A Controllable Membrane-Type Humidifier for Fuel Cell Applications—Part II: Controller Design, Analysis and Implementation

A membrane-based gas humidification apparatus was employed to actively manage the amount of water vapor entrained in the reactant gas supplied to a fuel cell stack. The humidification system utilizes a gas bypass and a series of heaters to achieve accurate and fast humidity and temperature control. A change in fuel cell load induces a reactant mass flow rate disturbance to this humidification system. If not well regulated, this disturbance creates undesirable condensation and evaporation dynamics, both to the humidification system and the fuel cell stack. Therefore, controllers were devised, tuned, and employed for temperature reference tracking and disturbance rejection. Two heater controller types were explored: on-off (thermostatic) and variable (proportional integral), to examine the ability of the feedback system to achieve the control objectives with minimal hardware and software complexities. The coordination of the heaters and the bypass valve is challenging during fast transients due to the different time scales, the actuator constraints, and the sensor responsiveness.