News / 8.1.2018

Finland’s largest rock cavern heat storage planned for Helsinki

Helen is planning to build a new energy storage facility in disused underground oil caverns located deep in the bedrock of Helsinki. The facility will be the largest of its kind in the country. The storage facility would accommodate over 40 times as much hot water as the amount of water in the pools at the Helsinki Swimming Stadium.

Three large oil caverns used for the storage of heavy fuel oil are located underground in Mustikkamaa. The oil storage facilities were decommissioned and vacated in 1999. Two of the rock caverns are identical and they can be converted for heat storage use. The volume of the storage facility would be about 260,000 m3.

Energy use has peaks in consumption at daily and annual levels. The energy system needs an increasing amount of flexibility due to renewable energy sources and efficient recycling of excess heat. Helen already has heat storage facilities at the Vuosaari and Salmisaari power plants. The planned rock cavern heat storage facility is ten times as large as the Vuosaari facility and it would raise the optimisation of Helen's energy production to a new level.

The district heating network and the existing energy system in Helsinki provide a good basis for new energy solutions, such as storage and flexibility. There is always call for storage, regardless of the methods of energy production.

Energy storage is a solution independent of the production technology and fuel, and it fits neatly into our current city energy system. Storage contributes to reducing the use of fossil fuels, increasing renewable energy and efficient recycling of excess heat, says Director Heikki Hapuli of Helen.

More efficient optimisation of production and recycling of energy

The logic of using the heat storage facilities currently in use in Helsinki is based on the balancing of daily consumption peaks. Due to the large energy capacity of the rock cavern heat storage, production can be optimised for a longer period, at a weekly level.

During the heat load of the coldest days of the winter, the start-up of separate natural gas and oil-fired heating plants can be avoided by using the storage facility. The charging and discharging capacity of the heat storage is 120 MW, i.e. the operating time is four days with full charging/discharging.

The designed storage project will be implemented if its technical and financial conditions are met.

Helen strives for climate-neutral energy production. The enormous and global climate challenge won't be resolved with any single technology or action, but climate change mitigation demands many strategies and everyone's commitment. Energy storage plays a significant role in increasing flexibility in the energy system.


  • Disused oil caverns are designed to be used as an energy storage facility.
  • Hot water is used for the storage of energy.
  • The effective volume of the rock cavern storage facility is about 260,000 m3.
  • The storage facility is ten times the size of the heat storage facility located at Helen's Vuosaari power plant or the cooling storage facility located under the Esplanade Park, and it can accommodate over 40 times the volume of water in the pools of the Helsinki Swimming Stadium.
  • The amount of energy stored is 11.6 GWh.
  • The charging/discharging capacity of the storage facility is 120 MW (sufficient for about four days).
  • The use of the rock caverns as a heat storage facility will have no impact on other activities in Mustikkamaa.

Infographic of the rock cavern heat storage

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