A membrane-based humidifier that uses cooling water of a fuel cell system to humidify the inlet air is modeled and analyzed in this paper. This four-state lumped model is simple and yet captures the humidification behavior accurately. A peculiar characteristic of this system is the fact that it exhibits nonminimum-phase (NMP) behavior. The reason the NMP behavior exists and the effect of system parameters on the location of the NMP zero are analyzed. A proportional control algorithm is proposed to reject the effect of system disturbances, and a feed-forward algorithm is developed to ensure proper humidifier operation under air flow rate changes. Because the NMP zero exists in the disturbance-to-output loop, the proposed algorithm was found to successfully eliminate the undershoot phenomena associated with the NMP zero. However, the disturbance-to-output loop is coupled with input-to-output loop, and the NMP zero could affect the feedback control design.

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