News / 27.7.2018

Cooling demand in Helsinki at a record level, underground reservoirs are utilised

Helen's district cooling production is at a record high as its customers' cooling demand has increased more than threefold from the June level. Cooling is produced at full capacity in the Katri Vala and the Esplanade heating and cooling plants and in the Salmisaari power plant. A sufficient cooling output for the customers is safeguarded with the huge underground reservoirs under the Esplanade Park and in Pasila.

On the hot July days, the cooling demand and Helen's production have risen to a record high, up to three times the normal levels. The average cooling output in June was 30–40 megawatts per day, but in the past few days it has risen to as high as 114 megawatts. This output would provide enough cooling for about 75,000 one-bedroom apartments. The difference is considerable compared with summers in the past few years. For example, the cooling need has doubled from the levels in July last year.

Helen is now producing district cooling in Helsinki at full capacity in the Katri Vala underground heating and cooling plant in Sörnäinen. Katri Vala is the world's largest heating and cooling plant producing heat and cooling in the same process. Cooling is also produced in the cooling plant at the Salmisaari power plant and in Helen's new heating and cooling plant under the Esplanade Park, which is currently undergoing commissioning tests.

Huge underground reservoirs are used in cooling

Underground reservoirs, i.e. cooling storage facilities, are utilised in the cooling of customers in Helsinki. Finland's largest district cooling reserve is located under the Esplanade Park at a depth of about 100 metres. The cooling reserve holds 25 million litres of water, which corresponds to the volume of an average-sized lake. The capacity of the Pasila cooling accumulator is about 11 million litres.

High daily variations: cooling during the day, charging the accumulators at night

The cooling need varies significantly according to the time of day and the temperature, and cooling production is adjusted accordingly.

"During a hot spell, we produce cooling during the night with an output of some 40 megawatts. The demand for cooling starts to rise in the early morning, and at 7am it is already about 70 megawatts. The 100-megawatt mark is exceeded already by ten in the morning," says Team Leader Mirka Mäkelä who is responsible for Helen's energy system development.

Cooling demand is at its highest in the afternoon, by which time it has already risen to a record 114 megawatts. The lower cooling demand during the early hours of the night enables utilisation of cooling reserves.
"The cooling reserves are charged at night to make sure that there is enough cooling for everyone when the cooling demand rises over the course of the day," Mäkelä explains.


  • The district cooling system in Helsinki is the third largest and the most rapidly growing cooling system in Europe.
  • Large commercial premises, data centres, shops and thousands of apartments are connected to district cooling.
  • Helen produces district cooling in the Katri Vala heating and cooling plant in Sörnäinen and in the Salmisaari cooling plant.
  • Helen is currently commissioning a new heat pump plant under the Esplanade Park. The plant will improve the output of cooling and renewable district heat production and increase the efficiency of the underground cooling reserve. At the same time, carbon dioxide emissions will fall by more than 20,000 tonnes.
  • Helen has two huge underground cooling water reserves, one under the Esplanade Park and one in Pasila.
  • The cooling reserves are charged at night and discharged during the day when the customers' cooling need is at its highest.
  • District cooling is the best cooling method in terms of the environment. In district cooling, surplus heat is recovered and utilised in district heat production.
  • The amount of heat collected from the excess heat of properties and utilised in district cooling would require solar collectors covering the size of 30 football pitches.
  • Helen's district cooling system is internationally awarded (e.g. Global District Energy Climate Awards).

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