Schematic diagram of the hybrid electric vehicle on which the proposed battery thermal management system is employed. Al-Zareer et al. Click to enlarge.

Temperature has an impact on the life cycle of a Li-ion battery, as well as its efficiency, safety and reliability. During charging and discharging, thermal energy is generated inside the battery, causing the battery temperature to increase. If internal heat generation is not removed from the battery, its temperature can reach relatively high levels resulting in electrolyte fires and battery explosions. The best operating temperature range for Li-ion batteries is between 25 ˚C and 40 ˚C, and the temperature distribution throughout the battery and from one module to another should be less than 5 ˚C.

Currently the main battery thermal management systems can be classified into three main types: liquid cooling, air cooling and phase change material (PCM) cooling. Air cooling and the liquid cooling systems can be further classified into two types: passive and active. PCM cooling system are usually passive systems. Because of the simplicity of the air cooling structure for EVs and HEVs, as well as the low cost and easy maintenance compared to other cooling systems, air cooling is one of the most widely used battery thermal management systems in EVs and HEVs. The Toyota Prius, the Honda Insight and the Nissan Leaf have used air cooling based thermal management systems.

… An air cooling thermal management system for battery packs is sufficient in many cases. However, during severe charging and discharging conditions, an air cooling system cannot meet the cooling requirements. A liquid has a higher thermal conductivity than air, which results in better cooling performance and which is better suited for large scale battery pack cooling applications.

… The use of phase change materials in battery thermal manage- ment systems is a recent development in battery cooling systems, and involves using the latent heat of the phase change to absorb the thermal energy generated by the batteries in the module. … In this paper, a PCM cooling system based on pressurized propane, changing phase from liquid to vapor, is introduced to manage the temperature of a battery pack of Li-ion batteries for hybrid electrical vehicle applications. The advantages of the proposed system are constant temperature phase change of the PCM system, the liquid cooling system having a high heat transfer coefficient and the air cooling system not needing an additional heat exchanger to reject heat to a heat sink.

—Al-Zareer et al.

For the most recent study, the team established a numerical thermoelectrochemical model to investigate the performance of the cooling system performance. The researchers investigated the effect of the level of the liquid in the battery pack and the effect of the saturation pressure of the liquid propane on the temperature distribution throughout the cylindrical battery.

The cylindrical cells array is partly submerged in the liquid propane. The thermal energy generated by the cells is transferred to the propane, cooling the batteries in the pack. The thermal energy generated by the cells is removed by the surrounding propane in two heat transfer modes.

The first mode involves boiling the liquid propane, where the level of the propane in the battery pack is maintained by the propane tank in the HEV. The propane in the vapor phase, which results from boiling of the liquid propane produces an upward flow due to the buoyancy force and the pressure difference made by the propane injectors, which inject the vapor propane into the combustion chambers of the electrical generator in the HEV.

Before the propane injectors there is a regulator tank that stores the propane vapor before sending it to the injectors. The upward flow of the vapor propane in the battery pack cools the reaming part of the batteries that are not submerged in the liquid propane.

Based on the results of their study, the team concluded:

• The propane-based cooling system provides good cooling performance, maintaining the temperature of the battery in the acceptable operation range. Covering only 5% of the total length of the battery by saturated liquid propane at a pressure of 8.5 bar maintains the maximum temperature of the battery below 39 ˚C for the charging and discharging cycles at a high rate of 7.5C for 600 s. The more propane covers the battery length, the lower is maximum temperature achieved. Covering 30% of the battery length by saturated liquid propane keeps the temperature below 34 ˚C.

• The temperature difference across a single battery in the pack can be maintained at less than 18 ˚C by covering only 5% of the battery length with saturated liquid propane. Increasing the percentage of battery length covered by propane reduces the temperature difference across the battery.

• Increasing the pressure of the saturated liquid propane from 8.5 bar to 10 bar, reduces the temperature difference across the battery. However, this also increases the maximum temperature in the battery. Increasing the pressure of the saturated liquid propane has a greater effect on the maximum temperature and the temperature difference in the battery for low liquid propane heights in the pack.

Resources

• Maan Al-Zareer, Ibrahim Dincer, Marc A. Rosen (2017) “Novel thermal management system using boiling cooling for high-powered lithium-ion battery packs for hybrid electric vehicles,” Journal of Power Sources, Volume 363, Pages 291-303, doi: 10.1016/j.jpowsour.2017.07.067