Researchers Demo Solar Liquid Hydrocarbon Fuel Production

ETH Zurich researchers have developed a novel technology that produces liquid hydrocarbon fuels exclusively from sunlight and air. For the first time worldwide they demonstrate the entire thermochemical process chain under real field conditions. The new solar mini-refinery is located on the roof of ETH’s Machine Laboratory building in Zurich, Switzerland.

Carbon-neutral fuels are crucial for making aviation and maritime transport sustainable. ETH researchers have developed a solar plant to produce synthetic liquid fuels that release the same CO2 during their combustion, recycling the CO2 as previously extracted from the air for their production.

CO2 and water are extracted directly from ambient air and split using solar energy. This process yields syngas, a mixture of hydrogen and carbon monoxide, which is subsequently processed into kerosene, methanol or other hydrocarbons. These drop-in fuels are ready for use in the existing global transport infrastructure.

The process chain of the new system combines three thermochemical conversion processes: Firstly, the extraction of CO2 and water from the air. Secondly, the solar-thermochemical splitting of CO2 and water. Thirdly, their subsequent liquefaction into hydrocarbons.

CO2 and water are extracted directly from ambient air via an adsorption/desorption process. Both are then fed into the solar reactor at the focus of a parabolic reflector. Solar radiation is concentrated by a factor of 3,000, generating process heat at a temperature of 1,500 degrees Celsius inside the solar reactor.

At the heart of the solar reactor is a ceramic structure made of cerium oxide, which enables a two-step reaction – the redox cycle – to split water and CO2 into syngas. The syngas mixture of hydrogen and carbon monoxide can then be processed into liquid hydrocarbon fuels through conventional methanol or Fischer-Tropsch synthesis.

Aldo Steinfeld, Professor of Renewable Energy Carriers at ETH Zurich, and his research group who developed the technology explained, “This plant proves that carbon-neutral hydrocarbon fuels can be made from sunlight and air under real field conditions. The thermochemical process utilizes the entire solar spectrum and proceeds at high temperatures, enabling fast reactions and high efficiency.” ETH’s research towards sustainable fuels is advanced by the effort at the mini-refinery plant.

The solar mini-refinery on the roof of ETH Zurich proves that the technology is feasible, even under the climate conditions prevalent in Zurich. It produces around one deciliter of fuel per day. Steinfeld and his group are already working on a large-scale test of their solar reactor in a solar tower near Madrid, which is carried out within the scope of the EU project sun-to-liquid. The solar tower plant was presented to the public in Madrid at the same time as the mini-refinery in Zurich.

The next project goal is to scale the technology for industrial implementation and make it economically competitive.

Philipp Furler, Director (CTO) of Synhelion and a former doctoral student in Steinfeld’s group said, “A solar plant spanning an area of one square kilometer could produce 20,000 liters of kerosene a day. Theoretically, a plant the size of Switzerland – or a third of the Californian Mojave Desert – could cover the kerosene needs of the entire aviation industry. Our goal for the future is to efficiently produce sustainable fuels with our technology and thereby mitigate global CO2 emissions.”

Two spin-offs already emerged from Aldo Steinfeld’s research group: Synhelion, founded in 2016, commercializes the solar fuel production technology. Climeworks, founded in 2010, commercializes the technology for CO2 capture from air.

This research is especially interesting in that the CO2 and water are sourced from the air. The energy needed for at least the first two steps is solar, while the third, methanol or Fischer-Tropsch synthesis, isn’t revealed., leaving some questions. One should note there isn’t a research paper, the team has gone directly to public demonstration, suggesting the team has proprietary technology best kept private.

Working the solar collection area backwards offers some thoughtful insight. A square kilometer or .62 mile square of area (247 acres) makes more than 5000 gallons of kerosene. Kerosene is more dense than gasoline and less dense than diesel. A 10 gallon yield, or 0.2% of the 5000 gallons would need about 21,500 square feet, about a half acre. Keep in mind the available solar irradiation in the Mojave Desert is quite different than Switzerland or even Madrid.

This technology looks pretty good. Congratulations are in order. Hopefully there will come an efficiency breakthrough. A published study could have helped with that.