The world’s largest sand battery just went live in Finland
August 27, 2025
With its 100-MWh capacity, Pornainen’s sand battery is the largest in the world
Polar Night Energy
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Finland has inaugurated an industrial-scale sand battery this week in the southern town of Pornainen, where it’ll take over heating duties from an old woodchip power plant for the municipality. It’s set to reduce carbon emissions from the local heating network by as much as 70%, and is the largest one of its kind in the world.
Developed by Finnish Firm Polar Night Energy – which also built the world’s first commercial sand battery a few years ago – this battery is about 42 ft (13 m) tall and 50 ft (15 m) wide. It serves as a storage medium for up to 100 MWh, with a round trip efficiency of 90%. That makes it about 10 times larger than the first-ever sand battery, and capable of storing enough heat for the whole town to use for a week.
This Thermal Energy Storage (TES) reservoir is a critical tool for places like Finland, which intermittently generate vast quantities of wind and solar electricity, but also face variations in energy demand and supply. The sand battery charges up when electricity is cheaply available and can hold a charge for months at a time, helping balance the energy grid during periods of high demand. You can see the battery being put together in Pornainen, in the video below.
World’s Largest Sand Battery
Here’s how it works: Excess electricity from renewable sources is used to heat the sand contained in a large insulated silo through a closed-loop air pipe system, to temperatures of up to 1,112 ºF (600 ºC). The sand gets hot and stays hot for a long time, acting as a battery.
Later, cool air is blown through the battery’s pipes, absorbing heat from the sand as it moves through the system. This heated air – which can reach temperatures of 752 ºF (400 ºC) – can then convert water into steam for industrial processes, or it can heat water for district heating using a device called a heat exchanger.
What Is a Sand Battery? Polar Night Energy’s Sand-based Thermal Energy Storage Explained
So in case you were wondering: no, this battery doesn’t store and provide electricity directly. Polar Night says it’s working on a system to convert this TES’ stored heat into electricity; it’ll likely involve the use of steam turbines.
This battery has actually been in operation since June, and the company says it’s already exceeded its efficiency targets in the early days of optimizing its functions. Many of Pornainen’s buildings, including its town hall, are currently heated by the new TES.
In the coming years, it’ll play a major role in helping the town achieve carbon neutrality – and it might inspire more municipalities in regions with similar climate and energy sources to adopt these giant batteries.
Source: Polar Night
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