Modeling and control of PEM fuel cell humidification systems

Maintaining proper membrane humidity is crucial to ensure optimal operation of a polymer electrolyte membrane fuel cell. Based on the cathode humidifier, a thermodynamic model was developed to describe the humidification phenomenon. The dry inlet reactants were either humidified by the fuel cell exhaust gas or cooling water. Steady state humidifier simulations were conducted to show that the model could be used for humidifier design optimization. Dynamic simulations were subsequently performed to predict the humidifier behavior during transient operations typical for automotive applications. It was found that the humidifier was a non-minimum phase (NMP) system when the inlet reactant flow rate changed. An experiment was constructed to validate the model, especially the existence of the NMP zero. The experimental results revealed that the system did have a NMP zero when the inlet air flow rate changed. The cause for the NMP zero was analyzed. A static feed-forward algorithm was developed to ensure proper humidifier operation under steady state conditions. The LQR technique and proportional control were used to reject disturbance and ensure RH to reach its steady states faster. Even though the NMP zero was from disturbance to output, it did affect the controller design due to the coupling effect between the disturbance and control input in the humidifier. A static feedforward plus proportional control strategy was proposed for an integrated fuel cell and dynamic humidifier system. Simulation results showed that the proposed control strategy could generate desired net power, maintain a required excess Oxygen level, and avoid fuel cell flooding.