Hyvinkää, a Finnish city of about 47,000 people north of Helsinki, is filling a former gravel pit with roughly 100 million gallons of tap water. The 380,000 cubic meters will form a giant heat storage basin that the local utility calls the world’s largest pit thermal energy storage. It is due to enter service in fall 2027.
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. Filling should last until late April 2027.
A 66-foot-deep basin at Kulomäki
The basin sits in the old Kulomäki gravel pit and reaches about 66 feet (20 meters) 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. It returns warm through the top diffuser, and discharging reverses that path. According to the project page, a middle diffuser is kept for a future heat pump or for cooler waste heat of around 158°F (70°C).
Electric boilers charge it in summer
Finnish summers bring plenty of cheap electricity but little need for heating. The utility will charge the store with heat from electric boilers and nearby industrial waste heat when energy costs little. In winter, when demand and prices peak, that heat goes into the district heating network.
“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, translated from Finnish. With the boilers, the company adds, the network can drop fossil fuels almost entirely.
Esa Vakkilainen is emeritus professor of energy systems at LUT University. In the utility’s 2025 construction release, he said storage had moved “from hours and days to the scale of a whole year.” He also pointed to data centers, which give off heat all year round.
18,000 megawatt-hours per charge
One full charge holds about 18,000 megawatt-hours of heat. The utility says that could warm about 1,000 detached houses for a year. It could also supply hot tap water to around 20,000 people over the same period.
A second figure, 90,000 to 100,000 megawatt-hours, is the heat expected to pass through the store each year. Operators will charge and drain it again and again. Its maximum charging and discharging power of 60 megawatts sets how fast it can respond on a cold day.
Like the thermal battery planned for a Kansas ethanol plant, the basin banks cheap energy as heat for later. In Hyvinkää, though, the storage medium is plain drinking water.
Double liners over groundwater
Because the site lies in a groundwater area, the project’s water permit requires close monitoring. The basin floor and embankments use a double liner with leak wells to flag any failure. Divers or a robot could repair a leak, the utility says.
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. That cools the groundwater and sends the recovered energy into the network, while 18 new observation pipes track conditions on the site and nearby.
Under the basin runs stainless steel piping from ISOPLUS Finland. Its delivery included 27 straight sections totaling about 1,063 feet (324 meters). The supplier says its large anchor elbows needed extensive design work before production.
Vantaa’s caverns will hold more
The “world’s largest” label applies only to pit storage in an open basin. In nearby Vantaa, Vantaan Energia is digging Varanto, three bedrock caverns each about 984 feet (300 meters) long. Its planned capacity of 90 gigawatt-hours is five times Hyvinkää’s single charge.
Vantaan Energia expects Varanto to be operational by 2030. At the other end of the scale, the same logic of storing energy as hot water lies behind ideas to turn ordinary home water heaters into hidden batteries.
Savings remain 2027 forecasts
The project costs €33 million, or about $37 million at current exchange rates. Finland’s Ministry of Economic Affairs and Employment granted roughly €7.7 million ($8.6 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.
Those numbers are projections for a basin that has not stored any heat yet. Crews still have to build the permanent cover between April and October 2027. Charging can start only in late summer 2027, so nobody knows yet how much heat the water will lose over the months.
In October, Kytömäki said construction was halfway done and on schedule, with no accidents on the site. L&T Teollisuuspalvelut Oy, part of Lassila & Tikanoja, is the main contractor for the first phase.
The technical details and schedule are published on Hyvinkään Lämpövoima’s project page.
Image: Hyvinkään Lämpövoima
