Design and Analysis of Air and Coolant Control for a Polymer Electrolyte Membrane Fuel Cell
Type of DegreeThesis
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The polymer electrolyte membrane (PEM) fuel cell is an alternative engine that can potentially replace the internal combustion engine in the vehicles of the future. When hydrogen, stored in a tank, and oxygen, from the air, chemically react in the PEM fuel cell, electricity is generated and water and heat are produced as byproducts. Air and thermal management of a PEM fuel cell system are two important issues for ensuring reliable operation and maintaining a high efficiency at continuously changing loads. Insufficient oxygen supply at dynamic loads on the cathode side causes oxygen starvation that can damage the thin layers of the cells. Conversely, excessive air supply increases the parasitic power dissipated by an air blower and this loss lowers the efficiency. Therefore, efforts to ensure the proper supply of oxygen have been proposed by different researchers who applied advanced control strategies that dynamically replenish the oxygen. However, those control strategies did not consider thermal effects. When operating PEM fuel cell stacks, heat produced continuously changes as the load current varies. Variation of temperature in the cell directly affects the rate of chemical reactions and water transport. Improper rejection of the heat might produce local hotspots and destroy the thin layers of the cell components. Conversely, elevated temperatures facilitate removal of water produced in the catalysts and increase mobility of water vapor in the membrane, which alleviates over-potentials. In addition, reduction of the parasitic power necessary for operating the electrical coolant pump can increase the efficiency. Therefore, development of a temperature control strategy could help to resolve concerns about reliable and efficient operation. This research project describes the design and analysis of control strategies for an air and thermal system that should reject excessive heat in the stack, minimize the parasitic power, and prevent oxygen starvation in the air supply system. The thermal circuit considered consists of a bypass valve, a radiator with a fan, a reservoir and a coolant pump, while a blower, and inlet and outlet manifolds, are the components for the air supply system. Finally, a classic proportional and integral (PI) and a state feedback control for the thermal circuit have been designed. Disturbances are compensated by a feed-forward element. The entire system is simulated and analytical results of the dynamic behavior are presented.