What a heat battery does
A heat battery stores energy as heat instead of as electricity. It heats a filler, crushed rock, sand, or brick, using ordinary electricity from the grid, and later blows air through the hot filler to release that heat when it is needed for warmth or steam.
Polar Night Energy and the utility Loviisan Lampo built one of these systems in Pornainen, Finland. It holds 100 megawatt hours of heat, a measure of how much energy it can store rather than how fast it can deliver it, inside 2,000 tonnes of crushed soapstone packed into a tank 13 metres high and 15 metres wide. It started running in June 2025. One year later, the operator reported that oil had been removed completely from the heating mix used to warm homes in the town, that use of biomass fuel had been cut by 60%, and that emissions from heating had been cut by 70%.
The comparison covers the first year the battery ran against the year before it started, as reported by the heating utility in the town.
Show the numbers
| Oil use cut | 100 |
| Biomass fuel cut | 60 |
| Emissions cut | 70 |
Hot water reached homes in the town for the whole first year without a single interruption, according to the operator.
Commissioning was exceptionally smooth, and reliability has been excellent: there have been zero interruptions in district heating delivery.
Mikko Paajanen, chief executive of Loviisan Lampo, the company that owns and runs the sand battery in Pornainen, Finland. Source 1.
The battery also let the utility buy cheaper power. CapMan Infra, the investment firm that financed the project, says it charged the battery at electricity prices 70 to 80% below the spot market average across the year, and by more than 90% below it in some months.
The same size battery, built for a factory instead of a town
Rondo Energy is building an identical size heat battery, 100 megawatt hours, at a Covestro chemical factory in Brunsbuttel, Germany. The Pornainen system heats a whole town. The Brunsbuttel system will supply only 10% of the steam that one factory needs.
Rondo says the project will cut up to 13,000 tons of carbon dioxide a year at that site. Construction began on 19 January 2026, and commissioning is planned for the end of 2026, so the system is still being built, not yet running.
This project shows how two of Europe's central challenges can be solved together.
Eric Trusiewicz, chief executive of Rondo Energy, on the heat battery being built at the Covestro factory in Brunsbuttel, Germany. Source 2.
How fast this technology is scaling
Polar Night Energy sand batteries have grown fast. The first one, in Kankaanpaa, Finland, started running in 2022 and holds 8 megawatt hours. Pornainen, 3 years later, holds 100 megawatt hours. Construction is planned to start in early 2026 on a 250 megawatt hour system near Vaaksy, Finland, for the utility Lahti Energia, due to finish in the summer of 2027.
The 2022 figure at Kankaanpaa is from source 3 and the 2027 planned figure near Vaaksy is from source 4, both companies of the same Polar Night Energy family of projects as the Pornainen figure the chart is filed under. The Vaaksy project is planned, not yet built.
Show the numbers
| Kankaanpaa, built 2022 | 8 |
| Pornainen, built 2025 | 100 |
| Vaaksy, planned 2027 | 250 |
Rondo Energy, the company building the Brunsbuttel factory battery, says its heat battery design can run for more than 40 years. Across all of its projects, the company counts 8 heat battery deployments among 11 commercial developments it has announced, adding up to 400 megawatt hours built or contracted, plus 3,000 megawatt hours more in partnerships still under discussion.