It's an unfortunate fact that some industries are far more difficult to clean up than others; steelmaking is one, aviation is another, and neither look set to wind down. Commercial passenger aviation represents 2.4 per cent of all greenhouse gas emissions and the industry is predicted to continue its expansion over the next 30 years. Recognising how this might stymie climate targets, world nations signed up to the International Aviation Climate Ambition Coalition at COP26 last November, commiting to reducing aviation’s CO2 emissions in line with a 1.5 C future. Now, the race is on to achieve carbon-neutral flight.

Energy storage in batteries is one proposed solution. Some prototypes have shown promise, but they’ve only been capable of powering small aircraft for short durations. Many engineers are instead determined to harness the power of hydrogen, and a swathe of hydrogen-propelled prototypes anticipate a maiden voyage this year. Phoenix, a two-seater craft being built at Delft University of Technology in the Netherlands, will attempt to travel more than 500 kilometres in a test flight this spring, while California-based ZeroAvia is trialling a 20-seater aircraft using a hydrogen-propulsion system and hopes to have the technology certified by the end of the year.

Meanwhile, a 40-seater, engineered by Universal Hydrogen, is due to take its first flight in September. Airbus is also developing three hydrogen-powered, commercial-scale aircraft, which the company hopes to have operational by 2035.

Hydrogen, in both its liquid and gaseous forms, is light and powerful. If the supply chain can be made greener and cheaper, it’s seen by some as the long-term solution to decarbonising aviation. But others point to its constraints, in terms of both safety and capacity. ‘Hydrogen I take with a pinch of salt because you have the issue of compressing a gas that’s highly explosive,’ says Thomas Brück, professor of synthetic biotechnology at the Technical University of Munich (TUM). Along with some others in the field, he has another idea: algae.

Brück is leading a collaboration between TUM’s AlgaeTec Center and Airbus to produce aviation biofuel from microscopic algae, which are grown in saline environments on marginal land unsuitable for crop cultivation. Brück’s team has already tested its fuel with Airbus’s planes. ‘There’s an efficiency advantage as the fuel can be co-designed with lightweight carbon-fibre materials, capable of achieving carbon-neutral flight,’ he says. When algae-based biofuels are burned, they will release greenhouse gases into the atmosphere, but the process is touted as carbon neutral because that carbon is drawn down during the algaes’ growth phase.

Sea kelp – a form of macroalgae – is another frontrunner. It can be grown at sea, removing the need to convert arable land and advocates point to the fact that it can be put to work using the petrochemical industry’s existing refining infrastructure. ‘Kelp forests also support diverse marine ecosystems, which in turn remove additional carbon from the atmosphere, enriching coastal areas with marine life,’ says Cindy Wilcox, co-founder, president and chief engineer at California-based company Marine BioEnergy, which is currently working on kelp-based biofuels. Through what it calls a ‘depth cycling’ technique – in which kelp is grown in the nutrient-rich oceanic depths – the company claims to have quadrupled traditional kelp yields. Wilcox claims that a mere 0.5 per cent of the world’s ocean would be required to cultivate enough kelp to replace all fossil-fuel-driven long-haul road vehicles and airplanes.

Brück, however, is sticking to the small stuff. ‘Macroalgae such as sea kelp are very good carbon sources. However, because of the seasonal growth, there’s a time period in which there’s little productivity,’ he says. ‘Microalgae have three to four times higher photosynthetic efficiency than terrestrial plants, and if grown in a saline environment, will be less prone to contamination.’

However, whether any of these ideas will really work is likely to come down to cost. Most people believe that scaling up any clean aviation fuel will require a green premium placed upon it by governments. According to Brück, algae-based biofuels are becoming increasingly cost-competitive, but ‘mass cultivation means new infrastructure. That’s the major economic hurdle for the industry right now.’ With aviation innovation stated as an ambition at COP26, technologists such as Brück and Wilcox hope that governments will inject the sector with support in 2022 in order to get clean planes airborne.