In Finland, Polar Night Energy has signed a contract to build the world’s largest sand thermal battery with a capacity of 250 MWh for the heating network of the town of Vääksy.
A contract has been signed with the local company Lahti Energia. Work will begin at the start of the new year and will be completed in 2027. The new thermal battery will become the largest in the world, using sand or a similar natural material as a heat transfer medium.
The new project follows the first successful commercial implementation of a similar thermal battery with a capacity of 100 MWh in Pornainen in southern Finland. The thermal battery in Pornainen has already been providing central heating to the district for about six months, having undergone a testing phase during the winter of 2024–2025.
The heat storage unit, measuring 13 × 15 meters, contains 2000 tons of soapstone — waste from the production of cladding material for saunas. At peak performance, it can deliver 1 MW of thermal power and stores heat from excess solar and wind energy by heating the sand with heating elements in the storage unit.
The facility in Vääksy will be slightly larger — measuring 14 × 15 meters and containing 2400 tons of local sand (the exact composition is not specified). At peak performance, it will be able to deliver 2 MW of heat with a storage capacity of 250 MWh of thermal energy.
The temperature inside the battery will reach 500 °C. Connecting the thermal battery to the heating network will help smooth out energy consumption spikes and utilize excess energy. In the future, the possibility of reverse conversion of heat into electricity is being considered, with preliminary agreements already reached with the national grid operator.
