Solar energy stored in ‘sand batteries’ could help get Finns through the long cold winter, which is set to be even tougher after Russia stopped its gas and electricity supplies.

The new technology has been devised by young Finnish engineers Tommi Eronen and Markku Ylönen, founders of Polar Night Energy, but could be used worldwide.

Though a number of other research groups are testing the limits of sand as green energy storage, the pair are the first ones to successfully rig it to a commercial power station.

Around 100 tonnes of the fine stuff, piled high inside a silo, went live at a power plant in the southwestern town of Kankaanpää in late May - just as Putin cut the country off in retaliation for joining NATO.

“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 says of the company’s inspiration.

“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 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.

It’s quite a simple structure to begin with. A tall, grey 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 super conductor. The hot air is then circulated in the container through a heat exchanger.

The sand can store heat at around 500C for several 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 which serves the city of Tampere. Homes, offices and even the local swimming pool all benefit.

“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.”

The large battery installed at the Vatajankoski power plant uses sand from a nearby sand pit.

“We wanted to find something that can be sourced nearly everywhere in the world,” the 33-year-old explains. 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 Dutch study published earlier this year. 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 builder’s sand was used for the prototype (to limit transport emissions), sand batteries work with any kind that has a high enough density, within certain thermodynamic parameters.

“We don’t want to compete with [the construction industry] for the good qualities of sand,” Markku confirms.

In fact, the focus is less on sand, and really all “about helping the wind and solar sectors to grow,” he adds. “We think that this is just one of the key components to make a society with a really high production of wind and solar power.”

With Finland’s geopolitical situation finely poised, and the world urgently needing to wean itself off fossil fuels, it’s no surprise that Polar Night Energy has been flooded with interest.

After the success in Tampere, the company has had lots of enquiries about building devices before winter, when the demand for heating starts in earnest. Their ambition extends well beyond western Finland, suggests Markku.

“We want to build hundred times larger storages around the world as fast as possible.”