Research On Power Battery Thermal Management Based On Solid-liquid Phase Change Heat Transfer Medium

With the continuous development of economy and society and improvement of people’sliving standards, the importance of energy-saving and emission reduction in transport sectorwill become increasingly prominent. The electric vehicles, with the dual advantages of lowpower consumption and zero emissions, have developed rapidly in recent years. Theperformance of the batteries, including traditional lead-acid and advanced nickel-metalhydride and lithium-ion battery, are significantly affected by the temperature. Excessive highand low temperature can significantly reduce the performance of battery. For the purpose ofextending the cycle life of power battery and improving the performance of electric vehicle,and then promoting the development and application of electric vehicles, in this thesis, theheat transfer characteristic of the difference battery thermal management systems, includingby using phase change medium or material （PCM）, heat pipe and heat pipe coupled with PCMas heat transfer medium, were investigated by experimental and numerical simulation. Inaddition, the heat transfer enhancement mechanisms of PCM on microscopic and mesoscopicscale were simulated and discussed by molecular dynamics and dissipative particle dynamicsmethod. The main research contents and conclusions are summarized as follows:1. The PCM-based thermal management system was designed for cooling the cylindricallithium-ion battery. And the single battery and battery module cooling systems were modeled.The effect of the thermal conductivity and content of the PCM and the environmental factorson heat transfer and distribution within single battery and battery module were investigated,respectively. The results showed that:（1） with the increasing of thermal conductivity of PCM,the heat transfer in PCM will be enhanced, the temperature gradient of the interface of batteryand PCM, and the maximum temperature of single battery and battery module were decreased.However, the maximum temperature of battery affected slightly after the thermal conductivityof PCM increased to a certain value.（2） The local temperature difference within the batterymodule will continue to increase if the PCM is insufficient, and the smaller the thermalconductivity of the PCM, the greater the increase. The local temperature difference decreaseswith the decrease of the phase change temperature of PCM when the ambient temperature isconstant and the PCM is sufficient. And the local temperature difference decreases with the increase of the ambient temperature if the phase change temperature of PCM is constant.（3）The heat in the battery and PCM can be transferred to the external environment within desiredtime when the battery during unused or charging condition. The heat from outer can beblocked effectively by PCM when the electric vehicle parked in the high temperatureenvironment. The local temperature difference increases with the increase of the velocitywhen the air was used as auxiliary way for cooling the battery, but the local temperaturedifference affected slightly when the thermal conductivity of PCM is high enough.2. The PCM-based cooling and heating systems for rectangular lithium-ion battery weredesigned and modeled. The relativity of the thermal conductivity between PCM and batteryand its effect on the heat transfer of the system were investigated. Based on the smoothsurface structure, the flat sintered heat pipes were used for cooling the rectangular battery andthe related heat transfer and distribution characteristic were studied experimentally. Theresults showed that:（1） the thermal resistance of the system can be reduced by increasing thethermal conductivity of battery. The heat transfer of the system cannot be strengthenedsignificantly with the exponentially increased thermal conductivity of PCM when the thermalconductivity of PCM and battery is same. However, the heat transfer in the PCM can bestrengthened and the local temperature difference of the battery module can be reduced.（2）For the battery heating/insulation condition, compared with air heating, the PCM heatingshowed short heating time, more uniform heat distribution and can meet the insulationrequirements if the battery shelved for a long time. The low thermal conductivity of PCM ishelpful for reducing heat loss in the insulation process and extend the insulated time while notconducive to increase the local temperature difference. Therefore, it is also necessary toenhance the heat transfer of PCM.（3） For the flat sintered heat pipes based battery coolingsystem, the concept of the effective cooling capacity and effective average temperaturecapability were first proposed. The effective cooling capacity and effective averagetemperature capability of the heat pipes must be considered simultaneously for controlling thebattery temperature rise and local temperature difference.3. The oscillation heat pipe （OHP）-based cooling system for rectangular lithium-ionbattery was designed. And combined with latent heat characteristics of PCM, the PCM/OHPcoupled battery cooling system was also designed. The effect of OHP placement and electrode direction on heat transfer and distribution of the battery were investigated experimentally. Theresults showed that:（1） to meet the requirements including heat dissipation and distributionevenly of the battery, the start temperature of OHP must be lower than the target temperatureof the cooling system and not higher than the corresponding maximum temperature when thebattery local temperature difference reached the target temperature difference.（2） The coolingeffect of PCM/OHP-based cooling system is better than OHP-based cooling system. Whenthe battery electrode close to the adiabatic end of heat pipe, the time of the maximumtemperature of the battery increased to target temperature becomes longer due to the heatwhich accumulated at high-temperature-side of battery can be conducted rapidly through theheat pipe. The time of the maximum temperature of the battery increased to target temperatureand the maximum temperature in the end of discharge are affected slightly by OHP placementwhen the electrode direction of different cooling systems is same.（3） The heat distributedmore evenly in the PCM/OHP-based cooling system when the battery electrode away fromthe OHP adiabatic end. The heat distributed more evenly in the OHP-based cooling systemwhen the battery electrode close to the OHP adiabatic end. In short, the PCM/OHP-basedcooling system showed a better stability. Heat transfer enhancement for PCM is also a keyfactor for heat dissipation in the PCM/OHP-based cooling system.4. Based on the common characteristics such as heat transfer enhancement in thedifference battery thermal management system, the research object in this section is focus onPCM. According to the ingredient of paraffin, the molecular model of pure alkane, binarymixed alkanes and alkane/water mixed PCM system were fabricated, respectively. The heatand mass transfer mechanisms were investigated by molecular dynamics simulation method.The results showed that:（1） the difference of simulated isobaric heat capacity ofn-nonadecane and n-eicosane at solid state with experiment value （from literature） does notexceed20%, and at liquid state, not exceed9.5%. The simulated thermal conductivity ofn-docosane, consistent with the fluctuation range of the experimental values in literature, isabout0.1⁰.4W·m^-1·K^-1when the temperature is288³18K.（2） The difference of simulatedphase change temperature of pure alkane with the values from literature and experiment doesnot exceed1%. The melting temperature of n-nonadecane and n-tetracosane were calculatedas308.5K and315.2K by self-diffusion coefficient, as308.2Kand314.6K, by isobaric heat capacity, when the mixed proportion of n-nonadecane and n-tetracosane is1:1and1:3,respectively.（3） In the alkane/water mixed PCM system, the addition of alkane will reducethe self-diffusion coefficient of the system that reflect the increasing of viscosity anddecreasing of thermal conductivity on macroscopic point of view. The twisting and stretchingof C-C bond and C-H bond become more intense during the phase change process of alkaneand lead to more energy absorbed or released on macro.5. The alkane and aluminum particles mixed PCM molecular model was fabricated byusing alkane as substrate material. The models such as encapsulated PCM and capsule mixedwith water composite PCM were fabricated by using alkane as core material and silicondioxide as shell material, respectively. The effect of particle size, hard and soft shell material,and the thickness of the shell on diffusion behavior of PCM was investigated. The resultsshowed that:（1） the particle size should not be too large and the content should not be toomuch, otherwise will prone to precipitation phenomenon, mixed uneven even cannot bemixed, and then increase the thermal resistance or distribute the heat uneven.（2） Theself-diffusion of the PCM capsules was affected by the hardness of shell material. The torsion,stretching and vibration of the core material can be restrict by hard shell material, and thenreducing the mobility of the core material after solid-liquid phase change.（3） Theself-diffusion of the PCM capsules was also affected by the thickness of shell material. Thetorsion, stretching and vibration of the core material can be restricted by excessive thick shelland then leading to weaken the mobility of core material.6. The coarse-grained models of different encapsulated PCM by using alkane as corematerial were fabricated. And the shell materials are melamine formaldehyde resin, methyltrimethoxysilane/3-aminopropyl trimethoxysilane, respectively. Based on the mesoscopicsimulation methods and theory, the aggregate mechanism of the PCM capsule and the relatedmain factors were investigated by dissipative particle dynamics simulations. The resultsshowed that:（1） according to the reported PCM capsule core and shell matreials, themesoscopic core-shell structure of n-dodecane/melamine formaldehyde resin-based capsulesPCM system was simulated. The encapsulation process and mechanism was analyzed, and themesoscopic morphology of the capsule with excessive core material was simulated.（2） Theevolution and the formation process of the PCM capsule which using alkane as core material and methyl trimethoxysilane/3-aminopropyl trimethoxysilane as precursor of shell materialwere simulated. With the similar core and shell materials, the content of core material in theDPD simulation is62.5%that the value is very close to the reported as64%. The possibletheoretical content of core material is about70%if the experimental conditions can beoptimized.In summary, the heat transfer characteristics in the battery thermal management systemsand the thermophysical properties of the main heat transfer medium in the systems wereinvestigated by experimental and numerical method on macroscopic, mesoscopic andmicroscopic point of view in this thesis. The research methods and results can providetheoretical guidance and reference for the new structure design of heat transfer medium andbattery thermal management system.