| Environmental protection and energy conservation are two key issues facing the current society.However,the conventional cars consume a large scale of fossil fuel,in the meantime,the exhaust emission is one of the principle reasons of the current haze.The introduction of new energy vehicles in the transport sector will help reduce the air pollution and energy consumption.The electric vehicle(EV)can effectively reduce the exhaust emission,which will be significant for the future development of the city.For EV,the core section is the power supply system.However,the onboard battery lifespan is not as long as the traditional gasoline engine.The capacity anxiety and the battery life are two concerns for the EV users.To extend the battery life is crucial during the EV promotion.Among them,the synergic electric power supply system,which contains the battery and supercapacitor,is one of the research directions for the battery lifespan extension study.In terms of the integrated design of the power supply system,the synergic power supply system is introduced because of the existing problems of the battery alone system,e.g.the battery capacity degradation.Based on the analysis and summarization of the function and characteristics of the synergic power supply,the configuration parameter and optimal control theory of the synergic power supply system are explored in the dissertation.A novel function named the continuous power energy(CPE)function is proposed.Considering that the Urban Dynamometer Driving Schedule(UDDS)and Highway Fuel Economy Test Cycle(HWFET)can cover the vast majority of the common daily driving cycles,both of these two cycles are used in the CPE function to estimate the power and energy requirement.Besides,the common driving phases,including the start up and the acceleration phase,are also integrated into the CPE calculation.By the mathematical calculation,the CPE function configures the appropriate power capacity and energy capacity for the synergic power supply system,which can make good use of the features of the supercapacitor,including the low energy density and high power density.Consequently,the battery can be protected effectively by the configured SC.Two controllable switches are added into the new proposed topology,and the topology of the synergic power supply system can be divided into seven operating modes.After this,though the topology contains a single un-directional DC/DC converter,the topology realizes the same function like the topology which adopts the bidirectional DC/DC converter.During the driving section,the hybrid energy storage system(HESS)has four operation modes,including the supercapacitor supplying the power alone,the battery and the supercapacitor co-supplying the power,the battery supplying the power alone(supercapacitor is idle),the battery supplying the energy to the motor and the supercapacitor at the same time.In the meantime,during the driving section,the HESS has three operation modes,including the supercapacitor recovering the braking energy alone,the battery recovering the braking energy alone,supercapacitor accepting the energy from both the braking energy and battery.The object of this dissertation is to extend the battery lifespan as long as possible by the supercapacitor assisting.The dissertation analyzes the charge transfer between the two plates of the lithium battery during the charging and discharging process,expounds the battery capacity decay principle,and establishes the mathematical model of the battery capacity degradation.The influence factors that affect the battery life are summarized.Based on empirical formula,the relationship between the different factors and battery capacity degradation are elaborated.After the completion of the synergic power supply topology and parameter configuration,according to the characteristics of the vehicle under different driving cycles,this dissertation adopts the different control optimization theories to optimize the synergic power supply system,including:1)Using the fmincon nonlinear solver to calculate the optimal result of the power distribution between the battery and the supercapacitor for a given driving cycle;2)Under the same driving cycle,the dynamic programming(DP)method is used to solve the optimal allocation of vehicle demand power between different power sources and compare the optimal result with fmincon method.Although DP principle cannot be realized in real-time power allocation,the DP optimal result can be regarded as the best optimal result and act as a perfect reference to evaluate the effect of different control methods;3)Based on the previous topology and parameters configuration,the automated searching method is used on the rule-based control strategy.The optimal logic thresholds in the synergic power control algorithm under specific driving conditions are calculated.In addition to the instant demanded power of vehicle,the real-time SOC of supercapacitor and the battery discharge power threshold are used to decide the operation mode of power source.After that,the demanded power allocation between the supercapacitor and lithium battery can be got.Not only the supercapacitor supplies the assisting power when needed(e.g.high demanded power),but also the supercapacitor can recover the energy quickly when the demanded power low,which is prepared for the next power peak situation.4)Popularize the above method to different driving cycles.Considering the various driving features in different driving cycles,three typical standard driving cycles are calibrated to determine the optimal parameter threshold combination separately in this dissertation.Develop the driving cycle recognition algorithm,and classify the corresponding typical driving cycle by analyzing the real-time driving states of the vehicle with three parameters,including the vehicle start-stop frequency per kilometer,the average speed and the variance of speed.Afterwards,the relevant key parameters are invoked based on the typical driving cycles recognition results accordingly.The result shows that the self-adaptive control strategy which is based on the driving cycle recognition,can mitigate the battery degradation largely when dealing the power distribution between the battery and supercapacitor.The mathematical model of each component is established based on the physical characteristics of lithium battery and supercapacitor cell.In this process,the charge and discharge characteristics of the battery and supercapacitor cell are tested.Taking a random driving route as example,calculate the battery degradation value after using the self-adaptive control method to allocate the power between the battery and supercapacitor,and compare the results of the battery capacity decay in different cases,including the battery alone,infinite supercapacitor,fmincon optimization and fuzzy control.The simulation result shows that the synergic power system with self-adaptive control is effective to decrease the battery capacity degradation.The research of this dissertation provides theoretical support for the synergic power supply configuration,topology and control of the electric passenger car,which is helpful to the application of synergic power supply in pure electric vehicle. |
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