Finland is filling a former gravel pit with 380 million liters of water to build the world’s largest thermal energy storage system

The Finnish city of Hyvinkää is transforming the former Kulomäki gravel quarry into a huge thermal energy storage reservoir. The project, carried out by the municipal company Hyvinkään Lämpövoima Oy, will use approximately 380 million liters of drinking water and is expected to become operational in autumn 2027.

Filling of the reservoir began in early October 2026 and is scheduled to be completed by the end of April 2027. The reservoir measures 225 by 165 meters, equivalent to approximately five football pitches, and is around 20 meters deep.

The water will be heated using electricity and industrial waste heat during periods when costs are low. In winter, when demand for heating increases, the stored heat will be fed into the city's district heating network.

The reservoir will be able to store approximately 18,000 megawatt-hours of heat, enough to cover nearly a quarter of local demand. The company estimates that the energy could heat 1,000 homes for a year or provide hot water for approximately 20,000 people. The water temperature will range between 90 and 140 degrees Celsius.