1. Introduction
In response to the crisis of fossil fuels and global air pollution, the transportation industry has begun to electrify. Rail transit took the lead in popularizing electric drive. As a result, the road transport sector has begun a transition from gasoline-powered vehicles to electric vehicles. Countries around the world have proposed plans to ban the sale of gas-powered cars. By the end of 2022, China has achieved the 2025 target of 20% new energy vehicle penetration ahead of schedule.Although hydrogen fuel cell technology has attracted attention as another new energy supply solution for new energy vehicles, pure electric vehicles using lithium-ion battery packs and drive motors as power systems are the mainstream of the industry [1]. Lithium-ion batteries are an important part of any vehicle’s energy supply system, whether it is a fuel cell vehicle or a pure electric vehicle.
Compared with traditional internal combustion engine vehicles, new energy vehicles have the advantages of high efficiency and cleanliness. However, there are some significant disadvantages, the most notable of which are strict standards for ambient temperature during use. All power sources have specific temperature ranges, whether they are drive motors, fuel cells or power lithium-ion batteries. The normal operating temperature range of lithium-ion batteries is −10 °C to +50 °C.Considering their performance and durability, automotive lithium-ion power batteries are sometimes limited to a narrow operating temperature range, that is, between +20°C and +40°C [2,3,4,5]. +45 °C to +60 °C is the ideal operating temperature for fuel cells [6]. The driving motor has a wider operating temperature range, with a maximum temperature of +130℃ [7,8]. Lithium-ion batteries inevitably generate Joule heat and chemical reaction heat during the charge and discharge process.The temperature rise of the battery caused by heat accumulation cannot be ignored because it will have a negative impact on the energy efficiency, safety and life of the battery. [9]. Therefore, new energy vehicle powertrains need to develop reliable battery thermal management systems (BTMS) that can adapt to cold and hot climates for global adoption.
Back in the early 2000s, Pesaran et al. [10] Recognize the importance of thermal management of batteries in electric and hybrid vehicles. For more than 20 years, researchers have created a variety of cooling technologies to manage battery temperature.These technologies include gases, liquids, phase change materials (PCM), heat pipes (HP), Peltier, and various combinations of these methods [11].The study found that the battery thermal management system must not only consider heat dissipation requirements, but also consider design indicators such as vehicle thermal management, battery preheating under low temperature conditions, power consumption, volume, temperature uniformity, etc. [12]. Although combining different cooling methods can effectively achieve cooling functions, optimizing single cooling methods such as liquid cooling and air cooling also has great potential.Liquid cooling offers clear advantages over air cooling and is the primary method used in commercial applications [13]. Passive cooling methods such as PCM and HP have received widespread attention due to their superior performance [14]. PCMs are available in a variety of designs, including cooling of lithium batteries, electronic components and photovoltaic panels [15,16]. Despite its advantages, PCM also has some disadvantages, such as low thermal conductivity, which makes it susceptible to failure due to heat accumulation, and low bulk density. On the other hand, HP materials have high manufacturing costs, complex installation, and difficult maintenance.Although many optimization solutions have been proposed [17]there is still a long way to go for practical commercial application [18]. Currently, liquid cooling is the most widely used battery temperature management solution due to its technical effectiveness, heat dissipation capabilities and cost-effectiveness.
The heat dissipation of cylindrical battery packs mostly adopts cross flow and series connection, liquid cooling or air cooling. Although the flow channel design of the cross-flow cooling method is relatively simple, the fluid cross-flow channel must pass through the cells, so a specific gap needs to be maintained between each cell. This takes up extra space and is detrimental to the energy density of the battery pack.In a series heat dissipation battery pack, due to the large temperature difference along the direction of fluid flow, the surface temperatures of each single cell are also unbalanced, resulting in reduced capacity and shortened battery pack life. [19]. Individual studies have proposed some forms of longitudinal flow heat dissipation for cylindrical cells, which can improve the uniform temperature performance of the battery pack; however, these solutions still require a certain gap between each cell and do not effectively improve the space utilization of the battery pack. Rate.
The longitudinal flow heat dissipation proposed in this article is a form of convection heat transfer. The heat transfer fluid flows along the axial direction of the battery in the holes formed by the closely arranged cylindrical battery pack, forming a convection heat exchange between the batteries. Cylindrical surface. This solution has three characteristics: First, the arrangement characteristics of cylindrical lithium-ion batteries. The arranged pores can be used as coolant flow channels to closely arrange the cells and effectively reduce the battery size. Encapsulate and improve the energy density of the cylindrical battery pack; Secondly, for each cell, the heat transfer area is the same, and the inlet temperature of the heat transfer fluid is also the same, so the battery pack has better temperature uniformity performance; Thirdly, heat transfer The fluid flows along the axial direction of the battery, and the entire side of the battery is available for heat dissipation, resulting in high heat transfer efficiency. The research contribution of this project is reflected in two aspects.
Compared with natural cooling and forced air cooling, liquid flow cooling batteries are widely used because of their high efficiency and reliability. [20].However, the additional weight of the coolant itself, the coolant flow channels, and the sealing elements used to avoid coolant leakage increases the manufacturing cost of the coolant circulation system, which is a disadvantage of the liquid cooling method [21]. For liquid cooling of cylindrical batteries, all methods currently proposed or used require a certain gap in the diameter direction between all individual cells to allow the coolant flow path to pass, which undoubtedly increases the size of the battery pack and reduces its volumetric energy density. The longitudinal flow heat dissipation method proposed in this article aims to eliminate this gap, taking advantage of the arrangement characteristics of cylindrical lithium-ion batteries, and using its arranged pores as coolant flow channels to allow the cooling medium to flow along the axial direction of the battery for heat dissipation. . This approach allows cylindrical cells to be packed closely together, not only reducing material and weight in the coolant flow channels but also increasing the volumetric energy density of the entire battery pack.
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Improving uniform temperature performance of cylindrical battery packs
The traditional cooling method for cylindrical batteries is cross-flow series cooling. The cooling medium flows along the diameter of the battery and is in direct or indirect contact with the sides of each battery. In the direction of flow of the cooling medium, heat is exchanged with each individual cell in turn. Therefore, the closer to the outlet, the higher the temperature of the cooling medium, and the lower the heat exchange rate with the battery surface. The worse the heat dissipation effect, the higher the surface temperature of the battery core. This is the biggest disadvantage of cross-flow cooling. In contrast, using longitudinal flow cooling, each battery can receive coolant with a uniform temperature, and there is no such problem.