Finnish engineers packed 2,000 tonnes of crushed stone left over from making fireplaces into a giant “sand battery” — electricity heats the material, which stores enough energy to cover roughly a week of winter demand on a local heating network

Forty truckloads of fireplace offcuts now heat a Finnish village through the dark half of the year.

Pornainen sits about an hour north-east of Helsinki and holds roughly 5,000 people. In a clearing next to a red-brick boiler house stands a corrugated steel silo, 13 metres tall and 15 metres wide, packed with 2,000 tonnes of crushed soapstone. Polar Night Energy, the Tampere company that built it, calls the thing a sand battery. The unit, commissioned in June 2025 for the district heating firm Loviisan Lämpö, is the largest of its kind anywhere.

No lithium, no cobalt, just hot rock in a shed.

What happens inside the silo

Cheap electricity heats air. That air moves through a closed loop of pipes buried in the stone, heating the core to 500C and leaving the energy stored there until it’s needed. When the village wants warmth, the flow reverses, pulls the heat back out and hands it to the district heating network through a heat exchanger. Buy power when it is cheap, spend it when it is not.

Stone beats water for one dull reason: boiling. Water gives up at around 100C, or 120C under pressure, while soapstone has no such ceiling. Polar Night Energy designs its systems to run as high as 650C, packing far more energy into the same volume. Sauli Antila, an operating partner at CapMan Infra, which owns Loviisan Lämpö, described the arrangement to Interesting Engineering as “a very static, robust, and simple system”.

The stone was going to be thrown away

Soapstone is what fireplace maker Tulikivi has left over after carving its heat-retaining stoves. Rather than buy virgin material, Polar Night Energy took the crushed waste and filled the silo over several weeks in 2024, with a quantity the company reckons weighs about the same as a thousand finished fireplaces. Forty truckloads went in. Nothing needs delivering after that, since the whole system runs on electricity, a detail confirmed in Polar Night Energy’s own account of the filling process.

One very bad winter

Finland’s 2025 to 2026 winter was punishing even by local standards, and the Nordic power market behaved accordingly. Company figures cited by Interesting Engineering show Finnish spot prices swinging inside a single week from 3 euros a megawatt-hour to 373. For a business model built on hoovering up cheap electricity, that is close to the worst case.

It held. During the coldest, priciest stretches, the site’s wood chip boiler took the lead and the sand battery played support. For the rest of the season, the roles swapped back. Chief executive Tommi Eronen told the same publication that the pairing still delivered low-cost, low-emission heat through the freeze.

A year of numbers

After twelve months, Polar Night Energy reported a 70 per cent drop in emissions across the Pornainen network, with chief operating officer Liisa Naskali saying the system had performed exactly as designed while cutting emissions cost-effectively. Oil use fell to zero. Wood chip burning dropped by 60 per cent. Storage covers close to a week of the town’s heat demand in winter and almost a month in summer, per CNBC. Round-trip efficiency lands between 80 and 90 per cent, by Polar Night Energy’s own account, with Antila putting recovery near 85 per cent even after weeks of sitting idle.

One installation, in one village, on one grid, is exactly that much evidence and no more.

The number that does the selling

Most thermal storage gets judged on how fast it discharges. Eronen argues the more useful figure is how long a system can spend charging. Pornainen takes roughly 100 hours to fill, compared with a few hours for rival systems. Cheap Nordic electricity arrives at awkward, unpredictable moments, so a slow-drinking battery catches more of it. The medium helps too. Because it’s industrial waste rather than engineered material, the cost per unit of stored heat sits well below a conventional hot water accumulator.

Villages are the demo, factories are the pitch

Antila is blunt about it: district heating is a niche, relevant mainly to Nordic countries and anywhere with a serious winter. Industrial process heat is the actual market. Food and drink plants, chemicals, pharmaceuticals and general manufacturing all need steam at 200C or hotter, currently get it by burning gas or oil, and have very few clean options at that temperature. A larger unit is already going up for Lahti Energia at Vääksy in Asikkala, rated 2 MW with 250 MWh of storage.

Back in Pornainen, the old oil boiler that used to back up the network has not fired once. It sits there, cold, next to a heap of warm gravel that used to be furniture.