On the design and advantages of a vapor chamber for battery packs and the discrepancy in vertical channel boiling correlations at small gap spacing

Abstract

The recent trend of increasing demand for rechargeable energy storage systems leads to the possibility of high power and energy electrical batteries. Especially for mobile applications, minimizing size and weight of the batteries is important, hereby increasing the power and energy density. This leads to higher heat losses, created by the internal resistance of the battery. During discharge, the chemical reactions in most batteries are exothermic which causes an even greater increase in heat generation. To avoid excessive temperatures, which can lead to lower efficiency, reduced lifetime and battery failure, adequate thermal management is needed. This paper introduces battery thermal management for prismatic cells through a vapor chamber. The battery cells are immersed in a non-conducting liquid, contained in a closed vessel. By absorbing heat from the battery, the liquid evaporates at the surface of the battery. The evaporated gas is condensed at the top of the vessel, by releasing heat to the environment. A lumped parameter model is made to simulate the response of a vapor chamber system on several load cycles and to compare with air cooling. Due to the evaporating heat transfer at the battery surface, the local heat transfer coefficient can be increased up to a factor 10 compared to air cooling. Load cycles of batteries often consist of peak currents being drawn for short times. This can cause a temporary increase in battery temperature. To avoid these peak temperatures, thermal buffering has been studied, for example by using the latent heat of melting PCMs. However this also results in an increase of the thermal resistance of the battery system. In a vapor chamber, the latent heat of evaporating refrigerant is used as a thermal buffer, without having a significant increase in thermal resistance. Analysis shows that peak temperature differences between the battery and the environment can be reduced by a factor up to 2 compared to air cooled batteries. The most important factor in modelling and designing the vapor chamber is the heat transfer coefficient of nucleate boiling in vertical rectangular channels and the influence of the distance between the cells on the heat transfer coefficient. Measurements and experimental correlations from literature do not correspond for small channels, where some perceive or predict increased heat transfer while others do not. An experimental setup is designed and being built to experimentally test the heat transfer coefficient for narrow vertical rectangular channels.