A German aviation firm claims to have carried out the world’s first liquid-hydrogen (LH2) powered piloted test flight, as part of a four-journey campaign using a demonstration aircraft fitted with a fuel cell-powered propulsion system.

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Stuttgart-based H2Fly carried out the campaigns from Maribor in Slovenia, using its piloted HY4 demonstrator aircraft, which had been fitted with storage for cryogenic liquid hydrogen as well as the fuel-cell system — which powered the plane for the duration of the flights.

One of the flights lasted more than three hours, H2Fly reported, significantly longer than the tests carried out by hydrogen aircraft developers ZeroAvia and Universal Hydrogen — of 23 and 15 minutes respectively — earlier this year.

Both the UK’s ZeroAvia and US-based Universal Hydrogen are testing larger, retrofitted passenger aeroplanes rather than demonstration craft and have only completed test flights with one of their planes’ two engines running on fossil fuel.

H2Fly’s flight test results found that the use of liquid hydrogen rather than gaseous would double the HY4 aircraft’s range from 750km to 1,500km.

“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 decarbonising commercial aviation.”

Liquid hydrogen requires much less space than gaseous hydrogen.

Compressed hydrogen at 350 or 750 bar contains 0.8 or 1.1kWh of energy per litre, respectively. Liquefaction increases the volumetric energy density to more than 2.3kWh/litre. This means that a fuel tank containing liquid hydrogen would contain about two to three times more energy — and therefore range — than a similar-sized tank containing compressed H2.

According to the EU-funded hydrogen liquefaction technology project, IDEALHY, state-of-the-art liquefaction technology today requires 12kWh of electricity to make a kg of liquid hydrogen — equivalent to 36% of the useable energy contained in 1kg of H2.

ZeroAvia, which uses gaseous hydrogen in its fuel cell propulsion systems currently being tested on a six-seater and a 19-seater aircraft, said it has plans to use liquid hydrogen in its larger models.

“We plan to certify a powertrain for 9-19 seat aircraft by 2025 that would use gaseous hydrogen as this is the quickest route to market,” a spokesperson for the company told Hydrogen Insight today. “However, for our engine for 40-80 seat aircraft with entry in service we will use liquid hydrogen as it improves substantially the volumetric constraints allowing greater range.”

Universal Hydrogen has flown a 40-seater aircraft using its fuel-cell propulsion system, but also used gaseous hydrogen for the test flight. The company is planning to convert the plane to use liquid hydrogen later this year.