CO2 neutral fuels are of central importance for more sustainable aviation and shipping. ETH researchers have built a solar plant that can be used to produce synthetic liquid fuels that only release as much CO2 during combustion as was previously extracted from the air. What is needed is a CO2 cycle in which exhaust gas is turned into new fuel so that the CO2 in the atmosphere does not increase further.
On the roof of the machine laboratory at ETH Zurich stands the solar mini refinery technology, which is directly connected to the solar system. Also connected is a Climeworks system, in which CO2 and water vapour are extracted from the ambient air, supplying the chemical components for the synthesis gas. Development is still in full swing. Solarkerosine was produced for the first time in 2014.
The production of solar fuels at ETH Zurich is based on three thermochemical conversion processes. All three are integrated in the new mini refinery on the laboratory roof. An adsorption-desorption process removes CO2 and water directly from the ambient air. Both are fed to the solar reactor in the focus of a parabolic mirror. The solar radiation is concentrated 3'000 times by the parabolic mirror, captured inside the reactor and converted into process heat at a temperature of 1'500 degrees Celsius. At the heart of the reactor is a special ceramic structure made of cerium oxide. The foam ceramic is the heart of ETH Zurich's development. There, in a two-stage reaction - the so-called redox cycle water and CO2 - are split and syngas is produced. Syngas can then be further processed into kerosene or methanol or other hydrocarbons - thus into fuel for aircraft or ships.
Large free areas and strong and constant solar radiation are necessary. That doesn't have to be the same in the desert, southern European countries have good conditions.
On large desert areas one could place mirrors without using agricultural land. Air and humidity would definitely be sufficiently available, says Philipp Furler (Technical Director Synhelion).
The demonstration plant in Zurich will not be able to produce large quantities of fuel. The reaction is not continuous. With better efficiency and a mirror area of one square kilometre, a daily production of 20'000 litres of aviation fuel could be expected, says Philipp Furler. Currently they can convert 5-6% of solar energy into fuel energy, the target is 25%.