Research On Fuel Cell System Modeling And Control Methods

Proton Exchange Membrane Fuel Cell (PEMFC) has advantages of zero release and high energy conversion efficiency.It can be used in small or medium-sized distributed generators.It has been widely applied and highly valued by academic field and enterprise field.Oxygen excess ratio (OER) is one of the most important performance indexes of PEMFC. The key is to control the inlet gas flow of anode and cathode in real time,and avoid oxygen saturation or starvation.Furthermore, normal operation of the system shall be ensured and the net output power needs to be maximized, such that the system efficiency can be improved.This thesis concerns the fuel cell modeling and control of OER. Major tasks accomplished and achievements are summarized as follows:(1). Through analyzing the structure of the fuel cell system together with its working condition, a simulation model is established using MATLAB/Simulink, which contains the fuel cell model,gas (air and hydrogen) supply model and the air compressor model. Based on these modules, a simulation platform for studying the control of OER is constructed.(2). Based on the relationship among flow rate,rotate speed and pressure,the compressor’s dynamic model is derived.In order to analyze the influence of compressor to the air supply system.(3). With respect to OER,we chose the compressor flow rate as the control objective, the input voltage of the compressor motor as the actuating variable,OER can thus be regulated and the efficiency of the system can be improved. Two control methods are investigated in this thesis:a PID controller and a sliding-mode controller. Transfer function of the PEMFC system is established which describes the dynamics of the plant.The model is simplified and the time-domains well frequency-domain characteristics are analyzed. This leads to the application of a PI controller. Based on the sliding-mode control theory, an SMC of nonlinear system is implemented. Saturation is introduced to reduce chattering which is a common problem of switching control strategies.Simulation results show that the SMC controller is robust against disturbances and can provide better performance under load variations and uncertainties.