The Sand Battery: How Finland Is Storing Heat in Plain Old Sand

Ask most people to picture a battery, and they'll think of lithium cells in a phone or an electric car. Finland has built something very different: a steel silo of sand that stores clean energy as heat, and it's already keeping a town warm.

It sounds like a gimmick, but it's real engineering; it's running commercially, and it may matter for countries like India. Here's what it is, how it works, where it falls short, and why it's worth knowing about.

What a sand battery actually is

First, a clarification. A sand battery doesn't store electricity the way a phone battery does. It's a thermal battery. Polar Night Energy describes its sand battery as a large-scale, high-temperature thermal energy storage that uses sand or sand-like materials as its storage medium and stores energy in the sand as heat.

The idea is simple. When wind and solar produce more electricity than the grid needs, that power is cheap and sometimes close to worthless. Instead of wasting it, you turn it into heat and park it in sand. When you need warmth later, you pull the heat back out.

How it works

Electricity heats air, which circulates through a closed loop of pipes embedded in stationary sand and transfers heat into it. When heat is needed, the air flows the other way, pulls the heat out, and passes it to the district heating network through a heat exchanger.

There's very little to break. One of the project's investors described it to Interesting Engineering as a static, robust and simple system with few moving parts.

So why sand? The answer is physics. Water can't be heated much past about 100°C at normal pressure (roughly 120°C under pressure) before it boils, while sand has no such limit and can be heated far higher without degrading. More heat per cubic metre means a more compact store. The Pornainen installation runs at 500°C, and Polar Night Energy's systems are designed for up to 650°C.

Sand also conducts heat poorly. That's a disadvantage in some applications, but here it helps, because its low heat conductivity reduces energy loss, according to Polar Night Energy's lead scientist. The outer layers stay relatively cool while the core is at maximum temperature, which keeps insulation demands manageable.

The story so far: three milestones

1. Kankaanpää, 2022: the first commercial unit.
The first commercial system was built by Polar Night Energy with the utility Vatajankoski. It's a 23-foot-tall steel container holding 100 tons of sand, with 100 kW of heating power and 8 MWh of capacity. According to Down To Earth, that is enough heat for about 100 homes and a public swimming pool in Kankaanpää, for roughly 80 hours on a full charge. The managing director of Vatajankoski said the heat loss was surprisingly low.

2. Pornainen, 2025: the big one.
The next project, with the district heating company Loviisan Lämpö, was a 1 MW / 100 MWh system. It's the world's largest sand battery. It's 13 metres high and 15 metres wide, and around 10 times larger than the Kankaanpää unit. Rather than virgin sand, it uses crushed soapstone, a by-product of a Finnish fireplace manufacturer. Its storage capacity is roughly a month of the town's summer heat demand, or about a week of winter demand.

3. The real-world test: winter 2025–26.
This is the part I find most convincing. Finland had an unusually cold winter, and spot electricity prices swung from 3 €/MWh to 373 €/MWh within a single week. That's a hard test for a system that depends on buying cheap power. Even so, the municipality reports that oil use fell 100 percent, emissions fell 70 percent, and woodchip combustion fell 60 percent. All public buildings are now heated through this network.

Efficiency is where sand batteries get interesting, but also where the claims need care. The round-trip efficiency at Pornainen (charging, storing, extracting) is reported at 80 to 90 percent, and an investor said thermal recovery stays around 85 percent even after weeks of storage. Treat that number with a caveat: it's heat in, heat out, not electricity in, electricity out. If you convert heat back to electricity, efficiency drops sharply. Polar Night Energy's own electricity-generating pilot (more on that below) targets 30 to 35 percent electrical efficiency.

That's why sand batteries make the most sense where heat is the end product: district heating, hot water, and industrial process heat.

One more detail most storage comparisons miss: the Pornainen battery charges over about 100 hours, while most competing systems have charging windows measured in hours. That lets it soak up cheap electricity whenever it appears, rather than only in a short window.

Where it's going next

• Bigger projects. A 2 MW / 250 MWh system for Lahti Energia in Vääksy, Asikkala is already underway and would be the largest sand battery built so far.
• Industry, not just towns. Executives call district heating a niche and see the larger market in industry, where factories make steam from gas or oil. Food and beverage, chemicals, pharma and general manufacturing are the near-term targets, with around 200°C steam seen as the sweet spot.
• Heat-to-power. Polar Night Energy is developing "Sand to Power," pairing turbines with the sand battery. A pilot in Valkeakoski concludes in 2027, after which the company plans commercial deployment. That pilot is a €4.2 million plant built with the district energy firm Valkeakosken Energia.
• Recognition. The Pornainen project received a Certificate of Merit at the 9th Global District Energy Climate Awards.

India doesn't have Finland's cold winters, so district heating isn't the obvious fit. But the Finnish developers saw another opening. They told Down To Earth the technology could offer big opportunities for process heating in Indian industries and for heating municipal buildings. Think textiles, dairies, food processing and chemicals, where boilers burn coal, gas or oil to make steam.

The wider storage picture is also relevant. At an energy roundtable, experts said India must add about 50 GWh of storage each year to reach a 2030 target of 250 GWh, with installed capacity still under 1 GWh. The same discussion argued that thermal storage can be far cheaper than lithium-ion and reduces dependence on imports. That's a roundtable's argument rather than a settled fact, but it explains the interest.

A few honest cautions for Indian readers:

• Don't confuse different "sand" technologies. An IIT Kanpur-incubated startup, Baud Resources, is working on gravity-based storage that lifts sand and industrial waste like coal ash between stations, storing energy as potential energy. That's a different idea from a heat-storing sand battery.
• Related research exists elsewhere. The US National Renewable Energy Laboratory has been testing a system that heats silica sand to 1,200°C and stores it in insulated silos. NREL says the storage cost is $2 to $4 per kWh of thermal energy, though it's less energy-dense than lithium-ion.
• Beware of sketchy numbers. Some websites claim India already has sand battery pilots in Tamil Nadu with specific efficiency and cost figures. I couldn't verify these in credible sources, so I've left them out. If you see them online, check before repeating them.

The honest limits

A sand battery isn't a replacement for the lithium battery in your phone, or for grid batteries that deliver electricity quickly. It needs electricity to charge, monitor and run its blowers. It shines when you need heat, when you have cheap surplus power, and when you have room for a big silo. And so far, the evidence comes mainly from one company's projects in a cold climate.

Still, the principle is hard to argue with. Sand is cheap, doesn't wear out the way chemical batteries do, and can even be recycled industrial waste. Sometimes the best technology is a well-insulated pile of something ordinary.

Read more

• Euronews, on the Pornainen sand battery: https://www.euronews.com/green/2025/06/15/sand-batteries-could-be-key-breakthrough-in-storing-solar-and-wind-energy-year-round
• Interesting Engineering, a site visit and winter performance data (April 2026): https://interestingengineering.com/energy/sand-battery-polar-night-energy
• Energy-Storage.news, on the 1 MW / 100 MWh project: https://www.energy-storage.news/larger-1mw-100mwh-sand-battery-set-for-commissioning-in-2025
• Down To Earth (India), on the original Kankaanpää battery: https://www.downtoearth.org.in/news/dirt-cheap-backup-why-finland-s-installation-of-the-world-s-1st-sand-battery-may-be-a-game-changer-87804
• Electrek, on the 2022 launch: https://electrek.co/2022/07/08/sand-energy-storage-wind-solar/
• Polar Night Energy, on the award: https://polarnightenergy.com/news/sand-battery-recognised-at-global-district-energy-climate-awards/
• Mercom India, on NREL's silica-sand storage: https://mercomindia.com/silica-offer-stable-inexpensive-energy-storage
• Outlook Business, on India's thermal storage debate: https://www.outlookbusiness.com/amp/story/industry/indias-storage-future-may-be-built-with-bricks-not-lithium
• pv magazine, on Baud Resources' sand-based gravity storage: https://www.pv-magazine.com/2024/04/25/indian-startup-develops-sand-based-gravity-energy-storage-system