In the face of heatwaves, how the cold water of the Seine refreshes Paris

"But it's like Batman!" exclaims a passerby in the chic and touristy 8th arrondissement of Paris, as a manhole-like plate slowly lifts, revealing a spiral staircase leading down to the depths.

Several dozen meters lower down lies the largest cold network in Europe.

This type of system is old: the UN headquarters in New York has been cooling with the East River since the 1950s. But it requires so much urban planning and construction that it remains rare. France had only about forty of them in 2023.

The process is the same as for a district heating network but acts in reverse, via a heat exchanger system between the pipes of the cold network and those of the building's internal network.

But unlike conventional air conditioning, it does not release hot air into the streets, and offers significant savings in energy consumption (-50%), water consumption (-65%), chemical use (-80%) and CO2 emissions (-50%), says Fraîcheur de Paris, a company co-owned by Engie, which manages around a hundred cooling networks abroad including Barcelona, ​​Singapore and Dubai, and the RATP.

"Maladaptations"

"With thermometers soaring, cities need (...) to replace stand-alone solutions" such as individual air conditioners, which contribute to further heating of the atmosphere and are "maladaptive," explains Raphaëlle Nayral, secretary general of Fraîcheur de Paris.

"In Paris, where heat waves could raise temperatures to 50°C by 2050, if we don't offer alternatives" to air conditioning, "we will make this city completely uninhabitable," she says.

In areas with high levels of air conditioning, studies show that they can increase the heat in urban centers by around 0,5 degrees, with projections of up to +2 to 3°C if the rate of equipment continues to increase. They also represent 7% of global greenhouse gas emissions, according to the UN, in particular because refrigerant gases, which are very warming for the atmosphere, tend to leak.

The 12 cold production plants in Paris send, via 110 km of underground pipes, water cooled to around 4°C to the 867 subscriber sites, including the National Assembly.

In summer, the Seine serves as a natural cooler for the buildings' cooling units. And when its temperature is below 8-10°C, the simple heat exchange with the river allows the distribution network to be directly cooled, which remains useful in winter for server rooms or shopping centers.

The "song of the Seine"

In the 8th arrondissement station, spread over four floors and 4 meters deep, water pumped from the Seine circulates through a network of black tanks and large green pipes with a shrill din.

"It's a bit like the song of the Seine that we hear," notes Ms. Raynal with a smile, amidst the roar of the compressors.

The process is subject to numerous standards: water from the Seine must never come into direct contact with that of the cooling circuits and must not be discharged into the natural environment if there is a difference of more than 5°C with the river, so as not to increase its temperature and damage its ecosystem.

In Paris, this cold network mainly serves commercial buildings (offices, department stores, museums, performance halls, etc.).

But the idea is to extend it to retirement homes, schools and daycare centers.

"We have our first hospital, the Quinze-Vingt hospital, which has just been connected. By 2042, the network should be more than doubled, with 245 km of distribution networks (...) to provide new shells of fresh air when the city is overheating," explains Ms. Nayral.

For individuals, this will be "longer term," due to the multiple renovations needed to connect residential buildings to the network.