Researchers at the Massachusetts Institute of Technology announced this week that they have developed a new battery-like system can store the the sun’s energy and release heat when needed at a later time.

In the near-term, the technology could provide a new energy source for communities in the developing world that don’t depend on the grid, or create a power system for people who live in cities who want to limit the amount of electricity they use.

The MIT scientists have developed a chemical composite that only releases stored energy when it reacts to light, a system that could take wasted energy from heavy machinery and use it later for cooking or heating a room.

“There are so many applications where it would be useful to store thermal energy in a way lets you trigger it when needed,” says professor Jeffrey Grossman, who worked on the project with MIT postdocs Grace Han and Huashan Li. The team’s findings have been published in this week’s Nature Communications.

The research comes at a point when decentralized renewable energy is growing as an alternative to the grid-based model of old. More and more people are exploring ways to sever ties from the energy grid, particularly in the wake of recent hurricanes that have hit local infrastructure.

Advancements in battery technology are helping to store up locally-generated energy.

MIT’s development uses a phase change material as its starting point. These store up energy when exposed to heat and turn into liquid, but they need a lot of insulation to avoid losing that energy. They’re also not that dependable, with a habit of unexpectedly turning back to solids and releasing their energy due to temperature changes.

With this new “battery,” the fatty acids that act as a phase change material are paired with an organic compound that responds to light. The arrangement melts when heated like normal, but when exposed to ultraviolet light it stays melted even after it’s taken away from the heat. A second light pulse activates the compound and causes the acids to return to their pre-heated solid state, releasing the thermal energy as they change back.

The blue LED activates the material and causes it to change phase.
The blue LED activates the material and causes it to change phase.

The system, which can store around 200 joules per gram, has a variety of applications for areas where grid power is not dependable. Users can place the “battery” in front of the sun, but it can also work with vehicle heat, industrial machines, or pretty much anything else that throws out wasted thermal energy. The stored power could then be used for heating a space, or drying out crops. The team notes that they have already had interest from people interested in using it for cooking in rural India.

“What we are doing technically is installing a new energy barrier, so the stored heat cannot be released immediately,” Han says.

The current system can handle a temperature change of around 18 degrees Fahrenheit. Internal testing shows the arrangement stores the heat for around 10 hours, a big improvement over other phase change materials that lose the energy in the space of a few minutes.

“There’s no fundamental reason why it can’t be tuned to go higher,” Han says.


Thermal energy storage offers enormous potential for a wide range of energy technologies. Phase-change materials offer state-of-the-art thermal storage due to high latent heat. However, spontaneous heat loss from thermally charged phase-change materials to cooler surroundings occurs due to the absence of a significant energy barrier for the liquid–solid transition. This prevents control over the thermal storage, and developing effective methods to address this problem has remained an elusive goal. Herein, we report a combination of photo-switching dopants and organic phase-change materials as a way to introduce an activation energy barrier for phase-change materials solidification and to conserve thermal energy in the materials, allowing them to be triggered optically to release their stored latent heat. This approach enables the retention of thermal energy (about 200 J g−1) in the materials for at least 10 h at temperatures lower than the original crystallization point, unlocking opportunities for portable thermal energy storage systems.

If you liked this article, check out this video where electricity is made from slow-moving water.