GKN Aerospace has revealed more details of the liquid hydrogen fuel cell propulsion system it is developing, which it says could eventually power a future zero-emission airliner with at least 96 seats.
Work on a 1MW-class demonstrator is being conducted under the UK Aerospace Technology Institute-funded H2GEAR programme, which will culminate in 2025 with a ground test of the full system.
But ahead of that milestone GKN this year will perform a series of tests to bring certain sub-systems, notably the cryogenic motor and electrical distribution network, to technology readiness level (TRL) 4.
“Our technology, which is unique in the industry, is that we are actually using the cryogenic temperature of the fuel as a heat sink; we are using it, via an alternative safer medium, to cool down our electrical network,” said chief technology officer Russ Dunn, briefing journalists on 24 April.
Cooling the electrical network to cryogenic levels lowers the resistance of the wiring, allowing higher power levels to be distributed at lower voltages and enabling the thickness – and therefore weight – of the wiring to be reduced; efficiency of the motor and its power density are also significantly improved.
At the heart of the design is liquid hydrogen, which needs to be stored at -253°C (-423°F). GKN makes use of that source of incredible cold to lower the temperature of an “intermediary substance” – in this case helium gas – which is in turn used to cool the motors and electrical wiring.
The use of cryogenic cooling is a “fundamental differentiating step” over the design of other fuel cell powertrains, says Dunn, and provides confidence that the system can be scaled up to an aircraft of at least 100 seats, or even beyond.
Although Airbus is also pursuing research into cryogenic cooling for electrical distribution as part of its ZEROe programme, Dunn declines to say whether GKN is participating in that effort.
“Airbus is very interested in what we are doing – I’ll leave it at that,” he says. “All I can say is that we have shared that concept with Airbus, and we are happy to continue to support them.”
In addition, Dunn says GKN has solved the issue of thermal management for the fuel cells. As a rule of thumb, fuel cells generate around 1MW of heat for every 1MW of power produced and integrating systems to deal with that waste heat runs the risk of adding substantial weight and drag to the aircraft.
“We have a solution that we believe works in terms of thermal management,” says Dunn, although he declines to offer more detail, citing a current lack of patent protection for the internally developed design.
Tests of the full powertrain – including an electric motor but absent a propeller – will take place at the University of Bath’s IAAPS powertrain test facility in Bristol in late 2025. GKN forsees potential service entry for such a system in around 2035.
GKN’s technology would be suitble for routes of around 2,160nm (4,000km) and its widespread uptake could help tackle more than 60% of aviation’s CO2 emissions, it argues.
Although GKN’s development focus has been on the powertrain, to fully understand the requirements for such a system it has also designed aircraft onto which it could be installed.
To date, these comprise a 48-seat propeller-driven aircraft and a larger 96-seater which is configured with ducted fans.
Dunn insists the designs are more than simply digital renderings and have been accompanied by extensive studies into their power requirements, aerodynamics, stability, and other considerations.
In common with other similar concepts, the hydrogen fuel tank, fuel cell and balance of plant would be stored in the rear of the fuselage, separated from the cabin by a bulkhead. The only things leaving that area, he says, would be the electricity produced by the fuel cells and the helium for the cryogenic cooling system.
Fuel cell powertrains are being developed as retrofit solutions by several other technology companies – notably Universal Hydrogen and ZeroAvia – but Dunn says GKN will not step into the aircraft modification market.
“I know people have looked at retrofitting onto existing platforms, but from our point of view it’s very inefficient to retrofit this sort of solution.
“It’s feasible to do it but if you really want to get the best out of the aircraft you are really talking about clean-sheet.”
A brand-new regional aircraft design would incorporate substantial efficiency improvements, helping to offset the additional cost of hydrogen fuel compared with kerosene, and the reduced passenger accommodation: GKN’s calculations suggest an aircraft currently sized to carry around 80 passengers would be needed for a fuel cell-powered 48-seater.
“You are going to need all of that efficiency to make this sort of concept feasible,” he adds.
“We are not saying [other developers] are barking up the wrong tree because we think they are doing some really exciting things, but they are working at the lower end of that zone; I think they are going to be constrained to retrofitting into a relatively niche market.
“Our aspirations are really to get zero-emissions for the masses,” says Dunn.
GKN also continues to analyse the likely requirements for hydrogen combustion engines, believing they will be need for larger aircraft. Its work on H2Jet, a two-year Swedish government-backed study, has recently concluded, he adds, and the company is now incorporating the findings from the project.
