Excess renewable energy will be stored as heat in a novel “sand battery” as part of a new plan to help decarbonise Finland’s electric grid.

Finnish start-up Polar Night Energy has struck a deal with district heating company Loviisan Lämpö for a 1MW/100MWh system in Pornainen, a municipality near Helsinki.

By serving as a means of storing excess wind and solar power for when it is needed most, Polar Night says the battery could cut the town’s emissions by nearly 70%.

“We want to enable the growth of renewable energy,” said Loviisan Lämpö CEO Mikko Paajanen.

The sand battery will he continued “keep the electricity grid balanced as the share of wind and solar energy in the grid increases.”

Various start-ups are developing similar methods of storing excess green power as heat in thermal energy storage projects.

Molten salt, liquid tin and bricks are among a plethora of mediums that developers are using for thermal energy storage, with the common denominator being a desire to use abundant and inexpensive materials.

Polar Night, founded in 2018, has developed its thermal battery technology using sand or sand-like materials that can be heated up to 1000°C and even higher.

The battery it is building in Pornainen will be made up of crushed soapstone – said to conduct heat better than conventional sand – that will be supplied by a local fireplace manufacturer for which the material is a by-product.

Polar Night builds a heat transfer system inside the sand enabling “effective energy transportation to and from the storage.”

“Proper insulation between the storage and environment ensures long storing period,” which Polar Night claims can stretch as long as months “with minimal heat losses.”

The sand batteries can be anything from tens to thousands of cubic meters in size and can be sited underground.

In this case, the battery will be around 13 metres high and 15 metres wide. The estimated duration for construction and testing is around 13 months.

Polar Night built its first commercial sand-based battery for a utility in western Finland, although that has just a fraction of the power of its new planned project.

Liisa Naskali, chief operating officer at Polar Night, said: “It's exciting to build a large-scale thermal energy storage, which will also act as a primary production plant in Pornainen's district heating network. This is a significant step in scaling up the Sand Battery technology.”

Antti Kuusela, Mayor of Pornainen, said his municipality “welcomes all innovative development projects that reduce emissions in district heating operations and contribute to network expansion.”