Construction of Finland’s largest rock cavern heat storage facility starts

In Mustikkamaa there are three underground rock caverns, which were excavated in 1982. They were used for storing heavy fuel oil until they were finally discharged and disused in 1999. Helen is now converting two connected caverns into Finland’s largest heat storage facility.

Procurement is underway, and demolition work in the oil caverns will start in the next few weeks. Construction will start in early 2019, and the heat storage facility will be completed for production use in 2021.

Thanks to the Mustikkamaa rock cavern heat storage facility, it will no longer be necessary to use and produce district heat consumed in Helsinki all at the same time.

“Heat or surplus heat produced with a high efficiency rate will be stored in the facility when the heating need in Helsinki is not at its highest. The heat of the storage facility can be used throughout the year. Especially in the winter, it may not be necessary to start up heating plants operating on oil or gas. The heat storage facility therefore allows reduction in the use of fossil fuels. The use of renewable fuels and CHP electricity will also be increased at the same time,” explains Project Manager Päivi Saajoranta. 

Based on computational comparisons with oil, the heat storage facility will reduce the use of fossil fuels by 1,000,000 litres of oil per year. The rock cavern heat storage facility will decrease Helen’s carbon dioxide emissions by 21,000 tonnes per year.

Heat storage facility will balance variable heat consumption

The heat storage facility will provide flexibility to the energy system as it will balance variable heat consumption through charging and discharging. When discharging the heat of the storage facility, the heat can be utilised as such in the form of district heat.

The effective water volume in the heat storage facility is 260,000 cubic metres and the total volume is 320,000 cubic metres. The charging and discharging capacity is 120 megawatts.

“The water in the heat storage facility is not connected to the water in the district heating network. Heat is transferred from the water in the facility into the water in the district heating network by means of heat exchangers. It takes four days to fill the heat storage facility with heat, and it can be discharged in full in four days,” explains Saajoranta.

The annual amount of stored energy is about 140,000 MWh, corresponding to the heat consumption of some 25,000 one-bedroom apartments.

The heat storage facility is controlled from the control room of the Hanasaari power plant or from the energy control room in Sähkötalo. The underground process areas of the heat storage facility are only visited for maintenance tasks during the outage of the facility.

Value of investment EUR 15 million

The value of the investment is about EUR 15 million, of which EUR 2.1 million will be met from the investment aid for new technology granted by the Ministry of Economic Affairs and Employment. The project employs dozens of people.

The Mustikkamaa rock cavern heat storage facility will hold over 40 times as much water as the pools in the Helsinki Swimming Stadium. The facility will be filled with water from the water supply network, which will take about three months.

The existing tunnel, which runs under the sea and the Sompasaari island from the Hanasaari power plant area to Mustikkamaa, will be utilised when the heat storage facility will be connected to the district heating network.

The heat storage facility will be located completely underground. No structures will be built above the ground level, and the storage of heat will not have an impact on other activities in Mustikkamaa.

Heat storage facility supports climate change mitigation

“Helen strives for climate-neutral energy production. Climate change mitigation requires several different methods, and energy storage contributes to reducing the use of fossil fuels, increasing renewable energy and efficient recycling of waste heat,” says Director Heikki Hapuli.

The Mustikkamaa heat storage facility will improve the energy efficiency of the energy system. It can be used for increasing profitable operating time in combined heat and power generation and improving its possibilities of operating on the market also in the future, safeguarding the security of energy production and supply.

Facts:

• District heat is stored in two combined rock caverns.
• Hot water with a temperature of 45–100°C is used for storing energy.
• The effective water volume in the cavern storage facility is 260,000 m3. It accommodates over 40 times as much water as the pools in the Helsinki Swimming Stadium.
• The amount of stored energy is 11,600 MWh, and the annual amount of stored energy is about 140,000 MWh. This corresponds to the annual heat consumption of some 25,000 one-bedroom apartments.
• The charging and discharging capacity of the facility is 120 MW, which is sufficient for about 4 days.
• The rock cavern heat storage facility will reduce Helen’s carbon dioxide emissions by 21,000 tonnes per year.
• The basic operating principle of the heat storage facility is continuous optimisation of production.
• The Mustikkamaa rock cavern heat storage facility will be completed for production use in summer 2021.
• Helen also has heat storage facilities at the Vuosaari and Salmisaari power plants.
• Helen is also planning a heat storage facility in Kruunuvuorenranta. This seasonal energy storage facility would operate according to a different principle than the Mustikkamaa heat storage facility.