Hydrogen-Powered Aircraft: A Technology Whose Time Is Coming

On January 19, 2023, a 19-seat Dornier 228 aircraft equipped with a ZeroAvia hydrogen-electric engine on its left wing and a Honeywell TPE-331 stock engine on its right flew successfully at ZeroAvia’s R&D facility at Cotswold Airport in Gloucestershire, U.K. The 10-minute flight included a takeoff, a full pattern circuit and landing.

The ZeroAvia hydrogen-electric engine was powered by two hydrogen fuel cell stacks and hydrogen tanks inside the Dornier’s cabin. Lithium-ion battery packs onboard provided peak power support during takeoff, plus redundant power for safe testing. In a commercial configuration, storage elsewhere in the aircraft would be used for the hydrogen system components and the seats restored.

The January 19, 2023, test flight used the largest ZeroAvia hydrogen-electric engine flown so far, which the company plans to submit for certification by the end of this year. The test flight proved that hydrogen-fueled aircraft are on their way to becoming a viable commercial reality.

“At this point, the aircraft industry is making significant progress in the development of functional hydrogen-powered aircraft,” said Alex Ivanenko, general manager of VTOL and new segments at ZeroAvia, and formerly CEO and founder at the hydrogen fuel cell stack innovator HyPoint, which ZeroAvia acquired in October 2022. “Many major aircraft manufacturers have committed funds and resources to develop hydrogen-powered aircraft, and the number of projects involving hydrogen fuel cell technology has grown significantly in the last two years. There is still a lot of work to be done, however, and significant challenges remain before the industry can move from concept designs to fully operational aircraft.”

Ivanenko’s assessment is echoed by Ellen Ebner, Boeing’s director of sustainability and future mobility. “The aircraft industry is in the beginning stages of creating a hydrogen-powered aircraft suitable for regional missions,” she said. “Commercial aircraft have a long development timeline, and the added challenge of fully developing and certifying hydrogen aircraft energy and propulsion systems means there is a great deal of uncertainty on entry-into-service for a hydrogen aircraft. Furthermore, the aircraft need to fly economically-viable missions and use economically-viable green (cleanly produced) hydrogen.”

Two Approaches to Hydrogen Power

ZeroAvia’s hydrogen fuel cell system is one of two ways being considered in the quest for clean hydrogen-powered aircraft.

The first approach consumes hydrogen to produce electricity, and then uses that energy to power electric aircraft engines. There are two ways to do this. “The first model involves replacing battery electric systems for novel aircraft with hydrogen fuel cells — LTPEM FC or turbo air-cooled HTPEM FC — which are capable of providing a more reliable and lighter weight source of energy than batteries,” said Ivanenko. “The second model involves replacing existing combustion engines with hydrogen-electric propulsion systems including electric motor, fuel cell and hydrogen storage. This model is becoming increasingly popular as it is more efficient, quieter and cleaner than conventional engines.”

The second approach uses hydrogen as a combustion-based replacement for kerosene ‘JET A-1’ aviation fuel. Both options are being investigated by Airbus under its ZEROe (Zero Emissions) initiative, first unveiled in September 2020. At its ZEROe web page (https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe), Airbus is proposing three kinds of hydrogen-fueled aircraft concepts, which would use both direct hydrogen combustion and hydrogen fuel cells.

The first ZEROe aircraft version would resemble a conventional low-wing Airbus twinjet but use two hybrid-hydrogen turbofan engines to provide thrust; with liquid hydrogen storage and its distribution system being located behind the rear pressure bulkhead (RPB). The second concept aircraft, with a high-wing, T-tail turboprop layout, would feature two hybrid-hydrogen turboprop engines with eight-bladed propellers to provide thrust, also with the storage/distribution located behind the RPB.

The third proposed ZEROe aircraft would employ a ‘Blended-Wing Body’ (BWB) with a very wide fuselage being part of the lift system. “The exceptionally wide interior opens up multiple options for hydrogen storage and distribution,” said the Airbus website. “Here, the liquid hydrogen storage tanks are stored underneath the wings. Two hybrid-hydrogen turbofan engines provide thrust.”

Airbus’ goal in pursuing its ZEROe initiative is to have viable zero emission hydrogen-powered aircraft in commercial service by 2035. “Real demonstrators recently announced by Airbus — including direct hydrogen combustion (gas turbine) and hydrogen fuel cells — will form a key part of this evaluation phase,” according to information provided by the company. “Airbus’ final decision on an actual aircraft configuration will be made when the technology’s maturity reaches an adequate level. We expect the technology to achieve this level in the next two to four years; around the 2024-2025 timeframe.”

As part of its ZEROe testing initiative, Airbus is reconfiguring its A380 MSN1 test aircraft — the first-ever A380 to roll off the production line — to conduct in-flight tests of a GE Passport turbofan engine modified to use liquid-hydrogen fuel.

A fascinating tour inside this ‘ZEROe demonstrator’ can be viewed online at http://www.airbus.com/en/newsroom/stories/2022-02-the-zeroe-demonstrator-has-arrived. “Our plan is to take this aircraft and modify it into a hydrogen propulsion flight laboratory,” said video tour host Glenn Llewellyn, Airbus vice-president, zero-emission aircraft. “Our ambition is to take this aircraft and add a stub in between the two rear doors at the upper level. That stub will have on the end of it, a hydrogen-powered gas turbine, and inside the aircraft there will be hydrogen storage and hydrogen distribution, which will feed this engine with hydrogen.”

Reconfiguring MSN1, whose original purpose was to put the A380’s own functions through their paces, is no small deal. It will include installing four hermetically sealed liquid hydrogen tanks at the rear of MSN1’s lower main deck, plus a distribution system to feed the hybrid hydrogen engine on its mounting stub.

“There will be a huge amount of instrumentation and sensors around the hydrogen storage distribution and hydrogen engine,” Llewellyn said in the video. The resulting flight test data will be studied by Airbus engineers on the ground, as well as being relayed to an onboard flight test station in real-time.

Changes will also be made to MSN1’s cockpit to manage and monitor the hydrogen propulsion system in flight. It will include a throttle “to change the amount of power at which the hydrogen engine will be operated at,” said Llewellyn. “On top of that, there will be a display which will allow the pilots to monitor the different key parameters of the system during ground and flight operations.”

Boeing has not released its plans for achieving hydrogen-powered flight to the same extent that Airbus has. What the company will say is that “Boeing is developing future flight concepts to understand the potential of new technologies and products to contribute to net zero emissions by 2050,” said Ebner. “Our technology programs generate models and data that we use to evaluate future flight concepts — candidates for future flight demonstration or potential products. Over the years ahead, we will continue to mature technologies to create the building blocks for a future air transportation system that may include products of many types and energy carriers.”

Challenges to be Overcome

“Designing a hydrogen-powered aircraft and its operations bring significant technical challenges related to creating the aircraft itself, fueling and servicing hydrogen-powered aircraft, and sourcing green hydrogen; that is, hydrogen produced using renewable energy to reduce lifecycle carbon emissions,” said Ebner. For instance, commercial planes must be radically redesigned to use this propulsion system because hydrogen requires more space and cryogenic conditions for on-aircraft storage. “Hydrogen has a low boiling point and must be chilled at -423 degrees Fahrenheit (-253°C),” she said.

As well, hydrogen takes up to four times the storage space used by jet fuel to deliver the same speed and range, even though it weighs less than half as much per unit of energy. Add the need to contain super-chilled liquid hydrogen in a safe way, and the practice of using wing fuel tanks may well be over.

Meanwhile, “there are challenges containing hydrogen throughout the fuel systems;” insulation engineering challenges also have to be resolved to protect hydrogen from heat during flight,” said Ebner. “Due to their small size, hydrogen molecules can leak through minute pores of welded seams and be absorbed into metal, leaking or making metal brittle in cryogenic conditions.”

“Beyond storage, hydrogen has to be put to use on the airplane,” she added. “That means reliably combusting it in airborne turbines, or developing much higher performance fuel cells and electric powertrains than exists today. Aircraft designs need to take these factors into account: accommodating hydrogen energy systems can impact flight physics and the ability to serve useful missions.”

Then there’s the issue of onsite hydrogen fuel storage and delivery to aircraft. That’s a subject that Prof. Josef Kallo has given much thought to. He is CEO of H2FLY, a Stuttgart-based company whose HY4 four-seat airplane powered by hydrogen fuel cells first flew on September 29, 2016. Somewhat resembling a glider, the HY4 uses a unique design with a two-seat passenger pod on each wing, and a single electric engine top-mounted on the wing’s center point.

H2FLY’s focus is on providing the powertrain for hydrogen-fueled aircraft. It has just announced a partnership with Stuttgart Airport to build the Hydrogen Aviation Center for aircraft development and flight testing at that location, which will be managed by H2FLY. Finding ways to fuel hydrogen-powered aircraft efficiently and safely will be part of that process.

“Commercial aircraft operations require a high level of fuel system loading and unloading, and fast turn-times between cycles,” said Kallo. “The reliability of the fuel system is thus safety critical. Moreover, hydrogen tanks will have to be reused much more often for commercial flight than in space travel. Today, hydrogen tanks are repurposed fewer than 10 times in spacecraft. By contrast, commercial aircraft reuse traditional jet fuel tanks more than 1,000 times.”

As part of its ZEROe initiative efforts, Airbus is looking at the logistics of storing hydrogen at airports and the best ways to transfer it safely yet efficiently to aircraft. “Deployment of infrastructure adapted to the aviation transition to hydrogen is mandatory,” said Karine Guenan, Airbus’ vice president of Zero Emission Ecosystem. “‘Hydrogen Hubs at Airports’ is a key part of the route to hydrogen deployment for aviation. Airbus is now collaborating with airports that are planning a stepped approach including using hydrogen to decarbonize all airport-associated ground transport — heavy goods logistics, buses, and tow trucks — in the 2020 to 2030 timeframe.”

Be Ready to Dispel Hydrogen Myths

A word to the wise — aircraft manufacturers, aircraft operators and airports alike need to be prepared to dispel Hindenburg-inspired myths about exploding hydrogen aircraft, which will inevitably be spread in the media and on the web. Although “hydrogen has to be proven to be every bit as safe and practical as traditional jet fuel when properly stored and handled,” Kallo said, the conspiracy-crazed machine that is social media will likely spread overblown fears as this new form of aircraft propulsion is about to go mainstream.

To justifiably address the fear of hydrogen among reasonably minded citizens (there appears to be nothing that can be done about the lunatic fringe), “an alternative set of airworthiness requirements will need to be established by governments,” said Kallo. “Equipment will also have to be subject to rigorous qualification testing to prove that new designs are capable.”

Certification Will Take Time

For safety reasons, the radical newness of hydrogen-powered aircraft will have to be carefully assessed and examined by regulators before such aircraft are allowed to enter commercial service.

This is not good news for those wanting to deploy this technology as soon as it is ready. “One of the biggest challenges associated with creating hydrogen-powered aircraft and servicing those aircraft is certification,” Ivanenko said. “The certification process for hydrogen-powered aircraft is lengthy and costly, and there is no existing framework for certification due to the relatively new nature of the technology. Additionally, there are currently no established procedures for servicing them, but because these aircraft are based on fuel cells and electric motors, they will definitely require less and cheaper service than combustion engines or turbines.”

Of course, none of the zero emissions associated with hydrogen-powered aircraft will matter if the hydrogen fueling these aircraft doesn’t come from non-polluting sources. This is one of the concerns associated with the electric car rollout. If the power they use is generated by coal-fired plants, such cars are not actually ‘greener’ than their gas-guzzling counterparts.

For its part, Airbus believes in hydrogen’s potential to fuel its future aircraft,” said Guenan. “The challenge today is to support its long-term scale-up to ensure there is enough low-carbon hydrogen available to fuel the aviation industry’s needs.”

How Close Are We to the Goal?

Having seen how much progress is being made towards practical hydrogen-powered commercial aircraft, an obvious question remains: How close are we to achieving this goal?

According to ZeroAvia’s Alex Ivanenko, “we are still a few years away from seeing a commercial airline offering scheduled services with hydrogen-powered aircraft. A number of aircraft manufacturers are developing prototype planes and powertrains powered by hydrogen fuel cells, including ZeroAvia, Piasecki Aircraft Corporation, and Airbus. However, there are still several technological challenges — none of them fundamental — that must be overcome. So I expect that the first hydrogen-powered commercial aircraft could enter service in the late 2020s.”

Since the launch of its ZEROe campaign in September 2020, Airbus has expressed its view that the technologies required to power a zero-emission aircraft will be mature enough for a target entry-into-service date by 2035. Moreover, the company believes that most technologies required for a zero-emission aircraft are emerging already in other industries and Airbus has been working on this for some time already, so it isn’t starting from scratch. Technology demonstrators will be developed over the next five to six years and a full-scale aircraft prototype should be developed by the late 2020s.

According to H2FLY’s Prof. Josef Kallo, it isn’t technology that will decide when hydrogen-powered commercial aircraft will enter service, but money and regulations.

On the money front, the fact that there are currently about “ten-thousands of planes flying that provide commercial transportation capacity” means that the cost of replacing them isn’t seriously prohibitive, as compared to replacing millions of gasoline-powered carts/trucks with electric models. “The investment to move to hydrogen-powered aircraft would be hundreds of billions of dollars, but on a global basis it could be done,” said Kallo. “We can have this done in 10 years.”

Getting hydrogen-powered aircraft certified is another matter entirely. “As an engineer, I can tell you that it’s not the technology but the regulatory work that will slow things down,” he said. To speed things up will require substantial political will on the part of governments and regulators everywhere, which could happen if the push to cut emissions becomes more urgent as climate change gets worse.

“Yes, there is a risk associated with hydrogen-powered aircraft technology, but the risk technologically is small,” concluded Kallo. “We just have to invest a lot of head-start money to get this technology to the same level of reliability and efficiency as the internal combustion engine, but without that engine’s emissions and noise levels.”

The bottom line: Hydrogen-powered commercial aircraft are within the realm of practical, doable reality. The ‘how’ of getting them into service appears to be entirely doable. It’s just a matter of ‘when’.