American engineers have developed a new material that can store solar energy during the day and release it later as heat, whenever it’s needed. This transparent polymer film could be applied to many different surfaces, such as window glass or even clothing.
Professor of Environmental Systems at MIT Jeffrey Grossman said rather than working on saving the sun’s energy as electricity, they concentrated on saving it as heat.
They key to their work is a molecule that can remain stable in either of two different configurations. 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.
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 then can be stimulated by a tiny pulse of heat, to revert to their original configuration and release much more heat in the process. The researchers modified the material’s chemistry to improve its energy density, its ability to form smooth, uniform layers, and its responsiveness to the activating heat pulse.
Grossman says the film could be useful for de-icing car windshields. While many cars have fine heating wires embedded in rear windows for that purpose, anything that blocks the view through the front window, even thin wires, is forbidden by law.
However, 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. He says BMW has expressed interested in that potential application.
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 said. “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.
Illustration courtesy MIT.