In our latest episode of Lexicon, we sat down with Annette Höglund-Dönnes, Chief Commercial Officer at Polar Night Energy, to explore how the company’s sand battery is helping municipalities and industries store renewable energy as heat, providing a low-cost and potentially transformative alternative to fossil fuels.

Read on to find out more. 

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What exactly is a sand battery?

For those unfamiliar with the concept, Höglund-Dönnes explains that a sand battery acts, essentially, as a giant thermal storage system.

“You can think about it as a very large and well-insulated hot water tank,” she told IE. “However, the energy is actually stored as heat. In our case, instead of water, we use sand as the storage medium,” she added.

So, electricity generated during periods of low demand or abundant renewable production is used to heat air, which can then be circulated through thousands of tonnes of sand inside a heavily insulated silo. This stored heat can later be discharged for district heating or industrial processes.

The concept is straightforward, but according to Höglund-Dönnes, people often underestimate the engineering involved.

“People assume it’s basic, but it’s actually a highly engineered system where heat transfer, airflow, insulation, and control systems all need to work precisely together,” she explained.

Why sand?

So, you might be wondering, “Why use sand at all?” The answer, it turns out, is deceptively simple; it is very common.

“Any sand can be used,” she says. “Even slag material from industrial processes can be used,” she added.

What’s more, unlike lithium-ion batteries, the sand itself does not degrade over time. That gives the technology one of its most attractive characteristics.

“There’s no degradation. The medium itself, the sand, really doesn’t change phases or anything,” she added.

But there are some other critical advantages too. “It doesn’t explode, and it’s not toxic. Those are also really great advantages,” she explained. 

The result is a storage medium that is abundant, inexpensive, safe, and capable of storing heat for extended periods.

Putting the technology to the test

And the technology is not just theoretical, it has already cut its teeth in the real-world. The company’s flagship installation in Pornainen, Finland, became operational in 2025 and recently endured one of the country’s coldest winters in years.

“We were very pleased to see that the battery performed at the upper end of what the simulations predicted,” said Höglund-Dönnes. “It actually exceeded the promised values,” she added.

For a company scaling its technology for commercial deployment, that result was a significant milestone. “I think seeing it in the real world and not just barely making it, but making it with a margin, was really encouraging,” she told us.

This, she explained, breaks the mold for emerging energy technology, which all too often never meets expectations. Pornainen, on the other hand, appears to have delivered that proof.

Scaling up is never simple

As Höglund-Dönneswent on to explain, Pornainen also represented a major leap in scale. The installation is roughly twenty times larger than Polar Night Energy’s previous deployment, creating a new set of engineering challenges.

“The biggest surprise for us was really how much more detailed engineering we had to do at that scale,” Höglund-Dönnes said. “It brought a whole new level of complexity, and we had to build capabilities to keep up,” she added.

Interestingly, she believes the complexity is often misunderstood. “The system is elegantly simple. The complexity is in the physics,” she told us.

That distinction is important because the company sees simplicity as one of its greatest strengths. The battery consists largely of steel, sand, insulation, and industrial components that are already widely available.

“It is not the complexity, it is the simplicity,” she said. “I think we are going to look back and say, ‘ Why didn’t we start sooner?’”

Beyond district heating

While district heating has provided an ideal first market, Polar Night Energy’s long-term ambitions are much, much bigger. The company sees industrial process heat as one of the biggest opportunities for decarbonisation.

Today, industries ranging from food and beverage production to chemicals and pharmaceuticals rely heavily on natural gas, oil, and other fossil fuels to generate heat. Many of those processes require temperatures between 100°C and 250°C, a range where thermal energy storage could potentially play a major role.

“We see a huge benefit in decarbonising where lots of natural gas or oil is used today,” explained Höglund-Dönnes.

Rather than requiring businesses to completely replace existing infrastructure overnight, she sees sand batteries working alongside conventional systems during the transition. “You wouldn’t need to take the entire step and decarbonise everything in one go,” she added.

Instead, companies could gradually reduce their dependence on fossil fuels while improving resilience and managing energy costs. “You could add the sand battery to operate in tandem with your current system. That already gives you a dual source for the fuel that you use,” she said.

Helping industry decarbonise with sand

As Höglund-Dönnes explains, one of the most exciting aspects of the technology is its ability to help companies take practical steps toward sustainability without radically changing the way they operate.

“A lot of business leaders are sitting with exactly this challenge today. How do they address the use of fossil fuels? How do they get rid of it?” she said.

The answer, she believes, lies partly in thermal energy storage. “Thermal energy storage is one of the technologies that’s going to help industries meet those environmental targets,” she told us.

That could be especially important as governments introduce increasingly ambitious emissions targets and regulations over the coming decades.

Building a circular energy system

Beyond decarbonisation, the company is also focused on circular economy principles. The use of industrial waste materials as a storage medium reflects a broader philosophy about how future infrastructure should be designed.

“Circularity is going to be key, no matter what we build,” she said. To this end, the company is committed to sourcing materials and expertise locally wherever possible.

“We’re really looking for local partners and suppliers,” she added.

In other words, she explained, the long-term goal is not simply building storage systems. It is creating local energy ecosystems where renewable electricity can be converted into heat and used close to where it is generated.

“How do we make sure local wind and local sunshine become local heat that can be used locally, with no transportation whatsoever?” she added.

Looking ahead

When asked what she hopes people will remember about the Pornainen project a decade from now, Höglund-Dönnes returned to the same theme that has appeared throughout the conversation: simplicity.

“I believe that the most important legacy is using nature’s own materials and a very simple and elegant way of storing energy in the form of heat,” she said.

“It’s not the complexity, it’s the simplicity. I think we’re going to look back and say, ‘ Why didn’t we start sooner?’” she explained.

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