Europe pushes Solar Fuels to help it end its reliance on Russian oil

By combining several optical and chemical-processing innovations, a European consortium is developing a pilot  system that will turn carbon dioxide and green hydrogen into fuels around the clock, using sunlight, artificial lighting, and plasmonics.

The reactor that the project, called Spotlight, is designed to work with the concentrated carbon dioxide stream from small- to medium-emission sources, which emit less than 1 megaton of carbon dioxide a year, and the end result will be methane and carbon monoxide, which can be turned into the fuel methanol.

“If it all turns out as we envision, it would be a really good technology to implement at chemical plants for decarbonization,” says Nicole Meulendijks of the Netherlands Organization for Applied Scientific Research (TNO), which is coordinating the project.

The project is one of many underway worldwide to make solar fuels. The goal is to use the sun’s energy to drive chemical reactions that can convert substances like water, carbon dioxide, nitrogen, and hydrogen into fuels. Solar fuels offer a way to bottle solar energy for long periods of time and then use it wherever, whenever. But known methods aren’t efficient enough to produce the fuels at a cost that's competitive with petroleum-derived fuels.

Artificial photosynthesis, which mimics the chemical processes in plants, is one heavily studied approach to produce solar fuels from abundant sources like carbon dioxide and water. Sun-to-Liquid, A-Leaf, and SofiA are some of the projects funded by the E.U. that have tried to develop and scale up solar fuel plants based on artificial photosynthesis.

Spotlight meanwhile uses an alternative technology, says Meulendijks. To start with the consortium includes parties across the entire value chain, from catalyst and photonic-device manufacturing to companies that provide green hydrogen and carbon dioxide. The partners have expressed their intention to commercially engage beyond research, she adds, which makes the final concept easier to take out of the laboratory and into the field.

The technologies involved cover the gamut, too. Sunlight will be harnessed using a solar oven built at the German Aerospace Center. The furnace is a large flat mirror that sends sunlight to a honeycomb-like array of movable hexagonal mirrors that concentrate the light by up to 5,000 times onto a reactor.

The reactor is a see-through device composed of two glass plates with flow channels sandwiched between them. The channels will be packed with a plasmonic catalyst material that the consortium researchers are developing. It is made of gold or other plasmonic metal nanoparticles that absorb specific wavelengths of sunlight and convert it locally into heat, which will drive the chemical reactions that convert the carbon dioxide and hydrogen pumped into the flow channels into fuel.

Unlike other solar fuels projects, which use concentrated sunlight to directly heat up a reactor, Spotlight's use of plasmon catalysts to convert sunlight into heat means that the absorption spectrum can be tuned by changing the type of metal and the size and shape of the nanoparticles, Meulendijks says.

“A mixture of different sizes and shapes of plasmon catalysts can easily cover the entire solar spectrum, which makes it possible to use all sunlight for the chemical process of choice.”

The plasmonic heating process is also why the system can get away with using a small array of mirrors rather than the vast fields of mirrors or lenses used to produce concentrated heat for conventional solar fuels. “The amount of land use is minimum,” Meulendijks says. “For example, an area of one to three [soccer] fields is needed for the photonic devices, which is feasible at chemical plant sites.”

Another unique feature of the Spotlight project is its endeavor to produce fuel for 24 hours, rain or shine. So as not to be limited by when the sun is shining, the consortium is developing LED-based light sources that emulate the sun. Lighting products leader Signify in Eindhoven, formerly Philips Lighting, is in charge of this. The challenge is to have the artificial light source mimic solar light as much as possible, and to adapt it completely to the spectrum of the catalyst material, says Meulendijks.

Now slightly over a year into the three-year project, the team is trying to optimize the reactor. For instance, they're figuring out how to pack the thin channels with as much catalyst powder as possible while still flowing gas through it as efficiently as possible.

In another five months, project members aim to have all the separate components - the reactor, light source, and catalyst materials - ready to be integrated together and tested at the German Aerospace Center at pilot scale.

If successful, Meulendijks says, the system has the potential to scale up to tackle much loftier ambitions. Ultimately at stake is the carbon emissions from the roughly 11,000 small-to-medium CO₂ sources around the world, she says. Those sources all together emit about 2.7 billion tonnes of carbon dioxide annually and account for 16 percent of emissions from all point sources.