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Welcome tothe HyFacts Short Course • Chapter S: • Safety of applications: Storages • Compiled by Markus Born, TÜV SÜD Industrie Service

Overview of hydrogen applications • (Source: Adapted from the draft NF M58-003)

Overview RCS - examples

Gaseous hydrogen storage • 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. • Hydrogen storage systems used in stationary applications, such as hydrogen buffers, are submitted to a higher frequency of hydrogen cyclic loading 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 (on-board fuel tanks and stationary storage) are typically permanently installed.

Hydrogen storagefor on-boarduse (1/2) • For hydrogen storage systems used as transportable containers as well as on board hydrogen fuel tanks, the reference pressure is the working pressure PW 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.

Hydrogen storagefor on-boarduse (2/2) • The test pressure PT is the pressure used to design a cylinder. The ratio between the test pressure and the working pressure is 1.5. That for on board application, the cylinder is designed for test 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 – BPR) is the safety factor and depends on the application and type of cylinder. For transportable composite material containers, the required BPR is 3. It is only 2,4 for metallic containers. • The specified BPR is 2,25 for tanks in carbon fiber composite. (Source: Air Liquide)

Hydrogen cylinders: Typesofdesign (1/2) • Different kinds of compressed hydrogen pressure vessels exist; they are designated by: • Type I: metal cylinder (steel or aluminum alloys) for 150 to 300 bar storage (1.1 wt% @ 200 bars) • Type II: thick metallic liner hoop-wrapped with fiber-resin composite (1.6 wt%) • Type III: thin metallic liner fully-wrapped with fiber-resin composite (5 – 6 wt% @ 350 – 700 bar) • Type IV: thin polymeric liner fully-wrapped with fiber-resin composite. (5 – 6 wt% @ 350 – 700 bar (Source: Air Liquide) (Source: Adapted from the website of Dynetek)

Hydrogen cylinders: Typesof design (2/2)

Behaviour of cylinders made of composite materials • Metals fail by crack propagation; this is often related to cyclic loading leading to fatigue crack growth. The lifetime of metallic pressure vessels can be counted in numbers of pressure cycles. • 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. • 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 and matrix cracks occur. (Source: 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)

Evolution offiberbreaks • For non-damaged cylinders, cyclic pressurization does not lead to any further damage in the composite, compared to that of holding the pressure constant. The figure shows that cycles of pressure did not alter the overall curve of damage accumulation which would have been obtained by holding the pressure constant. 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. (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)

Pressure Manufacturerspecifiedandtestedrelief Will not burst May burst Time Behaviour in fireconditions • 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. • 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. (Source: Air Liquide)

Effectofmechanicalimpact (1/2) • 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. • (Source: 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.

Effectofmechanicalimpact (2/2) • It is assumed that the typology of the defect depends on the parameters of the impact (mass, speed and shape of the impactor). It can reasonably be assumed 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. • (Source: Air Liquide)

Design and type approvalrequirements • In ISO 11119-3, certain tests for storage cylinders are described. • These are for example: • Hydraulic elastic expansion test • burst test • ambient cycle test • fire resistance test • vacuum test • environmental cycle test • environmentally assisted stress rupture test • flaw test • drop test • high velocity impact (gunfire) test • permeability test • torque test on cylinder neck bos • salt water immersion test • leak test • pneumatic cycle test

Non-destructiveexamination • Periodic testing is essential to evaluate damage caused by fibre breaks, de-lamination or mechanical damage. • For metal cylinders, every 10 years: inspection by hydraulic proof test at test pressure every 3 years: visual inspection • In carbon fiber reinforced composite pressure vessels, delayed failure anddefects resulting from accidental impact or cuts leading to delamination have to be detected. • Acoustic emission techniques: when a material undergoes stress, it produces sound waves. Testing methods based on the acoustic emission caption sound waves in a controlled fashion. • Ultrasonic testing: very short ultrasonic pulse-waves are launched into materials to detect internal flows or to characterize materials. Delamination in the composite can be detected by ultrasonic testing, while fiber breaks cannot.

Safety in fireconditions (1/2) • In fire conditions, compressed hydrogen pressure vessels need to be safely emptied to avoid burst. For this purpose, thermal fuses are used as ThermalPressure 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. The TPRD technology which is the most widely used is based on the fusion of eutectic materials. • Note:creep tests should be carried out in • order to make sure that eutectic materials • would not creep at high temperatures. (Source: Circle Seal)

Safety in fireconditions (2/2) • 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. • A TPRD functioningproperlyshould open only in fireconditions; itsopeningshould not betriggeredbyanothercause (such as a mechanicalimpact on the TPRD). Thiskindofinappropriateopeningis not likelytooccurwith TPRD usingeutecticmaterials, asthey open onlywhen a certaintemperatureisreached. On thecontrary, glassbulbscouldbebrokenbecauseof a mechanicalimpact. (Source: Dynetek) Note: since TPRDs do not reclose when the temperature drops, they are called non-reclosing pressure relief devices and should be replaced once they have been triggered – along with the storage which has been thermally attacked. • The securefunctionof TPRDs hastobetestedtogetherwith a cylinder!

Fireprotectionstrategy • 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 directingthejetfireto a givendirection. Thissafetysolutionmaybeusedparticularlyfor hydrogen bundles.

Stationary hydrogen storage • 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). Safetyvalvespreventthispressurefrombeingexceeded in service. • 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. (Source: Air Liquide)

Compressed hydrogen pressure vesselsused in stationary applications (1/2) (Source: Air Liquide)

Compressed hydrogen pressure vesselsused in stationary applications (2/2) • 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. • The Emergency Discharge Device is an extra safetydevicewhichcanbemanuallyactivated in fireconditions, but itisforemergencyuseonlyassignificantamountsof hydrogen arestored in thecompressed hydrogen vesselsforstationaryapplications.

Liquid hydrogen storage (LH2) • 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. (Source: Linde)

Hazardsandsafetymeasures (1/6) • Hydrogen embrittlement • In order to reduce the risks of hydrogen embrittlement, 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).

Hazardsandsafetymeasures (2/6) • Overpressure in the liquid hydrogen storage vessel • 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. • To prevent any rapid evaporation of liquid hydrogen, a highly efficient thermal insulation is required. Double walled vessel with vacuum and multilayer radiation shielding between both walls of the vessel are used. Besides, mechanical structures to support the inner vessel and withstand shocks and acceleration are thermally optimized to reduce conductive parasitic heat. • Aloss of vacuum between the walls of the liquid hydrogen storage vessel might lead to an increase of the pressure within the vessel. • Gas recirculation in the vessel might lead to an overpressure in the vessel

Hazardsandsafetymeasures (3/6) • Overpressure control: • 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 vessel 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.

Hazardsandsafetymeasures (4/6) • Cold embrittlement • An unexpected progression of cold fluids on non-resilient 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.

Hazardsandsafetymeasures (5/6) • Additional safety measures are taken: • Means shall be provided to minimize exposure of personnel topipingoperatingatcryogenictemperaturesbecause 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 , becausecryogenic 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. 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.

Hazardsandsafetymeasures (6/6) • Additional safety measures (continued): • In order to avoid release of liquid hydrogen to atmosphere lines discharging cold hydrogen (in gas or liquid form) should not be insulated up to the vent discharge connection. • 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. • Note: The same hazards related to fire and explosion as for gaseous hydrogen might occur in liquid hydrogen storagesystems. 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.

Cryo-compressed liquid hydrogen storage (CLH2) • Cryo-compressed storage refers to the storage of hydrogen at cryogenic temperatures in a vessel that can be pressurized, in contrast to current cryogenic vessels that store liquid hydrogen at near-ambient pressures. The cryo-compressed hydrogen storage enables to maximize hydrogen density. 20% density increase for 200bar / 20K cryo-compressed hydrogen 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. • Cryo-compressed storage would be more favourable for large stationary storage rather than for small mobile storage of automotive applications. • 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. • Hazards for cryo-compressed hydrogen storage systems are those occurring in hydrogen high pressure and cryogenic storage technologies such as cold embrittlement, boil-off and pressure increase, formation of oxygen-rich liquefied air.

Metalhydridestorage (MH2) (1/4) • 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 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. (Source: website of Hydrexia)

Metalhydridestorage (MH2) (2/4) • Metal hydrides compounds are formed when hydrogen is absorbed in the material. This absorption reaction is exothermic. 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 managementmeasures. • 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. 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. 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.

Metalhydridestorage (MH2) (3/4) (Source: website of McPhy Energy) (Source: J. Lu, 2008, “Light metal alanates and amides for reversible hydrogen storage applications”, Doctoral Thesis at the University of Utah)

Metalhydridestorage (MH2) (4/4) • Safety aspects: • Appropriate thermal management measures should be implemented, in order to prevent any steep increaseof the pressure in the closed system when exposed to high temperatures. • Metal hydrides may react spontaneously when they are exposed to air or water, which raises additional safety issues.

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