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1. Square Li-ion battery system based on PCM heat dissipation
Rao et al. also established a heat dissipation system model as shown in Figure 1 to study the effect of PCM thermal conductivity on the heat dissipation of prismatic batteries. The single battery used in the system is a square lithium iron phosphate power battery (xyz, 6.3cm×1.3cm×11.8cm) with an initial capacity of about 8A·h. After being placed for one year, the charge-discharge cycle test was performed again for 5 times, and 9 batteries with a capacity of (7.1±0.1) A.h were selected to form a 3S×3P battery module, and the paraffin/graphite composite PCM was filled between the batteries. When measuring the temperature of the single cell, 5 thermocouples are placed in the center of the xz plane, the end of the near pole and the end of the far pole; when measuring the temperature of the battery module, 9 thermocouples were placed at the high temperature and low temperature points of the single cell, and only the maximum temperature and the minimum temperature of each single cell were recorded. Use BS-9360 series secondary battery performance testing device (Guangzhou Kinte Industrial Co., Ltd./Guangzhou Electrical Apparatus Research Institute) to charge the battery, and use YTD-3200 battery performance testing device (Zhejiang Yuantong Electronic Technology Co., Ltd.) to discharge the battery, the single battery charging current is 2A, the cut-off voltage is 3.8V, the discharge current is 15A, 20A, 25A, 30A, 35A, and the discharge cut-off voltage is 2.1V. The Agilent data acquisition instrument (34970A) was still used to collect the surface temperature of the battery, and the comparative experiment used air cooling with a wind speed of about 2m/s.
2. Influence of PCM thermal conductivity on heat transfer
The discharge current is 35A, and the PCM phase transition temperature is 50℃. When the heat is dissipated in two ways, air natural cooling and PCM cooling, the change of the maximum temperature inside a single battery with the discharge time is shown in Figure 2. It is assumed that the thermal conductivity of the battery and PCM is the same (kPCM:kc=3.0:3.0). At the end of discharge, the maximum internal temperature of the battery exceeds 70℃ when air is naturally cooled, while the maximum internal temperature of the battery is 54.94℃ when PCM is used for cooling. In the first 400s of discharge, compared with the natural cooling of air, the maximum temperature of the battery is slightly lower when the PCM is cooled, but the difference is not significant, and the local temperature difference of the battery is not significantly reduced. With the occurrence of PCM solid-liquid phase transition, the PCM absorbs heat but the temperature remains basically unchanged, the maximum temperature inside the battery rises slowly, and the local temperature difference of the battery gradually decreases. Compared with the 42110 type battery, the thickness of the square battery in the xz and yz directions is smaller than the radius of the cylindrical battery. Due to the small thermal resistance, the maximum temperature inside the battery is only 4.94℃ higher than the PCM phase transition temperature. Due to the existence of the battery's own thermal resistance, when designing the battery thermal management system, it should be noted that the phase transition temperature of the PCM must be lower than the target temperature of the maximum temperature control inside the battery.
Figure 2 - Comparison of battery temperature changes with and without PCM
In the PCM heat dissipation model of cylindrical battery, the temperature change of the battery was studied when the thermal conductivity of PCM increased from 0.2W·m-1·K-1 to 3.2W·m-1·K-1. For the square battery, first, the thermal conductivity of the PCM remains unchanged, the initial thermal conductivity of the battery is recorded as kc, and the thermal conductivity of the battery is changed, and the thermal conductivity of the battery after the change is recorded as k'c. At the same time, the thermal conductivity of the battery remains unchanged, and the thermal conductivity of the PCM is changed. As shown in Figure 3, when 0<k'c: kc<0.5 and 1≤kPCM:kc≤10, the change of the maximum temperature inside the battery and the local temperature difference. By increasing the thermal conductivity of the battery itself, the maximum temperature inside the battery and the local temperature difference are significantly reduced. When the thermal conductivity of PCM is larger than that of battery, with the continuous increase of thermal conductivity of PCM, the decreasing trend of the maximum temperature inside the battery and the local temperature difference gradually slows down. The doubling of the thermal conductivity of PCM accelerates the heat transfer process in the solid-state PCM, but when the PCM undergoes a solid-liquid phase transition, the PCM temperature remains constant. Although the heat can be transferred from the inner PCM to the outer PCM in time, due to the limitation of the natural convection heat exchange between the outer PCM and the external environment, the maximum temperature inside the battery and the local temperature difference are not significantly reduced. Therefore, it can be seen that the key to reducing the internal and surface thermal resistance of the battery is to improve the thermal conductivity of the battery, and the improvement of the thermal conductivity of the battery involves the change of the physical properties of the material and the reduction of the battery capacity and other key parameters. When the battery is given, the enhanced heat transfer of the battery thermal management system is mainly realized by enhancing the heat transfer of the PCM. When the thermal conductivity of the PCM is greater than the thermal conductivity of the battery, increasing the thermal conductivity of the PCM cannot significantly enhance the heat dissipation of the system.
Figure 3 - The relationship between battery temperature change and PCM and battery thermal conductivity
The module adopts air natural convection cooling for heat dissipation, and the discharge current is 35A. When the battery electrodes are oriented in the same direction, it is easy to connect, but due to the local temperature difference between the proximal and distal ends of the prismatic battery, after the batteries are assembled into modules and battery packs, the local temperature difference of the entire module will further increase. In addition, after the electric vehicle is used for a long time, the unevenness of each single cell will increase, and the unevenness of heat generation will further increase the local temperature difference in the battery module. After the heat is transferred from the high temperature end of the battery to the PCM, due to the thermal resistance of the battery itself, the heat transfer in the PCM is enhanced, so that the heat is transferred from the PCM to the low temperature end of the battery, and the local temperature difference of the battery module may also be reduced.
When the thermal conductivity of battery and PCM kPCM:kc =4, the local temperature difference of the module approaches 5.0℃, y=0.023m, and the temperature distribution of the PCM at the xz section (-0.04m<x<0.04m) is shown in Figure 4. It can be seen from the figure that the temperature of the PCM near the electrode is significantly higher than that at the other end, but the maximum temperature difference is less than 0.8℃. At this time, since the thermal conductivity of the PCM is much greater than the thermal conductivity of the battery itself, after the PCM undergoes a solid-liquid phase change and absorbs heat, the heat transfer rate in the PCM is faster, which helps the heat transfer to the low temperature end of the battery.
Figure 4 - Temperature distribution inside PCM
Select the single battery with the largest heat generation in the module, when the thermal conductivity of the battery and PCM is kPCM:kc=4, y=-0.023m, at the xz section (-0.0315m<x<0.0315m), the temperature distribution inside the battery is shown in Figure 5. The maximum local temperature difference of the battery is 4.6℃. Since the heat in the PCM is rapidly transferred at the battery electrode end, the rapid accumulation of heat at the battery electrode end is avoided. The PCM with high thermal conductivity showed good cooling and temperature uniformity performance in the battery module.
Figure 5 - Temperature inside the battery