The Louvre, one of the greatest art museums in the world—home to the Mona Lisa—has an uneasy relationship with its neighbor, the River Seine. In 2016, the Louvre’s curators rushed to relocate priceless artworks after the Seine, which flows through Paris, breached its banks in a flood. That’s far from the only time floodwaters exacerbated by climate change have threatened the museum. It happened again, for example, in 2018.
But the Louvre also depends on the Seine. For the fickle river helps the museum maintain cool temperatures and humidity levels conducive to the long-term preservation of centuries-old art. It’s possible thanks to a sprawling network of pipes, pumps, and cooling machines that connect to the Seine. Hundreds of Parisian buildings rely on this network—but none more than the Louvre.
“For the district cooling, it’s the biggest client that we have,” says Raphaëlle Nayral, secretary general of Fraîcheur de Paris, the company that runs the cooling system. An average-size office building draws about half a megawatt of cooling power, whereas the Louvre soaks up 12 megawatts, Nayral explains.
Paris’ cooling network is one of just a few around the world that make use of bodies of water within, or near to, towns and cities. Water is 800 times denser than air, so using it to soak up heat can be extremely efficient. Plus, unlike traditional air conditioners, water-based cooling avoids dumping heat into the air directly outside a building, which heightens the temperature of built-up areas relative to their surroundings, known as the urban heat island effect.
While river- and lake-fed cooling networks are growing in Paris, Toronto, and the US, for example, there are headaches on the horizon. As the world warms, buildings are demanding more and more cooling—plus, some bodies of water are getting so hot in the summer that their usefulness for cooling purposes is dwindling. District cooling networks may seem an innovative response to climate change but, all the same, climate change could push some of these systems to their limits.
“In the old days, it was more like a luxury project,” says Deo de Klerk, team lead for heating and cooling solutions at the Dutch energy firm Eneco. Today, his company’s clients increasingly ask for district cooling as well as district heating systems. Eneco has 33 heating and cooling projects under construction. In Rotterdam, Netherlands, one of the company’s installations helps to cool buildings, including apartment blocks, police offices, a theater and restaurants, using water from the River Meuse.
It’s not hard to see why cooling technologies are getting more popular. A few years ago, Nayral moved out of Paris. She remembers the heat waves. “My routine during the weekend was to go to the parks,” she says. Nayral would sit there well into the evening—reading Les Misérables, no less—waiting for her apartment to cool down. Recently, she has increasingly found herself spending time in shopping malls, where air-conditioning is plentiful, in order to make it through searing hot French summers. This year, unprecedented heat waves hit France and other countries in Europe.
The city of Paris is now desperate to help its denizens find cool refuges during spells of extreme heat. A key component of Parisian climate adaptation plans is the river-supplied cooling network, the pipes for which currently cover a distance of 100 kilometers, though this is due to expand to 245 km by 2042. While around 800 buildings are served by the network today, those in charge aim to supply 3,000 buildings by that future date.
Systems such as Paris’ do not pump river water around properties. Rather, a loop of pipework brings river water into facilities where it soaks up warmth from a separate, closed loop of water that connects to buildings. That heat transfer is possible thanks to devices called heat exchangers. When cooled water in the separate loop later arrives at buildings, more heat exchangers allow it to cool down fluid in pipes that feed air-conditioning devices in individual rooms. Essentially, heat from, say, a packed conference room or tourist-filled art gallery is gradually transferred—pipe by pipe—to a river or lake.
The efficiency of Paris’ system varies throughout the year, but even at the height of summer, when the Seine is warm, the coefficient of performance (COP)—how many kilowatt-hours of cooling energy you get for every kilowatt-hour of electricity consumed by the system—does not dip much below 4. In the winter, when offices, museums, and hospitals still require some air-conditioning, the COP can be as high as 15, much higher than conventional air-conditioning systems. “It is absolutely magnificent,” boasts Nayral.
But those summer temperatures are increasingly a concern. This summer, the Seine briefly exceeded 27 degrees Celsius (81 degrees Fahrenheit), says Nayral. How can that cool anything? The answer is chiller devices, which help to provide additional cooling for the water that circulates around buildings. Instead of blowing out hot air, those devices can expel their heat into the Seine via the river loop. The opportunity to keep doing this is narrowing, though—because Fraîcheur de Paris is not allowed to return water to the Seine at temperatures above 30 degrees Celsius, for environmental reasons. At present, that means the river can accommodate only a few additional degrees of heat on the hottest days. Future, stronger heat waves could evaporate more of that overhead.
“We will have to make a decision. Can we run the chillers at a higher temperature range, so we can still have a heat exchange with the River Seine—or do we want to develop other facilities?” says Nayral. Those other facilities could include underground reservoirs of cool water. My suggestion of deepening or otherwise altering the Seine to increase the volume of water in it, potentially helping it to maintain lower temperatures, is politely rebuffed, however. “The banks of the river Seine are protected by UNESCO,” says Nayral.
I ask the question, however, because there are some district cooling networks that rely on much deeper water—which is consistently cool all year round. One example is the lake-source cooling system used by Cornell University in upstate New York. “People are so proud of this project,” says Fengqi You, professor in energy systems engineering at the university. Cornell’s network pumps water from the gigantic Lake Cayuga nearby, accessing it at a depth of 76 meters. The water there is as cold as a fridge, barely reaching 4 degrees Celsius at any time of year. So effective is this system that it can provide about 98 percent of the annual cooling load required by 113 buildings on Cornell’s campus.
This network does use some mechanical refrigeration, but it has an overall COP of around 20, says Cole Tucker, interim associate vice president for energy and sustainability at Cornell: “So, I mean, it’s pretty incredible.” However, even Tucker acknowledges that rising temperatures mean the demand for cooling is rising, and this will increase the university’s reliance on mechanical chillers.
Separately, the Canadian energy firm Enwave has recently boosted the capacity of a district cooling network in the city of Toronto. Like Cornell’s, this network also relies on cold lake water harvested at a considerable depth: 83 meters. “There really aren’t many systems like this globally,” says Carson Gemmill, vice president of solutions and innovation at Enwave. The water removed from the lake actually goes on to become drinking water at a treatment plant in this case. After being treated, the water picks up heat from city buildings when it intersects with heat exchangers and the urban cooling loop. “We just warm up that drinking water a little bit,” says Gemmill, noting that only heat is transferred—there’s no risk of contaminating the drinking water somehow.
Toronto’s network serves roughly 100 buildings at present, including hospitals, city buildings, commercial offices, and residential properties. Beefy tunnel-boring machines working beneath the city have allowed the system to expand over time. Gemmill explains that new pipes carrying cool water were placed within a concrete floor at the base of these specially constructed tunnels, which are around 3 to 4 meters in diameter.
Using large bodies of water as giant heat sinks makes sense, given how much heat they can absorb, says Martin Hendel, a researcher at ESIEE Paris, an engineering school within Gustave Eiffel University. However, district cooling networks including Paris’ can be expensive to install and maintain—meaning they work best in densely populated areas such as cities where large numbers of people will likely benefit from them. “These things are usually built for areas that have a lot of office space,” adds Hendel. “Office space tends to be cooled all year round.” That’s partly in response to the heat generated by servers and computer equipment.
Nayral acknowledges that expanding Paris’ cooling network is tricky. France’s capital is drenched in history—so digging in new infrastructure is often slowed by the requirement to allow archaeologists time to excavate any potentially important artifacts. This was also the case for London’s newest subterranean railway, the Elizabeth Line, originally known as Crossrail.
But Nayral points out that, in some areas, Paris’ network makes use of the large old tunnels of the city’s sewer system. There’s enough room to sling the cooling network’s pipes into these, she explains.
If the network is to expand as intended and more than double in just 17 years, then the engineers working on it will need to use every avenue, or tunnel, available to them. The need is serious—extreme heat is killing people in Paris and other cities. “In past centuries, access to heat was really a social gap between rich and poor,” says Nayral, noting that, today, it is perhaps access to cool spaces that increasingly divides us. But that’s where municipalities can step in. “This is the job of cities,” says Nayral, “to provide shelter.”
