Aircraft are powered by kerosene, which, like any fossil fuel, produces emissions. This is why the industry has long been researching alternatives to make aviation more environmentally friendly. Bio-kerosene from plants is now being complemented by a revolutionary concept known as “Sun to Liquid”: a reactor that produces kerosene from sun, water and carbon dioxide.

Despite the plethora of projects and ideas, electricity will not be able to power larger commercial aircraft in the foreseeable future due to the heavy weight of the batteries. There is still no way around the traditional fuel kerosene. One idea is therefore to produce kerosene synthetically, i.e. without crude oil. The revolutionary “Sun to Liquid” method could mean a breakthrough in fuel research. It produces kerosene, as the name suggests, from solar power. The other necessary “ingredients” are water and carbon dioxide (CO2). If the required CO2 is extracted from the surrounding air beforehand, this even compensates for the greenhouse gases generated later during combustion, making the fuel climate neutral.

This process was devised by researchers at the Swiss Federal Institute of Technology, ETH Zurich. In collaboration with other project partners, they first installed a mini refinery on the roof of their machine laboratory. Here, CO2 and water are extracted directly from the surrounding air and split using solar energy. Syngas – a mixture of hydrogen and carbon monoxide – is the product, which is then processed into kerosene, methanol or other hydrocarbons.



Aldo Steinfeld, Professor of Renewable Energy Carriers at ETH Zürich

“With this facility, we are proving that producing sustainable fuel from sunlight and air can work under real-life conditions too,” says Aldo Steinfeld, Professor of Renewable Energy Carriers at ETH Zurich, who developed the facility with his research group. “The thermochemical process uses the entire solar spectrum and runs at high temperatures. This enables fast reaction speeds and a high degree of efficiency.”
Even in the climactic conditions in Zurich, the mini solar refinery on the roof of the ETH proves that it works in practice. However, the output it produces is also mini – no more than deciliter drips out of the system per day – but the idea is scalable. Steinfeld and his research colleagues from Germany, the Netherlands and Slovenia are already testing the solar reactor on a large scale and as part of an EU project near Madrid, where there is much more sun.
Here, more than 160 mirrors, so-called heliostats, focus the sunlight on a reactor installed in a 20-meter tower. Its reactor chamber is heated by the sun’s rays to around 1500 degrees¬¬ – a peak value never reached before. With the help of a catalyst, a synthetic gas is then produced from water and the carbon dioxide taken from the surrounding air, from which kerosene can in turn be produced in a further step.
The project leader on the German side is Christoph Falter from the Bauhaus Luftfahrt e.V. research institute. What motivates him? “If the goals we have set ourselves of reducing greenhouse gases are to be achieved, we have to make significant improvements to fuels as well as technical improvements to the aircraft.” Falter is firmly convinced that solar fuels can make a significant contribution to reducing emissions. But these are expensive. A mechanical engineer and expert in the field of solar fuels, Falter expects production costs to be around two euros per liter of kerosene ¬– four times the selling price of conventional kerosene based on crude oil, which is offered for around 50 cents. “One reason for this is that, unlike gasoline for cars, there are no taxes levied on this fuel,” he explains.
Together with his team, he hopes that governments will soon be able to compensate for the price difference, for example, with a tax, through certificate trading, or with quotas for alternative fuels. “If you consider that the external costs of environmental damage are not taken into account in the case of conventional fuels, there is a good justification for introducing such measures,” says Falter. In addition, the technology for filtering carbon dioxide from the air is currently still very expensive. If the price for this fell, the price per liter of synthetic fuels would also fall.

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However, the Madrid facility is also only considered to be a somewhat larger test field. The researchers’ next goal is to take the technology up to an industrial scale and thus become competitive. For example, a solar plant with a surface area of one square kilometer could produce 20,000 liters of kerosene per day, but to cover the entire average demand of the aviation industry would mean working on a completely different scale. The facilities would have to be the size of a third of the Mojave Desert in California, or the whole of Switzerland.
Text by Behrend Oldenburg
Photos: Aero Engines, ETH Zürich
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