A new technology produces liquid hydrocarbon fuels exclusively from sunlight and air.

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Carbon-neutral fuels are crucial for making aviation and maritime transport sustainable. The new solar plant produces synthetic liquid fuels that release as much CO₂ during their combustion as previously extracted from the air for their production.

The system extracts CO₂ and water directly from ambient air and splits them 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 parabolic reflector bundles the light and directs it to the two reactors in the middle of the plant. (Credit: Alessandro Della Bella/ETH Zurich)

Proof of concept

“This plant proves that carbon-neutral hydrocarbon fuels can be made from sunlight and air under real field conditions,” explains Aldo Steinfeld, a professor of renewable energy carriers at ETH Zurich whose research group developed the technology. “The thermochemical process utilizes the entire solar spectrum and proceeds at high temperatures, enabling fast reactions and high efficiency.”

The solar mini-refinery on a Zurich roof proves that the technology is feasible, even under the climate conditions prevalent in the city. It produces around one deciliter of fuel per day (a little less than half a cup).

The fuel that the solar refinery produces. (Credit: Alessandro Della Bella/ETH Zurich)

Steinfeld and his group are already working on a large-scale test of their solar reactor in a solar tower near Madrid, carried out within the scope of the EU’s Sun-to-Liquid project.

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

“A solar plant spanning an area of one square kilometer could produce 20,000 liters of kerosene a day,” says Philipp Furler, director of Synhelion and a former doctoral student in Steinfeld’s group. “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.”

The research plant produces syngas, which can be processed into liquid hydrocarbon fuels through conventional methanol or Fischer–Tropsch synthesis. (Credit: Alessandro Della Bella/ETH Zurich)

How the solar refinery works

The process chain of the new system combines three thermochemical conversion processes:

• The extraction of CO₂ and water from the air.
• The solar-thermochemical splitting of CO₂ and water.
• Their subsequent liquefaction into hydrocarbons.

An adsorption/desorption process extracts CO₂ and water directly from ambient air. Both then enter the solar reactor at the focus of a parabolic reflector. Solar radiation is concentrated by a factor of 3,000, generating 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. This mixture of hydrogen and carbon monoxide can then be processed into liquid hydrocarbon fuels through conventional methanol or Fischer-Tropsch synthesis.

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

Source: ETH Zurich

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