The Stack Archive Feature

Heating Stockholm with excess data centre energy

Thu 22 Feb 2018 | Erik Rylander

Erik Rylander, head of Stockholm Data Parks at Stockholm Exergi, works on a scheme that utilises one of the most common by-products of data centres around the world – heat. There are very few other examples of this, yet it has the potential to change the way we think about data centres and the cities we live in. Ahead of his speaking session at Data Centre World, Rylander spoke to The Stack about how the scheme works, avoiding wasteful energy use, and how to spread the warmth across Europe.

Stockholm is a cold city. The average temperature in winter months is minus three degrees centigrade. Its citizens stay warm through its district heating system, which consists of 2800 kilometres of underground pipes.

Data centres produce a colossal amount of heat. The power required to run servers which fuel the internet and all its connected devices means that complicated systems of cooling are required, which in turn produces massive amounts of excess energy in the form of heat.

A cold city. Hot data centres. The solution seems clear when looking at the two side by side, but using the waste heat from data centres in provincial heating systems is a rare and innovative form of recycling.

Approximately 1% of Stockholm’s heating need is accounted for by the energy brought in by the scheme… Rylander has plans to bring that to 10% by 2035

Part of the reason it’s been able to happen in Stockholm is thanks to its labyrinth of underground pipes, which heat the city and means there is a ready-made infrastructure available to make the scheme work. However, the extent to which the city values heat is crucial.

According to Rylander, the approach is economically viable in any city where there is a demand for heat at least half the year round. There are major economies of scale at play here – the system is, unfortunately, less viable on a smaller basis – performing heat recovery on a single building for instance, faces the key issue that the heating requirements of the building would (in almost all cases) be significantly less than the levels that a data centre would provide.

As it stands, approximately 1% of Stockholm’s heating need is accounted for by the energy brought in by the scheme, for which the city’s energy utility organisation, Exergi, pays around $200,000 (approx. £142,000) per each megawatt of heat continuously delivered during the 6000 coldest hours of the year. Rylander has ambitious plans to bring that to 10% by 2035.

The practicalities

The scheme works through a partnership between Stockholm Exergi (formerly Fortum Värme a district heating and cooling utility), Ellevio (the power grid utility in Stockholm), Stokab (a dark fibre provider) and the city of Stockholm.

The data centre operators sell this at approximately 68 degrees centigrade, having raised it from its original temperature using a heat pump. This is captured by Stockholm Exergi and transported to the city’s residential areas.

Most data centres use a computer room air handler (CRAH), where a heat exchange is performed, such that the hot air is cooled while on the other side, the cool water is heated. Having captured the heat energy, the customer or Exergi will condition the heat to meet the requirements of the heating network.


The heat pumps that bring it to the required temperature would be the same equipment that produces the chilled water used by the data centre CRAH. Instead of relying on a chilled water CRAH, Exergi also performs the heat transfer in a hot air return plenum in an air-cooled system, which is done by placing coils in the plenum.

Though the process is quite technical, Rylander finds that his main difficulty is not the technical side of the job. His primary challenge, he says, is getting the message out to the market.

The benefits are not just commercial: the best way to fight climate change is to reduce the overall consumption of energy

Education, connection, remuneration

In a city that is exceptionally well connected to Europe, Stockholm could be a perfect location for serving the entirety of northern Europe. It is so well connected, in fact, that it’s possible to reach the markets where 50% of European GDP (around $10 trillion) is produced, in less than 30 milliseconds.

13 of the world’s largest global carriers have a presence in the city, and there are four separate paths to Europe, providing strong resilience. This alone should make Stockholm an extremely attractive market for data centre providers.

An additional incentive is the fact that Exergi actually pays for the excess heat, a fact that Rylander states is frustratingly little-known. Considering that heat is usually completely wasted, the $100 million in savings that a data centre can make over a 10-year period in Stockholm looks like an enticing prospect.

Tackling climate change

The benefits are not just commercial. The best way to fight climate change is to reduce the overall consumption of energy. In the current scenario, the data centre industry, according to some estimates, may consume more than 3000 TWh by 2025, which is on par with the electricity consumption of the U.S. today.

This means that a scheme such as this, which makes good use of what is typically wasted energy, fulfils one of two imperatives to tackle climate change: the use of additional sustainable electricity and the reuse of excess heat.

Rylander has another concern when it comes to the efficient use of energy in data centres. It’s often argued that the guaranteed corollary of the ever-growing Internet of Things is an increase in the number of distributed data centres, at the edge.

Rylander does not believe this is necessarily true, arguing that IoT architecture is still an ‘open question.’ He believes that not all the processing has to be done very close to an application – for instance in an autonomous car, most of the processing could happen within the car itself. Instead, the processing should be done in data centres which can be made as green as possible through the use of things like sustainable electricity and heat recovery.

Whichever way you lean on the question of IoT, it’s clear that better use of heat from data centres can help save money and save the planet. It’s up to the rest of the world to follow suit.


Erik will be speaking at the forthcoming Data Centre World which takes place on 21st and 22nd March 2018 at London’s ExCeL Centre. To hear from Erik and other data centre experts from around the world, register today for your FREE ticket.

Experts featured:

Erik Rylander

Head of Stockholm Data Parks and Heat Recovery
Stockholm Data Parks


business Data Centre feature green IoT
Send us a correction about this article Send us a news tip