Thermal Energy Storage: Material Absorbs Heat as it Melts and Releases it as it Solidifies

on December 20, 2018

Phys-OrgMIT researchers have demonstrated a new way to store unused heat from car engines, industrial machinery, and even sunshine until it’s needed. Central to their system is what the researchers refer to as a “phase-change” material that absorbs a large amount of heat as it melts and releases it as it resolidifies.

Once melted and activated by ultraviolet light, the material stores the absorbed heat until a beam of visible light triggers solidification and heat release. Key to that control are added molecules that respond to light by changing shape from one that impedes solidification to one that permits it. In a proof-of-concept experiment, the researchers kept a sample mixture in liquid form down to room temperature—fully 10 degrees Celsius below where it should have solidified—and then, after 10 hours, used a light beam to trigger solidification and release the stored thermal energy.

More than half of all the energy used to power mechanical, chemical, and other processes is expelled into the environment as heat. Power plants, car engines, and industrial processes, for example, produce vast amounts of heat but use a relatively small fraction of it to actually do work. And while sunlight delivers abundant radiant energy, today’s photovoltaic devices convert only a fraction of it into electricity. The rest is either reflected or absorbed and converted into heat that goes unused.

The challenge is finding a way to store all that thermal energy until we want to use it. Jeffrey Grossman, the Morton and Claire Goulder and Family Professor in Environmental Systems and professor of materials science and engineering, has been working on that problem for more than a decade.

A good way to store thermal energy is by using a phase-change material (PCM) such as wax. Heat up a solid piece of wax, and it’ll gradually get warmer—until it begins to melt. As it transitions from the solid to the liquid phase, it will continue to absorb heat, but its temperature will remain essentially constant. Once it’s fully melted, its temperature will again start to rise as more heat is added. Then comes the benefit. As the liquid wax cools, it will solidify, and as it does, it will release all that stored phase-change heat—also called latent heat.

PCMs are now used in applications such as solar concentrators, building heating systems, and solar cookers for remote regions. But while PCMs can give off abundant heat, there’s no way to control exactly when they do it. The timing depends on the temperature of the air around them.

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