News / 7.12.2020

Climate deed under our feet: filling of Mustikkamaa heat caverns is started

Helen has started to fill Finland’s largest heat storage facility with water. The heat caverns located underground in Mustikkamaa are a unique case even on a global scale and an important step on the carbon neutral path of Helsinki.

In Mustikkamaa, at a depth of 80 metres, there are two decommissioned oil caverns, which Helen is now filling with water. In future, the caverns will serve the customers of the district heating network by balancing consumption peaks throughout the year.

“This is an extremely smart way to convert energy infrastructure of the past for the needs of a carbon-neutral future,” smiles Unit Head Juhani Aaltonen who is responsible for the project.

For example, waste heat from waste waters and properties can be converted into district heat, stored in the water of the heat caverns and released for use as and when necessary. Heat will not dissipate on its own because the bedrock that is tens of metres deep acts as an excellent insulator.

The filling of the caverns will take more than three months due to their enormous total volume of 320 million litres. The caverns have such a large capacity that filling them with an ordinary kitchen tap would take more than 50 years. The heat contained in the water corresponds to the heating of 25,000 one-bedroom apartments all year round.

“A unique climate deed,” sums up Project Manager Päivi Saajoranta, continuing excitedly: “The heat caverns have attracted attention in all quarters, even overseas, because the underground implementation requires specific skills and expertise. There is nothing similar in use outside Finland.”

Not only raw materials, but also energy will be recycled in the future

The Mustikkamaa heat caverns are building a sustainable future where not only raw materials but also energy, which is produced in a number of different ways, are recycled. Helen aims to be carbon neutral in 2035, and the underground heat caverns are one important step on this path.

“A carbon-neutral future is implemented with a combination of many green technologies. Our heat caverns support all energy forms,” Aaltonen sums up.

Helen has numerous other projects that promote energy recycling, such as the large heating and cooling plants located underground in Sörnäinen and Esplanadi, as well as the plant under construction in Vuosaari, which will utilise the heat of sea water.

The Mustikkamaa heat caverns will be inaugurated next summer.


  • District heat is stored in two rock caverns in Mustikkamaa. The temperature of the water in the caverns varies between 50 and 90 °C.
  • The effective volume of the cavern storage facility is 260,000 cubic metres and energy capacity about 11,500 MWh. The charging and discharging capacity is 120 MW.
  • The discharging or charging of the hear caverns at full power takes four days.
  • The rock caverns will decrease Helen’s carbon dioxide emissions by 21,000 tonnes per year. The stored heat can be used for balancing demand peaks and that way cutting fossil heat production, for example, in cold winter days.
  • The production use of the Mustikkamaa heat caverns will start in summer 2021.
  • Helen is also planning a heat cavern in Kruunuvuorenranta. This seasonal energy storage facility would operate according to a different principle from the Mustikkamaa heat storage facility.

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