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:
- 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.
- 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.
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