Pipes carrying cold water in a district cooling plant

Plamen Galabov/Alamy

More than 77 million people in Europe enjoy district heating, where hot water is piped from a central plant to homes and buildings. The US has hundreds of these networks, mostly in multi-building campuses like universities, airports and business districts.

But as both Europe and North America swelter through yet another heatwave this week, many are wondering whether the mirror image of that technology, pipe networks of chilled water, could be a better cooling option than air conditioning.

“District cooling systems operate at twice the efficiency, or half the operating expense, of a typical in-building chiller plant,” says Rob Thornton at the International District Energy Association. “There’s emissions reductions, lower costs of manpower and… by concentrating the function to a central plant, you can serve dozens if not hundreds of buildings.”

Heatwaves are getting stronger, longer and more severe around the world, driving a boom in air conditioner sales. But in Europe, the fastest-warming continent, only about 1-in-5 households have air conditioning, and the lack of cool spaces can be deadly in an ageing population. The record heatwave in June, one of four major heatwaves there this summer, is estimated to have killed more than 20,000 people.

Air conditioning, while necessary for hospitals, schools and care homes, worsens planet-warming carbon emissions and urban heat islands. It isn’t a long-term solution, according to a 2025 report by the European Union Covenant of Mayors, an alliance of local governments. District cooling, on the other hand, can both cool cities and cut emissions, the group said.

Middle Eastern and Asian cities that face scorching heat for months of the year have already begun to build gigantic district cooling networks. Dubai’s state-backed district cooling company now pumps around 6 gigawatts of refrigeration — equivalent to freezing 1.5 million tonnes of ice per day — to more than 1700 buildings.

The US has about 500 of these district cooling networks, especially on university and hospital campuses, according to Thornton, and Europe has about 200, mostly in cities in France and Sweden.

The first such cooling systems were installed in Denver, Colorado, in 1889 and initially piped chilled ammonia or brine solutions. Such systems switched to water in the 1960s. They typically cool water to 6°C or 7°C in refrigeration tanks and pipe it to heat exchangers in each building in the network. These work like radiators in reverse, cooling the air in a home through contact with the cold water.

A simulated district cooling network of 37 public and commercial buildings consumed 15 per cent less energy and lowered operational costs by 10 per cent compared with individual air conditioning, according to a study in Hong Kong. The Paris district cooling system has cut electricity and emissions by half compared to individual cooling.

Office buildings often install air conditioning systems that are two to three times larger than is typically needed so that the AC doesn’t go out on the hottest day of the year, says Thornton. District cooling can shrink that huge pool of excess capacity.

To increase efficiency even further, district cooling systems in Geneva and Paris chill their liquid with water from Lake Geneva and the Seine, respectively, while one in Heerlen, the Netherlands, draws on flooded coal mines. In the US, some district cooling networks make ice at night, when electricity is cheap, and pipe it out once it melts during the day.

“There is… a functional advantage that district cooling brings to buildings that doing it yourself doesn’t bring,” says Thornton.

While most district cooling networks still serve public or commercial buildings, a few, like the one in Geneva, are starting to expand to residential buildings. But district cooling can cost hundreds of millions of dollars to build, mainly because streets and pavements need to be dug up to lay pipes. It is cheapest if it can be installed alongside an existing district heating system, according to Wangda Zuo at the Pennsylvania State University.

“It works for places with really dense buildings that have high demands, and ideally if you already have the tunnels to lay out the piping,” he says.

But a newer technology called an ambient loop system, which combines district heating and cooling in one, could enjoy more uptake in Europe. Upscale projects like Bankside Yards in London, which includes both commercial and residential buildings, have installed this technology, but for wider adoption, government backing will probably be needed, says Zuo.

The approach is a way to supercharge individual electric heat pumps, which typically cool or heat air they take from outside the home. An ambient loop system circulates water at whatever temperature the pipes are currently at, usually 10°C to 25°C (50°F to 77°F), although it must occasionally heat it in winter.

Then water-based heat pumps in each building cool or heat this liquid. So, rather than cooling outside air from 40°C to 20°C (104°F to 68°F) during a heatwave, for instance, the heat pump can convert water from 25°C to 20°C, which requires far less electricity.

“If you want to carry something from the foothill of a mountain to the top of a mountain, you have to work very hard,” says Zuo. “If the difference in height is low, it’s easier. With the ambient loop, the temperature difference is relatively smaller.”