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.
Water from the city mains began flowing in early October 2026 and the filling is expected to last until late April 2027. That cools the groundwater and sends the recovered energy into the network, while 18 new observation pipes track conditions on the site and nearby. The project costs €33 million at current exchange rates. Finland’s Ministry of Economic Affairs and Employment granted roughly €7.7 million in investment aid. The company expects fuel savings of several million euros a year and about 4,850 U.S. tons (4,400 metric tons) less carbon dioxide annually.
“Once completed, the heat storage facility will above all mean more stable heating prices for residents of Hyvinkää,” CEO Sanna Kytömäki said in the utility’s October announcement. Divers or a robot could repair a leak, the utility says. Hyvinkään Lämpövoima Oy, the city’s municipal heating company, expects the reservoir to cover close to a quarter of local heat demand. With the boilers, the company adds, the network can drop fossil fuels almost entirely. Since the site lies in a groundwater area, the project’s water permit needs close monitoring. The basin floor and embankments use a double liner with leak wells to flag any failure. Warmer groundwater next to the basin is the main environmental risk the company lists. A separate aquifer system with heat pumps will collect heat that seeps into the ground below.