FAQ Guide – Hydrogen Tank Inner Liner
- What is a hydrogen tank inner liner?
- What is the main function of a hydrogen cylinder inner liner?
- What are the different types of hydrogen tank inner liners available?
- What materials are commonly used for inner liners of hydrogen tanks?
- How are hydrogen tank inner liners typically manufactured?
- What are the key performance characteristics of a hydrogen tank inner liner?
- What factors should be considered when selecting an inner liner?
- What is the maximum pressure or temperature that an inner liner can withstand?
- What are the possible ways in which the inner liner is damaged?
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What is a hydrogen tank inner liner?
A hydrogen tank liner is a critical component of a hydrogen fuel storage system. The inner liner is essentially the innermost layer of the hydrogen tank and is responsible for containing the hydrogen fuel. The inner liner of a hydrogen tank serves as the base for a composite pressure vessel, and it is the central component of the pressure vessel. The inner liner is typically made of a material which can safely contain the high-pressure hydrogen gas. It serves as the inner shell of the tank, absorbs the hydrogen, encloses the hydrogen, and seals the contents of the pressure vessel against the surrounding environment. By winding the inner liner with carbon fibres, the hydrogen tank obtains its stiffness against pressures up to 700 bar.
What is the main function of a hydrogen cylinder inner liner?
The main function of a hydrogen tank liner is to provide a gas-tight storage space for the hydrogen fuel to contain and protect the hydrogen gas stored inside the cylinder. As hydrogen is a very volatile gas, it requires a high level of containment to ensure that it does not leak or escape. The inner liner plays a critical role in achieving this function, by providing a barrier that minimizes hydrogen permeation and loss during storage over a long period of time.
Inner liners especially made by polyamide, polyethylene, and cross-polyethylene have the advantage of minimal permeability against hydrogen. This means that they are able to provide a very good barrier against the volatile gas, resulting in minimal losses during storage.
- Polymer Liner: The thermoplastic inner liner serves as a gas-tight storage. When selecting materials for the inner liner, it is important to consider the barrier properties of the material to ensure that it can effectively contain the hydrogen fuel.
- Composite: The composite material is the load-bearing part of the composite pressure vessel and is used to support the load.
- Boss: The boss part made of metal represents the connection point between the liner and the filling and outlet valve.
What are the different types of hydrogen tank inner liners available?
Type I: The inner liner of a Type I pressure vessel consists mainly of a metallic (usually steel) wall. Nominal pressures for this type of vessel are typically in the range of 200 bar and are commonly used in the gas industry. There, such vessels are used as transport vessels as well as for stationary applications.
Type II: Type II pressure vessels have, in addition to the metal wall, a coating of resin-impregnated glass or carbon fiber. In the case of the Type II pressure vessel, this coating is placed completely around the cylindrical part of the vessel. On the one hand, this results in a slight weight advantage due to the lower wall thicknesses that can be specified. In addition, pressures of up to 1000 bar can be achieved, so that Type II pressure vessels are mainly used in stationary applications such as storage tanks at hydrogen filling stations.
Type III: Type III vessels have an inner liner made of metal (usually aluminum) and typically a carbon fiber wrapping around the entire vessel. In this case, the carbon fiber wrapping carries the major part of the load. This Type Is distinguished from Types I and II by its particularly high gravimetric energy density, so that it is used today mainly for mobile applications, for example in fuel cell vehicles. Typical pressures in mobile applications are 350 or 700 bar. It is, of course, also suitable for stationary applications, but the higher material costs compared with types I and II must be taken into account.
Type IV: The Type IV pressure vessel is the most recent pressure vessel that is currently manufactured in series. The liner is made of plastic (typically polyamide or polyethylene) and the shell is usually made of carbon fiber, as in the Type III vessel. This type of construction gives the pressure vessel a further weight advantage over the other three types. Applications for the Type IV pressure vessel are in both the transport and mobile sectors.
What materials are commonly used for inner liners of hydrogen tanks?
The materials used for inner liners of hydrogen tanks depend on the type of tank. The commonly used materials for each type of tank are as follows:
- Type I: Usually steel is used for the inner liner of Type I hydrogen tanks. These tanks are made of a single layer of steel with no reinforcing fibres.
- Type II: Usually steel is also used for the inner liner of Type II hydrogen tanks. However, unlike Type I, Type II tanks have a layer of reinforcing fibres, such as glass or carbon, that are embedded in a resin matrix to improve the strength of the tank.
- Type III: Usually aluminium is used for the inner liner of Type III hydrogen tanks. These tanks have a layer of aluminium alloy that is wrapped with reinforcing fibres, such as carbon or glass, and a resin matrix.
- Type IV: Polyamide, PE, XPE are commonly used for the inner liner of Type IV hydrogen tanks. These tanks are made of a plastic liner, typically polyamide, wrapped with layers of reinforcing fibres, such as carbon or glass, and a resin matrix. The plastic liner serves as the barrier for hydrogen gas, while the reinforcing fibres provide strength to the tank.
It’s important to note that the materials used for inner liners of hydrogen tanks must be compatible with hydrogen gas, which can cause embrittlement or stress cracking in certain materials. Therefore, the selection of materials for hydrogen tank liners is a critical aspect in ensuring the safety and reliability of hydrogen storage systems.
How are hydrogen tank inner liners typically manufactured?
The manufacturing process for hydrogen tank liners depends on the type of tank and the material used.
- For Type I and Type II tanks, which typically use steel or aluminium, the manufacturing process typically involves deep drawing, rolling, or welding. These processes are used to form the metal into the desired shape of the inner liner.
- For Type III tanks, which also use aluminium, the inner liner is typically formed by wrapping an aluminium alloy sheet around a mandrel and then wrapping reinforcing fibres and a resin matrix around the aluminium liner. The resulting liner is then cured to form a rigid structure.
- For Type IV tanks, which use a plastic liner wrapped with reinforcing fibres and a resin matrix, there are two major manufacturing processes for the inner liner: rotomolding and extrusion blow molding. Rotomolding involves heating a plastic resin in a mold that is rotated slowly in multiple axes to evenly distribute the resin and create a hollow liner. Extrusion blow molding involves extruding a plastic tube that is then inflated like a balloon into the desired shape of the inner liner.
Overall, the manufacturing process for hydrogen tank inner liners requires careful attention to detail and strict quality control to ensure the final product is safe and reliable for storing hydrogen gas.
What are the key performance characteristics of a hydrogen tank inner liner?
The key performance characteristics of a hydrogen tank inner liner include:
- Hydrogen permeation resistance: The inner liner of a hydrogen tank must have low permeability to prevent the leakage of hydrogen gas. The material used for the inner liner should have a high resistance to hydrogen permeation.
- Mechanical strength: The inner liner must be able to withstand the stresses and strains that occur during the filling, storage, and emptying of the tank. The material should have high mechanical strength, durability, and resistance to cracking, tearing, and puncture.
- Compatibility with hydrogen: The inner liner material should be compatible with hydrogen gas and should not react with it chemically. This is important to prevent any degradation of the inner liner material over time, which could result in hydrogen leakage.
- Resistance to high temperature and pressure: Hydrogen gas is stored under high pressure and at low temperatures. The inner liner must be able to withstand these conditions without any damage or deformation.
- Low weight: To maximize the amount of hydrogen that can be stored in a tank, the inner liner material should be lightweight. This is especially important for applications where weight is a critical factor, such as in aerospace or automotive applications.
- Manufacturing and processing: The inner liner material should be easy to manufacture and process into the required shape and size. It should also be cost-effective and readily available.
What factors should be considered when selecting an inner liner?
When selecting an inner liner for a hydrogen tank, several factors should be taken into consideration to ensure that the liner meets the specific requirements of the application. Some of the factors to consider when selecting an inner liner include:
- Cost and weight: The cost and weight of the inner liner can be important factors, especially in applications where weight and cost are critical design considerations.
- Choice of materials: The functional properties of the materials used for the inner liner should be appropriate for the specific needs of the application. For example, the thermal properties of the liner, such as heat aging resistance and extreme temperature resistance, are important considerations.
- Type, pressure rating, and permeation: The type of tank, pressure rating, and permeation requirements should also be taken into consideration when selecting an inner liner. Different types of tanks may require different materials or liner designs, and the pressure rating and permeation requirements may dictate specific liner properties or configurations.
What is the maximum pressure or temperature that an inner liner can withstand?
The maximum pressure or temperature that an inner liner can withstand depends on several factors, including the material used, the design of the tank, and the specific operating conditions.
For example, Type IV inner liners made of plastic materials such as Polyamide, PE, and XPE can generally withstand temperatures ranging from -40°C to 125°C.
Regarding pressure ratings, a Type IV pressure vessel (wrapped with carbon fibers) can normally be subjected to an operating pressure of 350 or 700 bar, with the burst pressure around 1,750 bar. The wrapping of the liner is designed to absorb the pressure and provide additional strength to the overall tank structure.
It is important to note that the maximum pressure or temperature that an inner liner can withstand should always be within the operating limits specified by the manufacturer and should be appropriate for the specific application. Any deviation from the specified limits can compromise the safety and reliability of the hydrogen storage system.
What are the possible ways in which the inner liner is damaged?
There are several possible ways in which the inner liner of a hydrogen tank can be damaged, including:
- Mechanical damage: The liner can be damaged due to impacts, scratches, abrasion, or other forms of physical damage. This can occur during handling, transportation, installation, or during the operation of the tank.
- Chemical damage: Exposure to certain chemicals or gases can cause damage to the liner material, leading to degradation, swelling, or other forms of chemical damage.
- Thermal damage: Excessive heat or cold can cause the liner material to expand or contract, leading to cracks, fractures, or other forms of thermal damage.
- Fatigue damage: Over time, the repeated cycles of pressure and temperature changes can cause fatigue damage to the liner material, leading to cracks, fractures, or other forms of fatigue damage.
Content contributed by Elkamet
Elkamet are experts in the field of tank development and manufacturing. Over the past years, they have applied their development skills into developing inner plastic liners for a variety of applications and have evolved as specialists in this area. For many years Elkamet is supplier for the inner liner of CNG tanks and in the recent years also deeply involved in many projects for H2 liners. From their manufacturing facilities in Wilhelmshütte (GER), Myslinka (CZ) and East Flat Rock (US), Elkamet, each year supply more than 700,000 parts produced in roto- and blowmolding to their customers. All their sites are certified according to IATF 16949.
Last update: 24.05.2023
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