DLR Says Lower, Slower Flights, New Fuels Can Decarbonize Long-Haul

The climate impact of long-haul aircraft could be reduced by as much as 70%—at the price of increased operating cost—by using different fuels and redesigning aircraft to fly lower and slower, concludes a study by German aerospace center DLR.

To calculate climate impact, the 30-month KuuL (climate-friendly ultra-efficient long-haul flight) project assessed more than just carbon dioxide (CO₂) emissions. Researchers also looked at non-CO₂ effects, such as contrails and emissions of nitrogen oxides (NOx) and water vapor.

The study simulated the effect of new-design aircraft conducting more than 3,000 long-haul flights per year over a period of 23 years and compared climate effect using the average ground-level warming of the atmosphere over 100 years, DLR says.

Long-haul flights carry about 10% of passengers each year, but generate approximately 40% of CO₂ emissions from air transport, the German research agency says. But emissions from long-haul flights are the hardest to abate, because of the size and range of the aircraft.

“Our research shows that just by switching from kerosene to SAFs [sustainable aviation fuels] reduces the climate impact by approximately 25%,” DLR researcher Martin Hepperle, KuuL project leader, says in a statement.

‘Green’ SAF produced using a CO₂-neutral process can reduce CO₂ emissions by 100% and also reduce non-CO₂ effects, as well as lower soot particle emissions—producing less prominent contrails. But the effect of NOx and water vapor are not reduced and climate impact cannot be brought to zero, DLR says.

Further reductions in climate impact are possible by switching to hydrogen as the energy source. This would reduce NOx emissions, which lead to production of additional ozone and other indirect effects on greenhouse gases, DLR says. But the hydrogen must be produced sustainably, using renewable energy.

But hydrogen combustion does produce water vapor, which results in contrails. “These are non-CO₂ effects that still need to be investigated in detail. However, they are expected to have less of an impact on climate than conventional contrails,” DLR says.

An aircraft fueled by liquid hydrogen (LH2) requires slightly more energy than one using SAF, but it takes 30% lower primary energy to produce green LH2 compared with green SAF. When this is taken into account, DLR concludes, the potential to use hydrogen for long-haul flights becomes apparent.

Flight altitude also has a strong effect on the climate impact from aircraft emissions, according to DLR. “If, in addition to a change of fuel, the maximum flight altitude is reduced by 2,000 m (6,500 ft.), a reduction in the climate impact of up to 70% can be achieved,” Hepperle says.

To remain energy-efficient at lower cruise altitude, the aircraft must be redesigned with a lower wing sweep and 15% lower flight speed. And while climate impact would be reduced, operating costs would be increased because the aircraft could perform fewer flights per day.

Flying lower and slower while also using SAF could reduce climate impact by up to another 40%, while using LH2 could provide another 10% reduction, DLR estimates. But operating costs could increase by up to 30-40% and long-haul flight times extend by up to 11-17%.

DLR’s modeling assumed green SAF and LH2 would cost twice as much as kerosene, pushing up operating costs. “However, the major technological challenges in the use of LH2 currently make it difficult to forecast whether SAF or LH2 will be most cost-effective and climate-friendly in the long term,” Hepperle says.

“In the long term, a compromise must be found here between energy demand, cost effectiveness and climate impact,” he says. A follow-on DLR project will aim to reduce modeling uncertainties and investigate technologies to lessen climate impact and development risk.