The contact area between the evaporation end of the sintered heat pipe and the square battery increases with the increase of the flattened thickness of the heat pipe, but the increase of the flattened thickness of the heat pipe will increase the heat transfer resistance; in addition, the failure of a certain heat pipe will increase the unbalanced heat distribution of the battery. The continuity in the structure of the pulsating heat pipe makes it possible to reduce the uneven heat distribution caused by the performance difference of different single heat pipes. And the pulsating heat pipe also has the advantages of small size, simple structure (no capillary core), low cost, good adaptability (the shape can be bent arbitrarily) and good heat transfer. The pulsating 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.
Figure 1 shows a battery cooling system in which a phase change material is coupled with a pulsating heat pipe. The pulsating heat pipe is sandwiched between the two batteries, and the phase change material (PCM) is filled into the space between the two batteries and the heat pipe, and the effect of the change of the battery electrodes on the heat dissipation of the system is studied. Since the PCM/OHP heat dissipation system has both the solid-liquid phase change heat storage of PCM and the liquid-vapor phase change heat transfer of the working fluid in the pulsating heat pipe, therefore, under each working condition, the cooling effect of the PCM/OHP cooling system is the best. When the heat generation power of the battery is the same, the battery electrode is close to the adiabatic section of the pulsating heat pipe, and the heat at the high temperature end is quickly dissipated through the pulsating heat pipe to avoid the local temperature of the battery being too high, thereby extending the time for the maximum temperature of the battery to rise to the target temperature, and at the same time reducing the maximum temperature of the battery at the end of discharge. Since the performance of the pulsating heat pipe used in the experiment does not fluctuate much in the battery scale, when the electrode orientation is the same, changing the placement method of the pulsating heat pipe has no obvious effect on the time when the maximum temperature of the battery in each system rises to the target temperature and the maximum temperature of the battery at the end of discharge.
Figure 1 - Battery cooling system coupled with phase change material pulsating heat pipe
In addition, PCM is used as a heat storage medium to speed up the transfer of heat 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., Wang et al. 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 bulky, and the use of heat sinks cannot meet the requirements of electric vehicle space compactness and installation flexibility. Combining the high latent heat of the PCM with the high thermal conductivity of the 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. Bogdan et al. established a mathematical model of an energy storage system coupled with a gravity heat pipe and PCM, and mainly analyzed the interface changes during the solid-liquid phase transition of PCM. In the experiment of Riffat et al., the sintered heat pipe was placed horizontally for heat dissipation of the thermoelectric cooling system. Compared with the traditional heat sink, the cooling coefficient (coefficient of performance) was significantly improved. Nithyanandam and Pitchumani used the thermal resistance network model to analyze the influence of parameters such as the diameter of the heat pipe, the length of the evaporation end and the condenser end on the energy storage efficiency of PCM; Shabgard et al. also established two thermal grid models and extended the corresponding analysis to the high temperature field, that is, PCM used in solar thermal power generation systems. Robak et al. compared the effects of heat pipes and finned tubes on the melting rate of PCM through experiments and found that the former is significantly better than the latter. Weng et al. compared the heat transfer between sintered heat pipe/PCM (tricosane) and traditional heat pipes for cooling electronic devices, and found that adding PCM can reduce fan power consumption by 46%. In their experiments, the PCM was wrapped in the adiabatic section of the heat pipe. As mentioned above, the performance of the PCM/heat pipe coupled heat transfer system is significantly better than the energy storage or cooling system using only PCM or heat pipe.