Going greener with liquid hydrogen

This isn’t the first time H2Fly has made headlines: The company was acquired by Joby in 2021 and broke a world record in 2022 when its hydrogen-electric demonstrator aircraft, a modified twin-fuselage Pipistrel Taurus designated HY4, reached an altitude of 7,230 feet.

On September 7, the companies announced that HY4, powered entirely by a liquid hydrogen-electric fuel cell system and cryogenically stored liquid hydrogen, had completed several piloted flights, including one that lasted more than three hours over Maribor, Slovenia.

H2Fly, based in Germany, said that by using liquid hydrogen rather than gaseous hydrogen, the aircraft range doubled to 1,500 kilometers, or about 932 miles.

JoeBen Bevirt, Joby Aviation founder and CEO, said, “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. 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.”

In June, H2Fly announced the development of its high-altitude (up to 27,000 feet) fuel cell system designed for eventual integration into commercial air travel. The fuel cells are being developed for aircraft in the megawatt-class range with 20 to 80 seats.

The flights were the result of Project HEAVEN, a European-government-supported coalition led by H2FLY and other European aerospace players to demonstrate the viability of using liquid hydrogen in aircraft.

Liquid hydrogen, when combined with oxygen in a fuel cell, produces heat and electricity with only water vapor as a byproduct. According to the Hydrogen Science Coalition, liquid hydrogen produces 3.3 times more energy per kilogram than jet fuel.

The challenge with liquid hydrogen, however, is that it is difficult to store. Liquid hydrogen boils at minus 253 degrees Celsius, meaning it must be stored at close to absolute zero temperature or minus 273.15 degrees Celsius/minus 460 degrees Fahrenheit. That’s where H2Fly’s cryogenic storage solution comes in.

Aircraft range is another challenge. Because of its density, liquid hydrogen is not able to produce the same amount of energy produced by jet fuel without significantly reducing the aircraft's range or installing extra cryogenic fuel tanks. These tanks are heavy. According to the Hydrogen Science Coalition, the extra tanks can reduce the payload of an aircraft 15 to 20 percent.

Peter Rez, Mooney Pilot and emeritus professor of physics at Arizona State University, explained in an email that although liquid hydrogen may seem appealing for aviation use, it faces significant hurdles before it can power commercial travel or general aviation.

“If one could use the same tanks as one uses for avgas or kerosene and fill them with liquid hydrogen the same volume would only store ¼ as much energy,” Rez wrote. “Given that the whole wing (and part of the belly) of long range intercontinental transport aircraft is a giant fuel tank, trans-Atlantic or trans-Pacific flight would no longer be possible.”

For general aviation, many aircraft have a wet wing, where the wing structure is the fuel tank. Rez noted, “That wouldn't be possible for a cryogen like liquid hydrogen where a double walled Dewar with a vacuum between the two walls is a minimum requirement for storage of a few hours. The volume available will then be further reduced.”

“As for H2FLY's aircraft,” Rez continued, “it makes no sense to use a fuel cell/electric motor/propeller for an aircraft that's supposed to cruise in the flight levels…. The overall efficiency would be about the same (maybe a bit higher ~ 40%) if the hydrogen were used as fuel for a high bypass ratio gas turbine.”