| Large capacity power Li-ion batteries are becoming preferred energy storage devices for electric vehicles, fuel cell vehicles and hybrid electric vehicles. Currently, it has drawn attention extensively, within the world-wide range. The electrochemical performance of the Li-ion battery chemistry, power and energy capability, cycle life and cycle life cost is impacted very much by the operating temperature. There exists an optimum operating temperature range with any kind of battery. High temperature rise and temperature imbalance during the charging and discharging processes of batteries are the main factor leading to the failure of battery pack. Therefore, thermal energy management is essential for improving thermal safety performance of the power batteries. The traditional PCM, such as paraffin, is taken as the most promising because of large latent heat, nontoxic, not corrosive, stable and low cost.Li-ion battery cooling experiment system is built to study the surface temperature of Li-ion battery at various discharge rates, thermal management systems and environment temperatures. The experimental results indicate that the higher the discharge rates, the higher the surface temperature of Li-ion battery. When the battery is discharged at high rate in low-temperature environment, pure paraffin have a bad cooling effect on Li-ion battery because of its low thermal conductivity. Paraffin/aluminum foam composite PCM has an ideal effect in limiting the temperature rise of the Li-ion battery during the discharge process. It always has a better effect on cooling battery than pure paraffin, whether at normal temperature or low-temperature environment.The surface temperature of the Li-ion battery is controlled by phase change heat storage process of PCM. Hence, it is necessary to investigate the heat storage properties of the composite PCM itself, such as the inner temperature distribution and the phase change time. To figure out how the addition of aluminum foam affects the heat storage process of PCM, an experimental system was built to measure the internal temperatures of pure paraffin and paraffin/aluminum foam composite PCM. The effect of the addition of aluminum foam on temperature uniformity along different directions, response time of phase transition and the heat storage rate of the PCM was discussed. The mechanisms of heat diffusion and natural convection dominate the melting process. The enhancement of heat conduction is more prominent than the suppression of natural convection of molten liquid by aluminum foam in the composite PCM, whereas natural convection prevails in the pure paraffin. The addition of aluminum foam largely improved the respond rate of phase transition in the composite PCM, especially in the direction of heat flow and the height direction, so that the heat storage capacity of PCM can be utilized more quickly and fully. According to the experimental results of heat storage process, the temperature distribution in the composite PCM is more uniform than that in pure paraffin. The experimental results also indicate that the addition of aluminum foam can accelerate the melting process. When the heat fluxes are7000W/m2,12000W/m2and15000W/m2, the addition of aluminum foam brings35.35%,22.14%,39.90%drop in the heat storage time, respectively.To further illustrate the phase transition mechanism of PCM, a1-D thermal model of phase change process is developed. The phase transition process of pure paraffin and the composite PCM is discussed, respectively. The inner temperature distributions in pure paraffin and the composite PCM during the phase transition are also obtained. A good qualitative and complete agreement is obtained between experimental and numerical results. |
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