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Typical battery thermal management systems based on phase change materials are as follows:
1. PCM/Aluminum foam battery thermal management system
The battery thermal management system based on PCM/aluminum foam was first proposed by Al-Hallaj et al. of Illinois Institute of Technology and applied to electric scooters. Figure 1 is a physical diagram of the battery thermal management system based on PCM/aluminum foam that they used in their experiments. Among them, (a) consists of 18 (3S×6P:S, Series; P, Parallel) single cells, and (b) consists of 24 (3S×8P) single cells. The single battery uses 18650 lithium-ion battery with a nominal capacity of 2.2A·h.
The main experimental conditions in the experiment are set as follows.
Charging process: The battery is first charged in constant current mode until the cut-off voltage of the single battery reaches 4.2V, and then switches to constant voltage mode charging. When the charging current of the single battery drops to 50mA, the charging is stopped. After the battery pack is fully charged, set aside for 1 hour.
Discharge process: The battery is discharged in constant current mode, and the cut-off voltage of the single battery is 3V. After the discharge is over, the battery pack is put on hold for 1h, and then recharged to enter the next cycle.
The battery pack was tested under four heat dissipation conditions, namely, natural convection, only foamed aluminum filled between batteries, PCM filled between batteries, and PCM/foamed aluminum filled between batteries. Taking a 3S×6P battery pack (voltage 12V, capacity 13.2A·h) as an example, after the battery pack has completed 5 charge-discharge cycles, the maximum temperature rise of the battery pack at different discharge rates is shown in Table 1. This shows that under the dual effects of the high latent heat of PCM and the high thermal conductivity of aluminum foam, the composite material can effectively reduce the temperature of the battery pack.
Table 1 - Maximum temperature rise of battery pack under different discharge rates
2. PCM/Copper foam battery thermal management system
As shown in Figure 2, the battery thermal management system based on PCM/copper foam (CF). The single battery adopts 42110 type lithium iron phosphate (LiFePO4) power battery with a nominal capacity of 10A·h. The battery pack contains 24 single cells. In order to avoid excessive local temperature in the middle of the battery pack, the batteries are arranged in a crossed arrangement of positive and negative electrodes. The battery pack voltage is 38V (12S×2P) and the capacity is 20A·h.
Figure 2 - Battery thermal management system based on PCM-copper foam
When preparing PCM/foamed aluminum, PCM/foamed copper and other composite materials, due to the small pore size of the foamed metal, the ordinary perfusion method will cause a large number of air bubbles to remain in the through-hole of the foamed metal due to the high viscosity of the liquid paraffin. This not only greatly reduces the charging amount of the PCM in the heat storage device, but also the residual air bubbles will cause a large thermal resistance and reduce the heat transfer performance of the entire battery thermal management system. Therefore, in the process of preparing the above-mentioned composite materials, a vacuum infusion process is generally used. The devices used in the vacuum infusion process mainly include a wax melter, a heating rod, a tee, a vacuum pump joint, a shell, a heating seat, etc. Two holes are opened on the shell, one hole is connected with a vacuum pump to vacuum the foam metal, and the degree of vacuum (Pa) is less than or equal to 10.0; the other hole is used to perfuse PCM such as paraffin. Vacuum the foam metal first and then pour the melted PCM into it. After vacuuming the top, pour it from bottom to top. The lower end of the entire housing assembly also uses a heating seat to heat the poured PCM to prevent curing.
3. PCM/expanded graphite battery thermal management system
Al-Hallaj et al. of Illinois Institute of Technology also designed a battery thermal management system based on PCM/expanded graphite, as shown in Figure 2, the experimental device based on the PCM/expanded graphite battery thermal management system used in their experiments. The single battery uses 18650 lithium-ion battery with a nominal capacity of 2.4A·h; the capacity of the battery pack (7S×2P) is 4.8A·h. The thermal conductivity of the PCM/expanded graphite composite is 16.6W·m-1·K-1, the latent heat of phase transition is 185kJ·kg-1, and the specific heat is 1.98kJ·kg-1·K-1, the bulk density of the composite is 789kg.m-3. Thermocouples measure the temperature of the middle (T1) and surface (T2) of the battery pack, respectively. The experimental results also prove that the use of PCM/expanded graphite has a better thermal management effect of the battery.
For the above-mentioned PCM/expanded graphite composite material for the battery thermal management system based on PCM/expanded graphite, Alrashdan et al. also specifically tested the thermomechanical properties of the composite material. The PCM/expanded graphite composite still maintains good mechanical properties after soaking at 22℃ for 12h, and the mechanical strength, thermal conductivity, and composite bulk density are improved to a certain extent compared with those before soaking. At the same time, it was noticed that after immersion at 45℃ for 12 h, the mechanical strength indexes of the PCM/expanded graphite composites decreased to varying degrees except the compressive strength. This shows that the working temperature of the PCM/expanded graphite battery thermal management system should not be too high, otherwise the mechanical strength of the PCM/expanded graphite composite will be seriously affected.
4. PCM/oscillating heat pipe battery thermal management system
PCM is used as a heat storage medium, in order to speed up the heat transfer process to the outside world, in addition to improving the thermal conductivity of the material itself, heat transfer can also be enhanced by inserting inserts into the structure. Akhilesh et al., Shatikian et al., Xiang-Qi Wang et al. all studied the heat transfer performance of heat sinks used for heat dissipation of electronic equipment when PCMs were built with aluminum fins. Compared with microelectronic devices/devices, the heat flux density of batteries is small but the volume is large, and the use of heat sinks cannot meet the requirements of electric vehicle space compactness and installation flexibility. Oscillating heat pipes (OHP) have the advantages of structural continuity, small size, simple structure (no capillary core required), low cost, good adaptability (the shape can be bent arbitrarily) and good heat transfer. The oscillating heat pipe can work at any inclination angle and heating mode, which further increases the matching ability of the battery pack with electric vehicles and other power equipment. Combining the high latent heat of the PCM with the high thermal conductivity of the oscillating heat pipe is expected to not only satisfy the heat dissipation without battery power consumption, but also further increase the compactness between the battery cells and battery modules in the battery pack.
Figure 4 is a schematic diagram of a rectangular battery thermal management system based on PCM/oscillating heat pipes (OHP). The evaporation end of the oscillating heat pipe is sandwiched between two adjacent cells, and the PCM is filled into the space between the two cells and the heat pipe. The condensing end of the heat pipe extends out of the battery and transfers the heat of the battery to the external environment. In the PCM/oscillating heat pipe battery thermal management system, the start-up temperature of the oscillating heat pipe is determined by the target temperature for battery heat dissipation and the target temperature difference. To meet the requirements of battery cooling and heat distribution balance, the start-up temperature of the oscillating heat pipe must be lower than the target temperature, and not higher than the maximum temperature of the battery when the local temperature difference of the battery reaches the target temperature difference.
