In the small Finnish town of Pornainen, an unusual silo just went online — and it might reshape how the world stores renewable energy. Standing 13 meters high and 15 meters wide, this insulated tower filled with crushed soapstone isn’t storing electricity. It’s storing heat — enough to keep homes, schools, libraries, and even the local swimming pool warm for weeks. Welcome to the era of sand batteries.
This facility, developed by Finnish startup Polar Night Energy, is now the largest of its kind in the world, capable of storing up to 1,000 megawatt-hours of heat and delivering 1 megawatt of thermal power. Its job? Capture excess wind and solar energy, convert it into heat, and hold onto that heat until it’s needed — whether that’s days or even weeks later. In summer, it can store up to a month’s worth of heat; in winter, about a week’s supply.
Why Sand?
Sand — or in this case, a crushed soapstone by-product from a local fireplace company — is abundant, cheap, and incredibly good at storing thermal energy. When hot air is passed through it, it traps the heat inside its dense, grainy structure. When needed, that heat is pushed back into the town’s district heating network — a system of pipes delivering hot water to homes and businesses.
The battery operates through resistive heating: excess renewable electricity heats the air, which is then blown through the sand to warm it up to approximately 500°C. Thanks to smart algorithms and sensors, the system accurately tracks the amount of energy stored and the rate at which it can be utilized.
“There’s really nothing fancy there,” says Markku Ylönen, co-founder of Polar Night Energy. “The complex part happens on the computer… so we know all the time how much energy is available and at what rate we can discharge and charge.”
A Game-Changer for Renewable Energy
One of the biggest challenges for solar and wind power is that they don’t produce energy all the time — the sun sets, the wind dies down. But heating homes still needs to happen even when the weather doesn’t cooperate. That’s where sand batteries come in. Unlike lithium-ion batteries (which are great for short bursts of power but expensive and polluting to produce), thermal batteries can store massive amounts of energy cheaply and cleanly.
This new installation in Pornainen is a tenfold expansion of an earlier prototype built in the Finnish town of Kankaanpää in 2022. That original project proved that the concept worked — and now, with this scaled-up version, the town’s heating network is set to cut emissions by up to 70%, replacing much of its reliance on oil and wood chips. The wood chip consumption alone is expected to drop by around 60%, thanks to the sand battery becoming the primary heat source for the town.
How It Helps in Daily Life
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Cleaner air: With less burning of fossil fuels and biomass, air quality improves.
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Cheaper bills: Heat from renewables tends to be more cost-stable than fossil fuels.
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Reliable warmth: Even during power shortages or gas supply disruptions (as seen when Russia cut off gas to Finland), heat remains available.
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Smarter cities: By connecting to district heating systems, towns can move toward climate neutrality — Pornainen’s goal by 2035.
Antti Kuusela, the town’s mayor, sees the battery as a crucial milestone. “Pornainen wants to be a front-runner in sustainable energy,” he says. “This project has gone very smoothly.”
Is Sand Really Sustainable?
There’s growing concern over sand mining for construction, but for thermal batteries, any high-density, heat-resistant material can do the job — it doesn’t have to be beach or river sand. In this case, the crushed soapstone used is a by-product, making it a circular, low-impact solution.
“We always choose the thermal medium based on the customer’s needs,” explains Annaleena Naskali from Polar Night Energy. “Sustainability and circular economy are key factors.”
What’s Next?
Polar Night Energy isn’t stopping here. The company is already planning larger projects and exploring how to convert stored heat back into electricity. Their vision? To build hundred-times-larger storage facilities around the world as quickly as possible, especially in places that face harsh winters or are trying to move off fossil fuels. And maybe, in the near future, we’ll look at a pile of crushed rock not as waste, but as a powerful tool in the clean energy revolution.
