1. Single-in and single-out flow channel
Compared with the cylindrical battery, the square battery has a regular shape and a flat surface, and the battery can be cooled by inserting a plate heat dissipation component (hereinafter referred to as a cold plate) between adjacent battery cells. The cold plate is generally made of aluminum, copper and other metal materials with high thermal conductivity, and has a variety of structures. One end of the square metal plate can be directly sandwiched between the batteries, and the other end extends out of the battery. The heat of the battery is dissipated through the high thermal conductivity of the metal material, and the plate extending out of the battery is cooled by air cooling. The high-efficiency cold plate type liquid cooling system mainly cools the battery by welding various shapes of flow channels inside the plate, so that the liquid flows through the flow channels; the flat tube structure can also be directly used, and the tubes are flattened and placed between adjacent cells.
In the cold plate liquid cooling system, the coolant does not directly contact the battery, which can effectively reduce the risk of short circuit and improve the safety of the battery pack. The flow channels in the cold plate can be divided into single-in-single-out flow channels, single-in-multiple-out flow channels, multiple-in-single-out flow channels, and multiple-in-multiple-out flow channels according to the liquid inlet and outlet forms. The coolant is generally water. The cooling liquid inlet and outlet of the single-in-single-out flow channel can be distributed on different sides, and the liquid flows in from the left and flows out from the right; the cooling liquid inlet and outlet can also be distributed on the same side, left in and left out or right in and right out. The advantages of the single-in-single-out runner structure cold plate are mainly simple structure and convenient installation; the main disadvantage is that the flow resistance in the tube is large, which is easy to increase the pump consumption, and when the battery size is large or the coolant flow rate is low, the temperature difference between the inlet and the outlet is large, which is not conducive to the balanced distribution of the battery temperature.
Yuan et al. used the numerical simulation method to analyze the cooling of the battery by the cold plate liquid cooling system when the cooling liquid inlet and outlet are distributed on the same side and different sides respectively. The velocity distribution of the coolant in the flow channel is shown in Figure 1, and the main channel connects four branches. The results show that the flow field is more uniform when the inlet and outlet are on the same side. When the inlet and outlet are on the same side, the flow rate of the coolant in channels 1 to 4 decreases sequentially. Square batteries are generally arranged on the same side of the positive and negative ears. In addition, during the charging and discharging process of the battery, the temperature of the electrodes is higher than that of other parts. Therefore, when the coolant inlet and outlet are on the same side, the battery tab (ie, the high temperature end of the battery) points to the flow channel 1, and the low temperature end of the battery points to the flow channel 4, which can reduce the temperature difference of the battery and improve the balance of the battery temperature.
2. Multi-in and multi-out flow channel
In the multi-in and multi-out flow channel plate liquid cooling system, there are two or more coolant inlets and outlets. When the size of the battery is large, a single-in, single-out flow channel is used. The larger the cooling fluid flow rate, the smaller the battery temperature difference, but at the same time the pump consumption is also larger. In order to reduce flow resistance and reduce pump consumption, multi-in and multi-out flow channels can be used. The disadvantage of the multi-in and multi-out flow channel is that the more coolant inlets and outlets, the more complex the system, and the greater the possibility of liquid leakage.
Xu Xiaoming and Zhao Youqun from Nanjing University of Aeronautics and Astronautics selected a plate-type liquid cooling system with dual-in and dual-out flow channels to conduct heat dissipation research on battery modules with 2 parallel and 12 strings. In the battery cooling system they designed, the coolant circulation path is: water pump - pressure gauge - flow meter - radiator - pressure gauge - water cooling plate - pressure gauge - valve - water tank - pressure gauge - water pump, deionized water was used as the coolant in the experiment. Since the temperature of the water pump is high during operation, it is placed on the water tank to reduce the temperature of the water pump. When building a liquid cooling experimental system, the possible liquid leakage and overpressure of the system should be fully considered, and the stability of the liquid inlet flow should also be considered. Subsequently, they also designed plate-type liquid cooling systems with single-in-single-out, three-in-three-out, and six-in-six-out runners. According to the principle of field synergy, as the number of coolant inlets and outlets increases, the speed uniformity of the cold plate is better, and the heat dissipation performance is gradually improved.
Liu of Tianjin University and Chen of the University of Michigan used silicone oil as a cooling medium to cool the Li-ion battery pack of an electric vehicle. The battery pack consists of 20 square cells connected in series with a total capacity of 20A·h. The cooling channel is arranged in the aluminum plate close to both sides of the battery pack, and the heat is taken out through the convection heat transfer between the silicone oil and the cold plate. The results show that when the Reynolds number is 1150, the ambient temperature is 20℃, and the discharge rate is 2℃, the temperature of the battery near the cooling channel is lower than 30℃ after discharge, while the temperature of the battery in the middle will reach 35℃.
The square Li-ion power battery cooling system based on plate microchannels, the cold plate material is made of aluminum with high thermal conductivity, the cooling liquid is liquid water, and the battery is discharged at a high rate of 5C at a constant current. The simulation results show that due to the high discharge rate, in the absence of a cold plate, that is, the battery is cooled by natural air convection, the maximum temperature of the battery after 720s of discharge reaches 77.33℃, and the local temperature difference is as high as 13.19℃; after adopting the two-channel cold plate liquid cooling system, when the inlet flow rate is 5×10-6kg·s-1, the maximum temperature of the battery drops to 63.43℃, and locally decreases to 9.6℃. In addition, the simulation results show that the performance of the liquid cooling system based on plate microchannels is affected by the number of microchannels, liquid inlet and outlet positions, liquid inlet flow and ambient temperature. The 6-channel liquid cooling system can meet the requirements shown in Figure 2 when the liquid enters from the pole side at a flow rate of 5×10-4kg·s-1. Figure 3 is a schematic diagram of different liquid flow directions, and the arrows point to the liquid flow directions. Figure 4 shows that when the inlet flow rate of each channel is 5×10-6kg·s-1, the battery is also continuously discharged for 720s at a rate of 5C. It can be seen from Figure 4 that the heat dissipation performance and temperature uniformity capability of Design 2 are the worst among all structures. At 720s, the maximum temperature of the battery is 63.28℃, and the local temperature difference is 13.94℃. Compared with design 2, design 1 shows better heat dissipation performance and temperature uniformity. The maximum battery temperature (58.40℃) of design 1 after discharge is 4.88℃ lower than that of design 2. However, the local temperature difference of the battery in design 1 is not the smallest among all structures, and the smallest local temperature difference is in design 3, which is 9.02℃. The inlet flow was increased to 5×10-4kg·s-1, and the results obtained are shown in Figure 5. It can be seen that after increasing the flow, the effect of the flow direction on the battery temperature becomes smaller. Compared with other structures, the maximum temperature of the battery in Design 1 after discharge is still the lowest among all structures, which is 30.61℃. In addition, after the inlet flow rate becomes 5×10-4kg·s-1, the local temperature difference of the battery of Design 1 becomes the minimum value among all structures, which is 4.94℃.
Figure 2 - The maximum temperature change curve of the battery under different inlet flow rates
Figure 3 - Schematic diagram of different liquid flow directions
Figure 4 - The maximum temperature change of the battery under different flows (the flow rate is 5X10-4kg·s-1)
Figure 5 - The maximum temperature change of the battery under different flows (flow is 5X10-4kg·s-1)
Because only using liquid water to cool the battery is difficult to ensure the normal use of the battery in extreme environments, Huo Yutao et al. used Al2O3-water nanofluid with a volume percentage of 1% as the cooling medium. The heat production of the battery is obtained from the experimental fitting, and the fitting result is shown in Figure 6. After 720 s of discharge, the maximum temperature of the cell decreased with the increase of the nanofluid inlet flow rate, as shown in Figure 7. The results show that in extreme working environments, in order to ensure the normal use of the battery, it is necessary to combine active cooling to reduce the temperature of the liquid at the inlet of the channel.
Figure 6 - Fitting of battery 5C discharge heat generation
Figure 7 - The maximum temperature change of the battery under different inlet flow rates
3. Serpentine channel cold plate
In the plate-type liquid cooling system based on the single-inlet-single-outlet flow channel, the flow resistance of the cooling liquid increases with the increase of the number of pipeline loops, but due to its simple structure, it has always attracted the continuous research of many scholars. As shown in the figure above, it is a cross-sectional view of the single-in, single-out serpentine runner cold plate (symmetrical along the xy plane). The flow channels are not distributed in parallel in the plate, but zigzag in a serpentine shape, can avoid the phenomenon that the cooling effect of the battery near the inlet end of the cooling liquid is good, and the cooling effect of the part near the outlet end is not good due to the low temperature of the cooling liquid at the inlet end and the high temperature at the outlet end.
For the structure of the serpentine channel, it can be reasonably designed according to the heat generation characteristics of the battery and the law of heat transfer and distribution. Jarrett and Kim from Queen's University in Canada conducted a numerical analysis of the battery heat dissipation characteristics of the serpentine channel cold plates with eight different structures as shown in Figure 8, found that the width of the cooling liquid inlet and outlet, the shape and distribution of the flow channel, etc. have a great influence on the temperature distribution characteristics of the battery. Even if the maximum temperature of the battery is not very different, the temperature distribution of different parts of the battery may be different due to the different structure of the serpentine channel. Therefore, in the design process of the serpentine channel structure, both the cooling of the battery and the uniform temperature of the battery should also be considered. In addition, Jarrett and Kim also obtained the optimized channel structure for the minimum cold plate average temperature, the minimum cold plate temperature standard deviation, and the minimum channel inlet and outlet pressure drop, respectively.
Based on the three optimal structures mentioned above, Jarrett and Kim studied the effects of heat flow distribution, heat flow size and channel inlet flow on the average temperature Tavg of the cold plate, the standard deviation of the cold plate temperature Tσ and the pressure drop Pfluid at the inlet and outlet of the channel.
According to the partial results simulated by Jarrett and Kim, the following three laws can be obtained.
(1) As the heat flow into the cold plate increases, the pressure drop at the inlet and outlet of the channel decreases, while the average temperature and temperature standard deviation of the cold plate increase.
(2) The increase of the inlet mass flow increases the pressure drop at the inlet and outlet of the channel, while the average temperature and temperature standard deviation decrease.
(3) Different heat flow density distributions will affect the pressure drop and temperature distribution at the inlet and outlet of the cold plate, but in practice, the heat generation of the battery is a complex process, and the channel design of the cold plate needs to be selected according to the actual situation.
4. Ultra-thin inner inclined fin microchannel liquid cooling plate
In the traditional straight channel cold plate, along the flow direction, the flow gradually changes from the inlet section to the fully developed section, and the convective heat transfer coefficient gradually decreases, resulting in an increase in the maximum temperature and an increase in the temperature gradient. On this basis, Jin et al. of the National University of Singapore and Xi'an Jiaotong University designed an ultra-thin inner inclined fin microchannel cold plate (LCP). When the flow rate and load are 0.1L·min-1, 220W and 0.9L·min-1, 1240W, respectively, the designed cold plate can reduce the surface temperature of the heaters on both sides to below 50℃. The internal structural dimensions of the cold plate are shown in Figure 9. The designed LCP consists of two plates, each of which contains the same size and number of inclined fins, as shown in Figure 9(a); the bevel angle and width of the inclined fins are optimized. The flow channel generally presents a U-shaped structure, as shown in Figure 9(b). Figure 10 reflects the two-dimensional simulation results of the flow distribution inside the microchannel cold plate in the case of double-in and double-out. It can be seen from the figure that, except for a few areas, the flow is relatively uniform as a whole.
The inclined fins in the cold plate break the fully developed end of the flow, so that the flow boundary layer develops periodically, and the bifurcated flow in the flow process accelerates the diffusion of heat to the center of the flow. The convective heat transfer coefficient of the flow inlet end is higher than that of the fully developed end, so the inner inclined fin microchannel cold plate has a higher heat transfer coefficient, and the flow of the T-shaped split port is more uniform than that of the straight channel.