DLR's global climate model simulations estimated a 26 percent reduction in radiative forcing by contrails when using 100 percent SAF compared to Jet A-1 fuel. These findings suggest that using SAF can significantly reduce the climate impact of aviation by decreasing non-CO2 effects such as contrails and by lowering CO2 emissions over the lifecycle of SAF.
"The results from the ECLIF3 flight experiments show how the use of 100 percent SAF can help us to significantly reduce the climate-warming effect of contrails, in addition to lowering the carbon footprint of flying - a clear sign of the effectiveness of SAF towards climate-compatible aviation," said Markus Fischer, DLR Divisional Board Member for Aeronautics.
Mark Bentall, head of Research and Technology Programme at Airbus, said, "We already knew that sustainable aviation fuels could reduce the carbon footprint of aviation. Thanks to ECLIF studies, we now know that SAF can also reduce soot emissions and ice particulate formation that we see as contrails. This is a very encouraging result, based on science, which shows just how crucial sustainable aviation fuels are for decarbonising air transport."
"SAF is widely recognised as a crucial solution to mitigating the climate impact of the aviation sector, both in the short term as well as the longer term. The results from the ECLIF3 study confirm a significantly lower climate impact when using 100 percent SAF due to the lack of aromatics in Neste's SAF used, and provide additional scientific data to support the use of SAF at higher concentrations than currently approved 50 percent," said Alexander Kueper, Vice President Renewable Aviation Business at Neste.
Alan Newby, Rolls-Royce, Director Research and Technology, added, "Using SAF at high blend ratios will form a key part of aviation's journey to net zero carbon dioxide. Not only did these tests show that our Trent XWB-84 engine can run on 100 percent SAF, but the results also show how additional value can be unlocked from SAF through reducing non-carbon-dioxide climate effects as well."
The research team reported its findings in the journal Atmospheric Chemistry and Physics (ACP). The ECLIF3 programme, which includes researchers from the National Research Council of Canada and the University of Manchester, conducted in-flight emissions tests and ground tests in 2021.
Emission and Climate Impact of Alternative Fuels (ECLIF)
DLR conducted extensive flight tests to characterize synthetic fuel emissions in 2015 with the ECLIF1 campaign. These tests continued in 2018 with ECLIF2 in collaboration with NASA, demonstrating that condensation trail climate impact can be reduced using a 50/50 blend of kerosene and SAF.
The ECLIF3 flights in 2021 used 100 percent SAF on an A350 to confirm how effectively unblended SAF reduces ice crystals in contrails. The Airbus A350-941, powered by Rolls-Royce Trent XWB-84 engines, served as the emission source aircraft. The DLR Falcon 20-E research aircraft, equipped with instruments to measure exhaust gases, aerosol particles, and contrail ice particles, followed the A350-941 at several distances to collect emissions data. The flight campaigns compared in-flight emissions of Jet A-1 fuel and Neste's SAF, HEFA-SPK.
Sustainable Aviation Fuels
The European Commission's ReFuelEU Aviation Regulation defines SAF as either synthetic aviation fuels, advanced biofuels from feedstock like agricultural or forestry residues, algae, and bio-waste, or biofuels from other sustainable feedstocks. SAFs are derived from renewable sources such as plant-based or waste-based fuels and potentially renewable synthesized e-fuels and hydrogen from renewable energy sources. These fuels are mostly free of aromatics, resulting in fewer soot emissions and fewer ice crystals in condensation trails, thus reducing aviation's climate-warming effects.
Soot, Ice Crystals, Contrails, and Climate Effects
Aircraft engines emit soot particles and volatile aerosols, with soot acting as a condensation nucleus for supercooled water droplets that freeze to form ice crystals, visible as condensation trails. These trails can persist for hours in cold, humid conditions, forming cirrus clouds with a warming effect. Using SAF can reduce these short-lived contrail-induced warming effects and provide long-term benefits by lowering CO2 emissions, as CO2 remains in the atmosphere for centuries, driving global warming.
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