A thermal sand battery stores energy as heat, not as electricity: electric heaters warm sand or another suitable granular material inside an insulated vessel, and a heat-transfer loop later carries that heat to a district-heating network or industrial process. That makes it a way to shift renewable electricity into useful heat when it is needed—not, in current heat-only installations, a source of electricity for the grid.
What a thermal sand battery stores
The stored energy is sensible heat: energy held in a mass of solid material at an elevated temperature. During charging, electricity runs resistive heaters and is converted to heat. A heat-transfer loop moves that heat into the insulated store, where the material retains it until demand arises.
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Despite the name, a sand battery does not have to contain ordinary beach sand. Suitable sand-like granular materials and some industrial by-products may work; Polar Night Energy’s large Pornainen installation uses crushed soapstone. The vessel and heat-transfer equipment matter as much as the choice of solid medium.
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- Charge: Electricity from the grid or local renewable generation powers resistive heaters. In Polar Night Energy’s description, circulating air in a closed-loop pipe system carries heat into the storage material. Polar Night Energy’s sand battery overview describes the system.
- Store: The heated solid sits inside an insulated silo or tank. The stored quantity is commonly expressed in megawatt-hours (MWh); this is capacity, not the rate at which the system can deliver heat.
- Discharge: The heat-transfer loop draws heat from the material and carries it to a heat exchanger. Depending on the installation, the exchanger can provide hot water, steam, or hot air to a heat network or process.
The useful output is determined by the connected heat system and its requirements. A district-heating network or industrial process must be able to use the delivered temperature and receive heat at the required rate.
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Can a sand battery make electricity?
The existing heat-storage examples described by the operators are designed to deliver heat. Converting stored heat back into electricity is a separate power-to-heat-to-power application, with additional conversion equipment and losses. It should not be assumed that a heat-only installation can supply electricity simply because it is called a battery.
Polar Night Energy’s Valkeakoski Sand to Power project is a pilot intended to test this conversion. The project page lists a 2025–2027 timeline and describes expected electrical efficiency of about 30–35% and combined heat-and-power efficiency of up to 90%. These are project estimates, not measured operating results: the page says conversion and efficiency are being tested, and that the pilot is built without a turbine, with performance simulated and modelled. The Sand to Power project page provides those qualifications.
What installed systems show
Published specifications illustrate the range of installations, but the figures below are reported by the named operator or developer, not independently reconciled. In particular, two pages describe the Kankaanpää installation differently.
| Installation | Published details | What the figures mean |
|---|---|---|
| Kankaanpää, Finland | Vatajankoski reports a steel tank about 4 metres wide and 7 metres high, containing 80 cubic metres of fine sand, with 8 MWh storage capacity and 100 kW heating power. It says the store supplies the district-heating network. | These are figures on Vatajankoski’s installation page: Vatajankoski’s energy-storage page. |
| Kankaanpää, Finland | Polar Night Energy’s overview describes the installation as 200 kW with 8 MWh capacity and around 100 tonnes of locally sourced sand. | These are Polar Night Energy’s separately published figures; they differ from Vatajankoski’s power and material descriptions. Polar Night Energy’s overview. |
| Pornainen, Finland | Polar Night Energy reports that Loviisan Lämpö commissioned the installation in June 2025. It lists 1 MW thermal power, 100 MWh capacity, dimensions of 13 metres high by 15 metres wide, and about 2,000 tonnes of crushed soapstone. | Developer-reported specifications for the larger installation, not a general design standard. Polar Night Energy’s overview. |
Power and capacity answer different questions. A 100 MWh store describes an amount of energy; a 1 MW thermal rating describes a delivery rate. Neither number alone tells you how long a particular installation can supply a heat user under real operating conditions.
Efficiency depends on the system and the service
Polar Night Energy reports 60–70% efficiency for the original Kankaanpää unit. It estimates approximate round-trip thermal efficiency of 85% for a 2 MW system and 90% for a 10 MW system. Those figures refer to different scales and should not be treated as one universal efficiency rating for sand batteries. The company’s overview gives the estimates.
Efficiency also depends on what counts as the useful output. A heat-only store supplying a heat network is not directly comparable to a system that first stores heat and then converts some of it into electricity. For power-to-heat-to-power, electrical conversion is an additional step; the Valkeakoski figures remain estimates while the pilot tests the process.
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Where thermal storage can be useful
A sand battery is most relevant where a site can use heat at the temperature and scale the system can provide. Potential applications include district heating and industrial processes that need elevated-temperature heat. Polar Night Energy’s 2024 technical white paper states that 36% of industrial process heat falls within the 60–400°C range; this is the company’s published estimate, not an independently established universal statistic. The March 2024 technical white paper provides the context.
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What determines the climate and economic case
Charging electricity is not automatically renewable or low-carbon. The climate benefit depends on the electricity used at charging time, while the economic case depends in part on electricity price and whether the system can serve a real heat demand. A Government of Yukon review notes that the economics of grid-charged thermal storage depend on time-of-use pricing with smart meters, and that emissions reductions depend on a largely clean grid. The Yukon thermal energy storage review discusses these conditions.
There is no established general cost comparison in these sources that makes thermal sand storage a universal winner over other storage technologies. Compare options by the service delivered, rather than by a single battery-efficiency label:
Quick Recap
- Output: heat, electricity after reconversion, or combined heat and power.
- Temperature match: whether the intended network or industrial process can use the delivered heat.
- Charging conditions: electricity availability, price, and carbon intensity when charging takes place.
- Integration: proximity to a heat user and the equipment needed to connect the store.
- Scale and duration: compare storage capacity in MWh and delivery power in MW separately, and at comparable system sizes.
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