31/03/2025
Attending the Institution of Mechanical Engineers’ ‘Engineering a Hydrogen Economy’ conference in Birmingham in early March provided a great overview of developments in both the production and use of hydrogen. The conference included a focus on Aviation, including presentations by Rolls-Royce and the Aerospace Technology Institute (ATI).
Hydrogen can be used as a fuel source for aircraft in the following two ways:
- Feeding hydrogen to an aircraft engine for combustion. The burning of hydrogen does not generate CO2 emissions.
- Using hydrogen in a fuel cell to generate electrical energy for use in powering an electric motor to drive a propulsion unit. As well as avoiding CO2 emissions, this also avoids NOx emissions.
When looking at hydrogen storage on-board an aircraft, cryogenic storage in liquid form is favoured over storage in gaseous form. However, storing hydrogen in liquid form requires fuel tanks insulated to maintain the hydrogen at temperatures of -253˚C or less. Further, as the energy density of liquid hydrogen is far lower than that of kerosene (~8.5 MJ/L versus ~35 MJ/L), the fuel storage volume required for an aircraft to achieve a given range will be greater for liquid hydrogen compared to kerosene. Fuel storage within the main fuselage of an aircraft rather than within the wings is likely to be necessary, as reflected in ATI’s FlyZero project which looked at three aircraft concepts employing liquid hydrogen fuel.
There remain challenges in the use of hydrogen in aviation, although these challenges also provide opportunities. Below, we have looked at three areas relating to the use of hydrogen in aviation by reference to some corresponding published patent applications:
1. Relieving pressure from gaseous hydrogen build-up in an aircraft fuel system without venting hydrogen to atmosphere
Where hydrogen fuel is cryogenically stored on an aircraft in liquid form, heat will be imparted to the hydrogen on passing from the fuel tank through the fuel system of the aircraft. The imparted heat may be sufficient to cause hydrogen in the fuel system to change phase from liquid to gaseous form and consequently expand. Without intervention, the expansion could impose excessively high pressures within the fuel system. Simply venting the gaseous hydrogen to atmosphere would be wasteful – and also potentially dangerous in view of the combustibility of hydrogen gas.
This first application relates to a hydrogen fuel system for an aircraft. The application describes a concept in which excess gaseous hydrogen in the fuel system is mixed with oxygen to form a mixture which is less flammable than hydrogen alone. The hydrogen / oxygen mixture is then fed to a catalyst bed to catalyse a reaction between the hydrogen and oxygen. The reaction that occurs is one of Catalytic Hydrogen Combustion, which commonly occurs at lower temperatures than flame-based combustion. The end result is an exhaust stream of high pressure steam rather than combustible hydrogen gas. Further, the exhaust stream has the potential to serve as an additional source of thrust for the aircraft.
2. Using byproducts of hydrogen combustion for an aircraft ice protection system
Although liquid hydrogen is cold – very cold – its combustion generates a lot of heat.
This second application relates to using a byproduct of combusting hydrogen – namely steam – in an ice protection system of an aircraft. The application describes how the steam may be used to both prevent ice formation and remove ice build-up on critical components of an aircraft engine; for example, for the surfaces of nacelles and guide vanes of a gas turbine engine. In one example, water vapour contained in the combustion products of a gas turbine engine burning hydrogen is condensed and sprayed into the engine inlet. The condensed water has a temperature higher than the ambient air flow entering the engine inlet. The spray of relatively warm water forms a film on the surfaces of the engine guide vanes to inhibit ice build-up. In another example, steam bled from the core air passage of a gas turbine engine burning hydrogen is fed to internally heat the lip of the engine nacelle to de-ice and/or prevent build-up of ice.
3. Reducing hydrogen permeation through the walls of fuel tanks and pipes
A particular problem with storing and transporting hydrogen is avoiding hydrogen losses due to permeation through the walls of the vessels and pipes used to contain and convey the hydrogen. Permeation losses arise due to the small size of hydrogen molecules. Whilst permeation losses are far less for liquid hydrogen compared to gaseous hydrogen, careful thought is needed when selecting materials for the containment of hydrogen. The benefits of minimising or preventing hydrogen permeation apply to storage and use of hydrogen on-board an aircraft, as well as to airport on-site storage and handling of hydrogen.
This third application relates to vessels made of composite material for storing hydrogen. The application is focussed on aircraft applications where a vessel contains liquid hydrogen. The application describes use of a layer of graphene for the purpose of supressing permeation of hydrogen through the wall of the vessel.
The challenges in adapting and designing aircraft and airport infrastructure for using hydrogen in place of kerosene will provide opportunities for innovation and the development of valuable new intellectual property (IP). We at Reddie & Grose are very much looking forward to working with clients in protecting their IP to help them achieve their commercial and environmental goals.
This article is for general information only. Its content is not a statement of the law on any subject and does not constitute advice. Please contact Reddie & Grose LLP for advice before taking any action in reliance on it.
