Design of Air, Water, Temperature and Hydrogen Controls for a PEM Fuel Cell System

Abstract

Polymer electrolyte membrane (PEM) fuel cell is the potential power source for vehicle applications, where supply of fuels and rejection of heat play significant roles in ensuring performance and durability. Operations of the fuel cells require four subsystems, air supply system, water and thermal management and hydrogen delivery system. Air supply system consists of a blower and a gas-to-gas humidifier. The air is supplied to the stack by the blower and humidified by a gas-to-gas humidifier using stack exhaust gas. Controls for the air supply system are designed based on a static feed-forward control and a state feedback control with integrator to maintain the oxygen excess ratio at a desired level regardless of any load applied. Flow rates of air controlled by a voltage of an electric motor driving a blower should be able to supply oxygen to dynamically follow changes of loads, whereby oxygen excess rate should be kept at a level that prevents oxygen starvation. At the same time, water in the stack should be maintained optimally to keep from low proton conductivity and water flooding in porous materials. Therefore, supplying air is humidified with a humidifier that captures moistures exiting the stack. In order to manipulate the amount of humidity, we propose to use an extra bypass valve, which opening is controlled to optimally maintain humidification of the membrane and avoid water flooding. Thermal management system consists of two thermal circuits because of cooling effectiveness, where the inner thermal circuit is made of a bypass valve, a heat exchanger, a water reservoir and a water pump, while the outer thermal circuit is made of a radiator along with a fan, a coolant reservoir and a pump. In order to maintain the stack working temperature at a desired temperature and reduce parasitic powers, a state feedback controller with integrator is employed. Fuel delivery system is a hybridized one that consists of two recirculation lines with an ejector and a blow in order to increase efficiency of fuel usage. The supply line is made of a hydrogen tank, a flow control valve and a low pressure regulator. Controls are designed to track a flow rate where pressure is kept at constant and purging operations are allowed. In this paper, controls for four subsystems of fuel cell system were proposed and examined on a dynamic one dimensional model for a stack that considers non isothermal and two-phase effects. Optimized state feedback controllers with integrator and observers are used to improve control performances and results are presented.