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Finland to Host 90 GWh Thermal Energy Storage System


Vantaa Energy plans to build a 90 GWh thermal energy storage facility in Vantaa underground caverns, near Helsinki. It says it will be the world's largest seasonal energy storage facility by all standards when completed in 2028.


Vantaa Energy, an urban energy company jointly owned by the cities of Vantaa and Helsinki, plans to build the world's largest seasonal heat storage system. At more than one million cubic meters, the underground heat storage system will have a total capacity corresponding to the annual heating demand of a medium-sized Finnish city.


The 90 GWh seasonal thermal energy storage will be built in Vantaa near Helsinki. A total of three caverns measuring 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 principle of operation of the seasonal thermal energy store “Varanto” is to store heat in underground caverns so that it can be used to heat buildings through the district heating network whenever necessary.


The underground caverns, with a total volume of 1,100,000 cubic meters, including process facilities, will be filled with hot water. Pressure will be created within the space, allowing water to reach temperatures of up to 140ºC without boiling or evaporating.

The development phase of the Varanto project began in the summer of 2021. At the time, Vantaa Energy said it was working with Finnish construction company YIT and engineering consultancy AFRY. Initially, the construction phase was to begin in autumn 2022 and the planned completion date was 2026.


Now, with all permits in place, the project is expected to begin construction in the summer of this year and be completed in 2028. The cost of the project is estimated to be around €200 million, and it has already received a subsidy to the investment of 19 million euros from the Finnish Ministry of Economy and Employment.


“The world is experiencing a huge energy transition. Wind and solar energy have become vital technologies in the transition from fossil fuels to clean energy. The biggest challenge of the energy transition so far has been the inability to store these intermittent forms of energy for later use,” said Jukka Toivonen, CEO of Vantaa Energy.

“Unfortunately, small-scale storage solutions, such as batteries or accumulators, are not enough; Storage solutions are needed on a large industrial scale. Varanto is an excellent example of this, and we are delighted to serve as an example to the rest of the world.”


Two 60 MW electric boilers will be built in collaboration with Varanto. These boilers will be used to produce heat from renewable electricity when electricity is abundant and cheap.


“Thanks to Varanto's intelligent control of electricity generation, waste heat and district heating, Vantaa will have a hybrid system that will allow us to make the most of different energy sources,” said Toivonen. “Our heat production system will work like a hybrid car: alternating between electricity and other forms of production, depending on what is most advantageous and efficient at all times.”


The developer claims that the use and distribution of stored heat is in line with the underground district heating networks in Finland and other Nordic countries, to which most homes are automatically connected.


District heating is by far the most popular form of heating buildings and homes in Finland. In Vantaa there are more than 600 km of underground district heating networks. About 90% of Vantaa's residents live in houses with district heating.

Relative to population, Finland is the largest producer of district heating in the Nordic countries. In 2023, a total of 37.3 TWh of district heating was produced in Finland. Of this amount, 53% was produced from renewable heat sources and 14% from waste heat.

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