Green sky thinking: the rise of hydrogen flight

In the ongoing push for cleaner, low carbon passenger aircraft, engineers have - in recent years - explored a wide variety of technology pathways: from the refinement of existing turbine technology, to the development of new sustainable fuels, and even the application of hybrid-electric battery systems.

But last year - as a sector laid low by the ravages of COVID-19 contemplated its future - the balance of power swung firmly in favour of an energy carrier that’s viewed as an increasingly important component of our future energy mix: hydrogen.

Whilst interest in hydrogen has been growing for some time, the most striking sign of its ascendance came in September 2020, when Airbus - the world’s largest aircraft manufacturer - announced that hydrogen fuelled propulsion systems would be at the heart of an ambitious plan to introduce a new zero-emissions commercial aircraft by 2035.

In a headline grabbing statement of intent, the aerospace giant unveiled concepts for three hydrogen-combustion powered airliners that offer a glimpse of what this future might look like.

Hydrogen has the same energy level as kerosene generating the same type of range and performance with one third of the weight

Grazia Vittadini - CTO, Airbus

Codenamed ‘ZEROe’ the three concepts include a turbofan design aimed at the 2000 nautical mile, 120 – 200 passenger segment; a smaller turboprop concept catering for flights of around 1000 nautical miles; and - most strikingly of all - a futuristic looking blended-wing aircraft that shows how the quest for low carbon aviation could usher in a revolution in airframe design.

Speaking during the online unveiling of the airliners, Airbus chief technology officer Grazia Vittadini said that whilst the company remains interested in various low carbon technologies, hydrogen is now emerging as the most promising route to zero emission aviation. Indeed, thanks to its compelling energy density, it could, she claimed, ultimately account for as much as 50 per cent of civil aviation’s decarbonisation. “Hydrogen has the same energy level as kerosene generating the same type of range and performance with one third of the weight,” she said.

The catch, of course, is in the volume. Indeed, at isoenergetic conditions the volume of hydrogen is four times that of kerosene. And as Vittadini explained, this means that one of the big challenges that lies ahead is tank design and integration.

Exactly how this will be overcome is yet to be decided, but all three of the Airbus concepts offer some suggestions on how it might be achieved: with the turboprop and turbofan concepts setting aside the aft section of the fuselage for fuel storage, whilst the blended wing concept illustrates how future airframe designs could evolve around this requirement.

Another fundamental component that will need to be de-risked and studied, said Vittadini, is hydrogen cryogenics. “Hydrogen is a gas  and it turns into liquid at -253C,” she said, “so it will be key to bring hydrogen to that temperature and keep it there throughout all phases of flight.”

These challenges and others are already under investigation she said, adding that Airbus aims to have initial results from a zero emissions demonstrator at some point this year. According to the company’s VP of engineering Jean-Brice Dumont it will be another three to five years before the firm settles on the likely candidate.

Crucially, Airbus’ vote of confidence in our planet’s most abundant element reflects a strengthening view across the sector. Professor Iain Gray, director of aerospace at Cranfield University, and a veteran of the UK aerospace industry, recently told The Engineer that he increasingly views hydrogen as the main contender: “I’m a great believer in hydrogen I do think its time has come in in aerospace and Airbus have made that firm commitment” he said.

Hydrogen also has some clear advantages over batteries when it comes to aviation, he said, something borne out by last year’s decision to cancel E-Fan X, the joint Airbus / Rolls-Royce hybrid electric aircraft project. “I think a useful conclusion Airbus made was that it showed that batteries and battery electric wasn’t going to be the long-term solution for the kind of A320 replacement type of product,” he said.  “I think it was a brave step to cancel it when they did, but on reflection it was probably the right time to draw the positive conclusions and move onto hydrogen.”

Glenn Llewellyn - Airbus’ VP for Zero Emission Aircraft - agrees that hydrogen now appears to have taken its place at the top of the low carbon pecking order. “We need an energy carrier that can be created using renewable energy and then carried on board an aircraft,” he said. “Our experience with batteries shows us that battery technology is not moving at the pace we want – and this where hydrogen comes in – it’s got several times more energy per kilogram than batteries have today.”

In the meantime, whilst Airbus’ historically ambitious plans are a number of years away from fruition, the distinction of being the first company to put a commercial scale hydrogen fuelled passenger aircraft in the air goes to one of the sector’s emerging stars: US startup, ZeroAvia

Late last summer - in what has been hailed as a landmark moment for low carbon flight - a modified six-seater Piper M-Class aircraft, powered by a hydrogen fuel cell powertrain (that combines oxygen from the air with hydrogen from the tanks  to form water and electricity) entered aviation history when it took to the skies above the firm’s R&D base at Cranfield airport in the UK.

The flight was conducted as part of the HyFlyer project, an Aerospace Technology Institute (ATI) funded initiative aimed at demonstrating low carbon powertrain technology for propeller aircraft. Led by ZeroAvia, the project also involves UK fuel cell specialist Intelligent Energy and the European Marine Energy Centre (EMEC) which has developed a green hydrogen refuelling system for the aircraft.

At the time of writing,

The company has also secured £12.3 million of government funding for a follow on project, HyFlyer II, that will see the company develop a certifiable 600kW hydrogen-electric powertrain for powering airframes carrying up to 19 passengers. This project is expected to  culminate with a 350 mile flight, bringing  the company a step closer to its ambition of decarbonising short sub-regional and regional flight. Renz said that the company could potentially be ready to launch a commercial product as soon as 2023.

Renz explained that like Airbus, ZeroAvia investigated a number of different technology pathways before concluding that hydrogen - in this case hydrogen electric - represented the best solution for this class of aircraft. “Batteries fall short because of the simple energy density,” he said. “You cannot store enough energy in the amount of mass you have available on an airplane. And If you look at the hybrid electric approach (burning jet fuel in a turbine and driving a generator that produces electricity) the more research that was done the more people realised that the cost and environmental credentials don’t improve substantially over a very efficient turbine engine.”

That’s when the group decided that hydrogen ticked the greatest number of boxes. “We realised from a cost perspective it looks very good because you get the maintenance cost benefits from electrification, from a fuel perspective it looks good because you can produce zero emission hydrogen, and also from an environmental perspective you don’t have problems like battery recycling. You can achieve something like 90 percent life-cycle emission reduction.”

Whilst ZeroAvia is a US founded firm, it’s notable that the bulk of the research activity has taken place in the UK. Indeed, according to Renz, the expertise at Cranfield and the wider UK aviation ecosystem has been key to getting the project off the ground. “We realised two things early on,” he said.  “One, that a lot of our supply chain is in Europe and two, a lot of the interest both by government and potential customers is in Europe.  We looked at the entire European offering and realised that the UK has the best set up in terms of talent, proximity to our supply chain and in terms of government ambition.”

Tapping into this expertise has been key to addressing some fairly fundamental engineering problems, not least, he said, overcoming the packaging challenges of accommodating a fuel cell system and hydrogen storage tank in a relatively small aircraft. “The airframe we chose is made to fly fast which generally means there is little space available,” he said, “fitting the fuel cell system in the available space was a big challenge for us so we spent a lot of time on that.”

Another key challenge has been around thermal management, and finding innovative ways to remove the waste heat generated by the fuel cell. “A good rule of thumb is fuel cell systems are 50 – 60 per cent efficient – for every watt you generate as useful power you pretty much generate a watt of waste heat – that means you have quite a lot of waste heat you need to get rid of and thermal management was really the second big engineering challenge.”

Significantly, the HyFlyer project team hasn’t restricted its focus to the aircraft, but has used the initiative to demonstrate the key components of a future hydrogen airport ecosystem: including green hydrogen production, hydrogen storage, and refuelling technology. Indeed, through the project,  EMEC has developed an end-to-end hydrogen refuelling system consisting of an electrolyser to produce hydrogen on site, a trail-mounted compressor for preparing the hydrogen for storage, and a specially developed refuelling truck.

Understanding and catering for these broader challenges will, said Renz, be key to taking the technology to the next level. “Today even if we had a commercially available product we couldn’t sell it because no airline would be able to fuel a hydrogen aircraft. So we need to start showing people that hydrogen is not some kind of miracle fuel, that you can produce it on site with electricity and water and you can store it on site.”

Iain Gray echoed the importance of this more joined up view, and is heartened that there appears to be a growing recognition of this across industry. “I think people are thinking a bit more about the ecosystem and how it all fits together rather than just a focus on point design of aeroplanes, and that makes it quite exciting,” he said.

It’s a trend that he believes will inevitably drive more cross sector collaboration; “This is likely to drive far greater levels of cross-sector collaboration than has traditionally been the case. The aerospace sector can’t solve all of its challenges on its own….the solution is going to come by people working together,” he said.

For Airbus’ Glen Llewllyn it all adds up to a very different and exciting future for engineers working in the sector: “In terms of partnering  and working across subjects, which as an industry we’ve not had to deal with in the past, this is going to be a really exciting opportunity,” he said.

What’s more, whilst there will inevitably be challenges in driving such a fundamental transformation, it’s important to note that that the sector will be able to tap into the existing momentum behind hydrogen in many other areas of the economy. “We’ve got some real strengths in the UK,” said Gray, “we’re strong on production and storage, and the use of hydrogen in transport is already deeply embedded. What we’re seeing is aviation starting to recognise what advantage it can take."

It seems paradoxical, but I would say to a young grad going into aerospace that there’s never been a better time to join the industry

Professor Iain Gray - Cranfield University

This certainly seems to be a factor that’s driving Airbus’ interest. “Hydrogen is an energy source that’s required by many industries in order to meet the Paris targets,” said Llewellyn, “the scaling that were’ going to see over the next few years is going to significantly bring the cost down and this makes it very interesting for aviation.”

It would be wrong to ignore the hardships and challenges that the pandemic has imposed on the civil aviation sector over the past 12 months. Many people working in the industry have lost their jobs, and some organisations dependent on its continued growth have simply ceased to exist.

But there is also a growing sense of momentum behind the sector’s push for a low carbon future, and that  - despite everything - makes it an exciting time. “It seems paradoxical, but I would say to a young grad going into aerospace that there’s never been a better time to join the industry,” said Gray. “You have a chance to shape things, to do things a bit differently and to get involved in companies that are going to be doing things that were inconceivable two years ago”

For Airbus’ Grazia Vittadini, there is no doubt that the industry is at a pivotal moment in terms of determining its low carbon future. And, as things stand, it looks as though hydrogen will be at the heart of this vision.  “There are many reasons to believe in hydrogen and our estimation is that it will contribute by more than 50 fifty per cent along our journey to decarbonising aviation,” she said. “it is one of the most promising technology vectors to allow mobility to continue fulfilling the basic human need for mobility in better harmony with our environment…..to enable future generations to enjoy flying just as much as we do.”