world's largest sand-based battery

Finland warms up the world's largest sand battery, and the economics
look appealing

A sand battery is a type of thermal energy storage system that uses
sand or crushed rock to store heat. Electricity -- typically from
renewable sources -- is used to heat the sand. That stored heat can
later be used for various ends, including to warm buildings.
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Like many Scandinavian towns, Pornainen operates a central boiler that
heats water for homes and buildings around town. The sand battery --
brought to life by the Finnish company Polar Night Energy -- can store
1,000 megawatt-hours of heat for weeks at a time, enough for a week's
worth of heating in the chilly Finnish winter. From storage to
recovery, only about 10% to 15% of the heat is lost, and the
temperature at the outlet can be up to 400 C.

https://techcrunch.com/2025/06/16/finland-warms-up-the-worlds-largest-sand-battery-and-the-economics-look-appealing/