The world's largest sand battery is heating a Finnish town

The small Finnish municipality of Pornainen, home to about 5,000 residents, has done something that most European towns are still drawing up plans to attempt: it eliminated fossil fuel use from its district heating system entirely, cutting overall emissions by 70% — all with a device that looks, at first glance, like an unusually large grain silo.

The technology is called a sand battery. It was built by Finnish company Polar Night Energy. And it works by storing heat — rather than electricity — using one of the most abundant materials on earth: crushed soapstone.

How it works

The Pornainen installation is a 13-meter-tall cylindrical steel silo filled with 2,000 tonnes of crushed soapstone. When surplus electricity is available — particularly during windy, sunny periods when renewable power is cheap — the silo's heating elements warm the soapstone to temperatures of up to 500 to 600 degrees Celsius.

The soapstone holds that heat for days or weeks at a time, losing energy only very slowly. When the town's district heating system needs heat, the silo releases it by cycling water or air through the hot stone mass, which transfers its stored thermal energy into the heating network that warms homes, schools, and businesses.

The key innovation is practical: thermal energy storage solves one of renewable energy's most persistent problems. Wind and solar generate power when conditions are right, not necessarily when demand peaks. By storing excess renewable electricity as heat, the sand battery decouples generation from consumption — letting a community capture renewable energy at its cheapest and use it at its coldest.

What it achieved

The results in Pornainen are dramatic. Before the battery, the town's district heating system burned oil and woodchip. After installation:

  • Oil use fell to zero — the oil boiler was entirely replaced
  • Woodchip burning fell by 60%
  • Emissions from district heating dropped 70%
  • The battery can meet the heating needs of all 5,000 residents for a month during the summer, and keep them warm for a full week through the depths of winter

"I am more convinced than ever that thermal energy storage technologies can play a very important role in helping municipalities reduce emissions," said Pornainen mayor Antti Kuusela.

Why soapstone

Soapstone — a metamorphic rock rich in talc — has excellent thermal properties for this application. It can be heated to very high temperatures without cracking, releases heat slowly and steadily, and is geologically abundant in Scandinavia. It is also the same material used in traditional Finnish saunas: Finns have understood for centuries that heated stone holds warmth exceptionally well.

The battery's crushed form maximizes surface area for heat transfer, while the insulated silo design minimizes losses to the surrounding environment.

The bigger picture

Pornainen is a proof of concept with implications far beyond Finland. Thousands of towns and cities across Northern and Eastern Europe rely on district heating systems currently fueled by gas, oil, or coal. The economics of sand batteries are improving as renewable electricity prices fall: the cheaper wind and solar power becomes, the more attractive it is to store that power as heat at scale.

Polar Night Energy has attracted significant international interest following the Pornainen installation, with municipalities in several countries exploring similar projects. The technology requires no rare materials, no lithium, and no complex chemistry — just rock, steel, and renewable electricity.

For the energy transition, sometimes the most elegant solutions are the oldest ones.