An increase in pressure drop on the cathode side of PEM fuel cell is a reliable indicator of PEM fuel cell flooding. Flooding, i.e., liquid water accumulation inside the cell causes a rather erratic cell potential behavior — steady voltage drops followed by sudden voltage increases. The pressure drop in this case behaves similarly, i.e., increases as the water accumulates, and drops as the water is expulsed from the cell, however with an obvious and detectable upward trend. In an opposite case of fuel cell drying, the cell potential steadily decreases as the ionic resistance increases, while the pressure drop initially decreases as the last of the liquid water is being expulsed, and thereafter remains constant. By monitoring both pressure drop and cell resistance in an operational fuel cell stack it was possible to reliably diagnose either flooding or drying conditions inside the stack, which were intentionally created by adjusting the humidification and stack temperatures. Pressure drop, therefore, may be used to define a control strategy, i.e. to make decisions on corrective actions. In most cases, the pressure drop is a linear function of flow rate when a single phase (gas) is flown through a fuel cell since the flow is well in the laminar regime. When the gas starts to carry water droplets, such as the case when the fuel cell operates with fully saturated gases, the pressure drop departs from linearity. In addition, slugs of water, possibly present inside the fuel cell passages, create unsteady conditions.

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