A new battery-like system can store heat and then release it when triggered by light. It could lead to devices that can store the sun’s heat during the day for use at night for cooking or heating, or that can capture waste heat from cars and industrial machines.

The technology, reported in the journal Nature Communications, would cut energy use and carbon emissions. And in the developing world, it would avoid the burning of fuels like wood and dung, smoke from which is a health hazard.

At the heart of the new heat-storage system are phase-change materials. These materials—certain metal alloys, molten salts, and fatty acids—melt when they absorb heat and this change from solid to liquid stores energy. When they cool and turn back into solids, they release energy.

Phase-change materials are already used to make thermal-storage systems. But long-term storage is a challenge since the materials tend to gradually lose their heat to the cooler surroundings. So they require complex, expensive insulation and packaging layers, plus the release of heat is hard to control.

To achieve a controlled heat-release system, materials scientists and engineers at MIT combined a conventional phase-change material, a fatty acid, with an organic compound called azobenzene. Azobenzene is known to change its molecular structure when exposed to light: it holds one shape when triggered by ultraviolet (UV) light, and switches to a different shape under visible light.

The hybrid material melts when heated, and it stays in this hot, melted state under UV light even when it is cooled down. It solidifies and releases heat only when exposed to visible light.

Small samples of the material tested in the lab held heat for over 10 hours. Conventional materials would lose heat in minutes.

A practical heat-storing device based on the material would be a container with windows and covers needed for controlled light absorption and dark storage, the researchers envision. A heat exchanger would transfer heat to the hybrid material, and a separate UV lamp would trigger the azobenzene shape-change process.

The container would be stored at room temperature with the covers closed for storage. To trigger heat release, the covers will be opened to expose the liquid composite to ambient light or blue LED light for a faster release.

Source: Grace G. D. Han, Huashan Li & Jeffrey C. Grossman. Optically-controlled long-term storage and release of thermal energy in phase-change materials. Nature Communications. 2017.

Photo: Andy Armstrong, Flickr