Compressed Hydrogen Storage for Fuel Cell Vehicles 2001-01-2531

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. The temperature change, however, was found to be negligible with regard to fuel cell thermal control issues. The departure from the ideal gas law was evaluated using the Redlich-Kwong equation of state. This provides the most accurate description of the PV=nRT relationship for simple equations of state. The pressure drop must be calculated from a number of factors such as: pipe material, bends within the pipe, length of pipe, and the number of valves (pressure regulators) the gas must pass through. The pressure drop and initial tank volume are used to calculate the remaining hydrogen - and hence the effective SOC for the CHG storage tank.

Primary results for the CHG fuel systems considered include: the temperature shows a change of ca. 13 K, the initial volume was calculated to be 264 Liters (69.7 Gallons) for 6 kg of H₂ stored at ambient temperature and 5000 psi, and the usable H2 depends on the pressure drop within the specific fuel system design. The system was used within an existing dynamic FCV model for fuel cell vehicle analyses.