In 2021, electric vertical takeoff and landing (eVTOL) air taxi developer Joby Aviation secretly acquired a company producing hydrogen aircraft fuel cell systems. This week, that company did something no other firm has done.
Joby subsidiary H2Fly on Thursday revealed it completed piloted flights of its four-seater HY4 demonstrator powered by liquid hydrogen, a milestone it says is the world’s first. The company made four takeoffs from an airfield in Maribor, Slovenia, using cryogenically stored liquid hydrogen to power a hydrogen-electric fuel cell system, which kept the aircraft in the sky for over three hours on one occasion.
Prior to this week’s tests, H2Fly had only flown with pressurized gaseous hydrogen. It said switching to the liquid form of the fuel source will double the HY4’s range from about 405 to 810 nm, a promising sign for a company that hopes to supply hydrogen-electric powertrains for zero-emissions medium- and long-haul commercial flights.
JoeBen Bevirt, founder and CEO of Joby, has repeatedly touted the benefits of hydrogen, hinting at the company’s desire to explore it as a fuel source. The company has no stated plans to integrate hydrogen fuel cells on its air taxi, but Bevirt gave its subsidiary some praise.
“H2Fly are pioneers in their field, and we’re proud of them achieving this watershed moment in the use of liquid hydrogen to power aircraft,” he said in a statement. “In the years to come, battery-electric and hydrogen-electric propulsion systems will enable us to build aircraft that are quieter and make mid- to long-range air travel possible with zero emissions. It’s critical we take action now and invest aggressively in these technologies for the health of our planet and future generations to come.”
The flight test campaign marked the culmination of Project HEAVEN, a European government-backed consortium created to explore the use of cryogenic liquid hydrogen in aircraft. H2Fly led the project, which also included partner Air Liquide, Pipistrel Vertical Solutions, and the German Aerospace Center. The tests were also funded by a pair of German federal ministries and the University of Ulm.
“This achievement marks a watershed moment in the use of hydrogen to power aircraft,” said Professor Josef Kallo, co-founder of H2Fly. “Together with our partners, we have demonstrated the viability of liquid hydrogen to support medium and long-range emissions-free flight. We are now looking ahead to scaling up our technology for regional aircraft and other applications, beginning the critical mission of decarbonizing commercial aviation.”
Compared to the pressurized gaseous hydrogen H2Fly used prior to these flights, the cryogenically stored liquid form has a higher energy density, enabling a lower tank weight and volume. That boosts the range and payload of the aircraft. Hydrogen fuel cells also produce electricity without combustion or emission and do not need to be recharged so long as they’re fed a supply of hydrogen and oxygen.
“Today’s success demonstrates the full potential of liquid hydrogen for aviation,” said Pierre Crespi, innovation director at Air Liquide, which designed, manufactured, and integrated the liquid hydrogen tank that powered the HY4. “Liquid hydrogen can be stored onboard and transported. Hydrogen is key to the energy transition and this new step proves that it’s already becoming a reality.”
With HEAVEN flight testing complete, H2Fly says it is now focused on the path to commercialization. The company recently announced a new H2F-175 fuel cell system—capable of providing a full power range at altitudes up to 27,000 feet—which it hopes will prove there are real-world applications for the technology beyond low-altitude demonstrations.
H2Fly will also open a Hydrogen Aviation Center, co-funded by the Ministry of Transport Baden-Württemberg, at Stuttgart Airport (EDDS) next year. One day, the center may provide fuel cell aircraft integration facilities and liquid hydrogen infrastructure to large swaths of the European aviation industry.
The Promises and Pitfalls of Hydrogen
Incredibly, the HY4 made its maiden voyage over half a decade ago in 2016. In April 2022, it set what H2Fly believes to be the altitude record for a hydrogen-powered aircraft, cruising at over 7,200 feet. Around the same time, HY4 completed a cross-country flight from Stuttgart Airport to Friedrichshafen Airport (EDNY) 77 sm (67 nm) away, the first for a hydrogen-powered passenger airplane between two commercial airports.
A few months prior to this week’s piloted test, H2Fly and Air Liquide also conducted on-ground coupling tests of an aircraft-integrated liquid hydrogen storage system, which validated the Joby subsidiary’s ability to install its architecture.
Looking ahead, H2Fly hopes to add hydrogen-electric propulsion to the European Union Aviation Safety Agency (EASA) CS-23 and CS-25 category aircraft. It has a partnership with Deutsche Aircraft to fly a 40-seat Dornier 328 demonstrator equipped with its fuel cell system in 2025.
But what will come of all this innovation?
H2Fly isn’t the only firm exploring hydrogen fuel systems; industry titan Airbus is also looking into applications for the technology, including a hydrogen-powered turbine engine. Another firm, ZeroAvia, is also in the mix, having recently flown the largest aircraft to be powered by a hydrogen engine, a 19-seat Dornier 228 demonstrator.
The firms have all been gripped by the allure of hydrogen, which is expected to greatly reduce emissions and help operators reach their ESG goals. It could also extend the range and payload of an aircraft, opening up new markets and use cases. But all that glitters is not gold, detractors say, poking a few holes in the hydrogen argument.
Some, like CleanTechnica’s Michael Barnard, worry the fuel source will be green but not cheap. As an example, he pointed to an effort by Air Liquide and others to establish a liquid hydrogen manufacturing facility for maritime shipping—that venture failed because of high production costs. Others point out that while hydrogen is environmentally friendly, its production often creates staggering emissions of its own, calling into question how “green” it truly is.
Liquid hydrogen in particular can also be difficult, costly, or even dangerous to transport. It requires specialized packaging to combat its high combustibility and may necessitate either an extensive logistics network or the construction of manufacturing sites near airfields. Neither option is ideal.
Further, there are some design and safety concerns around storing liquid hydrogen fuel near passengers. It needs to be kept in ball-shaped tanks that are as large as possible to prevent the liquid from boiling off, which creates challenges when designing an aircraft that also carries people.
Still, liquid hydrogen has the potential to curtail in-flight emissions (the ones that actually appear on company ESG reports) and, if the technology evolves as expected, open new business opportunities for operators. However, the industry will need to solidify the pipeline and limit hydrogen manufacturing costs and emissions before it becomes a viable alternative.
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