Dive Brief:

• Researchers at the Massachusetts Institute of Technology have developed a new material that releases heat when exposed to light.

• The phase change material could have uses ranging from providing a heat source for cooking in developing countries to capturing otherwise wasted energy from engines.

• In initial laboratory tests, the optically sensitive material was able to store thermal energy for at least 10 hours, the researchers reported.

Dive Insight:

Researchers at the Massachusetts Institute of Technology say a new chemical composite they have developed can capture, store and use lost thermal energy.

The “thermal battery” is a hybrid material that uses molecular switches that change shape in response to light. It is made by combining 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. Then, when triggered by another pulse of light, the material re-solidifies and gives back the thermal phase-change energy.

Similar phase change materials use waxes or fatty acids in low-temperature applications and molten salts in high temperature applications.

Molten salt is sometimes used in combination with concentrating solar projects that reflect sunlight, which is then used to melt salt that can be used later to drive a traditional steam turbine.

The technology has been heralded as a form of dispatchable solar power that some say could be cost competitive when deployed at utility scale. Companies such as SaltX Technology and Aalborg CSP are working on commercializing the technology. And companies such as NRG Energy have deployed utility-scale concentrating solar power projects, but they have run into problems, including intense light and heat that can attract and kill raptors.

The new material developed by the MIT researchers is likely years away from commercialization, but it provides an alternative to thermal energy storage. It could, in fact, be used to store heat from any source, releasing it when needed — for example, for cooking or heating after dark. “The trouble with thermal energy is, it’s hard to hold onto it,” MIT Professor Jeffrey Grossman said in the journal Nature Communications.

Grossman and his team – postdocs Grace Han and Huashan Li – added “little molecules that undergo a structural change when light shines on them” to traditional phase change materials.

When the hybrid material is exposed to heat in the presence of ultraviolet light, it turns into a liquid. If the material is then pulsed with UV light and kept in the dark, it can remain liquid — in essence trapping the thermal energy — for up to 10 hours. The thermal energy can then be discharged by exposing the material to visible light, such as the sun or a light bulb.

“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 said.

That kind of control could be useful in a variety of applications.

It would provide a means to store concentrated solar power with higher levels of control, for instance, Han told Utility Dive. But molten salt operates at much higher temperatures, “so we won't say that our material can completely replace the use of molten salt,” Han said.

Han said she and her colleagues are working on the engineering aspect of this concept to integrate the phase change material with diverse platforms such as fabrics and heat transfer fluids.

In the near term, the researchers are working with the Tata Trust to integrate the hybrid material into fabrics that could store and release heat. They are also working to incorporate the material into cookware that could be charged in the sun and used to cook indoors without burning scarce and noxious fuels. That is the main focus of the team’s research right now, providing a heat source for remote regions of India. “We are working with MIT Tata center to push this forward for commercial applications in a few years,” Han said.

As for concentrating solar power, Han said those applications are further out — at least five years away.