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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 713924, 15 pages
Review on Conventional Air Conditioning, Alternative Refrigerants, and CO2 Heat Pumps for Vehicles
1Department of Mechanical Engineering, Dong-A University, 37 Nakdong-Daero 550beon-gil saha-gu, Busan 604-714, Republic of Korea
2School of Mechanical Engineering, Yeungnam University, 214-1Dae-dong, Gyeongsan-si, Gyeongsanbukdo 712-749, Republic of Korea
Received 20 July 2013; Accepted 20 September 2013
Academic Editor: Hyung Hee Cho
Copyright © 2013 Moo-Yeon Lee and Dong-Yeon Lee. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
With the reinforced ODP and GWP regulations, clean refrigerants including CO2, energy saving technology for fuel economy, especially focused on development and control strategy for the effective air conditioning system, and heat pump for vehicles have been widely investigated. Recently, the automotive CO2 heat pump for the next generation vehicles as an alternative to that of internal combustion engines has been evaluated and studied as a good option. In this paper, first part is reviewed on the performance characteristics and improvement articles for the conventional automotive air conditioning systems and individual components as well as various alternative refrigerant articles for conventional automotive air conditioning systems. Second part deals with the feasibility of the automotive CO2 heat pump for next generation vehicles without internal combustion engines.
Because of the world energy crisis and environmental concerns, the internal combustion engines using fossil fuel energies are coping with serious regulations and limitations. The conventional automotive air conditioning systems for internal combustion engines are experiencing rapid changes to satisfy the protocols for emerging regulations and global warming. Many automotive makers and investigators have been looking for clean refrigerants and energy saving air conditioning system for conventional internal combustion engines. Consequently, in order to reflect these trends, CO2 and alternative refrigerants having lower both ODPs and GWPs have been studied for a long time as a replacement for various chemical refrigerants as well as currently dominantly used R-134a. Also, in order to minimize the energy consumption of the conventional automotive air conditioning system during the vehicle driving, numerous researches on individual effective components, optimized cycle operations, and enhanced control techniques have been widely investigated. Recently, the mobile CO2 heat pump for the next generation vehicles as an alternative to that for internal combustion engines has been evaluated and studied as a good option. Therefore, in the first part of this review, the performance and improvement articles on conventional automotive air conditioning system and individual components as well as various alternative refrigerants are reviewed. In the second part of this review, another issue with these efforts for conventional automotive air conditioning systems is alternative vehicles with power system using clean fuels with no carbon dioxide emission and improvement technology for fuel economy. Among these, green vehicles such as hybrid electric vehicles, fuel cell electric vehicles, and battery electric vehicles have a good potential for next generation transportation vehicles, which could replace the internal combustion engines. However, these vehicles suffer from short driving ranges and low operation efficiency of power system, especially high voltage battery packs when the heating and cooling system are operated. Especially, the heating problem under cold weather conditions is a critical issue of green vehicles because there is not the waste heat of the internal combustion engines. Most commercialized green vehicles adapt the high voltage PTC (positive temperature coefficient) heaters for cabin heating because of the quick warm-up performance . But this heating system caused the terrible driving ranges of the vehicles because the maximum efficiency of the PTC heater is 1.0 and the conversion efficiency (wheel to wheel efficiency) for heating in a cabin is under almost 50.0%. Namely, the usage of PTC heaters for heating could be shortened by the driving ranges of the vehicles from 30.0% to 65.0% under cold weather conditions . In order to overcome the short driving ranges and low operation efficiency of the green vehicles, heat pumps for cabin heating and cooling have been intensively studied worldwide by automotive makers and research groups and published in the open literature. The heat pump is a very attractive method of proving supplemental heat to the cabin which is to reverse the direction of the refrigerant flow in automotive air conditioning system . Namely, heat pump could be used as cabin heating system integrated with air conditioning system for cooling and system for both simultaneous cooling and heating in the vehicles. Bhatti  reported that heat pump was firstly used for heating in commercially produced electric vehicles in 1900s developed by Harrison Division for heating and cooling for a two-passenger subcompact coupe. Although this heat pump was used in concept vehicles, it was the first system used in commercially used vehicle. And, then, the heat pump also is used for comfort heating of electric vehicles. Domitrovic et al.  numerically investigated the performance characteristics of the heat pump using R12 and R-134a. They determined the cooling and heating heat transfer rate, COP, and compressor work with the variations of ambient temperatures. Hosoz and Direk  studied the performance characteristics of an R-134a automotive air conditioning system as an air to air heat pump using air as heat source. They suggested the experimental analysis of an integrated automotive air conditioning and air to air heat pump using R-134a, evaluated the performance for quantitative information on the losses, and pinpointed the components causing inefficiency using energy and exergy analysis technique. As a result, they developed the automotive air conditioning system capable of operating as a heat pump and concluded that the air to air automotive heat pump must be considered only as supplementary heating method to be used in energy efficient automotives lacking waste heat. In their next published article, Direk and Hosoz  studied experimentally energy and exergy analysis of an automotive heat pump using R134a as the working fluid and ambient air as a heat source. They tested steady state performances with various ambient temperatures at different compressor speeds. The results show that the heat pump can be used for supplementing the main comfort heating system of a vehicle producing insufficient waste heat. Lee et al.  experimentally investigated the performance of automotive heat pump for a large passenger electric vehicle which uses the wasted heat of electric devices for heating and air source for cooling. They reported the feasibility of integrating a heat pump into the air conditioning and heating system of an electric bus and tested performance characteristics of the developed heat pump under various weather conditions and parameters. Based on their experimental results, the observed heating and cooling performances of the automotive heat pump are sufficient as cabin heating and air conditioning of an electric vehicle with a short driving range.
Based on various researches on automotive heat pumps, conventional automotive air condition systems, and alternative refrigerants, the objective of this review covers the strictly selected available articles published in the open literature from 1994 to 2013 on conventional automotive air conditioning systems for internal combustion engines, alternative refrigerants, and automotive heat pumps. In addition, the feasibility on automotive CO2 heat pumps for the energy savings and environment regulations to adopt the coming next generation vehicles such as hybrid electric vehicles, fuel cell electric vehicles, and battery electric vehicles.
2. Mobile Air Conditioning
Extensive studies on conventional automotive air conditioning systems for internal combustion engines have been focused experimentally, theoretically, and numerically on both the performance analysis (prediction) including the alternative refrigerants and the performance enhancement technique including the optimized cycle operations and enhanced control techniques for conventional automotive air conditioning systems. The researches on performance and improvement of conventional automotive air conditioning systems with R-134a for internal combustion engines have been widely investigated and continuously improved over last years.
2.1. Conventional Air Conditioning
At first, numerous experimental articles on performance and enhancement for conventional automotive air conditioning systems have been published for a long time. Kaynakli and Horuz  tested experimentally the performance of the automotive air conditioning system consisting of the basic vapor compression refrigeration system as shown in Figure 1 and evaluated graphically the results obtained with varying of the compressor speeds, ambient temperatures, evaporator temperatures, and condenser temperatures. They designed the experimental setup consisting of the compressor with the belt-driven device coupled to an electric motor, a condenser, an evaporator, and an expansion valve. As a result, the power consumption of the compressor increased with the rise of the condenser temperature and the COP decreased. The cooling capacity of the evaporator increased with the rise of the air inlet temperature of the evaporator and the COP increased as well. Also, the cooling capacity of the evaporator increased with the rise of the ambient temperature. The cooling capacity of the evaporator and power consumption of the compressor increased with the rise of the compressor speed due to the increased refrigerant flow rates but the COP decreased because the increase of the compressor power consumption is greater than that of the cooling capacity of the evaporator. In summary, the cooling capacity of the evaporator increased with the rise of the air inlet temperature of the evaporator, ambient temperature, and compressor speed. The COP increased with the rise of air inlet temperature of the evaporator but decreased with the rise of the compressor speed and condenser temperature because the increase of the power consumption of the compressor is greater.
Preissner et al.  studied the suction line heat exchanger (SLHX) roles for improving the performance of the automotive air conditioning system with R-134a. In order to estimate it, authors built an R-134a prototype system with and without a SLHX and compared experimentally the performances under various test conditions. They mentioned that a proper SLHX design could be helpful to improve the performance of an automotive air conditioning system. A SLHX can improve the capacity as well as the COP by 5.0% to 10.0% at idle conditions (higher air temperature of 40.0°C at the condenser and low air velocity of 1.0 m/s) but its benefit nearly vanished at higher air velocity and lower temperature. Qi et al.  suggested the performance enhancement technique of the automotive air conditioning (MAC) system using R-134a using microchannel heat exchangers consisting of a condenser and an evaporator. Authors designed two retrofitted compact and high efficient heat exchangers (an evaporator is a microchannel type and a condenser is a newly developed subcooling parallel flow type) and compared them with the existing system. New evaporator and condenser compared with existed heat exchangers decreased by 17.2% and 15.1%, respectively, based on volume, and 2.8% and 14.9%, respectively, based on weight. Also, the enhanced automotive air conditioning system could supply better cooling capacity under all test conditions due to the higher performance of the newly developed heat exchangers and showed a higher COP than that of the baseline system under all test conditions except idle condition. In addition, the schematic diagram of micro-channel evaporator suggested by Qi et al.  is shown in Figure 2. In another interesting experimental article for improving the performance of the conventional automotive air conditioning system, Nelson and Hrnjak  experimentally evaluated the effect of the several new components including condensers with microchannel type, evaporators with microchannel type, internal heat exchangers, and scroll compressor instead of reciprocating type compressor to the automotive air conditioning system using R-134a. They suggested the best system the consisting of above-mentioned components. Cooling capacity and COP of the best system increased on average 14.3% and 10.6%, respectively. However, authors mentioned that the suggested best system might not be the best configuration possible for resolving the installation issues of actual vehicles. Li et al.  investigated the new method for controlling refrigerant flow in automotive air conditioning system using the electronic expansion valve (EEV). Authors suggested the new refrigerant flow control method employing fuzzy self-tuning proportional integral derivative (FSPID) to avoid the abruption of the changes of the air conditioning system response by disturbance according to the change of the automotive speed and ambient conditions. Basic concept of the FPSID is shown in Figure 3. The fuzzy logic control is to construct a set of rules based on a combination of human reasons, knowledge of plant characteristics, and the characteristics of the controller, from which it is used to draw conclusions. FSPID control tunes the PID parameters online to adjust the controller’s actions for meeting the real-time need as mentioned in their article (in 6th page). As a result, the evaporator discharge air temperature has dropped by 3.0°C as compared with that of the conventional PID control system. In addition, they provided the fundamental rationale for developing an automotive air conditioning control system using EEV. Mansour et al.  suggested the novel control strategy for multiple circuit of the new roof top bus air conditioning system operating on partial load conditions based on numerous experimental data. This new control logic is to achieve the energy savings of the system and thermal comfort of the passenger room by the temperature difference between the evaporator inlet air temperature and the set point temperature. As a result, it could be possible the automatic controller for a more reliable and efficient cooling road recognition to the faster response with changing thermal loads. Especially, the developed system could save the energy of 31.6% for 4500 hours of operation per year at a set point temperature of 21.0°C and the life cycle cost of the developed system is 20.0% lower than that of the conventional bus air conditioning system.
Numerous theoretical and analytical articles have been also studied on performance and enhancement for conventional automotive air conditioning systems as mentioned in Seo et al. . Maruyama et al.  studied numerically the capacity control of rotary type compressor for automotive air conditioning system. They developed the mathematical modeling to analyze the pressure characteristics of the chamber in the suction stroke of a compressor using energy equation of a dimensionless form and the capacity control characteristics of a compressor with the suction groove formed on the cylinder. Figure 4 shows the construction of the considered compressor model. It is a vane rotary type compressor having spacer ring in suction passage. As a result, the effective flow area of the suction groove determines the starting point of capacity control and the effective flow area of the suction port determines the slope of the refrigerating capacity drop rate as related to rotational speed. Tian and Li  developed the mathematical model of an automotive air conditioning system with a variable displacement compressor and evaluated the steady state performance. The simulated model showed a good agreement with the experimental data. Also, they considered the performance band of the special characteristics of a refrigeration system due to the frictional forces between the moving components of a variable capacity compressor. In other article by Tian et al. , they investigated the sensitivity analysis of the dynamic model to the automotive air conditioning system with a variable displacement compressor and a thermal expansion valve. They defined the conservative stable region (CSR) of the developed simulation model of the automotive air conditioning system and its size of CSR could be influenced by the initial system state and the direction and amplitude of an external disturbance. In addition, they concluded that the CSR could provide an absolute stable region for the external disturbance. Jabardo et al.  developed the steady state computer simulation model to evaluate the performance for the automotive air conditioning system with a variable capacity compressor and built the experimental bench to check and test the performance of the air conditioning system and individual components. Especially, effects on system performance of operating parameters such as compressor speed and return air temperature in the evaporator and condensing air temperature were experimentally considered and compared with the results obtained by the computer simulation program. Both results obtained from the computer simulation and experimental bench are well consistent within 20.0%. Moreover, most of them are well consistent within 10.0%. Lee and Yoo  described the performance analysis program for conventional automotive air conditioning system under various operating conditions. The simulated air conditioning system consisted of a laminated evaporator, a swash plate compressor, a parallel flow condenser, and an externally equalized thermostatic expansion valve (TXV) and proposed the model for combining the performance analysis programs of separate components of an automotive air conditioning system. The developed program was validated with the experimental data within 7.0% and effects of the refrigerant charge and condenser size on performances of the automotive air conditioning system were discussed. Authors concluded that an overcharge of 10.0% was most effective for various operating conditions and the most desirable condenser size to accomplish the best performance in view of the installation space and cost should be 90.0% of the condenser size for the standard conditions. Bhatti  provided analytically on enhancements of R-134a automotive air conditioning system with the intent to lower its total equivalent warming impact (TEWI) from the standpoint of environmental benignity of the system and compared the performances of R-134a automotive system with those of the proposed alternative systems (the flammable subcritical systems with R-152a, R-290, and R-717; the supercritical system with R-744; conventional open air cycle with R-729). Based on these comparisons, the TEWI of the realistically enhanced R-134a system is marginally higher than those of the flammable subcritical systems (R-717, R-152a, and R-290) but marginally lower than those of the supercritical system (R-744) and the conventional open air cycles (R-729). As a result, he concluded that the suggested R-134a system is the most pragmatic solution to deal with the issue of the automotive air conditioning system. Hosoz and Ertunc  developed the artificial neural network (ANN) model to predict the performance characteristics of automotive air conditioning (AAC) systems using R-134a and designed the experimental plant to test the performance of the ACC system under steady state conditions with the variation of compressor speed, cooling capacity, and condensing temperature. Based on the standard back propagation algorithm, the ANN model for air conditioning systems with R-134a was developed and used for predicting the compressor power, heat rejection rate in the condenser, refrigerant mass flow rate, compressor discharge temperature, and coefficient of performance (COP). Figure 5 shows the structure of the ANN for modeling the automotive air conditioning system. The predicted performance parameters by ANN model are good consistent with the experimental data within the range of 0.968 to 0.999 with mean relative errors of 1.5 to 2.5%.
Due to the Montreal protocol, the most common refrigerant in automotive air conditioning systems exclusively used is currently R-134a as an alternative of R-12. And recently, increasing concern over GWP (global warming potential) of R-134a and its effect on the environment have led many industries as well as conventional automotive air conditioning to look at other options. So, various clean refrigerants as an alternative refrigerant to R-134a have been considered on R-152a, R-161, HFO-1234yf, hydrocarbon mixtures, and so forth. At the first time, many published articles on alternative refrigerants to the conventional automotive air conditioning system were to find the most suitable alternative refrigerants to R-12. Kiatsiriroat and Euakit  studied numerically and experimentally the performance characteristics of an automotive air conditioning system with refrigerant mixture of R-22/R-124/R-152a for easy retrofit as an alternative for R-12. Authors proposed the mathematical models of each component for considered air conditioning system and simulated the system performance to find the best suitable composition of the refrigerant. In the present study, the mass fraction of R-152a was fixed at 23.0% due to its flammability and the other two varied under various operating conditions. The mass fractions of R-22 were mainly changed from 20.0% to 40.0% and the COP of the considered system increased with the decrease of the mass fraction of R-22. The suggested simulation model could be applied to predict the performance of the automotive air conditioning system with a blend of R-22/R-124/R-152a. Joudi et al.  theoretically investigated the most suitable alternative refrigerant to R-12 among R-134a, R-290, R-600a, and R-290/R-600a mixture (62.0%/38.0% based on molar basis) using computer simulation and experimentally validated for the use of R-290/R-600a as a drop in alternative to R-12 in a prototype automotive air conditioning system. They reported that performances of the R-290/R-600a mixture were very close to those of R-12. Especially, the compressor displacement, working pressure, energy consumption, and the COP between R-290/R-600a and R-12 showed very similar, whereas subcooling superheating, evaporator discharge temperature of air and the cool-down speed showed different. And recently published articles on alternative refrigerants to the automotive air conditioning system were to find the most suitable alternative refrigerants to R-134a. Ghodbane  reported an investigation of R-152a and hydrocarbon refrigerants as working fluids in the conventional automotive air conditioning system. Author evaluated the potential of R-152a and hydrocarbon refrigerants (R-290, R-600a, and RC-270) as an alternative to R-134a in automotive air conditioning systems and considered the secondary loop cooling system due to their potential flammability as shown in Figure 6. The key concept of the suggested system is to use a direct expansion cooling system (which is called the primary loop) to cool the thermofluid (which is called the secondary fluid) and it is possible to provide the required cooling inside the vehicle through the heat exchanger. The performances of a secondary loop system were compared with the results of the conventional air conditioning system using R-134a and R-152a as primary refrigerants and 50.0% ethylene glycol/water mixture as the secondary refrigerant and the COPs were 20.0% and 25.0%, respectively, lower than those of conventional air conditioning system using R-134a under road load and idle conditions. As a result, flammability aside, all four refrigerants were investigated as an alternative to R-134a. In addition, R-152a, RC-270, and R-290 showed superiority or marginal improvement as refrigerants when compared to R-134a but R-600a was not suitable for automotive air conditioning system due to the low COPs and high compressor displacement requirement. Baker et al.  suggested the R-152a refrigeration cycle for automotive air conditioning system and compared cooling and energy performance with that of a comparable R-134a system under typical wind tunnel test data. Refrigerant characteristics of the R-152a offers very similar to that of R-134a but with dramatically lower GWP. They reported that the cooling performance of R-152a system is equal or better than that of the R-134a system at a road speeds and equal at idle with less energy use. However, authors mentioned that the priority for usage of the R-152a is to resolve the safety issues of flammability and toxicity although the R-152a system shows as having the lower climate impact. Wongwises et al.  investigated experimentally on the possibility of the hydrocarbon mixtures to replace R-134a as the working fluid for an automotive air conditioning system. They designed the experimental apparatus to simulate the air conditioning system driven by the diesel engine with 2500 cc and tested 4.0 different ratios of hydrocarbon mixtures consisting of R-290, R-600, and R-600a as refrigerants. Especially, the compressor was coupled to the diesel engine with a pair of pulleys and a V-belt and states of the superheat vapor at the evaporator exit and the subcooled liquid at the condenser exit were maintained. Tested refrigerants were shown in Table 1. All tested hydrocarbon mixtures yielded a higher COP than R-134a and the mixture (R-290/R-600/R-600a: 50.0%/40.0%/10.0%) showed a highest COP under all test conditions. However, the results of R-290/R-600/R-600a: 100%/0%/0% could not be compared due to the instability during the test. Han et al.  investigated the cycle performance of the binary refrigerant mixture of R-131 and R-134a (named as M5, composition of 0.6/0.4 in mass fraction) as an alternative to R-134a used in conventional automotive air conditioning system and the thermodynamic properties of the tested refrigerant mixture were experimentally and theoretically investigated. The tested binary refrigerant showed much lower GWP than R-134a due to the low GWP of R-161 and also much higher volumetric refrigeration capacity and specific refrigeration capacity than R-134a due to the high values of them for R-161. The GWP of M5 is 527.0 and its temperature glide is very small of 0.17°C. Based on theoretical results, the COP of M5 was very similar to the R-134a and the specific refrigeration capacity and the volumetric refrigeration capacity of M5 showed 55.8% and 36.7% higher than those of HFC-134a, respectively, as shown in Table 2. Based on experimental results, the COP of M5 was a little higher than that of R-134a and the refrigeration capacity and the compressor power consumption of M5 showed 32.0% and 30.0% higher than those of HFC-134a, respectively. Theoretical cycle performances of M5 and R-134a showed in Table 2. As a result, M5 had comparatively lower GWP and a bit higher COP than R-134a as environmental performances and showed a good compatibility with the existing R-134a air conditioning system. However, authors suggested that the GWP of M5 should be reduced for the whole system. In another persuasive and latest article on the alternative refrigerant to replace the R-134a, HFO-1234yf thermodynamic properties such as boiling point, critical point, and liquid and vapor density are very similar to R-134a as shown in Table 3 [38, 39]. Consequently, a few studies applied in automotive air conditioning systems have been performed. Cho et al.  evaluated the drop in system for an automotive air conditioning system using HFO-1234yf. The experimental setup was designed to test the characteristics of the automotive refrigeration cycle and drop in test was carried out under variable compressor speed from 800 to 2500 rev/min. As a result, the cooling capacity and the COP was lower 4.0 ~ 7.0% and 3.0 ~ 4.0%, respectively, than those of R-134a system as shown in Figure 7. These trends of HFO-1234yf compared with R-134a have been already known in other papers including a recent research article by Zhao et al. . Bryson et al.  also tested the performance of R-152a and HFO-1234yf in the vehicle testing bench and compared it with that of R-134a. Both were the most likely synthetic drop-in replacement candidates although they had some flammability issues. Based on the experimental results, the COP and cooling capacity of R-152 and HFO-1234yf showed slightly higher and lower than those of R-134a. As above mentioned, the performance analysis and improvement of conventional automotive air conditioning system for international combustion engines have been still performed but researches on the automotive heat pump and most potential clean refrigerant for effective preparation of coming next generation vehicles without internal combustion engines have been widely progressed in automotive markets.
3. Mobile CO2 Heat Pumps
As before mentioned, HFCs such as R-134a for many industry as well as automotive industry have been used for a long time. However, due to increasing of the global warming and environment problem for refrigerants, newly clean refrigerants have been introduced and among these refrigerants, CO2 has received considerable attention in the field of automotive heating and cooling system (air conditioning and heat pump for heating) for cabin because of the favorable thermophysical properties including higher heat transfer properties and creating no damage to the environment. Even though many research articles have been published on cooling and heating performances, hot water supplier, and their optimization of the heat pumps using CO2 as refrigerant for both residential and commercial usages at various international journals, there are comparatively few published articles on the heat pump applications and air conditioning system using CO2 for automotive. However, fortunately, extensive research and development efforts have been devoted to CO2 due to its favorable heat transfer properties and no damage to the ozone layer.
3.1. CO2 Refrigerant
Based on both Montreal and Kyoto protocol bans for refrigerants and increased environmental relevance of automotive air conditioning systems, it has been apparent that CO2 showed the most favorable environmental characteristics among all alternatives as shown in Table 4 provided by Antonijević . He emphasized the fact that it is not necessary to capture CO2 refrigerant during the air conditioning system refilling or reparation and at the end of life, which simplifies handling. In addition, to maximize beneficial impact on the environment as an alternative to R-134a, which is the most widely used refrigerant of automotive air conditioning systems and heat pumps, CO2 candidate is considered as the most suitable refrigerant. McEnaney et al.  described the experimental comparison of automotive air conditioning system when operated with transcritical CO2 versus conventional R-134a. Both systems were designed based on equal heat exchanger core volumes, components volumes and weight, same materials, and face areas and the three energy balances were performed for airside, refrigerantside, and room calorimeter in wide operating ranges including both steady state and transient conditions. All data obtained by both R-134a and CO2 automotive air conditioning systems were for the same compressor speed and very close evaporating temperature, although the CO2 compressor operated at much lower pressure ratios than that of R-134a. As a result, CO2 automotive air conditioning system showed higher capacity than the R-134a baseline system. The CO2 automotive air conditioning system was sized to provide approximately equal capacity at the extremely high temperature idling condition, but its COP fell 10.0% short of the baseline system at that point. At outdoor ambient temperatures below 40.0°C, the COP of the CO2 automotive air conditioning system was 40.0% higher than that of the baseline R-134a system. Also, Pettersen et al.  developed the compact and lightweight heat exchangers for automotive air conditioning system using CO2. Efficient heat exchangers for CO2 air conditioning system were designed as microchannel or small diameter tube types for high refrigerant mass flux. The heat transfer coefficients for refrigerant side were higher than with fluorocarbons. Yamamoto and Komatsu  investigated experimental evaluation of the prototype CO2 system with swash plate displacement compressor and the R-134a system with variable speed compressor in wind tunnel under test conditions of 32.0 km/h, 64.0 km/h, and idle. They evaluated the pull down performance for both systems with constant outlet pressure type expansion valve and super heat control type expansion valve at the conditions of the ambient temperature of 40°C, the humidity of 25.0%, and solar radiation of 950.0 W/m2. The pull down performance of CO2 system showed a similar trend with the R-134a system during 32.0 km/h and 64.0 km/h but the performance must be improved at idle conditions. Bullard et al.  studied the transcritical CO2 automotive heat pump and automotive air conditioning system. Authors made a prototype of CO2 refrigeration system operating in both air conditioning and heat pump mode and considered test matrix to reflect quasi-steady state conditions of automotive heating up at moderately cold ambient weather ranges from −10.0°C to 20.0°C and indoor temperatures ranges from −10.0°C to 20.0°C. They reported some significant results on the feasibility of CO2 as a refrigerant for automotive heat pump and air conditioning system by experiment through the designed experimental set up as shown in Figure 8. Figure 8 shows the pictures of all elements of the first CO2 automotive air conditioning system in the schematic of the heat pump configuration. The heating capacity of the tested heat pump at relatively low ambient temperatures was not lacked and its capacity was not significantly decreased at lower operating temperatures. And the semitheoretical model on automotive transcritical cycle simulation for automotive air conditioning system with CO2 was developed by Brown and Domanski . Results obtained by their model were compared to experimental results provided by McEnaney et al. . Figure 9 shows the pressure and enthalpy diagram on the simulation results of the CO2 at the test conditions given in McEnaney et al. . The developed model could simulate the counter flow, parallel flow, and cross-flow of the evaporator and gas cooler and the simulation results were good consistent with the experimental data provided by McEnaney et al. . Brown et al.  evaluated the performance of CO2 and R-134a automotive air conditioning systems using semitheoretical cycle models. In order to do an equitable comparison, components of both air conditioning systems were equivalent except the additionally equipped liquid line/suction line heat exchanger for CO2 air conditioning system and the differences in thermodynamics and transport properties were accounted for the simulations. As shown in Figure 10, the important difference on both cycles was that discharge temperature and temperature change for CO2 were higher and greater than those for R-134a at the high side pressure heat exchanger and large CO2 glide was the reason for significant temperature mismatch because the refrigerant-to-air heat exchangers were cross-flow as mentioned in their research. As a result, the predicted results obtained by simulation analysis showed that COP of the air conditioning system using R-134a was better than that of air conditioning system with CO2. Also, the entropy generation in the gas cooler was the major cause of the lower performance for air conditioning system with CO2 than that of R-134a in the condenser. Liu et al.  designed the automotive air conditioning system with CO2 and evaluated the performances in the experimental test setup. The developed system consisted of the compressor with swash plate type, the gas cooler and evaporator with fin-tube types, a manual expansion valve, an internal heat exchanger, and an accumulator. The cooling capacity, compressor power consumption, and COP were measured and analyzed with the relation of the mass flow rate of CO2 due to the strong dependency between the system performances and mass flow rate of CO2. In addition, the system performances were considered with lubricants of PAG and POE for choosing the well-suitable lubricant with CO2. As a result, PAG oil worked very well although the cooling capacity of the tested system with POE oil was larger than that of PAG oil. Tamura et al.  studied CO2 automotive cooling and heating system for comparing the performance level of the conventional air conditioning system using R-134a as a refrigerant. Authors developed the prototype CO2 automotive cooling and heating air conditioning system for medium-sized vehicles as shown in Figure 11 and evaluated the system performance such as cooling and heating. The developed system showed a better performance than the existed R-134a system under various operating conditions and especially, its heating and dehumidifying COP were 1.3 times compared with the existed system. Chen et al.  studied theoretically CO2 vapor compression (VC) cooling system and hybrid ejector for vehicles to use the waste heat from exhaust gas and the VC subsystem to drive the ejector system and designed the schematic system of the hybrid ejector and CO2 VC cooling system as shown in Figure 12. Pressure and enthalpy diagrams of the CO2 vapor compression sub-system and ejector sub system using water as working fluid with some typical operating conditions are shown in Figure 13.
In addition, Ayad et al.  investigated the CO2 vaporization characteristics in the minichannel for the purpose of the automotive air conditioning CO2 evaporator. They developed the predictive model of the heat transfer coefficient of CO2 vaporization and made an experimental database of CO2 boiling heat transfer. One of interesting results is that the increasing mass velocity of does not significantly improve the heat transfer coefficient since it accelerates the apparition of dryout. It is an opposite trend of conventional refrigerants.
3.2. Mobile CO2 Heat Pump
Due to the superior refrigerant (thermodynamic and thermophysical) properties of CO2, it is adequate to use in a heat pumps with the high compressor discharge temperatures and relatively high density even at very low temperatures. Especially, the heating capacity and the COP of heat pumps using CO2 as HVAC of a passenger room for vehicles could be higher than using an electrical supplementary heating device as before mentioned in Antonijević . Kim et al.  provided a novel CO2 heat pump for use in FCEV (fuel cell electric vehicle) with the performance evaluation considering the heat exchanger arrangements. Authors designed the test facility with the cooling and heating loops consisting of a semihermetic compressor driven by electricity instead of conventional belt driven compressor, supercritical pressure microchannel heat exchangers with a gas cooler and a cabin heater, a microchannel evaporator, an internal heat exchanger, an expansion valve with EEV (electric expansion valve) instead of conventional TXV (thermostatic expansion valve) or capillary tube and an accumulator as shown in Figure 14. Figure 14 shows the schematic diagram of the test facility of a CO2 heat pump for fuel cell vehicles considering the heat exchanger arrangements. They experimentally suggested the steady state and transient performances under various operating conditions such as outdoor temperatures, indoor temperatures, heat exchanger arrangements, and compressor speeds. The electricity driven compressor, which is independently of the engine speed compared with a belt driven compressor, used in the tested heat pump using CO2 is an important operating parameter affecting the system performance. As a result, the change of the positions of the evaporator and the radiator as exterior heat exchangers showed a significant improvement in the system heating performance. In addition, the cooling and heating effectiveness and mutual interference were quantified for two different arrangements of exterior heat exchangers. They insisted that the heat pump must be efficiently utilized for the cabin heating of fuel cell vehicles in the absence of exhaust heat source of relatively high temperature as in internal combustion engines. In their another published article, Kim et al.  experimentally investigated the heating performance and its enhancement of the CO2 heat pump with heater core for fuel cell vehicles using the waste heat from the stack coolant and compared with them the conventional heating system and the heat pump without heater core. They also discussed the efficiency improvement of the CO2 heat pump using the concept of preheating the incoming air to the cabin heater as placed of the upstream position of the cabin heater. Authors described that the heating capacity of the developed CO2 heat pump with heater core using the recovered waste heat from the stack in the fuel cell vehicles was improved by 100% and 70.0%, respectively, of that of the heat pump without heater core and the conventional heating system under specified conditions as shown in Figure 15. Figure 15 shows the comparisons of performance for heat pump with and without heater core and conventional heating system with the variation of the heater core coolant flow rate at given conditions. Kim et al.  also considered the effects of operating parameters on the cooling performance of a CO2 air conditioning system for automotives. They designed the experimental CO2 air conditioning system consisting of a compressor, a gas cooler, an evaporator, an expansion valve, an internal heat exchanger, and an accumulator as a lab scale for test and tested with the gas cooler inlet pressure, the compressor speeds, the air inlet temperatures of the gas cooler, and the air inlet temperatures and the air flow rates of the evaporator for investigating the effects of operating parameters of the developed system. The experimental results for component performance test consisting of gas cooler and evaporator were provided to help the heat exchanger design for the optimized CO2 air conditioning system development and data for system performance test with the gas cooler air inlet temperature, the evaporator air inlet temperature, the evaporator air flow rate, and the compressor speed, which are quite matching the actual vehicle driving conditions, would be helpful for actual usage for the CO2 air conditioning system. In addition, they proposed the relation for optimum high pressure control algorithm for the transcritical CO2 cycle to archive the maximum COP for more effective cooling of the cabin for automotives. Based on the review of a few published articles, the CO2 heat pump for both air conditioning and heating system for a passenger room of vehicles could be possible and the CO2 as working fluid in the heat pump could be replaced the R-134a through many additional researches. In addition, the CO2 heat pump could be a good option of the next generation vehicles without the internal combustion engines.
Based on various and extensive researches on conventional automotive air conditioning system and alternative refrigerants including CO2 as a replacement for R-134a during a few decades, this review covers strictly selected articles published in the open literature from 1994 to 2013, even though every paper was not reviewed. This survey was divided into two parts. The first part is reviewed on the performance and improvement articles for the conventional automotive air conditioning system and individual components as well as various alternative refrigerant articles as an alternative to R-134a. The second part deals with the feasibility of automotive CO2 heat pumps for next generation vehicles without internal combustion engines. Extensive studies on conventional automotive air conditioning system for internal combustion engines have been focused experimentally, theoretically, and numerically on both the performance analysis (prediction) to air conditioning system and the performance enhancement technique including the alternative refrigerants for air conditioning system. Especially, during past decades, various clean refrigerants as an alternative to R-134a have been introduced and investigated on R-152a, R-161, HFO-1234yf, hydrocarbon mixtures, and so forth. Recently, due to the rapid increase of the global warming and environmental concerns, CO2 has received considerable attention as refrigerant for heat pumps and air conditioning systems for vehicles due to its favorable heat transfer properties and creating no damage to the ozone layer. In addition, the feasibility on automotive CO2 heat pumps for the energy savings and environment regulations to adopt the coming next generation vehicles has been investigated and automotive CO2 heat pump is evaluated as a good option.
This work was supported by the Dong-A University research fund.
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