| With the continuous development of industry technology and traffic trade,the invironmental pollution will become increasingly prominent.Especially,the fog and haze was lasting for long time in most regions of our country in recent years,which seriously eroded people's physical and mental health.Electronic vechiles,with dural advantages of low power consumption and zero emission,will be developed rapidly to relieve the energy shortage and alleviate environmental pollution caused by automobile exhaust.The key technology of electronic vechiles lies in the performances of the power batteries,which are significantly influenced by operation temperature.Excessively high or low operating temperature will affect the electrochemistry performance of the batteries,even cause thermal runaway,internal short-circuit etc.safety problems.In order to improve the operating performances and thermal safety of power batteries,in this thesis,the form stable composite phase change material expanded graphite(EG)/paraffin wax(PW)were prepared,subsequently,were applied into thermal management for power lithium ion batteries.The main research contents and conclusions in this thesis were summarized as follows:1.EG was prepared using graphite with different grain sizes of 50,80,100,200 meshes.EG/PW composite phase change materials with different mass fractions were prepared by absorbing liquid PW into porous EG.The binding characteristics between EG and PW were characterized,and the thermal conductivity and phase transition behavior of EG/PW composites were also investigated.The results showed that:(1)The mass ration of PW adsorption by EG prepared using graphite powder with grain size of 80 meshes is as high as 92.7 %,the chemical reaction didn't occur between EG and PW,there existed just a simple physical combination between them.(2)The thermal conductivity of EG/PW composite block is greatly enhanced with an increase in EG mass fraction,and exhibits anisotropic when the content of EG is equal to or more than 9 wt.%.The thermal conductivity of the EG/PW composite with 20 wt.% EG in axial direction is 7.22 W/m K,however,it is 10.10 W/m K in radial direction.(3)In comparison with pure PW,the supercooling of EG/PW composites decreases and their latent heat of phase change is slightly lower than the theoretical calculated value based on PW mass fraction2.EG/PW composite phase change materials doped with nano Cu powder and graphene were prepared,respectively.The effects of nano Cu powder and graphene with different content added into EG/PW composite were investigated on thermal conductivity enhancer of EG/PW.The effective thermal conductivity model of EG/PW composites doped with nano materials was constructed using fractal method and thermoelectric analogy technique.The results showed that:(1)The thermal conductivity of EG/PW composites remains nearly constant with an increase in nano Cu powder content,however,increases with the increasing of graphite content.(2)The effective thermal conductivity model of composite constructed based on fractal method and thermoelectric analogy technique can preferably predict the thermal conductivity of EG/PW composites doped with Cu nanoparticles,however,there appears significant deviation between predicted values of thermal conductivity based on model and experimental values of EG/PW composites doped with graphene.3.The mechanism of heat generation and the heat transfer model of phase change were analyzed,and the equivalent thermal conductivity,specific heat capacity and density of 26650 LiPFeO4 batteries were calculated,respectively.The heat transfer law and temperature controlling effect of different thicknesses EG/PW composites with various EG contents were investigated by experiment and numerical simulation,and the leakage rate of EG/PW block was calculated after they endured 10 times phase change cycles.The results showed that:(1)EG/PW composite significantly decreases the temperature rise in Li-ion batteries during phase transition of EG/PW composite,and the rising rate of batteries' surface temperature decreases with an increase in EG mass fraction,which is nearly close to zero when the EG content is up to 12 wt.%.(2)The leakage of EG/PW composite blocks decreases with increases to the EG mass fraction,and the leakage ratio is only 0.38 wt.% for composite with 30 wt.% EG.(3)EG/PW composites with 16~20 wt.% EG are viewed as the most promising alternative for 26650 Li-ion battery thermal management due to their shape-stabilized and the satisfying thermal management performance.4.A tube-shell battery pack with composite phase change material was first proposed for cooling cylindrical Li-ion battery.The effect of EG/PW composite in controlling temperature of the tube-shell battery pack was investigated via experiment and numerical simulation.The law of heat transfer and distribution inside the battery module was revealed,and the efficiency of heat dissipation of battery module was also evaluated during cooling process.The results showed that:(1)The maximum temperature of battery's surface is controlled within the scope of phase change temperature,and the maximum temperature difference across the battery pack is about 1 ?.(2)For the case of air natural convection,and T0=28 ? and 37 ?(ambient temperature),PCM solidification took about 60 min and 197 min,respectively.However,it took about 18 min and 80 min under the same environmental temperature for the forced air cooling.(3)The numerical results showed the air fluid flowed in a tortuous,zigzag manner across the aluminum tube bundles in the battery pack,improving heat transfer,which made the highest-temperature region dramatically decrease.However,the zigzag flow pattern also caused dead zones,eddy formation and reciprocating flow of the fluid in the back of the baffles,resulting in highest-temperature rigeion in the back of the baffles.(4)In comparison with the battery module provided by American All Cell Company,the tube-shell battery pack designed in this thesis has higher efficiency of heat dissipation.5.A new kind of aluminum honeycomb-enhanced EG/PW block was prepared for rectangular Li-ion battery thermal management.The heat generation charactristics of rectangular Li-ion battery and the effect of temperature controlling on rectangular Li-ion battery using aluminum honeycomb-enhanced EG/PW block were investigated by expereiments.The effects of phase change material with different thermal conductivities and phase change temperatures were investigated on thermal management for rectangular Li-ion battery.The heat dissipation efficiency of composite block was analysed by liquid cooling after discharge driving.The advantage of heat-coupling dissipation using phase change material and liquid was pointed out.The results showed that:(1)the addition of aluminum honeycomb into EG/PW block can further improve its thermal conductivity and structural strengthen.(2)The heat generation inside the rectangular Li-ion battery is not uniformity and the temperature in near the positive region is the highest,the maximum temperature difference is controlled within 3 ? across the battery surface using aluminum honeycomb-enhanced EG/PW block thermal management.(3)The maximum temperature and the maximum temperature difference of battery surface desrease with an increase in the thermal conductivity of phase change material,but the decreasing rate gradually becomes small.The composite phase change material with thermal conductivity of 3 W/m K is regarded as the alternative for rectangular Li-ion battery thermal management.(4)The melted phase change material can be solidified in fairly short time by liquid cooling,as compared to natural air cooling. |
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