Krzysztof Koziol, professor of composites engineering and head of composites and the advanced materials centre, at Cranfield University discusses ENABLEH2 and how the project has revitalised the enthusiasm in liquid hydrogen research for civil aviation.

Hydrogen was once the outsider in the race to deliver zero-carbon flight, just an eccentric alternative. Now it has become the frontrunner as a middle to long-term solution.

Liquid hydrogen has huge potential as a zero-carbon emissions fuel for the long-term: a much leaner combustion than any hydrocarbon fuel (fossil, bio or synthetic) that delivers ultra-low nitrogen oxide emissions (that contribute to acid rain and the ‘ozone hole’) and remove sulphur oxide and soot from the aviation emissions profile. The hard scientific evidence has been confirmed by the ENABLEH₂ (ENABLing CryogEnic Hydrogen-Based CO₂-free Air Transport) project — leading to EU approval for a hydrogen flight roadmap and the makings of a new regulatory framework.

The case is now there for transforming the industry infrastructure to suit a hydrogen and a hybrid hydrogen/electric future for the sector. But that still involves facing up to a number of tough technological challenges to build a very different hydrogen economy.

At the core of the hydrogen challenge, and the entire future viability of hydrogen flight, is its storage onboard aircraft and eliminating the threats to aircraft safety. Traditional processes and a reliance on existing materials won’t work.

What’s in store?

Storage tanks need to be able to withstand extreme variations in temperature: between -253ËšC and room temperature. Past studies have shown that -200ËšC has been a challenge in itself, moving the parameters to -253ËšC for liquid hydrogen brings a whole new level of problems.

Absolute reliability and resilience of the tank structure is essential for hydrogen flight of any kind. Given the nature of temperatures involved, a fracture would cause a huge explosion, not just within an aircraft but for a wider area. At the same time, the design of tanks needs to involve lightweight materials to minimise overall aircraft weight and fuel demands. Stainless steel tanks might provide safety but are unrealistic in terms of the added tonnage.

Cranfield University has led research into delivering hydrogen-powered aviation since the early 1990s, from production, storage and operations to propulsion. Following on from its role in ENABLEH₂, alongside players such as IATA, ICAO, International Airlines Group, Rolls-Royce, Siemens, Total, Heathrow Airport and Airbus, Cranfield has taken on leadership of the work on hydrogen for the UK-Aerospace Research Consortium (a consortium of 11 universities).

Over the past year, the University has been working on developing the most efficient and workable materials for next generation fuel tanks. This is part of the £40 million Airbus project to develop the world’s first commercial hydrogen aircraft by 2035. Airbus plans both a regional aircraft type design, operating routes of up to 1,850km, and a short-medium range (SMR) aircraft for routes up to 3,700km. The streams of research and development, jointly funded by the UK government’s Aerospace Technology Institute (ATI), are based around the company’s new Zero Emission Development Centre (ZEDC) for hydrogen technologies in Filton, Bristol.

In order to use hydrogen as fuel in the skies, there is the need for a design of a type VI cryogenic tank. This involves a new bill of materials and deployment of three levels of safety features — each one of which means exploring new generations of materials. With this approach there is the guarantee that even if active cooling or other systems fail, the materials being used will keep the liquid hydrogen at a safe temperature and pressure environment for long enough for the aircraft to land and issues be resolved.

One type of materials under development is based around a new form of self-healing polymers. Hydrogen is known for its diffusion into any type of material, consequently causing different forms of structural damage to whatever material it might be. To minimise safety concerns, there is the need for a material that is capable of both self-diagnosis and self-healing of damage caused to the structure. A self-healing polymer is particularly effective in repairing minor cracks and avoiding a worsening of the damage.

The next critical material for the tank is made up of aerogels: the synthetic porous ultralight materials derived from a gel which retain their form even when the liquid element is replaced with a gas. The nature of the materials makes them the lightest possible material yet created.

Finally, a two-dimensional graphene layer is being used to maximise the reduction of hydrogen leaks and to take the mechanical stability of the overall structure to another level. The one-atom-thick layer of carbon has a hexagonal nanostructure, which is estimated to be 200 times stronger than steel.

Extensive work is ongoing at Cranfield around the material development and molecular tuning to achieve the levels of performance needed, as well as the use of different combinations in order to find the optimal mix of tank materials for performance and safety. The first prototype of the type VI cryogenic hydrogen tank will be tested over the coming year, with flight testing (possibly from Cranfield airport) expected to start in 2026.

Delivering sustainability

The use of hydrogen fuels is increasingly looking like an essential element in delivering sustainable aviation for the long-term. Critically that means early guarantees of on-board safety — but it is also just one part of challenge around hydrogen. Hydrogen emissions are, in themselves, an added threat when it comes to climate change. Hydrogen molecules contribute to global warming indirectly, in short-term ways, by extending the lifetime of other greenhouse gases. Full hydrogen delivery systems will rely on networks of tank storage, pipelines and valves, each with the potential to involve loss through pressurisation, depressurisation, permeation leakage, and accidents.

With this in mind, the work in aerospace and the lessons in terms of materials, are going to be hugely important in making the hydrogen economy safe, viable and a genuine basis for sustainability.

www.cranfield.ac.uk