This Solar Tower Can Transform Water, Sunlight, and Carbon Dioxide Into Jet Fuel


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Aldo Steinfeld, a professor at ETH Zürich’s Department of Mechanical and Process Engineering, says his system for converting ambient air into jet-ready kerosene fuel using a solar refinery tower isn’t science fiction. It’s simply thermodynamics.

In a two-year proof-of-concept test atop ETH Zürich’s Machine Laboratory building in Switzerland back in 2019, Steinfeld’s mini solar refinery first showed how the process works, and laid the groundwork to scale the project. Both carbon dioxide (CO2) and water are extracted directly from ambient air and split using solar energy, as Steinfeld describes his work in a new paper, published late last month in the journal Joule. The result is syngas, a mixture of hydrogen and carbon monoxide, which is then processed into kerosene.

“The design of the solar reactor, the cornerstone technology, was the most challenging,” Steinfeld tells Popular Mechanics. “We evaluated the performance of the solar reactor based on five primary metrics and experientially validated its stable operation and full integration in the solar tower fuel plant.”

The plant grew from mini status to a larger-scale test when the team operated a solar refiner at the IMDEA Energy Institute in Spain in 2021. It will only keep growing. “The solar-to-fuel energy conversion efficiency needs to be increased to make the technology economically competitive,” Steinfeld says. Already he’s working on optimizing the structure with 3D printing to improve volumetric radiative absorption, which leads to higher energy efficiency.

To help turn the research project into a commercial reality, Synhelion, a spinoff company from ETH Zürich’s Machine Laboratory, is already planning to commission the world’s first industrial solar tower fuel plant in Julich, Germany. In March, Swiss International Air Lines announced it will be the first airline to fly with solar kerosene.

Proving the concept was an “important milestone toward industrial-scale production.” Steinfeld says one commercial-scale solar fuel plant could collect 100 MW of solar radiative power to produce about nine million gallons of kerosene per year. He’d need about 2.3 square miles to make that work. The total land footprint Steinfeld says is needed to create enough solar plants to “fully satisfy global demand” equals about half a percent of the area of the Sahara Desert.

The thermochemical process comes via three conversion units, all in a series. First it captures the ambient air to extract CO2 and H2O before a solar redox unit converts the CO2 and H2O—solar radiation heats the chemicals—into a syngas, a specific mixture of CO and H2. The third step is a gas-to-liquid synthesis unit, which converts the syngas into liquid hydrocarbons, usable as kerosene in jet fuel.

“We have successfully demonstrated the technical viability of the entire thermochemical process chain for converting sunlight and ambient air into drop-in transportation fuels,” Steinfeld says. “The overall integrated system achieves stable operation under real conditions of intermittent solar radiation and serves as a unique platform for further research and development.”

The process is carbon neutral because solar energy is used for production and releases only as much CO2 as was previously extracted for production. If the construction materials of the solar tower plant are created using renewable energy, he says the entire process can produce zero emissions.

Steinfeld says focusing on aviation fuel can help reduce carbon emissions from one of the leading industries contributing to that pollution. “These emissions can be avoided by substituting fossil-derived kerosene by solar-made kerosene,” he says. “Note that solar kerosene is fully compatible with the existing infrastructures for the fuel storage, distribution, and end-use in jet engines, and can be blended with fossil-derived kerosene. Thus, solar kerosene can help make aviation more sustainable.”

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