Polar Night Energy’s sand battery in Pornainen just survived its worst-case winter. Here’s what the data and the people behind it showed.
In late March, I visited Pornainen, a village of around 5,000 in Finland, as part of a press tour. To get to the world’s largest sand battery, you climb a spiral staircase bolted to the exterior of a dark corrugated steel silo set surrounded by forest, next to a red-brick building with a tall chimney. At the top, through a hatch, is something that doesn’t look like any heating system I’ve seen.
The space inside is low and crowded with massive pipes wrapped in crinkled foil insulation pressing upward. The silo holds 2,000 tonnes of sand, recycled waste from a fireplace manufacturer, threaded through with those pipes – enough thermal energy to cover the majority of the municipality’s heating needs throughout the year. At full charge, it reaches 500-600°C.
This is Polar Night Energy’s sand battery, commissioned in June 2025 for the district heating company Loviisan Lämpö, owned by CapMan Infra.
Finland’s winter of 2025–2026 was brutal, unusually cold even by local standards, and electricity prices hit levels that laid bare just how volatile the Nordic market can be. Polar Night Energy’s own data shows Finnish spot prices swinging within a single week from 3 €/MWh to 373 €/MWh.
For a technology whose entire commercial logic depends on buying cheap electricity and storing it as heat, this was the worst-case scenario.
“We had a very challenging winter,” Tommi Eronen, CEO and co-founder of Polar Night Energy, tells Interesting Engineering. “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.”
Sauli Antila, investment director at CapMan Infra, explains how the system handled the price swings. “During summer, we can store roughly one month’s worth of heating needs in one go. During winter, it’s a bit less than a week. But we have much greater resilience against energy price spikes.”
During the coldest, most expensive stretch, the wood chip boiler became the primary unit, and the sand battery supplemented it. The rest of the time, roles reversed.
The net result: Pornainen fulfilled all of its municipal climate targets with a single installation. Oil use dropped 100 percent, emissions fell 70 percent, and woodchip combustion was cut by 60 percent. According to the Mayor of the Municipality of Pornainen, Antti Kuusela, the municipality now heats all its public buildings, including a new sports arena opening in September 2026, entirely through this district heating network.
The operating principle is simple. Electricity heats air. That air circulates through a closed loop of pipes embedded in stationary sand, transferring heat into the material. When heat is needed, air flows in the reverse direction, extracting it and delivering it to the district heating network via a heat exchanger. Connecting the system to the existing network required only a few pipes and minor automation adjustments.
“There are not that many moving parts,” Antila tells me. “It’s a very static, robust, and simple system.”
Sand works because it doesn’t undergo phase change. Water tops out at around 100°C at atmospheric pressure, or roughly 120°C under pressure, before boiling. Sand doesn’t boil. It can be heated far higher without degrading. The Pornainen installation operates at 500°C; Polar Night Energy’s systems are designed up to 650°C. That ceiling means substantially more energy can be stored per unit of volume.
The outermost layers remain relatively cool even as the core reaches its maximum temperature, keeping insulation loads manageable and minimizing heat loss to the environment. Polar Night Energy models this distribution using multi-physics simulations, and its closed-loop heat transfer mechanism is patented.