A thermal sand battery stores energy as heat: electricity powers heaters, which warm sand or another granular material inside an insulated store. When heat is needed, a circulating heat-transfer loop carries it to a heat exchanger for district heating or an industrial process. The heat-only systems described here do not directly deliver electricity; converting stored heat back into electricity is a separate, pilot-stage effort.
What a thermal sand battery stores
Despite the name, a sand battery is not an electrochemical battery. It stores sensible heat in a large mass of hot solid material. Electricity is converted to heat during charging, and the insulated store holds that heat until a user needs it. The medium can be ordinary sand, a sand-like granular material, or another suitable material; Polar Night Energy’s large Pornainen installation uses crushed soapstone rather than sand.
How the charge-and-discharge cycle works
- Charge: Electricity from the grid or a local renewable source powers resistive heaters. In Polar Night Energy’s description, air circulates through pipes and transfers heat into the storage material.
- Store: The heated granular medium retains thermal energy inside an insulated structure. Charging can shift electricity use to times when renewable generation is abundant or prices are lower, but the climate and economic benefits depend on the electricity source and price.
- Discharge: The heat-transfer loop carries heat from the medium to a heat exchanger. The system can then provide hot water, process steam, or hot air, depending on its design and the connected heat user.
The output temperature depends on the system, so the store must be matched to the temperatures required by a district-heating network or industrial process. Polar Night Energy’s 2024 technical white paper estimates that 36% of industrial process heat falls within the 60–400°C range; that is the company’s estimate, not an independently established universal statistic.
What the installed examples show
Published project figures are reported by the operators and developer, not presented here as independently verified measurements. The Kankaanpää descriptions also differ, so their specifications should be kept attributed to the page that reports them.
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| Installation | Published details | What it serves or means |
|---|---|---|
| Kankaanpää, Finland — Vatajankoski | Vatajankoski describes a steel tank about 4 metres wide and 7 metres high, containing 80 cubic metres of fine sand. It lists 8 MWh of storage capacity and 100 kW of heating power. | The store supplies a district-heating network. |
| Kankaanpää, Finland — Polar Night Energy | Polar Night Energy’s overview describes the installation as 200 kW with 8 MWh capacity and around 100 tonnes of locally sourced sand. | This is a different published power and medium-quantity description from Vatajankoski’s page; the figures should not be combined as if they were one reconciled specification. |
| Pornainen, Finland | Polar Night Energy says Loviisan Lämpö commissioned the installation in June 2025. The developer reports 1 MW thermal power, 100 MWh capacity, dimensions of 13 metres high by 15 metres wide, and about 2,000 tonnes of crushed soapstone. | A larger thermal store serving heat demand; the listed specifications are developer-reported. |
Capacity and delivery power describe different things: MWh indicates how much energy a store can hold, while kW or MW indicates how quickly it can deliver power. Comparisons are meaningful only when the scale and output boundary are alike.
Does a sand battery generate electricity?
The Kankaanpää and Pornainen examples described above store and deliver heat. Their useful output is not electricity. Heat-to-electricity reconversion is being explored separately in Polar Night Energy’s Sand to Power pilot at Valkeakoski, Finland, listed for 2025–2027.
Polar Night Energy gives the pilot an expected electrical efficiency of about 30–35% and combined heat-and-power efficiency of up to 90%. These are project estimates, not demonstrated operating results: the company says the pilot is intended to test conversion and efficiency, is built without a turbine, and uses simulated and modelled performance. The figures therefore should not be treated as measured performance of a deployed electricity-generating sand battery.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to assess a sand-battery proposal
There is no single efficiency or cost figure that establishes whether a thermal store is the right choice. The relevant comparison is the service it provides and how well it fits the site.
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- Output: Establish whether the project supplies heat only, reconverts heat to electricity, or provides combined heat and power.
- Temperature match: Check that the delivered heat can serve the intended district network or industrial process.
- Charging conditions: Consider the electricity’s price and carbon intensity when the store charges. The Government of Yukon notes that grid-charged thermal storage economics depend on time-of-use pricing with smart meters, and emissions reductions depend on a largely clean grid.
- Integration: Account for connection to a heat network or nearby steady heat user, as well as charging and discharge equipment.
- Scale and duration: Compare storage capacity and delivery power separately, and use systems of similar scale and the same output boundary.
The available sources do not establish a general independent cost comparison with other storage technologies, so they do not support naming a universal cost or efficiency winner.
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