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

摘要

Analytical models were developed for the dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell (PEFC) systems for transportation applications. Focus is on heat transfer effects likely to limit rapid response of PEFC systems. Depending on the thermal mass, the heat exchangers and steam reformer can have time constants on 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 on the order of milliseconds. In vehicular applications, the response time of the turbomachinery, which is determined by rotational inertia, can be on the order of seconds or less. Dynamic reformer model was used to examine methanol conversion efficiency and 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. Thermal performance is considered in terms of the approach to steady-state temperature, possibility of catalyst overheating, and penalty in system efficiency incurred during startup 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. Depending on 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 catalyst bed and preheater, but only if the flow rate change is sufficiently fast relative to load changes.