Hydrogen storage

The very specific properties of hydrogen make its storage unconventional. At ambient temperature, it is an extremely light gas requiring storage at very high pressure (hundreds of bars) to have a meaningful mass within a reasonable volume. Because its liquefaction temperature is -253 °C, storage in liquid form requires cryogenic tanks. Hydrogen ability to adsorb on other material also authorises storage as a solid.

Be it compression, liquefaction or adsorption/desorption, the storage processes are energy intensive. Furthermore, extreme conditions (high pressure, low temperature) and hydrogen ability to permeate through numerous materials leads to very specific challenges.

The selection of the storage method is application dependent. For road vehicles, the tank volume is restricted by vehicle packaging, while a large hydrogen mass is preferred to provide a large range. For stationary applications, weight is less of an issue while leakage/purge prevention to avoid hydrogen accumulation are more critical. These very different requirements might lead to different storage solutions.

Storage as a compressed gas

Hydrogen storage in gaseous form is presently the most common approach because it is a light, well understood and relatively economical storage method. Such tanks are classified in types according to their composition and related pressure capabilities:

  • Type I: Full metal construction with pressure up to 300 bar
  • Type II: Mostly metal with some composite overwrap in the hoop direction, with a pressure capability of 200 bar
  • Type III: Metal liner (inner layer in contact with hydrogen) with a complete composite overwrap and a pressure capability of up to 700 bar
  • Type IV: Full composite construction with different materials for the liner and the overwrapping, capable of up to 700 bar

The material selection for the tank influences both the tank price and its weight. Type I tanks are the cheapest but are heavy; once fully filled, hydrogen only represents about 1.7% of the total installation weight. Type II tanks are almost as cheap and extend the hydrogen content to ~2.1% of total mass. Switching to tanks with a full composite overwrap extends the pressure capability, reduces the weight but also increases the piece cost by a factor of 8-10. A Type III tank can easily reach 700 bar and a hydrogen content of 4.2% of total mass. Finally, Type IV tanks have the same pressure capabilities and price as Type III tanks but are lighter, with a hydrogen content of 5.7% in mass within the tanks of a Toyota Mirai.

As 200 bar is the minimum operating pressure for compressed gas tanks, there is always and economic and environmental additional cost through hydrogen compression itself (see Life Cycle Analysis of Hydrogen Compression ).

High pressure also dictates the tank shape to be a cylindrical unit, because it is the best compromise between robustness to high pressure and acceptable packaging for integration within vehicles.

Type III tanks

Type III tanks are composite high-pressure hydrogen storage tank made of a metal inner layer and a carbon fibre wound outer layer. The liner (the inner layer) is generally made of aluminium alloy 6061 for its low sensitivity to embrittlement (dissolution and trapping of H atoms within metal leading to weakening of metal properties). Metallic end bosses can be aluminium or steel. Other aluminium alloys such as 2024 or 7075 also have good resistance to embrittlement but are prone to stress corrosion as soon as 3.5 ppm of water vapour are present in the hydrogen stream.

The carbon fibres of the outer layer are wound in a helical and hoop pattern around the inner layer to increase the structural strength of the liner by compressing it. During filling and at high pressure, the gas forces on the liner are mostly transmitted as traction within the fibres. An epoxy resin glues fibres together.

The fatigue strength of Type III tanks can be improved with autofrettage: an internal pressure past its elastic limit is applied to the aluminium liner to permanently deform it and to create permanent tension in the carbon fibres while keeping them in their elastic range. Once pressure is released, the carbon fibres create a compression stress within the empty liner while maintaining some tensile stress in the fibres. During fuelling, the compression stress partially offset hydrogen pressure effect on the liner, which on the long run reduces the effect of filling/emptying cycles on material fatigue.

For a Type III tank capable of storing 5 kg of hydrogen at 700 bar, the tank volume is about 126 L and the tank weight (excluding hydrogen) is about 151 kg.

Type IV tanks

Type IV tanks use two layers of synthetics materials, namely a high-density polymer for the inner layer and carbon fibre for the outer layer. Addition of glass fibre as a third layer around the carbon fibre layer has already been explored by some tank supplier to add further reinforcement.

The substitution of the metal inner layer by a polymer inner layer removes the embrittlement issues encountered with Type I-III tanks. However, a careful selection of the liner polymer is required to limit leaks as the carbon fibre outer layer is not impervious. Suitable polymers are polyethylene, nylon-6 also known as polyamide 6 (PA6) or PA6-based composites.

The outer layer is a carbon fibre filament winding around the inner layer and embedded in an epoxy resin gluing fibres together. This outer layer can be up to 25 mm thick. A high-temperature operation is necessary for a proper marrying of the resin and filaments along their whole length.

The carbon fibre filaments compress the inner layer, so that the inner layer expansion at filling is reduced. The fibres only operate in extension during tank filling and holding of high pressure. Accordingly, there are 2 main breakage modes: filament breakage and delamination between filaments and resin. Filament breakage is the main failure mode next to the inner layer while delamination is the main failure mode on the outer diameter.

The predominance of delamination on outer layers has the benefit of reducing the tank sensitivity to impacts. Impacts mostly damage fibres but on the outer diameter while fibre breakage is not the root cause of failure in this area.

The outer layer is the main cost item in a Type IV tank but also the main source of environmental impact. The selection of the carbon fibre and its manufacturing method are therefore 2 very active field of research to reduce cost and environmental burden.

For a Type IV tank capable of storing 5 kg of hydrogen at 700 bar, the tank volume is still about 126 L, but the tank weight (excluding hydrogen) is reduced down to 96 kg.

Storage as a liquid

Compared to gaseous state, liquid state always has a significantly higher density. For any fluid gaseous at ambient conditions, cooling to reach liquid state is the most obvious approach. However, when intense cooling is required (like for hydrogen), a combination of cooling and pressurising might be a better option. Finally, hydrogen offers a third option: adsorption on another fluid that is liquid at ambient conditions.

Cryogenic storage

Storing hydrogen in liquid form at ambient pressure is called cryogenic storage because hydrogen is only liquid below -253 °C at ambient pressure. Such a low temperature implies both a large energy consumption to reach the liquid stage and unusual requirements for storage tanks in order to avoid re-evaporation.

Cryogenic tanks for liquid hydrogen storage are not designed to withstand pressure but to provide an exceptional level of insulation to keep hydrogen liquid as long as possible. Their lack of pressure capability means that any pressure build-up due to evaporation requires venting. The losses associated with such a venting are called boiling off.

Whatever the quality of the insulation, heat transfers will happen and the tank will be completely vented on a sufficiently long timescale. Therefore, despite being a mature technology, cryogenic storage is not an appropriate method for long-term storage or any storage for an application with unknown delay until consumption.

The larger the tank, the lower the relative influence of heat losses. The tank shape is also key to limit losses: spherical tanks are optimal because they have the smallest external surface for a given storage volume. Accordingly, large spherical tanks are the preferred solution for industrial applications. Storing 5 kg of hydrogen in a spherical tank with 25 mm insulation material around it would occupy about 94 L and weigh about 67 kg.

Cold-compressed and cryo-compressed storage

Cold-compressed and cryo-compressed storage is a hybrid method between cryogenic storage and high-pressure storage. By operating above 300 bar, storage can operate between -233 and -193 °C, slightly reducing the heat loss issue, but mostly increasing the hydrogen density.

A side benefit comes from the tank ability to withstand pressure, so that boiling off can be largely reduced if not completely eliminated, significantly extending the duration of storage.

Estimations by the US Department of Energy indicate that storing 5 kg at 500 bar and -206 °C would require a tank of about 65 L capacity (almost a 50% gain over compressed hydrogen at 700 bar) weighing about 71 kg (about 25% lighter than the equivalent Type IV pure compression item).

Liquid Organic Hydrogen Carriers

Rather than storing hydrogen as a pure compound, it is possible to hydrogenate and dehydrogenate some classes of fluids, generally large aromatic hydrocarbons. These fluids have a high density and this density is almost unaffected by the addition of hydrogen.

While Liquid organic hydrogen carriers (LOHC) are generally considered to transport hydrogen over long distances in existing pipelines, they could also be used for storage. Because hydrogen is integrated within a well-known hydrocarbon, the handling and equipment are identical to the existing crude-based infrastructure.

The hydrogenation process is exothermic (releases heat) and runs at elevated pressure (30-50 bar) and moderate temperature (150-200 °C) over a catalyst. The dehydrogenation process is endothermic (absorbs heat), also runs at elevated pressure and at about 100 °C higher temperature, again over a catalyst. The heat requirement to release hydrogen makes it unsuitable for mobile applications, while the large quantities of required aromatics to store large quantities of hydrogen might be a health hazard.

Storage as a solid

Hydrogen itself cannot realistically be stored as a solid, but it can be absorbed on some classes of solids. There are two main classes: Metal Organic Frameworks and Metal Hydrides.

Metal–Organic Framework

Metal organic frameworks (MOFs) are the solid equivalent of LOHC but with the key disadvantage of only allowing hydrogen storage at very low temperatures (about -200 °C). Therefore, all the requirements from cryogenic storage apply, while pressure capability (20-30 bar) is required. The system weight and volume are similar to a Type IV compressed gas tank: for 5 kg of storage, tank weight is about 91 kg and tank volume is about 125 L.

Metal hydrides

Metal hydrides are compounds made of metal and hydrogen that are abundant and affordable. The hydrogen content can be increased or decreased to store or release hydrogen.

While the raw storage material can absorb mass fractions of hydrogen above compressed gas tank capabilities, the management of storage/release kinetics requires controlled heat fluxes within a sturdy tank, generally made of steel. The complete equipment is therefore generally heavier than a compressed gas tank for a given quantity of stored hydrogen.

A common base metal for metal hydrides is Magnesium, which is combined with others such as Nickel to create alloys improving storage/release kinetics and/or storage capacity.

In case of MgH2, hydrogen absorption releases 75 kJ/mol of heat, while releasing the hydrogen requires some heat addition. On a mobile application using an internal combustion engine or a fuel cell, there are heat sources from the propulsion system that can be used as heat source. However, the kinetics of hydrogen release is quite slow and dependent on heat flux, so that hydrogen release might become too slow to maintain operation of the heat source, inducing a complete shut-off.

Chemical storage

Because hydrogen very specific properties make it difficult to store, an alternative is to store the corresponding energy within other chemical compounds that can be produced from hydrogen without much losses. Adding hydrogen to nitrogen (from air) produces ammonia, while adding hydrogen to carbon sources (generally CO or CO2) leads to methane or methanol.

While methane (the main component of natural gas) and ammonia are still gases at ambient conditions, they are easier to store as compressed gases (methane at 250 bar contains as much energy as hydrogen at 700 bar) or liquefied at low temperature (ammonia is liquid at -33 °C, compared to -253 °C for hydrogen). Their handling is well-known because they are already used across various industries.

Methanol has the benefit of being a liquid at ambient conditions, so that it can be stored in a simple fuel tank (provided that polymer and rubber compatibility has been checked). However, the presence of carbon and oxygen in the molecule implies a low hydrogen content on a mass basis.

While attractive for storage purpose, the chemical storage restricts options at usage level. Hydrogen is a very versatile fuel that can be used in piston engines, turbine engines or fuel cells. The alternatives all have less advantageous properties that limit their use to specific applications preventing the definition of a single fit-for-all solution.

Safety of compressed hydrogen tanks

Compressed hydrogen storage tanks being the most widely used and the only sold for usage by the general public (Toyota Mirai, Hyundai Nexo), they have the most advanced homologation rules.

While not constituting an exhaustive list, the key tests for a high-pressure storage tank are:

  • Burst testing: forcing water into the tank up to bursting, measuring pressure to define its maximum pressure capabilities. Homologation requires that bursting only happens past 200% (or 225% depending on legislation) of nominal pressure (~1700 bar at normal conditions is often reported for tanks rated at 700 bar). While initial composite matrix failure occurs below tank nominal pressure (around 400 bar at normal conditions for a 700-bar tank), bursting occurs while fibre breakage dominates. Note that high temperature tends to further weaken tanks
  • Fatigue testing: defining the number of cycles that a tank can survive due to filling/emptying. Each filling creates expansion in the inner layer and traction in the fibres, while each emptying creates compression in the inner layer and releases traction in the fibres. Long-term, these cycles create liner damages, fibre breakage, delamination between fibres and resin
  • Fire testing: hydrogen must be safely released through a valve in case of surrounding fire, preventing any explosion
  • Penetration testing: hydrogen tank must survive a bullet impact without explosion, although induced leakage is authorised

High-pressure storage tanks have to be sized for more than their nominal pressure for safety reasons. If a tank is filled with a given mass of hydrogen under cold conditions and is then exposed to a hotter environment, gas heating creates additional pressure as it is trapped within a closed volume. A 700 bar tank can therefore experience up to 825 bar under hot conditions (hence reports of bursting pressure of ~1700 bar, i.e. ~200% of 825 bar).

Since ambient temperature influences both in-tank pressure and tank mechanical performances, all homologation legislations require tank testing between -40 °C and 85 °C for a tank initial filling at nominal pressure and 15 °C.

The filling process itself creates temperature variations during fuelling which can go beyond the above limits due to Joule-Thomson effect. A slow fuelling would reduce this risk but a fuelling time of around 4 min is specified to keep testing relevant with respect to actual operation for light duty vehicles.