MIT researchers have developed a new transparent polymer film that can store solar energy during the day and release it later as heat, whenever needed. The material could be applied to many different surfaces, such as window glass or clothing.

The new material solves a problem with renewable solar energy: the Sun is not available at night or on stormy days. Most solutions have focused on storing and recovering solar energy as electricity or other forms. The new finding could provide a highly efficient method for storing the sun’s energy through a chemical storage system, which can retain the energy indefinitely in a stable molecular configuration and release it later as heat.

Storing-releasing heat as molecular configurations

The finding, by a research team headed by MIT professor Jeffrey Grossman, is described in a paper in the journal Advanced Energy Materials.

The key is an azobenzene molecule that can remain stable in either of two different configurations: charged and uncharged. When exposed to sunlight, the energy of the light kicks the molecules into their “charged” configuration, and they can stay that way for long periods. Then, when triggered by a very specific temperature or other stimulus, the molecules snap back to their original shape, giving off a burst of heat in the process.

Built-in windshield de-icing

The “solar thermal fuel” material is highly transparent, which could make it useful for de-icing car windshields, says Grossman, the Morton and Claire Goulder and Family Professor in Environmental Systems and a professor of materials science and engineering.

While many cars already have fine heating wires embedded in rear windows for that purpose, anything that blocks the view through the front window is forbidden by law, even thin wires.

But a transparent film made of the new material, sandwiched between two layers of glass — as is currently done with bonding polymers to prevent pieces of broken glass from flying around in an accident — could provide the same de-icing effect without any blockage. German auto company BMW, a sponsor of this research, is interested in that potential application, Grossman says.

With such a window, energy would be stored in the polymer every time the car sits out in the sunlight. Then, “when you trigger it,” using just a small amount of heat that could be provided by a heating wire or puff of heated air, “you get this blast of heat,” Grossman says.

“We did tests to show you could get enough heat to drop ice off a windshield.” Accomplishing that, he explains, doesn’t require that all the ice actually be melted, just that the ice closest to the glass melts enough to provide a layer of water that releases the rest of the ice to slide off by gravity or be pushed aside by the windshield wipers.

The team is continuing to work on improving the film’s properties, Grossman says, improving its transparency and temperature increase (from 10 degrees Celsius above the surrounding temperature — sufficient for the ice-melting application — to 20 degrees). The new polymer could also significantly reduce electrical drain for heating and de-icing in electric cars, he says.

The work was supported by a NSERC Canada Banting Fellowship and by BMW.

Abstract of Solid-State Solar Thermal Fuels for Heat Release Applications

Closed cycle systems offer an opportunity for solar energy harvesting and storage all within the same material. Photon energy is stored within the chemical conformations of molecules and is retrieved by a triggered release in the form of heat. Until now, such solar thermal fuels (STFs) have been largely unavailable in the solid-state, which would enable them to be utilized for a multitude of applications. A polymer STF storage platform is synthesized employing STFs in the solid-state. This approach enables uniform films capable of appreciable heat storage of up to 30 Wh kg−1 and that can withstand temperature of up to 180 °C. For the first time a macroscopic energy release is demonstrated using spatial infrared heat maps with up to a 10 °C temperature change. These findings pave the way for developing highly efficient and high energy density STFs for applications in the solid-state.