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A vast cold-water reservoir hidden beneath Helsinki’s Esplanade Park is helping the city combat rising summer temperatures.
The underground site, managed by energy provider Helen, holds around 25 million litres of chilled water and supplies cooling to hundreds of buildings. Often called a “lake” due to its size, the tank forms part of Finland’s growing district cooling network.
A long service tunnel begins in the Punavuori district and stretches beneath the Kaartinkaupunki area, leading to the cold storage site. The space houses one of Helen’s heat pump plants, which also generates district heating.
“The system allows us to store and reuse energy efficiently,” said Timo Marttinen, production manager at Helen. “Cooling, heating, and electricity are all part of the same cycle, nothing is wasted.”
The concrete chamber, 80 metres long, 40 metres deep, and 8 metres wide, is filled with tap water drawn from Lake Päijänne. Water at the surface is around 12°C, while deeper layers remain close to 4°C.
When demand for cooling is low, surplus energy chills the reservoir water. On warmer days, the stored cold water is released to lower indoor temperatures.
Though Helsinki has a dense network of underground spaces, the cavern beneath Esplanade Park provided a rare combination of suitable geology and central location.
District cooling has expanded rapidly. Ten years ago, Helen’s network covered far fewer buildings. By the end of 2024, some 550 properties — including offices, commercial buildings, and 120 residential blocks, were connected.
“Interest among housing associations has grown,” said Henni Vornanen, team leader at Helen. “In new buildings, the systems can be added during construction. In older blocks, they’re often installed during major plumbing renovations.”
Helen’s cooling network is most extensive in central Helsinki, though growth has begun in other cities. Seinäjoki, for example, will open its first district cooling pipe this year.
Finland’s entire cooling network spanned just over 200 kilometres at the start of the year, according to Energy Industry Finland, while district heating systems cover around 15,000 kilometres.
Marttinen said that demand for cooling is not limited to summer.
“Server rooms, supermarkets and other commercial properties need cooling 24 hours a day, all year round,” he explained. “Previously, we generated cooling as needed. Now, these large water tanks give us flexibility in how and when we produce it.”
A second underground cooling reservoir already operates in Helsinki’s Pasila district. It is smaller, with a capacity of 11 million litres.
Another plant, located beneath Katri Vala Park in Sörnäinen, produces district cooling using purified wastewater. A new facility now under construction in Eiranranta will use low-temperature wastewater, reused after first passing through the Katri Vala plant.
Helen believes the demand for cooling will continue to grow.
“Longer heatwaves and increasing renovation and construction projects will support further expansion,” Vornanen said.
HT
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Source: www.helsinkitimes.fi
