Sun-powered generator makes liquid fuel out of thin air

Solar concentrators mounted on the university’s rooftop raise the temperature inside reaction chambers to thousands of degrees to turn CO2 into syngas, and later into kerosene or methanol. Credit: ETH Zurich.

Researchers from Switzerland have devised a method that simultaneously captures CO2 from the atmosphere and turns it into synthetic fuel, which can later be refined into methanol or kerosene. The entire process is powered by solar energy and, since the input carbon is extracted from the air, the fuel is essentially net-zero carbon neutral.

Although solving our climate emergency requires transitioning away from fossil fuel towards a 100% renewable energy system, this transition cannot happen overnight. The challenge is to minimize carbon emissions as much as possible during this critical transition period because the effects of CO2 buildup today will keep having an effect long into the future. Even if we miraculously stop burning all fossil fuels tomorrow, the damage done so far cannot be undue as greenhouse gases can remain stable in the atmosphere for decades, perhaps even centuries.

It’s likely we’ll be using fossil fuels for decades to come, especially in transportation. So, researchers at ETH Zurich have devised a single coherent system that addresses this need for sustainable liquid fuels.

In one box, the system first captures carbon dioxide and water vapor from the air at ambient temperature. The two ingredients are then sent to a second reactor where they’re converted into carbon monoxide and hydrogen using a similar heating/cooling cycle to that employed in the first step. The mix of carbon monoxide and molecular hydrogen is known as “syngas”. Finally, this syngas is introduced to another reaction chamber where it reacts in the presence of a copper-based catalyst, changing phase from gas to liquid and forming methanol or kerosene, depending on the concentration of each reactant.

Throughout the entire time, these reactions are powered by electricity and heat generated by solar panels and solar concentrators, respectively.

During a day of operation, the experimental setup produced 100 liters of syngas, which was processed into about half a decilitre (0.05 liters) of pure methanol. That may sound like a disappointingly low yield, however, this is merely a proof of concept rig and the researchers are confident they can considerably improve the efficiency from 5.6% currently to over 20%.

“The energy efficiency is still too low. To date, the highest efficiency value that we measured for the solar reactor is 5.6 percent. Although this value is a world record for solar thermochemical splitting, it is not good enough. Substantial process optimization is still required,” said Aldo Steinfeld, first author of the new study and Professor of Renewable Energy Carriers at ETH Zurich.

The resulting liquid fuel is carbon neutral because solar energy is used in its production and it releases only as much CO2 as was previously extracted from the air during combustion.

“The solar fuel production chain’s life-​cycle assessment indicates 80 percent avoidance of greenhouse gas emissions with respect to fossil jet fuel and approaching 100 percent, or zero emissions, when construction materials (e.g. steel, glass) are manufactured using renewable energy,” Steinfeld added.

The findings were reported in the journal Nature.

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