The Modeling Of Energy Storage Properties And SOC Estimation For Liquid Metal Batteries

Energy storage plays an important role in efficient utilization of renewable energy and the improvement of the smart grid.Liquid metal battery is a low-cost and high efficient energy storage technology,developed in recent years.And their unique advantages in large-scale energy storage attract broad attention of the academic elites and industrial circles.Based on the special internal state and external behavior,it's of great significance and importance to master liquid metal batteries' energy storage parameters and dynamic responses.Meanwhile,battery state of charge(SOC)estimation is also very necessary for strengthening the management of power consumption as well as prolonging cycle life.This thesis establishes the equivalent circuit model for liquid metal batteries to simulate their output characteristics.Based on the equivalent circuit model,the battery state of charge(SOC)estimation is precisely achieved.Furthermore,the battery model and SOC estimation are explored for the safe and efficient energy storage applications of liquid metal batteries.The main contents are summarized as follows:1.The voltage,impedance and life properties of liquid metal batteries are characterized by test data,which includes OCV-SOC curve,Voltage-Capacity curve in different rate,impedance spectroscopy,and cycle curve.According to the study,both capacity and voltage vary with charge/discharge rate.And the impedance spectroscopy can represent the electrochemical characteristics and mass transfer processes,which would provide references for battery modeling.Moreover,the cycle curve indicates that liquid metal batteries have high coulombic efficiency and low capacity fading rate.2.Fitting the impedance spectroscopy and equalizing the electrochemical elements to electrical ones,the second order Thevenin equivalent circuit model can be obtained.Then the parameter identification is accomplished by dealing with HPPC test data.As for the calculation of SOC,Ampere-Hour integral method is used.Besides,Rakhmatov model is added to correct SOC,of which the formula is developed into the iterative form so as to make the simulation easy to implement.Finish the battery model and run the simulation.The voltage error is about ±0.02 V.3.Establish state space expressions according to the battery equivalent circuit model.EKF method is improved to calculate the update matrixes by using initial values and observed datas.Finish the EKF model and then the state estimation can be obtained.The simulation shows that the error is about ±2% when the noise is Gaussian and white.In order to make up some divergence defects of EKF method,AEKF method is also adopted to add the noise adaptive modules to the filter.The AEKF model based on simulation shows that the error is about ±5% when the current and voltage noises are non-Gaussian.4.Preliminary investigation on the BMS and grid connection design based on the battery model and SOC estimation.The overall BMS design scheme of liquid metal batteries is introduced,in which AEKF method can be added to the program.And in the grid connection of liquid metal batteries,the battery model can be used.Therefore,converters can be chosen referring to the battery performance as well as the proper working range of SOC can be determined.This work can provide ideas for the following study on battery energy storage.