Biofuels should reduce greenhouse gas emissions. But air transport is ultimately relying on hydrogen, a source of energy with multiple constraints, which induces many changes.
Flying without (too) polluting: is it possible? And if so, when? These questions will weigh in at the 54th edition of the Paris Air Show (Seine-Saint-Denis), an unmissable event for aeronautics and space, which opens on Monday June 19. While the effects of climate change, linked to human activities, are increasingly visible, the aviation sector, responsible for 5% to 6% of global warming, intends to become cleaner. Faced with criticism, the representatives of the 193 States of the International Civil Aviation Organization (ICAO), a UN agency, managed to “a historic agreement” aiming for carbon neutrality by 2050 last October.
To achieve this objective, civil aviation intends to proceed in stages. Some airlines have already incorporated new-generation, less kerosene-intensive aircraft into their fleets. This work on fuel savings is supplemented by eco-piloting measures, studies on optimized trajectories, the possibility of carrying out ascents and descents continuously (and not in stages) and the electrification of everything that can to be on board the aircraft. In this effort, France does not intend to stay on the edge of the track. “We French, we must be the champions of ultra-low-energy aircraft”launched Emmanuel Macron, Friday, June 16, announcing the investment of 300 million euros each year between 2024 and 2030 to develop the aeronautics sector.
Promising, sustainable fuels remain little produced
But beyond these aspects, one of the main priorities concerns the csustainable aviation fuels, called in English sustainable aviation fuels (FAS). It is mainly about biofuels, made from used cooking oil, animal fat or plant waste. The gains are significant: The world explained, in 2021, that by using a biofuel from common household waste, the reduction in carbon dioxide emissions (CO2, the main greenhouse gas) amounts to 94% compared to the use of kerosene.
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Mixed with the usual kerosene, the biofuel allows planes to fly without having to modify them (as long as the mix is not too high). To illustrate the advances in the field, Airbus flew an Airbus A380 with 100% biofuels in March 2022. The manufacturer claims that all of its aircraft “are currently certified to fly with up to 50% SAF mixed with kerosene. The goal is to achieve 100% SAF certification by the end of this decade”.
For the moment, European regulations only require a 1% mixture. In 2050, it should be 70%, according to the EU’s REfuelEU Aviation roadmap. Achieving this will require significant efforts, as biofuel production remains “confidential”remarks the climatologist Nicolas Bellouin, executive director of the Climaviation project, in partnership with the general direction of civil aviation (DGAC), the Pierre-Simon Laplace Institute and the National Office for Aerospace Studies and Research (Onera) .
“We are very far from it, but it is because today the challenge is in production”, also estimated, at the end of 2022, Julian Manhes, specialist in biofuels at Airbus. This should start to take off with various projects, like the Grandpuits (Seine-et-Marne) site of TotalEnergies. In conversion, it must produce biofuel for aeronautics from 2025. Emmanuel Macron also announced on Friday the upcoming installation of a biofuel plant in the Pyrénées-Atlantiques, in Lacq. And the president assured that he could reach the EU’s goal of incorporating 6% SAF in fuels by 2030 : “We have secured the capacity to produce 500,000 tonnes in France.”
Another sustainable aviation fuel on the table is e-fuel, also called “synthetic fuel” or even “electrofuel”. It is manufactured after “an electrolysis of water to produce low-carbon hydrogen, which is then combined with CO2 (…) to produce a fuel”, exposes the Commissariat for Atomic Energy and Alternative Energies (CEA). This CO2 could be collected, for example, from cement factories, steelworks, incinerators, or even directly captured in the atmosphere, quotes the CEA. A technology still at an embryonic stage because “electrofuels are more complex to produce than biofuels”summarizes Maxence Cordiez, engineer at the CEA.
Hydrogen, the saviour?
The subject of SAF is struggling to convince Yves Gourinat, professor at Isae-Supaéro. “It is a possible transition. I believe in it in the short term, but not in the long term”believes the physicist, who sees sustainable fuels as a futile extension of the existing. “Thomas Edison did not invent the light bulb by reinventing the candlehe says. We have to change technology, change standards.”
For him, it is certain, the future of civil aviation goes through hydrogen. Same story on the industrial side. Marc Hamy, Vice President Corporate Affairs at Airbus, insisted on this point during a conference at the Airbus Air and Space Academy at the end of April.
“Hydrogen is a very, very, very interesting fuel for aviation (…) We are actively working on this solution.”Marc Hamy, Vice President Corporate Affairs at Airbus
during a conference at the Air and Space Academy
Why such an interest ? Hydrogen allows an aircraft not to emit CO2, but water vapour, and displays good performance: pFor the same weight, it generates three times more energy than kerosene. Two options emerge for its exploitation in aircraft. It could be transformed into electricity thanks to a fuel cell. It would power an electric motor that spins propellers. A viable runway for small aircraft, not for conventional commercial aircraft. The other solution, which would work for large devices, is to burn hydrogen in a heat engine.
The big problem concerns storage: hydrogen, eleven times lighter than air, takes up a lot of space. It is possible to keep it at high pressure to reduce its volume. This is most often how it is used in hydrogen cars. In order to save more space, it can be liquefied, being kept at -253°C. A process already used for space launchers. No wonder then that the engine manufacturer Safran, which works in space and aviation, mentions liquid hydrogen in its plan facing the climate challenge. However, even in its liquid form, hydrogen takes up four times the volume of kerosene. Huge tanks would then be needed to take it on board planes. This could involve devices with new architectures, larger, taller, with fewer seats. The avenues of work remain open.
On hydrogen, “there are quite significant efforts in the sector, even if we don’t yet have prototypes that allow us to say what’s coming out of it”, nuance Nicolas Bellouin. However, things could become more concrete soon. With the aim of finalizing a device for 2035, Airbus is considering a flight test stage in 2026. Despite everything, the executive director of the Climaviation project is skeptical. According to him, “hydrogen may be a niche”. Maxence Cordiez also shows reserve.
“Hydrogen is likely to play only a marginal role in decarbonizing aviation due to its low density.”Maxence Cordiez, engineer at the CEA
Hydrogen is not virtual. It is already found in space launchers, in buses (as in Pau), in certain cars. The manufacturer Rolls Royce manages to use it to run an aircraft engine on the ground. However, the environmental benefits of hydrogen are only valid if it is clean itself. Massively producing green hydrogen or exploiting natural deposits in an environmentally friendly way remain challenges, points out Laurent Catoire, director of the chemistry and processes unit at the National School of Advanced Techniques (ENSTA).
“The main problem is producing carbon-free hydrogen. What we know how to do right now is gray hydrogen, from fossil resources.”Laurent Catoire, director of the chemistry and processes unit at ENSTA
“The march is high for hydrogen”concedes Yves Gourinat, who nevertheless imagines a certification of the hydrogen plane within fifteen or twenty years. “The challenge is colossal”confirms Christophe Turpin, research director at the CNRS and specialist in hydrogen. With weighted optimism he commands attention on the global changes induced by this potential technological revolution. “It is a whole. It’s a first step to develop the technology, but the whole ecosystem has to follow, for production and supply”he remarks, insisting on a deployment which will inevitably be “very progressive”. “It’s the technology of the day after tomorrow. It’s the next move”he hammers.
A necessary reduction in traffic
Until then, the researcher recalls the need to use the aircraft “at the best” and proceed to a “big cleaning on internal flights”. A certain sobriety also underlined by Nicolas Bellouin. “The aviation market grew by 30% between 2014 and 2018 [avant la pandémie de Covid-19]he notes. If it continues at this rate, the market will have to be reduced.”
This reduction is imperative, according to Maxence Cordiez. Even before a hypothetical maturity of hydrogen, the engineer estimates that thebiofuels and e-fuels, which will emerge before hydrogen, “are above all a solution to decarbonize aviation once we have greatly reduced traffic”. For him, “We cannot reasonably think that they will allow the growth of air traffic to continue, or even to maintain it at its current level”.