Sui Zengguang1, Lin Haosheng1, Sun Qin2, Dong Kaijun2, Geng Man3, Wu Wei1
1. School of Energy and Environment, City University of Hong Kong, Hong Kong 999077
2. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
3. Guangzhou Gaolan Innovation Technology Co., Ltd., Guangzhou 510705
† 通信作者:吴 伟,E-mail:[email protected]
About author:SUI Zengguang (1990-), male, Ph.D. candidate, mainly engaged in the theoretical research of heat and mass transfer in heat and mass exchangers. Lin Haosheng (1996-), male, Ph.D. candidate, mainly engaged in energy storage technology, development and application of adsorption materials, heat and mass transfer theory research. Wu Wei (1988-), male, Ph.D., associate professor, mainly engaged in research on high-efficiency heat pumps, new working fluids, high-density energy storage, thermal management, solar thermal utilization, and zero-energy buildings. # The author has an equal contribution to the paper
Received: 2023-11-07 Revised: 2023-12-01
Funds: International Science and Technology Cooperation Project of Guangzhou Development District(2021GH07)
summary
As the power source of electric vehicles, the performance of the battery pack determines the safety and life of the electric vehicle, and an effective thermal management system plays a vital role in the safe operation of the battery pack. On the basis of the theory of numerical heat transfer, a thermal-flow-electric model of the battery pack liquid cooling system was established, and the flow field and temperature field distribution of the battery pack liquid cold plate under the working conditions of 0.5 C and 1.0 C were comprehensively analyzed. The results show that there is obvious flow resistance at the inlet and outlet, and the pressure difference between the inlet and outlet of the liquid cold plate is as high as 11.82 kPa, which leads to a significant increase in pump consumption. The temperature of the liquid cold plate showed obvious non-uniformity, the discharge rate increased from 0.5 C to 1.0 C, and the temperature inhomogeneity increased from 3.16 β °C to 5.57 β °C. At the same time, the temperature change of the battery pack under transient operating conditions is also taken into account. This study can provide a reference for the design and optimization of the thermal management system of battery packs.
Key words: battery pack; liquid-cooled radiators; numerical simulation; Thermal management
中图分类号:TK0; TB6 文献标识码:A 文章编号:2095-560X（2023）06-0499-07
Thermal Management Analysis of Lithium Battery Pack
SUI Zengguang1, LIN Haosheng1, SUN Qin2, DONG Kaijun2, GENG Man3, WU Wei1,†
1. School of Energy and Environment, City University of Hong Kong, Hong Kong 999077, China
2. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
3. Guangzhou Goaland Innovation Technologies Co. Ltd., Guangzhou 510705, China
Abstract
Battery pack is the power source of electric vehicles, and its performance determines the safety and lifespan of the electric vehicles. Effective thermal management systems play a vital role in the safe operation of battery packs. Based on the theory of numerical heat transfer, this work develops a thermal-flow-electrical coupling model of a battery pack liquid cooling system. It comprehensively analyzes the flow and temperature field distributions of the battery pack liquid cooling plate under 0.5 C and 1.0 C operating conditions. Results show obvious flow resistance at the inlet and outlet, causing the pressure difference between the inlet and outlet of the liquid cooling plate to be as high as 11.82 kPa, which significantly increases pump power.
The temperature distribution of the liquid cooling plate shows obvious non-uniformity, and the non-uniformity increases from 3.16 °C to 5.57 °C under steady-state operating conditions with the discharge rate increasing from 0.5 C to 1.0 C. In addition, this work also considers the temperature variation of the battery pack under transient conditions. This study can provide a reference for the optimization design of battery pack thermal management systems.
Key words: battery pack; liquid cooling radiator; numerical simulation; thermal management
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0 Introduction
Under the background of the mainland's low-carbon development of "carbon peak by 2030 and carbon neutrality by 2060", new energy technologies have been rapidly developed. Among them, the development of new energy electric vehicles effectively solves the energy and environmental problems caused by traditional vehicles. As one of the core components of electric vehicles, the performance of power batteries largely depends on the operating temperature. Excessive temperature will reduce the performance of this type of battery, and in severe cases, it will lead to extreme accidents such as thermal runaway and explosion. Therefore, how to ensure that the battery operates within a safe temperature range is an urgent problem to be solved [1].
At present, different thermal management strategies for battery packs have been developed rapidly. Among them, liquid cooling technology is favored by the industry because of its excellent cooling performance. Studies have shown that the design and deployment of liquid cold plates have a significant impact on cooling efficiency [2, 3]. With the help of the continuous development of numerical simulation tools, scholars at home and abroad have carried out a lot of research work on the optimal design of cooling plates [4]. The results show that the temperature inhomogeneity has a significant impact on the safe operation and life of the battery pack, and this inhomogeneity increases significantly when the battery works at higher current. At present, the research on liquid cooling system mainly focuses on channel geometric parameters, cooling plate structure, fluid flow distribution, etc. Recently, some advanced structural designs, such as leafy channels, microchannels, and serpentine channels, have also been proposed, providing new directions for the study of battery heat dissipation [5, 6, 7]. However, there are still limitations to the fluid-thermal-electrical coupling between liquid cooling and battery packs [8].
In this paper, numerical simulation tools are used to comprehensively evaluate the performance of the liquid cooling heat sink of a battery pack for a pure electric vehicle. Firstly, the three-dimensional steady-state model of the battery pack and the liquid cold plate was established by using Fluent software, and the flow and temperature field distributions of the liquid cold plate under the conditions of 0.5 C and 1.0 C were analyzed, and the temperature distribution inside the battery pack was discussed. Secondly, the thermochemical model of the battery pack was established by COMSOL software to explore the influence of the discharge process on the temperature field. This study can provide a reference for the optimal design of the heat dissipation structure of the battery pack.
1 Numerical research
1.1 Design Requirements
As shown in Figure 1, the battery pack studied mainly consists of a liquid cold plate and two battery modules. Each battery module consists of 26 cells, which are bonded with thermally conductive silicone to alleviate contact thermal resistance. The battery modules are placed on cooling plates and bonded with thermally conductive silicone. According to the structural dimensions and design parameters given in Table 1, the steady-state and transient numerical models of the battery pack are established.
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 | 图1 液冷散热电池包3D结构Fig. 1 3D structure of liquid-cooled heat dissipation battery pack |
 | 表1 结构尺寸及设计参数Table 1 Structural dimensions and design parameters |
1.2 Material Settings and Boundary Conditions
Taking the battery pack at 0.5 C and 1.0 C as an example, the thermal management capability of the liquid cold plate was evaluated. Steady-state analysis is performed for a given cell heating power. Fluent software based on the finite volume method has been shown to be able to accurately simulate such problems. According to the design parameters given in Table 1, the coolant is 50% glycol + 50% water, and the liquid cold plate inlet is given a flow rate and temperature of 5 L/min and 18β °C, respectively. The flow of coolant in the liquid cold plate channel is a forced single relative flow, and its related governing equations (continuity equation, momentum equation, energy equation) can be referred to the Fluent theory manual. The outlet is set to the pressure outlet boundary. In order to facilitate the study, the outer wall of the whole battery pack is set as a convective heat exchange boundary, the liquid cold plate and the upper cover plate are made of aluminum, and the upper wall and side wall of the battery module are set as thermal insulation boundaries. In the steady-state simulation, it is assumed that the heating power of the cell has a linear relationship with the discharge rate, that is, the heating power of the cell corresponding to 0.5 C is 12.5 W, and the value corresponding to 1.0 C is 25 W, and other boundary conditions remain unchanged. Detailed boundary conditions are shown in Table 2.
| | 表2 模型的边界条件Table 2 Boundary conditions of the model |
In the process of battery system design and development, the study of cell performance is particularly important. Among them, the real-time heat generation calculation of battery cells is a very important part of battery thermal management design. Therefore, in addition to the steady-state calculations based on the given heating power, the electrochemical-thermal-flow coupling model of the battery was established using COMSOL Multiphysics according to the working principle of the battery pack and the thermal effect principle.
In the model, the one-dimensional isothermal model of the battery module is used to calculate the heat production, and the influence of different magnifications on the temperature field distribution of the battery pack is quantitatively analyzed. According to Ref. [9], the boundary conditions for the electrochemical part were set as follows: the electrolyte consisted of ethylene carbonate:dimethyl carbonate solvent with a volume ratio of 1∶2, 2 mol/L LiPF6, and vinylidene fluoride-hexafluoropropylene copolymer [p(VDF-HFP)]. The anode material is graphite, and the cathode material is LiFePO4. The thickness of the electrolyte, cathode, and cathode is set to 52, 100, 174 μ m according to the case library; The active area of the electrode is 16 m2. Conservation of charge based on Ohm's law is used to calculate the potential in the conductive phase of electrons, where the charge transfer reaction is used as the source term or sink term. For the electrolyte phase of the porous electrode, the effective conductivity of the material was analyzed using the conductivity of σ m and the porosity ε of the material σ m, EFF, with the following expression [10]:
σm,eff=σmεγm,eff=m (1)
where γ is the Bruggeman coefficient, and the value is 3.3; The porosity ε value is 0.4. The diffusion coefficient is treated in a similar way. The rest of the parameter settings are based on the default settings of the COMSOL Material Library. In the thermal model, the temperature is set as the average temperature of the active battery material by non-local integral coupling. In order to ensure the accuracy and stability of the numerical simulation, the initial state of charge of the battery was set to 10%, that is, 1.0 C was set to 250 A. The square wave function is used to set the charge/discharge current at a discharge rate of 0.5 C/1.0 C, and the cycle time is 14 400 s (0.5 C)/7 200 s (1.0 C). The changes in battery potential and current density during the transient simulation are shown in Figure 2. The rest of the flow and heat transfer boundaries are the same as for steady-state calculations.
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| | 图2 0.5 C（a）、1.0 C（b）工况下电池电势和电流密度Fig. 2 Battery potential and current under 0.5 C (a) and 1.0 C (b) |
1.3 Mesh Model and Solution
The accuracy of numerical simulation results is highly dependent on the quality of the mesh. Due to the irregularity of the flow channel in the liquid cold plate, the model is divided by non-structural meshing. The whole computational domain consists of a solid domain and a fluid domain (Fig. 3), where the solid domain is composed of the battery cell, the upper cover, the liquid cold plate, and the thermally conductive silica gel, and the fluid domain only includes the cooling fluid. Since the solid domain only involves a simple thermal conductivity problem, it can be divided by a larger mesh. However, the flow and heat transfer problems in the fluid domain are more complex, and the mesh needs to be locally infilled. At the same time, in order to accurately capture the flow and heat transfer characteristics near the fluid-solid interface, a boundary layer mesh is used.
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| | Fig.3. Grid modelFig. 3 Mesh model |
The battery pack cools the battery cells by the flow of cooling fluid in the liquid cold plate, which belongs to the category of forced single counter-flow heat exchange. After preliminary calculations, the coolant belongs to the turbulent flow in the liquid cold plate, and the steady-state and transient simulations in this paper are simulated by the standard K-ε model. The pressure-velocity coupling is discretized by the Couple algorithm, and the turbulence and energy equations are discretized by the second-order Yingfeng formula. Three convergence criteria are set, i.e., the residuals of each process are less than 1 × 10-6, or the temperature difference and pressure drop of the coolant inlet and outlet are no longer changed. In order to optimize the calculation process, the inlet and outlet temperature and pressure difference were used as monitoring quantities, and the grid independence analysis was performed. As shown in Table 3, 12 300 100 mesh elements were used through the calculation of five sets of mesh models.
| | 表3 网格无关性验证Table 3 Grid independence verification |
1.4 Electrochemical model validation
Due to the lack of experimental data on heat production of lithium iron phosphate 280 Ah square cells, the electrochemical model of the cells was verified according to the discharge voltage-capacity curve of the cells in literature β [11]β. As shown in Figure 4, the discharge voltage-capacity curve of the established electrochemical model is in good agreement with the experimental data. Since all the material parameters are empirical parameters of the COMSOL material library, it can be assumed that the established electrochemical model can reflect the working characteristics of the lithium iron phosphate 280 Ah square cell.
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| | 图4 电化学模型验证Fig. 4 Validation of electrochemical model |
2 Results and Discussion
2.1 Steady-state analysis
According to the design parameters in Table 1, the temperature distribution in the battery pack under the two operating conditions (0.5 C and 1.0 C), as well as the velocity and pressure distribution in the fluid domain, are mainly discussed.
2.1.1 Temperature field analysis
Figure 5 shows the temperature contours of the battery pack under both operating conditions. Under the two operating conditions, the temperature of the inlet side of the battery pack is significantly lower than that of the outlet side, which is mainly caused by the inconsistent cooling capacity of the coolant at the inlet and outlet. When the discharge rates are 0.5 C and 1.0 C, the maximum temperature of the battery pack is 26.65 β °C and 33.48 β °C, respectively. In order to observe the temperature change inside the battery pack, the corresponding cross-sectional diagram is given. The results show that when the discharge rate is 0.5 C, the maximum temperature difference of the 54 cells is 7.52β °C. When the discharge rate is 1.0 C, the maximum temperature difference of the cell is 13.40β °C due to the increase of the heating power of the cell and the constant other boundary conditions. The temperature of the battery pack is unevenly distributed under both conditions, and this inhomogeneity increases with the increase of power, and the simulation results are consistent with the results in the literature [12]. Severe temperature inhomogeneity can significantly reduce the performance of the cell, and the temperature inhomogeneity can be reduced by optimizing the design of the fluid domain.
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| | 图5 0.5 C和1.0 C下电池包温度云图对比Fig. 5 Comparison of battery pack temperature contours under 0.5 C and 1.0 C |
Figure 6 shows a detailed contour of the temperature distribution of the liquid cold plate. Due to the abrupt change of flow direction and cross-sectional area, local high temperature phenomenon occurred under both conditions. When the discharge rate is 0.5 C, the maximum temperature of the liquid cold plate is 21.16 °C, and the maximum temperature difference is 3.16 °C. When the discharge rate is 1.0 C, the maximum temperature of the liquid cold plate is 23.57 °C, and the maximum temperature difference is 5.57 °C. The temperature distribution of the cooling fluid under both operating conditions is shown in Figure 7. Due to the influence of backflow, there are localized high temperature areas in the basin.
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| | 图6 0.5 C和1.0 C下液冷板温度云图对比Fig. 6 Comparison of liquid cooling plate temperature contours under 0.5 C and 1.0 C |
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| | 图7 0.5 C和1.0 C下冷却液温度云图对比Fig. 7 Comparison of coolant temperature contours under 0.5 C and 1.0 C |
2.1.2 Velocity field analysis
Because the boundary conditions under the two working conditions are only manifested in the different discharge rates of the cells, that is, the heating power of the cells is different. However, the temperature has almost no effect on the flow field and pressure field of the single-phase liquid, and it is considered that the fluid domain under the two working conditions has the same velocity field and pressure field. Figure 8 shows a streamline diagram and a cross-sectional velocity vector diagram of the cooling fluid. In the inlet and outlet areas, the fluid flow velocity is larger, and as the fluid enters the flow inside, the flow cross-sectional area increases and the fluid flow velocity decreases. At the same time, with a sudden increase in the flow area, the coolant will flow back. The slower flow rate of the fluid in the reflux region reduces the cooling capacity of the fluid in the region, resulting in higher temperatures in the region (as shown in Figure 7).
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| | 图8 流体域内冷却液流线图（a）与速度矢量云图（b）Fig. 8 Coolant streamlines (a) and velocity vector contour (b) |
2.1.3 Pressure field analysis
The pressure distribution of the coolant in the liquid cold plate is shown in Figure 9. The maximum pressure in the fluid domain is 12.56 kPa and the minimum pressure drop is -3.20 kPa, resulting in backflow in the fluid domain. Through the analysis of the pressure values at the seven sections in Table 4, it can be found that the inlet and outlet pressure drops are 11.82 kPa. The pressure changes uniformly in the flow channel, and the pressure loss is mainly concentrated at the inlet and outlet.
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| | 图9 流体域内冷却液压力云图Fig. 9 Coolant pressure contour in the fluid domain |
| | 表4 不同截面的压力Table 4 Pressure at different sections |
2.2 Transient Analysis
Since the temperature has almost no effect on the flow and pressure fields of the single-phase liquid, and the flow boundary conditions do not change, the fluid domain in the steady-state and transient calculations has the same velocity and pressure fields. Therefore, the velocity and pressure distributions within the fluid domain will not be repeated in this section. According to the design requirements in Table 1, the characteristics of the heating power and battery temperature in the battery pack under two working conditions (0.5 C and 1.0 C) are emphatically discussed.
As shown in Figure 10, there is a difference in the heating power of the battery during charging and discharging. At 0.5 C, the maximum heating power is about 3 kW/m3 when charging, and 4.93 kW/m3 when discharging. At 1.0 C, the maximum heating power is about 14 kW/m3 when charging and 17.5 kW/m3 when discharging. The results show that the battery pack can reach the maximum heating power under the discharge condition. The heating power increases faster in the charging phase than in the discharge phase. The heating power is kept low at the beginning of the discharge.
In summary, the temperature of the battery pack rises faster during the charging process than during the discharge process. Compared with the given steady-state heating power (12.5 W/cell at 0.5 C, about 4.85 kW/m3), the heating power in the transient simulation is slightly lower at 0.5 C and slightly higher at 1.0 C. As can also be seen from Figure 10, the battery temperature is lower most of the time than in the steady-state calculation because the battery is difficult to reach thermal steady-state (charging/discharging when the temperature is not equilibrate). For the 0.5 C condition, the maximum temperature is 22.32 β °C, and the average temperature is only 21.54 β °C. For the 1.0 C condition, the battery temperature is slightly higher than the steady-state calculation due to the higher heating power, and the maximum temperature reaches 35.17β °C in a short time during the discharge process, and the average temperature is up to 30.67β °C.
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| | 图10 0.5 C（a）和1.0 C（b）下电池最高温度、电池平均温度和发热功率对比Fig. 10 Comparison of the maximum battery temperature, average battery temperature and heating power under 0.5 C (a) and 1.0 C (b) |
Figure 11 shows the temperature distribution of the cell at the time of the highest temperature under the two operating conditions. When the discharge rate is 0.5 C, the 54 cells reach the maximum temperature of 22.32β °C at 7 200 s, and the maximum temperature difference is 2.91β °C. When the discharge rate increases to 1.0 C, the maximum temperature of the cell reaches 35.17 β °C at 7 200 s, and the maximum temperature difference is 10.25 β °C due to the increase of the heating power of the cell and the other boundary conditions remain unchanged.
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| | 图11 0.5 C和1.0 C下电池包最高温度时刻温度云图对比Fig. 11 Comparison of temperature contours at the highest temperature moment of the battery pack under 0.5 C and 1.0 C |
Figure 12 shows the maximum temperature of the liquid cold plate over time. When the discharge rate is 0.5 C, the maximum temperature of the liquid cold plate is 19.91β °C, and the maximum temperature difference is 1.91β °C. When the discharge rate is 1.0 C, the maximum temperature of the liquid cold plate is 27.48β °C, and the maximum temperature difference is 9.48β °C.
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| | 图12 0.5 C（a）和1.0 C（b）下液冷板最高温度对比Fig. 12 Comparison of the maximum temperature of the liquid cooling plate under 0.5 C (a) and 1.0 C (b) |
3 Conclusion
The thermal-fluid-electric model of a lithium iron phosphate battery was established by numerical simulation software, and the steady-state and transient analysis were carried out. The results show that the pressure difference between the inlet and outlet of the coolant reaches 11.82 kPa, and the pressure loss is mainly concentrated at the inlet and outlet of the liquid cold plate, and the coolant has obvious backflow phenomenon. At the same time, the cell temperature showed a significant inhomogeneity, which was further enhanced as the discharge rate increased. In order to ensure the stable operation of the battery, the flow pressure drop and temperature uniformity should be considered when optimizing the liquid cold plate.
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