Batteries are key to the future of the most abundant renewable energy sources, solar and wind, for one obvious reason: intermittency.

If electricity generators are to replace fossil fuels entirely, they will need to store much of this renewable energy for use when the sun isn’t shining and the wind isn’t blowing. Batteries, however, are expensive, especially if they involve lithium, which is not suitable for large-scale energy storage.

But a small, six-person start-up in Finland found an intriguing and inexpensive solution: it developed the world’s first commercial heat storage battery, made of a very cheap low-tech material.

“It’s a big mass of sand. In itself, the sand is not so special. It’s just sand,” said Matti Ulvinen, director of product sales at Polar Night Energy, the small company that developed the world’s first commercial sand battery.

“We use sand heated to high temperatures. The energy can then be stored for a long period of time.

The company installed the battery in a steel silo at the Vatajankoski power plant on the outskirts of the small town of Kankaanpaa in southwestern Finland. The silo, measuring 23 feet high and 13 feet in diameter, contains 100 tons of sand. The beauty of this material, according to the plant’s general manager, Pekka Passi, is that it retains heat without significant loss for months and does not evaporate.

“You can heat sand to a higher temperature than water. We can heat the sand up to 500 degrees centigrade so you have a lot more storage capacity in the same volume, in the same space,” he said.

The relatively small silo, which is a pilot project for Vatajankoski, should provide enough heat for around 100 homes, distributed via a district heating system – a local network of insulated pipes. Passi loves new technology.

“It’s pretty cheap. It doesn’t have to be a special sand, so it’s very inexpensive,” he said, “and I expect it to be low, very low maintenance.

Polar Night Energy, which developed and patented the battery’s closed-loop heating system, did not disclose the capital cost for commercial confidentiality reasons, but the company insisted that overall , it will be up to 10 times cheaper than existing high temperature storage methods. And because of those very high temperatures, the company’s Chief Technology Officer, Markku Ylonen, stressed that the battery won’t just benefit residential customers.

“Our system excels in high temperature applications, so we can deliver really cost effective high temperatures to industrial processes,” he said. “Steam for the food industry for cleaning and disinfecting equipment, hot air for the production of cement and steel.”

Storing heat when renewables are plentiful, for use when they are not, makes perfect sense for Finland with its long, dark, freezing winters. Especially now, Ylonen said, that Russia has cut off all gas supplies to the country after it applied to join NATO.

“It certainly stimulated interest in our system,” he said.

Ylonen also believes that the urgent need to make variable renewables more reliable has massively expanded the sand battery market. “I would expect the next one we build would be in Finland, but we hope to be global very soon. We actually trade in dozens of different countries, including America.

There is an apparent flaw in the new technology: there is a worldwide shortage of sand for construction. But the Finns argue that their battery will run on the lowest materials available – and there are billions of tons of them.