Performance Investigation Of Phase Change Material Based Battery Thermal Management

The global issues of energy shortage and environmental pollution have induced the growing interests of various new energy technologies.Among them,Li-ion battery(LIBs)has been widely employed because of its significant advantages.As a complicated electrochemical system,the multi physicochemical processes of Li-ion battery include heat and mass transfer as well as electrochemical kinetics.Along with the energy density getting higher and the expansion of application occasion,the problems of performance regressions and security issues induced from heat are becoming more and more outstanding.Thus,it is necessary to design an efficient and feasible battery thermal management(BTM)system to control the operating temperature of batteries within an appropriate range based on the temperature characteristics of batteries.The main contents and results are as follow:1.Firstly,the effects of temperature on battery discharge resistance,capacity and power are discussed.It is showed that the reliable and efficient performance of these batteries has high sensitivity to environmental temperature.The discharge capacity can be improved with the quickened movement of ions by increasing the temperature,while the performance is restricted significantly at low temperature because of the decreased dynamic conditions.Secondly,the performance and aging behaviors of LIBs under high and low temperature conditions are analyzed by in situ and ex situ postmortem approaches.It is the first time to investigate the reversible capacity loss of LIBs that cycled with different discharge profiles at low temperature.The results show that the capacity and power degradation is more severe under the condition of low discharge rate,not the widely accepted high discharge rate.2.Aiming at the common problem of phase change material(PCM)used in thermal management application,a novel form-stable and thermally induced flexible paraffin/olefin block copolymer/expanded graphite(PA/OBC/EG)composite PCM is proposed and successfully prepared.The results show that the as-prepared composite PCM has the advantages of good compatibility,thermal stability and shape stability.Besides,a mathematic method that treating PA/OBC blends as one component with equivalent physical properties is proposed and proved to predict the thermal conductivity of this kind of ternary material appropriately.Simultaneously,the good softness and flexibility achieved by triggering the phase transition of PA could lead to many deformation modes such as bend and compression,which are beneficial for thermal contact resistance decrease and installation improvement.Finally,the heat transfer characteristics are investigated experimentally.The heating and cooling processes are mainly affected by three factors,namely,thermal conduction,natural convection and absorption of latent heat3.A prototype of PCM based BTM system with an interference fit between battery and PCM is designed based on the thermally induced flexibility.Such a system has the advantages of simple assembly and good thermal contact.An experimental setup utilizing the thermal equilibrium method is performed to calculate the thermal contact resistance.The results show that different phase states of PCM have different thermal interface properties,and the thermal contact resistance decreases with the increase of heating power.In addition,both the heat dissipation performance under normal temperature condition and the heat preservation performance under low temperature condition are conducted for this system.Compared with the air based system,the maximum temperature of as-constructed system during 2.5C discharge process could be decreased by 28.8℃(from 72.2℃ to 43.4℃),and the soaking time from 45℃ to-5℃ could be extended by up to 130%.4.A three-dimensional thermal model of a prismatic battery with multi-layer structure is established and validated by the measured surface temperature of the battery.The proposed model is used to analyze and optimize the thermal performance of battery with different shape-stabilized PCM configurations.It is showed that the configuration that PCM wrapped around the battery has the best performing temperature rising and distribution due to the large thermal storage capacity of PCM and highest contact area between the PCM and the battery.In addition,there are critical values of PCM thickness and convective heat transfer coefficient at which the process of phase change is almost completed,and the temperature distribution is significantly different across these values.5.In order to optimize the thermal performance of BTM system under extreme operating conditions,a PA/ EG composites based battery module(PCM module),as well as the two dimensional thermal model is proposed.The effects of different density and EG mass fractions are investigated firstly under high-rate discharge condition.The results show that PA/EG composites with density as high as possible is the priority to achieve the best thermal performance.There exists an optimal mass fraction of EG in composite PCM due to the competitive relation between the phase change enthalpy and thermal conductivity.To further improve the overall performance,we also design a novel pyrolytic graphite sheets(PGS)-enhanced module(PCM/PGS module)where PGS forms the secondary thermal conductive network to improve the whole thermal homogeneity.As a result,the as-designed PCM/PGS module not only presents better heat dissipation performance and temperature uniformity,but also beneficial for energy saving.In the case of failure mode,to achieve the prevention of thermal runaway propagation,the introduction of PGS can transfer the heat absorbed in PCM near the trigger cell throughout the module,giving rise to less PCM consumption by a decreasing rate of ～71.4% in comparison with PCM module.