Temperature Control And Optimization Of Battery Thermal Management In Electric Vehicle

Power battery is one of the core components of electric vehicle,which is closely related to the safety,reliability,driving range and power performance of electric vehicle.With the continuous improvement of energy density of power battery,problems such as cycle life and charging and discharging performance of power battery greatly affected by temperature become prominent,especially in winter,summer and when the current is charged/discharged at high C-rates.Thus,it is urgent to research and develop a new and efficient battery thermal management system.In this paper,the battery thermal management system based on liquid cooling is studied from three aspects: single cell,battery module and thermal management system.Firstly,this paper studies the cylindrical lithium ion battery,and a models of two-dimensional and one-dimensional heat conduction problems with internal heat source and boundary conditions varying with time were established.By measuring the external temperature of battery,the model can achieve the estimation and prediction of the core temperature of battery.Because the analytical solution of two-dimensional problem is too complex,an approximate solution of one-dimensional problem is produced by truncating the series in the solution.The influence of initial condition,heat source,thermal boundary condition and thermal physical parameters on the core temperature change of battery is studied.The effect of initial condition on the core temperature of battery initially dominates,but it decays rapidly over time.The influence of heat source on the core temperature of battery mainly depends on the heat generation rate per unit volume.When the battery is discharged at high rates,it may become the main factor affecting the core temperature of the battery.The influence of thermal boundary conditions on the core temperature of the battery can be almost ignored in the initial stage,but will gradually dominate over time.The hypothesis of uniform temperature distribution leads to deviation of core temperature prediction,and its influence mainly lasts in the initial stage and transitional stage.Fortunately,the accuracy of the model will gradually recovered as time.The influence of heat source and boundary condition on the core temperature of battery lies in that both of them determine the variation trend of the core temperature of battery and the temperature difference between the core and the external of the battery.According to the change of boundary conditions and heat source with time,the information of core temperature and the temperature difference of the battery can be obtained by the model,which is the basis for the heat management system to adopt targeted heating and cooling strategies.Secondly,a battery module based on serpentine flat tubes is studied,and its flow and heat transfer characteristics are numerically studied.The influences of the thickness of the flat tube,the filler and the size of the battery on the maximum temperature and the maximum temperature difference of the battery module are studied.The flow uniformity of the battery module is evaluated by the coefficient of variation.The results show that for the same Reynolds number,as the thickness of the serpentine tube increases,the fluid velocity increases.Thus,the convective heat transfer is enhanced,the battery temperature are lower,and the temperature uniformity of the battery module is improved.But the pressure loss increases.It is found that the ratio of pump power to battery power consumed by working medium flow in the serpentine flat pipe is less than one per million,which can be completely ignored.The friction coefficient decreases with the increase of Reynolds number,and they are inversely proportional to each other.However,with the increase of Reynolds number,the decrease of friction coefficient gradually slows down,which means that the pressure loss caused by the increase of flow velocity will be more obvious.With the increase of Reynolds number,the Nusselt number increases gradually,indicating that heat transfer is enhanced,but the increase of Nusselt number slows down,which means that the improvement of heat transfer performance by increasing the flow rate is less and less obvious.The increase of flow rate only has obvious improvement of heat transfer performance in a certain range.In order to reduce the battery temperature at the inlet and outlet of the serpentine flat tube,a material with high thermal conductivity is filled.The results show that the maximum temperature of the battery module and the maximum temperature difference between the batteries decrease obviously with the increase of the thermal conductivity of the filler.As the size of the battery increases,the heat transfer area between the battery and the serpentine flat tube increases,so does the battery capacity.Under the same output power,the discharge rate of the battery will decrease accordingly.The final effect is that the battery size has no obvious influence on the temperature distribution of the battery module.The variation coefficient is used to evaluate the flow uniformity of the battery module.The results show that the flow of the battery module is most uniform when the wedge distributor and wedge collector are arranged diagonally.Thirdly,a new type of battery module with trapezoidal structure is proposed.The flow and heat transfer characteristics of trapezoidal battery modules with taper angle of 0°,60° and 90° is studied with numerical simulation method.It is found that the temperature uniformity of the trapezoidal battery module is improved for two reasons.The first reason is that the flow section gradually shrinks from the inlet to the outlet,which causes the acceleration of fluid flow and enhances the convection heat transfer.The second is the trapezoidal layout of the battery.More batteries are arranged at the inlet,and fewer batteries are arranged at the outlet,to make up for the cooling capacity decline caused by the temperature rise of the working medium.The effects of taper Angle,battery clearance and medium type on the velocity and temperature distribution inside the battery module were studied.Trapezoidal battery module with taper angle of 60° and 90° achieve more uniform temperature distribution than rectangular battery module.When the flow rate is small,the temperature uniformity of trapezoidal battery module with taper angle of 90° is the best,but it is sensitive to the change of the flow rate.With the increase of flow rate,the temperature uniformity of battery module with taper angle of 60° is better than that of battery module with taper angle of 90°.When using water cooling,the pressure drop of the battery module can be reduced by one or two orders of magnitude as compared to air cooling,and the temperature is not sensitive to the change of the battery clearance,so the compactness can be improved by reducing the clearance.The optimum cooling performance can be obtained by using a trapezoidal battery module with taper of 60°.Finally,an experiment system is built to investigate the flow and heat transfer of battery modules with taper angle of 0 °,60 ° and 90 °.Experiments have been carried out to test the effect of module structure,heat generation intensity and flow rate on the performance of the battery module.Battery module is studied under the conditions of steady state and transient state.The variations of maximum temperature and temperature difference between batteries are obtained.The results show that the cells near the inlet need less time to reach the steady-state.For the temperature difference between adjacent rows of batteries,the temperature difference between the first row and the second row of batteries is the largest,and the temperature difference between subsequent rows of batteries decreases.For rectangular battery module,there is a stagnation zone between the rear of the battery in the front row and the head of the battery in the behind row,resulting in poor cooling of the battery in behind row.Under the same discharge rate and mass flow rate,the maximum cell temperature difference of trapezoidal cell module is lower than that of rectangular cell when steady state is reached.In the transient process,the trapezoidal tends to reach the steady state more quickly than the rectangular battery module.