Thermal Performance Analysis of Low-GWP Refrigerants in Automotive Air-Conditioning System

Mechanical Power and Refrigeration Department (MPR), College of Technological Studies (CTS), Public Authority for Applied Education and Training (PAAET), Adailiya, Kuwait Refrigeration and Air Conditioning Department, Faculty of Industrial Education, Helwan University, Cairo, Egypt Mechanical Department, Faculty of Industrial Education, Suez University, Suez, Egypt College of Technological Studies (CTS), Public Authority for Applied Education and Training (PAAET), Adailiya, Kuwait


Introduction
e greenhouse gases of the refrigerants became the most issue in scientific research due to their impact on the environment. e industrial gases which are released in the environment are comparable with CO 2 gas, which is identified by a high global warming potential (GWP 100 ) equal to one. e gas with a higher value of global warming potential warms the earth more than the CO 2 gas. e current third generation of hydrofluorocarbon refrigerants in the automotive industry is characterized by a zero ozone depletion potential and a high global warming potential when released to the atmosphere. So, there was a great need to find out an alternative to the current R134a (GWP 100 � 1430) under the Kyoto protocol and the Montreal protocol [1]. Under the agreement of the Montreal Protocol, the phasedown of R134a was agreed by a reduction of the production rate as 7% in 2016, 37% in 2018, 55% in 2021, 69% in 2024, and 76% in 2027, with the phaseout virtually complete in 2030 [2]. From 2017, according to a directive of the European Union, automotive manufacturers manufactures cars with environment-friendly alternative refrigerants with GWP 100 ≤ 150. In the United States, by 2020 newly manufactured cars will be equipped with airconditioning system containing environment friendly refrigerants with GWP 100 ≤ 150 [2]. Globally, there are more demands than ever to reduce greenhouse gases to protect the environment and to find out a new generation of environmentally friendly refrigerants of low GWP, which is investigated here. So, the energy and exergy analysis of the automotive air-conditioning cycle which is based on the first law of thermodynamics to analyze the use of energy and on the dynamic analysis which is based on the second law of thermodynamics provides an alternative means of assessing energy efficiency for each part of the refrigeration cycle and compares operations rationally [3].
Many researchers in the past decade have concerned about the energy and exergy analysis of the refrigerants with a low global warming potential (GWP 100 < 150) according to Europe's recommendation. Cho and Park [4] studied experimentally the energy and exergy analysis of the refrigerant R1234yf and compared with R134a used in automotive air conditioning. e experiments were performed with a variable speed compressor, and the refrigeration cycles contained a heat exchanger. Both the cooling capacity and the coefficient of performance of R1234yf were reduced by 4-7% and 3.6-4.5%, respectively, compared with the R134a system. Jemaa et al. [5] performed a theoretical study using the Engineering Equation Solver (EES) software to analyze the chiller refrigeration cycle using R1234ze(E) as an alternative to R134a. Both the energy and exergy were analyzed at different evaporating and ambient temperatures. Zhang et al. [6] studied the energy and exergy of a zeotropic mixture of R32 and R236fa refrigerants used in a 4 kW chiller. e experiments with different concentration ratios of R32 to R236fa were carried. e exergy loss for each component of the chiller refrigeration cycle was discussed.
A review of alternatives to the R134a refrigerant was carried out by Verma et al. [7]. From the environmental point of view, the refrigerant with a low total equivalent warming impact factor (TEWI) of the investigated alternative refrigerants was the most suitable one in a straight drop-in substitute for R134a. Garcia et al. [8] performed a comparison study of the transient response for the R1234yf refrigeration cycle as a replacement for R134a. It was concluded that there was a similar dynamic behavior between the refrigerants R134a and R1234yf. e performance of the automotive air-conditioning system for the three types of refrigerants, R134a, R290, and R1234yf, at different operating conditions was studied by Navarro et al. [9]. It was concluded that a significant improvement in a compressor and volumetric efficiencies was obtained with R290 compared with R134a. Navarro-Esbrí et al. [10] investigated experimentally the R1234yf refrigerant in a refrigeration system as a replacement for R134a at different evaporating temperatures, condensing temperatures, and compressor speeds with and without a heat exchanger. ere was a reduction in cooling capacity by 9% when compared with R134a at the same operating conditions. Gomaa [11] carried out a comparative study between R152a, R1234yf, and R1234ze(E) with a baseline of R134a in an automotive airconditioning system. It was noted that the performance of R1234yf was very close to that of R134a when compared with the performance of R152a and R1234ze(E), respectively. e performance of the R1234yf refrigeration system in automotive air conditioning was studied by Lee and Jung [12]. ey noted that there was a reduction in COP of the refrigeration system with R1234yf by 4.0% lower than that of R134a.
Belman-Flores et al. [13] performed an energetic and exergetic study on a domestic refrigerator with R1234yf as a replacement for R134a. ey developed a thermodynamic computational model which enables to calculate the refrigeration cycle parameters at different operation conditions involving the exergy destruction ratio and exergy efficiency. e exergy destruction was mainly concentrated in the compressor, especially for the refrigerants R1234yf and R134a. Joybari et al. [14] conducted an exergetic study for a domestic refrigerator with R134a. It was found that the highest exergy destruction takes place in the compressor followed by the condenser, capillary tube, and evaporator.
Sánchez et al. [15] presented an experimental study on the energy performance evaluation of four low-GWP refrigerants compared with a high GWP of R134a as baseline.
e four refrigerants R1234yf, R1234ze(E), R600a, R290, and R152a were tested at different condensing and evaporating temperatures. Shaik and Ashok Babu [16] performed a theoretical study using the MATLAP code on four low-GWP alternatives of R22 in residential air conditioners. e thermodynamic performance of the investigated refrigerants was compared at different evaporating temperatures. Li et al. [17] performed a comparative study on energy efficiency of R717, R600a, and R1234yf as low-GWP refrigerants compared with R134a in domestic refrigerators. ey concluded that R1234yf has similar performance to R134a which can be considered as a drop-in alternative.
Vali et al. [18] performed an analytical study on the performance parameters of a refrigeration system with R22, R32, R134a, R152a, R290, and R1270. From environmental point of view, the R1270 was a more suitable refrigerant to replace R22.
In the present study, the experimental and theoretical investigation was carried out for three different types of refrigerants, which are considered as one of the most environmentally friendly refrigerants in automotive air conditioning applications. Due to the multiple operating conditions during the year of the automotive air conditioning, the study was extended to cover a wide range of compressor speed, refrigerant flow rate, and evaporating and condensing temperatures. e energetic and exergetic performance with a low-GWP 100 equal to 150 or less was the main point of interest. e low-GWP refrigerants of hydrofluorocarbon (R152a) and the two of a very low-GWP of the fourth generation refrigerants which are hydrofluoroolefins of R1234ze(E) and R1234yf were investigated in this research, with particular references to the present HFC-R134a (R134a). A weak double bond in hydrofluoroolefin refrigerants allows for short atmospheric life [19] while maintaining stability in the system, as illustrated in Figure 1. e atmospheric lifetime of the refrigerants is useful to measure the time it takes to leave the atmosphere as greenhouse gases. Table 1 describes the thermodynamic and environmental properties of the investigated refrigerants [19,20].  Figure 2. e closed-loop refrigeration cycle of R134a consists of a variable speed semihermetic compressor, air-cooled condenser, expansion valve, liquid receiver, filter drier, flowmeter, and evaporator. To control and vary the compressor speed, a frequency inverter was connected to the electric drive of the compressor. e power utilization of the compressor was measured with a wattmeter having an accuracy of ±1%. e second circuit was the ducted air-cooled condenser, which was equipped with measuring instruments to allow measurement of air temperature and velocity on the condenser airside. e duct was incorporated with a variable speed axial fan. e inlet temperature condition of the condenser varied according to the heat supplied from the electric heater which was inserted before the condenser coil. e third open-loop circuit was the duct-containing evaporator. e cooled air supplied from the evaporator coil passed through the duct in which the air duct was equipped with a variable speed axial fan and an electric heater to control the evaporator load. Two voltage regulators were used to adjust the airspeed and air temperature to a required value through the evaporator by controlling the voltage across the DC motor of the fan and the heater, respectively. Twenty thermocouples (type-k) of accuracy ±0.5°C were inserted upstream and downstream of the evaporator and condenser, respectively, in accordance with ASHRAE recommendation. A data acquisition system connected to the thermocouples was used to measure the temperature. e air velocity was measured in both evaporator and condenser ducts by a hotwire anemometer with an accuracy of ±0.1%. e refrigerant flowmeter with an accuracy of ±1% was connected through the refrigeration cycle to measure the refrigerant flow rate at different compressor speeds. e refrigerant pressure before and after the compressor was recorded with a high-and low-pressure gauge with an accuracy of ±1%.

Uncertainty Analysis
e implication of the experimental error specifies the error of the measuring and calculated quantities. For the different parameters, the uncertainty analysis was established according to Holman [21]. For independent variables (X 1 , . ., Y n uncertainties and W R was the uncertainty in the result , which can be calculated as e uncertainty values of measuring and calculating parameters are given in Table 2.

Energy Analysis
e energy balance was a statement of the energy conservation law (first law of thermodynamics) while the exergy balance was a statement of the energy degradation (second law of thermodynamics) [22]. e representative of thermodynamic refrigeration cycle is illustrated in Figure 3 in which the cooling capacity (Q evap ) was given as in which the refrigerant mass flow rate depends on the volumetric efficiency, stock volume, and specific volume of the refrigerant at the suction point as e volumetric cooling capacity (VCC) is defined as the cooling capacity per unit refrigerant volume at the exit of the evaporator, and it can be calculated as follows: e compressor power was given by At the compressor outlet, the actual specific enthalpy of the superheated vapor refrigerant (h 2 ) is calculated as e isentropic efficiency of the compressor (η is,comp ) was taken as 0.65 [23]. e coefficient of performance was defined as  Figure 1: Atomic bond of the HFC and HFO refrigerants [19].
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Exergy Analysis
Exergy analysis is a method for determining the availability of the energy that can be used in a certain system in which the deviation of the refrigeration cycle state from a given situation to the reference situation. Exergy analysis is an effective tool to find out where and how much of the input energy of a system was lost. e exergy balance was a statement of energy degradation (second law of thermodynamics). e following assumptions were considered in the system exergy analysis: (i) e steady-state conditions were satisfied for all system components. (ii) e pressure losses in the pipelines were neglected. (iii) e kinetic energy, potential energy, and exergy losses were not considered. (iv) e heat gain and heat loss from the system were ignored.
A graphical presentation of exergy balance through the refrigeration cycle of the automotive air-conditioning system is illustrated in Figure 4. e mathematical representative of the exergy (second law analysis) can be expressed as e exergy efficiency η Ex is a very useful measure of the extent to which the cycle approaches an ideal behavior. e exergy efficiency is the ratio of the actual COP to the maximum possible COP at the same operating condition [24]: e total energy destruction for all refrigeration cycle components can be expressed as e Engineering Equation Solver [25] software was used with the previous equations to develop a solution model of each investigated refrigerant with different input parameters at all state points of temperatures and refrigerant mass flow rate corresponding to and similar to that of experiments.

Results and Discussion
e results of the low-GWP refrigerants in the automotive air-conditioning system with R134a as baseline were presented as an energetic performance involving cooling capacity, compressor power, and coefficient of performance for different compressor speeds and evaporating and condensing temperatures. e exergetic performance is presented in the form of exergy destruction of each cycle component, total exergy destruction, and exergy efficiency at different cases of refrigerant flow rate and evaporating and condensing temperatures.

e Effect of Varying Compressor RPM.
e air conditioning in the automotive application and in most cases the compressor is semihermetic, which is usually connected to the engine crankshaft by a belt via two pulleys in which the compressor RPM varies according to crankshaft RPM, which in turn affects the refrigerant flow rate. e effect of varying compressor rotation of R134a on the compressor power at various condensing temperatures is shown in Figure 5. e compressor power consumption decreases with a lower value of the compressor speed and condensing temperature. e ambient air temperature affects the compressor power directly. As the condensing temperature increased by 5°C, the compressor power increased by 13% in which there was an increase in compressor power by 17% when the RPM of the compressor speed accelerated by 15%. Figure 6 illustrates experimentally both the cooling capacity and COP of R134a with the compressor RPM at various condensing temperatures. e cooling capacity increases with the increase of compressor RPM due to the increase in refrigerant mass flow rate. An increase in the condensing temperature by 5°C led to a reduction in the cooling capacity and the COP by 9% and 27%, respectively. e decrease in compressor RPM led to increase in COP of the refrigeration cycle, which can be revealed to the decrease in compressor RPM which yielded less friction between moving parts and consequently a higher isentropic efficiency of the compression process was obtained.
e investigation of a wide range of operating conditions on the automotive air-conditioning system with different refrigerant types of GWP 100 < 150 was studied using Engineering Equation Solver software (EES, 2017). A validation between the experimental and theoretical (EES) results was performed at the same operating condition which was in fair agreement. erefore, the characteristics of the refrigerants  Figure 4: Graphical presentation of exergy balance [24].

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R152a, R1234yf, and R1234ze(E) were investigated in comparison with a current high-GWP refrigerant of R134a.

Performance Criteria of Investigated Refrigerants.
e performance criteria of the low GWP refrigerants of R152a, R1234yf, and R1234ze(E) compared with R134a were specified to the effect of evaporating temperature, condensing temperature, and refrigerant flow rate on the energetic and exegetic parameters of the automotive air conditioning system. In particular, the most possible drop-in replacement refrigerant to R134a in automotive air conditioning application was R152a (GWP 100 less 10 times), R1234yf (GWP 100 less 358 times), and R1234ze(E) (GWP 100 less 238 times).

e Effect of Condensing
Temperature. In particular, the variation of the condensing temperature through the day, month, and season affects the system performance; therefore, a wide range of condensing temperature 20°C ≤ T c ≤ 45°C was considered. Figure 7 illustrates the cooling capacity of different refrigerant types at T e � 10°C and _ V � 0.0031 m 3 /s. At the same operating condition, it was noted that a reduction in cooling capacity was recorded for R152a, R1234yf, and R1234ze(E) compared with R134a by 3.6%, 3.8%, and 19%, respectively. e compressor power was impacted by changing of condensing temperature for different refrigerant types, which is illustrated in Figure 8. At the same operating condition, the compressor power consumption of R134a was higher than that of R152a, R1234yf, and R1234ze(E) by 8.5%, 1.6%, and 28%, respectively. It has been shown that the refrigerant R134a has higher values of both refrigeration capacity and compressor power than the proposed refrigerants at the same operating condition. In this case, the coefficient of performance (COP) is the most important factor to determine the performance characteristics of the proposed refrigerants, which is illustrated in Figure 9. e COP of R134a was lower than that of R1234ze(E) and R152a by 10.8% and 5.6%, respectively. It was confirmed that the COP of the refrigerant R1234yf wasvery close to the performance of R134a in which the COP of R134a was higher by 2%. e exergy destruction of the system compressor with different condensing temperatures is shown in Figure 10. e highest exergy destruction through the compressor was obtained for R1234yf followed by R134a while R152a has the lowest values of exergy destruction. Compared with the base refrigerant of R134a, the compressor exergy destruction of the refrigerant R1234ze(E) was higher by 6% while a reduction in the compressor exergy destruction was obtained by 14% and 29% for the refrigerants R1234yf and R152a, respectively, compared with R134a. e exergy destruction of the evaporator, expansion valve, and condenser was also calculated to form the total exergy destruction. e variation of the total exergy destruction of all refrigeration cycle components with the condensing temperature is illustrated   in Figure 11. e total exergy destruction of R1234yf and R1234ze(E) was quite similar in which the total exergy destruction of both refrigerants were higher than that of R134a by 12% while the total exergy destruction of R152a was lower than that of R134a by 26%. e effect of varying condensing temperature on the energy and exergy parameters is summarized as a sample of results in Table 3.

e Effect of Evaporating Temperature.
e evaporating temperature was different from one application to another, and it should be noted that the evaporating temperature affects the system performance positively. A wide range of evaporating temperatures which were applied in many air conditioning applications (− 15°C to 15°C) was tested for different refrigerant types of R134a, R152a, R124yf, and R1234ze(E). e compressor power consumption was influenced by varying evaporating temperature for all investigated refrigerants, as illustrated in Figure 12. Although the enthalpy difference (h 2 − h 1 ) in Equation (4) decreases as the evaporating temperature increases, the values of the compressor power increase as the evaporating temperature increases. is can be explained on the basis of equations (2) and (4), where the enthalpy difference (h 2 − h 1 ) decreases with the increase in the evaporating temperature, while the volumetric efficiency increases and the specific volume decreases in accordance with  R1234ze(E) Figure 9: Effect of condensing temperature on COP for investigated refrigerants.

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Equation (2). is led to an increase in mass flow rate being greater than the decrease in the enthalpy difference (h 2 − h 1 ), and consequently the compressor power consumption increases with evaporating temperature. is trend curve confirmed with Li et al. [17] and Llopis et al. [26]. e compressor power of R134a and R1234yf was quite similar. A reduction in a compressor power for R152a and R1234ze(E) by 8% and 26%, respectively, was occurred when compared with R134a.
ere are two important parameters that can characterize the most appropriate alternative refrigerants to R134a: volumetric cooling capacity and discharge temperature. Figure 13 illustrates the volumetric refrigeration capacity (VCC) and the discharge temperature versus the evaporating temperature for all investigated refrigerants. e volumetric refrigeration capacity expresses the cooling capacity per unit volume at the exit of the evaporator. It indicates the volume of refrigerants handled by the compressor. It was noted that R134a has the highest volumetric cooling capacity followed by R152a and R1234yf while the R1234ze(E) has lowest values of volumetric cooling capacity. is mean that the refrigerants R152a and R1234yf can be replaced by R134a on the same compressor size in which the refrigerant R1234ze(E) needs to resize the compressor for a given duty.
It is necessary to study the discharge temperature for the low-GWP refrigerant compared with R134a in order to clarify the steadiness and lifetime of the compressor. Figure 13 which illustrates the discharge temperature versus the evaporating temperature for all investigated refrigerants, it was noted that the refrigerant R152a has the highest discharge temperature, and it is impediment in replacement between R152a and R134a, while the discharge temperature of R1234ze(E) and R1234yf is lower than that of R134a which is considered an advantage in replacement between R1234ze(E), R1234yf, and R134a. e changing of the evaporating temperature affects the COP of the system positively, which is illustrated in Figure 14. As the evaporating temperature increased, the COP of the refrigeration system increased, which can be attributed to the increase in cooling capacity and was more rapid than the increase in compressor power. e refrigerant R1234ze(E) has the highest values of the COP among all the investigated refrigerants. It was confirmed that the thermal performance of the R1234yf refrigerant (GWP 100 � 358 times less than that of R134a) was the closest to the thermal performance of the R134a system and therefore a more environmentally sustainable refrigerant for automotive air conditioning. e exergy destruction for the system compressor with different evaporating temperatures is illustrated in Figure 15.

Referring to
e highest exergy destruction through the compressor was obtained for R1234ze(E) followed by R134a, while R152a has the lowest values of exergy destruction. Compared with the base refrigerant of R134a, the compressor exergy destruction of the refrigerant R1234ze(E) was higher by 6.5% while the compressor exergy destruction of R1234yf and R152a was reduced by 11% and 40%, respectively. e total exergy  Advances in Materials Science and Engineering destruction of the all-cycle components for different types of refrigerants and the evaporating temperature is illustrated in Figure 16. e total exergy destruction of R1234ze(E) was higher than that of R134a by 5.4% while the total exergy destruction of R1234yf and R152a was lower than that of R134a by 15% and 45%, respectively.

e Effect of Refrigerant Flow
Rate. e refrigerant volume flow rate variation which was produced as a result of varying compressor speed which in turn was originated from automotive crankshaft speed affects the performance of the automotive air conditioning. e effect of varying volume flow rate on the cooling capacity and the coefficient of performance is illustrated in Figures 17 and 18 for a typical condition of T e � 10°C and T c � 35°C. e refrigerant mass flow rate varied as the RPM of the crankshaft was changed, which depend on   the rate of fuel consumption of the automotive engine. In practice, the refrigerant mass flow rate ( _ m � ρ _ v) which is the refrigerant density multiplied by volume flow rate affects the cooling capacity and compressor power according to equations (1) and (2) in which the density varies according to variation in evaporating temperature. erefore, the influence of the refrigerant flow rate was evident to the coefficient of performance. As the refrigerant flow rate increases, the COP decreases which can be explained by the increase in compressor power with a flow rate greater than the increase in cooling capacity. It is noted that the refrigerant R1234ze(E) has the highest COP between investigated refrigerants. e total exergy destruction of the compressor, condenser, expansion valve, and evaporator for the different refrigerant types is illustrated in Figure 19.
e highest exergy destruction of the compressor was obtained for R1234ze(E), and the highest exergy destruction of the condenser was obtained for R1234fy, while the lowest value of exergy destruction for the compressor and condenser was obtained for R152a. e highest values of the exergy destruction for the evaporator and expansion valve were obtained for R1234ze(E) and R1234fy, respectively.
Exergy efficiency can give more logical ways to improve the energy performance of automotive air conditioning. For that reason, the exergy analysis which was performed for each cycle component should be considered integrated [6]. Figures 20 and 21 show the total exergy efficiency with both   condensing and evaporating temperatures. e total exergy efficiency was decreased with the condensing temperature while it was increased with the evaporating temperature. For both condensing and evaporating temperatures, the refrigerant R1234yf has the lowest exergy efficiency followed by R152a. e highest exergy efficiency was obtained with R1234ze(E) at different condensing temperatures while at different evaporating temperatures, the highest exergy efficiency was obtained for R134a. From the environmental, thermal, and exergy point of view, the refrigerant R1234yf has the best performance among all refrigerants that have been investigated to replace R134a in an automotive airconditioning system.

Conclusion
Energy and exergy analysis was presented for many environmentally friendly refrigerants as a drop-in replacement of current high-GWP of R134a in the automotive air-conditioning system. ree alternative refrigerants which is distinguished by zero ODP and GWP 100 < 150 were investigated, with particular reference to the current R134a refrigerant (GWP 100 � 1430). e exergy destruction of each component and the exergy efficiency at different condensing temperatures, evaporating temperatures, and refrigerant flow rates were presented, and the main conclusions are summarized as follows: (i) For all values of condensing and evaporating temperatures, a higher system COP was obtained at