Research On Controlling Of Thermal Management Performance Of EV's Battery By Heat Pump Direct Cooling

With the development of electric vehicle technology,battery thermal safety is getting more and more attention,and new thermal management technologies are constantly emerging.Among them,heat pump direct cooling thermal control technology is becoming one of the important solutions of thermal management due to its significant advantages such as efficient and rapid response and overheat emergency management and control.In this paper,an electric vehicle battery pack is used as a heat management object to conduct heat pump direct cooling experimental research,to explore the thermal performance response and control effects of the main control signals in the thermal management control process,furthermore,the compressor in the direct cooling of the heat pump is the main control object,focusing on the control strategy and experimental performance verification of the scheme of the electric vehicle under thermal conditions.The primary work of the research is to build a set of independent heat pump direct cooling battery thermal management system experimental platform.The thermal management object of the experimental bench is a full-size analog battery pack,the thermal management system meets the maximum heat generation matching capacity of the battery pack.The combination of the NI equipment thermal engineering data acquisition system and the vehicle prototype controller RapidECU ensures the real-time accurate measurement and control during the thermal management control process.On the experimental system,carry out experimental research on basic control and its thermal characteristics.Mainly explored the cooling performance of the compressor speed at different discharge rates of the battery pack,and the delay characteristics of the thermal management process.The experimental results show that under constant discharge rate,with the compressor speed increasing,the power consumption increases while the cooling capacity increases.Excessively high speed will often cause the system COP to decrease,and the cooling temperature of the battery will slow down,which is also consistent with battery heat production and heat transfer environment inside the system and battery.When the battery discharge rate changes linearly and dynamically,in order to achieve the best battery cooling effect,the slope of the compressor speed change should be in a certain gain relationship with the battery discharge rate change slope,so as to achieve the best performance.In fact,a certain thermal management system has certain heat transfer delay due to heat transfer inertia.As shown in this experiment,the delay time is positively related to the battery discharge rate,but inversely related to the compressor speed.Furthermore,taking the feedback control scheme of the compressor as an example,in response to the thermal management requirements of the electric vehicle during driving,carry out thermal characteristics research such as feedback control of compressor speed based on average battery pack temperature at constant speed,and over-control research experiments of compressor starting time with accelerator pedal position feed-forward control under acceleration.The results show that the feedback control algorithm of the experimental system can achieve good real-time cooling control of the compressor speed according to the battery temperature,and seek to reduce the power consumption of the compressor;At the same time,the vehicle driving speed is added as a correction variable to optimize the feedback algorithm,so that the feedback algorithm can cope with the thermal management of different vehicle speed conditions,and can greatly reduce the difference in the length of time that the battery reaches the thermal management target temperature under different heat production conditions,and improve the thermal response control ability.In addition,the feed-forward control algorithm uses the accelerator pedal position signal to pre-judge different battery heat production conditions,thereby controlling the thermal management system to start before the battery temperature surges,which greatly reduces the risk of thermal runaway of the battery due to sudden heat production.The research work is also based on the test driving conditions of electric vehicles-“New European Driving Cycle”(ie NEDC),designed a comprehensive thermal management compressor control scheme for objective conditions,and studied the real-time operation of the direct cooling method.Process thermal management control efficiency.It mainly includes the two main links of the start timing of the thermal management system under test driving conditions and the adjustment of the compressor speed after the start.Compressor start-up is determined by the start-up threshold temperature determined by the accelerator pedal position information through the feedforward algorithm and the average temperature of the battery pack fed back in real time,that is,the battery temperature reaches the current threshold temperature,the thermal management system is started,the adjustment of the compressor speed after starting is determined by the fuzzy algorithm of the battery real-time temperature and target temperature difference signal and the discharge rate.Through experimental comparison and analysis of the integrated thermal management compressor control program,the compressor is always at a higher speed of 3500 rpm and a lower speed of 1500 rpm.The results under 2 NEDC cycle conditions experiments can be obtained: Under the comprehensive thermal management compressor control scheme and laboratory environmental conditions,the start-up time of the heat pump system is about 820 s after the working condition starts.At this time,it has entered the high-speed working condition in the later stage,and the battery temperature rise begins to accelerate.This is also due to the objective transmission caused by thermal lag.At the end of the simulation experiment,the battery temperature under the comprehensive experiment control scheme reached the target temperature,and the power consumption was at a low level,compared with the simple non-real-time compressor control scheme,at a constant high speed of the compressor,the battery temperature is lower than the target temperature,the power consumption is greatly increased,and an excess phenomenon occurs;at a constant lower speed,the battery temperature will increase,under long-term operating conditions,it cannot meet thermal management requirements.Therefore,only the integrated control scheme of compressor control can further effectively respond to real-time driving conditions changes,which is beneficial to energy saving while meeting the thermal management response needs.In addition,the research also provides guidance for the further integrated collaborative control of heat pump direct cooling.