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Hydrogen cryogenic tank: a forward-looking concept for aeronautics

Given that 2% of global CO2 emissions come from aviation, and that CO2 emissions from aircraft have increased by 10% between 2014 and 2017, it is time to react and go green. The global energy transition is happening now. By 2050, aircraft will have to cut their CO2 emissions by 50%. The solution? Hydrogen, already being considered for ships and trucks, is an alternative solution for the production and storage of energy. But how can hydrogen be stored in an aircraft?

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Green aircraft in 2050

Hydrogen: the key to the future for our aircraft

Flying “green” while respecting the environment is the ambition of aircraft manufacturers by 2050. This will be possible thanks to electric rather than propelled aircraft. However, fitting batteries onto a aircraft due to their weight is inconceivable. Indeed, adding batteries to an aircraft would make it 20 times heavier. The solution: generate electricity on site. But how do you generate electricity in the air?

The choice landed on liquid hydrogen at -252.85°C., allowing less dangerous storage and larger quantities than its gaseous form. Today we are witnessing a growing utilization of hydrogen for electricity generation or as a primary fuel source.The advantage of hydrogen is its lightness, which makes weight reduction of the aircrafts possible, and therefore its energy consumption in flight.

In aeronautics, hydrogen will therefore be considered as an intermediate energy source to produce electricity. This process will be part of a virtuous circle since hydrogen will be produced from “green” electricity, itself produced from solar panels and wind power.

Thus, 3 different types of aircraft have been selected by Airbus for the development of “green” aircrafts:

First concept: the Turbofan. © Airbus

A turbojet with 120 to 200 passengers, which is equivalent to an A220 or an A320 with a range of more than 3500 kilometres.

A 100-passenger regional turboprop aircraft with a range of 1800 kilometres.

A flying wing with a capacity and an autonomy similar to that of a turbojet.

How to store hydrogen in an aircraft?

On a technical point of view, hydrogen is difficult to store. This gas has a very low density at ambient temperature and pressure: a volume of 11 m3 is needed to contain 1 kilo of hydrogen.

Storing it at room temperature and pressure is nonsense since it would take up too much space. Two options are therefore possible: either storing it under pressure in the gaseous state either cooling it to -252.85°C, its liquefaction temperature.

The optimal option is to liquefy it at a temperature lower than -253°C. In this case, the cryogenic tank is subjected to lower pressure as a liquid is less compressible than a gas. At the moment, this cryogenic temperature storage solution is used in the space sector, for hydrogen tanks, rocket fuel, and is being strongly considered in the aeronautics world. This solution is preferred to the pressurised tank for reasons of space and safety. Indeed, the density of liquid H2 is much higher than that of pressurised gas, making it possible to store the same quantity of hydrogen in a reduced volume. Moreover, this avoids the use of high pressures (700 bar) and the risks inherent in this solution.

A little reminder about cryogenics

Cryogenics is the study of low temperatures below -150°C. In short, it is the temperature at which gases liquefy.

Liquefaction temperature at atmospheric pressure:

  • Dioxygen (O2): -183°C,
  • Nitrogen (N2): -196°C,
  • Dihydrogen (H2): -253°C
  • Helium (He): -270°C.

Today we know how to reach these temperatures. The problem: the storage of these liquefied gases at these very low temperatures.

hydrogen cryogenic tank

Aluminium cryogenic hydrogen tank. Source : Stirweld.

Storage density of H2

Which material for a high-performance hydrogen cryogenic tank?

Steel, composite or aluminium?

Firstly, the material chosen for a cryogenic tank must absolutely comply with these criteria:

  • Damage, fatigue, ageing,
  • Chemical compatibility,
  • Resistance, stiffness, fragility,
  • Thermal expansion, permeability.

As a result, several materials are being considered for the hydrogen cryogenic tank. There is steel, which is too heavy and therefore not recommended, titanium alloys, which are too expensive, carbon fibre composite, which is non-recyclable and very expensive but very light, and finally aluminium, which is light, resistant and less expensive than composite. You can see that the battle is raging between composite and aluminium.

Composite poses two problems: its differential thermal expansion, which results in strong dimensional variations, the appearance of microcracks and therefore the permeability of the material. As for aluminium, its mechanical properties remain constant or even improve at low temperatures. As you can see, aluminium is the most suitable material for the manufacture of cryogenic tanks.

Indeed, due to its low density and good specific mechanical properties, aluminium would guarantee a lighter vacuum tank than cryogenic steel (of the nickel alloy steel type). Another positive point is that aluminium is infinitely recyclable, which is perfectly in line with current efforts to reduce the ecological footprint. In addition, aluminium has better resistance to low temperatures than steel. At very low temperatures, steel becomes as brittle as glass, which makes this material unusable for applications at very low temperatures. Conversely, aluminium is one of the only materials whose mechanical properties do not degrade at low temperatures. They even tend to improve. This is enough to explain aluminium’s very strong interest in cryogenic applications such as the storage of liquid hydrogen.

Comparative table: arc-welded steel, arc-welded aluminium and by FSW

Weldability of aluminium

One of the problems is the weldability of aluminium. Apart from its low density and good corrosion resistance, aluminium has a high thermal and electrical conductivity. These characteristics of aluminium make welding by conventional fusion welding processes more complex. Because of its high thermal conductivity, high welding energies are required. It is also important to bear in mind that aluminium has a strong tendency to deformation during welding and that defects may appear as a result of the welding operation (blowholes, porosities, hot cracking, weakening of mechanical properties, material degradation). In addition, before welding, the aluminium must undergo chemical etching of the refractory alumina layer.

Thus, with conventional welding techniques – TIG, MIG, MAG, electric arc – aluminium remains a difficult material to weld. However, in recent years we have seen the emergence of a new process for welding aluminium: friction stir welding or FSW. A breakthrough innovation that has shaken up the codes of our industry.

Friction stir welding: the solution for welding hydrogen cryogenic tanks

FSW is one of the best techniques for welding aluminium hydrogen tanks for aeronautics. As one of the most advanced trends in the welding world, FSW will make it possible to preserve the mechanical properties of aluminium. A FSW weld will have better mechanical properties (joint efficiency coefficient between 70 and 100%) than a MIG weld (a joint efficiency coefficient of 40-50%).

This process also allows a decrease in the post-weld defect rate, perfect repeatability and the possibility to weld high performance aluminium alloys. These high-performance aluminium alloys (2000 and 7000 series) are in fact reputed to be “non-weldable” by fusion. Today this is possible thanks to friction stir welding. Their use will allow a reduction in tank thickness and therefore a lighter structure. In the same way, FSW is part of this “green” aircraft dynamic since it is a clean process, with no smoke, no gas but also without the addition of material. This reduces the cost of consumables, leaving only one, the FSW tool.

The FSW, an innovative process in four steps:

Space in support of aeronautics

The themes tackled by the space sector are 100% transferable to the aeronautical field as far as the technology of hydrogen cryogenic tanks is concerned.

Decomposition of a rocket

Today most rockets have a main propulsion system based on the combustion of hydrogen and oxygen. Hydrogen, in its liquid form, powers the rocket engines. Indeed, during the first two minutes of a rocket’s take-off, thrust is provided by solid propellant boosters. These side boosters provide 90% of the power required for take-off. Then, the main cryogenic stage takes over. This stage consists of two tanks: a hydrogen tank and an oxygen tank. It is the combustion of oxygen and hydrogen that propels the rocket during the first ten minutes of flight.

The use of hydrogen is irreplaceable in space. It makes it possible to considerably reduce the mass and volume of launchers. Space is therefore definitely the example to follow in terms of cryogenic hydrogen tanks.

Hydrogen storage

The hydrogen is stored in a cryogenic tank in its liquefied form at -253°C. These tanks are made of aluminium. Due to its excellent mechanical properties at very low temperatures, aluminium is the perfect material for this application. The storage problems are the same as those in aeronautics.

For the past 15 years, rocket hydrogen cryogenic tanks have been welded by FSW with a retractable pin tool to fill the hole caused by the tool during the welding operation. FSW is preferred to other welding techniques for several reasons:

  • 100% leak-proof welding,
  • Use at high pressure (higher mechanical strength),
  • Easy replacement of TIG and MIG welding processes.
  • Extended weld life (improved fatigue resistance).

Today several launcher tanks have been welded by FSW. These include the Ariane 6 rocket, Space X’s Falcon 9, ULA’s Atlas V and Mitsubishi’s H-II. Others are in the process of being manufactured, such as Space X’s Falcon 9 FT & Heavy, the Angara A5, the Blue Origin, ULA’s Vulcan, Mitsubishi’s H-III and the Long March-5.

Green aircrafts are therefore a priority for aircraft manufacturers. It is urgent to reduce the ecological footprint of our aircraft and counter the boycott of them. We are talking here about biofuel, electricity, and other solutions. But the only one feasible today is hydrogen. Taking hydrogen in flight to create electricity is therefore the flagship development programme to go green by 2050. The storage of hydrogen will not be possible without a waterproof, light and space-saving tank. One of the materials in the race: aluminium. A lightweight material with improved mechanical properties at low temperatures and increasingly used. Its advantages: it is cheaper than composite and lighter than steel. What else can you ask for?

One problem could arise: its weldability. A welding technique solves this problem. Friction stir welding. A clean process that retains the mechanical properties of aluminium and guarantees perfect thermal conductivity and a perfect seal.

Stirweld, an expert in the field of friction stir welding or FSW  has joined the ESA incubator, ESA Business Incubation Centre in Northern France. This incubator, created in 2018, promotes the development of innovative companies and is based not on technology transfer but on the deployment of services and applications derived from space technology. The incubation of Stirweld within ESA will enable the development of new concepts of space parts welded by FSW. Before being incubated by ESA BIC, Stirweld already made 30% of its turnover in the space sector. One of its main customers is Ariane, a European aerospace company, for which Stirweld is a Tier 1 supplier.

Discover ESA BIC Nord France:

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If you would like to know more about FSW-welded hydrogen cryogenic tanks, please do not hesitate to contact us without any obligation.