Performance Investigation Of The Power Battery Thermal Management System Using Expanded Graphite Based Composite Phase Change Materials

Lithium ion batteries have been gaining recognitions as the primary technology for energy storage in electric vehicles and other motive applications. However, a significant amount of heat could be generated from lithium ion batteries, which may cause losses of capacity, shortage of cycle life and thermal runaways, and be accompanied by fires and explosions. An efficient thermal management system guarantees a high operation performance of power batteries, and the thermal management system using phase change materials(PCMs) is of the most promising thermal control technologies – taking advantages of the high latent heat and small temperature changes during a phase change, PCMs are capable of controlling the battery temperature at a specific value for a long period. Compositing PCMs with expanded graphite(EG) could greatly enhance their thermal conductivity, which helps significantly mitigate the temperature difference inside a battery pack and ensure that all battery cells be degraded at a similar rate so that the cycle life of the battery pack could be prolonged.Thermophysical properties of RT44HC/EG composites that are used in the power battery thermal management system, are characterized and the impacts of compositions of the composite PCMs on its thermal conductivity are specially studied. The result shows the composite PCM retains high phase change enthalpy, while the thermal conductivity is greatly enhanced with EG by 20~60 times and increases with the EG mass fraction and density of the composite. A mathematic model, as a function of the EG mass fraction and the density of the composite, is presented to predict the thermal conductivity of the EG-based composite PCMs. The model is proved to be simple and accurate, and its application could be extended to predict the thermal conductivity of a wide range of organics/EG composite. Besides, variations of thermal conductivity of composite PCMs with temperatures have also been observed. A sharp increase of thermal conductivity is observed during the phase change, which could be twice as the value at room temperature, allowing PCMs to be a super thermal conductor at some temperature.Second, the effects of the thermophysical properties of EG-based composites on the thermal management performance are studied. We find that the best phase change temperature for the PCM used in Li-ion battery thermal management is between 40 and 45 oC- which should be neither too low nor too high. Before the density is too high to cause a liquid leakage, increasing the density of composite PCMs helps reduce the peak temperature and the maximum temperature difference in a battery pack simultaneously. We also point out that a slight reduction of PCM mass fraction for the composite PCM with high densities would further reduce the temperature rise rate and the temperature difference in the battery pack, which is against traditional opinions that the only way to keep batteries cool for longer period is to increase the PCM mass fraction.We notice that the completely passive thermal management system may suffer from failure after several charge-discharge cycles of batteries operated under high currents, as the heat stored in PCMs cannot be dissipated rapidly. Here forced air convection is introduced as a supplementary cooling to solve this problem. The experiment result shows the temperature of the battery pack is kept within the optimal range, as active cooling intensifies the heat transfer between the PCM and the environment. A further study on airspeed effect confirms that active cooling and PCMs play different roles: the PCMs control the maximum temperature rise and temperature difference in the battery pack while the active cooling helps fully recover the latent heat of PCM during the period between cycles. The structure of this hybrid thermal management system is simple, but it shows more stable performance in the long term than the passive systems.Beyond the experiment studies, numerical models have been also developed to study the mechanism of the heat transfer inside both the passive and active-passive hybrid battery thermal management systems. A battery heat generation model, a model of thermal energy storage inside PCMs and a model of the simplified heat transfer of forced air convection are integrated together to describe the heat transfer in this activepassive hybrid thermal management system. The model is validated with experiment data and is used to theoretically analyze the roles played by active cooling and PCMs. Besides, a simplified thermal networks model to simulate the heat transfer in PCMs is also presented and validated, for the first time. Compared with numerical model, this model is accurate but much easier to compute, which could save the computation time by up to 98%.At last, an optimization method that combines the numerical model, central composite design of experiment and response surface methodology is presented to the minimize the weight and volume of the thermal management system with PCMs. A sensitivity analysis based on this method shows the battery configuration, constituents of composite PCMs and the location where the active cooling is applied and its intensity have influences on the performance of the battery thermal management system that combines PCMs and active cooling. After balancing all of those effects, the optimized designs are presented – which maintain high levels of performance while the mass and volume of the PCM being greatly reduced by 53~85% and 17%~56%.