Film Harvests Sunlight by Day, Releases Heat on Demand

MIT’s transparent polymeric film enables long-term, stable storage of solar heat via a chemical change.

MIT’s transparent polymeric film enables long-term, stable storage of solar heat via a chemical change.

 

We are increasingly hearing about the development of “smart materials” and the latest news is coming from a team of researchers at MIT. Materials and engineering professor Jeffrey Grossman, postdoc David Zhitomirsky, and graduate student Eugene Cho, have created a new transparent polymeric film that can store solar energy during the day and release it as heat hours or days later as needed. This film could be applied to many different surfaces, such as window glass or clothing.

 

Its most obvious use is in auto window de-icing. In fact, BMW, a sponsor of this research, is pursuing this potential application. 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.

 

According to the team, 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 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).

 

Essentially, the key 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 of time. 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.

 

According to the team, their new approach constitutes the first based on a solid-state material—in this case a polymer, and the first based on inexpensive materials and widespread manufacturing technology. Chemically-based storage materials—known as solar thermal fuels (STF), have been previously developed, including by this team, but the earlier efforts were designed to be used in liquid solutions and not capable of making durable solid-state films.

 

Manufacturing the new material requires just a two-step process that is very simple and scalable, according to the MIT team. To make the film capable of storing a useful amount of heat, and to ensure that it could be made easily and reliably, the researchers started with materials called azobenzenes that change their molecular configuration in response to light. The azobenzenes can then be simulated by a tiny pulse of heat, to revert to their original configuration and release much more heat in the process.

 

The team modified the material’s chemistry to improve its energy density—the amount of energy that can be stored for a give weight—its ability to form smooth, uniform layers, and its responsiveness to the activating heat pulse. The layer-by-layer solar thermal polymer film comprises three distinct layers—4-5 microns in thickness each. Crosslinking after each layer enables building up films of tunable thickness.

 

The MIT team is aiming to further improve the film’s properties. The material currently has a slight yellowish tinge, so the researchers are working on improving its transparency. And it can release a burst of about 10 degrees Celsius above the surrounding temperature—sufficient for the ice-melting application—but they are trying to boost that to 20 degrees. As is, this system may well be a significant boost for electric cars, which devote so much energy to heating and de-icing that their driving ranges can drop by 30 percent in cold conditions. The new polymer could significantly reduce the drain, according to the team.

 

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