There is no single sustainable alternative to jet fuel that could support flights on a scale equivalent to present day use, according to the briefing by the UK science academy, Net zero aviation fuels: resource requirements and environmental impacts.
The report considers four fuel options – green hydrogen, biofuels (energy crops and waste), ammonia and synthetic fuels – and their associated availability challenges, as well as likely costs, lifecycle impacts, infrastructure requirements and outstanding research questions. Batteries were not considered, as the Royal Society does not expect them to reach the required energy density for long-distance commercial flight by 2050.
Producing sufficient green hydrogen, which was previously identified by the Aerospace Technology Institute (ATI) as the most promising zero-carbon fuel for the near future, would require 2.4 to 3.4-times the UK’s 2020 wind and solar electricity generation, the report found.
Meeting existing aviation demand entirely with biofuels made from ‘energy crops’ – rapeseed, miscanthus and poplar wood – would require about half of UK agricultural land.
The findings underscore the challenges of decarbonising aviation, especially when resources are likely to be in international demand for a range of ‘net zero’ objectives. Addressing these challenges will require global coordination, the society said, particularly during the ‘transition period’ between current and future generation aircraft.
“Research and innovation are vital tools for the delivery of net zero,” said Professor Graham Hutchings FRS from Cardiff University, chair of the report working group. “But we need to be very clear about the strengths, limitations, and challenges that must be addressed and overcome if we are to scale up the required new technologies in a few short decades.
“This briefing tries to pull together those realities, to allow policymakers to understand the future resource implications of today’s policy and R&D decisions and to support international dialogue on this global technology transition.”
The fuel options all have their own strengths and weaknesses, according to the report. Green hydrogen produces no carbon dioxide (CO2), for example, but requires “substantial” modifications to aircraft. Producing enough to replace current aviation fuel would require incredible amounts of electricity, although renewable energy capacity is set to expand in the coming years.
Biofuels from energy crops and waste produce CO2, although that is mitigated by carbon capture in the biomass. Little modification would be needed to aircraft engines.
Waste feedstocks including sewage, solid municipal waste or forestry residues could contribute towards net zero fuel demand, but there is competition from established markets for these feedstocks. Significant investment in fuel production and collection infrastructure would also be required.
Global CO2 emissions from aviation were approximately 1,000m tonnes per year in 2018-19, representing 2.4% of global emissions. This dropped in 2020, before increasing to 720m tonnes in 2021. UK aviation accounted for 8% of the country’s greenhouse gas emissions in 2019.
The UK has committed to scale up manufacturing of sustainable aviation fuels (SAFs) and make domestic flying net-zero by 2040, but aviation is growing globally.
While alternative aviation fuels are likely to have an increased cost, persisting with traditional kerosene jet fuel is likely to become increasingly expensive as decarbonisation in other sectors accelerates, the report noted.
Research will also be important to understand the impact of non-CO2 emissions from jet engines, and the formation of contrails, which currently contribute significantly to warming by aviation. Alternative fuels might reduce these effects, but their impact is uncertain.
Wider considerations, including the development of new airframes to permit hydrogen or ammonia storage, the refuelling infrastructure, and safe refuelling and storage would also need to be investigated and adopted globally.
“How fossil fuel alternatives are produced is critical, as is how we measure their sustainability across the entire cycle of their use,” said working group member Professor Marcelle McManus from the University of Bath.
“We need consistency, and we need to apply this globally, because adopting any of these new technologies will create demands and pressures for land, renewable energy or other products that may have knock on environmental or economic effects.”
While the report considered the fuel required to maintain current levels of flights, reducing the number of journeys made could cut the requirements and make zero-carbon aviation more feasible.
The working group also included members from organisations including the ATI, the University of Oxford, Cranfield University and many others.
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