(155a) Modeling and Dynamic Analysis of Ethanol Reformer for Fuel Cell Applications

Fuel cells that utilize hydrogen are promising energy conversion units that have a high intrinsic efficiency. However there are operational difficulties in storing hydrogen. One way to alleviate this problem is to generate hydrogen in situ from a liquid fuel such as ethanol. Steam reforming of ethanol to produce hydrogen has several advantages. Ethanol is a sulfur-free compound, requires no pre-reforming, has high hydrogen-to-carbon ratio, has high energy density, is easy to store, is safe to handle and transport, has low toxicity and volatility, is commercially available and can be produced from renewable sources.  Previous work from our group has focused on steady state behavior of steam reformers for producing power. However, when power demand fluctuates with time as in the case of a power generator for emergency back-up power for domestic use or in a fuel-cell powered vehicle, the ethanol feed rate into the reformer also fluctuates. For this reason, it is important to study the dynamics characteristic of the reformer in order to ensure that the fuel-cell systems generates sufficient power to meet the fluctuating power demand. In this talk, a mathematical model for an ethanol reformer will be presented that captures the temporal and spatial variation of the species involved in the reforming reactions. The reformer is modeled as a tubular non-isothermal, non-isobaric packed-bed reactor operating at unsteady state. The partial differential equations resulting from this model are solved numerically after estimating the model parameters from the literature.  Simulation results are presented that show how the flow rate of hydrogen changes with time at the reactor exit. The reformer model is coupled with a fuel-cell stack model and the effect of fluctuating power demand from an experimentally determined power profile from a 3-bedroom house on the hydrogen and ethanol flow rates are studied. Based on these dynamic studies, a battery backup system is developed that stores excess power when the ethanol feed rate exceeds the power demand and utilizes power from the battery when the reformer is unable to provide the necessary hydrogen during increasing power requirement.