| Lithium-ion batteries(LIBs)are ideal energy storage components for electric vehicles(EVs)due to their large energy density and long cycle life.With the expansion of applications,the energy density and charge/discharge rate of LIBs are increasing,which brings more challenges to the thermal safety of batteries.Coupled battery thermal management system(BTMS)based on liquid coolant and phase change material(PCM)is a new type of battery cooling system,which can satisfy the security requirement of high-power LIBs at low power consumption,and provide a safe and efficient operating environment for EVs.Firstly,this paper proposes a coupled thermal management system based on composite PCM-paraffin/expanded graphite(PA/EG)and forced liquid cooling.Heat transfer characteristics of the coupled thermal management system are studied by experiment and numerical simulation,revealing the working mechanism of the coupled system.Then,the PCM and liquid coolant are modified.The flexible composite phase change material(FCPCM)with high thermal conductivity and nano phase change emulsion(NPCME)with high specific heat capacity are applied to replace the traditional carbon-based composite PCM and water coolant,further improving the cooling performance of the coupled system.Finally,a novel delayed liquid cooling strategy is proposed,which not only achieves a high temperature uniformity of LIBs under high power cases,but also greatly reduces the power consumption of the system.The main research contents and conclusions are as follows:1.A BTMS based on liquid cooling and composite PCM(PA/EG)is designed,and the relevant heat transfer model is established as well.Effects of liquid cooling system parameters(inlet temperature and flowrate)and thermophysical properties(thermal conductivity and phase change enthalpy)on battery temperature profiles are studied to reveal the cooling mechanism of the coupled system.It is showed that PCM is the core part to control the temperature temperature of the LIBs with high-rate discharging.As PCM with high phase change enthalpy can absorb large amounts of heat during the phase change temperature,the temperature rise and temperature difference of batteries can be effectively inhibited.Liquid cooling helps to release the latent heat of PCM,improving the stability of PCM in long-term operation.These two systems complement each other and they are indispensable.In addition,the battery temperature can be reduced by decreasing the coolant temperature or increasing the coolant flow rate,but the limited contact area between the liquid coolant and the battery gives rise to an uneven surface temperature distribution of the battery pack.Filling the composite PCM into the gap of the batteries can much increase the heat dissipation area of batteries.The composite PCM with high thermal conductivity also plays as a heat transfer medium,accelerating the heat transfer process from the battery to the liquid cold plate,thereby improving the temperature uniformity of the entire battery pack.Furthermore,it is found that there is a competitive relationship between phase change enthalpy and thermal conductivity of the composite PCM when the density is constant.For composite PCMs with low heat storage density,the cooling performance can be enhanced more effectively by increasing phase change enthalpy than thermal conductivity.2.Commercial indirect contact liquid cooling system usually places the cold plate at the bottom of battery pack,inducing a large axial temperature difference between the top and bottom of the battery.To solve the problem,this paper prepared a novel flexible composite phase change material(FCPCM)cooling pad,which applies paraffin(OP44E)as PCM,and hexagonal boron nitride(h-BN)as the heat transfer enhancer,respectively.FCPCM cooling pads are filled into the gap of batteries to improve the temperature uniformity of the battery pack.The proposed FCPCM cooling pad possesses high phase change latent heat(148.3 J/g)and thermal conductivity(2.6 W/(m·K)),and it can be easily stretched or bended when the temperature is beyond the melting point of FCPCM,which overcomes the poor mechanical properties of traditional carbon-based composite PCMs.FCPCM cooling pads which fill into the gap of batteries can absorb a large amount of heat,and accelerate the heat transfer rate in the axial direction,successfully controls the battery temperature near the phase change temperature,and greatly decreases the axial temperature difference of the battery pack.In addition,the thermally induced flexibility is beneficial for thermal contact resistance decrease and heat transfer enhancement between the battery and FCPCM.At a discharge rate of 5C,the maximum temperature and temperature difference of battery pack cooled by the coupled thermal management system are 9.1 ℃ and 4.7 ℃ lower than cooled by the single bottom cooling system.In addition,the heat dissipation performance of FCPCMs with different mixing ratios are compared,which shows that the mass fraction of OP44 E and h-BN are 60wt% and 20 wt% makes the best cooling performance.3.To improve the cooling performance of the liquid cooling medium and reduce the power consumption of cooling system,nano phase change emulsion(NPCME)with low supercooling degree and flow resistance is applied as a cooling medium in the battery thermal management system for the first time.NPCME is a latent functional thermal fluid,which is formed by dispersing PCM into water.Thus,the NPCME inherits the phase change characteristics of PCM,and the the fluidity of water,and its apparent specific heat capacity can reach 1.8 times of water.Owing to the high specific heat capacity,the NPCME can absorb much more heat under the same flow rate,thus better restraining the temperature rise of the battery than water cooling.Meanwhile,temperature difference between the inlet and outlet of the cold plate can be reduced by applying NPCME as the coolant,which contributes to a better temperature uniformity of the battery pack.Compared with the water cooling cases,the maximum temperature and temperature difference of the battery pack cooled by NPCME at9 C discharging are decreased by 3.5 ℃ and 1.3 ℃.Although the viscosity of the NPCME is higher than that of water,NPCME can attain the same cooling performance as water at a much lower coolant flowrate thus owning a lower pumping power than a water cooling system.4.As the liquid coolant flows through the battery module,the temperature of the coolant increases,leading to a large temperature gradient of the battery pack along the flow direction,this is a common issue of all liquid-based cooling systems.This paper presents a novel delayed liquid cooling stategy which can significantly reduce the temperature difference of the battery pack.The battery pack is passively cooled by PCM at the beginning of the discharging while the liquid cooling starts to work when the battery temperature reaches the PCM melting point.The delayed liquid cooling strategy shortens the period of liquid cooling,not only alleviating the accumulation effect of temperature difference caused by the flowing liquid,but also greatly reducing the power consumption of the liquid cooling system.Experiment results show that the maximum temperature difference of the battery pack at 4C discharging is decreased from 5.4 ℃ to 4 ℃,and the operation time of the liquid cooling system can be reduced by 310 s.The delayed cooling strategy fully takes the advantage of excellent temperature uniformity of PCM and powerful heat dissipation performance of forced liquid cooling,achieving the efficient combination of active and passive cooling. |
