Container Specifications for Metal Hydride Based Storage (25.2)

Criticality: High
Progress: Not Addressed
Score: 40
DOT Relevance: §178

Description of Key Area

Metal hydride-based hydrogen storage systems are presently being commercialized. While there are many different materials that may be used as hydrogen storage materials, they can be divided into two distinctive categories: rechargeable and non-rechargeable. The term rechargeable is used to describe a system which can be refilled by introducing hydrogen to the depleted system without the need to add or remove any other reactant or by-product and the system is designed to retain all material other than hydrogen. Non-rechargeable describes systems where to refill the system, the hydrogen-depleted material or by-products must be removed and fresh hydrogen-containing materials replenished. These systems are therefore designed to allow removal and addition of material other than just hydrogen. This part will discuss rechargeable systems; non-rechargeable systems are discussed in Item 25.3.

Rechargeable systems meet the description of entries UN 3468, Hydrogen in a metal hydride storage system and NA 9279, Hydrogen absorbed in metal hydride of 49 CFR 172.101, the Hazardous Materials table; this discussion will therefore apply to both. These entries have a hazard classification of 2.1 flammable gas with no listed subsidiary hazard. Neither table entry contains packaging instructions; therefore both require either approval of the Associate Administrator or issuance of a special permit before first shipment. Without any guidance on packaging, the OHMS must individually review and issue an approval or special permit for each system design and manufacturer/offeror for all metal hydride-based hydrogen storage systems.

The OHMS has issued several special permits for UN 3468 and NA 9279 systems. The special permits approved to date have included systems that utilize DOT specification 3AL (E 12650, E 13280, and E 13598), DOT specification 3E (E 13036) or ASME (E 13560) cylinders. It is anticipated that not all future applications for approval will utilize DOT specification cylinders.

Metal hydride-based hydrogen storage systems truly are systems. The cylinder or pressure container is only one part of the system that is needed for safe and proper operation and performance. For rechargeable systems, hydrogen gas reacts with another material, normally a solid phase material, to form a new hydrogen-containing compound, the “hydride” phase, in which hydrogen is chemically bonded. This reaction is normally exothermic or heat producing on hydride formation and endothermic or heat absorbing on hydrogen release. The systems will therefore typically contain a means of heat transfer between the contained material and an external heat sink. Means must also be provided to retain all material except for hydrogen gas. The hydride phase typically has a lower density than the non-hydride phase. The lower density means that the contained material literally swells on formation of the hydride phase. This could cause detrimental effects by overstressing the pressure container walls if a means is not provided to prevent non-uniform distribution of the material within. Most DOT specification cylinders require periodic requalification according to 49 CFR 180. Typically cylinders are requalified by visual inspection and the hydrostatic test method. The hydrostatic test method will most likely not be suitable for metal hydride systems. An ultrasonic method has been found to be suitable and was allowed by special permit E 13280. These are just a few examples of how metal hydride-based hydrogen storage systems differ from compressed gas and why there need to be requirements other than simply use of a specification cylinder.

Another way in which metal hydride-based systems differ from standard compressed gases is their response to pressure changes with changes in temperature. The change in pressure of a compressed gas due to a change in temperature is fairly linear and is approximated by the ideal gas law: DP = (nR/V)DT, where T is the absolute temperature and nR/V is constant for a closed cylinder. This is not the case for metal hydride-based systems. With these systems, the pressure and temperature are related through the van’t Hoff relationship: ln P = DH/RT – DS/R, where ln P is the natural log of pressure and DH and DS the enthalpy and entropy of reaction, respectively. The result is that, for example, using a material with reaction enthalpies typical of intermetallic hydrides (20 to 40 kJ/mole H₂), within the ambient temperature range expected during transport, the system gas pressure will approximately double with every 15 to 20°C (27 to 36°F) temperature increase. In other words, a system with a pressure of 1.7 MPa (250 psi) at 15°C (59°F) could have a pressure of approximately 6.9 MPa (1000 psi) at 55°C (131°F), whereas for a simple compressed gas the pressure change would be from 1.7 to 2.0 MPa (250 to 285 psi) for the same temperature rise. This pressure-temperature relationship must be accounted for when consideration is given to the pressure container and pressure relief device (PRD) selection.

Currently there are a lot of development activities being carried out by various companies and organizations around the world on these technologies. Entry level products have started entering the marketplace. The number of products, choice of hydride material and system designs available is expected to increase dramatically over the next few years. Without container specifications or at least a template or set of guidelines for use by the manufacturer for design, testing and manufacture and the OHMS for evaluating these systems, the effort required to review and approve or issue special permits for each may be burdensome.

Discussion of Criticality

This item has been assigned a criticality of high. Without system and testing specifications being developed for metal hydride-based hydrogen storage systems, there is no set of consistent minimum requirements to manufacturers and offerors follow. The absence of specifications or guidelines requires that OHMS personnel individually review and approve each system from each offeror and manufacturer. This could present a burdensome work load on the OHMS if this technology is found to be able to meet current expectations leading to many requests for approval.

While it is considered critical that system specifications or at least guidelines be developed for rechargeable metal hydride-based hydrogen storage systems, it is also recommended that the specifications or guidelines be designed so as to not prohibit new and innovative designs. This technology is relatively new and is evolving. New advanced materials and designs are expected. The specifications should therefore be performance-based and avoid being too prescriptive, while ensuring a minimum level of safety.

Discussion of Progress

The OHMS has performed system reviews and has issued several special permits for metal hydride-based hydrogen storage systems. The special permits include DOT-E 12650, E 13036, E 13280, E 13560 and E 13598. DOT-E 13036 is for a system that is not allowed to be recharged, i.e., for a single use only. The review for the other special permits, which allow recharging, have included consideration of stress on the cylinder walls and have therefore required either in-process testing (E 12650, E 13280 and E 13598) or periodic strain monitoring (E 13560). While in the initial stage of introducing these products, in-process testing is reasonable, as more systems are developed and the systems become more ubiquitous, it might become impractical.

Efforts are being carried out on developing consensus standards for metal hydride-based hydrogen storage systems. The efforts include:

1. The ISO technical committee for hydrogen technologies (TC 197) has a working group drafting a standard for transportable reversible metal hydride hydrogen storage systems (ISO 16111). This document is currently in the approval stage as a committee draft (“CD”) for advancement to the draft international standard stage (“DIS”). In parallel to the CD approval, the document is being considered for publication as a technical specification; with possible publication of the TS much earlier than possible for the International Standard. Once the international standard is approved, the technical specification will be withdrawn. This document only considers stand-alone containers.
2. CGA has also considered developing a standard for portable metal hydride hydrogen storage systems. This effort is early in development and the expected publication date is unknown.

IEC TC 105 on fuel cell technology and UL’s STP 2265 have considered some additional requirements and testing for “Micro” systems that might be used with low-power portable fuel cell products.

The CGA pamphlet S-1.1-2001, referenced in 49 CFR 171.7, does not contain any guidance for PRD selection for metal hydride systems. More recent versions of pamphlet S-1.1 do contain guidance, however it is based on what has been approved by the OHMS in special permits and not on what experts of metal hydride systems consider to be the most appropriate PRDs for use.

Recommendations

It is recommended that the OHMS develop a minimum set of design and test criteria for rechargeable metal hydride-based hydrogen storage systems. These criteria should be provided to potential manufacturers and offerors for use in their design and testing of the storage systems and would help ensure consistency in application of rigor in determining the minimum level of safety. Following guidance in current versions of CGA pamphlet S-1.1 for PRD selection is not recommend. It is preferred that these criteria be performance-based. Ideally they would be based on ISO 16111, underdevelopment by an international committee of experts.

Current cylinder markings will not be appropriate for metal hydride-based hydrogen storage systems. This is partly due to fact the pressure container design must account for stress from factors other than just gas pressure and that gas pressure does not vary according to the gas law with changes in temperature. Current cylinder testing and markings relate to cylinder service pressure, test pressure and PRD settings. Metal hydride systems will not have the same relationship between these pressures. A new standard for relating markings with service pressure, test pressure and PRD settings of metal hydride-based systems will need to be developed. Guidance on marking is given in ISO 16111.

To help ensure that the standards being developed for metal hydride-based hydrogen storage systems meet the need of OHMS, it is recommended that the OHMS assign personnel or contractors to actively participate on the applicable development committees. These would include ISO TC 197 working group 10 and the Compressed Gas Association’s Hydrogen Fuel Technology committee.

From experience obtained from systems approved under these guidelines, they could, at an appropriate future time, be refined and used as a basis for a New Rule Making Proposal for conversion into regulations and incorporated into 49 CFR 173.