The world's largest sand battery in the city of Pornainen has confirmed its design performance after its first full year of operation: the district heating system has reduced climate emissions by 70%, completely abandoned oil, and reduced wood chip combustion by 60%.
Technology: 2000 tons of stone instead of oil and chips
At the heart of the installation is a steel silo, 13 meters high and 15 meters wide, filled with 2000 tons of crushed soapstone. The material was not chosen by chance: it is a byproduct of the Finnish fireplace manufacturer Tulikivi, meaning the battery is also an example of a circular economy. The stone is heated to 500–600 °C using renewable electricity purchased on the spot market during low-price hours – when wind and solar power plants generate excess energy.
The heat can then be stored for days and weeks thanks to multilayer insulation. As needed, it is transferred to Pornainen's district heating system, which previously ran on oil and wood chips. The storage capacity is about 100 MWh of thermal energy with a power output of 1 MW: this is enough to cover the needs of a city with a population of about 5000 residents for approximately a month in the summer and about a week in the winter.
Results of the first year
According to the operator, the operational results exceeded expectations:
- -70% climate emissions from the heating system;
- -60% biomass (wood chip) consumption;
- -100% oil consumption – oil boilers have been decommissioned;
- >85% efficiency and 100% reliability of heat supply.
The project's economics were equally impressive: charging the battery cost on average 70–80% less than the average market price of electricity, and in some months, the difference exceeded 90%.
Solving the "eternal" problem of green energy
The main significance of the technology lies in combating the instability of renewable energy sources: the battery stores energy when the sun shines and the wind blows, and releases it when needed. This allows for an increased share of renewables in the grid without compromising the reliability of heat supply.
"I am more convinced than ever that thermal energy storage technologies can play a very important role in helping municipalities reduce emissions," said Pornainen Mayor Antti Kuusela. He also emphasized that the installation enhanced the city's energy security: "The network has gained greater flexibility and a better ability to cope with various disruptions."
What's next
The success in Pornainen, where the battery was built by Polar Night Energy for the Loviisan Lämpö heating network, has already become a basis for replicating the technology in other municipalities in Finland and abroad. For small towns dependent on expensive imported fuel, the sand battery is one of the cheapest ways to decarbonize: it contains no rare metals, toxic materials, or degrading chemical elements – only stone, steel, and insulation.
