Research On Energy Management Method Of Hybrid Energy Storage Device

The energy and environmental crisis has promoted the development of the automobile industry in the direction of new energy.Pure electric vehicles are green,pollution-free and environment-friendly means of transportation,and are the main direction of the strategic transformation of my country’s automobile industry.During the driving process of the vehicle,the high-frequency,large-scale and rapidly changing charging and discharging process will lead to the accelerated decay of the life of the lithium-ion power battery,which limits the development and application of pure electric vehicles.The characteristics of supercapacitor with long cycle life and high current charging and discharging capabilities can complement the high energy density advantages of lithium-ion power battery.The lithium-ion battery/supercapacitor hybrid energy storage system can meet the dual requirements of high power and high energy for pure electric vehicles.The energy management method of lithium-ion battery/supercapacitor hybrid energy storage system is of great significance for protecting lithium-ion battery,giving full play to the advantages of supercapacitor,and improving system efficiency and stability.The main contents of this thesis are as follows:(1)The structure and working principle of the battery are analyzed,and then the performance characteristics of different types of batteries are analyzed.,the effects of operating conditions on the discharge characteristics and stability of lithium-iron phosphate batteries were mainly studied.The results show that factors such as different temperatures,different discharge rates and different types of discharge current have a great influence on the output energy of lithium-iron phosphate batteries.Similar to the method used to study lithium-iron phosphate batteries,the research results of supercapacitor show that factors such as different temperatures,different discharge rates and different types of discharge currents have little effect on the output energy of supercapacitor.(2)Focused on the problem of capacity attenuation of power battery caused by excessively high discharge current during electric vehicle driving,a fully active hybrid energy storage system composed of Lithium-ion power battery,supercapacitor and multi-port DC/DC converter is constructed firstly.Then taking supercapacitor state of charge(SOC),vehicle demand power and vehicle speed as input variables,and lithium-ion power battery output power as output variable,a fuzzy logic-based energy management strategy for energy storage system is proposed by constructing 45 fuzzy rules,which ensures the peak power demand of the vehicle during starting and acceleration,and so as to avoid the impact of high-frequency current fluctuations on the life of the power battery.Experimental results under Highway Fuel Economy Test(HWFET)show that the proposed fully active dual-energy source hybrid energy storage system and the energy management strategy based on fuzzy logic can effectively protect Lithium-ion power battery from large current fluctuations,thereby extending lithium-ion power battery life.(3)Considering the cost,structure and control complexity,a semi-active hybrid energy storage system is designed.In this system,the lithium-ion battery is connected to the DC bus through a DC-DC converter,and the supercapacitor is directly connected to the DC bus in parallel.A model is established for the hybrid energy storage system,and then a model prediction energy management method is proposed to reduce the power loss rate of the hybrid energy storage system and improve the stability of the DC bus voltage.The energy management method with multiple constraints not only reduces the loss of the hybrid energy storage system,stabilizes the DC bus voltage,but also limits the discharge current amplitude of the lithium-ion battery.The experimental results under the HWFET condition show that,compared with the pure battery energy storage system,under the premise of ensuring the stability of the DC bus voltage,the proposed model predictive control strategy based on the semi-active hybrid energy storage system can effectively reduce the current amplitude of40.81% lithium-ion battery pack,and improve the efficiency of the hybrid energy storage system by 1.74%.