Just a Phase?


Thermal energy storage offers tremendous potential for a variety of technologies. A common approach to thermal storage uses a phase change material. Phase change materials (PCM) are substances—often fatty acids, paraffins, or salt hydrates—that store and release energy by melting and solidifying at a specified temperature.

Typically, input heat melts the material and the phase change, or the transition from solid to liquid, stores energy. When the PCM is cooled back down below the melting point, it turns back into a solid. At this point, the stored energy is released as heat.

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One of the challenges of this method of thermal energy storage is controllability. Most PCMs pass through their phase change temperature uncontrollably, which causes them to lose stored heat rapidly.  But researchers at MIT have recently found a way to regulate the process by including molecules that change shape in response to light.

Postdocs Grace Han and Huashan Li and Professor Jeffrey Grossman combined phase change materials and a light-sensitive dopant that works like a molecular switch to control the release of energy. They recently published their findings in the journal Nature Communications.

The hybrid material, which can store about 200 joules per gram, melts when heated. After being exposed to ultraviolet light, it stays melted even when cooled back down. When triggered by another pulse of light, the material resolidifies and gives back the thermal phase change energy on command. 

“By integrating a light-activated molecule into the traditional picture of latent heat, we add a new kind of control knob for properties such as melting, solidification, and supercooling,” Grossman told MIT News.

The researchers explain that in its chemically stored form, thermal energy can remain stable for long periods of time until the trigger is activated. Therefore, the photo-switching molecule is a remarkable advancement that makes the controlled dispatch of this latent energy possible.

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