Up in the Air…?: The challenge of sustainable aviation

Green design

In the short term, the easiest way to reduce the carbon impact of flying is to burn less fuel. One of the simplest methods is through software such as SkyBreathe, which automatically optimises flight plans for fuel-efficiency.

Better design also makes a difference. Some current-generation planes are up to 20% more energy efficient than older designs, mainly through improvements to the bypass ratio: the ratio of thrust generated by the fan you see spinning at the front of aeroplane engines. The higher the bypass ratio, the less fuel you need. Modern aircraft are also more aerodynamic and employ lighter materials¹².

However, limitations to how much it’s possible to optimise current configurations mean that a radical change in fuel efficiency will require an equally radical shift in aircraft design. Last year, KLM unveiled a blended-wing design—what it calls a “Flying-V”—that effectively places passengers inside the wings¹³. KLM said that the lighter, more aerodynamic design should consume 20% less fuel than an Airbus A350 while maintaining the same cargo and 314-passenger capacity.

But when a radical new design can cost $30 to $40 billion¹⁴, introducing an entirely new aircraft platform is a significant challenge. “There’s a duopoly between Boeing and Airbus,” said R. John Hansman, professor of aeronautics at the Massachusetts Institute of Technology. “And in the current world where there’s a backlog for airplanes, a company can’t really justify all kinds of technical risks. I think the barrier to entry for a full-scale airplane is enormous.”¹⁵ Moreover, these new designs would still rely on traditional jet fuel, and rising passenger demand in the coming decades is likely to offset any increased efficiencies.
 

Hydrogen: new heights, or all hype?

The need for an energy-dense power source in aviation means that we’ll likely need liquid fuels for longer distance flying for decades to come. But those fuels don’t necessarily have to be of the fossil variety. One particularly promising liquid alternative is hydrogen, which has the energy density required to keep a plane in the air while emitting zero carbon at point of use. The only waste product is water.

Expressing his doubts about batteries, Glenn Llewellyn, vice president of zero-emissions technology at Airbus, said, “We don’t believe that it’s a today-relevant technology for commercial aircraft and we see hydrogen having more potential.”¹⁶ Demonstrating that confidence, Airbus recently unveiled three designs for zero-emission commercial aircraft that could enter service by 2035: a turbofan design powered by a modified gas-turbine engine for up to 200 passengers with a range of 2,000+ nautical miles, a blended-wing body design with a similar passenger capacity and range, and a turboprop design with a 100-passenger capacity and a range of over 1,000 nautical miles.

Even here, though, challenges remain. Chief among these is that steam methane reforming (SMR)—the most common method of producing hydrogen—emits up to 150g of greenhouse gases per kilowatt-hour of energy¹⁷. Electrolysis provides a carbon-neutral way to produce hydrogen (provided that the electricity required is generated by nuclear or renewables), but is currently more expensive than SMR. Although investors are increasingly focusing on green hydrogen as regulatory changes, including the EU Green Deal and China’s new carbon-neutral target, highlight the importance of increasing scale while reducing costs, multiple sectors including steel, cement, chemicals, heating and other forms of long-distance transport are all competing for this limited supply. So, while projects such as BIG HIT on Orkney and Denmark’s HyBalance are proving that it is possible to produce hydrogen efficiently, more research and investment is required before a sustainable hydrogen-based aviation industry can become a reality.

Maintaining liquidity

Another class of alternative liquid fuel is much closer to seeing widespread use. Sustainable Aviation Fuels (SAFs)—including biofuels—are made from a variety of raw materials including waste fats and oils, vegetable oils, wood chips or even household waste. Although the burning of SAFs still emits carbon, the raw materials from which they're made are created by pulling carbon from the atmosphere. That means SAFs’ overall GHG emissions are still 65-95% lower than traditional fossil-kerosene fuels¹⁸. And best of all, SAFs work with the aeroplanes of today.

But, as things stand, first-generation biofuels lack the sustainability that would make them a true game-changer. They remain expensive—two to five times more costly than conventional jet fuel¹⁹. And then there’s the problem of scale. Today, SAFs constitute just 0.1% of global kerosene, and even the most optimistic estimates suggest that they could make up a mere 4 - 8% by 2035²⁰.

That said, between increased scale and production innovations, the widespread use of SAFs could be made viable. While there are six certified kinds of SAF, Hydroprocessed Fatty Acid Esters and Free Fatty Acid (HEFA)—produced from used cooking oils, animal fats, and vegetable oils—is currently the cheapest and most commercially mature. In the short- to medium-term, HEFA could be scaled up through better collection of waste fats and oils and by diverting raw vegetable oil from other uses. However, it's still a long way from being able to meet the ever-rising demand for aviation fuel, and no SAF is currently approved for use at more than a 50% blend with conventional jet fuel.

In the longer term, we’ll need to develop more advanced production methods that make use of sustainable biomass, such as agricultural and forestry residues. In comparison to HEFA, such methods are relatively immature. For example, the first commercial-scale plants designed to produce SAFs using the Alcohol-to-Jet and Gasification/Fischer-Tropsch methods are only just reaching the planning or building stages, in many cases with a projected output of less than 0.1 metric tons per year. From there, the transition to multiple commercial plants will take at least a decade of project development, construction, and commissioning. Meanwhile, less mature technologies that promise increased efficiency or make use of cheaper feedstocks—such as pyrolysis and hydrothermal liquefaction-—still have many production and investment hurdles to clear, and so are much further away from commercial viability.

Beyond the six existing certified SAFs, electrofuel—created by using renewable electricity to extract hydrogen from water and fuse it with carbon captured from the atmosphere—is another promising route. Since its “ingredients” are abundant, electrofuel could have the potential to meet all aviation fuel demand. But if that potential is to be realised, significant R&D will be required to bring down the cost of equipment and improve the conversion efficiency that will, as things stand, create a prohibitively expensive form of fuel.

Despite all this, relative market readiness plus compatibility with existing technology makes SAFs the most promising low-carbon solution for long-haul aviation, at least in the short term. And they’re set to receive a boost from new policy and regulatory commitments. Many European countries are introducing SAF mandates and, as part of EU green deal, the ReFuelEU initiative²¹ has proposed to implement a blending mandate—which would see the imposition of a gradually increasing proportion of SAFs relative to conventional fuels—along with enhanced incentive and support mechanisms. 

Changing course

As we’ve seen, significant questions remain around how we might achieve truly sustainable air travel. While we’re closest to a revolution in the short-haul space, 80% of emissions come from flights longer than 1500 km²²; which, due to the energy density problem, will require liquid fuels for a long time to come. And though SAFs and hydrogen are promising avenues of research, significant technological challenges lie ahead. In the meantime, faster, more sustainable ground-based travel options, such as Hyperloop and maglev trains, are likely to be useful in bridging the carbon gap, as well as changing attitudes to reduce travel demand and boost sustainable tourism.

Since all problems are soluble given the right knowledge, sustainable aviation is only a matter of time. Whether that time comes sooner or later will depend upon how we invest in the challenge.

¹Breaking Travel News (2020) ‘Airbus unveils three zero-emission aircraft concepts’. Available here.
²PR Newswire (2020) ‘ZeroAvia Completes World First Hydrogen-Electric Passenger Plane Flight’. Available here.
³Ritchie, H. and Roser, M. (n.d.) ‘Emissions by sector’, Our World in Data. Available here.
⁴International Civil Aviation Organization (n.d.) ‘Trends in Emissions that affect Climate Change’. Available here.
⁵International Energy Agency (2020) ‘The Covid-19 Crisis and Clean Energy Progress’. Available here.
⁶Quicke, A. and Jones, E. (2020) ‘Grounded: Civil aviation emissions reductions under COVID-19 in Australia and globally and the potential long-term impacts to emissions in the sector’, The Australia Institute. Available here.
⁷Turk, D. and Kamiya, G. (2020) ‘The impact of the Covid-19 crisis on clean energy progress’, International Energy Agency. Available here.
⁸Reuters (2018) ‘EasyJet expects to be flying electric planes by 2030’. Available here.
⁹France-Presse, A. (2018) ‘Norway aims for all short-haul flights to be 100% electric by 2040’, The Guardian. Available here.
¹⁰Sparks, E. (2020) ‘Will sustainable air travel ever be possible?’, Lonely Planet. Available here.
¹¹Groves, D. (2020) ‘Sky’s the limit — battery technologies for commercial electric air travel’, Lexology. Available here.
¹²McKinsey & Company (2020) ‘The future of air travel’. Available here.
¹³Chow, D. (2019) ‘Futuristic “Flying-V” airplane concept puts passengers inside the wings’, NBC News. Available here.
¹⁴See McKinsey & Company (2020).
¹⁵See Chow, D. (2020).
¹⁶Ryan, C. (2020) ‘Airbus Unveils Hydrogen Designs for Zero-Emission Flight’, Bloomberg. Available here.
¹⁷Nicholls-Lee, D. (2019) ‘Flight risk: can we take the carbon out of air travel?’, The Guardian. Available here.
¹⁸Bauen, A., Bitossi H., German, L., Harris, A., Leow, K. (2020) ‘Sustainable Aviation Fuels: Status, challenges and prospects of drop-in liquid fuels, hydrogen and electrification in aviation’, Johnson Matthey Technol. Rev., 2020, 64, (3), 263–278. Available here.
¹⁹ See Bauen et al. (2020).    
²⁰ Sustainable Aviation (2020) ‘Sustainable Aviation Fuels Road Map’. Available here.
²¹DG Move (2020) ‘ReFuelEU Aviation - Sustainable Aviation Fuels’. Available here.
²²See Sparks, E. (2020).

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