The permeability or crossover characteristics of a typical perfluorosulfonic acid base type membrane are used for the temporal and spatial estimations of nitrogen concentration along the anode channels of a polymer electrolyte membrane fuel cell stack. The predicted nitrogen accumulation is then used to estimate the impact of local fuel starvation on stack voltage through the notion of apparent current density. Despite the simplifying assumptions on the water accumulation and membrane hydration levels, the calibrated model predicts reasonably well the response of a 20-cell stack with a dead-ended anode. Specifically, the predicted voltage decay and the estimated gas composition at the anode outlet are experimentally validated using the stack-averaged voltage and a mass spectrometer. This work shows that the crossover of nitrogen and its accumulation in the anode can cause a considerable decay in stack voltage and should be taken into account under high hydrogen utilization conditions.
A control-oriented mathematical model of a polymer electrolyte membrane (PEM) fuel cell stack is developed and experimentally verified. The model predicts the bulk fuel cell transient temperature and voltage as a function of the current drawn and the inlet coolant conditions. The model enables thermal control synthesis and optimization and can be used for estimating the transient system performance. Unlike other existing thermal models, it includes the gas supply system, which is assumed to be capable of controlling perfectly the air and hydrogen flows. The fuel cell voltage is calculated quasistatically. Measurement data of a 1.25kW, 24-cell fuel cell stack with an integrated membrane-type humidification section is used to identify the system parameters and to validate the performance of the simulation model. The predicted thermal response is verified during typical variations in load, coolant flow, and coolant temperature. A first-law control volume analysis is performed to separate the relevant from the negligible contributions to the thermal dynamics and to determine the sensitivity of the energy balance to sensor errors and system parameter deviations.
Key technical challenges in hybrid fuel cell power system applications are the power management and the thermal control. In this paper, a charge-sustaining supervisory power controller is developed, which minimizes the warm-up duration of a fuel cell/battery hybrid power system by optimally controlling the power split between the fuel cell system and the battery, as well as the operation of an auxiliary heater. The controller is implemented as a model-predictive feedback law. First, a control-oriented, mathematical model of the system is established and partially validated with experimental data. An optimal control problem is then stated, and from the necessary conditions of Pontryagin's minimum principle a solution is derived. The operation of the controller is demonstrated in the simulation, and the controller's functionality is analyzed in detail. As the controller has a feedback structure and as it requires only low computing power, it is adequate for an on-board, real-time application.
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