Studies of transient behavior of proton exchange membrane fuel cells (PEMFC).

摘要

The Proton Exchange Membrane Fuel Cell (PEMFC) is a technology with growing interest. The PEMFC is the most fascinating among other kinds of fuel cells for its high power density and zero emission pure water products. The use of PEMFCs will expose it to transient conditions. For instance, acceleration or deceleration in vehicle applications and turning on or off dishwashers in stationary applications may cause transient conditions of operation of PEMFCs. This dissertation presents experimental data that may be used to understand PEMFC behavior during these transients and these data may be used to verify the numerical simulations and models of PEMFC designs. The electrical load was changed with fixed inlet flowrates for the anode and cathode, and this caused hydrogen and air, stoichiometries to change. The transient experiments showed conditions and stoichiometric changes that gave the overshoot and undershoot behaviors.; Data are presented to show the effects of voltage changes on the current response with four different cases of stoichiometry changes: from excess to normal, from normal to excess, from normal to starved, and from starved to normal. An overshoot behavior was observed when the cell stoichiometry changed from normal to starved condition. With a triple path flow field this overshoot was followed by an undershoot and this second order behavior is a result of, in this case, the air flowing back into the cell at the end of anode side to balance pressure. We named these phenomena as "vacuum effects" when the current density shows "undershoot" after "overshoot" behavior. For other conditions an undershoot behavior was observed when the voltage changed to cause a change from starved to normal conditions. In contrast, only exponential first order behavior was observed for voltage changes between excess and normal conditions.; Various cell voltage ranges and change rates are presented to compare the overshoot and undershoot behaviors. Experiments were performed to explain the effect of different cell voltage ranges, from a region of kinetic limitation to a region of I-R limitations and from a region of mass transfer to I-R limitations. While these descriptions are qualitative, global, and do not reflect local limitations, the observed behaviors can be explained and grouped with these descriptions. The higher the cell voltage, indicating a change from kinetic to I-R limitation yields the larger the magnitude of overshoot peak. The effect of flow field designs is also presented for two different flow fields: single path and triple path. The structure of the flow field is shown to affect the transient behavior of PEMFC so that the triple path yields a smaller magnitude in the overshoot and undershoot peaks.; The "vacuum effect" caused after the overshoot peak is quantified with the reservoir tube connected at the end of anode (hydrogen outlet). Different sizes and shapes of reservoir tubes affect the time dependent length of overshoot peaks as well as "undershoot" after overshoot. The bigger the reservoir tube size the longer the overshoot peak and the dimmer the "undershoot" after overshoot behavior. The anode dilution effect on the transient response is presented in this dissertation. Various concentrations of hydrogen diluted with nitrogen shows the different shapes of overshoot peaks.