Jul 26, 20223 min
Hydrogen-powered aircraft could prove to be the best way for the air travel industry to meet its bold commitment to net-zero carbon emissions by 2050. For that to happen, hydrogen must be stored safely on aircrafts. One solution for safe storage involves helium.
The International Air Transport Association, an airline membership organization, in 2021 set a goal of achieving net-zero carbon emissions by 2050 which is not an easy route. Researchers are focusing on hydrogen-powered aircraft since it is the most obvious way in decarbonizing air travel.
Molecular hydrogen is an attractive option because, whether combined with oxygen in a fuel cell to create electricity to power motors or combusted in liquid or gaseous form, the main byproduct is water vapour. While both water vapour and carbon dioxide contribute to atmospheric warming by trapping heat radiating from the Earth, water molecules remain in the atmosphere a handful of days before dissipating or falling to Earth as precipitation instead of lingering for decades as carbon dioxide molecules do. Not only that, hydrogen molecules unleash three times the amount of energy of traditional jet fuel by mass.
This illustration depicts Airbus’ first round of flight tests with the ZEROe demonstrator, a modified A380 aircraft that will help guide the design of Airbus’ future passenger airliner. Plans call for flights in 2026 in which a hydrogen combustion engine on the demonstrator’s rear fusalage would burn liquid hydrogen fuel from four 100-kilogram tanks located inside the fuselage.
Paul Gloyer, president of Gloyer-Taylor Laboratories, or GTL, strongly believes that companies need to move towards composite fuel tanks as compared to metallic versions as composites are the best method of storing molecular hydrogen as an aviation fuel.
But that advantage is offset by a volume problem: It would take almost 3,000 litres of unpressurized, ambient-temperature gaseous hydrogen to release the same amount of energy as a litre of conventional jet fuel, according to Airbus, which is studying aluminium fuel tanks for a future hydrogen passenger airliner. This volume could be reduced by pressurizing hydrogen and storing it at cryogenic temperatures to keep it in a liquid state.
Packing the molecules together in liquid form and keeping them that way requires extremely low temperatures — at least minus 253 degrees Celsius. This in turn calls for a multilayer tank that needs, for long-distance flights, an active cooling system that basically works like a refrigerator by circulating cryogenic fluid such as helium gas through high and low pressures.
GTL announced in March that it had built a 2.4-meter-long cryogenic tank weighing in at 12 KG when empty. And to keep the inside temperature of the tank from rising during flight — which would result in boiling, turning the liquid hydrogen into gas — the tank is double-walled with a vacuum in between the layers to reduce heat transfer from the tank’s exterior.
Despite the potential for lighter-weight designs that composites afford, Airbus is one of the companies sticking with metallic tanks for the time being. As part of its ZEROe initiative to design and build a hydrogen passenger airliner that would begin service in 2035, the company is developing cryogenic aluminum tanks at its various Zero-Emission Development Centers throughout Europe.
Storing hydrogen tanks in an aircraft’s fuselage could sound scary to passengers as hydrogen, like other fuels, is highly flammable and could easily be ignited. To ensure safety, Airbus plans to place active detection sensors for hydrogen and oxygen in the ZEROe demonstrator that would be capable of detecting any leaks in milliseconds. Additionally, the hydrogen valves will be insulated to prevent leakage, and there will be active and passive hydrogen ventilation systems to avoid hydrogen from concentrating in any part of the aircraft.
Ultimately, the decision of whether to choose metals or composites will come down to a variety of factors. Some companies may consider the composite tank technology not mature enough and therefore risky for near-term applications, or that one material works better than the other for a particular aircraft market.