Sweden's homes heated by a giant underground thermos
February 27, 2025
In an undisclosed location somewhere below the streets of the southern Swedish city of Vasteras, lies a system of man-made tunnels and caverns. Meticulously carved into the bedrock in the late 1960s and early 70s, their original Cold War purpose was to hide a stash of oil. Today, they're used to store something else: very hot water.
A visit from the Swedish king when they opened for business late last year points to the importance of what is essentially a giant heat battery designed to keep the city warm.
Come the winter, when temperatures can drop as low as -20 degrees Celsius (minus –4 degrees Fahrenheit), warmth is one thing the 160,000-strong city on the Svartan river needs. Like many urban centers in Sweden, Vasteras operates a district heating system. In other words, instead of each home having its own small boiler, several huge ones are housed in the local power plant. This is a co-generation plant that burns household waste and wood to produce electricity — and heat for 98% of the buildings in the city.
"It's a very efficient way to use fuel", says Lisa Granstrom, who is in charge of the heat and electricity strategy at Malarenergi, the city-owned energy company that runs the power station.
What is a co-generation plant?
In a standard power station, the heat needed to make steam to spin a turbine goes to waste. But in a co-generation plant, like the one in Vasteras, that same heat is re-used in the form of hot water. This is fed into a 900-kilometer (about 560 miles) network of pipes connected to homes, so residents can have toasty houses and warm showers. Not just in the winter.
"During the summer, we need to have the power plant running, because everyone wants to have a hot shower — even on a warm summer day," Granstrom says.
The technology is roughly twice as energy efficient as a conventional power plant, with 90% of the energy put in, being used.
In fact, running the plant can produce more energy than the city uses in summer, and some of that heat gets lost. In the winter, however, there are days when it struggles to produce enough heat, meaning additional boilers would need to be fired up. Running on fossil fuels, that implies CO2 emissions. Malarenergi saw the caverns as a way to change things.
A climate-friendly underworld
Back in the Cold War, having declared itself neutral, Sweden had to ensure it had enough fuel to keep its economy running in case a full-scale war broke out. So among other measures, it built the site in Vasteras to stockpile enough oil to supply that particular city for an entire year. In 1985, amid thawing political tensions, the store was decommissioned and the oil was drained, leaving the caverns to idle. Until 2019, when Malarenergi decided to offer them a new lease of life.
"We had to clean them from all the oil residue that was still in there," Granstrom remembers. "After that, we needed to do all the piping inside the caves." Only then could they reseal and fill them with water, a process which took five months since they are big enough to hold as much liquid as 100 Olympic-sized swimming pools.
The whole project cost $15.5 million (€14.7 million). But it's an investment the company hopes to recoup within five to 10 years because it saves on fuel by taking the warm water from the top of the cavern to the heat exchanger, where its heat is passed into the district heating network.
Though it is freezing outside, even the walls of a tunnel above the caverns are warm. The bedrock serves as an insulator that makes sure only very little of the heat stored in the water escapes. "Like coffee in a thermos, this water will also stay hot," Granstrom says.
The excess heat from summer can now be used to heat the water in the caverns and in winter, it can be tapped into to keep the city warm for up to two weeks, depending on the outside temperature.
This not only means that even less energy is lost. The company also estimates it saves about 1,600 tons of CO2 emissions every year, about as much as 460 people in Sweden emit, because the emergency boilers can stay off.
Can other places build cavern heat storage?
"It's great that this large storage is implemented, and we get more experience from it," said Sven Werner, a retired professor at Sweden's Halmstad University, and district heating expert.
"People try to avoid risk," he told DW. "So if somebody has taken the risk and it's been successful, you have followers."
Of course, not every city has an abandoned oil storage site waiting to be reclaimed, but it's also possible to build heat storage caves from scratch.
The Varanto project in the southern Finnish city of Vantaa, which is due to become operational in 2028, aims to excavate a cave system that holds more than three times as much water as the one in Vasteras.
Werner says some Nordic countries — especially Sweden, Finland and Norway — have the right conditions to build lots more cave storage. "We have crystalline bedrock," he said. "And it's ideal to make large rock caverns in this kind of bedrock."
Places with less solid bedrock may have to resort to different underground heat storage solutions, such as drilling wells to heat up natural underground layers of fractured rock, sand or gravel that hold groundwater.
It's a technology in use across many places in the Netherlands, but is also how the parliament building in Germany is heated. Another solution, which is particularly popular in Denmark, is to dig out a huge pit, line it with waterproof material and use it to store hot water.
"District heat is very local," says Granstrom. "You have to have adapt your local solution to your local settings."
Edited by: Tamsin Walker