Hydrogen is the most promising technology for less polluting aviation. The first flights have proven successful but its production remains very energy consuming, its storage challenging and the existing infrastructure unsuitable. See how industry, researchers, and EU projects are taking up these challenges to get ready for take-off
You might have given up driving for the sake of the environment, or you might have chosen to invest in an electric vehicle. But are you ready to give up flying? It appears that aviation is one of the most polluting and least climate-friendly industries. According to figures published by the European Commission, if global aviation were a country, it would rank in the top 10 emitters, and someone flying from Lisbon to New York and back generates roughly the same level of emissions as the average person in the EU does by heating their home for a whole year.
But people are not going to stop travelling, and they will certainly not stop using aircraft. So, how can the aviation industry contribute to meeting ambitious targets such as the one set by the EU to reach climate neutrality by 2050? There is no unique definitive solution, but several projects are being discussed and tested in order to make aviation much cleaner. Right now, hydrogen is all the rage in the industry.
Some of the companies which have successfully tested hydrogen-propelled aircraft are Universal Hydrogen in California and ZeroAvia in Great Britain. At the same time, Airbus “expect to make the necessary decisions on the best combination of hydrogen technologies by 2025”. Hydrogen is the most common element in the universe and is already widely utilised in the fertiliser and chemical industries. But extracting it is no joke, and it requires a massive amount of energy. As Dominic Eagleton from the ONG Global Witness puts it, “If you do it badly, then it’s a very high emitting fuel.” He explains that the main production process today implies using fossil gas: “For every ton of hydrogen that’s produced, you’re looking at a minimum of eight tons of CO2 release up to about 14 tons, depending on how it’s done. And a really important point here is this is just carbon dioxide emissions we’re talking about, which occur at the point of production. But if you look at the supply chain for fossil gas, that releases a lot of methane, which is a very damaging greenhouse gas because it’s more than 80 times more potent as a climate pollutant over a 20-year period.”
The solution would be the so-called “green hydrogen”, which is, however, a minimum amount of the hydrogen currently produced. According to the International Energy Agency, IEA, “In 2022, 70% of the energy requirement for dedicated hydrogen production was met with natural gas and around 30% with coal (mostly used in China, which alone accounted for 90% of global coal consumption for hydrogen production). Low-emission hydrogen production represented less than 1% of total hydrogen production in 2022, despite growing 5% compared to 2021.”
The problem is that producing green hydrogen, which means exclusively through renewable energy, is very expensive. For a producer, it implies building their own renewable energy plant – a wind farm, a solar farm, a hydroelectric plant… – and plugging the electrolyser directly into it. Right now, an easier way to produce hydrogen is to plug into the grid, which means the electricity may come from renewable energy but also from fossil energy. This is what ZeroAvia is doing at the moment, but they are planning to go fully green by installing their own solar panels on the hangar at their R&D facility, where the company has built a microcosm of hydrogen infrastructure for airports, with pipelines and refuelling trucks. As one of the challenges of hydrogen-powered aviation is that the entire infrastructure will have to be rethought, not just the aircraft.
Another challenge is that hydrogen has lower volumetric energy density, which means that more onboard fuel storage is needed to cover the same distance compared to current jet fuels. Existing fuel tank volumes would only allow for very short routes, and the location of current fuel tanks in an aircraft’s wings would not support hydrogen storage in gas or liquid forms. ZeroAvia is seeking to resolve this problem in order to enable it to retrofit planes with fuel cells – the aim being to achieve huge environmental benefits, explains the head of External Affairs, Dominic Weeks: “With hydrogen, you have zero carbon emissions even if you burn it in a combustion engine. But if you use it in a fuel cell, it’s a more efficient use of the fuel. And it also has no emissions other than water vapour. So, having assessed it, we saw that there was a pathway to take the technologies that have been pretty well tested in road transport – fuel cells – and develop those further for aerospace and to have something flying much sooner than many other people thought.” The final goal is to get certification by 2025, which is why they also decided to retrofit existing aircraft rather than develop new ones or wait for a new airframe to come onto the market.
Still, there is one main challenge remaining: the volumetric storage, says Weeks. “That’s why the move to liquid hydrogen for aircrafts above 20 seats is so important,” he adds. It’s the aim of the OVERLEAF project: to develop a game-changing liquid hydrogen storage tank for the aviation industry. In order to increase the gravimetric index of the liquid hydrogen and so save space, the tank will be split in two parts: “The inner tank is responsible for the storage of the liquid hydrogen. It has to be under cryogenic temperatures, while the outer tank, which is not under cryogenic temperatures, is the structural part of the tank, that has to be able to support all the loads that we have on the tank itself,” says Noelia González, a researcher at Aimen, one of the partners of the project.
This means the inner tank will have to be at a temperature of 20 K, the equivalent of -253.15° C. The project must take into account other challenges, such as the weight of the tank, which is crucial in aviation, the costs, and of course the safety level. A new material is also being developed, which will also take into account circularity. The aim, says Emma Celeste Lope Retuerto, project coordinator, is to do the final test in Romania at the beginning of 2025. If it works, this disruptive technological solution should be able to be installed in all kinds of aircraft, including for long distances.
So what’s the future of aviation with hydrogen-propelled aircraft? Here is the prediction of Dominic Weeks from ZeroAvia: “The big change that hydrogen electric aircraft will bring is that the lower operating costs will make more routes viable economically. So more people will be connected, more aircraft flying. And it could be a renaissance for regional turboprop aircraft over the next few years, which fly in an underused portion of airspace. And crucially, they also fly where basically there isn’t contrail formation, so that water vapour doesn’t have an environmental impact released in those altitudes. So, we’re going to see an increase in traffic, which is obviously something that needs to be managed, but there are huge potential benefits from that. But this kind of technology will do two things: it will eliminate air quality impacts entirely. So there will be no air quality impacts on the health of people living proximate to airports. And it will also reduce the noise of the aircraft, because there is no engine noise.”
A paradise? We’ll see. In the meantime, hydrogen fuel at the moment remains the most promising technology to reduce pollution and fight climate change in the aviation industry without having to give up connecting with different cultures in far-away countries.