MIT researchers hope a new chemical composite will allow solar heat energy to be stored for later release. This thermal “battery” is intended for use in parts of the developing world, to aid in evening tasks such as cooking and after-dark heating.

Scientists are using a method known as phase change material (PCM), where input heat melts the material and its phase change stores energy. When the PCM is cooled back down below its melting point, it turns back into a solid, at which point the stored energy is released as heat.

Materials such as waxes, fatty acids and molten salts have been successfully used for this process. However, these materials require a great deal of insulation and pass through that phase change temperature uncontrollably. MIT’s system uses molecular switches that change shape in response to light; when integrated into the PCM, the phase-change temperature of the hybrid material can be adjusted with light, allowing the thermal energy of the phase change to be maintained even well below the melting point of the original material.

“The trouble with thermal energy is, it’s hard to hold onto it,” Professor Jeffrey Grossman explained.

His team developed add-ons for traditional phase change materials. The trick was to find a way to integrate these molecules with conventional PCM materials to release the stored energy as heat, on demand. Researchers combined fatty acids with an organic compound that responds to a pulse of light. The light-sensitive component alters the thermal properties of the other component, which stores and releases its energy. The hybrid material melts when heated, and after being exposed to ultraviolet light, it stays melted even when cooled back down. Next, when triggered by another pulse of light, the material re-solidifies and gives back the thermal phase-change energy.

“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,” said Grossman.

The system could make use of any source of heat, not just solar.

“The availability of waste heat is widespread, from industrial processes, to solar heat, and even the heat coming out of vehicles, and it’s usually just wasted,” MIT postdoc researcher Grace Han explained. “What we are doing technically is installing a new energy barrier, so the stored heat cannot be released immediately.”

In its chemically stored form, the energy can remain for long periods until the optical trigger is activated. In their initial small-scale lab versions, they showed that the stored heat can remain stable for at least 10 hours, whereas a device of similar size storing heat directly would dissipate it within a few minutes.

The organic phase-change material has already garnered interest for cooking applications in rural India. Such systems could also be used for drying agricultural crops or for space heating.