Can the aviation industry cut its carbon emissions?  It’s a complex, controversial issue, but the point of departure is clear.  Thanks to cheaper flights, the sector is growing by 4% a year and its 2.5% slice of global carbon emissions is set to rise rapidly.  Moreover, its role as eco bad guy is hard to challenge when you compare the 67 grammes of CO₂ per passenger kilometre produced by Ryanair, for example, with the relatively paltry 6g generated by high-speed rail.

But the debate soon runs into turbulence, buffeted by the crosswinds of contrasting opinions (often passionately expressed) and conflicting signals.  What should we make of events in October, for instance, when British Airways announced plans to offset carbon emissions for all domestic flights as a step towards zero emissions by 2050, but the UN aviation agency’s assembly failed to agree on a long-term emissions target for the sector?

Consensus and joined-up thinking are in short supply.  In Europe, hopes are pinned on the CORSIA carbon offsetting scheme, yet critics question whether it will significantly limit the increase of emissions – let alone cut them.  Financial instruments such as taxing kerosene and ‘frequent-flier’ levies also have their champions, but the industry is (unsurprisingly!) less enthusiastic.  Similarly, ‘flight-shaming’ people into flying less has vocal supporters but is far from universally popular and has serious backlash potential.

Undeniably, some huge steps have already been made, especially in making aviation more fuel-efficient.  Since 1990, emissions per passenger kilometre have halved and progress continues.  For example, a new Rolls-Royce power gearbox delivering 25% better fuel efficiency in large turbofan engines is set to enter service shortly.  Think too of fuel-friendly operational innovations such as continuous climb/descent (to avoid ‘stepping’ after take-off and before landing) or simple measures like ensuring seats are lighter and flights are fuller.  None, though, is the really big game-changer needed to make serious, rapid inroads into the industry’s carbon footprint.  Sights must be set on a different destination: ditching kerosene.

New heights for biofuels?

If you’d been landing your aircraft at Oslo International Airport in 2016, you’d have found a new refuelling option available.  The airport was the first major hub to offer ‘sustainable aviation fuel’ (SAF), in this case made from camelina oil.  Today, more airports are offering SAF and airlines including KLM, All Nippon Airways and United Airlines are trialling it.

Over its lifecycle, SAF is 80% less carbon-intensive than kerosene.  Thousands of flights using SAF blended with kerosene have already taken place and there’s no shortage of potential sources, including energy crops (such as camelina), farming/forestry waste, algae and even used cooking oil.  And plenty of activity is driving SAF forward.  For instance, Project Solaris (supported by South African Airways and Boeing) aims to extract aviation fuel from nicotine-free tobacco, while Europe’s first dedicated SAF plant is due to be up and running by 2022.

In 2018, though, SAF accounted for less than 0.1% of aviation fuel consumption and some early targets look set to be missed, including the EU’s 2Mt/year by 2020.  Clearly, barriers need to be tackled.  One is high cost.  Another is the thorny(!) issue of land-use changes needed to support large-scale energy crop cultivation.  SAF may be on the up and have a key role to play in helping deliver the International Air Transport Association’s aim of halving the sector’s carbon emissions by 2050, but climbing higher, quicker, clearly presents challenges.

Synthetic solutions

In 2018, the Intergovernmental Panel on Climate Change (IPCC) ‘Special Report on Global Warming of 1.5°C’ (see IFRF blogpost) grabbed global attention.  Among many eye-catching findings, one flew somewhat under the radar: the potential of synthetic aviation fuel.  Also known as ‘electrofuel’ or ‘power-to-jet’, the attractions of producing clean, synthetic kerosene using only water, renewable electricity and CO₂ captured from the air are obvious.  Electrolysis splits the water into oxygen and hydrogen, which is combined with carbon from the CO₂ to produce fuel.

It’s an exciting prospect, but in commercial terms it’s currently just that: a prospect.  Though it can be produced using existing technology and used in existing aero-engines with little modification, it’s very expensive.  According to estimates, blended with conventional kerosene on a 50:50 basis by 2050, synthetic kerosene could double global spend on aviation fuel.  Research is under way to address the barriers – one European consortium which includes the Royal Schiphol Group is now doing exactly that – but the inevitable conclusion is that this fascinating long-term option isn’t a shorter-term game-changer.

Electric blue

Is aviation ‘electrifiable’?  Electrification is, of course, seen as a big part of the solution to today’s eco-challenges in spheres such as road transport.  But could it also transform our skies?  It’s a topic currently buzzing with activity, interest and ingenuity.  In September, for instance, the UK’s Secretary of State for Transport challenged the British aviation industry “to create an electric revolution in our skies”.  This was especially timely since, at this summer’s Paris Air Show, electric planes had caused quite a stir, including the small Israeli-built ‘Alice’ prototype powered by five electric motors incorporating lithium-ion batteries – one motor on each wingtip and three more on the tail.

Elsewhere, E-FanX, a hybrid fossil fuel/electric plane developed by a consortium including Airbus, Rolls-Royce, Siemens and Cranfield University, is due to take its first flight in 2021.  Indeed, Airbus is investing hundreds of millions of euros in electric technology, even though large commercial electric-powered aircraft are probably two decades away.  But if the overall prospect of electric planes still appears somewhat distant, there’s one proven power-producing technology that may be relatively close to getting off the ground…

High on hydrogen?

In a $6 million NASA-funded project, the University of Illinois is currently working on an all-electric aircraft powered by fuel cells fed with cryogenically liquefied hydrogen fuel – the main attraction being the weight advantage compared with batteries.  And while hydrogen liquefaction is a somewhat niche technology, there are signs this might be changing.  For example, the Cranfield-led ENABLEH₂ project, whose partners include Heathrow Airport, aims to re-fire enthusiasm for liquid hydrogen research for civil aviation applications.

But back to fuel cells.  When the world’s first hydrogen fuel cell-powered aircraft took off from Stuttgart in September 2016, it was arguably a genuine trailblazer.  In 2018, for instance, Singapore-based HES Energy Systems unveiled plans to build a small plane incorporating ultra-light fuel cells powered by liquid or gaseous hydrogen, while California-based ZeroAvia claims to offer “the first practical true zero-emission aviation powertrain” that harnesses batteries and hydrogen fuel cells.  Nor should we dismiss the possibility of burning hydrogen directly in jet engines, which could require relatively limited changes in engine design (as well as an extensive PR campaign to bring the flying public on board!).

The hurdles facing the fuel options I’ve touched on shouldn’t be understated.  Yet potential always exists for ground-breaking research to unleash a big leap forward.  As ever, it’s a question of incentives, investment, policy and will.  But as this piece shows, there’s no shortage of options.  If the timescales of their major, commercial-level impacts can be cut from decades to years, there may just be clean, blue skies ahead sooner than we imagine.