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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 282313, 9 pages
http://dx.doi.org/10.1155/2013/282313
Research Article

Cooling Performance Characteristics on Mobile Air-Conditioning System for Hybrid Electric Vehicles

1Korea Automotive Technology Institute, 74 Yongjung-Ri, Pungse-Myun, Dongnam-Gu, Chonan-si 330-012, Republic of Korea
2Department of Mechanical Engineering, Dong-A University, 37 Nakdong-Daero 550 Beon-Gil Saha-Gu, Busan 604-714, Republic of Korea

Received 4 October 2012; Accepted 29 January 2013

Academic Editor: Hakan F. Oztop

Copyright © 2013 Ho-Seong Lee and Moo-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.

Abstract

This study investigates the cooling performance characteristics of the mobile air-conditioning system using R744 (CO2) for the hybrid electric vehicle as an alternative to both the R-134a and the conventional air-conditioning system. The developed air-conditioning system is operated with an electric driven compressor in the battery driving mode and a belt driven compressor in the engine driving mode. The cooling performance characteristics of the developed system have been analyzed by experiments under various operating conditions of inlet air temperature, air flow rates for the gas cooler side and evaporator side, and electric compressor revolution respectively. As a result, cooling performances of the tested air-conditioning system for the EDC driving mode (electricity driven compressor) were better than those for the BDC driving mode (belt driven compressor). The cooling capacity and cooling COP of the tested air-conditioning system for both driving modes were over 5.0 kW and 2.0, respectively. The observed cooling performance of the tested air-conditioning system may be sufficient for the cabin cooling of hybrid electric vehicles.

1. Introduction

Many automotive companies are developing green cars without an internal combustion engine, because international regulations on the use of fossil fuel resources have become stricter with increases in global warming and glacier melting. Also, conventional vehicles have two major problems; one is the use of fossil fuels for driving, and the other is the use of chemical refrigerants in the air-conditioning system for cabin cooling. So, many automotive makers are developing green cars that do not use fossil fuels. Although the classifications of green cars have not been officially defined internationally, we generally classify electrically driven vehicles as electric vehicles, fuel cell electric vehicles, and hybrid vehicles [1, 2]. Among these, the hybrid vehicle is the most commercially accessible alternative to fossil fuel vehicles because many techniques for the realization of the mass production have been developed by leading automotive makers. The hybrid vehicle uses two driving power systems consisting of an internal combustion engine and a high-voltage battery pack system for the driving motors. The two driving systems are operated optionally depending on the driving conditions, including road conditions and outdoor temperatures. And these driving types for the hybrid vehicle could give a stable driving distance. In general, the high-voltage battery pack system in the hybrid electric vehicle is used in the short driving mode for less than 60 km driving distance at low driving velocity and the installed engine is used in the long driving mode for more than 60 km driving distance at high driving velocity. The driving method of the hybrid electric vehicle could reduce both unpredictable and predictable problems during the driving of the fully electric driven vehicles such as the fuel cell electric vehicles and electric vehicles.

To overcome the two problems of conventional vehicles, the hybrid mobile air-conditioning system using R744 (CO2), a natural refrigerant, as the vehicle refrigerant is developed for cabin cooling of the hybrid vehicle in this study. The R744 (CO2) has been considered as an alternative of CFC and HCFC because it has negligible ODP (Ozone depletion potential) and GWP (global warming potential) as well as many merits such as no toxicity, no flammability, high volumetric capacity, and better heat transfer properties [3, 4].

Various types of air-conditioning systems for conventional vehicles have been investigated. Kim et al. studied the performance of an automotive air-conditioning system using R-134a and R-152a, respectively, by experiments. They reported the possibility of R-152a as an alternative refrigerant to the current R-134a in automotive air-conditioning system [5]. Lawrence et al. compared the on-vehicle performances of an R-152a and an R-134a heat pump system using engine coolant. They reported that the performances and capacities of both R-152a and R-134a heat pump systems were almost identical [6]. Lee et al. studied the effects of operating parameters on the cooling performance of a transcritical R744 (CO2) mobile air-conditioning system. They reported that the mobile air-conditioning system using R744 (CO2) gave higher cooling performance than that using R-134a [7]. Steven Brown et al. studied the comparative analysis of an automotive air-conditioning system operating with R744 (CO2) and R-34a [8].

However, there is a limited amount of data available on the cooling systems for the hybrid electric vehicle, even including the results from the evaluation by Cho et al. on the cooling performance of an electric air-conditioning system using R744 (CO2) for an electric vehicle [9]. Therefore, this study investigates the feasibility of integration and the cooling performance characteristics of the mobile air-conditioning system using R744 (CO2) for the hybrid electric vehicle as an alternative to both the R-134a and the conventional air-conditioning system. The developed system is operated with an electric driven compressor in the battery driving mode and a belt driven compressor in the belt driving mode. Experiments to analyze its cooling performance characteristics have been performed under various operating conditions of inlet air temperature, air flow rates for gas cooler side and evaporator side, and electric compressor revolution, respectively. The results were then compared with cooling performance characteristics of the conventional air-conditioning system. In addition, the developed air-conditioning system using R744 (CO2) with the belt driven compressor can be alternative to the conventional air-conditioning system.

2. Experimental Setup and Data

2.1. Test Setup

Figure 1 shows the schematic diagram of the basic test setup used to measure the performance of the hybrid mobile air-conditioning system for the hybrid electric vehicle. The test setup mainly consisted of an electric driven compressor, a belt driven compressor, expansion valve with electronic control type, two psychrometric calorimeters for a gas cooler, and an evaporator, which provided precontrolled ambient temperature and air flow rate. The psychrometric calorimeter, equipped with an air-handling unit including a cooling coil, a heating coil, and a humidifier, was set to 25°C and 40°C to an accuracy of °C. The psychrometric calorimeter was controlled by using the PID control method. Both a multiflow type evaporator and a multiflow type gas cooler were installed in the psychrometric calorimeters to control the air-side inlet conditions. The electric driven compressor for R744 (CO2) was a variable speed type, whose driving current was measured by the inverter driver (SV-IG5A) manufactured by LS industrial Systems and whose work was calculated based on the power input and the supply current. The belt driven compressor was coupled to a traction motor, which simulated engine speeds of 900 and 1800 rev/min, and the compressor work was calculated considering the energy conversion efficiency of the traction motor based on the power input and the current supplied to the traction motor. The power inputs were separately measured exactly to evaluate the hybrid mobile air-conditioning system. During the experiments, the major operating parameters were monitored graphically and numerically in real time. In order to calculate and evaluate the performance of the hybrid mobile air-conditioning system, temperature, pressure, and mass flow rate were measured. The refrigerant R744 (CO2) was used in the system. Table 1 shows the specifications of the hybrid mobile air-conditioning system for a fuel cell electric vehicle. Table 2 shows the test conditions in this study. In order to reflect the various driving conditions in summer season, the inlet air temperature and air velocity of the gas cooler were set to 25°C ~ 40°C and 2.3 m/s ~ 6.0 m/s, respectively, and the inlet air temperature and air flow rate of the evaporator were set to 25°C ~ 40°C and 4 m3/min ~ 7 m3/min, respectively, with relative humidity of 50%. The air flow conditions of the gas cooler and evaporator were selected based on FEM and HVAC module test results [10, 11]. The compressor speed is set to 900 rev/min ~ 1800 rev/min for BDC mode and 2000 rev/min ~ 6000 rev/min for EDC mode. The above test conditions reflected real conditions of conventional passenger vehicles: the idle condition and 100 km/h driving condition. The test results between the hybrid electric vehicle and the conventional vehicle were compared. Table 3 shows the accuracy and uncertainties of the experimental parameters and measured data. Refrigerant and air temperatures were measured with thermocouples. The thermocouples were calibrated to an accuracy of ±0.1°C. The refrigerant flow rate was measured by a Coriolis type flow meter with an accuracy of ±0.2% and an upper limit of 650 kg/h. This flow meter was installed between the outlet of the gas cooler and the inlet of the expansion valve to minimize measurement errors. Pressure sensors, which can measure absolute pressure up to 25 Mpa with an accuracy of ±0.1%, were installed at the inlet and outlet of each component. In order to verify the measured data on the cooling capacity and the cooling COP, an uncertainty analysis was performed for the 95% confidence level, set by the standards of ANSI/ASME (1985) and Moffat [12, 13]. The precision limits and bias limits of all the parameters associated with cooling capacity and cooling COP were estimated. The average uncertainties of the experimental data on cooling capacity for BDC and EDC modes and cooling COP for BDC and EDC modes were 4.3% and 4.5%, and 5.6% and 5.8%, respectively. In addition, the mobile cooling system with R744 (CO2) is designed for simultaneous usages at both EDC and BDE modes. Basically, the electricity driven compressor with inverter driver and EXV 2 were operated at EDC mode and the belt driven compressor and EXV 1 were operated at BDC mode.

tab1
Table 1: Component specifications of the hybrid mobile air-conditioning system.
tab2
Table 2: Test conditions.
tab3
Table 3: Accuracy and uncertainties of the experimental parameters and measured data.
282313.fig.001
Figure 1: Schematic diagram of the test setup.

2.2. Design Point

As shown in Table 1, the tested hybrid mobile air-conditioning system uses R744 (CO2) because it is environmentally benign with nearly negligible global warming potential (GWP) and zero ozone depletion potential (ODP), although a transcritical CO2 cycle shows intrinsic disadvantages in comfort cooling due to large expansion losses and higher irreversibility during the gas cooling process as mentioned in Cho et al. [14]. The core size of the gas cooler is  mm3 and the core size of the evaporator is  mm3. The compressor for BDC mode is an electric driven scroll compression type with an inverter driver and the compressor for BDC mode is a belt driven reciprocating type with a traction motor. The accumulator has a volume of 500 mL and a maximum pressure of 12.5 MPa at a temperature of 90°C. Both the evaporator and the gas cooler with a multiflow fin tube heat exchanger were installed for heat exchange between the refrigerant and air. And the gas cooler was operated in the transcritical region. And an accumulator was installed to ensure a stable operation during the change of a driving mode and to ensure a sufficient degree of overheating between the compressor suction and the outlet of the evaporator. In order to ensure the subcooled temperatures over 5.0°C while vehicles are driven on unpaved roads, the two EXVs are properly operated with the indoor and outdoor temperatures. The mobile air-conditioning system for a cabin in the conventional passenger vehicle was designed considering the subcooled temperature of at least 5.0°C between the condenser outlet and the inlet of the expansion device for a stable cooling performance in various real driving conditions including high way and off-road conditions, as mentioned by Lee et al. and Park et al. [15, 16].

2.3. Data Reduction

The refrigerant side heat capacity was calculated by the refrigerant enthalpy method (ANSI/AMCA 210, 1985, and ASHRAE Standard 116, 1983) [12, 17]. Equation (1) was used to calculate the refrigerant side heat capacity. The air side heat capacity was calculated by utilizing both the air flow rate and enthalpy difference, which were calculated by (2). Equation (2) was used to calculate the air side heat capacity:

The cooling COP (coefficient of performance) of the hybrid mobile air-conditioning system was calculated by (3) for the electric driven compressor and (4) for the belt driven compressor:

3. Results and Discussion

An electrical air-conditioning system using two individual R744 (CO2) compressors was developed to cope with the cooling load in the cabin of the hybrid electric vehicle under hot weather conditions and tested for different driving modes. Both cooling test results are compared with the results of the conventional air-conditioning system using the belt driven compressor and R-134a founded in an internal combustion engine. By using the refrigerant charge matching method in the air-conditioning system [18, 19], the refrigerant charge of the air-conditioning system was set at 1.5 kg for the evaporator air temperature of 25°C, air flow rate of 4 m3/min, and relative humidity of 50%. The same setting was used for the air temperature of 25°C and air velocity of 2.3 m/s in the gas cooler and compressor speed of 4000 rev/min in the EDC mode and compressor speed of 900 rev/min in the BDC mode. In addition, the test conditions chosen for the system matching were normal operation conditions for a cabin air-conditioning system for the summer season.

As the front end module (FEM) in hybrid electric vehicle designs, the arrangement of the cooling radiators for the engine, traction motor, high voltage battery pack system, and so forth, and the arrangement of the heat exchanger (gas cooler or condenser) for the air-conditioning system are important factors and their performances may affect each other because of the limited space of the FEM in the vehicle. Accordingly, slim heat exchangers should be used to satisfy the increased number of heat exchangers in the hybrid electric vehicle. A slim gas cooler of 10 mm depth was developed and installed in this study.

Figure 2 shows the pressure drop characteristics of the gas cooler and evaporator for R744 (CO2) according to mass flow rate. The pressure drop at the gas cooler increased a little with the rise of the compressor speed, but the increase was not significant. In addition, the pressure drop at the gas cooler at all compressor speeds increased with the rise of the mass flow rate because of the increased compression ratio and maximized at 3.2 bar. This value is a little higher than that of the condenser using R-134a in the conventional air-conditioning system, but it does not reduce the operation and performance of the air-conditioning system using R744 (CO2) because R744 (CO2) has a lower critical temperature and higher critical pressure than R-134a. Namely, its vapor pressure of R744 (CO2) is much higher than R-134a. High pressure and density would be improved by the heat exchanger effectiveness; this was consistent with the result of Cho [20]. The pressure drop of the evaporator using R744 (CO2) was similar to that of the evaporator using R-134a. The pressure drop of the evaporator increased by an average of 38.2% with the rise of the compressor speed from 3000 rev/min to 6000 rev/min.

fig2
Figure 2: Pressure drop characteristics of the gas cooler and the evaporator according to mass flow rate.

Figure 3 shows the effects of the inlet air temperature of the gas cooler on the cooling capacity and cooling COP for different driving modes. For the tests, the driving modes were set, and the compressor speed was chosen to be 4000 rev/min in the EDC mode and to be 900 rev/min in the BDC mode for the idle (stop) condition, which was not affected by vehicle driving conditions. The cooling capacity and cooling COP at all tested conditions increased with the rise of the gas cooler inlet pressure but decreased with the rise of the inlet air temperature of the gas cooler from 25°C to 40°C due to the decrease of the gas cooler’s heat transfer efficiency between the refrigerant and air temperature. This result is consistent with the results of Yang et al. [21]. The cooling capacity and cooling COP in the EDC mode were a little higher than those in the BDC mode. At the gas cooler inlet temperature of 25°C and inlet pressure of 9 Mpa, the cooling capacity and cooling COP in the EDC mode were 1.15% and 2.34% higher than those in the BDC mode, respectively.

282313.fig.003
Figure 3: Effects of the inlet air temperature of the gas cooler on the cooling COP and cooling capacity according to gas cooler pressure.

Figure 4 shows the effects of the inlet air temperature of the evaporator on the cooling capacity and cooling COP for different driving modes. At all driving modes, the cooling capacity and cooling COP increased with the rise of the evaporator inlet temperature and air flow rate due to the increased rate and efficiency of the heat transfer in the evaporator. The cooling capacity at evaporator air flow rate of 4 m3/min increased by 42.6% in the BDC mode and 45.9% in the EDC mode with the rise of the evaporator inlet temperature from 25°C to 40°C.

fig4
Figure 4: Effects of the inlet air temperature of the evaporator on the COP and cooling capacity according to gas cooler pressure.

The cooling capacity at evaporator inlet air temperature of 25°C increased by 22.4% in the BDC mode and 22.1% in the EDC mode with the rise of the evaporator inlet air flow rate from 4 m3/min to 7 m3/min. In this study, the effect of the air flow rate on the variations of cooling COP was smaller than that of the air temperature. In addition, at the evaporator inlet air temperature of 25°C and air flow rate of 4 m3/min, the cooling capacity and cooling COP in the EDC mode increased by an average of 6.4% and 10.71%, respectively, compared to those in the BDC mode. The cooling COP increased more than the cooling capacity because of the lower work of the electric compressor in the EDC mode. Generally, in the system, the efficiency of the electricity driven compressor with inverter driver was superior to that of the belt driven compressor because the compression ratio of the electricity driven compressor could be properly controlled with the variations of the outdoor and indoor temperatures.

Figure 5 shows the effects of the electric compressor speed on the cooling capacity and the COP. In the EDC mode, the cooling capacity of the evaporator at all compressor speeds increased with the rise of the gas cooler inlet pressure due to the increased heat transfer efficiency of the gas cooler, and the cooling COP also increased. However, the cooling COP decreased with the increase of the compressor speed due to the increased compressor work. This is because the refrigerant mass flow rate increased from 138.4 kg/h to 171.1 kg/h with the rise of the compressor speed from 3000 rev/min to 6000 rev/min, and the outlet air temperature of the gas cooler increased with the gas cooler inlet pressure. In addition, the increase rate of the cooling COP at all compressor speeds decreased with the decrease of the cooling capacity increasing rate. As shown in Figure 5(a), the mobile air-conditioning system using an electric compressor with an inverter driver could be actively controlled to adjust cooling loads in the vehicle cabin as the hybrid electric vehicle is driven in the EDC mode. This means that the cooling COP of the mobile air-conditioning system is superior to that of the conventional air-conditioning system in the BDC mode [18]. Lee et al. mentioned that the heat transfer efficiency of an electric air-conditioning system using an inverter driven compressor is 10% higher than that of the conventional air-conditioning system using a belt driven compressor coupled to the engine because of the actively controlled compression ratio with TXV (thermal expansion valve) and compressor frequency by the inverter driver. This result is very consistent with the result obtained from this study. In the BDC mode, the cooling capacity increased but the cooling COP decreased with the rise of the compressor speed because the compressor work increased rapidly. The cooling capacity increased by an average of 5.8% with the rise of the compressor speed from 900 rev/min to 1800 rev/min because the compression ratio of the belt driven compressor and the heat transfer efficiency of the gas cooler increased, but the cooling COP decreased by an average of 4.7% because the compressor work increased by an average of 11.1% with the rise of the compressor speed.

fig5
Figure 5: Effects of the compressor speed on the COP and cooling capacity according to the gas cooler inlet pressure.

Figure 6 shows comparison of performances of the developed air-conditioning system using R744 (CO2) and the conventional air-conditioning system using R-134a. In order to compare the test results of R-134a mentioned by Lee et al. [18], the two systems were compared for evaporator inlet air temperature of 42°C, relative humidity of 50%, air flow rate of 7.5 m3/min, gas cooler (or condenser for R-134a) inlet temperature of 27°C, and gas cooler inlet air velocity of 4.0 m/s. And the electric compressor speeds in the EDC mode under the given conditions were selected to be 4000 rev/min and 5000 rev/min. As a result, the cooling COP of the electric air-conditioning system using an inverter driven compressor was higher than that of the belt driven air-conditioning system for refrigerants R744 (CO2) and R-134a. However, in case of the air-conditioning system using a belt driven compressor coupled to the engine, the air-conditioning system using R744 (CO2) showed better performance than that using R-134a because the cooling capacity of the refrigerant using R744 (CO2) was in general greatly superior to that of the R-134a [22]. Therefore, the two air-conditioning systems using R744 (CO2), considered for a cabin of the hybrid electric vehicle, showed better cooling performance than the conventional air-conditioning system using R-134a. In other words, these air-conditioning systems using R744 (CO2) could be very helpful to overcome the short driving problem of the hybrid electric vehicle because they would consume less energy from the high-voltage battery system (considering that energy consumption is closely related to the driving ranges) and provide higher cooling COP. In addition, during tests, the cooling COPs of the EDC mode, the BDC mode for R744 (CO2), and the BDC mode for R-134a decreased by 7%, 33.2%, and 15.7%, respectively. That is, the decreasing rate of the cooling COP at the EDC mode was lower than those at the other modes because the volumetric efficiency of the tested electric scroll compressor using inverter driver was over 90%.

282313.fig.006
Figure 6: Performance comparison of the developed air-conditioning system using R744 (CO2) with the conventional air-conditioning system using R-134a.

4. Conclusions

This study investigated the performance characteristics of the hybrid mobile air-conditioning system using R744 (CO2) for the hybrid electric vehicle. Two air-conditioning systems using R744 (CO2) for cabin cooling of the hybrid electric vehicle are optionally selected depending on the driving modes. In this paper, the inlet air conditions of both the gas cooler and evaporator and the compressor speeds for BDC and EDC varied. Experimental results showed that the cooling capacity and cooling COP of the hybrid mobile air-conditioning system using R744 (CO2) were sufficient to meet the cooling load of the hybrid electric vehicle under warm weather conditions. In addition, the cooling capacity and the cooling COP for both driving modes of the tested system were over 5.0 kW and 2.0, respectively.(1)The cooling capacity and cooling COP at all tested conditions increased with the rise of the gas cooler inlet pressures but decreased with the rise of the inlet air temperature of the gas cooler from 25°C to 40°C due to the decrease of the heat transfer efficiency of the gas cooler between the refrigerant and air temperature.(2)The cooling capacity at the evaporator inlet air temperature of 25°C increased by 22.4% in the BDC mode and 22.1% in the EDC mode with the rise of the evaporator inlet air flow rate from 4 m3/min to 7 m3/min.(3)The cooling capacity at all compressor speeds increased with the rise of the gas cooler inlet pressure due to the increased heat transfer efficiency of the gas cooler, and the cooling COP also increased.(4)The cooling COP of the electrical air-conditioning system using an inverter driven compressor was higher than that of the belt driven air-conditioning system using R744 (CO2) and R-134a. However, in case of the air-conditioning system using the belt driven compressor coupled to the engine, the air-conditioning system using R744 (CO2) showed better performance than that using R-134a

Nomenclature

BDC:Belt driven compressor
COP: Coefficient of performance
EDC: Electricity driven compressor
: Enthalpy difference, (kJ/kg)
: Pressure drop, (Mpa)
: Mass flow rate, (kg/min)
:Heat capacity, (W)
: Temperature, ().
Subscripts
:Air
:Inlet
:Outlet
:Pressure
:Refrigerant.

Acknowledgment

This work was supported by the Dong-A University Research Fund.

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