Study of the Electric Vehicle Heat Pump Air-conditioning System

Transcription

1 AVEC 1 Study of the Electric Vehicle Heat Pump Air-conditioning System Po-Hsu Lin, Automotive esearch & Testing Center No.6, Lugong S. 7th d., Lukang, Changhua County, 50544, Taiwan(.O.C) Phone: # 360 Fax: alvin@artc.org.tw Air-conditioning system is one of the important accessory systems of electric vehicle which provides cooling and heating function for the passengers. The PTC heater in the present electric vehicle shows poor energy efficiency because of the electric-thermal conversion and the multiple-stage heat transfer mechanisms. In order to overcome the drawback of the PTC heater, a modified heat pump system structure was developed to recover the waste heat which is generated by the subsystems during operation. In this paper, the electric vehicle battery pack is chosen as the target subsystem. A battery model was built and corrected according to the experimental data. The final simulation results show that the maximum heat generated by the chosen battery pack under 1.5C discharging is about 356 W which is nearly 0% of the heat energy for the heating function. Therefore, the modified system can maintain the heat pump system efficiency under cold weather and avoid the condenser from frosting. Topics / Green-Car System Control 1. INTODUCTION Air-conditioning system which provides the cooling and heating function is one of the important accessory systems of electric vehicle (EV). In the present electric or hybrid vehicle, an electric compressor and a PTC heater are adopted to provide cooling and heating function. The refrigerant is compressed by the electric compressor and then flows through the condenser, the expansion, the evaporator and finally returns to the electric compressor. During the operating process, the heat inside the cabin is adsorbed by the evaporator and exhausted to the environment by the condenser. The electric compressor is able to vary the rotational speed according to the heat load of the cabin. Therefore, the efficiency of the cooling subsystem is able to increase by reducing the unnecessary electric energy. The electric compressor is now a mature technology and product so the research topic of the cooling function focuses on the energy-saving control algorithm. The PTC heater generates heat by consuming electricity to heat-up the medium which is usually air or water. And then the mediums warm up the air that enters the cabin by heat convection and conduction. Although the PTC heater could provide sufficient heat energy to warm up the cabin, low energy efficiency is presented due to the complex heat transfer mechanisms. The power consumption of a PTC heater could be up to 5kW where the electric compressor only consumes kw to provide cooling function. The use of the PTC heater reduces the electric vehicle mileage from 50% to 70% where the electric compressor is from % to 50% [1]. Therefore, a solution which could provide high efficiency heating function is needed to replace the PTC heater. In order to overcome the drawback of the PTC heater, a modified heat pump system structure was presented by Automotive esearch & Testing Center (ATC). The modified structure is aimed to recovery the heat generated from the subsystems on the EV and uses to heat up the cabin. In this paper, the battery pack of ATC electric vehicle is chosen as the subsystem and the maximum recovered heat is simulated to study the possibility of the proposed structure.. MODIFIED HEAT PUMP SYSTEM STUCTUE Heat pump system is a full-developed technology and a lot of products have been designed based on this technology. However, some of the characteristics of the heat pump system make it hard to use in the vehicle, i.e. low heat absorption and frosting of condenser due to the lack of heat energy under extremely weather condition. Therefore, the heat pump system needs to be properly designed to overcome these drawbacks. In this study, the suitable temperature range for the heat pump system is chosen as -10~40 and the system needs to provide sufficient air-conditioning effect to keep the cabin temperature remaining within 18~8. The specification of the system is then designed based

2 AVEC 1 on the available heat sources and the vehicle specifications. The available heat sources include the radiation heat from the sun, heat convection from the environment and the heat from the passengers, et al, which was carried out by Chang []. The vehicle specifications, ex. the area of the windows and vehicle surface, will change the amount of the heat transferred in or out of the cabin. In this study, the China-Motor Varyca is chosen as the target vehicle to evaluate the heat pump system specifications which is shown in Table 1. The demanded cooling and the heating ability for the vehicle were calculated based on the demanded operating temperature range, demanded setting temperature of the cabin and the vehicle specification. The final results show that the system needs to provide 5kW for the cooling function and 4kW for the heating function respectively. Table 1 Discharging Experiment Conditions Specifications oof area (m ) Side area(m ) Bottom area(m ) 6.55 Tail area(m ) Front glass area(m ) Front glass angle( o ) 5.33 ear glass area(m ) ear glass angle( o ) Side glass area(m ) Passenger No. 5 However, the system may not recover enough heat from the extreme cold weather. Therefore, the additional heat sources are needed to be found for solving the drawbacks. Since the subsystems of the EV generate heat during operating, they could be used as the heat sources to the heat pump system. In order to recover the waste heat from the subsystems, the heat pump system structure is needed to be modified. Fig. 1 shows the modified heat pump system structure diagram. An additional heat exchanger (HE), an expansion (EXV), several check s and solenoid s are added into the original air-conditioning system. The structure could perform 8 different circulations which are able to adjust the temperature of the cabin and subsystem respectively according to the combination of the check s. The HE and EXV are placed adjacent to the target subsystem to provide cooling or heating function according to the subsystem temperature. EAAC controller additional HE and then use to heat-up the cabin by the evaporator. Since part of the heat energy comes from the subsystem, it is expected that the heat pump system could work under cold weather and avoid condenser from frosting. 3. LI-ION BATTEY HEAT GENEATION MODEL The subsystems on the electric vehicle, i.e. propulsion motor, inverter and battery pack, generate heat during operating. Therefore, the temperature of these subsystems is usually controlled by air-cooling or liquid-cooling mechanism. Among those subsystems, the efficiency of the battery is highly sensitive to its temperature. Therefore, the temperature of the battery is needed to be controlled within a proper range. In other words, the generated heat during operating is needed to be properly removed. In this study, the battery pack was chosen as the subsystem. The heat generated by the battery cell as well as the battery pack was studied for the generated heat. In order to simulate the battery temperature behavior during discharging, the heat generating mechanism is needed to study. In the research of Li-ion battery temperature variation [3], three different heat sources which come from the chemical reaction, the polarization resistance and the internal resistance are modeled respectively. During the discharging, the reaction heat is sum of the heat generated at both positive and negative pole which is derived as Q reac = Q / F I = QI (1) where Q is the sum of the heat generated at both the negative and positive pole, F is the Faradays constant and I is the discharging current. The heat generated by the internal and polarization resistance can be expressed as Eq.() and (3) Q ploar = I pd () Qint = I (3) er e where pd is the polarization resistance and e is the internal resistance of the cell. Therefore, the overall heat generated during discharging is the sum of Eq. (1) to Eq. (3) which is derived as Condenser Front 4-way Ex. Valve 1 Electric Compressor Evaporator Cabin Front Fan Temperature measurement Unit Ex. Valve a Heat Exchanger Aux. b System Fig. 1 ATC modified heat pump system structure Therefore, the modified heat pump system can recover the waste heat from the subsystem by the Vent Cabin Outside ear Q z = Q reac + Q polar = QI + Q int er td = Q reac pd e (4) where td is the sum of the internal and polarization resistance. 4. SIMULATION ESULTS AND DISCUSSION In this paper, the Finite-Volume-Method (FVM) was adopted to simulate the battery temperature variation behavior. The heat generated during discharging, as shown in Eq.(4), was used as the

3 AVEC 1 boundary condition of the simulation. The battery cell models were built and coded in the SC/Tetra software with 400k grids. The model of the battery module, which includes 16 cells, was built based on the cell model and the number of the grids of the battery module is 60k. In order to obtain an accuracy simulation result of the battery pack temperature and the generated heat, several experiments were executed to obtain the actual temperature behavior of the battery cell and module. The experiment conditions are listed in Table 1. Each experiment was repeated for three times to obtain an arithmetic average result of the cell and module temperature variation. The experiment data was then used to correct the model of the battery cell and module. Table 1 Discharging Experiment Conditions Discharging Discharging Capacity Current Period 15A (1C) 15 min Cell 15Ah.5A (1.5C) 10 min 60A (1C) 15 min Module 60Ah 90A (1.5C) 10 min 4.1 Experiment and simulation of cell and module temperature Fig. and 3 shows the experiment and corrected simulation results of the single cell temperature variation under 1C and 1.5C discharging. The experiment data starts from different value due to the different environment temperature when the experiment is executed. However, the experiment results show a similar temperature increasing tendency. As shown in Fig., all the experiment data shows that the cell temperature increases.7 with different initial temperature. The simulation result also shows the same temperature increasing magnitude as experiment data. Furthermore, the experiment results show that the cell temperature increases with a smoother slope before 0 seconds. Because the heat is generated inside the cell, the temperature of the battery core will be increased first and then the surface Cell Temperature Variation under 1C Discharging Experiment Elapsed Time (sec) Fig. Cell temperature variation under 1C discharging Fig.3 shows the temperature variation of the cell under 1.5C discharging. The experiment and the simulation results show the similar temperature increasing magnitude of 4.3. In addition, the experiment cell temperature increases with a constant slope which is different from the results with 1C discharging. Since the heat generated under 1.5C is higher than under 1C according to Eq. (4), it takes shorter time for the battery surface to heat up. Temeraure (C) Cell Temperature Variation under 1.5C Discharging Experiment Elapsed Time (Sec) Fig.3. Cell temperature under 1.5C discharging Fig.4 shows the cell temperature of the battery module under discharging and it can be seen that the temperature of the cells is different from others. Because of the connecting order of the cells inside the module, the cell which is closed to the output joint will have a higher temperature than the other cells due to the maximum current loading. Since the temperature of the cell in the module is different, an arithmetic average of the cell temperature is used to be the representative temperature of the module. Therefore, the simulation result of the module temperature is also an arithmetic average of the cell temperatures. 溫度 ( o C) 經過時間 ( 秒 ) Fig. 4 Cell temperature variation in a module Fig. 5 and 6 show the temperature variation of the module under 1C and 1.5C. The initial module temperature is the environment temperature which the experiment is executed. It can be seen that the module temperature increases 4.6 and 6.4 respectively. The temperature increasing tendency of the simulation is close to the experiment result. Therefore, the model of the battery cell and module is reliable to retrieve the actual condition of the battery pack

4 AVEC Module Temperature Variation under 1C Discharging Experiment Elapsed Time (Sec) Fig. 5 Module temperature under 1C discharging Module Temperature Variation under 1.5C Discharging Experiment Fig. 7 Temperature profile of the battery pack Elapsed Time (sec) Fig. 6 Module temperature under 1.5C discharging 4. result of battery pack heat removal There are totally 400 cells in the ATC electric vehicle battery pack. However, only part of the battery pack is able to be simulated due to the limitation of computer ability. The pack model is built based on the correcting cell model and the number of the grids of the model is 74 million. There are three fans that drag the air into the pack from the top and two fans that draw the air outside the pack. The battery pack model is then used to simulate the temperature variation and the maximum heat that could be took out by the outlet fan. With the initial condition of environment temperature (5 ) and 1.5C discharging current, four temperature profiles with different layer of cells in the pack at the 600 second is shown in Fig. 7. It can be seen that the cells which are close to the fans, whether inlet or outlet, have a lower temperature than other cells. Furthermore, the cells at the higher layer also show a lower temperature than the lower layer. Due to the alignment of the cell in the battery pack, Fig. 7 shows a highly non-uniform temperature distribution. Fig. 8 shows the air flow filed diagram of the pack. It can be seen that the air velocity in the channel which is beneath the fan is higher than other place. In other words, the heat generated in the pack will not be totally removed. Therefore, the maximum temperature in the pack is 34.8 which is 9.8 higher than the environment. The simulation result shows that the heat which could be removed by the outlet fan is 356W. Therefore, we estimates that the heat recovered from the ATC EV battery will be twice of the simulation result, i.e. 71W. Fig. 8 Air velocity distribution inside the battery pack 9. CONCLUSION In this study, a modified heat pump system structure is presented. The heat pump system adopted the additional heat exchanger to recover the waste heat generated by the electric vehicle subsystems. The battery pack is one of the subsystems which the temperature is needed to properly control to maintain in a proper range. Therefore, the waste heat is able to be used for the source of the heating function. The temperature variation and generated heat of the electric vehicle battery during discharging was studied. The models for the battery were coded on the SC/Tetra platform and the Finite Volume Method was used to simulate the behavior of the model. Several experiments were executed with two different discharging depths for temperature of the cell and the module. The results were then used to correct the simulation models. The modified model shows great results about the temperature behavior in compare with the experiments. EFEENCES [1] Chang, T.B., et al., The Investigation of Maximum Cooling Load and Cooling Capability Prediction Methods for the Air-condition System of Vehicle in Taiwan, Journal of Vehicle Engineering, Vol.4, No. 1, 007, pp [] Nakane, S., et al., Air Conditioning System for Electric Vehicle, J. Society of Automotive

5 Engineering of Japan, Vol. 64, No. 4, 010, pp [3] Chang, Z. J., et al., Effect of Internal esistance on Temperature ising of Lithium-Ion Battery, Chinese Journal of Power Sources, 34 (010) AVEC 1