The transparent polymer film could be applied to many different surfaces, such as window glass or clothing.
The film could mean that clothing could, on demand, release sufficient heat to keep you warm. The material could be used in a car windshield to store the sun’s energy and then release it as a burst of heat to melt away a layer of ice.
Although the sun is a virtually inexhaustible source of energy, it is only available about half the time we need it - during daylight. For the sun to become a major power provider for human needs, there has to be an efficient way to save it up for use during nighttime and stormy days. Most such efforts have focused on storing and recovering solar energy in the form of electricity, but the new finding could provide an 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 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).
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 specific temperature or other stimulus, the molecules snap back to their original shape, giving off a burst of heat in the process.
In the researchers’ platform for testing macroscopic heat release, a heating element provides sufficient energy to trigger the solar thermal fuel materials, while an infrared camera monitors the temperature. The charged film (right) releases heat enabling a higher temperature relative to the uncharged film (left).
The storage materials, known as solar thermal fuels (STF), have been developed before, including in previous work by Grossman and his team. But the 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, explained Zhitomirsky. The fresh 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.