Engineers tackle safety and cost concerns to refuel hydrogen planes as quickly as possible

Anyone who has taken a flight recently knows that airlines try to have their aircraft on the tarmac for as little time as possible. The entire industry is predicated on the economics of fast turnarounds, including ‘deplaning’ passengers, getting new ones on, refuelling and cleaning as quickly as possible. On some short-haul routes, the turnaround can take as little as 35 minutes.

Operators of the current crop of kerosene-powered planes have managed to get the process down to a fine art, but air travel is set to change over the next few decades. More research will be needed, though, to figure out how to achieve the same speedy turnarounds with hydrogen, an entirely different fuel source.

“In principle, the same steps will take place during the turnaround of a hydrogen-powered aircraft as during that of a conventional aircraft,” says Tim Schunkert, a researcher at the German Aerospace Center’s Institute of Air Transport. “However, the refuelling process will be different and could influence the parallel execution of other ground operations, potentially changing the overall turnaround sequence.”

In September, the Civil Aviation Authority (CAA) selected a partnership between Beyond Aero, London City Airport and the University of East London to investigate how hydrogen-powered aircraft could be turned around at a busy airport. The project is looking at how Beyond Aero’s proposed six-to-eight passenger hydrogen-electric aircraft, which uses gaseous hydrogen, could be handled once it lands and before it takes off again. The smaller aircraft are an interesting testbed because they use the gaseous form of the element – larger commercial aircraft may need to use liquid hydrogen, bringing a different set of engineering problems.

For aircraft using liquid hydrogen, fuel must be kept at about -253°C. Refuelling connections and transfer lines need to be purged before fuel transfer to remove any contaminants, while they also need cooling to prevent problems when the cryogenic fuel flows through them. That requires a lot of extra work – and time – for ground crews.

The amount of fuel involved also differs for hydrogen-powered aircraft. Jet fuel contains about 9,700kWh of energy per cubic metre, compared with 2,360kWh for liquid hydrogen. “This means that roughly four times the volume needs to be transferred into the aircraft for the same energy content,” says Schunkert.

Engineers cannot simply ramp up the pumping speed to compensate. Liquid hydrogen sits close to its boiling point and speedy pumping can vaporise it, leading to cavitation or the formation of vapour bubbles that can interfere with the equipment.

But just because a hydrogen plane needs four times as much fuel does not mean it needs to take four times as long to fill. A simulation by researchers at Delft University of Technology estimated that refuelling a proposed hydrogen turboprop could take less than 20 minutes. That is within the turnaround times needed at airport terminals, provided refuelling can happen alongside other ground operations.

From there, the engineering problems get more complicated. Zahra Jaffary, a PhD researcher at Adelaide University, has studied what hydrogen refuelling would mean for people working around aircraft. In her research, conducted with Mirjam Wiedemann, she brought together aviation experts, including a refueller, to identify the risks. “Obviously, from the engineering side, there are quite a lot of points that we already understand about hydrogen,” she says. “But what we do not understand is how it will react at an airport environment, especially in terms of the human factors.”

Those concerns include whether refuellers can work safely under time pressure and exactly how close other workers and aircraft can be. There are also risks when an aircraft is not being refuelled – liquid hydrogen gradually warms and turns into gas, increasing pressure inside the tank. Some may need to be released through controlled venting to avoid disasters. Hydrogen may also need to be released when refuelling equipment is cooled down – all of which makes it less economical as an option.

Airports have more to worry about than safety and financial concerns. Existing kerosene infrastructure cannot be repurposed for liquid hydrogen, and Schunkert says deliveries by road would be difficult to scale up, while long-distance liquid-hydrogen pipelines are impractical. One option could be to pipe gaseous hydrogen to an airport, liquefy it locally and build dedicated cryogenic tanks and distribution systems.

For now, the CAA’s London City project is an attempt to understand how the pieces might fit together – but not necessarily in the immediate future. “I see hydrogen as a long-term solution, rather than something that can be implemented on a large scale in the near future,” says Schunkert.

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