Microsoft heats Finnish homes with heat from its data centers.

In Mäntsälä, Finland, a data center heats two-thirds of the population. The town, just north of Helsinki, was the first to implement on a large scale the idea of local engineer Ari Kurvi, who is obsessed with recovering waste heat from data centers. Since it began operating a decade ago, the Nebius Group data center has dramatically reduced energy costs for residents. Now his idea is spreading, and some of the world's largest technology companies are focusing on recovering heat from data centers in an effort to become more sustainable. The largest project in the world is the one launched by Microsoft just outside Helsinki: a cluster of data centers that, once completed, will provide heating for 100,000 homes, 40% of the needs of Espoo, Finland's second-largest city. The home of Nokia aims to be carbon neutral by 2030 and has already shut down a coal-fired power plant because it is no longer needed.

With a harsh climate and an electricity grid dominated by renewable sources—hydroelectric and wind—the Nordic countries have become a magnet for technology companies. Here, data centers can often be cooled simply with outside air, reducing energy consumption. Another advantage is that electricity prices in the region are low, but the most attractive aspect is the ubiquity of district heating systems. These networks are extremely energy efficient, especially when powered by infrastructure that generates a lot of waste heat, such as a subway or a large data center.

At the Espoo facility, which Fortum is building on the Microsoft campus and is slated to be operational by the end of the year, heat from the data centers will produce warm water between 25 and 35 degrees Celsius, which will be fed into a recovery plant equipped with around forty water-to-water heat pumps. The pumps extract the heat, sending water at 86°C to two large electric boilers. These heat it to 115°C, a temperature sufficient to heat homes. The possibility of leveraging district heating (operational since 1954) was the reason that prompted Microsoft to build its first Finnish center here. "These data centers are actually large ventilators, which allow us to convert electricity into heat at low cost, which is not trivial: it is easier to produce zero-emission electricity than zero-emission heat," comments Kai Mykkanen, mayor of Espoo and former Finnish Minister for Climate.

Other major waste heat recovery projects are underway in Sweden and Ireland. South of Stockholm, for example, Conapto opened its fourth and largest data center last year: the 20-megawatt facility feeds excess thermal energy into the city's district heating system, which distributes it to 10,000 homes. Despite the technology's potential, however, there are limitations to its implementation, chief among them the geographic discrepancy between data centers and district heating systems, which are concentrated in urban centers, where land prices are higher. In Norway, the installation of data centers immersed in forests—like a gigantic Google project south of Oslo—faces strong opposition precisely because of the lack of nearby consumers.

The heat available from data centers in Europe is expected to reach at least 200 terawatt-hours per year by 2050 (and perhaps even sooner), four times the current level, according to Brian Vad Mathiesen, professor of energy planning at Aalborg University in Denmark: "Obviously, not all of this heat can be reused, but parameters should be imposed on data centers from their construction onwards." In Germany, a new energy efficiency law will impact the evolution of heat recovery technologies: the regulation requires larger data centers to use 10% of waste heat from 2026, and 20% from 2028. The fact remains that heat recovery does not make data centers climate-friendly, but merely less harmful.