| Environmental pollution and energy shortage have caused a growing demand for high energy density battery energy storage systems.Lithium-ion batteries are assumed to offer the greatest potential for electric vehicle due to their advantages of high specific energy and power,long cycle life,no memory effect and fast charging and discharging speed.However,Lithium-ion batteries are very sensitive to temperature.Uneven temperature will result in partial deterioration of batteries.Excessive temperature will not only reduce the service life of batteries,but also threaten the safety of batteries,and even cause permanent damage.Therefore,to prolong the cycle life of batteries and maximize the performance of batteries,it is necessary to adopt the battery thermal management system to make the batteries work efficiently.Firstly,the working principle and thermal conduction mechanism of Li-ion battery are analyzed briefly.Experiments were carried out on the battery,including the temperature rise of charge and discharge,the extraction of the thermophysical properties and the heat generation rate.Based on the determination of physical parameters,the finite element model of lithium ion battery was established,and the numerical simulation of different discharge current and heat transfer coefficient was performed.The simulation results of different discharge currents are compared with the experimental results,which suggested that the numerical results can reflect the actual temperature field of Li-ion battery accurately.Further,the research results of different heat transfer coefficients revealed that forced liquid cooling had the best heat dissipation performance.Secondly,a novel double-layered reverting tree-shaped channel structure was proposed to cool Li-ion battery due to the temperature gradient generated by the traditional channel structure(parallel straight channel and serpentine channel).The advantages of low power consumption and good heat transfer of the tree channel structure were highlighted by comparing the tree-shaped channel structure with the traditional channel structure.Then,the effects of the structural parameters(width ratio,length ratio,bifurcation angle and channel thickness)and the inlet mass flow rate on the thermal performance and flow loss of the double-layered tree-shaped channel were studied numerically,the optimizing surface of the structural parameters was obtained.It provided a guiding range for tree-shaped channel design.To reduce the difference of heat dissipation between the cells,the above-mentioned channel was improved to a symmetrical double-layer tree-shaped channel structure.Based on the sensitivity analysis of the structure parameters and the inlet mass flow rate to the performance of the channel,the optimal design of the liquid cooling system of the battery module was implemented by using Non-dominated Sorted Genetic Algorithm-Ⅱ(NSGA-Ⅱ).The transient performance indicators(convective heat transfer coefficient and skin friction coefficient)were chose to directly reflect the heat transfer process of the liquid cooling system.At the same time,the battery module was confined to the optimal temperature range,and the maximum temperature difference between the cells was within the optimal range.Since there is no direct functional relationship between objective function,constraint function and design variables,the performance indicator values of Latin hypercube sampling points were calculated numerically,and the response surface approximation surrogate model was adopted to approximate the objective functions and constraint functions.The Pareto representative solutions showed that the battery module cooling system with better comprehensive performance can be obtained by reasonable design of the structural parameters and the inlet mass flow rate. |
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