There’s turbulence ahead for the aviation sector, with emissions soaring following the COVID-19 pandemic, aircraft fleet set to expand and an array of energy challenges. What role does sustainable aviation fuel play in this mix?
The aviation sector is responsible for around 2.5 cent of global CO₂ emissions, contributing around four per cent to global warming.
In the last few decades, the growth of the sector has been more rapid than other modes of transportation such as rail, road or shipping.
In 2022, as global travel rebounded from the COVID-19 pandemic, aviation emissions approached 800 Mt CO₂ – roughly 80 per cent of the pre-pandemic rate.
It’s estimated that the global commercial aircraft fleet will expand by 3.3 per cent each year, increasing from 29,000 to 42,000 within the next decade.
Furthermore, a significant number of aircraft fleet projected to be operational in 2040-50 is in service today, with limited possibilities for retrofitting to reduce emissions.
To mitigate emissions growth and align with net zero targets by 2050, a multi-thronged approach is required. This includes uptake of sustainable aviation fuels (SAF), advancements in airframe and engine design, operational efficiencies and strategies to manage demand.
But the high energy-to-weight ratios that large airliners must possess makes decarbonising long-haul commercial aviation one of the most thorny challenges for engineers to navigate.
Here’s why decarbonising the aviation sector will be critical to the energy transition.
A question of energy density
Liquid aviation fuels have a significantly higher energy density compared to many alternatives, which is a key advantage in aviation. This high energy density allows aircraft to store more energy in a smaller volume or weight, resulting in the greater range and fuel efficiency required for long-haul flights.
Jet fuel has an energy density of approximately 43 MJ/kg.
Alternative fuels, such as biofuels or gaseous hydrogen, tend to have lower energy densities, which can impact the range and efficiency of aircraft designed for these fuels.
Liquid hydrogen has a higher energy density when measured by weight. However, the process of turning hydrogen into a liquid uses a significant amount of energy – between 20-60 per cent of its energy value – reducing how efficient it is as a fuel. There are also only two hydrogen production plants in Australia.
However, hydrogen fuel cells are a practical solution technology for medium-range aircraft applications up to around 2000-3000 km in the not-too-distant future, according to Andrew Moore, co-founder and Chief Engineering Officer of AMSL Aero, an Australian company pioneering hydrogen-powered electric aircraft.
“There are still challenges with cryogenic storage of liquid hydrogen on aircraft and integration with fuel cells, but these issues are close to being solved,” he said.
Meanwhile, the energy density of lithium-ion batteries, the most common type of battery used in experimental electric aircraft, is significantly lower than jet fuel.
The potential emissions reductions from battery-powered aircraft are substantial. A plane charged using renewable energy could reduce emissions by 90 per cent compared to conventional aircraft powered by jet fuel.
Batteries are also more efficient, with around 70 per cent of the energy used to charge a battery able to be used to power an electric plane.
But the weight comparison between jet fuel and batteries highlights the challenge of using electric power for large commercial aircraft.
For example, if a Boeing 737 was electrified via the batteries available today, all the aircraft’s passengers and cargo would have to get the boot to make room for battery storage to fly the plane for less than an hour.
Given jet fuel has around 50 times the energy density of today’s batteries, that equates to one kg of jet fuel versus 50 kg of batteries. Bridging this gap will require lighter lithium-ion batteries or new battery technologies with much higher energy storage capacity.
“Incremental improvements are needed to balance energy density, fast-recharging capability and lifespan,” Moore said.
“A battery must combine high-energy density with high-power density to meet the power demands of take-off while also providing sufficient energy for long flights.”
Future fuel needs
In 2019, fuel consumption for aviation amounted to 436 billion L. While it’s early days for SAF production, SAF production is set to hit 1.5 million Mt this year, which equates to around 0.5 per cent of aviation fuel needs. But this is triple the amount of SAF produced in 2023.
But this could shift as SAF mandates come into effect. While Australia is yet to mandate SAF, the European Union (EU), Singapore, the US and UK will be introducing them in coming years – pushing up demand for SAF to 4.5 million Mt in 2030.
For example, under the EU’s ReFuelEU Aviation initiative, airlines are required to start using a two per cent SAF blend by 2025. The mandate increases to six per cent by 2030, 20 per cent by 2035 – reaching 70 per cent by 2050. Major airlines such as Lufthansa, KLM and Air France are expected to comply with these regulations.
SAF can reduce carbon emissions by up to 80 per cent over its life cycle compared to standard jet fuel, with factors such as the type of sustainable feedstock used, the production method and supply chain deliveries influencing this reduction.
The most common SAF feedstocks include waste oils, fats, agricultural residues and lignocellulosic materials. The agricultural land required to produce bio-based feedstocks such as oil crops or agricultural residues could be substantial. For example, to produce 12.3 million t of SAF, around 42.4 million t of rapeseed biomass would be needed, covering 68 per cent of the UK’s agricultural land.
But on the home front, Australia’s vast renewable energy resources and growing green hydrogen production will play a key role in biofuel refining processes. As the hydrogen economy expands and green hydrogen production ramps up, hydrogen itself will serve as a feedstock – synthesising with CO₂ to produce SAF. It’s estimated that there’s enough feedstock to support up to five billion litres of SAF production by 2025, potentially reaching 14 billion L by 2050.
Through leveraging a combination of feedstocks and technologies, Australia can meet a significant and growing share of its jet fuel demand.
But worldwide production needs to increase and requires a greater diversity of feedstock to meet future demand.
“There is a limit to how much can be produced from used cooking oil and tallow, so more research is needed into sustainable oil crops,” said Heidi Hauf, Chair of GreenSkies and former Asia-Pacific Sustainability Lead at Boeing.
“International alignment on sustainability standards is important, but Australian conditions need consideration due to our different farming practices.
“We have had to work on our water consumption more than other parts of the world so we often have crops that are much lower in water usage.”