A new way to store the suns heat is using modified carbon nanotubes. These can store solar power indefinitely, later to be recharged by the sun.

We convert the suns heat to electricity, but if we learn to store it in chemical form, it is advantageous. This is because it can be stored for a long period of time   without losing any of the stored energy.

Until now, the chemicals needed for this storage often degraded within a few cycles, or included an expensive and rare element called ruthenium.

However, last year, MIT associate professor Jeffrey Grossman and four co-authors solved how to use diruthenium, the top chemical for reversibly storing solar energy, to store this. This made it easier to understand the process and see which materials could be used this way.

Now, Grossman and fellow researcher Alexie Kolpak have solved how to do this. The duo used carbon nanotubes, small tubular structures of pure carbon, along with a compound called azobenzene. The resulting molecules were produced using nanoscale templates to control their physical structure and find new properties in the separate materials.

This new chemical system is less expensive than earlier options, as well as more efficient at storing energy.  These nanofabrication methods can control the molecules interactions, increasing the amount of stored energy, the length of time they can store it, with total, independent control.

This type of solar energy uses a molecule that changes structure when exposed to sunlight. It can forever remain stable in that form until a stimulus can quickly bring it back to its other form. This releases its stored energy in a burst of heat. This works similarly to a rechargeable battery.

An advantage of this new approach it combines energy harvesting and storage into a single step. This makes it much simpler. A limitation, however, is that while this can be used for heating, producing electricity would require an extra step.

The team is very actively looking at a range of new materials, Grossman says, and calls this the tip of the iceberg.

Yosuke Kanai, assistant professor of chemistry at the University of North Carolina at Chapel Hill, says the idea of reversibly storing solar energy in chemical bonds is gaining a lot of attention these days. The novelty of this work is how these authors have shown that the energy density can be significantly increased by using carbon nanotubes as nanoscale templates. This innovative idea also opens up an interesting avenue for tailoring already-known photoactive molecules for solar thermal fuels and storage in general.

The teams success is outlined in a paper with their new findings published online in the journal Nano Letters.

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