Numerical study on a water cooling system for prismatic LiFePO₄ batteries at abused operating conditions

Highlights

• A novel modular water cooling system for Li-ion batteries is proposed.

• Cooling performances with different parameters were numerically studied.

• Abused battery operating conditions were considered.

• Battery temperature <32.5 °C and greatly uniform distribution were obtained.

Abstract

Cooling of the Li-ion battery module is a critical issue for electric vehicles in regard with the battery performance and lifetime, particularly at abused operating conditions. A water cooling system consisting of two novel cover plates with T-shape bifurcation structures and eight traditional cold plates was proposed and investigated for cooling of prismatic LiFePO₄ battery modules. A computational fluid dynamics simulation model of the cooling system was established and validated by experimental data. The results show that a cold plate with four circular mini-channels flowing water can keep the maximum temperature of a single battery around 35 °C at 1 °C discharging rate and ambient temperature of 40 °C. Effect of the coolant flow rate is much less significant than the inlet coolant temperature on the cooling performance. Two sets of the proposed water cooling systems can significantly reduce the maximum temperature of a battery module consisting of 15 batteries. Moreover, great temperature distribution uniformity between different batteries can be achieved because the novel cover plate is able to distribute water evenly into each mini-channel of the cold plates. The cooling system can keep the maximum temperature of the module below 32.5 °C and the difference between the highest and lowest temperatures around 1.5 °C at abused operating conditions.

Introduction

Electric vehicles (EVs) are considered as a promising alternative technology for traditional internal combustion engine vehicles (ICEVs) because of higher energy utilization efficiency and lower emissions of pollutant [1]. The Li-ion battery pack is one of the most important components in EVs as the energy storage unit which can support fast acceleration and long driving mileage. The lifetime, usable capacity and safety of a Li-ion battery pack are significantly influenced by the operating temperature and the temperature difference between individual batteries [2]. In-plane non-uniform temperature distribution in a large single battery can also influence its performance [3]. However, a large amount of heat is generated in EVs during discharging due to chemical reactions and ohmic resistance [4]. Generally, operating temperature in the range of −20 and 60 °C is required for Li-ion batteries to avoid temperature-related degradation and possible thermal runaway [5]. Pesaran et al. [6] suggested a narrower temperature range of 15 and 35 °C to maintain the optimal battery performance. Moreover, the temperature difference between individual cells in a module is required to be less than 5 °C [7]. Therefore, the battery thermal management system (BTMS) is necessary to effectively and efficiently remove excessive heat from the batteries.

Three major cooling techniques for Li-ion batteries are air cooling, liquid cooling and phase change material (PCM) cooling. Besides, emerging cooling technology such as mist cooling combining forced air and water evaporation [8] and hybrid cooling integrating liquid cooling and PCM [9], [10] were also in development. Generally, air cooling and liquid cooling are more commercially mature than PCM cooling. During high discharging rate or high ambient temperature, the passive PCM cooling also has the risk of running out of available latent heat [11]. Although air cooling has advantages including low cost, weight, complexity and ease to maintain, it is not suitable for batteries working at abused operating conditions such as high discharging rate or high ambient temperature due to low heat transfer characteristics of air [12]. Packaging is also an issue of air cooling thermal management systems since a large manifold for air supply is needed [13]. Recent improvements in both air cooling and liquid cooling concentrate in novel design and parametric study of BTMSs to reach optimized heat dissipation performance in regard with low maximum temperature and high temperature uniformity of different batteries in a module.

Numerical simulation is widely employed to investigate the temperature distribution and thermal behavior of Li-ion batteries at various operating conditions [14]. For air cooling of pouch batteries, Park [15] numerically compared the performances of BTMSs with traditional U-shape and Z-shape ducts. Fan et al. [16] numerically studied effects of the gap spacing and air flow rate on thermal management of an air cooling battery module containing eight prismatic Li-ion batteries operating under a modeled aggressive driving profile. It was found that the temperature rise of batteries can be reduced by lowering the gas spacing or increasing the air flow rate. A trade-off is required to achieve the satisfied cooling performance at acceptable fan power consumption. Yu et al. [17] proposed a novel BTMS design with two directional air flows. Numerical and experimental results indicate both the maximum temperature and temperature uniformity are improved. Recently, fins [18] and metal foams [19], [20] were embedded into the flow channels in order to enhance heat transfer. Giuliano et al. [21] experimentally studied the performance of an air cooling thermal management system employing metal-foam based heat exchanger plates inserted between adjacent batteries. The temperature rise can be restricted to less than 10 °C for 200 A charging-discharging cycles at the air flow rate of 1100 mL/s. However, it is worth noticing that the tested ambient temperature is only 22 °C. Saw et al. [20] employed the CFD technique to study a metal foam cooling system for Li-ion battery module and optimized pore density and porosity of the metal foam. For liquid cooling, Lan et al. [22] proposed a new BTMS design with mini-channel aluminum tubes twisting around a single Li-ion battery. Water flows inside the mini-channels to cool down the battery. Numerical results show that the maximum temperature of 27.8 °C and temperature difference of only 0.8 °C can be achieved at the discharging rate of 1 C. Zhang et al. [23] further experimentally verified the cooling performance of such a BTMS design for a battery pack. A sheet of flexible graphite was inserted between the cell wall and the tubes in order to enhance heat transfer. Experimental results indicate that great cooling performance was achieved and the temperature difference between batteries was below 5 °C. Another commonly used design concept is to insert a cold plate with mini-channels into neighboring prismatic batteries. Huo et al. [24] and Qian et al. [25] investigated effects of the number of channels, flow direction, and water velocity. Except for straight mini-channels, other types of channels were also investigated. Jarrett and Kim [26] numerically studied and optimized a cold plate with a serpentine channel. Jin et al. [27] examined the cooling performance of a cold plate by adding oblique fins in the straight channels and the results show that heat transfer was greatly enhanced.

Most of the previous research on liquid cooling of prismatic battery modules assumed the coolant entering each cold plate was uniform. However, the method to ensure uniform distribution of coolant in separate cold plates is usually not discussed. Moreover, the maximum temperature and temperature distribution uniformity in a battery module operating at abused conditions, such as high discharging rate and ambient temperature, are not thoroughly investigated. The present study proposed a novel modular designed liquid cooling system consisting of a cover plate to uniformly distribute water in different cold plates. Numerical methods were employed to investigate the cooling performance for a battery module consisting of multiple large capacity (70 Ah) prismatic lithium iron phosphate batteries at 1 °C discharging rate and ambient temperature of 40 °C. Effects of the ambient temperature, number of mini-channel in a cold plate, coolant type, inlet coolant temperature and flow rate were studied.