Material Insights · Understanding the Material World

Sensible heat storage: Engineering and performance

Brilliant heat-to-heat, poor heat-to-power: the key trade-off

What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.

As a heat-in, heat-out device a sensible store is very efficient and its power and energy can be sized separately. Turning the heat back into electricity is a different story, with much lower efficiency.

Round-trip efficiency

Favourable

Heat-to-heat efficiency is high because the only loss is leakage. Rondo claims above 97% for its 100 MWh unit (company claim, as reported), IRENA expects molten salt to exceed 92% and tank or solid stores to reach about 90%, and the Dronninglund pit store reached over 90% storage efficiency. Polar Night Energy quotes about 80% for its sand battery (company figure), and an Energy Innovation analysis assumes about 95% with heat losses below 1% a day for some designs. If heat is turned back into electricity through a steam turbine, efficiency falls sharply: Siemens Gamesa's rock store ran at about 25%, with over 42% hoped for at larger scale.135102123

Brick store, heat out (company claim) above 97%5 Molten salt, expected over 92%1 Sand battery (company figure) about 80%3 Industrial thermal battery, assumed / standby loss about 95%; under 1% a day23 Rock store, electricity out about 25% today; over 42% expected21

Power and energy decoupling

Favourable

Charging power is set by heaters and blowers and energy by the mass of material, so the two can be sized separately. Pornainen pairs 1 MW of output with 100 MWh, about 100 hours (calculated), while a 10-foot Kraftblock module holds about 4.2 MWh at 1 MW. Extra energy is cheap: molten salt storage costs USD 26.1 to 40 per kWh of heat and large Danish pit stores about EUR 30 per m3 of water. A US study sized stores to deliver 36 hours of heat with charging capacity 3.5 times the output, to soak up wind and solar hours.1271016

Pornainen power to energy 1 MW / 100 MWh, about 100 hours (calculated)2 Molten salt storage cost USD 26.1-40 per kWh heat (2020)1 Pit store benchmark about EUR 30 per m3 at 100,000 m3 or more10 Modelled US industrial store 36 hours; charging 3.5 times output16

Footprint and siting flexibility

Mixed

Solid stores are compact and can go next to the user: Pornainen is about 13 m tall and 15 m wide, and Kraftblock's material holds up to 1.2 MWh per m3. Siemens Gamesa's rock bed held only about 0.2 MWh per m3. Water stores are bulky: Vojens needed 210,000 m3, so pit stores suit rural district heating. Aquifer stores need suitable groundwater layers, with Dutch wells 40 to 250 m deep, and building mass is fixed by the building itself.27101121

Pornainen dimensions about 13 m tall, 15 m wide2 Energy density, Kraftblock / Siemens Gamesa rock up to 1.2 / about 0.2 MWh per m3721 Dutch ATES well depth 40-250 m11

Safety and thermal runaway risk

Favourable

There is no thermal runaway or toxic off-gas of the kind that drives lithium battery codes. The hazards are ordinary industrial ones: very hot surfaces, pressurised steam, hot air ducts and, for molten salt, a corrosive and oxidising liquid. Antora notes that carbon is used in fireproofing and is non-flammable. Such stores are governed mainly by boiler, pressure-equipment and building codes rather than by battery fire standards such as NFPA 855.913

Carbon block stability (company data) solid up to 3,500 C9

Sources

  1. Innovation Outlook: Thermal Energy Storage IRENA, 2020
  2. World's largest Sand Battery now in operation (Pornainen press release, company data) Polar Night Energy, 2025
  3. Solution for clean energy's big problem (Pornainen reference, company data) Polar Night Energy, 2025
  4. Rondo Energy's 100 MWh solar-powered heat battery now operational (news report of company claims) ESS News, 2025
  5. Kraftblock thermal energy storage from upcycled industrial by-products (ACerS Bulletin feature) American Ceramic Society Bulletin, 2023
  6. Antora Energy launches ready-to-scale industrial decarbonisation technology, establishes new ultra-high temperature record Steel Technology, 2023
  7. Seasonal pit heat storage: cost benchmark of 30 EUR/m3 solarthermalworld.org, 2019
  8. 35 years of Dutch experience on ATES (Godschalk, SpeicherCity GeoTHERM workshop) University of Halle / SpeicherCity, 2024
  9. Study on the hybrid energy storage for industrial park energy systems, Table 2: thermal storage materials (National Science Open 3(3)) National Science Open, 2024
  10. Industrial Electrification in the United States (CAELP report) E3, 2024
  11. Review of Carnot Battery Technology Commercial Development (Novotny et al., Energies 15(2), 647) MDPI, 2022
  12. Industrial Thermal Batteries: Decarbonizing U.S. Industry While Supporting a High-Renewables Grid (Rissman) Energy Innovation, 2023