3D Printed Reactor Core Makes Solar Fuel Production More Efficient

Using a new 3D printing technique, researchers at ETH Zurich have developed special ceramic structures for a solar reactor. Initial experimental testing show that these structures can boost the production yield of solar fuels.

In order to make air travel more environmentally friendly, intensive research is being conducted into sustainable fuels. In recent years, ETH researchers have developed a technology that enables the production of a carbon-neutral liquid fuel from sunlight and air. The centerpiece of this technology is a solar reactor capable of splitting CO₂ and water, which had previously been extracted from the air.

The researchers have devised a 3D printing method for creating the ceramic structures of the solar reactor. This enables more efficient transport of solar radiation into the reactor, thereby doubling the fuel production capacity.

In 2019, the researchers demonstrated the entire thermochemical process chain under real conditions for the first time, in the middle of Zurich, on the roof of the ETH Machine Laboratory. These synthetic solar fuels are carbon neutral because they release only as much CO₂ during their combustion as was drawn from the air for their production. Two ETH spin​offs, Climeworks and Synhelion, are further developing and commercializing the technologies.

At the heart of the production process is a solar reactor that is exposed to concentrated sunlight delivered by a parabolic mirror and reaches temperatures of up to 1,500°C. Inside this reactor (which contains a porous ceramic structure made of cerium oxide), a thermochemical cycle takes place for splitting water and CO₂ captured previously from the air. The product is syngas — a mixture of hydrogen and carbon monoxide, which can be further processed into liquid hydrocarbon fuels such as kerosene (jet fuel) for powering aviation.

Until now, structures with isotropic porosity have been applied, but these have the drawback that they exponentially attenuate the incident solar radiation as it travels into the reactor. This results in lower inner temperatures, limiting the fuel yield of the solar reactor.

Now, researchers from the group of André Studart, ETH professor of complex materials, and the group of Aldo Steinfeld, ETH professor of renewable energy carriers, have developed a novel 3D printing methodology that enables them to manufacture porous ceramic structures with complex pore geometries to transport solar radiation more efficiently into the reactor’s interior. The research project is funded by the Swiss Federal Office of Energy.

The researchers say that hierarchically-ordered designs with channels and pores that are open at the surface exposed to the sunlight and become narrower towards the rear of the reactor have proven to be particularly efficient. This arrangement enables the reactor core to absorb the incident-concentrated solar radiation over the entire volume. This in turn ensures that the whole porous structure reaches the reaction temperature of 1500°C, thereby boosting the fuel generation.

These ceramic structures were manufactured using an extrusion-​based 3D printing process and a new type of ink with optimal characteristics developed specifically for this purpose, namely low viscosity and a high concentration of ceria particles to maximize the amount of redox active material.

Successful Initial Testing

The researchers investigated the complex interplay between the transfer of radiant heat and the thermochemical reaction. They were able to show that their new hierarchical structures can produce twice as much fuel as the uniform structures when subjected to the same concentrated solar radiation of intensity equivalent to 1,000 suns.

The technology for 3D printing the ceramic structures is already patented, and Synhelion has acquired the license from ETH Zurich. “This technology has the potential to boost the solar reactor’s energy efficiency and thus to significantly improve the economic viability of sustainable aviation fuels,” Steinfeld says.

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