Air Supply Control For Fuel Cell Systems

With the increasing demand for fossil fuel and the widespread concern about environmental issues, there is a growing interest in the development of alternative clean energy. Polymer electrolyte membrane fuel cell (PEMFC) has become one of the most potential sustainable energy technologies because it can convert the chemical energy from the reaction between hydrogen and oxygen into electricity without any pollution. A fuel cell system is composed of several subsystems, such as air supply system, fuel supply system, water cooling system and so on. To improve the reliability and economical efficiency of the fuel cell, it is crucial to research on the modeling and control of the fuel cell subsystems, especially the air supply control. In this thesis, the air supply system is studies, including the following parts:(1) Development of the fuel cell test platform:A hardware-in-the-loop test platform of low temperature water-cooling fuel cell is built, including the air supply loop, the water cooling loop, and other auxiliaries. System monitoring program of the air supply system is developed based on Labview. The availability of the platform is verified by open loop experiments that integrate hardware and software systems.(2) System model development:An improved 3rd-order model of PEMFC air supply system is developed based on the reaction mechanism of the fuel cell. A dynamic model of vane compressor is built to assist the controller design, by utilizing system identification based on the hardware-in-loop test platform. For the optimization of the fuel cell system net power, the models of the fuel cell voltage and power are extended to describe the relation between the optimal oxygen excess ratio and current demand. The model parameters are identified via experiments.(3) Controller design and implementation:Since the air supply system is nonlinear, a feedback linearization control method and a 2nd-order sliding mode based cascade control strategy are proposed in this paper. The system with feedback linearization controller is proved to be globally stable by Lyapunov based theory. From simulation results, the feedback linearization controller has faster response and higher control precision. Then it is applied to the test platform, achieving good control performance. Moreover, the control strategy is improved based on the voltage and power models to optimize the net power of the fuel cell system. Experimental results validate that the improved controller promotes the net power as much as 8% while controlling the oxygen excess ratio at its setpoint under load’s current variations.