Thermal Management of Densely-packed EV Battery Set

EVS28 KINTEX, Korea, May 3-6, 2015 Thermal Management of Densely-packed EV Battery Set Abstract Z. Lu 1, X.Z. Meng 1, W.Y. Hu 2, L.C. Wei 3, L.Y. Zhang 1, L.W. Jin 1* 1 Building Environment and Equipment Engineering, Xi an Jiaotong University, 20 Xianning West Road, Shaanxi, 710049, China. Email: lwjins@gmail.com 2 Chinese Association of Refrigeration, 67 Fucheng Road, Beijing, 100142, China. 3 Shenzhen Envicool Technology Co., Ltd., 9 Building, Hongxin Industrial Park, Shenzhen, 518129, China. *Corresponding author: lwjin@mail.xjtu.edu.cn The modern development of electric vehicles requires higher power density to be packed into battery box. It is always expected that the battery can be arranged as much as possible, which leads to the thermal management issue due to the heat generation inside the battery packs. It is more serious when the battery system is running at high power modes, such as large current charging/discharging and energy recovery processes, etc. As an extreme temperature affects performance, reliability, safety and lifespan of batteries, thermal management of battery system is critical to the success of all electric vehicles. The working temperature range and temperature uniformity are major factors to maintain battery working at its ideal conditions. In this research, air cooling for a battery box is investigated numerically, of which 252 Li-ion batteries (32650) are packed densely in a space with dimensions of 121(x) 380(y) 462(z) mm. The objective is to explore the air cooling capability on the temperature uniformity and hotspots mitigation subject to various flow paths, heat generation, and air inlet conditions. Different flow paths are designed and the results of the thermal characteristics are compared and analyzed. It shows that a proper designed air cooling system is able to maintain Li-ion batteries at optimal operating temperature and to minimize the hotspot for low and moderate heat generation at 4.375 W m -2 and 8.75 W m -2. However, as for high heat generation at 16.5 W m -2, the temperature of battery packs inside battery box depends mainly on the inlet flow temperature with reasonable flow paths. This implies that the battery thermal condition can be successfully controlled using air cooling method if the inlet flow can be pre-cooled by EVs air-conditioning system or a dedicated mini refrigeration system. Keywords: battery packs, forced air cooling, flow paths, numerical simulation 1 Introduction In response to energy crisis and environmental problems, the pure electric vehicles, with the advantages of low power consumption and zero emissions, have developed rapidly in recent years. As the only power of pure electric vehicles, the battery working conditions directly affect the EVS28 International Electric Vehicle Symposium and Exhibition 1

performance of electric vehicle. Due to high energy density, high voltage and low self discharging and so on, Li-ion batteries are becoming an attractive applications for pure electric vehicles. As for Li-ion battery, the optimal operating temperature is about 20-40 o C [1]. Nowadays, the battery can be arranged as much as possible in battery box to meet required higher power density of electric vehicle, which could result in serious thermal management issue because of heat generation inside the battery box. For instance, high temperatures have the devastating effects on the battery packs that the life of battery packs can be severely shortenedbattery life cut in half for every 10 o C increase. Moreover, battery packs also need to be operated at uniform temperatures because its fluctuations results in the difference of charge/discharge behavior, which lead to electrically unbalanced modules and the reduction of battery packs performance [2]. Therefore, it is important to maintain the battery packs optimum temperature and temperature uniformity for ensuring battery stability and extending battery lifespan through proper thermal management system, which is critical to safe and efficient operation of electric vehicles. Several thermal management systems have been carried out in the last years to keep the battery packs at an optimum temperature with small variations [3-12]. These thermal management methods are mainly divided into air cooling, liquid cooling and phase change cooling manners. At present, compared to other cooling methods, air cooling is a common method because of reliable and simple battery cooling system. In this study, air cooling for a densely-packed battery box is investigated numerically, where 252 Li-ion batteries (32650) are arranged in a space with dimensions of 121(x) 380(y) 462(z) mm. Numerical approach is performed using CFD Fluent code. The objective is to explore the air cooling capability on the temperature uniformity and hotspots mitigation under various flow paths, heat generation, air inlet velocity and temperature in order to provide some specific guidance for thermal characteristic analysis of densely-packed battery packs. Nomenclature U Velocity vector Ø General variables ρ Density of fluid [kg m 3 ] Γ ϕ generalized diffusion coefficient Source term Ѕ ϕ 2 Configuration of Densely- Packed Battery Box The thermal management issue of a denselypacked battery box is studied in this paper, whose dimensions are of 121(x) 380(y) 462(z) mm. In this study, the densely-packed battery box is designed to house 252 Li-ion batteries (32650) arranged into six rows and has five air baffles to fix batteries. Three kinds of flow paths, namely, 15 vents, 17 vents and 59 vents are investigated under various air inlet conditions and heat generation by batteries. The details of the densely-packed battery box are shown in Fig. 1. Figure 1: The schematic diagram of densely-packed battery box configuration with three flow paths EVS28 International Electric Vehicle Symposium and Exhibition 2

3 Numerical Simulation 3.1 Airflow modeling In this study, the air flow and temperature distributions in the densely-packed battery box are numerically simulated using Fluent 6.3.26. The flow is assumed to be steady, threedimensional, incompressible and turbulent. The Boussinesq approximation is used to model the buoyancy effects. Turbulence is resolved using the standard k-ε turbulence model. All the variables (velocities, temperature, turbulent energy and dissipation energy) to be solved are denoted by ϕ. The general transport equation for ϕ can be written as [13]: div U div grad S (1) t where ρ is the density of the fluid, U = (u, v, w) is the velocity vector, Γ ϕ is the generalized diffusion coefficient and Ѕ ϕ is the source term. With properly prescribed Γ ϕ, Ѕ ϕ and ϕ, Equation (1) can be taken as the continuity, momentum, energy or other scalar equations. 3.2 Boundary conditions In this study, the boundary conditions include velocity-inlet, outflow-outlet and no-slip condition at all walls of battery box and battery surfaces. In addition, all walls of battery box are taken as adiabatic wall and the heat fluxes of battery surfaces are set based on different heat generation corresponding to different charge/discharge rates. 3.3 Numerical scheme The grid is generated using the Gambit 2.4.6 preprocessor and the discretization of the computational domain is achieved using an unstructured mesh. The solution method is based on the following main hypothesis: the diffusion terms are second-order central-differenced and the second-order upwind scheme for convective terms is adopted to reduce the numerical diffusion. The coupled velocity-pressure terms are resolved using the SIMPLE algorithm. 4 Results and Discussion 4.1 Grid independency analysis The grid independency analysis is carried out to ensure that the numerical results are not influenced by the cell numbers. We take same grid numbers for the battery box with different flow paths due to similar geometry. Therefore, three kinds of grid numbers of battery box with 15 vents, namely, 4118732 (coarse), 4671297 (regular) and 5203321 (fine) are chosen to investigate grid independency analysis for numerical simulation. Figure 2 presents the trends of temperature variation along z direction at location of x = 74.9 mm and y = 133.6 mm. It is obvious that the temperature differences between the regular mesh and the fine mesh are rather small. Therefore, the regular meshes are used for densely-packed battery box under different flow paths in this article. Figure 2: The trends of temperature variation along z direction at location of x = 74.9 mm and y = 133.6 mm. 4.2 Flow fields and temperature fields analysis for 15 air inlets Figures 3(a) and 3(b) show temperature fields and flow fields of the densely-packed battery box with 15 air inlets under the heat flux of battery surfaces and airflow rate set at 8.75 W m -2 and 22.4 m 3 h -1 respectively. It can be seen that the high temperature area of battery packs is around the center and at the bottom near air outlet; the temperature of battery packs at the top of the box is relatively low due to the effect of air inlet. The maximum temperature of battery packs is 316 K and the maximum temperature differences is 22 K. From a closer view of Fig. 3(b), it is observed that the air velocities are rather small at the bottom of battery box and there are downdrafts at the last two rows of battery packs. These observations are in agreement with the temperature fields shown in Fig. 3(a) that the temperature of battery packs at the bottom of battery box increases along the airflow direction and reaches the maximum; battery surface temperature decreases with the growth of height. Therefore, when the heat flux of battery surfaces is 8.75 W m -2, the air cooling performance of a densely-packed battery box with EVS28 International Electric Vehicle Symposium and Exhibition 3

15 air inlets could not meet the requirements of operating temperature for Li-ion batteries. (a) temperature field Figures 5(a) and 5(b) present the horizontal temperature profiles along z direction (airflow direction) at locations of x = 74.9 mm, y = 133.6 mm and x = 46.2 mm, y = 183.4 mm for these two kinds of flow paths. The positions of selected temperature points are shown in Fig. 6. As expected, the temperatures of selected points (T 1, T 2, T 3, T 4, T 5, T 6) far away from air vents for 17 air inlets with increasing more than 2 K, in comparison with that of 15 air inlets. While, the temperatures of selected points (T 7, T 8, 9, T 10, T 11, T 12) near air vents have similar values for these two kinds of flow paths. Based on the above analysis, it is found that additional two air inlets could reduce the maximum temperature of battery packs at the bottom of battery box. However, the smaller air inlet velocity significantly affects heat dissipation of battery packs from all vents. Figure 3: The temperature field and velocity field for 15 air inlets at 293 K 4.3 Flow fields and temperature fields analysis for 17 air inlets According to the results of 15 air inlets, additional two air inlets are designed to improve air cooling performance at the bottom of battery box. Figure 4(a) shows the temperature contours of the densely-packed battery box with 17 air inlets under same conditions with 15 air inlets. It is observed that the high temperature area of battery packs is near the center and the maximum temperature of battery packs at the bottom of battery box decreases due to the effect of airflow. However, the maximum temperature of battery packs is 318 K which is about 2 K higher than that of 15 air inlets. Combination with air velocity fields shown in Fig. 4(b), it can be seen that the air velocity in the central of battery box is slightly lower than that of 15 air inlets, which may affect heat dissipation of battery packs far away from all air vents. (a) temperature field Figure 4: The temperature field and velocity field for 17 air inlets at 293 K EVS28 International Electric Vehicle Symposium and Exhibition 4

(a) 17 air inlets (b) 15 air inlets Figure 5: The horizontal temperature profiles along z direction at location of x = 74.9 mm, y = 133.6 mm and x = 46.2 mm, y = 183.4 mm (a) 17 air inlets (b) 15 air inlets Figure 6: The schematic diagram of different selected temperature points 4.4 Flow fields and temperature fields analysis for 59 air inlets In order to avoid the smaller air inlet velocity affecting the heat dissipation of battery packs from air vents, 59 air inlets are designed to keep all battery packs close to air vents. Figures 7(a) and 7(b) show temperature fields and flow fields of a densely-packed battery box with 59 air inlets under same conditions with above two flow paths. As expected, the maximum temperature of the battery packs is 310 K, which occurs at the central of cells. It indicates that this flow path can significantly reduce the maximum temperature of the battery packs higher than 6 K and 8 K, in comparison with the maximum temperature of 316 K for 15 air inlets and 318 K for 17 air inlets. Therefore, when the heat flux of battery surfaces is 8.75 W m -2, this flow path can improve the heat dissipation performance of forced air cooling system. From Fig. 7(b), it can be seen that although the air velocity is smaller than that of above two flow paths, the airflow around battery packs is relatively uniform that can effectively avoid the heat dissipation problem of battery packs from air vents. From Fig. 8, it can be observed that the temperature profile of selected points (T 1, T 2, T 3, T 4, T 5, T 6) is similar to those of T 7, T 8, T 9, T 10, T 11, T 12, which is different from above two flow paths, i.e., 15 and 17 air inlets. This observation is in agreement with uniform flow fields around battery packs. The positions of selected temperature points are shown in Fig. 9. EVS28 International Electric Vehicle Symposium and Exhibition 5

(a) temperature field Figure 7: The temperature field and velocity field for 59 air inlets at 293 K Figure 9: The schematic diagram of different selected temperature points with 59 air inlets 4.5 Flow fields and temperature fields analysis for 59 air inlets at different heat generation by battery packs Figure 10 shows the temperature fields and flow fields of a densely-packed battery box with 59 air inlets under air inlet temperature and airflow rate set at 293 K and 22.4 m 3 h -1 respectively. It can be seen that the maximum temperature and maximum temperature difference of battery packs are 301.5 K and 8 K respectively. Table 1 compares the thermal characteristics of battery packs for 59 air vents at different heat fluxes of battery surfaces. Obviously, the maximum temperature and maximum temperature difference increase along with increasing the heat flux of battery surfaces. Compared with heat flux of battery surfaces set at 16.5 W m -2, the heat dissipation performance of battery packs could meet the operating temperature requirement of Li-ion battery when heat flux of battery surfaces is 4.375 W m -2 or 8.75 W m -2, and the air inlet temperature is 293 K. Therefore, as for low and moderate heat generation, a proper designed air cooling system is able to maintain Li-ion batteries at operating temperature and to minimize the hotspot. However, as for high heat generation, the lower air inlet temperature or the higher air inlet velocity is needed to improve forced air cooling performance of a denselypacked battery box with reasonable flow paths. Figure 8: The horizontal temperature profiles along z direction at location of x = 74.9 mm, y = 133.6 mm and x = 46.2 mm, y = 183.4 mm with 59 air inlets EVS28 International Electric Vehicle Symposium and Exhibition 6

Therefore, as for low heat generation, the higher air inlet temperature could satisfy the requirements of Li-ion batteries operating temperature. Table 2 shows the comparison of thermal characteristics of battery packs for 59 air vents at different air inlet temperatures. As expected, the maximum temperature rises gradually with the increase of air inlet temperatures. While, the maximum temperature difference does not change. It indicates that air inlet temperatures have little effect on temperature uniformity (a) temperature field Figure 10: The temperature field and velocity field for 59 air inlets at 4.375 W m -2 (a) temperature field Table 1: The comparisons of thermal characteristics of 59 air vents at different heat generation Heat flux of battery surfaces (W m -2 ) 4.375 8.75 16.5 Airflow rate (m 3 h -1 ) 22.4 22.4 22.4 The maximum temperature (K) 302 310 324 The maximum temperature Difference (K) 9 16 30 4.6 Flow fields and temperature fields analysis for 59 air inlets at different air inlet temperatures Figures 11(a) and 11(b) show temperature fields and flow fields of a densely-packed battery box with 59 air inlets under the heat flux of battery surfaces and airflow rate set at 4.375 W m -2 and 22.4 m 3 h -1 respectively. It can be seen that the maximum temperature and maximum temperature difference of battery packs are 311.5 K and 8 K respectively. Figure 11: The temperature field and velocity field for 59 air inlets at 303 K Table 2: The comparison of thermal characteristics of 59 air vents at different air inlet temperatures Air inlet temperature(k) 293 298 303 Airflow rate(m 3 h -1 ) 22.4 22.4 22.4 The maximum temperature(k) 301.5 306.5 311.5 The maximum temperature difference(k) 8 8 8 EVS28 International Electric Vehicle Symposium and Exhibition 7

Conclusions The forced air cooling of a densely-packed battery box is investigated by numerical simulation to explore the air cooling capability on the temperature uniformity and hotspots mitigation under various flow paths, heat generation and air inlet conditions. Based on the above research, the following conclusions may be drawn: (i) As for low and moderate heat generation, a densely-packed battery box with a proper designed air cooling system is able to maintain Li-ion batteries at optimal operating temperature within the range of 293 K - 313 K; while, as for high heat generation, the lower air inlet temperature or the higher air inlet velocity is needed to improve forced air cooling performance of a densely-packed battery box with reasonable flow paths; (ii) Despite of the lower air inlet velocity, the forced air cooling performance of a denselypacked battery box with 59 air inlets is stronger than the other two kinds of flow paths discussed in this article, which could provide uniform temperature fields and mitigate hotspots. It indicates that effective heat transfer area may have a more significant effect on forced air cooling performance than air velocity; (iii) The proper flow path that makes air as cooling medium contacted effectively with battery surfaces is critical to forced air cooling performance for a densely-packed battery box (iv) The air inlet temperature is not critical for maintaining the temperature uniformity; as for low heat generation, the moderate air inlet temperature should be chosen to cool denselypacked battery box. temperature uniformity, Journal of Power Sources, 196(13)(2011), 5685-5696. [4] A. Jarrett., et al., Design optimization of electric vehicle battery cooling plates for thermal performance, Journal of Power Sources, 196(23)(2011), 10359-10368. [5] D. Linden, Handbook of batteries and fuel cells, New York. 1984. [6] A. S. Keller., et al., Thermal characteristics of electric vehicle batteries, Self, 2013, 08-27. [7] C. Alaoui, et al., A novel thermal management for electric and hybrid vehicles, Vehicular Technology, IEEE Transactions on, 54(2)(2005), 468-476 [8] A. A. Pesaran., Battery thermal management in EV and HEVs: issues and solutions, Bttery Man,43(5)(2001), 34-49. [9] S. Al-Hallaj., et al., Thermal modeling of secondary lithium batteries for electric vehicle/hybrid electric vehicle applications, Journal of Power Sources, 110(2), 341-348. [10] S. A. Khateeb., et al., Thermal management of Liion battery with phase change material for electric scooters: experimental validation, Journal of Power Sources, 142(1)(2005), 345-353. [11] R. Sabbah., et al., 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, 182(2008), 630-638. [12] R. Kiziel., et al., Passive control of temperature excursion and uniformity in high-energy Li-ion battery packs at high current and ambient temperature, Journal of Power Sources, 183(2008), 370-375. [13] W. Q. Tao., Numerical heat transfer, Xi an Jiaotong University Press, Xi'an, 430-447, 2001. Acknowledgments You may list acknowledgments here if appropriate. References [1] H. Teng, et al., An analysis of a lithium-ion battery system with indirect air cooling and warm-up, SAE Tech. Pap. 4(3)(2011), 15. [2] A. A. Pesaran, et al., Thermal performance of EV and HEV battery modules and packs, National Renewable Energy Laboratory, 1997. [3] R. Mahamud, et al., Reciprocating air flow for Liion battery thermal management to improve Authors Mr. Z. Lu is currently a Master Student studying in the Department of Building Environment and Equipment Engineering of Xi an Jiaotong University. His research is related to the battery thermal management. Dr. X.Z. Meng is a Senior Engineer working at the School of Human Settles and Civil Engineering of Xi an Jiaotong University. His research includes both numerical and experimental heat transfer. EVS28 International Electric Vehicle Symposium and Exhibition 8

Mr. W.Y. Hu obtained his Master Degree from the School of Power and Energy Engineering of Xi an Jiaotong University. He is the Deputy General Secretary of Chinese Association of Refrigeration. He is Mr. L.C. Wei obtained his Master Degree from the School of Power and Energy Engineering of Xi an Jiaotong University. He is the Chief Engineer of Shenzhen Envicool Technology Co. Ltd. in charging of the development of cooling system of EV and HEV. Dr. L.Y. Zhang is an Associate Professor working the Department of Building Environment and Equipment Engineering of Xi an Jiaotong University. Her research interests include refrigeration system and building environment. Dr. L.W. Jin obtained his Ph.D. Degree from Nanyang Technological University, Singapore. He is now a Professor at the Department of Building Environment and Equipment Engineering of Xi an Jiaotong University. His research focuses on the thermal management of energy equipment. EVS28 International Electric Vehicle Symposium and Exhibition 9