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by Lorenz Keller
Finland commissions world's largest sand battery
1.9.2025
Translation: machine translated
Soapstone quarry instead of fossil fuels: The world's largest sand battery has been connected to the grid in Pornainen. The plant supplies central buildings and homes via the district heating network and stores energy from the wind and sun for weeks on end.
An energy project that could fundamentally change the way we use renewable heat has gone into operation in the southern Finnish municipality of Pornainen. The world's largest sand battery was officially inaugurated on 25 August 2025. It supports the climate-neutral heat supply of the municipality, which has around 5,000 inhabitants.
How does a sand battery work?
In contrast to conventional batteries, the system does not store electricity, but heat. Excess electricity from wind and solar power is used to heat air to up to 600 degrees Celsius. This hot air flows through a closed pipe system into an insulated steel tank filled with around 2000 tonnes of crushed soapstone. The material comes from the Finnish fireplace industry and can store the energy for weeks.
Source: Polar Night Energy
When heat is needed, cooler air flows through the tank, absorbing the stored energy and reaching temperatures of up to 400 degrees Celsius. The heat is then channelled into the district heating network via heat exchangers.
The battery delivers a heating output of around one megawatt and stores up to 100 megawatt hours of thermal energy. This covers the community's heating requirements for around a week in winter or almost a month in summer.
Less wood, no oil
The system replaces an old wood chip heating system, eliminates the need for oil and reduces wood consumption by around 60 per cent. This reduces the district heating network's CO2 emissions by almost 70 per cent. That is around 160 tonnes per year. For the operator Loviisan Lämpö, this not only means lower operating costs, but also greater security of supply.
Source: Polar Night Energy
Mayor Antti Kuusela also emphasises the importance of the project: the sand battery is a key component in achieving the goal of climate neutrality by 2035.
Why sand or soapstone?
Although the technology is generally referred to as «sandbattery», the plant in Pornainen actually relies on soapstone quarry. The material is a waste product from the fireplace industry and is available locally in large quantities.
Granular materials such as sand and soapstone are thermally inert and can store heat for a particularly long time. As no rare raw materials or complex chemicals are required, the process is considered robust, sustainable and scalable. At the same time, the use of by-products promotes the circular economy.
Source: Polar Night Energy
Unlike batteries based on rare earths or chemical components, the technology is robust and scalable. According to the manufacturer, the large-scale system achieves a high level of efficiency and exceeds the guaranteed efficiency targets in the first year of operation.
Conversion to electricity as the next step
While the plant in Pornainen only supplies heat, Polar Night Energy is planning another pilot project in Valkeakoski. This will be the first time that stored heat can be converted back into electricity
The process, known as «Power-to-Heat-to-Power» (P2H2P), uses steam turbines to generate electricity from the stored heat. Significantly higher temperatures are required for this process, which is why the new plant is being designed accordingly.
The plan is to achieve an electrical efficiency of 30 to 35 per cent, comparable to conventional power plants. In combined heat and power operation, the overall utilisation of the energy used could even reach up to 90 per cent. Construction will start in autumn, with the first tests scheduled for early 2026. The project is being funded by Business Finland and will run until 2027.
If the pilot project is successful, sand batteries could not only supply heat in the future, but also contribute to the power supply. This would potentially be a real game changer for the energy transition.
Header image: Polar Night Energy
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