Water balance limitations in transportation PEM fuel cell systems

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Water balance limitations in transportation PEM fuel cell systems

Sharma, Rohit / Fletcher, James H. / Lear, William E.

In order for the potential of Proton Exchange Membrane (PEM) fuel cells to be realized, several important system-level issues must be solved. One of the requirements placed on automobile engine operation is to operate satisfactorily under a wide range of ambient conditions and power demands. Under some combinations of temperature, altitude, and load, the water vapor lost in the exhaust is greater than that produced via the hydrogen oxidation process in the fuel cell stack. Unless significant liquid water reserves are carried - not an attractive design option - then the membranes will become dry, with catastrophic results. In the present paper, an analysis is shown which allows the water discharge rate to be calculated as a function of ambient conditions, overall stoichiometry, and exhaust temperature. The analysis is design-independent, in that specific design choices only affect the stoichiometry and exhaust temperature parameters. The condition of zero net water discharge is emphasized, since that corresponds to the boundary between sufficient water and the catastrophic dry-out process. Results indicate that moderate-to-high altitude operation will most likely require pressurization of the fuel cell stack.