Water Content Estimation And Control Of PEM Fuel Cell Stack And The Individual Cell In Vehicle

With energy crisis and environmental pollution growing more serious,saving energy and being environmentally-friendly are attracting our attention.Proton Exchange Membrane Fuel Cell(PEMFC),as an electrochemical energy conversion device,is a very promising alternative for improving our life and environment,thanks to its low emission,low operating noise and high efficiency etc.Government and enterprises all over the world start fundamental research projects of the PEM fuel cell,implement the demonstration of fuel cell vehicles and even promote commercialization of the fuel cell system with ever greater efforts,while the fuel cell durability and system cost still limit its marketization process.To improve the durability,the fuel cell must operate in optimized states continuously but the water content inside a fuel cell is full of uncertainty.Managing the water content also becomes a key problem for design and control of the fuel cell engine.However,the PEM fuel cell is a complex system with gas,water,electricity,heat and force being coupled.Estimating water content based on mathematical model of multi-physics lacks effective information to enhance accuracy and to solve the model analytically or numerically both requires much computation efforts;AC impedance provides significant support for water estimation but its implementation is a great challenge.In hence,it is necessary to study how to combine the multi-physics model and AC impedance together to have a better estimation of water content and optimize the system control.From the aspect of electrical characteristic,this paper proposes a model for water content inside the Membrane Electrode Assembly(MEA)of a fuel cell.The mathematical connection between the polarization curve and equivalent circuit model proves the importance of MEA and its feasibility to study water content.Optimizing the structure of MEA realizes the internal water content recirculation of a fuel cell and improves its performance under middle or lower current density.This model quantifies the influence of water content on performance of MEA and variation of equivalent circuit model is an excellent indicator of water content.Besides,this paper put forwards the comprehensive online AC impedance measurement method,including current excitation method,weak voltage and current signal processing method and method for analyzing measurement error.Experiment validates accuracy of the proposal and it can be applied to the fuel cell powertrain system.This paper analyzes the sensibility of the water content of a fuel cell with self-humidifying MEA to operating conditions experimentally and the high frequency impedance and statistical characteristic are proposed as indicator of water content.This paper simulates the online water content estimation algorithm for a fuel cell with MEA of no self-humidifying ability.In addition,this paper establishes the dynamic model of the air supply system of a fuel cell engine and decouples the closed-loop control of the air supply system and the water content estimation.The experiment on a fuel cell system validates the proposed method for searching optimized operating conditions and the water management strategy.The innovation of this paper includes the following aspects: A water content model of the MEA is proposed which quantifies the influence of water content on performance and electrical characteristic of a fuel cell;The online AC impedance measurement method for a fuel cell stack and each cell is proposed and experimentally validated with comparable accuracy to the commercial AC impedance equipment and application feasibility;The water management strategy oriented for the fuel cell stack and each cell is put forward and experiment validates the closed-loop control of water content in the MEA and improvement of performance and uniformity the fuel cell.The work in this paper explores the combination of the multi-physics model and AC impedance to estimate water content online and optimize the system control,which may have certain meaning for application and research of a fuel cell system.