World’s largest sand battery survives its worst winter, ready for roll out

Inside a dark corrugated steel silo in a forest clearing in Pornainen, a village of 5,000 people in southern Finland, 2,000 tonnes of crushed soapstone sit wrapped in pipes and foil insulation, heated to between 500°C and 600°C. Outside, during the winter just passed, Finnish electricity prices swung between €3 and €373 per megawatt-hour in a single week. The sand battery, as Polar Night Energy's engineers call it, kept the town warm through all of it.

Commissioned in June 2025 for district heating company Loviisan Lämpö, the Pornainen installation is the world's largest sand battery — a 1 MW / 100 MWh thermal storage system standing 13 metres tall and 15 metres wide.

The 2025-26 Finnish winter was unusually cold even by local standards, and electricity prices hit levels that exposed just how volatile the Nordic market can be.

"We had a very challenging winter," said Tommi Eronen, chief executive and co-founder of Polar Night Energy. "It was super cold, even compared to our standards, and electricity prices were very, very high. But still, thanks to the peak energy capacity, and the combination of the wood chip boiler and this system, we were able to produce very low-priced district heating and very low emissions."

The battery has delivered on its two headline promises: the use of oil in Pornainen's heating network has been completely eliminated, and annual CO₂-equivalent emissions from the local heating system have fallen by approximately 70% — around 160 tonnes per year. Consumption of wood chips has dropped by roughly 60%, with the existing biomass boiler retained as backup during peak demand periods.  

How it works

The principle is straightforward, and that simplicity is much of the point. Electricity — ideally cheap, surplus renewable power from wind or solar — heats air through a resistive heating process. That hot air circulates through a closed loop of pipes embedded in the stationary sand or soapstone, transferring heat into the material. When heat is needed, the air flows in the reverse direction, extracting it and delivering it to the district heating network via a heat exchanger.

"There are not that many moving parts," said Liisa Naskali, Polar Night Energy's chief operating officer. "It's a very static, robust, and simple system."

Sand works because, unlike water — which boils at around 100°C — it can be heated to far higher temperatures without degrading or changing phase. This allows the storage of heat at densities that water-based thermal systems cannot match. The Pornainen system achieves a round-trip efficiency of 80 to 90%, with the battery taking approximately 100 hours to charge fully — a week-scale storage horizon that is significantly longer than most thermal storage systems.

The system's profitability is managed by an AI optimisation layer. Finnish telecommunications company Elisa handles the charging and discharging decisions through an AI-driven solution that automatically identifies the most economically advantageous moments to store and release heat, based on electricity spot prices and Finland's grid operator Fingrid's reserve markets.

"Our AI-driven solution automatically identifies the most economically viable moments to charge or discharge the sand battery. This brings Loviisan Lämpö significant savings and revenue, making the sand battery a truly profitable investment," said Jukka-Pekka Salmenkaita, vice president of AI and special projects at Elisa Industriq.

The soapstone used as the thermal medium is recycled waste from Tulikivi, a Finnish fireplace manufacturer — a circularity that reduces both cost and embedded carbon in the storage medium itself.

The scale question

At 100 MWh capacity, the Pornainen battery is ten-times larger than the world's first commercial sand battery, which Polar Night Energy built in Kankaanpää in 2022. In summer, it can cover almost a month's heat demand for Pornainen; in winter, close to a week. The Mayor of Pornainen, Antti Kuusela, said the municipality now heats all its public buildings — including a new sports arena opening in September 2026 — entirely through this district heating network.

That record is already being superseded. Polar Night Energy has signed an agreement with Lahti Energia to build a 2 MW / 250 MWh sand battery in Vääksy, Asikkala — 2.5 times the capacity of Pornainen — with construction under way and completion scheduled for summer 2027. The new system will use approximately 2,400 tonnes of locally sourced natural sand and stand roughly 14 metres tall and 15 metres wide.

"This is a solid foundation for scaling sand battery technology to even larger capacities, and across new industrial sectors," Naskali said. "The lessons learned are directly applicable to scaling into new regions."

The technology's limits

Sand batteries address one part of the renewable energy storage problem — heat — with unusual elegance. Storing heat in insulated sand is in many cases more efficient than converting electricity to heat, back to electricity and back to heat again, avoiding two conversion losses in the chain. The 80 to 90% round-trip efficiency compares favourably with pumped hydro and is competitive with lithium-ion battery storage for thermal applications.

But the current technology delivers heat, not electricity. Polar Night Energy is working on closing that gap through a pilot in Valkeakoski testing power-to-heat-to-power conversion — the full cycle of storing electricity as heat and recovering it as electricity — though the company acknowledges this remains a more complex engineering challenge.

"District heating is a niche," the company's chief operating officer said, signalling that industrial heat applications — where temperatures above 100°C are needed for manufacturing processes — represent the next frontier for the technology.

The Pornainen project has received recognition from Time magazine's Best Inventions of 2025 list, the Capgemini Nordic Sustainability Tech Award, and a special award at the COP30 climate conference through the Mission Innovation Net-Zero Industries Mission.  

For a town of 5,000 people in a Finnish forest, Pornainen has become an unlikely reference case for one of the more genuinely practical answers to a question that has haunted the renewable energy transition since its beginning: where does the heat go when the sun is not shining and the wind is not blowing? The answer, it turns out, may be a pile of hot rocks.