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Near term (ca. 2005) Fuel Cell Vehicles (FCVs) will primarily utilize Direct-Hydrogen Fuel Cell (DHFC) systems. The primary goal of this study was to provide an analytical basis for including a realistic Compressed Hydrogen Gas (CHG) fuel supply simulation within an existing dynamic DHFC system and vehicle model. The purpose of this paper is to provide a tutorial describing the process of modeling a hydrogen storage system for a fuel cell vehicle. Three topics were investigated to address the delivery characteristics of H2: temperature change (ΔT), non-ideal gas characteristics at high pressures, and the maximum amount of hydrogen available due to the CHG storage tank effective “state-of-charge” (SOC) -- i.e. how much does the pressure drop between the tank and the fuel cell stack reduce the usable H2 in the tank. The Joule-Thomson coefficient provides an answer to the expected ΔT during expansion of the H2 from 5000 psi to 45 psi.

Vehicle manufacturers claim that fuel cell vehicles are significantly more fuel-efficient and emit fewer emissions than conventional internal combustion engine vehicles /1/. A computer model can help to explore and understand the underlying reasons for this potential improvement. In previous published work, the UC Davis Vehicle Model for the case of a load-following Indirect Methanol Fuel Cell Vehicle (IMFCV) has been introduced and discussed in detail /2/. Because of possible technical barriers with load following vehicles, as well as near term cost issues, hybrid fuel cell vehicle concepts are widely discussed as another fuel cell vehicle option. For load following vehicles, the questions of fast start up and fuel processor dynamics in extreme transient situations, (e.g., during phases of hard acceleration) are not totally resolved at this time. For both of these performance issues, a hybrid design could offer at least an interim solution.