The thermal management of the power battery with air as the medium is to let the air traverse the battery pack to take away or bring heat to achieve the purpose of heat dissipation or heating. The battery cooling method using air as the medium is also called air-cooled cooling. According to whether the electric vehicle needs to provide auxiliary energy, it can be divided into active and passive heat dissipation methods. It can also be divided into natural convection and forced convection according to the causes of air flow outside the battery pack.
For the air-cooled cooling system, it can be combined with the design of the driving characteristics of the whole vehicle, and the heat can be taken away according to the natural wind formed by the speed of the vehicle, or forced air ventilation can be generated by fans and fans to dissipate heat from the battery. According to the arrangement of the batteries, it can be divided into two ventilation modes, serial and parallel. Studies by Pesaran et al. show that parallel ventilation works better. At the same time, the air-cooled heat dissipation is also affected by the thickness of the runner. Considering the specific requirements of cost and car space, air-cooled heat dissipation is generally regarded as the first choice for electric vehicle battery heat dissipation. The Toyota Prius battery pack uses parallel ventilation air cooling as suggested by Pesaran et al. According to the test data of the National Renewable Energy Laboratory of the United States, when the NMH battery used in the battery pack works in an environment of 25℃, the maximum temperature can be controlled below 45℃, and the battery temperature difference can be controlled below 5.0℃. Mahamud and Park designed a reciprocating air-cooled heat dissipation system for Li-ion (LiMn2O4/C) batteries, and the temperature difference between the batteries dropped by 4℃. And compared with the ordinary unidirectional flow air-cooled cooling system, the maximum temperature of the battery is reduced by 1.5℃ in 120s. In China, the work related to battery heat dissipation mainly focuses on air-cooled heat dissipation. Jiao Hongjie et al. developed a parallel HEV with Ni-MH battery ventilation cooling device. Lou Yingying designed a 36V plum-shaped battery pack for HEV, and studied the air-cooled heat dissipation characteristics by combining experiments and numerical simulations. Liang Changjie conducted experimental and numerical research on the temperature field uniformity of Ni-MH batteries. The maximum temperature difference of the battery pack decreased from 20.4℃ to 4℃, and the maximum temperature decreased from 65℃ to 53.45℃. Xu Chao studied the heat dissipation of the LiFePO4 battery pack of a hybrid electric bus (SWB6116HEV), and proposed ways to increase the heat dissipation effect by adding a fan at the rear of the battery pack, improving the baffle, and widening the cooling air duct between the batteries.
In general, the main advantages of an air-cooled cooling system are as follows:
(1) The structure is simple and the quality is relatively small;
(2) There is no possibility of liquid leakage;
(3) Effective ventilation when harmful gases are generated;
(4) The cost is lower.
2. Passive and active
Passive air cooling usually refers to the direct use of the natural wind formed by the speed of the vehicle to take away the heat of the battery without using any external auxiliary energy. The method is simple, easy to implement, and low in cost, and the heat exchange in the heat dissipation process is mostly based on natural convection. However, for effective cooling, the shape of the battery or the shape of the battery package needs to be specially designed and used to increase the heat dissipation area of the battery.
The heat exchange in the heat dissipation process of active air cooling is mainly forced convection. Therefore, if the space around the battery module allows, a local radiator or fan can be installed, or an auxiliary or car's own evaporator can be used to provide cold air. The principle is shown in Figure 1. The method reduces the packaging design requirements of the battery, and can be used in more complex systems. The position of the battery on the vehicle is also not restricted, and the structural design of the entire vehicle is less affected.
Figure 1 - Schematic diagram of active air cooling
3. Alternate ventilation
With the increase in the size of battery modules and battery packs, whether serial ventilation or parallel ventilation, it is easy to cause excessive temperature differences between battery cells at different positions. By changing the air inlet and air outlet, the temperature difference between different battery cells can be further reduced. Alternate ventilation is a heat dissipation method that cools the battery by periodically changing the position of the air inlet and the air outlet. The principle is shown in Figure 2. Through the special structure design, the air passes through the left or right side of the battery intermittently, which helps to avoid the phenomenon that the temperature on the left or right side is locally too high.
Figure 3 shows the profile of the temperature distribution of a row of batteries during alternate ventilation. Assuming that τ is an air intake cycle, at the initial moment (tc=0), the right air intake; when tc=τ/4, change to the left air intake; when tc=τ/2, continue to maintain the left air intake;
When tc=3τ/4, it is changed to the right air intake; when tc=τ, the right air intake is continued. Compared with single-side ventilation, the above-mentioned alternate ventilation can significantly reduce the temperature difference between the leftmost and rightmost batteries, reducing the unbalanced heat distribution in the battery module.
Figure 3 - Contour plot of temperature distribution of alternately ventilated cells