A new concept of thermal management system in Li-ion battery using air cooling and heat pipe for electric vehicles

Highlights

• Heat pipe copper sheets (HPCS) for the battery thermal management system is designed.

• A battery thermal management system is simulated and validated with experiments.

• Temperature variation of a battery module in various initial conditions is reported.

Abstract

This paper presents the concept of a hybrid thermal management system (TMS), including air cooling and heat pipe for electric vehicles (EVs). Mathematical and thermal models are described to predict the thermal behavior of a battery module consisting of 24 cylindrical cells. Details of various thermal management techniques, especially natural air cooling and forced-air cooling TMS are discussed and compared. Moreover, several optimizations comprising the effect of cell spacing, air velocity, different ambient temperatures, and adding a heat pipe with copper sheets (HPCS) are proposed. The mathematical models are solved by COMSOL Multiphysics®, the commercial computational fluid dynamics (CFD) software. The simulation results are validated against experimental data indicating that the proposed cooling method is robust to optimize the TMS with HPCS, which provides guidelines for further design optimization for similar systems. Results indicate that the maximum module temperature for the cooling strategy using forced-air cooling, heat pipe, and HPCS reaches 42.4 °C, 37.5 °C, and 37.1 °C which can reduce the module temperature compared with natural air cooling by up to 34.5%, 42.1%, and 42.7% respectively. Furthermore, there is 39.2%, 66.5%, and 73.4% improvement in the temperature uniformity of the battery module for forced-air cooling, heat pipe, and HPCS respectively.

Introduction

Nowadays, climate change is one of the most critical issues worldwide [1]. The transportation industry is responsible for 24% of the direct carbon dioxide (CO₂) emission from fuel combustion in 2018 [2]. A possible solution is the usage of electric vehicles (EVs) as an alternative to reduce some parts of the air pollution problem through original equipment manufacturers (OEMs) [3]. Lithium-ion (Li-ion) batteries are introduced to feed electric motors in the EVs as a source. Li-ion batteries usage has been developed rapidly because of the advantages of acceptable recyclability, high energy, and power density. They have been widely applied in different fields comprising of electronic products and EVs [4]. Despite positive features, using Li-ion batteries in high-current applications and different ambient temperatures [5], [6] lead to overheating because of excessive heat generation [7]. Li-ion batteries should work within a specific temperature and thermal specifications. The most effective temperature in which Li-ion batteries should operate is in the range of working temperatures between 15 °C to 40 °C [8]. An inappropriate thermal management system (TMS) for battery systems leads to heat accumulation, which could overheat the battery module. If this excessive heat generation cannot be controlled, the lifetime and efficiency reduction will be inevitable, and in the worst case, more severe safety issues may lead to the explosion of the battery module [9]. Additionally, the temperature uniformity among all the cells in the module is essential. Non-uniformity of temperature distribution within battery modules may cause electrical imbalance over time, which causes the mismatch between the state of charge of cells [10] and the reduction of battery module performance. For example, a 10 °C to 15 °C temperature variation in the battery module causes a 30% to 50% degradation [11]. Generally, the TMS of battery systems is divided into active and passive cooling systems. On the one hand, active cooling comprising of the air and liquid cooling systems that need an external source of energy [12]. On the other hand, passive cooling, such as phase change materials (PCM) does not consume any external energy [13]. Air cooling is one of the most suitable TMS because of the low manufacturing cost, simple layout requirements, and high reliability. Generally, it is divided into two classes, natural air cooling and forced air cooling. A number of researchers experimentally and numerically discussed the effect of natural and forced air cooling on the cylindrical cells [14], [15]. Moreover, several studies [14], [16], [17], [18], [19] have been considered the influence of airflow pattern, airflow temperature, type of cell arrangement, air velocity, inlet, and outlet position. Nevertheless, the air-cooling system is not practical in some cases comprising stressful conditions, high ambient temperatures, and high charge/discharge rates due to the low heat transfer coefficient of the air [20]. Therefore, using a high heat transfer device to increase the cooling efficiency of the system in the design of many air cooling applications is necessary. Giuliano et al. [21] built a metal-foam heat exchanger for lithium titan battery to increase the heat transfer coefficient for air cooling applications. Mohammadian et al. [22] designed a kind of aluminum pin fin heat sink for prismatic Li-ion batteries. Li et al. [23] designed experimentally and numerically a battery air cooling thermal management system using a double silica cooling plate with copper mesh. The mentioned methods are generally used in many air cooling applications to increase the heat transfer, however, they increase the volume and weight of the system. Therefore, finding less volume, weight and more compact ways of cooling for Li-ion batteries is necessary. It is found that heat pipe can transfer the heat very efficiently [24], [25], [26], [27]. In recent years the heat pipe cooling technology has been used vastly in battery modules [4], [20], [28], [29], [30], [31], [32], [33]. Burban et al. [34] studied experimentally an unlooped pulsating heat pipe for the electronic thermal management field with hybrid vehicle applications. Rao et al. [35] used the plate heat pipe that could control the temperature of LiFePO₄ battery within 50 °C in the heat generation rate of less than 30 W. Besides, it maintained the temperature difference of module in 5 °C. Tran et al. [36] considered and analyzed the effect and cooling performance of heat pipes and fins in different inclinations on the evaporation of a Li-ion battery. Wu et al. [37] studied the effect of the natural convection, forced convection, and tubular heat pipe cooling method. They found that the heat pipe cooling method performed the best in controlling temperature rise. However, the temperature on the surface of the cells was undesirable due to the low effective heat transfer area between the heat pipes and the battery surface. Heat pipe TMS is often used for prismatic and pouch cells as it requires a large contact area and volume. In fact, for cylindrical shape cells, a new contact design is needed. Feng et al. [38] experimentally monitor the thermal and strain of cylindrical Li-ion battery pack, using a new design combined heat pipe and fins to manage it. They found that heat pipes and fins with the fan system can control the temperature of the whole pack to reach the operation temperature requirement with low energy consumption. Gan et al. [39] experimentally designed a TMS embedded with the heat pipe for a battery module with cylindrical cells. They used a conduction element to increase the contact area between heat pipe and cells. Wang et al. [40] presented a study using the heat pipe and the conduction element for thermal management of the cylindrical module. They found the best optimization is achieved by 19 mm battery spacing, 4 mm conduction element thickness and 120° circumference angle and 60 mm conduction element heigh. Nonetheless, due to difficulty in the design of heat pipe TMS for cylindrical cells in all the mentioned cooling systems the condenser needs a separate unit that increases the volume and weight of the system. In present work, in order to decrease the volume and more compact cooling system, several optimal designs have been done experimentally and numerically with air cooling, L shape heat pipe, and heat pipe copper sheet (HPCS). The new design of HPCS is proposed for TMS of a battery module with cylindrical cells in which the condenser located inside the module. The copper sheets are designed for thermal contact between flat heat pipe and battery to enhance the heat transfer surface. CFD model is conducted to analyze the temperature distribution of the battery module. The results show that all cooling designs not only decreases the maximum temperature of the module but also increase the uniformity. The paper is organized as follows. Section 2 represents the objective of the research. In Section 3, the experimental setup is represented. The simulation of the battery module model is described in Section 4. Results, discussion, and optimization of the air-cooling system are explained in Section 5. Lastly, a relevant conclusion is drawn in Section 6.