Progress towards sustainable fuel production from air CO2 and solar energy: Swiss researchers have succeeded in significantly increasing the yield of their already developed solar system by using a reactor made of printed ceramic structures. The test results give hope that this could significantly improve the economic efficiency of the CO₂-neutral production of kerosene etc., say the scientists.
As is well known, humanity mobilizes carbon that was deposited ages ago: until now, fuels such as gasoline or kerosene have been produced from fossil fuels such as petroleum. When they burn, this adds carbon dioxide to the Earth's atmosphere. In order to counteract climate change, scientists are therefore working on processes to produce fuels that are at least climate-neutral: When burned, only as much CO₂ should be released as was previously absorbed from the atmosphere during production. A research team from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) has been working on a promising process for several years now.
Hot reactions in focus
The core of their system for producing fuel from air CO2 and solar energy is a solar reactor, which consists of a porous, ceramic structure made of cerium oxide. Concentrated solar radiation is directed onto this element by parabolic mirrors. This causes the reactor to heat up to its operating temperature of up to 1500 degrees Celsius. It is then supplied with water and CO₂ that was previously obtained from the air. A cyclic thermochemical reaction then takes place in the hot reactor due to the action of the cerium oxide as a catalyst: the water and the CO₂ are split and so-called syngas is created - a mixture of hydrogen and carbon monoxide. It can then be further processed into liquid hydrocarbon fuels, such as aircraft kerosene.
Video: How the solar system works. © ETH Zurich
The developers have already proven that the system works with a test facility on the roof of the ETH. However, there has been one shortcoming on the way to commercializing the concept: the effectiveness and yield of the system still leave a lot to be desired. The limiting factor was the structure of the solar reactor, say the developers. So far they have used a conventionally manufactured ceramic element with uniform porosity. However, its characteristics meant that it was not heated optimally by the incoming solar radiation: the ideal temperatures for the thermochemical reaction were not reached inside the reactor, which limited the performance.
Sophisticated structures increase efficiency
The solution to the problem has now been made possible by further developments in the area of 3D printing: highly complex ceramic shapes can now be produced using the additive process. Scientists have now adapted this technology to print delicate ceria ceramic structures to create reactor cores that allow solar radiation to be transported more efficiently inside. To do this, they first developed a special “ink”. It is a paste consisting of polymers and the cerium oxide particles and has a consistency that enables 3D printing. Layers are applied, which then dry so that the blanks retain a stable shape. At the end, the printed reactor cores are then fired at over 1600 degrees Celsius to form a solid ceramic unit.
For the tests, the team produced different geometries using this method. As they report, hierarchical structures with channels and pores ultimately turned out to be optimal. The sun-exposed surface is
It is designed to be quite open, and the following sections become more and more dense. This arrangement allows the concentrated solar radiation to be better absorbed across the volume of the reactor. As a result, the entire porous structure then reaches a reaction temperature of 1,500 degrees and thus contributes to fuel production, the researchers explain.
As can be seen from their results so far, the effect of the new concept on the thermochemical reaction performance is significant: It was shown that twice as much fuel can be produced with the printed hierarchical ceramic structures compared to the previous uniform structures with the same concentrated solar radiation. This represents important progress on the path to commercializing the concept, which two of the university's spin-off companies are now devoting themselves to. The technology for 3D printing of the ceramic structures has already been patented, writes ETH Zurich. Finally, co-author Aldo Steinfeld says: “This technology has the potential to significantly increase the energy efficiency of the solar reactor and thus significantly improve the economic viability of sustainable aviation fuels,” says the scientist.
Source: Swiss Federal Institute of Technology Zurich (ETH Zurich), specialist article: Advanced Materials Interfaces, doi: 10.1002/admi.202300452
https://doi.org/10.1002/admi.202300452