Dynamic analyses of a methanol to hydrogen steam reformer for transportation applications.

Abstract

Improving the dynamic response of the steam reformer in a fuel cell power plant designed for transportation applications will enable the power plant to operate in a transient manner with a reduced need for supplementary batteries and their associated cost, weight, and life cycle limitations. As an aid to improving the dynamic response of the steam reformer, two dynamic models have been developed to characterize and improve those aspects of the design that limit its ability to respond to the varying output requirements occurring in vehicular applications. The first model is a phenomenological model of the reformer. This model is used to show the effect of the reformer response speed on basic vehicle characteristics such as the supplementary battery requirement and the overall vehicle weight. The second model is a first principles model which identifies important physical parameters in the steam reformer. The first principles model is used with a design optimization procedure to determine the values of the steam reformer design parameters which will yield the fastest response time to a step input in hydrogen demand under a variety of initial conditions. Results of this analysis suggest that a steam reformer optimized for fast response could have response times on the order of 15 to 20 seconds. A sensitivity analysis suggests that this response can be achieved primarily by reducing the thermal capacity of the reformer and improving the rate of heat transfer to the gaseous constituents within the reformer. With a steam reformer response time on the order of 15 to 20 seconds, ultracapacitor and flywheel technology become a more attractive supplementary energy storage method due to their superior life cycle and power density characteristics when compared with traditional chemical batteries.