Finland is building a 100-million-gallon water pit to store 18,000 MWh of heat, enough to supply 20,000 people

Finnish city of Hyvinkaa with about 47,000 residents is filling a former gravel pit with roughly 100 million gallons of tap water. The 380,000 cubic metres will fill a 66-foot-deep basin to form a giant heat storage that the local utility calls the world's largest pit thermal energy storage, according to a report by Ecoticias.Hyvinkään Lämpövoima Oy, the city’s municipal heating company, expects the reservoir to cover close to a quarter of local heat demand. Water from the city mains began flowing in early October 2026 and the filling is expected to last until late April 2027.Preparing for the wintersThe basin is located in the Kulomaki gravel pit and reaches about 66 feet deep.

Its cover will measure 738 by 541 feet (225 by 165 meters). The utility compares that area to about five soccer fields. Inside, the water will stay between 104°F and 194°F (40°C and 90°C).

Three diffusers set at different heights move it in and out of the basin. During charging, pumps pull cold water from the bottom and send it through a heat exchanger.