Thermal energy storage

Also known as Heat storage, Sand battery, Heat battery, Thermal battery

Thermal energy storage keeps heat in materials such as sand, bricks, rock or molten salt so that it can be used later for industrial processes or district heating. Operating systems now exist in Finland and at a steel plant in India, but most performance figures still come from the operators or from modelling.

Pilot and early commercialDemonstratedReviewed 21 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

A thermal energy storage system takes heat, often made from electricity or captured as waste heat, and keeps it for later use. A useful comparison is a hot water bottle that stays warm for hours, but on a much larger scale. Most systems use cheap, heavy materials such as sand, crushed rock, bricks or molten salt. These are heated, then insulated so the heat does not escape. When heat is needed, air, water or steam carries it out to a factory or a heating network.

Heat is a different job from electricity. Many factories use large amounts of heat for drying, cooking and making materials, and many towns are heated through pipes called district heating networks. Storage lets a site use heat when it is needed rather than when it is produced. In Finland, the operator Loviisan Lämpö reports that a sand battery at Pornainen now supplies heat to a local district heating network.

Technical explanation

Most thermal storage systems use sensible heat. The temperature of a solid or liquid store rises as energy is added, and the energy stored depends on the mass of the material, its heat capacity and the temperature change. Sand, crushed soapstone, rock and bricks are solid media; molten salt is a liquid medium. Electric resistance heaters are used to charge the NREL sand design. In the NREL design, the sand cycles between ambient temperature and more than 1000 °C, with the particles heated to 1200 °C for charging. Rondo states that its bricks store heat at up to 1500 °C and deliver heat at up to 1500 °C.

Efficiency depends heavily on the metric used. A heat-in to heat-out figure is different from electricity-in to electricity-out. As reported by SolarPACES, NREL says stored thermal energy in an insulated silo remains above 95% efficient over multiple days, with about 1% of heat lost per day. For electricity-in to electricity-out, Gifford and colleagues (2024) modelled a notional 135 MWe plant with particle storage and found annual round-trip efficiencies above 50%, falling by about 1.5% when duration rose from 10 to 100 hours. A 2025 review lists 93% to 98% for two-tank molten salt systems, but it does not state the metric or the insulation and temperature conditions behind those figures, so the values should not be compared directly.

Operating projects are at an early stage. The Pornainen unit in Finland is rated at 1 MW and 100 MWh. At Tata Steel's Jamshedpur plant in India, two Kraftblock units with a combined 20 MWh capacity store waste heat at up to 500 °C. The Pornainen figures come from the operator, and the Jamshedpur figures are Kraftblock's as reported by Canary Media. The sources reviewed here do not include independently measured, multi-year performance data.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Reported, unverified Stated by a company or team but not independently confirmed.

Polar Night Energy reports that the Pornainen sand battery, commissioned by Loviisan Lämpö in June 2025, provides 1 MW of thermal power and 100 MWh of storage capacity, using about 2,000 tonnes of crushed soapstone.

Sources: [1] · Article text · medium confidence

The operator says the Pornainen system can cover almost a month of summer heat demand, and close to a week of winter demand, in the district heating network it serves.

Sources: [1] · Article text · medium confidence

Rondo states that its Rondo Heat Battery stores and delivers heat at temperatures up to 1500 °C and has a service life of more than 40 years without performance degradation.

Sources: [2] · Product description · medium confidence

As reported by SolarPACES, NREL says stored thermal energy in its insulated sand silo has an efficiency above 95% over multiple days, and that an insulated silo loses about 1% of its heat per day. The 95% figure refers only to heat remaining in the silos.

Sources: [5] · medium confidence

A 2025 review lists round-trip efficiencies of 93% to 98% for two-tank molten salt systems and 75% to 90% for encapsulated phase-change materials, but does not state the efficiency metric or the insulation and temperature conditions behind these figures.

Sources: [4] · Table 1 · medium confidence

Kraftblock's heat battery at Tata Steel's Jamshedpur plant, charged with waste heat from sinter cooling, has a 20 MWh capacity, stores heat at up to 500 °C, and has run for a one-year test since May 2025.

Sources: [6] · medium confidence

Proposed A design or plan that has not yet been built or tested.

Gifford and colleagues modelled a notional 135 MWe grid-electricity plant with particle thermal storage and report annual round-trip efficiencies above 50%, with a decrease of about 1.5% when storage duration rises from 10 to 100 hours.

Sources: [3] · Abstract · medium confidence

Kraftblock and Tata Steel project that the system will cut about 22,000 tonnes of CO2 a year and eliminate about 110 GWh of fossil gas use a year. These are company projections, not measured results.

Sources: [6] · low confidence

The NREL team estimates a levelised storage cost of 7 cents per kWh for a stand-alone sand system built from scratch, against a funding goal of 5 cents per kWh.

Sources: [5] · low confidence

Limitations and open problems

  • The round-trip efficiency figures are not like-for-like. Heat-in to heat-out figures above 95% (NREL, via SolarPACES) differ from the electricity-in to electricity-out figure of above 50% from modelling (Gifford and colleagues, 2024).
  • Most performance and emissions figures come from the operators or companies, including Polar Night Energy, Rondo and Kraftblock. The sources reviewed here include no independently measured, multi-year operating data.
  • NREL's 100-hour sand design is being tested only at 10-hour pilot scale with 100 kW of power, and the full-scale design rests on modelling and planning.
  • Heat-only storage needs a steam or other power block to produce electricity, which adds cost and reduces overall efficiency.
  • The cost figures are targets or estimates. NREL's 5 cents per kWh funding goal and its 7 cents per kWh estimate for a stand-alone system are not verified project costs, and the sources reviewed here publish no cost for the Tata Steel system.
  • Heat losses depend on insulation quality. NREL reports that an insulated silo loses about 1% of its heat per day, so practical storage duration depends on the insulation design.

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • Reusing retired coal power plant sites, with their existing steam equipment and grid connections, to store heat and generate electricity. NREL's Zhiwen Ma has suggested this could meet the 5 cents per kWh cost goal, but this has not been demonstrated.
  • Grid-scale electricity storage using a sand or particle store linked to a steam turbine, as in NREL's notional 135 MWe plant. This remains a modelled design.
  • Decarbonising high-temperature industrial heat in steelmaking and other heavy industries that now rely on fossil gas, using captured waste heat or surplus renewable electricity.

Product concepts: Shared underground heat store for apartment blocks

Timeline

  1. 2022Polar Night Energy launches the Kankaanpää sand battery in Finland, the smaller unit that the 2025 Pornainen system is about ten times larger than. Sources: [1]
  2. 16 Nov 2022Rondo Energy announces the Rondo Heat Battery, which stores heat in bricks at temperatures up to 1500 °C. Sources: [2]
  3. Jun 2024Gifford, Wang, Ma and Braun publish a modelling study of particle thermal storage for grid electricity in Applied Energy. Sources: [3]
  4. 18 Sept 2024NREL begins a 10-hour pilot-scale demonstration of its sand-based storage, aiming for a 100-hour design. Sources: [5]
  5. May 2025Kraftblock's 20 MWh heat battery at Tata Steel's Jamshedpur plant begins a one-year test run. Sources: [6]
  6. 11 Jun 2025Loviisan Lämpö announces that the 1 MW, 100 MWh sand battery at Pornainen, Finland, is commissioned and in operation. Sources: [1]

Futures this could help bring about

AI illustration
How far away?

Factories heated by stored renewable heat

Pilots · 3 of 8 steps done

Glossary

Sensible heat storage
Storing heat by raising the temperature of a solid or liquid, such as sand, rock or molten salt, without changing its state.
Round-trip efficiency
The share of energy recovered after a store is charged and discharged. The metric must be stated, because heat-in to heat-out and electricity-in to electricity-out give different values.
Levelised cost of storage (LCOS)
The total cost of a storage system spread over the energy it delivers over its life, usually stated per kWh.
District heating
Heating several buildings from one central heat source, which is delivered through insulated pipes.

Sources

  1. company announcement
    World's Largest Sand Battery Now in Operation
    Polar Night Energy, 11 Jun 2025. Accessed 21 Oct 2026.
  2. company announcement
    Product launch: the Rondo Heat Battery (RHB), providing the world's lowest-cost, zero-carbon industrial heat
    Rondo Energy, 16 Nov 2022. Accessed 21 Oct 2026.
  3. peer reviewed paper
    Modeling electrical particle thermal energy storage systems for long-duration, grid-electricity storage applications
    Jeffrey Gifford, Xingchao Wang, Zhiwen Ma et al.. Applied Energy, Jun 2024. DOI: 10.1016/j.apenergy.2024.123521. Accessed 21 Oct 2026.
  4. review article
    Comprehensive review of emerging trends in thermal energy storage mechanisms, materials and applications
    Collins Chike Kwasi-Effaha, Ogheneochuko Okpako. Frontiers in Energy Research, 28 Aug 2025. DOI: 10.3389/fenrg.2025.1651471. Accessed 21 Oct 2026.
  5. news
    Long-duration thermal energy storage in sand begins NREL demo
    Susan Kraemer. SolarPACES, 18 Sept 2024. Accessed 21 Oct 2026.
  6. news
    Global giant Tata Steel is using a heat battery to curb emissions
    Canary Media, 27 Feb 2026. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.

Changes on 21 Oct 2026
Published on owner instruction (content loop iteration 7)fd22377
Sources and claims re-checked against originalsfbb434c
Initial profile drafted from primary sources7e66249