Research Of Cooling System Battery Pack On Hybrid Electric Bus

With the graveness of environment pollution and energy shortage, researches of hybrid electric vehicle (HEV) are getting more and more attention. The research of cooling system of vehicle batteries, which are one of the power sources of HEVs, has great value because the working performance and cycle life of battery are highly influenced by their operating temperature.This dissertation gives a research on the cooling system of a SWB6116HEV hybrid electric bus which uses LiFePO4 batteries. According to the design requirements of thermal management and the temperature property of LiFePO4 batteries, the cooling system is analyzed with CFD simulation and the corresponding experiment is carried out. The main work of this dissertation can be concluded below:(1) The temperature property of LiFePO4 batteries is analyzed. The experiment of a single battery is taken to measure its thermal parameters and a program is coded in LabVIEW to collect and record the temperature data automatically. The speed of heat generation which is treated as the function of current intensity and the equivalent special capacity of a single battery are acquired through the processing of experiment data. Besides, the orthotropic conductivities of the battery are calculated on the principle of series and parallel thermal resistance in heat transfer.(2) Current cooling system which uses natural air ventilation is simulated with CFD software Fluent and the corresponding experiment took on the hybrid bus is carried out to verify the simulation. The comparison of experiment data and simulation results shows that they agree with each other. This reveals that the thermal fluid field can be predicted through CFD simulations. In addition, both results of experiment and simulation show that the cooling system doesn't meet the demands of thermal management of battery pack in the condition of extreme weather in summer so that the cooling system has to be improved.(3) Three improvements are taken in order to reduce the resistance of flow and increase the amount of air.①Add cooling fans at the end of battery pack and take the cooling air from the carriage of hybrid bus.②Improve the structure of the board in battery pack.③Widen the tunnels between batteries. The effects of these three improvements above are simulated by CFD computation. The results show that the maximum temperature in the battery pack will be lower than 40℃which means that the LiFePO4 batteries could work in their best range of operating temperature.(4) The peak value of heat generation speed, which is appeared when the batteries are under heavy loads, is calculated with the experiment data of urban cycles. According to the results of steady simulation which is based on the peak value of heat generation speed above, the cooling system is optimized once more by adding an additional cooling fan at the end of battery pack. The additional fan only works when the battery pack is under heavy loads in order to increase the capability of cooling system temporary. Steady and unsteady computations are taken to simulate the decreasing process of temperature in the battery pack. The results show that the optimization has a good effect and give a threshold value of temperature using to control the additional fan.