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MIT Technology Review

A “Battery” for Solar Heat

New system could save the warmth of a sunny day and release it when it’s cold.

February 23, 2016

Imagine if your clothing could, on demand, release just enough heat to keep you warm, allowing you to lower your thermostat and still stay comfortable. Or picture a car windshield that stores the sun’s energy and then releases it to melt away a layer of ice. A team of MIT researchers says both scenarios may be possible soon, thanks to a new material that can store solar energy during the day and release it as heat whenever it’s needed. A transparent polymer film, it could be applied to many different surfaces, such as window glass or clothing.

The key to long-term, stable storage of solar heat is a chemical change, say MIT professor Jeffrey Grossman and postdoc David Zhitomirsky, who published their findings earlier this year. Whereas heat itself inevitably dissipates over time no matter how good the insulation around it, a chemical storage system can retain the energy indefinitely in a stable molecular configuration, until its release is triggered by a small jolt of heat (or light or electricity).

The system uses a type of molecule that can remain stable in either of two different configurations. When it’s exposed to sunlight, the energy in that light kicks the molecules into their “charged” configuration, and they can stay that way for long periods until their energy is released on demand.

Already, the preliminary system might be a significant boon for electric cars, which devote so much energy to heating and to deicing windshields that their driving range can drop by 30 percent in cold conditions. The new polymer could significantly reduce that drain, Grossman says.