Intelligent Energy

A thermal battery for storing solar

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MIT researchers want to give solar energy a rain check with a rechargeable heat battery. In search of materials to chemically store the sun's energy, they give nanotubes a try.

A major concern for renewable power industries is storage. Figuring out how to save that energy for those rainy, windless days and nights would benefit bottom lines, grid stability, and ultimately, the Earth. For solar's sake, MIT engineers have been making inroads in chemically storing the sun's energy as heat rather than electricity. The idea is to develop a thermal battery that can take in and release solar energy on demand, and do it over and over again.

Last November, a research team led by Jeffrey Grossman demonstrated how this could be done with help from ruthenium, a very expensive metal that isn't easy to find. So they've been on the lookout for a suitable substitute, a nice and cheap substitute.

They may have found it: a combination of carbon nanotubes and azobenzene.

Publishing this week in Nano Letters, Grossman and Alexie Kolpak, a post doc, describe how the material stores 10,000 times more energy within the same amount of space than its predecessor, the ruthenium compound. According to the researchers, its energy density is akin to that of a lithium-ion battery. And like lithium-ion batteries, the potential device would be rechargeable.

The material performs by switching between two stable energy states, one that brings in the charge and one that releases it. When sunlight strikes the nanotubes, it changes their molecular structure. The new shape can hold steady for long periods of time, storing the energy within its molecular bonds. Though the structure is stable, flipping the switch and releasing the heat needs only a little push.  A catalyst, even just a flash of light, the researchers say, could do the trick. Add sunlight to repeat.

Grossman says in a statement:

You’ve got a material that both converts and stores energy. It’s robust, it doesn’t degrade, and it’s cheap.

The practical applications are significant. The energy stored could heat water and homes or produce steam to generate electricity. While promising, however, this research is at its dawn. The solar industry will likely see many nights pass before the production of such nanotube batteries is perfected.

In the meantime, Grossman and his team so they will continue their search for material with similar properties.

Related on SmartPlanet:

Images: Grossman/Kolpak

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Melissa Mahony

Contributing Editor

Contributing Editor Melissa Mahony has written for Scientific American Mind, Audubon Magazine, Plenty Magazine and LiveScience. Formerly, she was an editor at Wildlife Conservation magazine. She holds degrees from Boston College and New York University's Science, Health, and Environmental Reporting Program. She is based in New York. Follow her on Twitter. Disclosure