Scientists in Japan have found a substance that reversibly and quickly stores and releases low-grade heat without decomposing, an advance that could lead to more efficient reuse of industrial waste heat.
The study, a collaboration between scientists from Tohoku University’s Institute for Materials Research and Tokyo-based Rigaku Corporationis detailed in nature communication†
The researchers used a layered manganese oxide mineral that contains potassium ions and water of crystallization. The mineral is said to be quite similar in composition to birnessite, which is commonly found on the Earth’s surface. The team fabricated their compound in the form of an insoluble black powder and then examined the crystal structure using an X-ray diffractometer and a transmission electron microscope. They then examined how the structure of the compound changed when heated or cooled, and how much and how quickly heat energy was stored and released.
Heating the material to 200︎OC dehydrated it by giving the stored water molecules the energy it needed to be released into the surrounding atmosphere. When the dehydrated material was then cooled to below 120OC in a dry container and then exposed to moist air, it absorbed water molecules and released its stored heat.
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“This ‘intercalation’ mechanism, where water molecules are reversibly introduced into a layered material, is very beneficial for heat storage,” said Tetsu Ichitsubo, a materials scientist at Tohoku University. “It is very fast, reversible and the structure of the material is well maintained. Also, oxygen in the atmosphere does not degrade the layered manganese oxide crystal and water does not dissolve it. This makes it an excellent candidate for the reuse of residual heat in industrial environments.”
According to Tohoku University, “birnessite-type layered manganese dioxide with crystal water” showed better overall performance compared to other compounds currently under investigation for heat storage.
“Our material has a long life, can reversibly store and release large amounts of heat per unit volume, and charges and discharges quickly,” Ichitsubo said in a statement.
The researchers will now work on increasing the amount of water molecules the material can absorb in order to increase the amount of heat energy it can store.