White Light, Stored Heat

Researchers have developed a new material for solar thermal energy applications, a collector that can serve as its own heat battery. As a result, the technology could help smooth out the production of electricity from solar power over a day and night cycle, or during cloudy weather.

The essential idea, says MIT postdoctoral research associate Timothy Kucharski, involves a molecule containing a kind of spring-wound hinge. Exposing the molecule to a burst of sunlight latches the solar energy in place, like arming a mousetrap. The molecule can then be left idle until its energy is needed, at which point a simple chemical catalytic reaction springs the molecular hinge and releases the stored solar energy as heat.

Kucharski and co-investigators used the dye azobenzene, which he says has long been considered a possible solar thermal collector material. However, most solar thermal target molecules have never realized their potential. Some lose their ability to store energy after only a few charge-discharge cycles. Others, like azobenzene, don’t store enough energy in their native configuration to be practical for solar energy applications.

However, the new work (published in Nature Chemistry) used the azobenzene molecules like little toothpicks pushed into the walls of a long drinking straw. The “straw” here is the single-walled carbon nanotube, which let the researchers configure tight arrays of azobenzene that achieve impressive battery-like energy storage. The group found their proof-of-concept array captured and stored 56 Watt-hours per kg of thermal energy, while by comparison lead-acid car batteries store 33-42 Wh/kg. (However, it’s important also to note that lead acid batteries store and output electric energy, while the azobenzene “batteries” perform the more challenging task of actually storing heat.)

“We’ve demonstrated a proof of principle for being able to increase the energy density through this kind of packing,” Kucharski says. “The problem is that with this particular system, it’s not ready for scaling up yet.”

Their prototype, Kucharski says, works well but there's room for improvement. Single-walled carbon nanotubes offered an attractive substrate to test their idea on, he says, but the nanotubes also absorb solar energy themselves, which generates unnecessary heat and noise and reduces the system’s overall efficiency.

So he says his group is now investigating other template materials, including graphene and various polyethylenes. They also are now considering a wide assortment of other solar collecting-and-storage molecules beyond azobenzene, as well as adding new components to the azobenzene molecule itself, such as adding hydrogen bonds and oxygen-hydrogen (aka hydroxyl) groups.

Prototyping even just a handful of templates with the many imaginable solar thermal collector molecules, of course, would be impossible. But Kucharski says his group has also developed powerful software that can simulate the properties of the solar thermal materials and rapidly winnow down the field to a workable number of promising candidates that they can then test in the lab.

If the group’s work can be developed into solar thermal technology—and they’ve submitted patents already—Kucharski says it may not resemble anything like the solar water heaters on the market today.

For instance, he says, picture a window with microfluid chambers in it that allow a liquid version of this collector/battery material to flow through it.

“If you have this as a fluid, with the minimal amount of solvent, then you can flow it from the storage tank through a window that’s exposed to the light,” Kucharski says. “As it’s flown through, it gets charged by the sun. Then it gets sent to a storage tank where it stays charged. When you need it you send it through a reactor with the immobilized catalyst in it—you spend a little bit of heat, and extract more heat from it. Then you can run that [back to the tank], and you close the loop.”

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