Could liquid hydrogen be the fuel that transforms the future?
According to the International Energy agency (IEA), aviation accounted for 2% of global energy-related CO2 emissions in 2022, having grown faster in recent decades than rail, road or shipping. To curb growth in emissions, many technical measures related to low-emission fuels, improvements in airframes and engines, operational optimisation and demand restraint solutions are needed over the next decade in order to get the sector on track to meet its Net Zero emissions target by 2050.
Sustainable aviation fuels (SAF) offer a bridge for existing fleets, but are currently expensive, production-limited and still produce CO2. Hydrogen, by contrast, is carbon-free and surpasses kerosene for gravimetric efficiency, the crucial energy-to-weight ratio in aerospace. Liquid hydrogen also requires 80% less fuel tank space than gaseous to deliver the same energy due to its reduced volume. FlyZero, a research programme run by the UK Government-funded Aerospace Technology Institute (ATI), has validated the feasibility of hydrogen-powered aircraft through physics-based models of three study aircraft of increasing size and range.
Hydrogen-powered aircraft will, however, require fuel cell or adapted turbine-based powertrains, cryogenic tanks and extensive hydrogen infrastructure at airports, posing a myriad of challenges to current innovation and technology development efforts.
THE HYDROGEN CONVERSATION
Hydrogen’s huge potential for the aerospace sector has, naturally, started many conversations around the technological developments, regulation, funding and research and testing programmes that will be required to effectively leverage liquid hydrogen as a fuel for future aircraft. Speaking at Advanced Engineering in November, Peter Young, Chief Engineer of Ultrafan at Rolls-Royce, is observing positive signs in this area. “The thinking and understanding of how we handle hydrogen is developing within the industry, and it’s something we need to do together,” he said. “As organisations, we are all grappling with a relatively new substance and what it might do to traditional aerospace materials. We need to learn, and the fact that there’s an openness to doing so is positive. From a regulatory point of view, I’m interested to see what develops in terms of ensuring the right level of safety, and I think there is going to be a big shift in terms of regulation which will require certain expertise.”
The High Value Manufacturing (HVM) Catapult is undertaking a lot of work in this space through its seven centres of innovation to grow the UK’s hydrogen supply chain. The catapult’s awareness modules on hydrogen offer insights into hydrogen production, storage and distribution, as well as the opportunities for industry in these areas.
Katie Milne, Project Director for Hydrogen at the HVM Catapult, added to the conversation, saying: “One of our partners is the Aerospace Technology Institute (ATI), who we are working with on its hydrogen capability network and its objectives around developing UK testing infrastructure. We are working with the UK aerospace sector to build a case around liquid hydrogen to say that it needs to be pulled forward as fast as possible.”
TEST AND DEVELOPMENT
Rolls-Royce has committed itself to being at the forefront of the development of hydrogen combustion engine technology capable of powering a range of aircraft, including those in the narrow-body market segment, from mid-2030. Most recently, the company successfully proved that a full annular combustor of a Pearl 700 engine running on 100% hydrogen could be combusted at conditions that represent maximum take-off.
The company’s UltraFan demonstrator aero engine is also enabling the exploration of hydrogen power solutions, Young said: “We’re looking at composite materials, additive manufacturing, metal injection moulding (MIM) – there’s a whole raft of different technologies to improve component efficiency and effectiveness.”
This early on in the liquid hydrogen development process, it’s important to look at the evolution of technology in other industries, he added, “In the car industry, we’ve reached the point where we have electric and hybrid electric – electrification is the main route for improving efficiency there. Everything needs an associated business case, and while we are developing some of these new technologies in aerospace, it is currently a challenge to see how we can achieve a high enough efficiency to prove that this technology is viable at scale.”
ENSURING SUITABLE INFRASTRUCTURE
In addition to developing, testing and regulating technologies for the use of liquid hydrogen on aircrafts, another vital piece of the puzzle is infrastructure.
“We’re beginning to work with airports, as that is a big issue,” explained Milne. “Liquid nitrogen has four times the volume of kerosene for the same type of mission, so you’re going to need monitoring structures and decisions need to be made about whether that liquid nitrogen is produced by electrolysis in airports or shipped in, and where it is liquefied. These are huge infrastructure challenges.”
Hydrogen production will also be a key consideration going forwards, she added, “Hydrogen production and SAF has to be derived from somewhere. How the provision and prioritisation for production will play out will be interesting, as you will have aerospace applications, automotive vehicles, infrastructure requirements and so on. I think from an aerospace perspective, there is a lot of uncertainty of the ‘roots’ which need to be explored.”