This 'sand' battery stores renewable energy as heat

The heat can then be used to warm water in the winter when energy is more expensive.

Polar Night Energy

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.

Similar to traditional storage systems for renewables, Polar's technology stores energy from wind turbines and solar panels that isn't used at once. To be precise, it stores energy as heat, which is then used for the district heating network that Vatajankoski services. Sand is inexpensive and is very effective at storing heat at about 500 to 600 degrees Celsius. Polar says its technology can keep sand "hotter than the stoves in typical saunas" for months until it's time to use that heat during Finland's long winters.

As the BBC explains, the resistive heating process used to warm the sand generates hot air circulated inside the structure. When it's time to use the stored energy, the battery discharges that heated air to warm water in the district's heating system, which is then pumped into homes, offices and even pools. At the moment, Polar's sand battery only serves a single city, and it's still unclear whether the technology can be scaled up. The BBC also says that its efficiency "falls dramatically" when it comes to returning electricity to the grid instead. It's early days for the technology, though, and other companies and organizations might be able to find solutions for those issues.

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