A team of researchers from the Massachusetts Institute of Technology (MIT) has developed a material that can store solar energy during the day and release it later as heat. The transparent polymer film can be applied to different surfaces, yielding potential applications from de-icing window glass to heating clothing.

The researchers, led by Jeffrey Grossman, professor of materials science and engineering, found that the key to enabling long-term, stable storage of solar heat is to store it in the form of a chemical change rather than storing the heat itself. Whereas heat dissipates over time no matter how effective 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 jolt of heat, light or electricity.

Manufacturing the material is a two-step process that is “very simple and very scalable,” says graduate student Eugene Cho. The system is based on work that was aimed at developing a solar cooker that could store solar heat for use after sundown. The team realized that if the heat-storing material could be made in the form of a thin film, then it could be “incorporated into many different materials,” he says, including glass or fabric.

The solar thermal fuel polymer film comprises three layers, each 4 to 5 microns in thickness. Image credit: Jeffrey Grossman/MIT.The solar thermal fuel polymer film comprises three layers, each 4 to 5 microns in thickness. Image credit: Jeffrey Grossman/MIT. To make the film capable of storing a useful amount of heat, and to ensure that it could be manufactured easily and reliably, the team started with materials called azobenzenes that change their molecular configuration in response to light. The azobenzenes can be stimulated by a pulse of heat to revert to their original configuration and release more heat in the process. The researchers modified the material’s chemistry to improve its energy density—the amount of energy that can be stored for a given weight—its ability to form smooth, uniform layers and its responsiveness to the activating heat pulse.

The material they ended up with is transparent, which could make it useful for de-icing car windshields. While many cars already have heating wires embedded in rear windows, anything that blocks the view through the front window is banned by law. 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 in an accident—could provide the same de-icing effect without any blockage. 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 in the sunlight. Then, “when you trigger it,” using 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. “Accomplishing that, he says, 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.

The team is continuing to work on improving the film’s properties. The material currently has a slightly yellowish tinge, so work is underway to improve its transparency. And while the material can release a burst of heat about 10 degrees Celsius higher than the surrounding temperature—sufficient for the ice-melting applications—they are trying to boost the temperature to 20 degrees Celsius.

The researchers say that the system as it currently exists could be a boon for electric cars, which devote considerable energy to heating and de-icing. As a result, their driving ranges can drop by 30 percent in cold weather. The new polymer could reduce that drain, Grossman says.

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