Greta Thunberg’s zero-carbon Atlantic crossing is not an option for most. But it might be in years to come, if experiments with hydrogen, solar and batteries pay off

Flight risk: can we take the carbon out of air travel?

“Flight shaming is not going to solve all our problems,” says Liam Megill at the start of another marathon day trying to make aviation history. “You can’t take a train to the [United] States, so instead we have to make flying sustainable.”

Greta Thunberg demonstrated this week that you can travel long haul without burning fossil fuels. But not everyone can travel on a zero-carbon yacht.

In the global battle against carbon, aviation is one of the toughest challenges of all. Grids are getting ever greener thanks to renewables, cars are going electric, even construction is exploring new ways to become more sustainable.

But air travel is popular, dirty and, most importantly of all, hard to power with anything other than hydrocarbons.

That’s where Megill comes in. The German-Canadian is seeking the holy grail of air travel – a manned aircraft powered by a liquid hydrogen fuel cell. The technology, engineered by the non-profit organisation AeroDelft which he co-founded in 2017, promises a sort of atmospheric alchemy: a power source whose principal emission is water vapour.

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It’s 8am and Megill has an 18-hour day ahead of him, culminating in a nail-biting control and systems test flight at nearby Rozenburg, the Netherlands, and then back to the workshop until 2am to finalise the wing and tail mould designs, paint part of the internal fuselage and run some simulations. Making aviation history is hard graft, but if the team’s latest endeavour, Project Phoenix, is successful, flying could become not just the safest form of long-distance transport, but eventually also the greenest.

Facebook Twitter Pinterest The fuel tank inside aircraft, whose principal emission is water vapour. Photograph: Project Pheonix

A shiny white, yellow and blue prototype for Project Pheonix’s motor glider was unveiled in April to a ticketed audience, and boasts a range greater than 2,000km, at a cruising speed of 200km/h, on just one tank of liquid hydrogen.

Inside Phoenix’s simple, tented workshop within Delft’s Tec Factory, the ambitious project, says Megill, is “slowly becoming a reality”, as daily deliveries – from 100 metre rolls of fibreglass to tiny electrical components – begin piling up. The full-scale, two-seater aircraft should be ready for testing by 2021, and Megill, a qualified flight instructor, may even pilot it himself.

The design features a pioneering “boundary layer suction”: 14m tiny perforations on the wing to reduce drag. Safety features include sensors and vents, in case of a hydrogen leakage, and an electric battery should the fuel cell fail. Revisiting hydrogen since the Hindenburg (1937) and Challenger (1986) disasters can be a hard sell. “In the public’s mind, they still have hydrogen as a very scary, difficult substance to use,” acknowledges Megill. “[But] it isn’t all that much.”

Hydrogen, the lightest and most abundant element, produces three times more energy per kilo than kerosene, although in gas form (at 700 bar), is six times the volume and requires heavy compression tanks. Phoenix keeps the hydrogen in liquid form by storing it at -253C (-423F), halving the space needed.

But hydrogen’s trump card is that, when mixed with oxygen to create electricity, it emits nothing but water vapour, unlike conventional jet fuels, which are spewing out ever larger amounts of carbon dioxide every year.

AeroDelft is investigating measures such as storage tanks to ensure that all water is released at low altitude or in dry air to prevent cloud creation which can have a greenhouse effect.

Liquid hydrogen has been flight tested as far back as 1988, with the Soviet Union’s Tu-155. But like Boeing’s Phantom Eye 25 years later, it relied on a combustion engine, losing about 70% of the fuel in forces of friction and heat. Phoenix’s liquid hydrogen fuel cell, explains Megill, will be twice as energy efficient as a fuel-burning system.

Facebook Twitter Pinterest The first test flight of Solar Impulse 2 in Abu Dhabi, UAE, a plane whose electric batteries are 97% energy efficient and powered by the sun. Photograph: Solar Impulse | Revillard | Rezo.ch

Sceptics point out that most hydrogen is still produced using steam methane reforming (SMR), which even with carbon capture and storage, still creates as much as 150g of greenhouse gases per kWh. Creating hydrogen via electrolysis is more costly and the process (excluding manufacturing)is carbon neutral only if the electricity is generated with renewables.

“If you produce hydrogen in a dirty way, it has no advantage over kerosene,” says clean technology expert Bertrand Piccard, who, in 2016, completed a carbon-free flight around the globe in Solar Impulse, a plane whose electric batteries were 97% energy efficient and powered only by the sun.

Piccard’s achievement was widely heralded. But the solar option looks a long way from becoming a mass-market alternative. Scaling up the single-seat aircraft, whose solar-cell-covered wingspan was already as large as a jumbo jet, is still problematic.

“It’s not tomorrow that we fly fully solar aeroplanes with passengers … but it will come,” says Piccard, who points to fuel-saving aircraft towing systems (ATS) and route-optimising software such as SkyBreathe as “an intermediate way” to reduce the carbon cost of flying.

The other technology under exploration is battery-powered aircraft. Norway announced last year that it wants all short-haul flights to be electric by 2040. But sceptics counter that battery cells will never be enough to power big aircraft and long-haul flights, and that hybrid solutions will have to come first.

Though hydrogen production is increasingly efficient and projects such as Big Hit on the Orkney Islands and HyBalance in Denmark are good examples of hydrogen being produced efficiently, fully sustainable hydrogen-based aviation is still some way off.

“Hydrogen would be facing a strong headwind if it wants to be the solution to sustainable aviation, especially in the timescale that we know we have to act,” says Julian Beach of Green Fuels, the world’s leading manufacturer of biodiesel equipment.

Facebook Twitter Pinterest The Phoenix Project aircraft. Photograph: Project Pheonix

Biofuels, for their part, have been extensively criticised for the huge amounts of land that would be required to produce sufficient quantities of fuel to make a difference.

In 2017, 69 countries, including the Netherlands and the UK, voluntarily signed up to Corsia (the Carbon Offsetting and Reduction Scheme for International Aviation) targets, which include more than halving aviation’s CO2 emissions by 2050.

For Megill, a carbon-free future for aviation requires time, courage and collaboration. “Our goal is to show that it works and hopefully convince others to make the infrastructure to make green hydrogen,” he says. He hopes that by 2030 the technology that the Phoenix has demonstrated could be used to create a commercial passenger aircraft, but a major investor would need to take over.

A 1:3 scale model of Phoenix, with a wing span of 5.7 metres, will make its maiden flight next year, doubtless highlighting once again that carbon-neutral flying is technically possible, and that liquid hydrogen is a potentially sustainable fuel. Until investors see the value in that, flying less is probably still the best path of travel.

This article is part of a series on possible solutions to some of the world’s most stubborn problems. What else should we cover? Email us at theupside@theguardian.com