Builders are preparing the way to fill 2,400 tonnes of local sand into a silo under construction off a rural road between two large lakes in southern Finland in what will, when completed, be the second heat storage battery for renewable energy operating in the region.
Currently designed to power local district heating networks, the “sand battery” is the brainchild of startup Polar Night Energy. It now has two utility partners in the region implementing the technology. A sand battery has been operating since 2025 in the city of Pornainen; the second is under construction an hour’s drive away in the larger town of Vääksy.
A sand battery is a large silo filled with an inert, non-combustible material that absorbs and retains heat from superheated air. The air circulates through a network of pipes and ducts coiled around the silo.
This thermal energy can come courtesy of excess electrical energy from renewable sources such as wind or solar. This power often comes at the wrong time, creating a use-it-or-lose-it problem. This problem has dogged renewable energy for decades, driving a quest to develop creative energy storage solutions.
Polar Night Energy’s 1 MW Pornainen sand battery facility, the company’s second commercial facility.Photo courtesy of Polar Night Energy
A sand battery uses electricity to power resistance elements (like a toaster, only bigger) that heat the air flowing into the battery. The sand inside, or in the case of the Pornainen battery, recycled crushed soapstone from a local fireplace manufacturer, can contain this heat. Internal temperatures can reach 500°C to 600°C or 930°F to 1100°F.
The stored heat is sent, on demand, through a heat exchanger. The output can be heated air, water or steam, explains Annette Höglund-Dönnes, commercial director of Polar Night Energy, based in nearby Tampere. Sand battery components include steel, pipes, an electric fan and resistors, insulation and a steel carriage.
The idea came to Tommi Eronen and Markku Ylönen, now CEO and CTO of Polar Night Energy, respectively, when they were students in the early 2010s. They developed it in a backyard. By 2022 they had a pilot battery up and running, successfully increasing the runoff of a data center for heating purposes.
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That same year, public utility Loviisan Lämpö, which is owned by a private equity firm, bought the heating business from a company that serves the 5,000 people in Pornainen. It was then that the mayor of the city, Antti Kuusema, received a call. If the city was willing, Lämpö intended to install a 2,000-ton, 1 MW/100 MWh sand battery unit. It was in operation in 2025, replacing the oil and chip boiler that the city used, along with biomass, to operate its district heating network.
“Our experience has been very good. The idea for us as a small municipality has been to try to reduce carbon emissions in all operations,” says Kuusela. So far, it has meant a 60% to 70% reduction in carbon emissions in district heating and no more oil burning.
Chart courtesy of Polar Night EnergyDistrict heating networks are environment specific: they are common in Scandinavia, less so elsewhere, but there are about 2,500 in the United States (including the Manhattan Steam Tunnel System). They distribute heat from a central source, whether fossil, geothermal or nuclear, through a spaghetti network of insulated pipes to surrounding residential or industrial areas.
As of July 2026, the base was in place at the Vääksy sand battery site, which will be a 2 MW/250 MWh facility, and construction was progressing for operation in spring 2027. Polar Night Energy recently signed a distribution agreement to expand its reach into the Baltic states, partnering with US investors Solsiden Wangard Family Office and SIA Soldag, a construction based in Latvia. Plans call for the next sand battery to be even larger, perhaps 10 MW.
Sand batteries are now used for heat in cold weather, but the technology is not strictly tied to that, says Höglund-Dönnes. They could find industrial uses in a shipyard, university campus or in dairies for pasteurization.
“You just have to have the need for heat,” he says.
