Finland’s Giant Sand Battery Heats Up the Future

In the sleepy Finnish village of Pornainen, a 42 foot tall and 50 foot wide building is filled with 2,000 tons of crushed soapstone. This isn’t a silo, it’s the world’s largest sand battery, a revolutionary thermal energy storage device that went live in June 2025. Polar Night Energy developed this behemoth for Loviisan Lämpö and it’s the backbone of Pornainen’s district heating network, reducing carbon emissions by 70% and eliminating the need for oil completely.

Sand batteries sound like something out of a sci-fi novel, but the idea is ridiculously simple. Excess electricity, usually from renewable sources like wind or solar, heats air which is pumped through a closed loop piping system inside the silo. This air heats the sand – or in this case soapstone, a byproduct of fireplace manufacturing – to 1,112°F. The sand retains heat for weeks, maybe months with minimal loss. When needed, cool air is pumped through the same pipes and absorbs the stored heat and takes it to a heat exchanger which heats water for homes, schools and municipal buildings. It’s a brilliant way to store energy without using traditional batteries.

Pornainen’s sand battery has a 100 megawatt hour capacity, ten times bigger than the world’s first commercial sand battery developed by Polar Night Energy in Kankaanpää in 2022. With a thermal power output of 1 megawatt it can provide heat for a month in the summer or a week in Finland’s harsh winters. This means it can replace an old woodchip power plant, reducing the town’s wood chip usage by 60% and avoiding around 160 tons of CO2 equivalent emissions per year. This is a game changer for a 5,000 person town aiming to be carbon neutral by 2035.


Renewable energy sources like wind and solar are plentiful but intermittent, delivering excess electricity when demand is low and falling short during peak periods. The sand battery absorbs extra electricity when it’s cheap and stores it as heat for later use. Elisa, a Finnish telecom operator, uses artificial intelligence to optimize this process, charging the battery when electricity is cheap and discharging it when heat is most needed. This saves money and helps the grid by participating in Finland’s reserve markets which ensures stability as renewables grow.


Over 100 workers and 40 subcontractors spent a year building this monster and Polar Night Energy delivered it as a turnkey solution. Soapstone from Tulikivi’s fireplace production was used which adds a layer of sustainability by reusing industrial by-products. This fits with Finland’s goal of carbon neutral circular economy by 2035 where waste becomes a resource that powers homes. Business Finland even funded the project for its innovative approach.

Unlike traditional batteries this system doesn’t store electricity – it stores heat. Polar Night Energy is exploring ways to convert that heat back into electricity with steam turbines but for now the battery’s main job is to heat. Since June it has been heating Pornainen’s town hall, schools and library and has exceeded expectations with a round trip efficiency of 90%. That means only a small fraction of the energy used to heat the sand is lost when it’s extracted so it’s a very effective solution.
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