Storing Solar Energy as Heat
Tom Lombardo posted on April 13, 2014 | 18007 views

Sunlight is a great source of energy when it’s available, but saving it for a rainy day is easier said than done. Convert it to electricity and you only get about 20% of what’s available. Store that in a battery and you lose even more. And batteries have limited lifespans.

Alternatively, you could store the sun’s heat in molten salts, water tanks, or another thermal mass. The key word there is mass, and you need a lot of it to store much heat. Even then the heat dissipates over time, making heat storage temporary at best.

Chemical engineers at MIT are working on a way to store heat long term, not with thermal mass and insulation, but through chemical structures. Using molecules known as photoswitches, a team led by professors Jeffrey Grossman and Timothy Kucharski hopes to create a rechargeable thermal battery, capable of storing heat indefinitely. This would allow solar energy to be used for pollution-free heating and cooking, even at night.

A photoswitch is a molecule whose chemical shape can be altered by adding energy to it. Once altered, it retains its new shape until triggered by another event, where it reverts back to its original state and releases the stored energy. Not only does it retain the energy long term, the molecule can be reused over and over with no degradation. In theory, it would have an infinite number of recharge cycles.

Grossman’s team has been studying photoswitches for several years, and their early computer models showed that azobenzine molecules attached to carbon nanotubes would make a good thermal battery. They were concerned, however, that the energy density wouldn’t be sufficient. Recent experiments, the results of which are soon to be published in the journal Nature Chemistry, have surprised the researchers by exhibiting a much higher energy density than expected - very close to that of Li-ion batteries. It turns out that the computer models were based on single carbon nanotube structures with azobenzene molecules attached. In the lab, however, the carbon nanotube structures interacted with each other, and the azobenzine molecules interlocked like a three-dimensional zipper. The interaction between molecules created much higher energy densities than the researchers had theorized. Grossman says, “Now we’re looking at whole new classes of solar thermal materials where you can enhance this interactivity.”

The energy storage-and-release process is entirely clean and infinitely renewable, but it’s not very efficient … yet. Better materials and manufacturing methods are needed in order to make molecular thermal energy storage economically feasible. This research brings that reality a big step closer.

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