Huge cavern will keep a city warm

Storing heat for an entire community is the goal of this giant thermal project.

Huge cavern will keep a city warm
source: Vantaa Energy

The world’s largest district heating storage facility will begin construction this year in Finland’s fourth-biggest city.

Vantaa Energy is building the Varanto facility (which means ‘vault’ in English) in the city of Vantaa. Here, it will store waste heat in underground caverns to be recirculated during winter to heat buildings within the community.

Heat consumption varies enormously between seasons in Nordic countries such as Finland. The cavernous vault will attempt to even out this variability by storing industrial waste heat from data centres, cooling processes and waste-to-energy assets that are generated all year round.

Due to be completed in 2028, the facility will be built within Vantaa’s own bedrock, where a total of three caverns about 20 meters wide, 300 meters long and 40 meters high will be excavated to store over a million cubic meters of 140-degree water.

"Unfortunately, small-scale storage solutions, such as batteries or accumulators, are not sufficient; large, industrial-scale storage solutions are needed."

To store the hot water, the bottom of the caverns will sit 100 meters below ground level, and once filled with hot water the pressure within will allow the water to reach temperatures of up to 140 degrees without boiling or evaporating.

With 600 kilometres of underground district heating networks in Vantaa, about 90% of its residents live in a home heated by district heating. The new thermal energy storage caverns will be almost as large as two Madison Square Gardens (New York), according to Vantaa Energy.

“The world is undergoing a huge energy transition. Wind and solar power have become vital technologies in the transition from fossil fuels to clean energy,” Vantaa Energy CEO Jukka Toivonen said.

“The biggest challenge of the energy transition so far has been the inability to store these intermittent forms of energy for later use. Unfortunately, small-scale storage solutions such as batteries or accumulators are not sufficient; large, industrial-scale storage solutions are needed.”

The project cost has been estimated to be about 200 million euros and a 19-million-euro investment grant has been awarded by Finland’s Ministry of Economic Affairs and Employment. 

The tech
The Varanto storage facility will be built in three caverns within the bedrock. The total thermal capacity of the fully charged seasonal thermal energy storage is estimated at 90 gigawatt-hours – which would heat a medium-sized Finnish city for a year. Two 60-MW electric boilers will also be built to produce heat from renewable electricity when electricity is abundant and cheap. By intelligently balancing electricity generation, waste heat and district heating, Vantaa Energy says the facility will work like a hybrid car: alternating between electricity and other forms of production to achieve maximum efficiency.