H2Fly To Apply Lessons Learned From Liquid-Hydrogen Flights

H2Fly conducted four liquid-hydrogen-fueled test flights with its HY4 fuel-cell propulsion testbed.

Credit: H2Fly

After conducting the first piloted flights of an electric aircraft powered by liquid hydrogen, Germany’s H2Fly is taking the lessons learned into development of a megawatt-class fuel-cell powertrain for demonstration on a Deutsche Aircraft D328 regional turboprop.

Stuttgart-based H2Fly completed four flights of its HY4 testbed aircraft from Maribor, Slovenia, in September using liquid hydrogen (LH2) to power its fuel-cell propulsion system, which had previously flown using pressurized gaseous hydrogen. One flight lasted 3 hr., the HY4 landing with some of its 12 kg (26 lb.) of LH2 remaining.

The flights completed the four-year, €6.9 million ($7.4 million) Heaven project, an EU-supported effort to demonstrate a high-power-density hydrogen fuel-cell powertrain for aircraft. Further ground demonstrations with the LH2-fueled HY4 are planned before the aircraft is retired, H2Fly co-founder Josef Kallo said.

“Now we are focusing on building the liquid hydrogen storage for the 328. We have identified the contractors for the materials, the supplier for the valves and partners to work on the evaporator, and now we will push forward on the hydrogen storage system [for the 328] and on the new H2F-175, which is our own customized fuel-cell system,” he says. The HY4 uses fuel cells provided by Germany’s EKPO.

Air Liquide developed the double-walled, vacuum-insulated LH2 dewar tank installed in the cockpit area of the right-hand fuselage in the four-seat, twin-fuselage HY4. An internal heater pressurizes the LH2 in the tank, and a heat exchanger using waste heat from the fuel cell drives an evaporator that vaporizes the hydrogen for delivery to the fuel-cell cathode at 6.5 bar (94 psi) absolute pressure.

Evaporator control is identified as one of the technical achievements of the Heaven program. “The storage system needs an evaporator because the hydrogen in a liquid state has to go into the gaseous state. What we learned is this evaporator has to be controlled very exactly,” Kallo says.

“That is done by evaporating enough hydrogen, which then is directly used, and we control the pressure at the fuel cell. Including the energy that goes into the evaporator–which is mainly from the heat of the fuel cell–that pressure control has to be done extremely precisely so that we have a very stable pressure at the fuel cell,” he says. “The simplicity of the controller was astonishingly good.”

Control of the evaporator, pressure and fuel-cell power output was sufficiently fast and precise to eliminate the need to use a buffer battery to handle power transients. “We expected this, but we could show that during the 3-hr. cruise flight we did not need the battery,” Kallo says. 

The research has also provided insights into the procedures required to safely fuel an aircraft with LH2. “We have three other projects in parallel to Heaven related to the functionality of the full powertrain. These projects are now ending, and what we see is an assessment that the dynamics of the fuel supply, fuel cell and powertrain are matching with each other. This is one important outcome,” he says.

“A second important outcome is that liquid hydrogen refueling, even with a very basic ground support unit, can be handled. It’s not very fast. It takes 20 min. to refill. But the point is it’s not rocket science. It’s just learning procedures and checklists,” Kallo says. Flights from Maribor also showed an LH2-powered aircraft could be handled like a conventional operation at a busy international airport. 

“What we definitely see is that liquid hydrogen may require a little bit more planning,” he added. “So if you want to have seven daily flights, then you would need something like a defueler if you go for the first flight and you have to turn back.” A hydrogen defueler would also be required when aircraft are taken out of service for maintenance and overhaul.

Graham Warwick

Graham leads Aviation Week's coverage of technology, focusing on engineering and technology across the aerospace industry, with a special focus on identifying technologies of strategic importance to aviation, aerospace and defense.