Varanto – the world’s largest cavern thermal energy storage

When Varanto is completed in 2028, it will be the world’s largest seasonal thermal energy storage facility in terms of size (1,1 million cubic meters) and capacity (90 GWh).

Vantaa Energy is building a seasonal thermal energy storage facility in Vantaa, Finland. Seasonal thermal energy storage is called Varanto. When completed in 2028, it will be the largest in the world by all standards (1,1 million cubic meters and 90 GWh). The operating principle of the seasonal thermal energy storage is to store heat in underground caverns so that it can be used to heat buildings via the district heating network whenever it is needed.

Giant underground facility enables unprecedented energy storage

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 1.2 million electric car batteries.

Towards a hybrid district heating system

Varanto will help Vantaa Energy to do more from less, as we can store previously wasted heat for use at a later time. Varanto is a giant step towards the hybrid heating system to be built in Vantaa. 

In such a system, the smartest possible generation method in Vantaa can be chosen for each moment and the different energy sources can be taken full advantage of. At its best, the heat-producing system will work like a hybrid car, sometimes running on electricity, sometimes on other production forms. District heating network acts as the power transmission and is steered by intelligent control. This way, Vantaa Energy can secure an affordable and stable price for our customers while eliminating emissions.

Estimate of costs and schedule

The project, valued at around 200 million euros, is financed by Vantaa Energy, but has already been awarded a 19-million-euro investment grant from Finland’s Ministry of Economic Affairs and Employment. 

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.