air conditioning system in the automobile industry. Low specific heat and high latent heat is preferred for the refrigerant. At low temperatures high latent heat increases the refrigerating effect per kg of refrigerant and also if specific heat is high it will decrease the refrigerating effect per kg of refrigerant. To run the whole air conditioning system at a lower cost it is very important that the pressure maintained in the evaporator and condenser is low enough and also at times to avoid leakages due to high pressure in the condenser. Hence refrigerant used in the air conditioning system should have low condensing pressure so that the power required for compression would be small. Otherwise, the condenser parts have to be designed to withstand high pressure which will automatically lead to a rise in the capital cost of the equipment. 172 S.S. Vali et al. Refrigerant’s critical pressure should be higher than the condenser pressure to avoid heat rejection, diminution of the zone of condensation and large power requirements. High vapour density or low specific volume is required for preferred refrigerant to reduce the size of the compressor and at the same time velocity could be kept low as in this case smaller diameter condenser tubes can be used so as to make the whole air conditioning system compactible. Speaking about the compatibility of the air conditioning system high thermal conductivity refrigerant is preferred so that the area of heat transfer in evaporator and condenser can be minimized. Toxicity is one of the important factors that should be looked upon especially when refrigerants being used in the air conditioning system of the automobile. Refrigerant should not be poisonous to human beings, that is, to both the driver and the passengers and also the foodstuff. As toxicity depends mainly upon the concentration and exposure limits, exposure of such toxic refrigerants if used would be high to driver and passengers inside the cabin area. The refrigerant should not be flammable and explosive at all. It should not make combustion in air. For example, freon, carbon dioxide, SO2 are non-flammable. Methane, butane and other hydrocarbons are flammable. Ammonia will form an explosive mixture when the concentration in air is between 16 to 25%. Refrigerant should be non-reactive as well as non-corrosive. Refrigerant should not react with any kind of materials that are being used in the refrigeration cycle like compressors, condenser tubes, control valves, and evaporators. For example, HCL is formed when CCl2F2, CH4Cl reacts with water, as a result HCL dissolves the copper from condenser tubes and they get deposited on the compressor pistons and ultimately the life of machinery gets deteriorated. It should have high solubility with the lubricating oil. If both lubricating oil and refrigerant are not miscible since oil is heavier it will settle down in the evaporator and ultimately heat transfer will get reduced. To avoid this problem oil separator has to be installed or overflow drain has to be employed to remove the oil that will float on the surface of the refrigerant if the density of the refrigerant is more than the lubricating oil used. Then in such a case, it will get difficult to keep our air conditioning system compacted in an automobile. Ozone depletion potential (ODP), it gives information about the relative amount of depletion a refrigerant can cause to the ozone layer. Hence ODP of the refrigerant should be as low as possible. For example, R-22 has an ODP of 0.055. GWP of the refrigerant should be as low as possible. GWP stands for global warming potential which measures the amount of heat trapped in the atmosphere by a refrigerant. For example, GWP of Carbon Dioxide is 1.0 whereas for methane it’s 72, which means methane can trap 72 times more heat than the carbon dioxide if the same amount of them were introduced in the atmosphere. The refrigerant of preferred properties should be readily available and must be cheap. High COP should be given by the refrigerant in the working temperature range so as to decrease the running cost of the system. After briefly studying what all the properties a refrigerant should have it’s also important to study what kind of refrigerants were used in the air conditioning system of the automobile industry and what kind are still in use, best alternatives and optimal working conditions of the refrigerants. When talking about the refrigerants we always refer to CFCs, HCFCs, and HFCs. With the course of time and detailed study on all such refrigerants, each kind of them got phaseout were retrofitted with much effective alternative refrigerant (Ghezloun et al., 2013; Wu et al., 2018). Kyoto protocol and Montreal protocol were important contributions for ruling out the use of CFCs and HCFC’S by shedding light on their major drawbacks, which is ozone layer depletion by 173 Automotive air-conditioning system technology chlorofluorocarbons (CFCs) and global warming by hydrochlorofluorocarbons (HCFCs) which are an entity of greenhouse gases. Both CFCs and HCFCs were popularly used as refrigerants in the air conditioning system of the automobile industry until they got phased out due to such protocols. CFC’s were used as refrigerants in most pre-1994 model year vehicles and HFC’s in 1995 and up model year vehicles. 3.2 CFC refrigerants CFC’s were one of the first kind of refrigerants that were used in the automobile industry. CFC refrigerants are as follows: R11, R12, R13, R13B1, R113, R114, R500, R502, and R503. Out of these most commonly and widely used refrigerant in the automobile was R-12. This was one of the first types of mainstream refrigerants to be used in an automobile. The air conditioning system in automobile running with the use of R12 refrigerant consists of a semi-hermetic compressor with a cooling output of 8.3kW at the evaporating temperature of –10°C and air condenser of a lamella construction with forced air circulation (Havelský, 2000). The condensing unit used in such an air conditioning system was placed in a chamber with a constant temperature of 32°C. Semi-hermetic compressors are basically used if deep freezing is required. Intake of suction gas is done directly into the compressor and cooling of motor is done by an integrated ventilation unit. An automatic measuring system equipped with a multimetre and a switch of measuring position which can be evaluated by computer was used to measure temperatures in different places of the cooling cycle after their stabilization in the set range of the highest and lowest evaporating temperature. Values obtained experimentally were compared the values of refrigerants R401A & R409A mixture, R134a, and R22 over the whole range of evaporating temperatures. Higher values of energy efficiency determined by the COP than the refrigerant R12 was observed. Hence on the basis of carried out experiments it is observed that operation of the refrigerating systems with substitute refrigerants R134a, R401Aand R409A is more efficient than using R12 from the energy point of view. The researchers have presented various studies on R12 performance and its replacements (Dhar and Gadhi, 1980; Factor and Miranda, 1991; Noga and Šimunek, 2009; Padilla et al., 2010; Rachidi et al., 1997; Tashtoush et al., 2002; Valiron et al., 2003). These experiments concluded from an efficiency point of view. When it comes to refrigerants, that is CFC’S, HCFC’s and HFC’s ecological impact is also taken into consideration. Substitute refrigerants for R12 are evaluated from the ecological point of view. For this, we need to be familiar with some terms like TEWI and GWP. TEWI stands for Total Equivalent Warming Impact are used to express contributions to global warming. It is defined as the sum of the direct emissions and indirect emissions of greenhouse gases. GWP stands for global warming potential. It measures how much heat a greenhouse gas traps in the atmosphere. Table 1 Ecological impact by substitute refrigerants of R12 GWP (100 years) P (W) E (kWh) TEWI (kg CO2) R12 1,800 4,093 7,367 15,264 R134a 1,200 3,827 6,887 30,961 R401A 1,080 3,907 7,033 30,579 R409A 1,440 3,854 6,937 32,841 Refrigerants Source: Wu et al. (2018) 174 S.S. Vali et al. Experimental results of ecological impact by substitute refrigerants of R12 are presented in Table 1 (Wu et al., 2018). P is the refrigerating system input, whereas E is the annual consumption of the driving energy. From the results it can be concluded that it is considerably less harmful to use refrigerants R134a, R401A and R409A as compared to R12 and its mixtures with R134a in the given air conditioning system. It is also noticed that refrigerants R401A, R409A and 134a in comparison to R12 have a stronger influence on the indirect contribution to global warming. In the year 1994, R12 got phased out as it’s a part of a larger group of chemicals called CFCs. R12 got banned in 1989 under the Montreal protocol with the aim of reducing ozone depletion by such chemicals. 3.3 HCFC refrigerants Since CFCs were phasing out in its time slowly HCFC’s were found to be better alternatives. These are refrigerants that contain Hydrogen, Chlorine, Fluorine, and Carbon. They cause only 10% of ozone depletion as compared to CFCs. They are energy efficient, less toxic and cost-effective and were safe to use. With the emerging use of HCFC’s world’s CFC consumption had fallen to about 75%. Following are the HCFC refrigerants: R22, R123, R124, R401A, R401B, R402A, R403B, R408A, R409A, R414B, and R416A. Out of these most common and widely used refrigerant used in the air conditioning system of the automobile industry was R22 and also a mixture of R22, R124, and R152a which was proposed as an alternative for CFCs. A detailed study had been performed on the performance of the vapour compression system of an automobile with a refrigerant mixture of R22, R124 and R152a (Kiatsiriroat and Euakitt, 1997). The performance of the system was checked at different working conditions by adjusting the mass fraction of R22 in that taken mixture of refrigerants. The percentage of composition was taken in the blend of refrigerants was 50, 30 and 20% mass fraction of R22, R124 and R152a (HFC) also composition of R22, R124 and R152a with mass fraction of 30, 47 and 23% was preferred as an alternative for R22 (Kiatsiriroat and Euakitt, 1997). From the results, it was found that the energy required from the engine to drive the compressor increases with the compressor speed and the mass fraction of R22. Figure 8 Primary energy consumption of the engine for driving the compressor (see online version for colours) Automotive air-conditioning system technology 175 It was also noticed that at different mass fractions of R22 the pressure ratio increases with the speed and it decreases at a lower mass fraction. Ultimately as the pressure ratio increases compression work also increases. Figure 8 depicts the mentioned results (Kiatsiriroat and Euakitt, 1997). It was also observed that at different mass fractions of R22 the pressure ration increases with the speed and it decreases at a lower mass fraction. Ultimately as the pressure ratio increases compression work also increases. Hence, it was concluded that the COP of the system increases with the reduction of the mass fraction of R22. More specifically it was concluded that suitable and more efficient conditions can be obtained for the mass fraction of R22 in the range of 20–30%, as it was found that COP of the system slightly increases when the mass fraction of R22 is less than 30% in spite of having low atmospheric concentrations and some major advantages over already phased out R12, unfortunately, HCFCs are greenhouse gases. Around the 1970s, it was found that the ozone layer was getting damaged by chlorine and a hole had formed due to the excessive use and venting of CFCs and HCFCs into the atmosphere. Hence due to the major contribution to releasing greenhouse gases and causing global warming under the Kyoto protocol, HCFCs also got phased out. Soon R22 was not allowed to be used in the newly produced machines and by the year 2020, refrigerant R22 will be completely phased out. The researchers have presented theoretical performance studies on various HFC blends, HFC/HC blends and HC blends as R22 replacements used in air conditioning applications (Shaik and Babu, 2017a, 2017b, 2017c, 2018a, 2018b; Shaik and Setty, 2019; Vali and Babu, 2018a, 2018b). These studies concluded alternative refrigerants from the volumetric refrigeration capacity, discharge temperature, COP and GWP point of view. 3.4 HFC refrigerants As chlorine was found to damaging the ozone layer, HFCs are the first generation of alternative refrigerants that do not contribute to ozone layer depletion but they do act as greenhouse gases (Dang et al., 2017; Dione et al., 2018). HFC’s meet the needs of the air conditioning system in the automobile industry in safety, efficiency availability, and reliability and affordability aspects and hence there is no phase-out of these refrigerants due to ozone layer depletion. Unfortunately, these refrigerants have fairly high GWP hence even HFCs face the need for alternatives under the Kigali Amendment (October 2016). HFC’s were started getting used in the automobile industry from 1995 and up model year vehicles. Below mentioned are HFC refrigerants: R23, R134a, R404A, R410A, R417A, R422A, R422B, R422D, R507 and R508B. Among these refrigerants most popularly used refrigerants in the refrigerant industry is R134a and R410A. R134a was primarily developed to replace R12 in the air conditioning system of the automobile industry. It has zero ODP and a GWP of 1430. A detailed study had been conducted about the performance of the experimental automotive air conditioning system under various working conditions using R134a as a refrigerant (Gill and Singh, 2017). The experiment was conducted at the compressor speeds of 600, 800, 1,000, 1,200, 1,400 rpm and at the condensing temperature of 50–60°C for each thermal load (Esen, 2012; Meng et al., 2018; Naresh and Balaji, 2018; Sotomayor and Parise, 2016). From the result of the conducted experiment, it was found that R134a refrigerant showed the preferred thermal performance. From the result, it was also observed that R134a has a low mass flow rate due to which charge of R134a is slightly low. 176 S.S. Vali et al. More specifically the results showed that the compression rate of the system with R134a refrigerant system for 60°C condenser temperature is higher than 50°C condenser temperature for the same cooling capacity (Navarro et al., 2013). But it was noticed that the COP of the system with R134a for 60°C condenser temperature is 28–30% lower than the R134a system for 50°C condenser temperature at 1,000 rpm and 2,400 W cooling load. It is seen that with the increase in evaporator load, COP of the system increases and it decreases with the increasing compressor speed and condensing temperature (Bostanci et al., 2018). And also, the higher the evaporator load and condensing temperature, the higher was the rate of exergy change in the compressor. The mass flow rate of the system for 60°C condenser temperature is 4–5% higher than the R134a system for 50°C condenser temperature at 1,500 W cooling load with 600 rpm. Figures 9 and 10 depict the above mentioned experimentally found results. Figure 9 COP as a function of compressor speed (see online version for colours) Figure 10 Refrigerant mass flow rate as a function of the compressor speed (see online version for colours) Automotive air-conditioning system technology 177 Even though R134a does not cause any danger to the ozone layer, it’s a greenhouse gas with a considerably high GWP potential of 1,430 when released into the atmosphere (Gasche et al., 2012). Hence many countries have begun to use an alternative and phase out the use of R134a in the coming years. It is estimated that after the year 2025 use of R134a refrigerant will be discontinued in new vehicles (Kabeel et al., 2016). For some widely used refrigerants which got phased out like R12, R22 and which are on the verge of getting phased out like R134a, some new alternative refrigerants are found. Unfortunately due to certain limitations in working conditions, some of the alternatives are still in the experimental phase and some got implemented in the newly produced vehicles. The following are the proposed alternative refrigerants: R-1234yf, HFC-152a, and CO2 aka (R744). 4 Alternative refrigerants to automotive VCR air-conditioning systems 4.1 Refrigerant R1234yf Refrigerant R1234yf also known as HFO-1234yf, was jointly introduced in the field of refrigerants by Honeywell and DuPont companies. It is one such alternative that is getting quite successful in becoming an alternative for R134a refrigerant. The many researchers focused on performance of R-134a and R1234yf refrigerants (Akram et al., 2013; Cao et al., 2017; Daviran et al., 2017; Fortkamp and Barbosa, 2015; Gill and Singh, 2018; Needham and Westmoreland, 2017; Qi, 2013; Wang, 2014; Zilio et al., 2011). Ever since the idea R1234yf refrigerant as an alternative for R134a got proposed, R1234yf refrigerant has become an important area of research for many researchers. As R1234yf was found to have similar properties as R134a refrigerant many studies and experiments have been conducted on it to check and compare its working conditions to that of R134a (Golzari et al., 2017; Javidmand and Hoffmann, 2016). After conducting many tests and experiments it was concluded by many researchers that the evaporation heat transfer coefficient of R1234yf is nearly as same as that of R134a. Also, it was revealed by the experiment that the condensation heat transfer coefficient of R1234yf was lower than that of R134a by 12–13%. A detailed R1234yf ‘drop-in’ experiment in R134a automotive air conditioning system was performed. It was observed from the experiment that the R1234yf system can show a better COP than the R134a system under the same capacity of cooling if condenser and evaporator heat transfer is increased by 20%and 10% (Lim et al., 2017). It was observed from the experimental results that the COP and cooling capacity of the R1234yf automotive air conditioning system was lower by 0.8–2.7% and approximately 4% than that of the R134a system. The compressor volumetric efficiency of R1234yf was about 5% lower than R134a because of the higher frictional pressure drop in suction tubes. Based on the simplified thermodynamic cycle, the performance improvement potential of the R1234yf automotive air conditioning system was studied. That is, the effect of superheat at evaporator outlet, subcooling at condenser outlet on system COP and cooling capacity was the main objective. Before studying the effect of superheating and subcooling theoretical performance of R1234yf system was compared with that of R134a. A comparison between both the refrigerant systems is presented in Table 2. 178 Table 2 S.S. Vali et al. Performances of MAC system for refrigerants R1234yf and R134a under various typical conditions Deviation (%) R1234yf R134a Deviation (%) R1234yf R134a Deviation (%) High speed R134a City R1234yf Idle Qc (kW) 2.488 2.696 –7.7 4.503 4.952 –9.1 5.852 6.544 –10.6 Wc (kW) 0.802 0.828 –3.1 1.946 2.015 –3.4 3.466 3.604 –3.8 COP 3.100 3.257 –4.8 2.314 2.457 –5.8 1.689 1.816 –7.0 Performance parameters It is observed from Table 2 that both cooling capacity and COP of R1234yf refrigerant system were smaller than that of R134a for all operating conditions of a vehicle. The results were noted from the effect of superheat and subcooling as mentioned above. As subcooling and compressor performance was fixed so as to observe the superheat effect on the system, it was found that superheat has very little influence on system performance. As the system’s COP and cooling capacity were increased only by 2.6% and 3.7% with the increase in superheat from 1K to 10K. Hence it was concluded that it would be almost very difficult to match up with the values of cooling capacity and COP of the R134a system by just increasing superheat (Lim et al., 2017; Neto et al., 2014). Also, with the increase in superheating it reduced the mass flow rate of the refrigerant slightly because of low vapour density with higher refrigerant vapour temperature whereas our preferred R1234yf refrigerant needs high refrigerant mass flow rate (Qi, 2015). Whereas in the case of subcooling the COP and cooling capacity were increased by 15% with the increase of subcooling from 1K to 10K. When compared with the R134a system, it was found that it is possible to match up with cooling capacity and COP of the R134a system with subcooling. For instance, with a subcooling of 7K on the R1234yf system, it had the same COP as the R134a system with 3K, which is COP = 2.4. Even though similar COP and cooling capacity to that of R134a was attained with the increase in subcooling it comes with a drawback that refrigerant quality at evaporator inlet was found to be decreased which in return leads to larger enthalpy difference in the evaporator (Sedrez and Barbosa, 2015; Vaghela, 2017). It was also absorbed that cooling capacity could increase by 72.8% with compressor volumetric efficiency from 0.55 to 0.95. Power consumption could be reduced leading to the increase in COP if compressor isentropic efficiency was improved. Hence from all the above-mentioned changes, it can be concluded that R1234yf can be a better alternative provided some above-mentioned modifications. 4.2 Refrigerant HFC152a It can be a very effective alternative for R134a as it is found to have similar operating characteristics to R134a and cools much better. The automotive air conditioning system running with this refrigerant typically needs two-thirds of normal charge. The major benefit is that it has 10 times less global warming rating than R134a though much higher than that of CO2, which is 120 (Koytsoumpa et al., 2018). It’s one and only main drawback due to which it is still in the experimental phase is that it’s slightly flammable. Automotive air-conditioning system technology 179 In January 2004, a self-contained unit for off-road construction equipment that featured an oil-driven compressor that used HFC-152a as a refrigerant was used for the first time. Several performance studies reported that the performance of R152a, used in automobiles was comparatively better than R134a (Ghodbane, 1999; Yoo and Lee, 2009; Lee, 2015). However, utilization of R152a in automobiles is limited due to its slightly flammable nature and safety problem is still a matter of conversation in the automobile industry. 4.3 CO2 aka R744 Many researchers have reported work on R-744 (Gullo et al., 2017; Hazarika et al., 2018; Hu et al., 2016; Janotkova and Pavelek, 2006; Jin et al., 2017; Lorentzen, 1994; Tamura et al., 2005; Wang et al., 2018; Zhang et al., 2017a, 2017b; Zhiyi et al., 2017). One of the main drawback for using CO2as a refrigerant is that requires extremely high operating pressure that is 1,800 psi on the high-pressure side and 350–400 psi on the low-pressure side as compared to R134a which requires 300-400 psi (Jin et al., 2011). This high-pressure requirement is due to the fact that CO2 does not condense that easily in the refrigeration circuit and remains in the gaseous state itself. Even leak detection possess a major challenge as CO2 in the atmosphere maybe higher than the amount emitted by a leak from an air conditioning system, hence it gets difficult to detect. Speaking of the reasons for which it is considered as an alternative is it has nearly no impact on global warming or ozone depletion. It is also non-toxic in small amount and can be dangerous over a concentration of 5%. It is also non-flammable and cheap. Mercedes is supporting R744 as an alternative refrigerant. Previously, many researchers tried to improve performance of existing refrigerant with addition of mixtures and nanofluids to the refrigerants (Jin et al., 2011; Lorentzen and Pettersen, 1993; Majgaonkar, 2016; Megdouli et al., 2017; Seshaiah and Reddy, 2016; Sivashanmugam, 2012; Wang and Li, 2007; Wu et al., 2017; Wuebbles, 1994; Xuan and Li, 2000; Yeo et al., 2012). 5 Conclusions This paper presented a detailed review of the automotive air-conditioning system. From this study, the following conclusions can be drawn. Although significant research on various alternative automotive air conditioning systems had carried out from many years still conventional VCR air conditioning system is the most commonly used system in automobiles. Several performance studies on various refrigerants (R134a, R152a, CO2 and R1234yf) used in the automotive air conditioning system operating with various conditions revealed the better COP with R152a in comparison with R134a whereas COP of R1234yf and CO2 was observed to be slightly lower than R134a as presented in Table 2. From Table 1, GWP100of R1234yf (4) and CO2 (1) were observed to be very low as compared to GWP100of R134a (1,300). Although GWP100 of R152a (140) was lower than that of R134a (1,300), the safety measures must be followed while using R152a due to its mildly flammable nature (ASHRAE A2 group). It is noticed that the increase of refrigerant charge will increase the cooling capacity but the further increment of refrigerant charging decreases the cooling capacity of the system. One of the important factors noticed is that subcooling of the condenser temperature will approximately lead to the achievement of optimal conditions of cooling capacity. With an increase in 180 S.S. Vali et al. compressor speed and refrigerant charge compression work is increased.R-1234yf, HFC-152a and CO2 are the primarily proposed alternatives. All their properties, working conditions and limitations of which they are still in the experimental phase are briefly discussed. It is concluded that with certain modifications in the air conditioning system of the automobile industry using R1234yf and similar, the better COP and cooling capacity could be achieved. This work provides a road map towards the selection of eco-friendly alternative air conditioning systems to be used in automobile industries. And also it gives useful information to the investigators to carry further research work on alternative automotive air conditioning systems working with various alternative low GWP refrigerants in order to investigate the performance of the system. References Akram, M.W., Polychronopoulou, K. and Polycarpou, A.A. 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