Engineers are used to the idea of working with fuels and materials where there is a risk of fire but a relatively low risk of explosions. Until recently, in spite of the rapid development of hydrogen technologies, few people have been talking about explosion safety, especially in aviation.
Hydrogen has been used for centuries in industries like mining, chemicals, and water — but not in the quantities, types, and conditions that are planned for civil aviation.
In general, the storage, transfer, and combustion of hydrogen requires special care because of its highly explosive nature when confined and mixed with air (oxygen). Hydrogen has one of the widest flammability ranges and one of the lowest ignition energies among common fuels. Made up of tiny molecules, hydrogen is known for its diffusion into any type of material, causing different forms of structural damage to whatever material it might be. Using traditional metals for example, such as steel or aluminium, the hydrogen works its way into the lattice of the metal and causes it to become brittle over time, leading to cracking. With polymers using a carbon-fibre and glass-fibre tank with a coating inside, the hydrogen will penetrate through the lining and without additional care to the level of tank emptying and filling, the coating will buckle and ultimately fail.
Now, the need for higher energy densities for hydrogen to match traditional kerosene fuels means storage at high pressures and extreme temperatures (up to 10 bar and below -253 degrees centigrade) that present a new and difficult series of challenges for engineers.
With the focus on hydrogen as a critical technology for long-term decarbonisation of global aviation, safety is suddenly high on the agenda. The commitment of the sector has been demonstrated, for example, by Airbus’ development of the ZEROe aircraft, which aims to be the first hydrogen-powered zero-emission commercial aircraft targeted for entry into service by 2035.
There are precedents in engineering for working in extreme conditions and extreme parameters, such as in space and subsea environments; and there is confidence that the use of hydrogen will be safe when the materials, equipment and standards have been adopted with the appropriate care
In April 2024, an Aerospace Technology Institute event series drew attention to the major challenges around adopting hydrogen-powered aviation based on roundtable discussions with the representatives of major stakeholders, such as Rolls-Royce, GKN Aerospace, the Health and Safety Executive, and BOC.
The events identified the following bottlenecks for scaling innovation: establishing test facilities, characterising material properties, and determining safe operating conditions. In other words, the industry doesn’t yet fully understand how hydrogen will interact with the conditions involved.
There are precedents in engineering for working in extreme conditions and extreme parameters, such as in space and subsea environments; and there is confidence that the use of hydrogen will be safe when the materials, equipment and standards have been adopted with the appropriate care. Regulatory bodies like the EASA are working closely with manufacturers to develop new safety standards for hydrogen-powered aircraft, while collaborations like the Clean Sky 2 initiative are fostering innovation across the sector.
The demand for professionals with hydrogen-related skills and expertise started to increase. Thousands, if not tens of thousands, of engineers will be needed with a specialist understanding of hydrogen in order to deliver the necessary infrastructure of production, transport, and storage, and to maintain standards of safety throughout day-to-day operations.
In March 2024, £69 million was invested into the creation of the Cranfield Hydrogen Integration Incubator (CH2i), the first major hydrogen technology hub tasked with demonstrating the potential of hydrogen as a net zero aviation fuel. As part of this growing network of practical activity around hydrogen for aviation, the university is working alongside industrial partners to offer courses disseminating the state-of-the-art knowledge and experience of engineers working with hydrogen-related technologies.
More widely, there is a need for academia and industry — where there is still only a relatively small number of experienced professionals — to pool their practical hydrogen knowledge and experience in order to train the next generation of engineers and develop the necessary specialisms. In universities, for example, sophisticated computational modelling tools are available to play through hundreds of different scenarios to assess the particular risks around hydrogen use in different circumstances and in combination with different materials, gases and powders, as well as the potential impacts of an accident. Specifically, computational fluid dynamics can be used to design sensor placement for leakage detection, to model released gas propagation, and to design equipment for explosion protection.
With its zero emissions potential, hydrogen in both gas and liquid forms is going to play an important role in the green energy landscape. An evolution in engineering expertise will be essential to creating a very low risk environment in aviation, proving viability and safety for wider applications.
Dr Tamás Józsa, Lecturer in Computational Fluid Dynamics, Centre for Computational Engineering Sciences, Cranfield University
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