(100d) Potential Diffusion-Based Failure Modes of Hydrogen Storage Vessels for On-Board Vehicular Use

Hydrogen-fueled fuel cell vehicles offer an environmentally attractive technology alternative to reduce greenhouse gas emissions driven, in part, by the transportation sector. The use of hydrogen/fuel cell integrated systems could also benefit other mobile applications such as forklifts and yard tractor utility vehicles. This research aims at quantifying the risks associated with gaseous diffusion through the walls of on-board vessels containing hydride materials as hydrogen storage media. The storage vessels of interest to this study include Type-III (made of carbon fiber reinforced composite wrapping with an inner metallic liner) and Type-IV (same as Type-III but with a polymeric liner instead of the metallic liner). In Type-III vessels, the liner could be aluminum or stainless steel and in Type-IV, high density polyethylene (HDPE) is commonly used as the liner material. Two failure modes related to gaseous diffusion are considered in this research. The first failure mode represents diffusion of the contained hydrogen gas through the vessel walls to the environment. Hydrogen could also leak through the vessel's welds, end fittings such as O-rings or other seals. The severity of consequences of this potential failure mode varies depending on the degree of confinement in which the permeated hydrogen will gradually accumulate, hydrogen leak rate, liner thickness, liner temperature, vessel internal pressure and the proximity to an ignition source. The second failure mode represents potential diffusion of oxygen from the surrounding air to the contained hydride inside the vessel. Most of the hydride materials, such as NaAlH4, MgH2, LiBH4, and LiAlH4, are air reactive (pyrophoric) and the severity of consequences depends on the amount of permeated oxygen inside the vessel as well as the pyrophoricity of the hydride. The consequential failures that may result from this failure mode may include over-temperature, vessel over-pressurization, or loss of containment integrity. It is also possible that the diffused oxygen gas may react with the contained hydrogen gas to form water vapor which would exacerbate the severity of hydride reactions. Also in Type-III vessels, there is a chance that the diffused oxygen could react with the metal liner and form oxides. In this work, consequence analysis using experiments and modeling is utilized to evaluate risk associated with these oxygen diffusion scenarios. Detailed event tree models are presented for each gaseous diffusion failure mode as an accident initiating event. The frequency of occurrence of the generated risk sequences will be quantified. The safety insights to be drawn from this study should assist risk analysts, OEM, Codes & Standards development teams, and other stakeholders in making risk-informed decisions with respect to identifying the optimum type of liner material, liner wall thickness and other design components of the hydride storage vessels for on-board vehicle applications.