I recently booked my first flight since the COVID-19 pandemic devastated the aviation industry. Who can forget the airports full of grounded planes, with staff and pilots laid off? According to the International Civil Aviation Organization (ICAO), the number of air passengers worldwide fell by 60% between 2019 and 2020. And although numbers climbed back to 2.3bn in 2021, they were still 49% below pre-pandemic levels.

But despite the problems, sustainable air travel has made big progress over the last two years. Many airlines and carriers have exploited the opportunity afforded by the drop in passenger numbers to scrap older, less economic and less efficient planes. Planes spew out carbon dioxide (CO₂) and nitrous oxides (NOx), which also helps form ozone in the upper troposphere. They also emit particulates and leave water-vapour trails (contrails), both of which trap heat.

Sustainable air travel has made big progress over the last two years.

With airlines and plane manufacturers keen to improve their environmental credentials, one simple solution is to power aircraft with bio-fuel, known as sustainable aviation fuel (SAF) in the trade. Existing aircraft can use jet fuel mixed with 50% SAF without needing to be modified in any way. Doing so can slash emissions by up to 80% compared to ordinary jet fuel, with Rolls-Royce and Boeing having already carried out test flights on 747s fitted with Trent engines using 100% SAF.

Alternative fuels

Unfortunately, these greener fuels are up to five times more expensive than jet fuels so they won’t succeed without tax incentives or investment by the fuel industry to make them cheaper. The Finnish firm Neste, for example, is using old cooking oils as a feed stock for its SAF, claiming that more than 370,000 commercial flights have used SAF since 2016. Neste can currently make about 150 m litres of the fuel a year, but that’s still a tiny fraction of what’s needed – and there’s only so much used cooking fat available before SAFs compete with global food supplies.

Those concerns are one reason why the Biden administration has launched the Sustainable Aviation Fuel Grand Challenge, which aims to produce 3 billion gallons of SAF a year by 2030. But even if that ambitious goal is met, SAFs will only cut the direct CO₂ emissions from planes; they do nothing for NOx, water vapour or contrails. The obvious solution is hydrogen, which emits almost no CO₂, very little NOx and just a bit of water vapour. Its energy density (140 MJ/kg) is triple that of kerosene (43 MJ/kg) and far higher than lithium-ion batteries (0.95 MJ/kg).

On the downside, hydrogen is a gas at room temperature, which means it has to be liquefied or compressed so it can be stored in the fuselage. That’s why I think electric batteries could be an answer, for smaller planes at least. Last year Rolls-Royce broke two world records for the fastest all-electric plane, hitting speeds of 555 km/h over a distance of 3 km. The plane used a 400 kW axial flux electric motor from the Oxford-based automotive powertrain supplier YASA.

Unfortunately, today’s batteries are so heavy and bulky that battery-electric planes will probably only be useful for short flights. But according to a recent report from the UK’s Aerospace Technology Institute (ATI), the aviation sector can become carbon-neutral by 2050 using a combination of SAFs and ultimately “green” hydrogen (i.e. hydrogen not derived from fossil fuels) using fuel cells, gas turbines and hybrid systems. The ATI believes that a mid-sized hydrogen-powered plane could be flying by 2035 and a narrow-body aircraft by 2037. The former could fly 280 passengers from London to San Francisco directly.

If half of the world’s commercial planes were hydrogen-powered by 2050, the ATI reckons that the aviation sector’s carbon emissions would fall by 4 × 10⁹ tonnes (4 Gt). That would be equivalent to four years’ worth of emissions from all existing conventional planes, with potentially 14 Gt being saved by 2060. It’s not a pipe dream: many companies contributed to the ATI’s report, including Airbus, easyJet, Eaton, GE Aviation, GKN Aerospace, Reaction Engines and Rolls-Royce.

Elsewhere, Reaction Engines, IP Group and the UK’s Science and Technology Facilities Council launched an intriguing new joint venture at the COP26 conference in Glasgow. They want to see if the exhaust heat from a plane can be used to make hydrogen from ammonia fuel, creating a blend that mimics jet fuel and can be used in existing aircraft engines. Meanwhile, Aviation H2 in Australia is using liquid ammonia combustion in modified jet engines and aims to have a converted Dassault Falcon 50 plane in the skies by mid-2023.

Retrofitting existing aircraft with hydrogen-electric powertrains makes good economic and environmental sense.

Beyond this kind of “green ammonia”, perhaps the cleanest option of all is being pioneered by UK/US start-up ZeroAvia, which has re-fitted an existing turboprop Dornier 228 commuter plane with hydrogen fuel cells and electric motors. Compared to jet engines, its approach could cut operating costs by 60% and maintenance costs by 75% – and, of course, with zero emissions. ZeroAvia has already carried out the first zero-emission six-seater plane flight, signed a partnerships with British Airways and United Airlines, and raised over $130m in funding.

Retrofitting existing aircraft with hydrogen-electric powertrains makes good economic and environmental sense given that there were 23,000 commercial aircraft in service around the world in 2017. But I am also intrigued by a potentially revolutionary new plane design being developed by US start-up Otto Aviation. Its ultra-aerodynamic laminar flow Otto Celera 500L craft can – the company claims – take six passengers more than 8300 km at speeds of 740 km/h using a single propeller, flying 8–9 times further per litre of fuel than a similar jet can.

If developments like these succeed, perhaps “green flying” will one day be possible.