Smart solutions for cleaner flying

By German Aerospace Center (DLR)
Researchers at the German Aerospace Center (DLR) found out: Small changes in flight altitude and airspeed, together with the choice of energy source, can already enable significant environmental impact reductions.
© Tom Brunberg/unsplash

Long-haul flights carry only around 10 percent of all passengers each year but generate approximately 40 percent of the carbon dioxide emissions due to air transport. This is due to the long distances and flight times involved. Even small changes in flight altitude and airspeed, together with the choice of energy source, can significantly reduce their climate impact. In addition, aircraft specially designed for flight at different altitudes will also make a decisive contribution to climate compatibility on long-haul routes. Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have arrived at these conclusions as part of the KuuL (Klimafreundlicher ultra-effizienter Langstreckenflug; climate-friendly ultra-efficient long-haul flight) project. This work is part of DLR's extensive research commitment in line with its aviation strategy for climate-compatible flight.

Reduction of climate impact with existing aircraft is possible

In their calculations, the project participants compared kerosene, sustainably produced, carbon-dioxide-neutral synthetic fuels – referred to as Sustainable Aviation Fuels (SAFs) – as well as liquid hydrogen (LH2) and, in conjunction with slightly reduced flight altitudes and speeds, developed aircraft designs required for the various energy sources. The researchers compared the climate impact – that is, the influence of the emissions on climate change – in each case using the average ground-level warming of the atmosphere over 100 years. To do this, they simulated the effect of the newly designed aircraft over 3000 long-haul flights per year and over an operating period of 23 years.

It was quickly confirmed: "No completely new aircraft need to be built for the use of SAFs. Our research shows that just switching from kerosene to SAFs reduces the climate impact by approximately 25 percent, without the need for new aircraft," explains Project Leader Martin Hepperle from the DLR Institute of Aerodynamics and Flow Technology. "The actual climate impact of carbon dioxide emissions can be reduced by 100 percent if the SAFs are produced using a carbon-dioxide-neutral process. This means that during the production of these synthetic fuels, only as much carbon dioxide is emitted as was previously removed from the atmosphere by plants or other processes," Hepperle explains.

Smart solutions for cleaner flying
The reduction of the climate impact of aviation by adjusting flight altitude, airspeed and choice of energy source leads to rising operating costs© DLR

The use of SAFs can also reduce the impact of non-carbon-dioxide effects. Condensation trails and the resulting contrail cirrus are the most significant factor. Since the combustion of synthetic fuels leads to reduced soot particle emissions, SAFs produce less prominent contrails. However, the effect of nitrogen oxides and water vapour is not influenced by this. Therefore, the climate impact cannot be brought to zero by simply changing the energy source.

By switching to hydrogen, further reductions are possible in terms of climate impact, since here, in addition, nitrogen oxide emissions from air transport can be greatly reduced. These emissions lead to the production of additional ozone and a number of other indirect effects on greenhouse gases. "But here, too, we have to produce the hydrogen sustainably – that is, not from fossil fuels," explains Hepperle. Of course, water vapour emitted in the atmosphere when hydrogen is combusted also produces contrails. These are non-carbon-dioxide effects that still need to be investigated in detail. However, they are expected to have less of an impact on the climate than conventional contrails.

New aircraft designs are needed in the long term

However, the long-term climate impact resulting from aircraft engine emissions is also strongly dependent on the flight altitude. "If, in addition to a change of fuel, the maximum flight altitude is reduced by 2000 metres, a reduction in the climate impact of up to 70 percent can be achieved," says Hepperle. However, at this altitude, the aircraft design must also be modified due to the higher air density, and, in particular, the sweep angle of the wings must be reduced. The flight speed would also have to be reduced by up to 15 percent to remain energy efficient. "However, this would then require the development of new aircraft," explains Hepperle. As the climate impact is reduced, however, the operating costs increase because the aircraft can perform fewer flights per day. "In the long term, a compromise must be found here between energy demand, cost effectiveness and climate impact," says Hepperle.

Smart solutions for cleaner flying
Lower and slower flying aircraft require a reduced wing sweep angle© DLR
Aircraft designs for hydrogen propulsion

While aircraft powered by kerosene or SAFs with the same fuel systems resulted in very similar designs, according to calculations by the project participants, aircraft with liquid hydrogen as fuel differ significantly from conventional aircraft. "Above all, the tank volume as well as integration and safety pose a special challenge for long-haul aircraft," explains Hepperle. Although the mass of hydrogen for the same amount of energy is much lower than for kerosene or SAFs, larger and heavier tanks with special thermal insulation would be required, as well as much more complex fuel systems and new types of combustion chambers in the engines. To accommodate the large hydrogen tanks, the fuselage diameter would also have to be increased.

However, contrary to the researchers' expectations, the study showed that LH2 is also a prospect for long-haul flights and could be an alternative to SAFs in the long term. An aircraft fuelled by LH2 requires slightly more energy than one powered using SAFs. "If one takes into account the approximately 30 percent lower primary energy demand in the production of LH2 compared to SAFs, the potential for this technology path becomes apparent," explains Hepperle. "However, the major technological challenges in the use of LH2 currently make it difficult to forecast whether SAFs or LH2 will be more cost effective and climate-friendly in the long term."

In a follow-up project, researchers from the various disciplines will further reduce existing uncertainties in the models, for example with regard to the potentially greater aerodynamic loads at lower altitudes, and investigate new technologies to minimise climate impacts and development risk for industrial application.

In addition to the DLR Institute of Aerodynamics and Flow Technology, the KuuL project involved the Institutes of System Architectures in Aeronautics, Air Transport, Propulsion Technology and Atmospheric Physics.