Lahti Energia has initiated a joint project with Polar Night Energy to build the world’s largest thermal sand battery with a capacity of 250 MWh. This innovative system will serve as a thermal energy reserve for the district heating network in Vääksy, southern Finland.
The technological challenge of the sand battery
With a thermal power of 2 MW, the new facility seeks to largely replace the use of natural gas and biomass in heat generation. According to company estimates, this will enable a 60% reduction in annual emissions from the grid, through an 80% reduction in natural gas consumption and a reduced dependence on wood chips.
The battery system will operate with some 2,400 tons of natural sand, capable of reaching temperatures in excess of 500 °C. This thermal mass allows heat from renewable sources to be stored for long periods and released at times of high demand.
Once completed in the summer of 2027, the system will have a 14-meter-high, 15-meter-diameter structure. Polar Night Energy will act as prime contractor for the project, with work to begin in early 2026.
In addition to supplying heat at low cost and with low emissions, the thermal battery thermal battery will also contribute to balancing the national power grid. By integrating into the reserve markets operated by Fingrid, it will be able to stabilize supply in times of variability in wind and solar generation.
Finnish technology has gained ground as a viable option for emission-free energy storage. Polar Night Energy’s thermal battery was included in TIME magazine ‘s Best Inventions of 2025 list and recognized at international forums such as COP30.
The project is supported by Business Finland and enhances Lahti Energia’s strategy of offering sustainable and competitive solutions for its customers. The company is owned by the Municipality of Lahtiand operates under circular economy and energy efficiency principles.
Source and photo: Polar Night Energy