A new way store solar heat

A new way to store solar heat

10:28 AM, 12th January 2016
A new way to store solar heat
The layer-by-layer solar thermal fuel polymer film comprises three distinct layers (4 to 5 microns in thickness for each). Cross-linking after each layer enables building up films of tunable thickness.

CAMBRIDGE, US: Imagine if your clothing could, on demand, release just enough heat to keep you warm and cozy, allowing you to dial back on your thermostat settings and stay comfortable in a cooler room. Or, picture a car windshield that stores the sun’s energy and then releases it as a burst of heat to melt away a layer of ice.

According to a team of researchers at MIT, a new material 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 clothing.

Most efforts have focused on storing and recovering solar energy in the form of electricity, but the new finding could provide a highly efficient method for storing the sun’s energy through a chemical reaction and releasing it later as heat.

The finding, by MIT professor Jeffrey Grossman, postdoc David Zhitomirsky, and graduate student Eugene Cho, is described in a paper in the journal Advanced Energy Materials.

The key to enabling long-term, stable storage of solar heat, the team said, 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 well 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).

Molecules with two configurations

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. 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.

Such chemically-based storage materials, known as solar thermal fuels (STF), have been developed before, including in previous work by Grossman and his team. But those earlier efforts “had limited utility in solid-state applications” because they were designed to be used in liquid solutions and not capable of making durable solid-state films, Zhitomirsky said. The new approach is the first based on a solid-state material, in this case a polymer, and the first based on inexpensive materials and widespread manufacturing technology.

“This work presents an exciting avenue for simultaneous energy harvesting and storage within a single material,” said Ted Sargent, university professor at the University of Toronto, who was not involved in this research.

The system is based on previous work that was aimed at developing a solar cooker that could store solar heat for cooking after sundown, but “there were challenges with that. 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, including glass or even fabric,” said Cho.

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 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, ability to form smooth, uniform layers, and its responsiveness to the activating heat pulse.

Shedding the ice

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 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, he said.

The team is continuing to work on improving the film’s properties, Grossman said. The material currently has a slight yellowish tinge, so the researchers are working on improving its transparency.

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

© MIT News

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