New process can store solar power

CHAPEL HILL, N.C. — A team of researchers has found a potential solution to one of the fundamental problems of generating large amounts of energy from the sun’s rays: how to store some of the power so it’s available at night.

The scientists found a new way to use solar energy to split molecules of water into its atomic-level components: oxygen and hydrogen. The hydrogen can then be burned for fuel, generating only water as waste, which can then be recycled to be split again.

The hydrogen could be created and used by infrastructure similar to generators and solar arrays that are already familiar, said Tom Meyer, who led the research and is director of the federally funded Energy Frontier Research Center at the University of North Carolina-Chapel Hill.

“Part of a solar array, instead of just making electricity during the day, could in fact be making chemicals,” he said. “So when the sun goes down, you just run the chemicals through your power plant, and you extract the energy back out as you need it.”

Only a fraction of the energy being generated in the U.S. now comes from solar or from wind, which also is intermittent. But it’s enough to already create some distribution problems, and the issue is expected to increase as more renewable energy sources are added to the national grid.

Meyer had been researching how to turn solar energy into fuels for several decades while also working on other projects, he said. Parts of the process had long been apparent, but it took the addition of nanoparticle engineering to capitalize on the potential.

That’s where Gregory Parsons’ group at North Carolina State University came in, said Meyer. Parsons is director of N.C. State’s Nanotechnology Initiative.

The process uses two basic components. One, a molecule called a chromophore-catalyst assembly, absorbs sunlight and then begins breaking down the water molecules. The other, a nanoparticle to which thousands of chromophore-catalyst assemblies are attached, is part of a film that shuttles away electrons, a vital part of deconstructing the water molecules.

It didn’t work smoothly, though, until Meyer turned to Parsons’ team, which found a method for coating the nanoparticles with extremely thin layers of titanium dioxide. The scientists found that the nanoparticles could then carry away electrons quickly enough to make the process work well. They also figured out how to build a protective coating that keeps the chromophore-catalyst assembly tethered firmly to the nanoparticle, as the two pieces had tended to break apart.

The process currently generates hydrogen equal to about 1 percent of the energy in the sunlight received. But now that it’s clear the concept works, the researchers think there’s little doubt that they can improve the efficiency, Meyer said. They hope to reach their goal of at least 15 percent, which is similar to the efficiency of current commercial solar cells.

Also, they plan to explore the potential for using the same process to turn the greenhouse gas carbon dioxide into a carbon-based fuel such as methanol that could be burned in a closed loop, generating carbon dioxide to be used again.

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