Chapter S:
Safety of applications, including coverage by
standards: storage
1
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
1.1
Hydrogen storage
Hydrogen storage is a key challenge for the development of the hydrogen energy
economy. Hydrogen can be stored either as gaseous hydrogen, as liquid hydrogen
or as metal hydride.
1.1.1
Usage of storage systems
Hydrogen storage systems are used for different purposes: as transportable
containers for the supply of hydrogen, as on-board hydrogen fuel tanks, or as
stationary storage (see section 0 for examples of such applications).
1.1.1.1
Conditions of use of storage systems
Hydrogen storage systems have different conditions of use, depending on whether
they are used as transportable containers for the supply of hydrogen, as on-board
hydrogen fuel tanks, or as stationary storage.
For instance, hydrogen storage systems used in stationary applications, such as
hydrogen buffers, are submitted to a higher frequency of hydrogen cyclic
loading (frequent hydrogen inflows and outflows) than storage systems used for the
other two applications.
The storage systems intended for supply of hydrogen are connected and
disconnected for use and filling, whereas those for the other two applications (onboard fuel tanks and stationary storage) are typically permanently installed.
1.1.1.2
Pressure definitions for different types of service
The definition of the reference pressure differs, depending on the type of usage of
hydrogen storage systems.
1.1.1.2.1
Pressure definition for hydrogen storage systems used as transportable
containers and as on-board hydrogen fuel tanks
2
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
For hydrogen storage systems used as transportable containers as well as on
board hydrogen fuel tanks, the reference pressure is the working pressure P W at
15°C, defining the amount of hydrogen when the storage system is full.
It is important to keep in mind that pressure depends on temperature. The value of the
gas pressure at 15°C sets the value of the gas density in the storage vessel.
Therefore, the reference pressure corresponds to a hydrogen density.
Example: the reference pressure of a vehicle tank is of 700 bar at 15°C.
Other kinds of pressure are defined for transportable containers and cylinders:

The test pressure PT is the pressure used to design a cylinder; it is used for
proof test (after manufacturing and during periodic inspection). The ratio
between the test pressure and the working pressure is 1.5. Note however that
for on board application, the cylinder is designed for test (sustained load and
cycles) at 1.25 times the working pressure.

The burst pressure is the pressure for which the cylinder bursts. The ratio
between burst pressure and working service pressure (burst pressure ratio –
BRP) is the safety factor and depends on the application and type of cylinder.
For transportable containers in composite material the required BPR is 3,
without regards to the type of fiber, whereas it is only 2,4 for metallic
containers.
For on-board application, the specified BPR is 2,25 for tanks in carbon fiber
composite.
Figure 1: Definition of cylinder pressures:
example for a working pressure at 200 bar (Source: Air Liquide)
3
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Note: pressure vessels used as transportable containers or as on-board hydrogen fuel
tanks do not have any pressure safety valve. Depending on the application and the
region of use, they may be equipped with thermally activated non-reclosing pressure
relief device, designed to open only in fire conditions.
1.1.1.2.2
Pressure definition for hydrogen storage systems used in stationary
applications
For hydrogen storage systems used in stationary applications, the reference
pressure is defined as the maximal pressure that the system is designed to
bear, called design pressure, or Maximum Allowable Working Pressure (MAWP).
Safety valves prevent this pressure from being exceeded in service.
1.1.2
Compressed hydrogen pressure vessels
Since the energy density of gaseous hydrogen can be improved by storing hydrogen
at higher pressures, gaseous hydrogen is compressed up to pressures higher than
150 bar, and then stored in pressure vessels. In automotive applications, working
pressures up to 700 bar are required to get sufficient energy density on-board.
1.1.2.1
Types of compressed hydrogen pressure vessels and their applications
As explained in section 1.1.1, compressed gaseous hydrogen pressure vessels can
be used either in stationary or in mobile applications.

Examples of compressed hydrogen pressure vessels used in stationary
applications are 50 L steel bottles bundles, fixed tube bundles or tube
trailers, used for instance to supply refuelling stations with hydrogen.

Examples of compressed hydrogen pressure vessels used in mobile
applications are cylinders used in automotive applications.
4
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Type I
Figure 2: Examples of compressed hydrogen cylinders, bundles and tube trailers
(Source: Air Liquide)
Caption:
On the left: steel cylinder (50L @ 200 bar) storing 0,8 kg hydrogen
In the middle: bundle containing 18 cylinders
On the right: tube trailers
Note: in automotive applications, the expected lifetime of a cylinder is of about 20
years.
Reference:

Biennal Report on Hydrogen Safety, June 2007
5
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Compressed hydrogen pressure vessels used in mobile applications: focus
on cylinders
1.1.2.2
1.1.2.2.1
Types of design (1, 2, 3, 4)
Different kinds of compressed hydrogen pressure vessels exist; they are designated
by:

Type I: pressure cylinder made of metal (steel or aluminum alloys) for 150 to
300 bar storage

Type II: thick metallic liner hoop-wrapped with fiber-resin composite

Type III: thin metallic liner fully-wrapped with fiber-resin composite

Type IV: thin polymeric liner fully-wrapped with fiber-resin composite.
Figure 3: Different types of cylinders design
(Source: Adapted from the website of Dynetek)
The choice of pressure vessel material and technology results of a balance between
technical performances needed by the application and cost. Indeed, composite
materials made of fibres and resin are more expensive than steel or aluminium
alloys (they are still produced in limited series or are under development); but they are
light and they increase the ability of the cylinder to withstand high pressures,
allowing to improve compactness. In carbon fibre reinforced composite pressure
the fibres support almost all the mechanical stress in the walls of the pressure
vessels; while the liner (either metallic or polymeric) guaranties the tightness.
Therefore, the use of composite pressure vessels is mainly driven by weight issues for
transportable applications and by the need for materials able to withstand high
pressures.
6
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Type I pressure vessels, used to store hydrogen for industrial applications, have a low
gravimetric storage density1: 1.1 wt% at 200 bars. Type II vessels have an improved
gravimetric density (1,6%) and they can withstand high pressure so they are an
interesting alternatives for 850 bar fuelling station buffers. Type III and IV pressure
vessels have a higher gravimetric storage density than type I and II pressure vessels:
about 5-6 wt% at 350-700 bars. As an example, 4 m 3 hydrogen can be stored either in
a 30 kg cylinder type I at 200 bar (corresponding to a 1 wt% gravimetric density), or in
a 7 kg cylinder type IV at 300 bar (corresponding to a 5 wt% gravimetric density).
Type III and IV pressure vessels are the most suitable solutions for on-board
storage or mobile applications requiring both low weight and compactness, i.e.
working pressures up to 700 bars. Filament wound carbon fiber / resin
composite materials are the most suitable for high pressure hydrogen
containers.
Type I
Figure 4: Steel and composite cylinders (Source: Air Liquide)
Caption:
On the left: steel cylinder type I
On the right: composite cylinder type IV
In the following section, the report focuses on composite cylinders only as they are the
most suitable solutions for on-board storage or mobile applications.
1
The gravimetric storage density is the ratio of the weight of hydrogen stored in the vessel to the sum of the
weight of hydrogen stored and of the weight of the tank equipped with valve – multiplied by 100. Its unit is
w%.
7
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
1.1.2.2.2
Behaviour of cylinders made of composite materials
Degradation from static and cyclic loads
Metals fail by crack propagation; this is often related to cyclic loading leading to fatigue
crack growth. Therefore, it is often taken for granted that the lifetime of metallic
pressure vessels can be counted in numbers of pressure cycles.
The degradation mechanisms of composite pressure vessels differ from the ones of
metallic pressure vessels, as carbon fiber composites are materials with mechanical
properties and failure modes different from metals.
In composite pressure vessels, carbon fibers are embedded in a viscoelastic matrix
which takes over the load where a fiber has broken. When the material is subjected to
a static load, the matrix relaxes over time, leading to a delayed failure of fibers where
they are the weakest. This progressive failure of the reinforcing fibers is at first
randomly distributed in the composite. At very high loads which exceed those applied
in normal services, or for very long period of sustained load, if allowed to progress
sufficiently, the failure of these fibers will lead to clustering of fiber breaks and
instability of the pressure vessel. Fiber break is thus the critical parameter to
monitor. In addition, the composite can delaminate 2 and matrix cracks occur.
2
Delamination is a mode of failure for composite materials.In laminated materials, repeated cyclic stresses,
impact, and so on can cause layers to separate, forming a mica-like structure of separate layers, with
significant loss of mechanical toughness.
8
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Figure 5: Failure processes in composites
(Reference: A. R. Bunsell, Mines ParisTech, F. Barbier, Air Liquide, A. Thionnet,
MinesParisTech, H. Zejli, MinesParisTech, B. Besançon, Air Liquide, 2006, “Damage
accumulation and lifetime prediction of carbon fiber composite pressure vessels”,
Proceedings of the 14th International Conference on Nuclear Engineering, Miami,
July 17-20, 2006)
For non-damaged cylinders, cyclic pressurization does not lead to any further
damage in the composite, compared to that of holding the pressure constant. Figure
6 shows that the pressure cycling does not contribute significantly to the damage
accumulation taking place, as the curve is the same as if no pressure cycling would
have been performed.. It is the effect of the pressure and time which provokes
damage accumulation and not the number of pressure cycles.
For damaged cylinders (with fiber breaks or delaminations for instance), a study is
currently carried out in order to determine whether cyclic pressurization might
lead to further damage in the composite. If yes, specific cylinder protection devices
or appropriate lifetime in term of cycles number would be needed.
9
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Figure 6: Evolution of fiber breaks as the pressure vessel is held at constant pressure
for 25h, with pressure cycling at the same pressure between these periods
(Reference: A. R. Bunsell, Mines ParisTech, F. Barbier, Air Liquide, A. Thionnet,
MinesParisTech, H. Zejli, MinesParisTech, B. Besançon, Air Liquide, 2006, “Damage
accumulation and lifetime prediction of carbon fiber composite pressure vessels”, 14th
International Conference on Nuclear Engineering, Miami, July 17-20, 2006)
Reference:

A. R. Bunsell, Mines ParisTech, F. Barbier, Air Liquide, A. THionnet,
MinesParisTech, H. Zejli, MinesParisTech, B. Besançon, Air Liquide, 2006,
“Damage accumulation and lifetime prediction of carbon fiber composite
pressure vessels”, Proceedings of the 14th International Conference on
Nuclear Engineering, Miami, July 17-20, 2006
10
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Behaviour in fire conditions
Tank manufacturer provides a pressure relief curve which will prevent burst in
bonfire test conditions. This allows the pressure vessel integrator to design an
overpressure relief system which will relieve pressure at least as quickly as specified,
hence avoiding burst in fire conditions.
Pressure
Manufacturer
specified and tested
relief
Will not burst
May burst
Time
Figure 7: Pressure relief curve
(Source: Air Liquide)
In order to determine the shape of these curves, fire tests are carried out on
composite cylinders. The safety devices of the cylinders are removed and the vessels
are exposed to fire. These tests give valuable information to researchers about the
behaviour of the composite structure under strong mechanical strength with the
increase of temperature.
Fire tests have shown that the hydrogen pressure increase within the cylinder is
not responsible for the bursting of cylinders. The bursting of cylinders in fire
conditions is caused by the loss of mechanical resistance of the cylinder wall.
It has been also shown that the behaviour of composite cylinders in fire conditions
depends of the cylinder brand (each manufacturer has one method to define the
structure of the cylinder wall). During fire tests, the hydrogen pressure increase in the
cylinder ranges from a negligible pressure increase to an increase of 25%, depending
on the cylinder provider. In the same way, the time required for a cylinder to burst
under fire conditions also depends on the cylinder provider.
Normative reference: ISO/DIS 15399 (gaseous hydrogen - cylinders and tubes for
stationary storage)
11
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Effect of mechanical impact
Tests are carried out in order to identify the effects of mechanical impacts on
composite cylinders. For instance, in a report of 2000, the INERIS, Air Liquide and
the CEA/CEREM relate that the burst pressure of a cylinder impacted by a vehicle at
65 km/h has not been modified by this impact. The burst pressure of the impacted
cylinder was of 1701 bar, while the measured burst pressures of two new cylinders
were of 1760 and 1692 bar. However, we should not forget that much of the kinetic
energy (144kJ) has been absorbed by the deformation of the vehicle bumper.
Figure 8: Impact of a vehicle on a composite cylinder
(Source: pictures provided by Air Liquide)
When subjected to a mechanical impact, composite structures are damaged at the
point of impact or in an area close to that point. Examples of mechanical impacts
are shocks between cylinders in bundles, cylinder fall due to hazardous handling, or
impacts from forklifts.
It is assumed that the typology of the defect depends on the parameters of the
impact (mass, speed and shape of the impactor). Microstructure observations are
currently carried out in order to analyze the typology of the defect after impact.
12
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Nevertheless, we can reasonably assume that a mechanical impact creates a local
failure of the fibers leading to a modification of the stress field of the composite
structure. The evolution of such a defect under cyclic or static loading is under study
Tests are also carried out in order to identify the extreme impacting conditions
leading to the total destruction of the cylinder.
Figure 9: Example of impact tests carried at the INERIS
(Source: pictures provided by Air Liquide)
Reference:

Final report by J. Chaineaux (INERIS), C. Devilliers (Air Liquide), P. SerreCombe (CEA/CEREM), « Sûreté des dispositifs de stockage de l’hydrogène
sous haute pression équipant des véhicules routiers » / « Safety of high
pressure hydrogen storage vessels used for road vehicles », December 2000
1.1.2.2.3
Safety measures
Design and type approval requirements
In order to guarantee that cylinders are suitable for use, different tests are carried
out on them. These tests are described in standards, which impose specifications
on materials and design. Depending on the application, different standards are used.
Examples of standards are:

ISO/DIS 15869 P for hydrogen on-board storage in automotive

ISO/DIS 15399 for gaseous hydrogen cylinders and tubes for stationary
storage

EN12245 for transportable composite cylinders
13
© HyFacts 2012/13 – CONFIDENTIAL – not for public use

ISO 11119-3 for composite gas containers between 0,5 L and 150 L water
capacity.
Standards explain the tests to which cylinders should be submitted, and the
acceptance criteria for each test. Some of these tests are cyclic tests, such as the
ambient cycle test or the environmental cycle test.
As an example, some of the tests described in ISO 11119-3 are listed in Table 1, as
well as the corresponding acceptance criteria.
Table 1: List of some of the tests described in ISO 11119-3
(Source: ISO 11119-3)
Caption: PB is the burst pressure and PT is the test pressure.
Test name
Test procedure
Acceptance criteria
Water (or another fluid) is used as
the test medium. The pressure in
the cylinder is increased gradually
and regularly until the test pressure
Hydraulic
elastic
PT is reached. The test pressure is
held for at least 30 s.
expansion
test
The elastic expansion is
The cylinder should not show an
elastic expansion in excess of
110% of the average elastic
expansion for the batch at
manufacture
AND
determined from the difference in
volume measured at 10% test
pressure PT and at the test
there should be no leak or failure
to hold pressure
pressure.
Burst pressure PB ≥ 2 PT
AND PB ≥ minimum design burst
Three cylinders are tested
hydraulically, to destruction at burst
Burst test
pressure PB, by pressurizing.
pressure specified by the
manufacturer
AND the burst should not result in
separation at the joint (for
cylinders without liners
manufactured from two parts
joints together)
Ambiant
cycle test
Two cylinders are subjected to a
For cylinders with PT ≥ 60 bar:
hydraulic pressure cycle test to test
Cylinders should withstand
pressure PT (or to the maximum
(without failure by burst or
14
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
developed pressure at 65°C for
leakage) N pressurization cycles
cylinders with PT ≥ 60 bar).
to test pressure PT or Nd
The cylinders are subjected to
successive reversals between an
upper and a lower pressure.
pressurization cycles to maximum
developed pressure. How to
calculate N and Nd is detailed in
ISO 11119-3.
For cylinders with PT ≤ 60 bar:
The two cylinders should
withstand 12 000 pressurization
cycles to test pressure PT
Two cylinders are fitted with valves.
The cylinders are charged to 2/3 of
the test pressure PT,
Fire
resistance
test
The cylinders are fire resistance
tested in both the vertical or
horizontal position. The cylinder
The cylinder should not burst
during a period of 2 minutes from
the start of the fire test.
and valve are exposed to fire
engulfment, but the relief device is
shielded from direct flame
impingement.
Note 1: this list of tests extracted and summarized from the ISO 11119-3 is not
exhaustive. Other tests are: vacuum test, environmental cycle test, environmentally
assisted stress rupture test, flaw test, drop test, high velocity impact (gunfire) test, fire
resistance test, permeability test, torque test on cylinder neck boss, salt water
immersion test, leak test, pneumatic cycle test, water boil test.
Note 2: All required tests are mentioned in the directive 79/2009 (see section Error!
Reference source not found. on European directives).
15
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Non destructive examination
Periodic testing is essential to evaluate damage caused by fibre breaks, delamination or mechanical damage.
Testing techniques for pressure vessels are based on long experience with
metal pressure vessels. Metals fail by crack propagation; this is often related to
cyclic loading leading to fatigue crack growth. Therefore, tests for metal pressure
vessels are designed so as to identify any incipient crack growth. Every 10 years, an
inspection by hydraulic proof test at test pressure is carried out.
A visual
inspection of the pressure vessels containing gaseous hydrogen is also done every 3
years.
However, the mechanisms leading to failure in metals are very different from
those in composites. As explained in the section 1.1.2.2.2, in carbon fiber reinforced
composite pressure vessels, the fibers are embedded in a viscoelastic matrix so
that over time the relaxation of the matrix can alter the stress states in the fiber
and lead to their delayed failure. This is the mechanism which has to be
addressed in tests used to determine the long term reliability of carbon fiber
composite pressure vessels. In addition, defects resulting from accidental impact
or cuts leading to delamination in the composite have to be detected.
The best techniques for testing composite pressure vessels are acoustic emission
techniques and ultrasonic testing. Both are nondestructive testing methods:

Acoustic emission techniques: when a material undergoes stress as a
result of an external fore, it produces sound waves. This is the phenomenon of
acoustic emission. Testing methods based on the acoustic emission stimulate
and caption sound waves in a controlled fashion.

Ultrasonic testing: very short ultrasonic pulse-waves with center frequencies
generally ranging from 0,1 to 15 MHz are launched into materials to detect
internal flows or to characterize materials. Ultrasonic testing is not widely used
yet, but this method is currently being studied in the frame of several projects.
Delamination in the composite can be detected by ultrasonic testing, while
fiber breaks cannot.
Reference:

A. R. Bunsell, Mines ParisTech, F. Barbier, Air Liquide, A. THionnet,
MinesParisTech, H. Zejli, MinesParisTech, B. Besançon, Air Liquide, 2006,
“Damage accumulation and lifetime prediction of carbon fiber composite
pressure vessels”, Proceedings of the 14th International Conference on
Nuclear Engineering, Miami, July 17-20, 2006
16
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Safety in fire conditions
In fire conditions, compressed hydrogen pressure vessels need to be safely
emptied to avoid burst. For this purpose, thermal fuses are used as Thermal
Pressure Relief Devices (TPRD). TPRDs are systematically used for composite
pressure vessels storing hydrogen under high pressures. The TPRDs currently
used work according to one of the following principles:

Fusion of eutectic materials. When a certain temperature is reached, the
eutectic material within the TPRD melts, which triggers the release of
hydrogen. The pressure within the cylinder then decreases.
Figure 10: TPRD with eutectic material
(Source: Circle Seal)
Note: creep tests should be carried out in order to make sure that eutectic
materials would not creep at high temperatures.

Break-down of a glass bulb (part of the TPRD) containing a liquid. When
the boiling temperature of the liquid contained in the glass bulb is reached, the
liquid vaporizes. This increases the glass bulb pressure up to its bursting
pressure. The bursting of the glass bulb then triggers the release of hydrogen.
Figure 11: Glassbulb
(Source: Dynetek)
17
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
The TPRD technology which is the most widely used is based on the fusion of eutectic
materials.
Since hydrogen is stored at lower pressures in metallic vessels than in composites
vessels, safety objectives are met without requiring the use of any TPRD. Besides,
ensuring the reliability of TPRDs in fire conditions is not easy to achieve: important
laboratory tests are carried out to assess their performance. If the performance is not
assessed, the TPRD may become an additional potential point of leakage. A TPRD
functioning properly should open only in fire conditions; its opening should not be
triggered by another cause (such as a mechanical impact on the TPRD). This kind of
inappropriate opening is not likely to occur with TPRD using eutectic materials, as they
open only when a certain temperature is reached. On the contrary, glassbulbs could
be broken because of a mechanical impact.
The secure function of TPRDs has to be tested together with a cylinder (see
79/2009, annex IV, d).
Note: since TPRDs do not reclose when the temperature drops, they are called nonreclosing pressure relief devices and should be replaced once they have been
triggered – along with the storage which has been thermally attacked.
A fire protection strategy is developed. It includes various fire protection
solutions (devices) which can be combined, such as:

TPRD

Coatings improving the fire resistance of vessels (under research). Such
coatings would be used to in conjunction with thermal fuses, in order to delay
bursting and provide additional time to empty the vessel safely. Indeed,
emptying the vessels too quickly means that the hydrogen flow rate would be
high and the flame length so big that the hydrogen jet fire may constitute a risk
of fire propagation. Thus, the safety objective is to limit the flame length. This
can be done by limiting the flow rate of the hydrogen released by the TPRD.

Devices directing the jet fire to a given direction. This safety solution may
be used particularly for hydrogen bundles.
18
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Compressed hydrogen pressure vessels used in stationary applications
1.1.2.3
1.1.2.3.1
Description
Compressend hydrogen pressures vessels are used in stationary applications. See
examples of hydrogen buffers in Figure 12.
Figure 12: Hydrogen buffers
(Source: Air Liquide)
1.1.2.3.2
Hazards and safety measures for hydrogen buffers
The safety measures for hydrogen buffers are:

Integration minimizing risk of flame impingement on vessels

Pressure safety valve – for protection against overpressure

Separation distances

Emergency discharge valve (manual) – for use in case of fire conditions in the
vicinity

Availability of water for spraying tanks in case of fire conditions.
19
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
The Emergency Discharge Device is an extra safety device for compressed
hydrogen pressure vessels used for stationary applications. It can be manually
activated in fire conditions, but it is for emergency use only as significant amounts of
hydrogen are stored in the compressed hydrogen vessels for stationary applications.
1.1.2.3.3

Coverage by standards and certification
For buffers: ISO 15399 (Gaseous hydrogen - cylinders and tubes for
stationary storage)
1.1.3
Liquid hydrogen storage
1.1.3.1
Description
Since the density of liquid hydrogen is higher than the density of gaseous hydrogen,
storing liquid hydrogen instead of gaseous hydrogen reduces the required storage
volume.
In liquid hydrogen storage systems, hydrogen is stored in vessels at cryogenic
temperatures (-253°C). These cryogenic vessels are metallic double-walled
vessels with a high vacuum or material insulation, sandwiched between the walls.
Figure 13: Liquid hydrogen tank system
(Source: Linde)
Reference:
Biennal Report on Hydrogen Safety, June 2007
20
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
1.1.3.2
Hazards and safety measures (with respect to hydrogen, cryogenic
temperatures, and pressure)
The main hazards which might occur in liquid hydrogen storage systems are listed
below.
Hydrogen embrittlement
In order to reduce the risks of hydrogen embrittlement (explained in section
Error!
Reference source not found..), the container should be built according to
established standards:
•
Materials shall be compatible with hydrogen.
•
If a liquid hydrogen vessel needs to be warmed up for elimination of impurities
or maintenance, this shall be done according to a procedure ensuring that the
inner vessel is never exposed during the warming process to a pressure
greater than 2 barg when the inner temperature is greater than –150°C (to
prevent the risk of hydrogen embrittlement cracking of austenitic steel).
Overpressure in the liquid hydrogen storage vessel
An overpressure in the liquid hydrogen storage vessel – leading to pipe / equipment
failure - might occur in some hazardous conditions.

Parasitic heat input leads to a rapid evaporation of the liquid hydrogen
(also called boil-off) and subsequently to an increase of pressure. As
liquefied hydrogen expands when it is vaporized, the pressure of the circuit
increases when cold fluid evaporates. The typical boil-off of hydrogen trailers
or stationary storage is of about 0.5% per day in volume.
To prevent any rapid evaporation of liquid hydrogen, a highly efficient
thermal insulation of the liquid hydrogen storage vessel is required. Double
walled vessel with vacuum and multilayer radiation shielding between
both walls of the vessel are used. When vacuum-jacketed insulation cannot
be used, alternative insulation used shall be classified as incombustible or
non-flammable (M0 or M1 class). Besides, mechanical structures to support
the inner vessel and withstand shocks and acceleration are thermally
optimized to reduce conductive parasitic heat.
A loss of vacuum between the walls of the liquid hydrogen storage
vessel might lead to an increase of the pressure within the vessel.
21
© HyFacts 2012/13 – CONFIDENTIAL – not for public use

Gas recirculation in the vessel might lead to an overpressure in the vessel.
There are systems controlling the pressure in the storage vessels: when there is
an excessive hydrogen pressure within the vessel, hydrogen is released so that the
pressure in the storage vessl can decrease. For this purpose, burst disks were used;
nowadays, safety valves are used. In typical hydrogen trailers or stationary storage,
the safety valve pressure is set to 13 bar. The liquid hydrogen storage vessels shall
have two primary overpressure protection safety systems operating in redundancy.
Once the burst disk has been opened to release hydrogen, the liquid hydrogen
storage vessel can no longer be used. Indeed, as the hydrogen pressure within the
vessel decreases, air might come into the vessel and condensate, which could lead to
the formation of a flammable oxygen-hydrogen mixture.
A hydraulic overpressure might also occur in the storage vessel, when it is
overfilled. The safety measure corresponding to this hazard is a device warning the
person filling the liquid hydrogen storage vessel when the tank is going to be
overfilled.
Cold embrittlement
An unexpected progression of cold fluids on non-resilient3 materials (such as carbon
steels) can weaken them, making them burst at any minor shock.
Appropriate materials to avoid cold embrittlement (316 stainless steel...) should
be used. They are used on the parts of the equipment which might be exposed to
liquid hydrogen, i.e. on the equipment upstream the hydrogen vaporizer (including the
liquid hydrogen storage tank itself).
Note: in the event of a problem on the vaporizer, isolation measures on the liquid
hydrogen storage vessel should be taken. This would avoid the presence of liquid
hydrogen downstream the vaporizer, on pieces of equipments which are not made of
appropriate materials.
3
Resilience is the property of a material to absorb energy when it is deformed elastically and then, upon
unloading to have this energy recovered.
22
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Additional safety measures are taken:

Means shall be provided to minimize exposure of personnel to piping
operating at cryogenic temperatures because the contact of personnel with
liquefied hydrogen and cryogenic atmosphere might lead to injuries: skin
frostbite, lung damages by breathing cold atmosphere, hypothermia.

Purging on the ground should be avoided, because cryogenic liquid spills
might lead to massive hydrogen clouds and to fogs (due to humidity
condensation) creating slipping risks.

Cold sections of liquid hydrogen installations that must be removed from
service should be purged with warm hydrogen or helium at ambient
temperature prior to being purged with nitrogen or other inert gas (see
section about maintenance safety in part 3). Indeed, nitrogen coming in
contact with liquid hydrogen might condense and freeze, which would
obstruct piping and valves. Following installation or repair work, any
residual air shall be purged from the system with helium or nitrogen prior to
the introduction of hydrogen. When purging the system with nitrogen, a purge
with helium or warm hydrogen shall be performed prior to the cool down with
cold hydrogen for start-up.

To avoid leaks, liquid hydrogen piping should be all welded or flanged
and have appropriate expansion loops to allow for thermal contraction
due to temperature variations. The number of compression fittings on liquid
pumps should be minimized to facilitate disassembly and repair of
components.

Lines discharging cold hydrogen (in gas or liquid form) should not be
insulated up to the vent discharge connection, in order to avoid release to
atmosphere of liquid hydrogen.

Air might condensate because of un-insulated piping and equipment
potentially operating at below air condensation: an oxygen-rich liquefied air
might form. To avoid the inflammation of the flammable mixture, the exterior
of cryogenic bare piping, fittings and valves shall be kept free of oil and
grease and such equipment should not be installed above asphalt
surfaces or other combustible materials.

Visual inspections are regularly carried out on vessels containing liquid
hydrogen.
23
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Note: The same hazards related to fire and explosion as for gaseous hydrogen might
occur in liquid hydrogen storage systems. However, the risk of fire or explosion is
higher as gas vapors at low temperature are heavier than air and may accumulate in
low points before they warm up.
1.1.3.3
Coverage by standards and certification

Installation: ISO/DIS 20100

Liquid hydrogen specific requirements: clause 5.3

Safety distances: clause 14.3

Storage vessel sub-system: list of standards of TC 220, in appendix Error!
Reference source not found.. (ISO/TC 220 gives construction standards
which can be applied for hydrogen cryogenic storage).
1.1.4
1.1.4.1
Cryo-compressed hydrogen storage
Description
An alternative concept to liquid hydrogen storage or to compressed gaseous hydrogen
storage is cryo-compressed storage. It refers to the storage of hydrogen at
cryogenic temperatures in a vessel that can be pressurized 4, in contrast to current
cryogenic vessels that store liquid hydrogen at near-ambient pressures. The cryocompressed hydrogen storage enables to maximize hydrogen density. The potential
density increase is significant for 200bar / 20K cryo-compressed hydrogen: 20%
versus saturated liquid hydrogen at 1 bar, factor 6 versus 200bar / 300K compressed
hydrogen.
The implementation of the cryo-compressed technology requires having a vessel able
to operate at pressure and cryogenic temperature with optimized thermal
insulation for long time autonomy.
Even if there is currently no commercial offer for cryo-compressed storage, the
association of pressurized and cryogenic storage has been considered recently by
automotive players for on-board hydrogen storage. However, as increasing storage
size increase storage autonomy, cryo-compressed storage would be more
favourable for large stationary storage rather than for small mobile storage of
automotive applications.
4
There is no “cryo-compressed state”; it corresponds either to a sub cooled liquid for conditions below the
critical pressure, or to a supercritical cold gas for conditions above the critical pressure and the normal
liquefaction temperature.
24
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
The storage increase for hydrogen delivery (mobile trailer, stationary storage)
would be a potential application for cryo-compressed storage. The challenge is to
have well thermally insulated vessels to maintain cold temperature.
1.1.4.2
Hazards and safety measures
Hazards for cryo-compressed hydrogen storage systems are those occurring in
hydrogen high pressure and cryogenic storage technologies (see sections 1.1.2
and 1.1.3), such as cold embrittlement, boil-off and pressure increase, formation of
oxygen-rich liquefied air...
To reduce these risks, structural materials resistant to pressure and cryogenic
temperature are required. The use of carbon or fibre glass composite vessel will
remain mandatory to minimize weight and the compatibility of materials with
cryogenic operation (thermal cycling, tightness) has to be carefully addressed.
Thermal insulation is necessary to avoid boil-off and have long storage autonomy.
Cryo-compressed hydrogen storage is not covered yet by RCS.
1.1.5
1.1.5.1
Metal hydride hydrogen storage
Description
Hydrogen storage in metal hydride is a solution offering the potential for storage of
hydrogen for stationary, transport or portable applications. Metal hydrides are
chemical compounds formed when hydrogen gas reacts with certain metals such as
magnesium, nickel, copper, iron or titanium. Hydrogen bonds with metallic
compounds, forming a weak attraction that stores hydrogen until heated.
The main advantage of metal hydride hydrogen storage is that the volume density of
metal hydrides is higher than the volume density of liquid or gaseous hydrogen.
However, metal hydrides still store little energy per unit weight.
Stationary metal hydride storage is still under development, but transportable
metal hydride storage is already developed and covered by RCS.
25
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Figure 14: Cast magnesium alloys with novel nano-structure developed
by the start-up Hydrexia (Source: website of Hydrexia)
References:

Biennal Report on Hydrogen Safety, June 2007

Website of Hydrexia
1.1.5.2
Behaviour of materials
Metal hydride compounds are formed when hydrogen is absorbed in the material.
This absorption reaction is exothermic; the formation of metal hydride compounds
is accompanied by a heat release. In order to absorb hydrogen to the maximum
capacity of the hydrogen storage metal hydride material, heat must be removed from
the material thanks to appropriate thermal management measures.
The reaction of formation of metal hydride compounds is reversible. When heat is
applied to the metal hydride material, hydrogen is released. Important reaction
enthalpies are required for the release of hydrogen: megawatts to half a gigawatt are
handled during the refuelling of on-board vehicular systems with metal hybrids 5.
Thermal management is thus an issue for metal hydride hydrogen storage.
The hydrogen absorbing behaviour of metal hydride alloys is characterized using a
Pressure – Temperature curve (see Figure 15). For a given temperature, if the
pressure is above a certain level (the equilibrium pressure), hydrogen is absorbed and
a metal hydride is formed. If the pressure is below the equilibrium pressure, hydrogen
is desorbed, and the metal returns to its original state. For a given metal hydride
material, the equilibrium pressure depends upon the temperature.
5
Source: http://www1.eere.energy.gov/hydrogenandfuelcells/storage/metal_hydrides.html
26
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Figure 15: Pressure – Temperature curve for magnesium hydrides
(Source: website of McPhy Energy)
The equilibrium pressure curve depends on the metal hydride material which is
considered (see Figure 16). The metal hydride materials could be chosen so that
they have a potential for hydrogen absorption and release in the optimum
operating pressure-temperature window of an application.
Figure 16: Pressure – Temperature curve for different metal hydrides
(Reference: J. Lu, 2008, “Light metal alanates and amides for reversible hydrogen
storage applications”, Doctoral Thesis at the University of Utah)
27
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
For instance, for Proton Exchange Membrane fuel cell vehicular applications, the
optimum operating pressure – temperature window is in the range of 1 to 10 atm and
25 to 120°C. As showed on the Figure 17, a few metal hydride materials have a
potential for release in this operating pressure-temperature window.
Figure 17: Pressure – Temperature curve for different metal hydrides
(Source: Website of the US Department of Energy)
References:

Biennal Report on Hydrogen Safety, June 2007

Website of McPhy

Website of the US Department of Energy
1.1.5.3
Hazards and safety measures
Appropriate thermal management measures should be implemented, in order to
prevent any steep increase of the pressure in the closed system when exposed to
high temperatures (see Figure 15).
Besides, metal hydrides may react spontaneously when they are exposed to air or
water, which raises additional safety issues.
28
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
1.1.5.4
Coverage by standards and certification
Stationary metal hydride storage is still under development, but transportable
metal hydride storage is already developed and covered by standards:

ISO 16111:2008 (Transportable gas storage devices -- Hydrogen absorbed in
reversible metal hydride)

UN Packing instructions n°205
29
© HyFacts 2012/13 – CONFIDENTIAL – not for public use