This 'sand' battery stores renewable energy as heat
 

A company in Finland has created an an unusual storage solution for renewable energy: One that uses sand instead of lithium ion or other battery technologies. Polar Night Energy and Vatajankoski, an energy utility in Western Finland, have built a storage system that can store electricity as heat in the sand. While there are other organizations researching the use of sand for energy storage, including the US National Renewable Energy Laboratory, the Finns say theirs is the first fully working commercial installation of a battery made from sand.

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