# Sensible heat storage: Engineering and performance

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

Source page: https://www.materialinsights.org/energy/e_sensible_heat/sg-eng/

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.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2020/Nov/IRENA_Innovation_Outlook_TES_2020.pdf)^[3](https://polarnightenergy.com/reference/solution-for-clean-energys-big-problem/)^[5](https://www.ess-news.com/2025/10/20/rondo-energys-100-mwh-solar-powered-heat-battery-now-operational/)^[10](https://www.solarthermalworld.org/news/seasonal-pit-heat-storage-cost-benchmark-30-eurm3)^[21](https://www.mdpi.com/1996-1073/15/2/647)^[23](https://energyinnovation.org/wp-content/uploads/2023/07/Industrial-Thermal-Batteries-Report-1.pdf)
 Brick store, heat out (company claim) above 97%^[5](https://www.ess-news.com/2025/10/20/rondo-energys-100-mwh-solar-powered-heat-battery-now-operational/) Molten salt, expected over 92%^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2020/Nov/IRENA_Innovation_Outlook_TES_2020.pdf) Sand battery (company figure) about 80%^[3](https://polarnightenergy.com/reference/solution-for-clean-energys-big-problem/) Industrial thermal battery, assumed / standby loss about 95%; under 1% a day^[23](https://energyinnovation.org/wp-content/uploads/2023/07/Industrial-Thermal-Batteries-Report-1.pdf) Rock store, electricity out about 25% today; over 42% expected^[21](https://www.mdpi.com/1996-1073/15/2/647)

#### 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.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2020/Nov/IRENA_Innovation_Outlook_TES_2020.pdf)^[2](https://polarnightenergy.com/news/worlds-largest-sand-battery-now-in-operation/)^[7](https://ceramics.org/wp-content/bulletin/2023/pdf/articles/F-kraftblock_mar23_hi-res.pdf)^[10](https://www.solarthermalworld.org/news/seasonal-pit-heat-storage-cost-benchmark-30-eurm3)^[16](https://www.ethree.com/wp-content/uploads/2024/10/CAELP-E3-Industrial-Electrification-Report.pdf)
 Pornainen power to energy 1 MW / 100 MWh, about 100 hours (calculated)^[2](https://polarnightenergy.com/news/worlds-largest-sand-battery-now-in-operation/) Molten salt storage cost USD 26.1-40 per kWh heat (2020)^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2020/Nov/IRENA_Innovation_Outlook_TES_2020.pdf) Pit store benchmark about EUR 30 per m3 at 100,000 m3 or more^[10](https://www.solarthermalworld.org/news/seasonal-pit-heat-storage-cost-benchmark-30-eurm3) Modelled US industrial store 36 hours; charging 3.5 times output^[16](https://www.ethree.com/wp-content/uploads/2024/10/CAELP-E3-Industrial-Electrification-Report.pdf)

#### 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.^[2](https://polarnightenergy.com/news/worlds-largest-sand-battery-now-in-operation/)^[7](https://ceramics.org/wp-content/bulletin/2023/pdf/articles/F-kraftblock_mar23_hi-res.pdf)^[10](https://www.solarthermalworld.org/news/seasonal-pit-heat-storage-cost-benchmark-30-eurm3)^[11](https://speichercity.geo.uni-halle.de/files/2025/03/20240219_SpeicherCity_GeoTHERM-Workshop_ATES-for-Germany_04_Godschalk_35-years-Dutch-experience-on-ATES.pdf)^[21](https://www.mdpi.com/1996-1073/15/2/647)
 Pornainen dimensions about 13 m tall, 15 m wide^[2](https://polarnightenergy.com/news/worlds-largest-sand-battery-now-in-operation/) Energy density, Kraftblock / Siemens Gamesa rock up to 1.2 / about 0.2 MWh per m3^[7](https://ceramics.org/wp-content/bulletin/2023/pdf/articles/F-kraftblock_mar23_hi-res.pdf)^[21](https://www.mdpi.com/1996-1073/15/2/647) Dutch ATES well depth 40-250 m^[11](https://speichercity.geo.uni-halle.de/files/2025/03/20240219_SpeicherCity_GeoTHERM-Workshop_ATES-for-Germany_04_Godschalk_35-years-Dutch-experience-on-ATES.pdf)

#### 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.^[9](https://www.steel-technology.com/news/antora-energy-launches-ready-to-scale-industrial-decarbonization-technology-establishes-new-ultra-high-temperature-record)^[13](https://nso-journal.org/articles/nso/full_html/2024/03/NSO20230051/T2.html)
 Carbon block stability (company data) solid up to 3,500 C^[9](https://www.steel-technology.com/news/antora-energy-launches-ready-to-scale-industrial-decarbonization-technology-establishes-new-ultra-high-temperature-record)

#### Sources

1. [Innovation Outlook: Thermal Energy Storage](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2020/Nov/IRENA_Innovation_Outlook_TES_2020.pdf) IRENA, 2020
2. [World's largest Sand Battery now in operation (Pornainen press release, company data)](https://polarnightenergy.com/news/worlds-largest-sand-battery-now-in-operation/) Polar Night Energy, 2025
3. [Solution for clean energy's big problem (Pornainen reference, company data)](https://polarnightenergy.com/reference/solution-for-clean-energys-big-problem/) Polar Night Energy, 2025
4. [Rondo Energy's 100 MWh solar-powered heat battery now operational (news report of company claims)](https://www.ess-news.com/2025/10/20/rondo-energys-100-mwh-solar-powered-heat-battery-now-operational/) ESS News, 2025
5. [Kraftblock thermal energy storage from upcycled industrial by-products (ACerS Bulletin feature)](https://ceramics.org/wp-content/bulletin/2023/pdf/articles/F-kraftblock_mar23_hi-res.pdf) American Ceramic Society Bulletin, 2023
6. [Antora Energy launches ready-to-scale industrial decarbonisation technology, establishes new ultra-high temperature record](https://www.steel-technology.com/news/antora-energy-launches-ready-to-scale-industrial-decarbonization-technology-establishes-new-ultra-high-temperature-record) Steel Technology, 2023
7. [Seasonal pit heat storage: cost benchmark of 30 EUR/m3](https://www.solarthermalworld.org/news/seasonal-pit-heat-storage-cost-benchmark-30-eurm3) solarthermalworld.org, 2019
8. [35 years of Dutch experience on ATES (Godschalk, SpeicherCity GeoTHERM workshop)](https://speichercity.geo.uni-halle.de/files/2025/03/20240219_SpeicherCity_GeoTHERM-Workshop_ATES-for-Germany_04_Godschalk_35-years-Dutch-experience-on-ATES.pdf) 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))](https://nso-journal.org/articles/nso/full_html/2024/03/NSO20230051/T2.html) National Science Open, 2024
10. [Industrial Electrification in the United States (CAELP report)](https://www.ethree.com/wp-content/uploads/2024/10/CAELP-E3-Industrial-Electrification-Report.pdf) E3, 2024
11. [Review of Carnot Battery Technology Commercial Development (Novotny et al., Energies 15(2), 647)](https://www.mdpi.com/1996-1073/15/2/647) MDPI, 2022
12. [Industrial Thermal Batteries: Decarbonizing U.S. Industry While Supporting a High-Renewables Grid (Rissman)](https://energyinnovation.org/wp-content/uploads/2023/07/Industrial-Thermal-Batteries-Report-1.pdf) Energy Innovation, 2023

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_sensible_heat/index.md)
- [Overview](https://www.materialinsights.org/energy/e_sensible_heat/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_sensible_heat/correlation/index.md)
- [Science and technology](https://www.materialinsights.org/energy/e_sensible_heat/sg-sci/index.md)
- [Economics](https://www.materialinsights.org/energy/e_sensible_heat/sg-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_sensible_heat/sg-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_sensible_heat/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_sensible_heat/links/index.md)