Britain’s most learned scientific body has injected a strong dose of realism into the debate over low-carbon jet fuels. In an 80-page report published in March, the Royal Society boldly stated that, right now, “there is no clear net-zero alternative to jet fuel.”
The report has not been well-received by the airlines or manufacturers. But Guy Gratton, associate professor of aviation and the environment at Cranfield University, makes no apologies. “It’s no good burying our heads in the sand,” he said. “For instance, only 0.02 percent of aviation demand can currently be met by biofuels.”
Gratton was part of the leadership team for the report. Yet he is no eco-warrior. He is a lifelong pilot and aeronautical engineer who has specialized in airborne scientific research. He readily acknowledges that aviation contributes just 2.5 percent of global CO2 emissions. Meanwhile, it supports $2.7 trillion (3.6 percent) of the world’s GDP and generates 65 million jobs.
Airliners have already flown on a 50/50 mix of kerosene and biofuel derived from waste products. But the report includes startling calculations on the scale of the feedstock required to meet the UK’s 12.3 million-tonne annual jet fuel requirement. For instance, even 80 percent of the country’s used cooking oil would produce only 0.6 percent of the need. The ratios do not appear much better for other potential sources, such as straw or forest waste.
Meanwhile, the report answers the question of whether the aviation industry can grow its own feedstock with a resounding no. For instance, meeting the UK demand by cultivating rapeseed (canola) would require two-thirds of Britain’s agricultural acreage. Moreover, as Gratton notes, every feedstock has other uses and other markets.
So if synthetic biofuels represent only a partial, short-term solution, what are the alternatives? The report did not consider battery power solutions. The required energy density will likely not be achieved in time to enable the introduction of viable new airframe designs in significant numbers by 2050. Instead, it examines hydrogen, ammonia, and e-fuels.
Prototype conversions of small twin-turboprops to hydrogen/hybrid power have already flown, and Airbus, for example, plans to introduce to market a hydrogen-powered airliner by 2035. The report notes that hydrogen can burn as a gas in engines to produce thrust or be fed into fuel cells to provide electricity to drive propellers or fans.
But the supply-demand conundrum also applies to hydrogen. Maximum sustainability requires “green” hydrogen produced by electrolysis of water using renewable energy. Meeting the UK’s fossil jet fuel demand would require huge additional investment in wind or solar power—between 2.4 and 3.8 times the country’s current renewable energy capacity. It would also need lots of de-ionized water. “We need new aircraft, and I think it can be done, but I wouldn’t trivialize the problems,” said Gratton.
Ammonia presents a possibility. It carries significantly higher volumetric energy density than hydrogen, the report notes. The maritime industry has begun investigating, and a few companies are converting ships to ammonia power from modes such as electricity and oil. “It might work for aviation, but we need a lot of research,” said Gratton.
One could produce synthetic e-fuel through the reaction of captured CO2 with water. It could present a totally net-zero solution, but it is significantly more energy-intensive than even hydrogen. To meet UK demand would require five to eight times more than the country’s current entire renewable energy capacity.
Of note, the report devotes space to logistics and training issues. “Almost certainly, integrating technologies into aircraft will be the most demanding, but airport and energy distribution infrastructure cannot be disregarded,” it notes. This is not just about fuel storage and pipework. Hydrogen-powered airliners will likely appear in different shapes, requiring adaptation of parking stands. “Almost every profession will need retraining,” said Gratton.
In sum, aviation is achieving 2 percent fuel efficiency gains each year, but annual growth amounts to 5.3 percent, according to Gratton. Therefore, the sector’s emissions double about every 21 years. “Aviation is not yet a majority greenhouse gas emitter, but we must stop it from becoming one,” he said. Other than reducing the amount of air travel or relying on long-term offsets, the options remain limited and uncertain. “And every solution is more expensive than now,” he concluded.