Water management in a polymer electrolyte membrane fuel cell using an anode recirculation system

The performance of a polymer electrolyte membrane (PEM) fuel cell depends on operating variables such as temperature, pressure and humidity. Designing a water management system to provide optimal humidity conditions inside the PEM fuel cell system can achieve high efficiency and ensure long life. In this work, we focus on the water management of a PEM fuel cell system through modeling, system analysis and control design. In the first part of the dissertation, we develop a zero-dimensional isothermal model that predicts the experimentally observed flooding and drying conditions in the anode and cathode (the two electrodes) at various loads. Using this model, the equilibria of the lumped water mass in the two electrodes are analyzed at various flow conditions of the fuel cell to determine stable and unstable (liquid water growth) operating conditions. Two case studies of water management through modification of cathode inlet humidity and anode water removal are then evaluated using this model. The desired anode water removal and the desired cathode inlet humidity are specified based upon (i) the water balance requirements, (ii) the desired conditions in the electrodes, and (iii) the maximum membrane water transport at those conditions.; The second part of the dissertation deals with the problem of meeting the water management requirements for various loads using an anode recirculation system as the anode water removal mechanism. A phenomenological model of the anode recirculation system is developed and used for sizing the components of the recirculation system to meet the steady state water management requirements at several loads. A state feedback controller is designed for the PEM fuel cell stack with the recirculation system to satisfy both the steady state and transient requirements for water management and pressure regulation across the membrane during load changes. In an attempt to reduce the measurements required and to eliminate the need for a state observer, a static output feedback controller is then designed. It is shown that the performance achieved by the static output feedback using anode side pressure measurements and an anode humidity measurement is comparable to the one achieved by a state feedback controller. The output feedback controller is gain scheduled to account for the changes in plant dynamics due to the condensation of water in the cathode electrode.