1. Flow field design of battery pack thermal management system
The rate of heat dissipation per unit area of the battery pack to the heat transfer medium is expressed as
Among them, h represents the convective heat transfer coefficient on the surface of the battery pack, and the subscripts bat and amb represent the surface of the battery pack and the heat transfer medium, respectively.
First, the design of the flow field determines the order in which the heat transfer medium flows through different positions of the battery pack, which will affect the value of the Tbat-Tamb term, thereby affecting the local heat dissipation rate at different positions. Second, the design of the flow field determines the flow velocity of the heat transfer medium at different locations, and the flow velocity will affect the h term of the local convective heat transfer coefficient. Third, the design of the flow field determines the local shape of the flow channel, which will also affect the value of the local convective heat transfer coefficient h. Therefore, the rationality of the flow field design has a significant impact on the thermal management effect of the battery pack.
(1) Path design of flow field – serial flow channel and parallel flow channel. According to the passage of the heat transfer medium inside the battery pack, the flow field can be divided into serial flow channel type and parallel flow channel type, as shown in Figure 1. In the serial flow channel design, the heat transfer medium passes through each single cell or battery module in strict order, while in the parallel flow channel design, the heat transfer medium enters the battery pack box and passes through the parallel flow channels. Divide the current through different battery sub-modules in parallel. For serial runner designs, the battery modules behind the runners will not be able to dissipate heat effectively because the medium will gradually be heated in the serial runners. It has been pointed out that the parallel flow channel design results in better temperature uniformity at different locations of the battery pack compared to the serial flow channel.
(2) Velocity design of flow field—speed regulation and pressure regulation of parallel flow channels. For the parallel flow channel design, the flow rates of different flow channels must be as uniform as possible to reduce the non-uniformity of temperature at different positions inside the battery pack. Two methods to ensure uniform flow rate: speed regulation method and pressure regulation method, and the optimal combination of the two methods is given. The speed regulation method refers to reducing the width of each channel in turn in the direction of increasing the number of parallel channels to adjust the flow resistance of the heat transfer medium, so that the heat transfer medium can redistribute its flow according to the resistance of each channel, so as to achieve the purpose of adjusting the flow rate distribution. The pressure regulation method changes the pressure difference on both sides of different channels by changing the inclination angle of the inlet and outlet collector plates, thereby indirectly adjusting the flow rates of different channels.
The thermal management system of the lithium-ion power battery system is mainly divided into air cooling and liquid cooling according to the different cooling media. Among them, liquid cooling has better cooling effect, but it needs to arrange special pipes, has many parts, complicated control and high cost. The gas cooling system has low heat transfer rate and low volumetric efficiency, but is widely adopted due to its simple design, simple control and low cost. Due to the small convective heat transfer coefficient of gas, it is more difficult to use gas to heat or cool battery systems than liquids. Therefore, the design of gas cooling systems should be optimized to the greatest extent possible for battery packs. Taking the development of a power battery cooling system for an electric vehicle as an example, the optimization scheme of the air-cooled cooling system is proposed, and the final optimization scheme is determined by the simulation results.
2. Problems and solutions of cooling system
The air-cooled cooling system of an electric vehicle power battery is shown in Figure 2. There are two main problems in this cooling system: one is that the temperature difference between the battery modules is too large; the other is that the pressure loss is too large, and the structure of the air channel needs to be optimized. The main reasons for these two problems are: the arrangement of the battery modules is asymmetric, and the air flow between the battery modules is inconsistent; the battery module adopts a double-layer structure, which generates heat accumulation; there is a sudden contraction or expansion in the air channel. The cross section changes suddenly, the structure does not have enough corner radius at the corner, and the air cannot transition smoothly. In order to solve these two problems, the air-cooled heat dissipation system of the battery system is optimized: the battery module is arranged in a single-layer structure, the structure of the air channel is arranged symmetrically, and the cross-section is changed by using a small shrinkage angle and multiple cross-sections. , and design a large corner radius at the corner. There are two types of improved schemes: scheme one, the air inlet of the air channel is set at the top left side, the air outlet is set at the bottom end of the right side, and the air inlet and outlet are located on both sides; scheme two, the air inlet port of the air channel is located at the top left side, The air outlet is at the bottom left side, and the air inlet and outlet are at the left end. In the two schemes, the battery modules are arranged in a single-layer symmetrical arrangement, and a large rounded transition is designed at the turn of the air inlet, and a small-angle contraction is used to reduce the pressure loss. The improved scheme is shown in Figure 3 and Figure 4.
3. Evaluation indicators of air-cooled cooling system
In the design process of the air cooling system of the battery pack, it is necessary to evaluate it with relevant indicators to determine whether the optimization scheme is feasible. The main indicators are the maximum temperature difference of the system, the maximum temperature of the system, and the pressure difference between the inlet and outlet. The maximum temperature difference of the system refers to the difference between the highest temperature and the lowest temperature of all the cells in the lithium-ion power battery system, which reflects the uniformity of the cooling system and ensures that the cooling effect of each cell is consistent. The maximum temperature of the system refers to the maximum temperature of all cells in the lithium-ion power battery system, which can represent the cooling effect of the cooling system to a certain extent. The inlet and outlet pressure difference refers to the pressure difference between the air inlet and the air outlet in the air cooling system, which is closely related to the structure of the air flow channel of the cooling system.
4. Simulation analysis
The flow field and thermal field simulation are carried out for the two optimization schemes respectively, and the temperature cloud map, pressure cloud map and velocity cloud map are formed, as shown in Figure 5, Figure 6, and Figure 7. By comparing and analyzing the simulation results of flow, pressure, temperature, etc., the advantages and disadvantages of the two schemes are judged according to the evaluation indicators of the cooling system, and the one with better cooling effect is selected. The key parameters of the two optimization schemes are compared in Table 1. From the data in Table 1, it can be seen that the maximum temperature difference of the system, the maximum temperature of the system, and the flow uniformity of the scheme 2 are better than those of the scheme 1, but the pressure difference between the inlet and the outlet of the scheme 2 is slightly different. Therefore, the second solution with the same side design of the air inlet and outlet has a better heat dissipation effect, and further optimization design can be made on the basis of the second solution to obtain a better heat dissipation effect.
|Scheme||Maximum temperature difference (K)||Maximum temperature (K)||Inlet and outlet pressure difference (Pa)||Flow unevenness (%)|