The Värtaverket plant in northern Stockholm burns wood chips at temperatures hot enough to keep a quarter of a million homes warm through a Swedish winter. It is the largest biomass-fired combined heat and power plant in the European Union, and it is, by quiet design, the second-largest piece of climate infrastructure on the Stockholm balance sheet. The largest is the network around it.
Stockholm Exergi, the joint venture between the City of Stockholm and the Finnish utility Fortum that runs the network, operates the most extensive district-heating system in Sweden. The pipes connect approximately ninety per cent of the city’s buildings, distributing heat from Värtaverket and a constellation of smaller plants across more than three thousand kilometres of insulated pipe. The system has been gradually rebuilt over forty years from a fossil-fuel base into something more peculiar: a heat market with multiple suppliers, including the city’s data centres.
How a heating grid became a marketplace
The Open District Heating programme that Stockholm Exergi launched in 2014 turned the network on its head. Rather than treat the grid as a one-directional flow from generation to consumer, the company started buying excess heat from anyone who could deliver it at the right temperature. Data centres, supermarkets, ICA refrigeration sites and a handful of industrial facilities now sell their waste warmth to the grid at market-set prices. By the 2024 reporting cycle, third-party heat injection accounted for roughly fifteen per cent of the network’s annual delivered energy, a figure projected to cross twenty per cent by the late 2020s.
The data-centre integration is the part of the model that other European cities study most. Stockholm Exergi has standardised the connection: a heat exchanger, a metering arrangement, a contract with a tariff that reflects the marginal cost of the grid’s otherwise-fossil heat sources. The bigger Stockholm data centres recover heat at temperatures around forty degrees Celsius and feed it into the return loop. Stockholm Exergi raises it to delivery temperature using heat pumps. The deal works because the city’s electricity is overwhelmingly low-carbon hydro and nuclear; the heat-pump uplift does not undo the carbon savings.
The carbon-capture pivot
The most ambitious move of the past three years has been the Bio-Carbon Capture and Storage project at Värtaverket. Announced in 2022 and shortlisted for an EU Innovation Fund grant of approximately one hundred and eighty million euros, the plan is to capture roughly eight hundred thousand tonnes of biogenic carbon dioxide per year by the late 2020s and ship it to undersea storage in the Norwegian sector of the North Sea. The arithmetic is unusual: because the feedstock is biomass, capturing the resulting carbon makes the plant net-negative across its lifecycle, on the assumption that the wood is harvested sustainably.
The economics depend on policy. The EU Emissions Trading System does not yet credit biogenic carbon removals at full value, and Sweden’s national framework for negative emissions is still being negotiated in parliament. Stockholm Exergi’s CEO Anders Egelrud has been candid in interviews that the project is being built ahead of the regulatory framework, in the hope that the framework arrives before the construction does.
What the model can and cannot export
Helsinki and Copenhagen run district-heating systems of comparable density; Vienna’s is more state-controlled but smaller per capita; Munich and Hamburg have networks that are still partially fossil. The Stockholm model travels well in two parts: the open marketplace for waste heat, and the standardised data-centre integration. The Swedish electricity context, however, does not export. A city with a coal-heavy or gas-heavy grid cannot rely on heat-pump uplift to clean its district-heating output. The Stockholm playbook works because Sweden built its low-carbon electricity grid in the 1970s and 1980s and has had forty years to forget how unusual that was.
What Stockholm Exergi will tell you, off the record, is that the harder problem is none of the technical ones. It is contracting. Persuading three dozen data-centre operators, six supermarket chains, and a handful of industrial sites to sign tariff agreements with a city utility takes longer than building the heat exchangers. The technical work was done by 2017. The marketplace is still being built.
Sources: Stockholm Exergi annual reports 2017-2024, City of Stockholm climate strategy documentation, EU Innovation Fund 2023 award announcements, Open District Heating programme technical specifications.