In Pornainen, Finland, 2,000 tons of crushed soapstone began heating a town this year, so wood chips and oil dropped out of its district network almost completely
A gray steel tower rises almost forty feet above a quiet municipal yard outside Helsinki.
Inside it sits nothing more exotic than crushed rock.
Two thousand tons of it, packed tight and glowing hot in the dark.
The structure sits in Pornainen, a small municipality in southern Finland, built for the local utility Loviisan Lämpö to replace an aging wood chip plant.
For decades that rock would have ended up as construction waste.
Instead it is now the main heat source for an entire town.
What makes a pile of stone capable of running a district heating network through a Nordic winter, with almost no fuel burned at all, is the question worth answering first.
How does crushed rock actually hold a town’s heat?
The mechanism is almost embarrassingly simple. Electricity, often bought when it is cheap or abundant on the grid, runs through heating elements buried in the rock bed. That heat does not need to go anywhere right away, because the crushed soapstone packed inside the steel shell holds its temperature for weeks with only modest losses, which is the whole trick behind the system.
So when the town needs warmth, a network of pipes draws hot air back out of the rock and transfers it into the water that circulates through homes and public buildings. There is no combustion at any point in that second step, which is what separates this from a wood chip boiler or an oil furnace doing the same job, and it is also why the maintenance burden stays so low.
The soapstone itself is not a new material chosen for drama. It is a byproduct of fireplace manufacturing, the kind of leftover rock that normally gets landfilled once a stove maker finishes cutting the stone it actually wants.
What does the installation actually look like on the ground?
The structure sits in Pornainen, a small municipality in southern Finland, built for the local utility Loviisan Lämpö to replace an aging wood chip plant. The tower itself is close to forty three feet tall and roughly fifty feet across, a scale that required dozens of subcontractors and more than a hundred workers on site during construction, since pouring foundations for a vessel that size in Finnish soil is its own slow engineering job.
The unit delivers 1 MW of thermal power and holds 100 MWh of capacity. The developer, Polar Night Energy, says that is ten times larger than the company’s first commercial unit built in Kankaanpää, and that gap in scale is itself part of the story.
In practical terms, that capacity means the storage tower alone can cover close to a month of Pornainen’s summer heat demand. Even in deep winter, it carries the network for days before needing a recharge.
What happened once the system actually switched on?
The commissioning happened in June, with the facility fully inaugurated later that summer, and it has been running as Loviisan Lämpö’s primary heat source ever since. That is a meaningful shift from a pilot project to the backbone of a working utility, one that most thermal storage demonstrations never reach.
The measured environmental effect has been just as concrete. According to the developer’s own one year update, the Sand Battery built for Loviisan Lämpö has now been in operation for one year, and with its help, climate emissions from the Pornainen district heating network have been reduced by 70 percent. Oil use in the network has been eliminated outright and wood chip burning has fallen sharply alongside it.
Mikko Paajanen, the chief executive of Loviisan Lämpö, framed the project around the utility’s results. “Our cooperation with Polar Night Energy has been very successful,” he said.
Where does a rock pile like this still run into limits?
None of this means thermal rock storage is about to replace lithium batteries or gas turbines outright. The system produces heat, not electricity, so it solves one specific problem, keeping buildings warm, rather than feeding a grid directly, which is a narrower job than the headlines around it sometimes suggest.
That distinction matters for anyone comparing it to the battery projects covered elsewhere on this desk, including the flywheel storage now being pushed to higher speeds in Shanghai.
Even so, the economics invite scrutiny rather than skepticism. The storage cost works out to roughly five dollars per MWh, a figure that looks remarkably low next to most electrochemical storage, though it reflects one project at one scale rather than a proven industry average. A pilot to convert the stored heat back into electricity is already being discussed, but until that is tested, the honest claim stays narrower.
What does this mean for other cold climate grids watching closely?
Because the hardware is mechanically simple, insulated steel, heating elements, crushed rock, the appeal for other utilities is less about novelty and more about easy replication. A town with a district heating network and a spare patch of land does not need exotic chemistry to try something similar.
That same bet on unglamorous materials doing the heavy lifting echoes other projects this desk has tracked, including solar panels anchored by gravity rather than driven posts at the Ivy Landfill, where engineers chose the simplest fix available over a more complex one.
The operators in Pornainen now say the facility has held full reliability through its first stretch of operation. Whether other cold climate utilities follow is still open, but the tower outside Helsinki has already answered the question that mattered first.
Hugo is an engineer with strong technical expertise. Multilingual from an early age, his writing combines technical clarity with a strong interest in science and energy.
