Patent Description:
Lightweight energy storage is a key topic for next generation aircrafts. Hydrogen offers high energy densities, whereas the storage technique (cryogenic, compressed, solid state/absorbed) is a key issue. Hydrogen can be compressed and/or cooled down to cryogenic temperatures to increase the volumetric and gravimetric energy density. Usually complex tank systems are needed with individual requirements to the materials, design and working principle e.g. regarding operational safety.

Compressed and cryogenic hydrogen are the techniques of choice for today's vehicles, like cars or airplanes. Cryogenic tanks can achieve the lowest added weight wherein about <NUM> - <NUM> tank weight is needed per kg stored H2. Conventional tanks work with applied inner pressure to avoid gas ingress from outside. As tank material typically metals, metal alloys and composites are in use. Full composite tanks can be challenging because of the long in-service life of civil aircraft. Hydrogen leakage may also be an issue.

<CIT> discloses a ship containment system for storing and transporting liquefied hydrogen. The system comprises a spherical cargo tank arrangement within the ship's hull, wherein the cargo tank is supported by a skirt arrangement mounted on the hull of the ship through which the tank is mounted in the ship without direct contact between outer layer of the cargo tank and the hull.

<CIT> discloses a method for protecting a cryogenic tank, the tank comprising two concentric casings between them delimiting an inter-wall space in which a so-called low pressure obtains, the method involving detecting an increase in pressure within the inter-wall space and in response generating a stream of inert gas within the inter-wall space.

<CIT> discloses a cryogenic fuel storage system for an aircraft with a cryogenic fuel tank having a first wall forming a storage volume capable of storing a cryogenic liquid fuel; an inflow system capable of flowing the cryogenic liquid fuel into the storage volume; an outflow system adapted to deliver the cryogenic liquid fuel from the cryogenic fuel storage system; and a vent system capable of removing at least a portion of a gaseous fuel formed from the cryogenic liquid fuel in the storage volume.

<CIT> discloses an apparatus and method for storing gases and liquids having a high transmigration rate through containment surfaces due to their molecular size. An inner pressurized retaining enclosure for the storage of gas and liquids has a second enclosure wall about the first enclosure defining a gap between the respective enclosure surfaces which is filled with a containment gas or liquid having a molecular size greater than the porosity of the storage wall and stored gas or liquid. The containment gas effectively impedes the transmission of the storage gas and liquid through the first inner pressure retainer enclosure by creating a pressure barrier on its outer surface.

<CIT> discloses a container for containing gas, having a double-walled housing, which defines an inter-space between the inner and the outer walls, the inter-space being filled with a fluid at a higher pressure than the pressure of the hydrogen or helium contained within the inner wall.

<CIT> discloses an aircraft having a propulsion system which utilizes a dual fuel system having an inner tank for containing a cryogenic fuel and an outer tank surrounding the inner tank for containing a second fuel wherein the second fuel is a fuel having a low freezing point and a high boiling point which acts as an insulator for the cryogenic fuel in the inner tank.

It is the object of the invention to improve hydrogen tanks for use in vehicles, such as aircraft.

The object is achieved by the subject-matter of the independent claim. Preferred embodiments are subject-matter of the dependent claims.

The invention provides a hydrogen tank assembly for a vehicle, preferably for an aircraft, the hydrogen tank assembly comprising.

The hydrogen tank assembly further comprises an inert gas separator that is configured to separate hydrogen from the inert gas and a recirculation line that is arranged to transport the separated inert gas from the inert gas separator back to the inert gas source.

Preferably, the inert gas pressure is <NUM> %, more preferably <NUM> %, of the hydrogen pressure. Preferably, the inert gas pressure is <NUM> %, more preferably <NUM> %, smaller than the hydrogen pressure.

Preferably, a pressure difference between the hydrogen tank pressure and the inert gas pressure is not greater than <NUM> mbar, more preferable not greater than <NUM> mbar, still more preferably not greater than <NUM> mbar.

Preferably, the hydrogen tank assembly further comprises an inner tank wall cover that is configured for allowing the inert gas to pass towards the inner tank wall for flushing hydrogen and/or configured as a thermal insulation.

Preferably, the inner tank wall cover is made of a material chosen from a group of materials consisting of open porous foam material, perforated honeycomb material, dry fiber layer material, thermal insulation material having channels.

Preferably, the inert gas source is fluidly connected by an inert gas supply line, the inert gas supply line having an end portion for discharging the inert gas. Preferably, the end portion is arranged adjacent to the inner tank wall and/or within the tank wall cover. Preferably, the inert gas is helium.

Preferably, the inner tank wall is configured as a liner. Preferably, the inner tank wall is made of a mono-resin material. Preferably, the inner tank wall is made of a fiber composite material. Preferably, the inner tank wall comprises at least one compensation member configured for reducing or preventing thermal shrinkage of the inner tank wall and/or the the inner tank wall cover. Preferably, the compensation member is configured for reducing or preventing relative shrinkage between the inner tank wall and the inner tank wall cover. Preferably, the compensation member is made of an elastic meta-material that is configured for reducing or preventing thermal shrinkage of the inner tank wall. Preferably, the compensation member is made of a textile fiber material. Preferably, the textile fiber material is a nonwoven fiber material.

Preferably, the hydrogen tank assembly further comprises a main hydrogen line that is arranged for allowing a main hydrogen consumer to be supplied with hydrogen from the hydrogen tank volume. Preferably, the main hydrogen consumer is an aircraft engine.

Preferably, the hydrogen tank assembly further comprises at least one leakage line that is arranged to collect hydrogen that has leaked from the hydrogen tank volume into the shroud volume. Preferably, the leakage line comprises a leakage sensor for determining a leak rate of the hydrogen. Preferably, the leakage line comprises a check valve. Preferably, the leakage sensor is arranged upstream of the check valve. Preferably, the leakage line discharges the hydrogen into the environment.

Preferably, the hydrogen tank assembly further comprises at least one auxiliary hydrogen line that is arranged for allowing an auxiliary hydrogen consumer to be supplied with hydrogen from the tank volume. Preferably, the auxiliary hydrogen line is fluidly connected to the main hydrogen line. Preferably, the auxiliary hydrogen consumer is a hydrogen fuel cell and/or an aircraft engine. Preferably, the auxiliary hydrogen line is fluidly connected to the leakage line, preferably via a check valve.

Preferably, the hydrogen tank assembly further comprises a selector valve that is configured to allow flow of hydrogen from the tank volume to any of the main hydrogen line, the hydrogen consumer, the fuel cell, the inert gas separator, and the environment.

The invention provides an aircraft comprising at least one engine and a preferred hydrogen tank assembly, wherein the engine is supplied with hydrogen from the hydrogen tank volume.

One idea is to provide a hydrogen tank with a double walled pressure tank system. An inert gas, such as helium, is in the outer chamber and hydrogen is in the inner chamber. The helium may have a smaller pressure than the hydrogen.

In order to reduce weight of pressurized cryogenic hydrogen tank systems composite materials offer a unique potential. If applied directly as tank wall different coefficients of thermal elongation (abbreviated: CTE) of the resin matrix and the fibers may cause cracks at cryogenic temperature. Together with the pressure difference between the hydrogen volume and the outside unwanted leakage rates may occur.

The invention proposes to reduce the driving force for leakage to a minimum by putting a counter pressure between the inner tank wall and the outer tank wall. The counter pressure shall be put by pressurized helium.

This basic configuration allows very low leakage rates - even if cracks occur in the inner tank wall. The remaining leaking hydrogen can be constantly purged thereby keeping to concentration very low. This very low concentrated hydrogen can completely be blocked by the outer tank skin that is also shielded from the cryogenic hydrogen and is thus resistant to thermally induced micro-cracks.

The inner tank wall may have a thermal insulation that allows for flushing near the inner tank wall. Materials such as open porous foams, perforated honeycombs, dry fiber layers, channels in the insulation material, etc. may be used to this end.

The pressure difference may be chosen low enough, so that the inner tank can be replaced by a thin mono-resin-material liner. The thin liner is adaptive enough to avoid micro cracks due to different CTE. In order to avoid thermal shrinkage of such thermoplastics, a compensation mechanism using elastic meta-materials may be employed.

Furthermore, it is possible to regulate the temperature driven hydrogen boil off by controlling the helium inlet temperature.

Boiled off gas mixture having hydrogen and helium can be turned into energy, water and helium via an appropriate fuel cell e.g. a proton exchanger membrane fuel cell (PEM-FC). Feedback of the helium gas back into the purging loop is also possible.

The helium has a flushing function and reduces or prevents accumulation of hydrogen in the insulation material.

The inner pressure of the inner tank wall can be kept slightly above the outer pressure of the inner tank wall by a back-pressure valve. This valve may also automatically control the helium feed. The helium feed can be constant flow controlled.

Any leakage of the hydrogen tank wall can be measured via the pressure drop in the helium system.

The outer skin may be used to integrate the tank mechanically into the main structure of the vehicle, e.g. aircraft. The outer skin may be adapted to bear structural loads. Furthermore, a leakage sensor can be employed to anticipate the tank health status.

Embodiments of the invention are described in more detail with reference to the accompanying schematic drawings. Therein the only Fig. depicts an embodiment of a hydrogen tank assembly <NUM> for a vehicle (not depicted) according to the invention.

The hydrogen tank assembly <NUM> comprises a hydrogen tank volume <NUM> for cryogenic hydrogen. The hydrogen can be stored therein at cryogenic temperatures and with a hydrogen pressure of preferably <NUM> bar or slightly above. Under these conditions the hydrogen is generally a liquid that boils of hydrogen gas.

The hydrogen tank assembly <NUM> includes an inner tank wall <NUM> that defines the hydrogen tank volume <NUM>. The inner tank wall <NUM> is made of a thin thermoplastic resin layer. The inner tank wall <NUM> includes a plurality of compensation members <NUM>. The compensation members <NUM> may be disposed on the outer circumferential surface of the inner tank wall <NUM> and extend in an axial direction of the inner tank wall <NUM>. The compensation members <NUM> are preferably configured to prevent thermal shrinkage of the inner tank wall <NUM>. The compensation members <NUM> may be made of an elastic meta-material, which are known per se.

The hydrogen tank assembly <NUM> has an inner tank wall cover <NUM>. The inner tank wall cover <NUM> may be configured as a thermal insulation <NUM>. The inner tank wall cover <NUM> is configured to allow an inert gas, such as helium, to pass through the inner tank wall cover <NUM> and flush hydrogen that leaked out of the hydrogen tank volume <NUM> away from the inner tank wall <NUM>. The inner tank wall cover <NUM> can be made of open porous foam, perforated material, or dry fiber material.

The thermal insulation <NUM> may also include a glass sphere material that is known in the field of cryogenics. The glass sphere material is usually a bulk material comprising a plurality of microscopic glass spheres. The glass spheres enclose a vacuum. The glass sphere material may be arranged within preformed chambers, such as those of a honecomb strucuture or the like. The vacuum in the glass spheres provides excellent thermal insulation while the "porosity" of the glass sphere material allows sufficient gas movement.

It is also possible that the tank wall cover <NUM> and/or the thermal insulation <NUM> has a plurality of channels. The channels are preferably configured in plane of the respective tank wall cover <NUM> or thermal insulation <NUM> such that the tank wall cover <NUM> and/or the thermal insulation <NUM> are divided into a pluralitly of sections. The extent of the sections along parallel to the inner tank wall <NUM> is determined by the pressure differential between the inner tank volume <NUM> and the shroud volume <NUM> (see below) such that the pressure drop of hydrogen passing through the tank wall cover <NUM> or thermal insulation <NUM> does not exceed the pressure differential.

The hydrogen tank assembly <NUM> comprises an outer tank wall <NUM> (also called outer tank skin). The outer tank wall <NUM> surrounds the inner tank wall <NUM> and defines a shroud volume <NUM>. The shroud volume <NUM> can be pressurized with an inert gas, such as helium, having a predetermined inert gas pressure. The inert gas pressure is chosen to be slightly smaller than the hydrogen pressure, e.g. <NUM> mbar less than the hydrogen pressure.

The hydrogen tank assembly <NUM> includes an inert gas source <NUM> that stores and pressurizes the inert gas. The inert gas source <NUM> is fluidly connected to the shroud volume <NUM> via an inert gas supply line <NUM>. The inert gas supply line <NUM> has an end portion <NUM> for discharging the inert gas. The end portion <NUM> is arranged in the vicinity of the inner tank wall <NUM> or within the inner tank wall cover <NUM>.

The hydrogen tank assembly <NUM> includes a main hydrogen line <NUM> that fluidly connects to a main hydrogen consumer <NUM>, such as an aircraft engine (not depicted). The main hydrogen line <NUM> is configured for supplying the main hydrogen consumer <NUM> with hydrogen.

The hydrogen tank assembly <NUM> includes an auxiliary hydrogen line <NUM>. The auxiliary hydrogen line <NUM> includes a selector valve <NUM>. The selector valve <NUM> allows the auxiliary hydrogen line <NUM> to fluidly connect to multiple outputs. The auxiliary hydrogen line <NUM> can fluidly connect to the environment for discharging excess hydrogen, to the main hydrogen line <NUM> or to a fuel cell <NUM>. A check valve may be installed where needed, in order to avoid back flow.

The hydrogen tank assembly <NUM> has a leakage line <NUM>. The leakage line <NUM> fluidly connects the shroud volume <NUM> to the auxiliary hydrogen line <NUM> via a check valve <NUM>. The leakage line <NUM> collects hydrogen that has leaked through the inner tank wall <NUM>. The leakage line <NUM> has an end portion <NUM> that is arranged in the vicinity of the inner tank wall <NUM> or within the inner tank wall cover <NUM>. The leakage line <NUM> may include a leakage sensor <NUM> that is arranged downstream from the end portion <NUM>, preferably adjacent to the check valve <NUM>.

The fuel cell <NUM> may be supplied with a mixture of hydrogen and helium via the auxiliary hydrogen line <NUM>. The fuel cell <NUM> transforms hydrogen and oxygen in a manner known into electrical energy and water. The inert gas does not react. The fuel cell <NUM> therefore also acts as an inert gas separator <NUM> that is configured for separating the inert gas from the hydrogen. The fuel cell <NUM> is also capable of generating a pressure that causes hydrogen or a mixture of hydrogen and the inert gas to flow from the tank volume <NUM> or shroud volume <NUM>, respectively, toward the fuel cell <NUM>. The fuel cell <NUM> may thus act as a pump without moving parts.

The hydrogen tank assembly <NUM> includes a recirculation line <NUM> that fluidly connects the inert gas separator <NUM> with the inert gas source <NUM>, so as to recirculate the inert gas back in to the system.

Claim 1:
A hydrogen tank assembly (<NUM>) for a vehicle, the hydrogen tank assembly (<NUM>) comprising
- an inner tank wall (<NUM>) that defines a hydrogen tank volume (<NUM>) configured for storing cryogenic hydrogen at a predetermined hydrogen pressure;
- an outer tank wall (<NUM>) that defines a shroud volume (<NUM>) which surrounds the inner tank wall (<NUM>); and
- an inert gas source (<NUM>) that is fluidly connected to the shroud volume (<NUM>) and configured for pressurizing the shroud volume (<NUM>) to an inert gas pressure that is smaller than the hydrogen pressure,
characterized by an inert gas separator (<NUM>) that is configured to separate hydrogen from the inert gas and a recirculation line (<NUM>) that is arranged to transport the separated inert gas from the inert gas separator (<NUM>) back to the inert gas source (<NUM>).