Dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell systems

Analytical models have been developed to study the dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell (PEFC) systems for transportation applications. Attention is focused primarily on the heat transfer effects which are likely to limit rapid response of PEFC systems. Depending on the thermal mass, the heat exchangers and the steam reformer can have time constants of the order of several seconds to many minutes. On the other hand, the characteristic time constants associated with pressure/density disturbances arising from flow rate fluctuations are of the order of milliseconds. The dynamic reformer model has been used to examine the methanol conversion efficiency and the thermal performance during a cold start. Response times are determined to achieve 50-100% of the steady-state methanol conversion for two catalyst tube diameters, The thermal performance is considered in terms of the approach to steady-state temperature, possibility of catalyst overheating, and the penalty in system efficiency incurred during the start-up time. For the complete reference PEFC system various turn-down scenarios were simulated by varying the relative rates of change of fuel cell loading and system flows. It is shown that depending on the relative rates of cell loading changes to flow rate changes overheating of the catalyst can occur due to excess heat transfer in the reformer preheater. This can be controlled by an additional water quench between the catalyst bed and preheater, but only if the flow rate change is sufficiently fast relative to the load changes.