Research On Basic Control Characteristics Of Refrigerant-based Direct Cooling For Battery Thermal Management

Battery thermal management of refrigerant-based direct cooling technology in the electric vehicle is highly efficient and compact,and outstanding at temperature control capability,also can be coupled with vehicle air-conditioning system.So it has become a battery thermal management solution with great potential,and has received attention.Based on the refrigerant-based direct cooling experiment,this paper explores its basic influence and function characteristics,especially discusses its control strategy,on the basis of refrigerant-based direct cooling operation conditions.By using the corresponding control algorithm,analyzes its compressor variable speed control strategy,provides a basis for further optimization and energy saving of refrigerant-based direct cooling.The research work first built a refrigerant-based direct cooling heat management experiment bench,simulating the typical working condition process,formulating the control mode,and using the model-based V-flow to realize the controller algorithm development,carry out the corresponding refrigerant-based direct cooling control process research.Using the feedforward enhance predictive ability,embedded feedback target correction,and multiple feedback signals are used to adjust the threshold between thermal management system modes.On the design of the refrigerant-based direct cooling control system,start research with its operating characteristics.Firstly,develope the influence of the compressor speed and the variability of the condensing fan speed,using compressor speed as the main control.Experiments show that in the refrigerant-based direct cooling process of this experiment,it is especially important to match the heat load to a reasonable compressor speed,showing strong sensitivity.If the rotation speed is low,the battery will continue to rise its temperature.On the contrary,if the rotation speed is high,the surface temperature of the direct cooling plate is lower and the uniformity of battery is worse,and the system COP is worse too.The experiment shows that under the premise of preventing the large temperature III rise of the battery,the longitudinal temperature difference of the battery using the higher compressor speed is increased by more than 75%.In the experiment of continuous reduction of battery load,the hysteresis of heat/cold transfer in the refrigerant-based direct cooling process is further verified.It also defines the compressor speed control under variable load,besides restrain the battery temperature rise and also avoid the overshoot of the battery temperature drop.In order to prevent the temperature of the cold plate from being too low and then decreasing the temperature difference of the battery,as well as the excess energy consumption,In the overshoot experiment the difference of average COP caused by the change of different rotational speeds was 2.54 and 3.10,that is important to maintain the stability of battery temperature as an important goal for refrigerant-based direct cooling.Further research on thermal management lie in the hysteresis caused by discharging rate change and cold quantity transfer lag caused by compressor speed changes through experiment analysis,the experiments show that the delay of heat transfer process need marked through the corresponding calibration experiment,so that the compressor cooling capacity is relatively matched with the battery heat generation and its transmission In the control process.On the basis of defining and analyzing the delay characteristics of thermal management behavior,Then this paper carried out the experiment on compressor speed follow the working condition of discharge rate raise/lower,and can get the battery temperature is relatively stable.manifests the refrigerant-based direct cooling process due to the complexity of the evaporation phase change heat transfer on battery pack heat exchange plate of,the heat transfer characteristics of its easy to be instability and need more powerful refrigerants rheology control.Combined with the experimental bench of refrigerant-based direct cooling thermal management and control system,selecte the hardware of MPC5554 as thermal management controller.According to the functional requirements of thermal management and control,programming and realize the platform control.At the same time,Meca and Labview were used to strengthen the monitoring and calibration functions,then complete the construction of the refrigerant-based direct cooling control bench system.Among them,the software architecture of sensing?decision?output in thermal managment programming,using modular design and V-model,and then download to the thermal management controller,to achieve experiment control.Finally,the typical road spectrum conditions are simulated by bench experiment,and respect to battery heat load the compressor speed is used in refrigerant-based direct cooling process.Design variable speed control scheme of compressor,The compressor speed adopts fuzzy control algorithm.Based on the calibration of compressor speed under variable heat load,the appropriate compressor speed is selected to the heat load by judging the battery discharge rate and battery temperature.Matlab was used to complete the online simulation of the fuzzy controller,and then the control strategy was download into the refrigerant-based direct cooling controller to prove the rationality of the control strategy.Experimental conditions with NEDC characteristics were designed and useing the analog battery to reproduce,The characteristics of fuzzy variable compressor speed control are verified under experimental conditions,and the fuzzy control structure was compared and analyzed with the experimental working conditions of fixed compressor speed of 1200r/min and 4000r/min.The analysis shows that the fuzzy variable compressor speed control scheme can not only restrain the large temperature rise of the battery,but also optimize the battery temperature difference and reduce energy consumption.While choose low compressor speed can not effectively prevent the battery temperature rise,if choose high compressor rotation speed can achieve rapid cooling to the battery,but at the same time will cause battery exchange plate surface's temperature goes too low,the battery temperature is too low means that energy consumption increases.The data shows that by using the fuzzy variable speed control the battery longitudinal temperature optimization levels by more than 30%,and energy consumes less than 1/3 compare with the fixed high compressor speed strategy.Therefore,the reasonable and timely variable compressor speed control strategy can not only inhibit the battery temperature rise,but also achieve battery temperature consistency optimization and good energy saving.