Passive Cooling of High Capacity Lithium-Ion batteries

Under the pressure of fossil fuel shortage and environment pollution, the world's industries are forced to shift their attention to green energy sources, transportation being the main actor in the consumption of fossil fuel. Therefore, increasing attention has been paid to the development of electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV) as environment friendly alternatives to the traditional internal combustion engine vehicles. In addition, an electric motor has a much higher efficiency (over 90%) compared to an internal combustion engine with an efficiency of 30%. The key of developing clean energy vehicles is to find a viable electric energy storage device with high specific energy density to support high driving mileage, as well as high specific power to support fast acceleration. Moreover, safety, cost and lifetime are important factors to take into consideration in the choice of the energy storage system. Taking into account all the cited parameters, lithium-ion batteries seem to be the best among other cell chemistries, which make them the preferred energy storage systems for vehicular applications. The top priority to be ensured when designing electric vehicles is safety. It is essential at every step from cell design, to module assembly and is very dependent on temperature. To obtain optimum performance, the operating temperature of Li-ion battery should be kept between 20 degrees C and 40 degrees C. Operating at higher temperatures deteriorates the performance, lifespan and safety of Li-ion batteries and may endure thermal runaway under extreme conditions. A battery thermal management system (BTMS) is therefore required to avoid thermal runaway and performance degradation of Li-ion batteries and to increase their lifespan. A passive battery thermal management system is developed and integrated in a high capacity battery module for cooling purposes. It is a cooling system with no energy consumption based on copper sintered heat pipes with water as working fluid. Heat pipes combine both the principles of phase change and thermal conductivity. In the evaporator section, the liquid present in the porous structure of the heat pipe wall turns into vapor by absorbing heat form the battery surface. The vapor flows along the heat pipe to the condenser section turning into liquid by releasing latent heat to ambient air. The heat transfer between the condenser and the ambient air are intensified by adding fins, thus increasing the exchange area. The studied cells have a capacity of 60 Ah and a nominal voltage of 3.2 V. In order to analyze the thermal behavior of the battery, several K-type thermocouples are placed on the battery surface as well as on the thermal management system. This paper presents the experimental and theoretical study of the battery module operating in repetitive charge/discharge cycles at different current rates as well as for driving cycles. A detailed description of the experimental setup and procedure will be presented.