Performance investigation of a passive battery thermal management system applied with phase change material

https://doi.org/10.1016/j.est.2021.102279Get rights and content

Highlights

  • A passive, low-cost battery thermal management system with PCM units was designed.

  • It exhibited good heat storage capability and temperature equalization capability.

  • After thermal insulation, a temperature retaining function could also be achieved.

  • The effects of the parameters of the PCM unit on its thermal performance were got.

  • The methods and results could provide reference for the design of BTMSs.

Abstract

Lithium-ion batteries will produce a lot of heat during working, which can increase the temperature and temperature difference in the battery module, and seriously impair the capacity, life, and safety of the batteries. Therefore, an effective thermal management system should be implemented to control the temperature of the batteries, while reducing the energy consumption and cost of the system as much as possible. In this paper, a passive, low-cost battery thermal management system consisting of pouch lithium-ion cells and phase change material units is designed. Based on the heat production process of the battery and heat transfer theory of the phase change material, a numerical model is built and the performance of the thermal management system is analyzed. It is found that the phase change material units can effectively decrease the maximum temperature and the maximum temperature difference on the cell at the end of discharge, and thus have good heat storage capability and temperature equalization capability. After insulating heat dissipation from the surfaces of the phase change material units, a temperature retaining function can also be achieved. The effects of the design parameters of the phase change material unit on its thermal management performance are further investigated, including the thermal conductivity, viscosity, and latent heat of the phase change material, the thickness of the phase change material unit, and the thermal conductivity of the unit shell. It could provide theoretical references and technical means for the design and implementation of passive, low-cost battery thermal management systems applied with phase change materials.

Introduction

The power battery is the core component and main energy source of Electric Vehicles (EVs). Its performance is closely related to the vehicle's driving range, power output, energy consumption and charging efficiency, et al [1]. The optimal working temperature of the lithium-ion batteries widely used on EVs is 10-40 °C, and the maximum temperature difference on the battery or in the battery system is better to be controlled within 5 °C [2]. Higher temperatures, lower temperatures, or greater temperature differences can affect the capacity, life, and consistency of the batteries. However, lithium-ion batteries will produce a lot of heat during the charging or discharging process, causing the temperature of the battery system to rise rapidly. While in cold weather, the temperature of the battery system may drop below 0 °C. Besides, the differences in the thermal conductivities on the different directions of the battery, the differences in the heat dissipation conditions on the battery and in the battery pack, and the inconsistencies in the production, operation, and aging process of the batteries will increase the temperature difference. Therefore, it is necessary to implement a Battery Thermal Management System (BTMS) to adjust and balance the temperature of the battery system.

At present, the methods commonly used in vehicles for BTMSs include natural cooling, air cooling, and liquid cooling [3]. The effect of liquid cooling is the best, but the structure is complex and the cost is high. So, it is mainly used in middle-class and high-class vehicles (such as Tesla Model S). Natural cooling and air cooling have simpler structures and lower costs, so they still have a lot of applications in junior-class and even some middle-class vehicles (such as Nissan Leaf and Roewe Marvel X), which have broad markets and amazing sales. However, the cooling effects of these two methods is usually not as good as liquid cooling, so it is difficult for them to meet the requirements of high-performance and long-life battery systems and extreme-fast charging technology in the future [4].

Compared with these conventional thermal management methods, the Phase Change Material (PCM) can absorb or release a lot of latent heat by phase transition, and maintain a constant temperature during the process [5]. Meanwhile, it can be designed according to different scenarios with flexible structures and lower costs, and thus has good application prospects in the BTMSs on vehicles [6]. For example, Wang et al. [7] applied the PCM to the thermal management system of cylindrical lithium-ion cells, and found that compared with natural convection, the PCM could control the maximum temperature and maximum temperature difference on the cells more efficiently, and significantly lowered the overall temperature of the battery module. Lv et al. [8] found that the MT of the battery module with PCM was significantly lower than that of the battery module without PCM, and the cycle life was longer. Lian et al. [9] studied the characteristics of the microcapsuled PCM suspension applied to the liquid cold plate. It was found that compared with pure water, the microcapsuled PCM suspension could reduce the MT of the liquid cold plate and improve the temperature uniformity. Rao et al. [10] investigated the effects of the PCM parameters and environmental conditions on the temperature distribution of cylindrical lithium-ion cells. It was found that when there was enough PCM, the lower of the PCM melting point, the lower of the local temperature difference on the battery module. But when the PCM was not enough, the local temperature difference would gradually increase after the PCM completely melted. Xiao et al. [11] prepared a kind of polymer phase change material (PoPCM). This kind of PCM had great latent heat and stable solid-solid phase change behavior. It was difficult to deform or leak, and showed good and stable heat dissipation performance, which could be used in the thermal management system of a battery module. Yang et al. [12] performed numerical calculations and visualization experiments on the melting process of the PCM around a cylindrical cell. It was found that when wrapped with a metal shell, the PCM began to melt from the surface contacted with the cell, and formed a tapered peak during the melting process. Choudhari et al. [13] proposed a design of inserting fins into the PCM module. The results showed that the fin structure enhanced both the heat transfer in the PCM module by conduction and the heat transfer out of the PCM module by convection. So it could improve the heat dissipation capability of the PCM module on cylindrical cells.

Although the latent heat of the PCM makes it an important material that can be applied to thermal management systems, the relatively lower thermal conductivities of commonly used PCMs limit their performance. When rapid thermal response is required, adding heat conduction medium into the PCM can usually improve the response speed. For example, Samimi et al. [14] added carbon fibers into the PCM and applied it to a BTMS of cylindrical lithium-ion cells. Experiments showed that the carbon fibers increased the thermal conductivity of the PCM, which could reduce the MT on the cells while making the temperature distribution more uniform. Y. Azizi et al. [15], Farid Bahiraei et al. [16], G. Karimi et al. [17] studied the effects of aluminum wire mesh panels, carbon-based nanoparticles, metal nanoparticles and metal matrix on the properties of PCMs in BTMSs. Besides, as the thermal capacity of the PCM is still limited, if it can be integrated with other heat dissipation or heat conduction methods, better thermal management effects could be achieved, such as air cooling [18], liquid cooling [19], heat pipe cooling [20], etc. Lv et al. [21] designed a serpentine composite PCM module. It had greater surface area and more air flow channels, which could improve the cooling capability of the PCM module and the cooling performance of the battery module.

It can be seen from the above that it is possible to apply PCMs to the BTMSs on vehicles. However, current research in this area mainly focused on cylindrical or prismatic cells, but few studies discussed its applications on pouch lithium-ion cells, which have been widely used in EVs. In terms of structure design, a considerable part of research integrated PCMs with other heat conduction mediums or heat dissipation methods. They adversely increased the structure complexity, production costs, or energy consumption to different levels, and reduced the reliability and feasibility of practical applications on vehicles. Therefore, it is necessary to further apply the PCM on the BTMS of pouch lithium-ion cells, and discuss its design method for better performance, less energy consumption, higher feasibility, and lower cost.

In this paper, the PCM of paraffin is applied on pouch lithium-ion cells, and a passive, low-cost BTMS consisting of 4 cells and 3 PCM units is designed. Taking the internal resistance variation of the cell during discharge and the heat production rate of the tabs into account, the thermal management performance, heat dissipation function and temperature retaining function of the BTMS are studied. By changing the thermal conductivity, viscosity, latent heat of the PCM, the thickness of the PCM unit and the thermal conductivity of the unit shell, the effects of these parameters on the Maximum Temperature (MT) and Maximum Temperature Difference (MTD) on the cell are investigated. It could provide theoretical references and technical means for the design and implementation of passive, low-cost BTMSs applied with PCMs on vehicles.

Section snippets

Model and governing equations of the battery

Fig. 1 shows the pouch lithium-ion cell used in this study. The cell is wrapped with a plastic aluminum film. The active material in the positive electrode is LiFePO4, and the active material in the negative electrode is graphite. The positive and negative tabs are on the same side of the cell. The anode material is graphite, the cathode material is LiFePO4. The electrolyte material is LiPF6. Table 1 shows the basic parameters of the cell. The interior of the cell is a multilayer structure

Thermal management performance of the PCM unit

Since the cell close to PCM unit 1 has the worst heat dissipation condition, the temperature on the surface of this cell in contact with PCM unit 1 is taken to evaluate the thermal management performance. During the 5 C discharging process, the MT and MTD on the cell are demonstrated by the solid lines in Fig. 6. Meanwhile, the MT and MTD on the corresponding cell in the battery module without PCM units are also calculated, as demonstrated by the dotted lines in Fig. 6.

It can be seen that for a

Effects of design parameters on the thermal management performance of the PCM unit

Although the structure of the PCM unit is simple, it still has many adjustable design parameters, including the thermal conductivity, viscosity, latent heat of the PCM, the thickness of the PCM unit, and the thermal conductivity of the PCM unit shell. Here, the MT and MTD on the cell near PCM unit 1 are used as the indicators of the thermal management performance. The surfaces of the PCM units exposed to the air are thermally insulated.

Conclusion

In this study, a passive, low-cost BTMS applied with PCM units is designed. The performance and functions of the thermal management system are analyzed, and the effects of the design parameters on the MT and MTD on the cell are investigated. The following conclusions are obtained:

  • 1

    The PCM unit can effectively reduce the MT and MTD on the cell at the end of 5 C discharge. The MT is reduced from 324.10 K to 316.84 K, with a decrease of 7.26 K. The MTD is reduced from 2 K to 1.16 K, with a decrease

CRediT authorship contribution statement

Yanan Wang: Conceptualization, Methodology, Software. Zhengkun Wang: Data curation, Writing - original draft, Software. Haitao Min: Writing - review & editing. Hua Li: Visualization, Investigation. Qingfeng Li: Supervision, Validation.

Declaration of Competing Interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgments

This work is supported by Foundation of State Key Laboratory of Automotive Simulation and Control (20181102) and Shandong Provincial Natural Science Foundation, China (ZR2020ME138).

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