World’s largest and hottest underground thermal storage facility under construction in Vantaa

The state-owned energy company Vantaan Energia has begun excavating what will become the world’s largest underground thermal storage facility, designed to store heat at unprecedented temperatures, *Yle Uutiset* reports.

The massive project, named *Varanto* (Finnish for “reserve”), involves carving a network of caverns into bedrock near Helsinki’s Ring III highway. When complete, the three-chamber system will have a combined volume equivalent to nine Finnish Parliament buildings—or roughly the size of the Olympic Stadium in Helsinki. The facility will store water heated to 140°C, significantly hotter than conventional thermal storage systems, allowing it to retain far more energy per cubic metre.

**”It would be a lie to say we aren’t a little nervous,”** admitted Kalle Patomeri, Vantaan Energia’s director of energy business. **”What will we find underground? How will the natural material behave under such high temperatures and pressure?”** Despite the challenges, Patomeri expressed confidence in the project, noting that its planning phase consumed five to ten times the resources of any previous power plant built by the company.

### A buffer against volatile energy markets
The primary function of *Varanto* is to shift energy from periods of low demand (and low prices) to times of peak need. While seasonal thermal storage is not new, the scale and temperature range of this project are unprecedented. Once fully charged, the facility could supply heat to a mid-sized city for an entire year.

The project addresses two key challenges: **seasonal waste heat** from Vantaa’s year-round waste incineration plant, which has little use for excess heat in summer, and **fluctuating electricity prices**. With energy companies increasingly relying on electric boilers for heat production, *Varanto* will help mitigate cost spikes—electricity tends to be most expensive precisely when heating demand peaks. The facility will also absorb excess heat from data centres and potential future green hydrogen plants, which generate heat on their own schedules, not when it’s coldest outside.

**”Energy markets have been in tremendous upheaval in recent years, and that undoubtedly affects *Varanto*’s future profitability,”** Patomeri acknowledged. **”But unlike a battery, this system adapts. As the market changes, so can its function.”**

### A costly but necessary investment
With an estimated price tag of **€300 million**—nearly equal to Vantaan Energia’s entire annual revenue—the project dwarfs similar efforts. Finland’s Ministry of Economic Affairs and Employment has contributed €19 million in investment aid, a fraction of the total cost.

For comparison, Helsinki’s **Mustikkamaa thermal storage** (completed in 2021 within a repurposed oil cavern) holds one-third the volume at a cost of €15 million. A more recent project in **Hyvinkää**, currently under construction in a gravel pit, will store one-third the energy at a cost of €33 million—just one-tenth of *Varanto*’s budget.

Patomeri remained undeterred by the scale of the investment: **”We’ve built large facilities before. Our first waste-to-energy plant had a similar price tag, and it proved to be a good decision. We’re confident this will too.”**

### By the numbers
– **Total volume:** 900,000 m³
– **Heat capacity:** 90 GWh
– **Discharge power:** 200 MW
– **Electric boiler capacity:** 2 x 60 MW
– **Estimated cost:** €300 million