Material Insights · Understanding the Material World

Sensible heat storage: Science and technology

Why the oldest storage trick needs no scarce minerals

What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.

Sensible heat stores hold energy by making a cheap, plentiful material hotter: water, rock, sand, brick, carbon or molten salt. The physics is simple and well proven, and the materials are among the least scarce of any storage technology.

Mechanism

Favourable

Energy goes in as heat and raises the temperature of a solid or liquid; the amount stored is mass times specific heat times the temperature rise. Water stores 4.2 kJ per kg per degree C, but only between about 20 and 80 C, so a tonne swung by 60 C holds about 70 kWh. Solids can run much hotter: the Pornainen sand battery in Finland keeps 100 MWh in about 2,000 t of crushed soapstone, roughly 50 kWh per tonne, and brick and carbon stores run at over 1,000 C, with Antora quoting up to 2,400 C for its carbon blocks (company figure).2691328

Specific heat, water / rock 4.2 / 0.9-1.1 kJ per kg per C13 Water, 60 C swing about 70 kWh per tonne (calculated)13 Pornainen soapstone store 100 MWh in about 2,000 t, about 50 kWh/t (calculated)2 Top storage temperature, company data Rondo up to 1,500 C; Antora above 1,800 C demonstrated (2023), up to 2,400 C quoted (2026)6928

Material scarcity and toxicity

Favourable

No critical or conflict minerals are needed. Water costs about USD 0.005 per kg and rock USD 0.02 to 0.3 per kg. Several developers use waste: Kraftblock's pellets are up to 85% recycled material, mainly steel slag, and Pornainen's soapstone is an offcut from fireplace making. Rondo says its bricks are made of oxygen, silicon and aluminium, and Antora calls its carbon blocks earth-abundant (company statements). The exception is molten nitrate salt, which is corrosive and freezes if allowed to cool.4671320

Material cost, water / rock USD 0.005 / 0.02-0.3 per kg13 Recycled content, Kraftblock pellets up to 85%7

Degradation and ageing

Favourable

There is no chemistry to wear out, so capacity barely fades. The real loss is heat leaking through insulation, which matters most for seasonal stores: in Danish pit stores, losses linked to charging and discharging range from 6% to 24% a year. Hot solids can crack or crumble under repeated heating, and salts corrode tanks. In an EU-funded test Kraftblock had run 4,700 cycles with no degradation observed, against a target life of over 30 years (10,000 to 15,000 cycles); Polar Night Energy designs for 30 or more years (company figure).3810

Annual heat loss, Danish pit stores 6-24% of heat charged and discharged10 Cycle test, Kraftblock 4,700 cycles, no degradation observed (EU project report)8 Design life over 30 years (Kraftblock target; Polar Night Energy, company figure)38

Variants compared123591012192528

VariantTypical temperatureExample size or deploymentMain use
Hot-water tanks20-80 CWidespread; cheapest mediumHomes, district heating
Seasonal pit storesup to about 90 CVojens, Denmark: 210,000 m3 of waterSummer solar heat for winter
Aquifer stores (ATES)usually below 25 Cover 2,800 systems worldwideBuilding heating and cooling
Molten saltup to 700 C targetedover 21 GWh installed (IRENA 2020)Solar thermal power plants
Sand and rock400-750 CPornainen 100 MWh (2025); Siemens Gamesa Hamburg 130 MWh pilot (2019)District heat; power pilot
Brick and carbonabove 1,000 C; up to 2,400 C quoted (company figures)Rondo 100 MWh in California (2025); Antora 5 GWh at Big Stone, South Dakota (company data, 2026)Industrial steam and heat
Building thermal massroom temperature (21-24 C band)pre-cooling cut cooling costs 5-12% on a modelled day in a US officePre-heating or pre-cooling

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. New industrial Sand Battery filled with soapstone equal to 1,000 fireplaces Polar Night Energy, 2024
  5. Rondo Energy's 100 MWh solar-powered heat battery now operational (news report of company claims) ESS News, 2025
  6. Thermal storage company Rondo plans 90GWh ramp-up of heat battery gigafactory (news report of company plans) Energy-Storage.news, 2023
  7. Kraftblock thermal energy storage from upcycled industrial by-products (ACerS Bulletin feature) American Ceramic Society Bulletin, 2023
  8. KRAFTBLOCK: scalable, modular, high temperature thermal energy and power storage (EIC Accelerator, periodic report) European Commission CORDIS, 2025
  9. Antora Energy launches ready-to-scale industrial decarbonisation technology, establishes new ultra-high temperature record Steel Technology, 2023
  10. Seasonal pit heat storage: cost benchmark of 30 EUR/m3 solarthermalworld.org, 2019
  11. High-Temperature Aquifer Thermal Energy Storage (HT-ATES) Projects in Germany and the Netherlands: Review and Lessons Learned (Dobson et al., Energies 18(23) 6292) eScholarship (LBNL), 2025
  12. 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
  13. Siemens Gamesa inaugurates electric thermal energy storage system (press release) Siemens Gamesa, 2019
  14. Antora Energy announces U.S. thermal battery manufacturing facility Business Wire, 2023
  15. Large-scale simulation-based parametric analysis of an optimal precooling strategy for demand flexibility in a commercial office building (Lu, Adetola and Bhattacharya, Energy and Buildings) PNNL / OSTI, 2024
  16. Antora Energy closes US$550 million Series C to scale US thermal battery deployment (news report of company data) Energy-Storage.news, 2026