Sustainable apartment block

A very Finnish thing’: Big sand battery to store wind and solar energy using crushed soapstone

The battery will be able to store a week’s heat demand in winter - how does it work?

A huge sand battery is set to slash the carbon emissions of a Finnish town. The industrial-scale storage unit in Pornainen, southern Finland, will be the world’s biggest sand battery when it comes online within a year. Capable of storing 100 MWh of thermal energy from solar and wind sources, it will enable residents to eliminate oil from their district heating network, helping to cut emissions by nearly 70 percent. "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,” says Liisa Naskali, COO at Polar Night Energy, the company behind the innovation. “This is a significant step in scaling up the sand battery technology.”  

Sand batteries are getting bigger in Finland

The new 1 MW sand battery has a precursor. In May 2022, Polar Night Energy rigged a smaller design to a power station in Kankaanpää town. Launched just as Russia cut off gas supplies in retaliation for Finland joining NATO, the project was a timely example of how renewable energy could be harnessed in a new way. Euronews Green previously spoke to the young Finnish founders, Tommi Eronen and Markku Ylönen, who engineered the technology. “We were talking about how - if we had the liberty to design a community for ourselves - how could we solve the energy problem in such a confined environment?” Markku said of the inspiration behind Polar Night Energy in 2018. “Then quite quickly, especially here in the north, you run into the problem of energy storage if you're trying to produce the energy as cleanly as possible.” The friends started playing around with ideas, landing on sand as an affordable way to store the plentiful electricity generated when the sun is shining, or the wind blowing at a high rate. Finding a way to store these variable renewables is the crux of unleashing their full potential. Lithium batteries work well for specific applications, explains Markku, but aside from their environmental issues and expense, they cannot take in a huge amount of energy. Grains of sand, it turns out, are surprisingly roomy when it comes to energy storage. The sand battery in Pornainen will be around 10 times larger than the one still in operation at Vatajankoski power plant in Kankaanpää. The start-up also previously connected a pilot plant to the district heating network of Tampere city.  

So how do sand batteries work exactly?

It’s quite a simple structure to begin with, Polar Night Energy said of its prototype. A tall tower is filled with low-grade sand and charged up with the heat from excess solar and wind electricity. This works by a process called resistive heating, whereby heat is generated through the friction created when an electrical current passes through any material that is not a superconductor. The hot air is then circulated in the container through a heat exchanger. The sand can store heat at around 500C for several days to even months, providing a valuable store of cheaper energy during the winter. When needed, the battery discharges the hot air - warming water in the district heating network. Homes, offices and even the local swimming pool all benefit in Kankaanpää, for example. “There’s really nothing fancy there,” Markku says of the storage. “The complex part happens on the computer; we need to know how the energy, or heat, moves inside the storage, so that we know all the time how much is available and at what rate we can discharge and charge.”  

How will the sand battery serve residents in Pornainen?

Having refined its charging algorithms, Polar Night Energy is now ready to scale up the storage tech in Pornainen. Once completed, the new battery will be integrated with the network of Loviisan Lämpö, the Finnish heating company that supplies district heating in the area. "Loviisan Lämpö is moving towards more environmentally friendly energy production. With the Sand Battery, we can significantly reduce energy produced by combustion and completely eliminate the use of oil," says CEO Mikko Paajanen. The project also aligns with Pornainen’s plans for carbon neutrality. Many of its buildings, including the comprehensive school, town hall, and library, rely on district heating. Mayor of Pornainen Antti Kuusela says the municipality “welcomes all innovative development projects that reduce emissions in district heating operations and contribute to network expansion.” In total, the sand battery is expected to knock off 160 tonnes of carbon dioxide equivalent emissions per year. As well as weaning the town off oil, woodchip burning is expected to drop by 60 per cent as a result. The battery’s thermal energy storage capacity equates to almost one month’s heat demand in summer and a one-week demand in winter in Pornainen, Polar Night Energy says. Construction and testing of the 13 metres high by 15 metres wide battery is estimated to take around 13 months, meaning it should be keeping residents warm well before winter 2025.  

Is sand a sustainable material?

“We wanted to find something that can be sourced nearly everywhere in the world,” Markku said. But is sand as ubiquitous as we might think? Demand for the construction material is set to soar by 45 per cent in the next 40 years, according to a recent Dutch study. Building sand is typically extracted from rivers and lakes, and ‘sand pirates’ are speeding up its loss from these ecosystems. But as far as the Finnish engineers are concerned, it doesn’t really matter where the sand comes from. Though builders’ sand was used initially (to limit transport emissions), sand batteries work with any sand-like material that has a high enough density, within certain thermodynamic parameters. In Pornainen, Polar Night Energy has found a sustainable material in crushed soapstone; a by-product of a Finnish company’s manufacture of heat-retaining fireplaces. "Tulikivi is a well-known and traditional company,” says Naskali. “The soapstone they use is a very Finnish thing.” "We always choose the thermal energy storage medium based on the customer's needs. Examining and testing different materials is crucial for us to use materials that are suitable in terms of properties, cost-effectiveness, and promotion of circular economy," she adds. Polar Night Energy has big ambitions to take its technology worldwide. As Markku told us back in 2022, “we want to build a hundred times larger storages around the world as fast as possible.”