The Future of Advanced Air Mobility

H2Fly's Breakthrough Flight Demonstration Bolsters the Case for Liquid Hydrogen Propulsion

Hydrogen propulsion pioneer H2Fly’s efforts to bring the technology to the air transport sector were energized by the breakthrough flight of its HY4 demonstrator aircraft at Maribor Airport in Slovenia last week. Co-founder and CEO Josef Kallo was visibly pleased when talking to media on the sidelines of the event, describing it as “a major milestone” and “world’s first.”

The demonstration flight, which followed earlier test flights and ground tests, validated what Germany-based H2Fly and its partners had hoped for, namely that cryogenic liquid hydrogen storage technology could be safely applied to aeronautical applications. The message is that we can use liquid hydrogen to support medium and long-range emissions-free flight," said Kallo. We have demonstrated that a liquid-hydrogen storage system with a fuel cell system can be used to propel an electric motor in an aircraft.

“I can proudly say that we are the first to operate liquid hydrogen-powered flights of a fuel cell electric aircraft,” he continued. Kallo acknowledged that many other companies are venturing into the development of hydrogen-powered flight. “We know we have competitors, we know about our competitors," he commented. "I have no information that some other [companies] will fly [with liquid hydrogen] in the next week, the next month, the next year."  

Universal Hydrogen, for instance,  has operated several flight tests this year of its 40-seater De Havilland Canada Dash 8-300 hydrogen-electric demonstrator, powered on one side by a megawatt-scale hydrogen fuel cell propulsion system. ZeroAvia in July completed the 10th flight of the Dornier 228 testbed aircraft, with one Honeywell TPE331 turboprop engine replaced by the prototype hydrogen-electric powertrain ZA600.

While both these California startups plan to move to liquid hydrogen (LH2), their test aircraft so far have used only gaseous hydrogen storage. The bulk of H2Fly’s test campaign also used hydrogen in gaseous form.

The HY4 demonstrator aircraft—a retrofitted Pipistrel twin fuselage electric Taurus 4G model—has completed over 115 take-offs since 2016, of which five were LH2-powered flights. These included the 10-minute demonstration flight on September 7, which AIN observed, and a flight on September 4 that lasted three hours and one minute. The three-hour flight around Maribor airport was executed successfully without any battery support during cruise flight and “completely obliterated our record of 2020 [of two-hour flights powered by gaseous H2],” Kallo remarked. It used about 10 kilograms (22 pounds) of hydrogen.

H2Fly co-founder and CEO Josef Kallo with the HY4 demonstrator aircraft.
H2Fly co-founder and CEO Josef Kallo with the HY4 demonstrator aircraft. (Image: AIN/Cathy Buyck)

“Depending on the meteorological conditions and the flight altitude, the aircraft uses about three kilograms of LH2 per hour," he said. "So, for an eight-hour flight, we would need about 24 kilograms of liquid hydrogen." The HY4’s aluminum storage tank, designed and supplied by France’s Air Liquide, can hold up to 24 kilograms of LH2.

Kallo asserted that using cryogenically stored liquid hydrogen instead of gaseous hydrogen, which enables significantly lower tank weights and volume, leads to a longer range. “The HY4 demonstrator aircraft using gaseous hydrogen gives us a range of around 750 kilometers [466 miles]. Now, with the new liquid hydrogen tank of Air Liquide, we have a range of about 1,500 kilometers [932 miles],” he said, adding that he believes “that in most applications we can go even 2,000 kilometers [1,240 miles].

Building the Business Case

Based on pre-pandemic data, airliners fly some 600 billion revenue passenger kilometers on flights under 2,000 kilometers in Europe (the 27 EU states plus the UK) yearly. “This represents a major addressable market,” maintained Kallo. For operators, it offers the opportunity to lower carbon dioxide emissions and reduce noise pollution. For aircraft manufacturers, the H2Fly boss estimates that replacing fossil kerosene with hydrogen on regional aircraft flying routes of less than 2,000 kilometers could create a market of new powertrains, hydrogen storage systems, fuel cells, and electric motors valued at around €10 billion ($10.7 billion) per year in Europe.

“This is a multibillion-euro market we can go into if we achieve the upscale of the [fuel cell] technology from 120-kilowatt electric power to megawatt power. This [a one-and-a-half-megawatt powertrain] will enable a 40-seater regional aircraft,” Kallo said. “The goal is to go zero-[carbon] emission up to 2,000 kilometers,” he added, pointing out that he believes it will be possible based on the learnings from HY4’s comprehensive test campaign.

“As an engineer, I can say that a 40-seater hydrogen-powered plane is a question of money and time. After that it becomes visionary,” he remarked. Visionary though not impossible, Kallo said, is the upscaling to four-megawatt-plus-four-megawatt powertrains, which could propel larger aircraft for 80 to 100 passengers.

The HY4 flights are the culmination of Project Heaven, a European Union-supported consortium formed to demonstrate the feasibility of using liquid, cryogenic hydrogen with fuel cells in aircraft. The group is led by H2Ffly and includes the partners Air Liquide, Slovenia’s Pipistrel Vertical Solutions, the German Aerospace Center (DLR), Germany’s EKPO Fuel Cell Technologies, and Seville, Spain-based Center for Social Innovation Fundación Ayesa. Additionally, the HY4 has received funding from the German Federal Ministry for Economic Affairs and Climate Action, the German Federal Ministry for Digital and Transport, and the University of Ulm, where Kallo is a professor.

Kallo and a handful of fellow engineers and researchers at the University of Elm and the DLR founded H2Fly in 2015. The company, which U.S. eVTOL developer Joby Aviation acquired two years ago, is based in Stuttgart and plans to open its Hydrogen Aviation Center at Stuttgart Airport next year.

Aiming Higher and Farther

As part of its upscaling venture,  H2Fly in June announced a project to develop a next-generation aviation-grade high-performance fuel cell system. The new H2F-175 systems will be capable of providing their full megawatt power range in flight altitudes of up to 27,000 feet and will be integrated into a Dornier 328 demonstrator aircraft as part of the German government-funded “328 H2-FC” program to develop a hydrogen fuel cell system for flights in the EASA large aircraft class (CS-25). Flight tests are expected to begin in 2025. The 10 partners in the DLR-led project are all German, and include H2Fly, Deutsche Aircraft, and Diehl Aviation, among others. The country’s Federal Ministry for Economic Affairs and Climate Action has allocated €30 million in funding.

Two pilots flew H2Fly's HY4 technology demonstrator aircraft.
Two pilots flew H2Fly's HY4 technology demonstrator aircraft in a September 7 flight at Maribor in Slovenia. (Image: AIN/Cathy Buyck)

No EU funds are involved in the 328H2-FC project, Kallo confirmed. At least not for now. The bloc’s public-private aviation research and innovation partnership, Clean Aviation, next year will launch projects into technologies for the use of hydrogen in aviation and developing climate-friendly aviation. “We are confident a lot of funding could come from that side,” he said. Project Heaven received funding from the EU’s Clean Hydrogen joint undertaking, the successor to the Fuel Cells and Hydrogen partnership.

H2Fly’s long-standing partner in the development of the HY4 and Project Heaven, Slovenia-based light aircraft electric manufacturer Pipistrel (now part of Textron eAviation), “unfortunately” is not involved in the 328H2-FC program and H2Fly’s present upscaling project, said Kallo. The company played a crucial role in securing the necessary permits and authorizations from the Slovenian CAA for the HY4 test campaigns. “The CAA of Slovenia was very strict and very cooperative,” Kallo told AIN. “They want to understand how the new technologies work and they really want to push forward the opportunity to build rules and regulations for a hydrogen-powered plane.” EASA was not involved, he said.

Kallo expects that the 40-seat, 2,000-kilometer-range liquid hydrogen-electric regional airliner will be used in commercial service in 2029-2030. He acknowledged that besides solving the technological challenges, meeting this goal will require obtaining certification and the availability of LH2. “Hopefully, by that time we will have LH2 widely available at airports,” he noted, adding that public acceptance of hydrogen-powered aircraft also will play a role.

Much like its competitors Universal Hydrogen and ZeroAvia, which both have secured large orders for their H2 retrofit kits, H2Fly sees its future hydrogen fuel-cell propulsion system mainly as a retrofit solution for existing airframes. “The Dornier 328 is a very efficient 40-seater aircraft but given the high-volume needs [of the hydrogen storage system], we will need new airframe solutions,” Kallo said, describing it as an “exciting chance” for aircraft manufacturers to develop concepts and designs. Kallo told AIN that the 328 H2-FC project intends to use the Dornier 328 demonstration aircraft as it is, noting that “we would gain range if we would take seats out.” Reducing the number of seats in the cabin or reducing the cargo belly-hold to make a place for the LH2 storage and associated fuel cell powertrains might not be welcomed by airlines as they always seek to maximize payload. “We will have to be competitive with fossil fuel-powered aircraft,” Kallo admitted. “In my opinion, the pressure from the market to save CO2 will become stronger in the next couple of years.”

Upscaling the fuel cell technology from that of a four-seat electric Pipistrel for a 40-seater Dornier turboprop will also require the upscaling of the LH2 onboard storage and supply to airports.

“We showed that we can safely handle liquid hydrogen at an airport, put it in an aircraft, and operate it in an aircraft during taxi and flight,” said Pierre Crespi, innovation director at Air Liquide Advanced Technologies. “We firmly believe that hydrogen will be part of the game to decarbonize flight,” he added.

Project Heaven and the successful HY4 test campaign marked the first time the French group company and world leader in industrial gases used LH2 in a commercial airport. Air Liquide produced the green liquid hydrogen in France and brought it to Maribor. “We brought 3.5 tonnes of LH2, so plenty to refuel this aircraft, which only needed 12 kilograms for the demonstration flight,” he quipped. Air Liquide used the standard LH2 nozzles to refuel the HY4 but the company is developing future nozzles “because we will need to refuel bigger and more aircraft in the future,” Crespi told AIN.