News / 11.5.2021

Gigantic heat caverns in Mustikkamaa have now been filled with water

The filling of underground heat caverns in Mustikkamaa with water has been completed this week, and the use of the heat caverns for heating homes in Helsinki is a step closer.

Helen started to fill the heat caverns with tap water in early December. The filling of the caverns took more than three months due to their enormous total volume of 320 million litres. The caverns have such a large capacity that filling them from an ordinary kitchen tap would have taken more than 50 years.

The current estimated temperature of the water storage facility is about +30 degrees. Slow heating of the bedrock has started, and actual heating of water in the heat cavern will start in April. The aim is to connect the heat caverns into the current district heating system of Helsinki in July.

The heat caverns balance the consumption peaks in the district heating network throughout the year. For example, waste heat from waste waters and properties can be stored in the heat cavern and released for use as and when required. In future, the temperature of the water in the heat cavern will vary between +45 and +100 degrees according to usage situation.

- Helen is proceeding fast towards a carbon-neutral future, and the underground heat caverns are an important step on this path. The use of old oil caverns as an energy storage facility is a good example of Finnish innovation that is unique even on a global scale. The emission-free energy system of the future is made up of many pieces, and storage is an important element in the system. The heat caverns support all heat production forms, says Helen’s Director Timo Aaltonen.

Carbon-neutral production is also currently built at Helen’s heating and cooling plant in Sörnäinen with the arrival of two new heat pumps. A seawater heat pump and a bioenergy heating plant are being built in Vuosaari. The construction of the geothermal heating plant in Ruskeasuo will start this spring. Helen is also investigating, for example, waste heat from the Kilpilahti industrial area and extensive utilisation of seawater heat pumps.


  • The heat contained in the water in the Mustikkamaa cavern heat storage facility corresponds to the heating of 25,000 one-bedroom apartments all year round.
  • Heat will not escape from the heat caverns because the bedrock that is tens of metres deep acts as an excellent insulator.
  • The rock caverns will decrease Helen’s carbon dioxide emissions by 21,000 tonnes per year.

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Carbon neutrality Heating