Design and Simulation of Liquid-cooling Plates for Thermal Management of EV Batteries

EVS28 KINTEX, Korea, May 3-6, 2015 Design and Simulation of Liquid-cooling Plates for Thermal Management of EV Batteries Dai Haifeng 1,2, Sun Zechang 1, 2, Wei Xuezhe 1,2,Yang Shuqiang 2 1 Clean Automotive Engineering Center, Tongji University, Shanghai, tongjidai@gmail.com 2 School of Automotive Studies, Tongji University, Shanghai Abstract To enhance the performance and extend the life time of EV batteries, the working temperature of battery should be controlled in an optimized range. This object is usually implemented with a thermal management system. This paper makes an investigation of the state-of-art thermal management technologies for EV batteries. Generally the liquid cooling with an elaborated design has a good performance especially for those applications with high power demand and under arduous temperature conditions. Based on the analysis of basic fluid principles, this paper makes a detailed study on the design and optimization of the liquid-cooling plates. Four different types of cooling plates with different fluid channels (S-type and linear type) are designed to facilitate the study. The comparison and analysis of different structures of cooling plate is made with the help of the CFD. The numerical results from all the with the commercial software ANSYS FLUENT is given and discussed. The battery heat generation is approximated by only the joule and reversible (entropic) losses in battery cells. Results show that, with a good design, the cooling plate can achieve both reasonable temperature range and distribution. However, a lot of extra attentions should be given to reduce pressure loss and avoid eddy zones. Keywords: liquid-cooling plates, thermal management, design and, EV battery 1 Introduction in 14pt bold As one of the most critical parts of electric vehicles (EVs), traction batteries have drawn more and more attention from all over the industry. The performance and safety of the vehicle is highly dependent on battery systems. In real applications, battery temperature influences the availability of discharge power, energy life extension and safety. Therefore, batteries should operate within an ideal temperature range that is optimum for performance and service life. Generally a thermal management system (TMS) is used to meet that demand, and the main objectives of the TMS include (1) to limit the absolute temperature of battery cells below the allowed maximum value and (2) to make the temperature distribution among battery cells even. Heat source of the battery pack includes two parts, the first one is the heat generated by the battery itself during charging and discharge and the second on is the environmental heat power conducted or radiated into the battery pack [1, 2]. No matter where the heat comes from, the thermal management should guarantee that the temperature of the battery pack is in a reasonable range. In addition to considering the absolute temperature of a battery pack, uneven temperature distribution in EVS28 International Electric Vehicle Symposium and Exhibition 1

a pack should also be carefully considered, because the temperature imbalance among the cells could lead to different characteristics of each cell. Recent years, there have been a lot of researches on the thermal management of battery pack [3-5]. Generally, traditional TMS can be realized with air cooling, liquid cooling, heat pipe, phase change material (PCM). With a good design, the air cooling system can performs well in some applications, such as those with low power demand. However, there are some disadvantages of the air cooling solution. First, when the battery works at a high temperature or with large charge/discharge current, air cooling will not be very effective, and the non-uniform distribution of the temperature in a pack becomes inevitable. Second, the ratio of space utilization of the aircooled battery pack is very low, which often leads to the difficulties of the layout design of the whole vehicle. Third, it is hard to get a high level of ingress protection for the air-cooled battery packs. Pure PCMs nowadays are still not very suitable for the cooling system design because of low thermal conductivity although they generally have large heat storage capacity. To enhance the thermal conductivity of PCMs, a lot of efforts have to be done, which makes the system too complicated [6-8]. The liquid cooling design is now getting more and more attention because it good performance, especially for those applications with high power demand and under arduous temperature conditions. To use liquid as the heat medium is more effective than using air. For the same flow rate, the heat transferring rate for most applications is higher than air [9-12]. Although liquid cooling is promising in EV applications, the cooling system may not perform as expected due to inappropriate design. Based on the analysis of basic fluid principles, this paper makes a detailed study on the design and optimization of the liquid-cooling plates. Four different types of cooling plates are designed to facilitate the study. The comparison and analysis of different structures of cooling plate is made with the help of the CFD. The numerical results from all the with the commercial software ANSYS FLUENT is given and discussed. The battery heat generation is approximated by only the joule and reversible (entropic) losses in battery cells. 2 Structure design and CFD model 2.1 CFD model The cooling plate is designed to cool down three battery modules. The battery module is composed of 10 battery cells connected in series, each cell having a nominal capacity of 40 Ah. Heat generated by each battery cell is 28 W, which is considered to be the harshest condition. That means that if the cooling plate works well in this situation, it will be suitable for all working condition in vehicular applications. The battery cell is prismatic and with a hard case. The size of each cell is 100mm 32mm 180mm (length thickness height). The basic parameters of battery cells for CFD is shown in Table 1. Table 1: Key thermal parameters of battery cells for CFD Value Heat conductivity (length direction, 4.323 W/mK) Heat conductivity (thickness 0.867 direction, W/mK) Heat conductivity (length direction, 5.02 W/mK) Heat capacity (kj/k) 1115 Table 2: Detailed parameters of the cooling plate Value Cooling plate material aluminum Heat conductivity of cooling 202 plate (W/mK) Specific heat capacity of 871 cooling plate (J/kgK) Density (kg/m 3 ) 2719 Cooling liquid water Viscosity (kg/ms) 0.001003 Heat conductivity (W/mK) 0.6 Specific heat capacity (J/kgK) 4182 Density (kg/m 3 ) 998.2 Battery Module number 3 Plate size (mm mm mm) 450 380 10 Radius of fluid inlet and outlet 7 (mm) Power of heat source (W) 864 The cooling plate takes the heat from battery cell away from the bottom of the battery cells, as shown in Fig.1, and the size of the cooling plate is 450mm 380mm 10mm. Aluminum is selected EVS28 International Electric Vehicle Symposium and Exhibition 2

as the material of the cooling plate, and to simplify the and analysis, water is selected as the heat medium. Detailed parameters of the cooling plate are listed in Table 2. To make a detailed analysis of the cooling effect, four structures are designed and simulated, as shown in Fig.1. In Fig.1, 1, 2 and 3 are the contact interface of the battery module and cooling plate, which means the heat source will be located at those places. 4 is the fluid channel in the cooling plate and 5 is the division fin among fluid channels. The height and width of the fluid channel of structure 1 and structure 2 are both 7 mm 7 mm, and the thickness of division fin is 3.5 mm. There are totally 26 fluid channels in these plates. The height and width of the fluid channel of structure 3 and structure 4 are both 6.5 mm 7 mm, and the thickness of division fin is 3 mm. There are totally 30 fluid channels in these plates. The inlets and outlets of the four plates are all round, with a radius of 7 mm. (c) design-3 (d) design-4 Fig. 1: Structure designs of four cooling plates (a) design-1 (b) design-2 2.2 CFD The CFD model is export into ANSYS to generate the fluid domain and the mesh. The mesh types are tetrahedron and pyramid, and around 1.6 million meshes are generated. The meshed model is then imported into Fluent to implement the. Some assumptions are made, (1) the radiation effect is neglected, (2) the heat exchange between cooling plate and environment is ignored, (3) distribution of liquid velocity in inlet is considered to be even, (4) the derivatives of all variables are considered to be zero on the plane of outlet, and (5) the water is considered to be incompressible. Since the numerical calculation of the liquid cooling system is essentially a coupled problem of heat exchange with liquid fluid and heat conductivity in solid, the governing equations are generally the N-S equations of three dimensional, steady state and incompressible flow of liquid. Computational domain includes the cooling plate and liquid inside the plate. The model is k turbulence model, k ( kui) ui k [( u ) ] Gk (1) t x x x i j k j EVS28 International Electric Vehicle Symposium and Exhibition 3

( ki ) t xi 2 u C [( ) ] x x k k j i 1 u Gk C2 j (2) where k is turbulence energy, is turbulence dissipation rate, is the liquid density and u the velocity of the fluid, C1 =1.44, C2 =1.44, C u =0.09, k =1.0, =1.3. During computation, the SIMPLE algorithm is used with high Reynolds equation and wall function. 2.3 Model verification Before analyzing the results of each design, we should guarantee that the model and computation could be converged. The termination of a CFD is controlled by specifying limits on the calculation residuals. In that case, the would stop once the objective functions reach a steady converged value. We make a convergence test of the models, and find that, when the residual error of velocity functions and energy functions are less than 10-5 and 10-7 respectively, the model can be considered to be converged. (a) Temperature change of cooling liquid (b) Temperature change of cooling plate Fig.2: Calculated temperature changes of cooling liquid and cooling plate Take the structure 4 as an example, we set the initial temperature and the environmental temperature are all 300 K, and the velocity at inlet is 4 m/s, when the is converged, the average temperature of the cooling plate is 300.35 K, and average temperature of water is 300.1078 K, the iteration numbers is 8900. The result is shown in Fig.2. 3 Results and discussions The initial temperature of the battery cells are set 300 K and the velocity of the fluid at inlet is 1 m/s. The results of the designs are shown in Fig.3 and Fig.4. Fig.3 gives out the temperature distribution on the cooling plate and Fig.4 illustrates the distribution of fluid velocity inside the cooling plate. We can see a clear difference among the design although the conditions are all same. In the first structure design, the highest temperature of the cooling plate is 305.755 K and located at the center. The mean temperature of the cooling plate and water flow are 302.465 K and 301.233 K respectively. Thus, the average temperature difference between cooling plate and water flow is 1.232 K. The uneven temperature distribution is mainly caused by the uneven fluid velocity distribution, as shown in Fig.3 (a). The average fluid velocity is 0.22 m/s, however, the velocity of the fluid in the middle channels is less than 0.1 m/s. The heat exchange rate decreases with the low fluid velocity, which in turn leads to the temperature rises. This is one of the common drawbacks of the linear type cooling plate design. The pressure loss of the fluid through the cooling plate is 6.1 kpa. Result shown in Fig.3 (b) indicates that, the highest temperature of the second design is located at the two sides of the plate, nearby the outlet. The highest temperature is 305.123 K, while the average temperature of the cooling plate and cooling liquid are 303.08 K and 300.80 K respectively. The temperature difference between the cooling plate and liquid is 2.28 K. From Fig.4(b), we can find that the fluid velocity of the middle channels is larger than that of the side channels. The average velocity is 0.17 m/s, while the velocity of the middle channel is 0.43 m/s. The velocity decrease from the middle channel to the side channels, and the velocity of the outmost channel is about 0.08 m/s. Thus, it is obviously to see the velocity difference among the channels, which in turn makes an uneven temperature distribution. The pressure loss is about 3.3 kpa in this case. The third and fourth structures are similar with the S-type flow channels, and actually the fourth type is an optimization of the third type. Comparing with the first and second designs, the third and fourth ones performs better. EVS28 International Electric Vehicle Symposium and Exhibition 4

(a) Temperature distribution of design-1 (b) Temperature distribution of design-2 (c) Temperature distribution of design-3 (d) Temperature distribution of design-4 Fig.3: Temperature distribution of the four types of cooling plates Fig.3(c) shows the temperature distribution of the third design. The temperature increases in the direction of water flow, and the highest temperature is in the last module the water goes through, which is 302.58 K. The average temperature of the cooling plate is 301.604 K, and the average temperature of the cooling liquid is 300.80 K. Thus the temperature difference between cooling plate and cooling liquid is 0.804 K. The result shown in Fig.3(c) also indicates a better temperature distribution than the first and second designs. The velocity distribution shown in Fig.4(c) illustrates that the velocity distribution in the first module is very good, and the velocity of two channels in the second module is not as good as expected, and even worse, in the third module, the velocity of the third module is unfavorable. The unfavorable fluid velocity in those channels is caused by the angle of fluid channel in the cooling plate, which is a little bit large. The liquid velocity of this design is 0.83 m/s and the pressure loss is 10.8 kpa. Fig.3(d) shows the temperature distribution of the fourth design. And with a similar regular as the third design, the temperature increases in the direction of water flow. The highest temperature point is located at the outlet of the cooling plate, which is 301.895 K. The average temperature of the cooling plate and the liquid are 300.978 K and 300.374 K respectively, which makes the temperature difference between plate the liquid to be 0.604 K. Compared with the third design, this design improves the velocity distribution inside the cooling plate. The fluid velocity is 0.69 m/s and the pressure loss is 9.5 kpa. The improvement is caused by the optimization of the angle of fluid channel in the cooling plate. The round corner is used to reduce the fluid resistance. No matter what kind of design is used, the pressure loss at the corner of the fluid channel is larger than other points. There are usually eddy zones at those corners, which forbid the fluid taking away any heat. Thus, in the cooling plate design, a lot of efforts should be made to avoid those corners, and to optimize the water flow in the cooling plate. The key parameters of the four cooling plates can be calculated by FLUENT with a post-processing after the process. The parameters include fluid-solid contact area, the highest temperature, the average temperature, the average velocity and the pressure at inlet and outlet etc., and the results are all listed in Table 3- Table 6. EVS28 International Electric Vehicle Symposium and Exhibition 5

Table 3: Key thermal parameters of the cooling plate- 1 calculated by FLUENT with a post processing after Solution-1 Fluid-solid contact area(m 2 ) 0.288515 Heat flux of fluid-solid coupling 2994.65 Highest temperature on plate (K) 305.755 Average temperature on plate (K) 302.465 Average temperature of water(k) 301.233 Average plate-liquid temperature 1.232 Average fluid velocity (m/s) 0.22 Pressure loss (kpa) 6.1 Table 4: Key thermal parameters of the cooling plate- 2 calculated by FLUENT with a post processing after Solution-2 Fluid-solid contact area(m 2 ) 0.28715 Heat flux of fluid-solid coupling 3008.88 Highest temperature on plate (K) 305.123 Average temperature on plate (K) 303.08 Average temperature of water(k) 300.80 Average plate-liquid temperature 2.28 Average fluid velocity (m/s) 0.17 Pressure loss (kpa) 3.3 Table 5: Key thermal parameters of the cooling plate- 3 calculated by FLUENT with a post processing after Solution-3 Fluid-solid contact area(m 2 ) 0.33103 Heat flux of fluid-solid coupling 2610.04 Highest temperature on plate (K) 302.58 Average temperature on plate (K) 301.604 Average temperature of water(k) 300.80 Average plate-liquid temperature 0.804 Average fluid velocity (m/s) 0.73 Pressure loss (kpa) 10.8 Table 6: Key thermal parameters of the cooling plate- 4 calculated by FLUENT with a post processing after Solution-4 Fluid-solid contact area(m 2 ) 0.339405 Heat flux of fluid-solid coupling 2545.63 Highest temperature on plate (K) 301.895 Average temperature on plate (K) 300.978 Average temperature of water(k) 300.374 Average plate-liquid temperature 0.604 Average fluid velocity (m/s) 0.69 Pressure loss (kpa) 9.5 4 Conclusions Liquid cooling is a very good option for the thermal management design of traction batteries in EV applications, especially for those working with high power demand or at high environment temperature. A good design of the liquid cooling plate would improve the performance of the cooling system. This paper discussed the detailed design, CFD and analysis four elaborated liquid cooling plates. Simulation results show that, the cooling plates with the S-type fluid channels outperforms those with linear-type fluid channels in temperature range and distribution control. However, the corresponding pressure loss of the S-typed plates is larger than the linear-typed plates. Moreover, there might be some eddy zones in the S-typed plates if there is not an impropriate design of the fluid channels. Thus, when designing a liquid cooling system with cooling plates, a lot of attentions should be given to the reduce pressure loss and avoid eddy zones. Acknowledgments This work is financially supported by the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) under Grant No. 20100072120026 and Shanghai science and technology committee under grant no 12dz1202202. References [1] Williford RE, Viswanathan VV, Zhang JG. Effects of entropy changes in anodes and cathodes on the thermal behaviour of lithium ion batteries. Journal of Power Sources. 2009. 189(1): 101-107 [2] Saito Y. Thermal behaviours of lithium-ion batteries during high-rate pulse cycling. Journal of Power Sources. 2005. 146(1-2): 770-774 [3] Zhonghao Rao, Shuangfeng Wang. A review of power battery thermal energy management. Renewable and Sustainable Energy Reviews. 2011. 15(9) : 4554-4571 EVS28 International Electric Vehicle Symposium and Exhibition 6

[4] Todd M.Bandhauer, Srinivas Garimella, Thomas F.Fuller. A critical review of thermal issues in lithium-ion batteries. Journal of The Electrochemical Society. 2011. 158(3) : R1 R25 [5] Wu MS, Liu KH, Wang YY, Wan CC. Heat dissipation design for lithium-ion batteries. Journal of Power Sources. 2002;109(1):160 166 [6] Sabbah R, Kizilel R, Selman JR, Al-Hallaj S. Active (air-cooled) vs. passive (phase change material) thermal management of high power lithium-ion packs: limitation of temperature rise and uniformity of temperature distribution. Journal of Power Sources. 2008. 182(2): 630-638 [7] Khateeb SA, Farid MM, Selman JR, Al-Hallaj S. Design and of a lithium-ion battery with phase change material thermal management system for an electric scooter. Journal of Power Sources. 2004. 128: 292-307 University, Shanghai, China in 1994. He is now with the Clean Energy Automotive Engineering Centre, Tongji University. His research interests include electric vehicle powertrain systems and electro-hydraulic braking system. Wei Xuezhe received his B.S. degree and M.S. degree in electrical engineering and Ph.D. degree in vehicular engineering from Tongji University, Shanghai, China, in 1994, 1997 and 2005 respectively. He is now with the Clean Energy Automotive Engineering Centre, Tongji University. His research interests include energy storage system design and optimization, battery modelling and state estimation. Yang Shuqiang received his B.S. degree in Mechanical Engineering in 2011 and M.S. degree in vehicular engineering in 2014 from Southwest Jiaotong University and Tongji University respectively. He is now with Shanghai Volkswagen. [8] Alrashdan A, Mayyas AT, Al-Hallaj S. Thermomechanical behaviors of the expanded graphitephase change material matrix used for thermal management of Li-ion battery packs. Journal of Materials Processing Technology 2010; 210 (1): 174-9 [9] Josh Payne, Mark Niedzwiecki, Satish Ketkar, Igor Isayev. Thermal characterization & management of PHEV battery packs. SAE paper: 2009-01-3069 [10] Anthony Jarrett, II Yong Kim. Design optimization of electric vehicle battery cooling plate for thermal performance. Journal of Power Sources. 2011. 196(23): 10359-10368 [11] Ho Teng, Kim Yeow. Design of direct and indirect liquid cooling systems for high-capacity, high-power lithium-ion battery packs. SAE paper: 2012-01-2017 [12] Kim Yeow, Ho Teng, Marina Thelliez, Eugene Tan. Thermal analysis of a li-ion battery system with indirect liquid cooling using finite element analysis approach. SAE paper: 2012-01-0331 Authors Dai Haifeng received his B.S. degree in mechanical engineering and Ph.D. degree in vehicular engineering from Tongji University, Shanghai, China, in 2003 and 2008 respectively. From 2008, he has been with the Clean Energy Automotive Engineering Centre, Tongji University. His research interests include energy storage system design and optimization, battery modelling and state estimation. Sun Zechang received his B.S. and M.S. degree in automatic control from Harbin Institute of Technology, Harbin, China, in 1976 and 1981 respectively, and received his Ph.D. in electrical engineering from Tongji EVS28 International Electric Vehicle Symposium and Exhibition 7