World’s largest cavern thermal energy storage built in Vantaa

Vantaa Energy is building a seasonal thermal energy storage facility in Vantaa, Finland. When completed in 2028, it will be the largest in the world by all standards and its thermal energy capacity could fully charge as many as 1.3 million electric car batteries.


The fluctuating nature of renewable energy is one of its main issues. Renewable energy sources like solar and wind power depend on the optimal weather conditions, which means it may not always be available when needed. This may become a problem, especially in regions where heat consumption varies significantly between seasons.

In order to deal with this issue, Vantaa Energy is building a seasonal thermal energy storage facility in Vantaa. The operating principle of the facility called Varanto, meaning ‘vault’ or ‘reserve’, is to store heat in underground caverns so that it can then be used to heat buildings via a district heating network whenever it is needed.

Providing renewable energy on-demand

The seasonal thermal energy storage facility will be built in Vantaa’s bedrock, where a total of three caverns about 20 meters wide, 300 meters long and 40 meters high will be excavated. The bottom of the caverns will be 100 meters below ground level. The capacity of the caverns is huge; their total volume is 1,100,000 cubic meters, including process facilities. The underground caverns will be filled with hot water. Pressure will be created within the space, allowing the water to reach temperatures of up to 140 degrees without the water boiling or evaporating.

The total thermal capacity of the fully charged seasonal thermal energy storage is 90 gigawatt-hours and is equivalent to as much as 8 million euros worth of district heat. This capacity could heat a medium-sized Finnish city or town for as long as a year. Broken down into smaller energy units, this amount of energy is equivalent to 1.3 million electric car batteries.

Planned Hybrid District Heating System in Vantaa. © Vantaa Energy

Varanto allows Vantaa Energy to store previously wasted heat for use at a later time and is a giant step towards the hybrid district heating system to be built in Vantaa. The system will enable the smartest possible generation method to be chosen for each moment and the different energy sources can be fully utilised.

At its best, the heat-producing system will sometimes be running on electricity, sometimes on other production forms. The Vantaa Energy district heating network acts as the power transmission and is steered by intelligent control, securing affordable and stable prices while eliminating emissions.

The project, valued at around 200 million euros, is financed by Vantaa Energy, but has been awarded a 19-million-euro investment grant from Ministry of Economic Affairs and Employment of Finland. Construction of the storage facility’s entrance is expected to start in summer 2024, and the seasonal thermal energy storage facility could be operational as early as 2028.

Finland a leader in energy innovation

Finland has become a leader in innovative energy and storage solutions. The strong focus on research, supportive policy environment, well-functioning collaboration ecosystem and Finnish technological expertise are some of the reasons behind Finland’s success. Finland’s energy innovation and renewable power strengths also have a significant role in reaching the ambitious climate targets.

Vantaa Energy is one of Finland’s largest urban energy companies, aiming for carbon negativity in energy production in 2030. The new seasonal thermal energy storage facility Varaamo is the most significant project aiding emission reduction. Achieving carbon negativity requires capturing carbon dioxide, which is currently being explored bu Vantaa Energy.

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