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MIT and Harvard team develop material that stores sun’s heat

13 April 2014

Researchers from MIT and Harvard University have developed a material that can absorb the sun’s heat and store that energy in chemical form, ready to be released again on demand. A paper describing the new process is published in the journal Nature Chemistry.

While the material could produce electricity, it would be inefficient at doing so, but for applications where heat is the desired output—e.g., for heating buildings, cooking, or powering heat-based industrial processes—this could provide an opportunity for the expansion of solar power into new realms. In essence, it makes the sun’s energy, in the form of heat, storable and distributable, siad Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering at MIT, and co-author.

Some molecules, known as photoswitches, can assume either of two different shapes, as if they had a hinge in the middle. Exposing them to sunlight causes them to absorb energy and jump from one configuration to the other, which is then stable for long periods of time.

These photoswitches can be triggered to return to the other configuration by applying a small jolt of heat, light, or electricity; when they relax, they give off heat. In effect, they behave as rechargeable thermal batteries: taking in energy from the sun, storing it indefinitely, and then releasing it on demand.

The new work is a follow-up to research by Grossman and his team three years ago, based on computer analysis. Translating that theoretical work into a practical material proved daunting; in order to reach the desired energy density, it is necessary to pack the molecules very close together, which proved to be more difficult than anticipated.

The team tried attaching the molecules to carbon nanotubes (CNTs), but found it difficult to get them packed onto a CNT in that kind of close packing for which they were striving. However, though the best the researchers could achieve was a packing density less than half of what their computer simulations showed they would need, the material nevertheless seemed to deliver the heat storage they were aiming for. Seeing a heat flow much greater than expected for the lower energy density prompted further investigation, said Timothy Kucharski, a postdoc at MIT and Harvard, and lead author.

After additional analysis, they realized that the photoswitching molecules, called azobenzene, protrude from the sides of the CNTs like the teeth of a comb. While the individual teeth were twice as far apart as the researchers had hoped for, they were interleaved with azobenzene molecules attached to adjacent CNTs. The net result was that the molecules were actually much closer to each other than expected.

The interactions between azobenzene molecules on neighboring CNTs make the material work, Kucharski said. While previous modeling showed that the packing of azobenzenes on the same CNT would provide only a 30% increase in energy storage, the experiments observed a 200% increase. New simulations confirmed that the effects of the packing between neighboring CNTs, as opposed to on a single CNT, explain the significantly larger enhancements.

This realization, Grossman says, opens up a wide range of possible materials for optimizing heat storage. Instead of searching for specific photoswitching molecules, the researchers can now explore various combinations of molecules and substrates.

While further exploration of materials and manufacturing methods will be needed to create a practical system for production, Kucharski says, a commercial system is now “a big step closer.”

The team also included MIT research scientist Nicola Ferralis, assistant professor of mechanical engineering Alexie Kolpak, and undergraduate Jennie Zheng, as well as Harvard professor Daniel Nocera. The work was supported by BP though the MIT Energy Initiative and the US. Department of Energy’s Advanced Research Projects Agency – Energy.

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