In the air, it's like on the ground: the CO2 emissions have to go down. Transport accounts for almost one third of total EU CO2 emissions. Civil aviation accounts for 13.5 percent, as calculated by the European Environment Agency for 2016. The aircraft is particularly in the criticism because of all types of traffic when flying the largest amount of greenhouse gases emitted per passenger kilometer: according to Umweltbundesamt a quarter more than by car. Because we fly more often, the harmful emissions are constantly increasing. But alternative engines and synthetic fuels could enable emission-free flying.

The Institute of Aircraft Design at the University of Stuttgart is conducting research into alternative drives for small passenger aircraft and short routes in the e-Genius project. A so-called two-seater aircraft flew in 2015 purely from battery electric Stuttgart to Milan over the Alps. "We had lunch in Italy, while we had electricity and flew back the same day," says Ingmar Geiss, responsible for the aircraft's electric drive. As the crow flies, the distance is just 325 kilometers, the maximum range of the aircraft is 400 kilometers. "Battery electric drives already work quite well with today's technology in two-seater aircraft," says Geiss. Currently, the e-Genius is converted to a serial hybrid drive: A small battery is just the start, the aircraft is powered by a diesel generator.

The clean alternative to the diesel generator is a fuel cell: no carbon dioxide, no nitrogen oxides, no particles - this technology is free from harmful emissions. Provided the hydrogen is produced with green electricity.

Fuel cell drives require new aircraft configurations. Instead of kerosene in the wings, the hydrogen is stored in the hull, and under the wings hanging several small electric motors instead of two or four large engines. This propulsion technology can help the climate the most, because it is suitable for all distances of the aviation: First for short and medium distances, after technological advancement even for long-distance flights. "Short- and medium-haul flights are the most widespread flights, which is why changes in the propulsion systems for these aircraft are quickly and positively impacting the environment," says Jörg Sieber, Head of Innovation Management at German engine manufacturer MTU Aero Engines in Munich. Munich residents are already working on a fuel cell for short-haul and medium-haul aircraft; by 2050, the first aircraft could take off with this drive, says Sieber.

Kerosene without harmful emissions

Until then, biofuels can help reduce harmful emissions. They are based on vegetable oils and are mixed with fossil fuel. But few airlines mix their fuels - although CO2 emissions are 60 to 80 percent lower and soot emissions are only half. One of the reasons: "Because biofuel is much more expensive than conventional kerosene, only a few airlines fill up with green kerosene," says Sieber.

An alternative to biofuel is synthetic kerosene, called power-to-liquid . It burns without harmful emissions. It is produced by means of excess energy, which is therefore available in low-consumption times. A problem of the energy turnaround is that electricity from wind and sun is often generated when demand is low. Electricity can not be stored in large quantities. It makes sense to use it to produce synthetic crude oil and from it fuels such as kerosene. "That's how the energy and mobility turnaround was helped," says Professor Roland Dittmeyer, head of the Institute of Micro Process Engineering at the Karlsruhe Institute of Technology. His institute has developed a system together with partners in which green kerosene is produced by green electricity from CO2. A few days ago, the first barrel was bottled.

"Artificial kerosene can be mixed with up to 50 percent fossil kerosene without having to change anything on aircraft engines," says Dittmeyer. The engines could also be completely powered by power-to-liquid , but must be seals made of other materials to withstand the novel fuel. "This is technically feasible and the cleanest solution, because then flies is largely climate-neutral," says the professor.

According to Dittmeyer, the Karlsruhe proceedings could be ready for the market in about five years, and the existing tank infrastructure could continue to be used without any changes. The production of synthetic fuel, however, is energy-intensive, the expected costs therefore mainly dependent on the price of electricity and are at about 1 to 1.50 euros per liter, well above the current market prices for fossil kerosene.

The largest European aircraft manufacturer Airbus has since last year converted a four-engine regional jet to a flying laboratory for electric flight research. "One of the four engines will be replaced by an electric drive, and the electric motor will be powered by batteries and a kerosene generator," says Olivier Maillard, project manager at Airbus for E-Fan X - the name of the electric project. The engine builder Rolls Royce is an industrial partner.

Unlike the e-Genius, the Airbus plane will be a parallel hybrid, so the different drives work simultaneously. The e-engine delivers two megawatts and thus a similar performance as the kerosene aggregates. For Airbus machines, long-haul flights would not be possible with today's battery technology, but short- and medium-haul routes would be. "For these routes, we work on purely battery-electric as well as hybrid drives and exclude any alternatives," says Maillard. In the next 10 to 15 years, these technologies could be ready for the market.