# Thermochemical storage: Science and technology

Why chemical heat packs several times more than hot water

Source page: https://www.materialinsights.org/energy/e_thermochem_store/sg-sci/

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.

Thermochemical stores hold heat as a chemical bond: heat drives a reaction apart, and recombining the products later releases it. Densities are several times those of hot water or molten salt and standby losses are near zero, but cycle stability and reactor design keep the technology at laboratory and pilot scale.

#### Mechanism
Favourable

Heat splits a compound into two parts that are kept apart: a salt and water vapour, a metal oxide and oxygen, or lime (CaO) and CO2 in calcium looping. Nothing leaks while they are separate, so heat can be held for months, and recombining them releases it. Calcium looping absorbs 178 kJ per mole of limestone, calcining at 920 to 950 C and giving heat back at 600 to 850 C, for about 390 to 490 kWh per tonne of CaCO3. Cobalt oxide stores about 850 kJ per kg, against 155 kJ per kg for molten salt used as sensible heat. Salt hydrates for homes are screened to release heat above about 50 to 65 C and recharge below about 100 to 120 C: potassium carbonate stores about 1.3 GJ per m3 (about 360 kWh per m3, calculated) at material level.^[3](https://pure.tue.nl/ws/portalfiles/portal/65069212/1_s2.0_S0306261917304750_main.pdf)^[4](https://zaguan.unizar.es/record/120012/files/texto_completo.pdf)^[5](https://www.energy.gov/eere/solar/articles/lessons-learned-devolping-thermochemical-cycles-solar-heat-storage-applications)^[6](https://pubs.rsc.org/en/content/articlehtml/2021/ra/d1ra05472d)
 Calcium looping, reaction heat and density 178 kJ/mol; 390-490 kWh per tonne CaCO3; about 1.2-1.4 GJ/m3 after 10-20 cycles^[4](https://zaguan.unizar.es/record/120012/files/texto_completo.pdf) Cobalt oxide vs molten salt (sensible) 850 vs 155 kJ/kg^[5](https://www.energy.gov/eere/solar/articles/lessons-learned-devolping-thermochemical-cycles-solar-heat-storage-applications) Manganese oxide (Mn2O3/Mn3O4) 202 kJ/kg; charge above 950 C^[6](https://pubs.rsc.org/en/content/articlehtml/2021/ra/d1ra05472d) Salt hydrates K2CO3 / MgCl2, material level 1.30 / 1.93 GJ/m3 (open system)^[3](https://pure.tue.nl/ws/portalfiles/portal/65069212/1_s2.0_S0306261917304750_main.pdf)

#### Material scarcity and toxicity
Mixed

Limestone, manganese and potassium carbonate are abundant and cheap. Cobalt oxide performed best in early US tests but cobalt is a critical mineral, which is why research moved to manganese-iron oxides. Some salts cause problems: magnesium chloride can release corrosive hydrogen chloride above about 140 C and absorbs water until it dissolves, and calcium chloride hydrates cannot be used above about 40 C because they melt. Spent lime is benign.^[3](https://pure.tue.nl/ws/portalfiles/portal/65069212/1_s2.0_S0306261917304750_main.pdf)^[5](https://www.energy.gov/eere/solar/articles/lessons-learned-devolping-thermochemical-cycles-solar-heat-storage-applications)^[6](https://pubs.rsc.org/en/content/articlehtml/2021/ra/d1ra05472d)
 Re-oxidation of manganese oxide, undoped vs 10% iron oxide 6% vs 100%^[5](https://www.energy.gov/eere/solar/articles/lessons-learned-devolping-thermochemical-cycles-solar-heat-storage-applications)

#### Degradation and ageing
Unfavourable

This is the central weakness. Natural limestone sinters and loses activity quickly: one model uses a residual conversion of only 7.5%, so most of the lime stops reacting after many cycles, and magnesium-based stabilisers are being tested. Titanium-doped manganese oxide samples lost 53% to 63% of their oxygen uptake (measured as weight gain) over 30 cycles, and only cobalt oxide showed full, repeatable re-oxidation in US tests. Salt hydrates fare better in some cases: the CREATE project cycled potassium carbonate to simulate 20 years of seasonal use with no signs of degradation.^[4](https://zaguan.unizar.es/record/120012/files/texto_completo.pdf)^[5](https://www.energy.gov/eere/solar/articles/lessons-learned-devolping-thermochemical-cycles-solar-heat-storage-applications)^[6](https://pubs.rsc.org/en/content/articlehtml/2021/ra/d1ra05472d)^[7](https://iea-shc.org/article?NewsID=304)^[8](https://dspace.lib.cranfield.ac.uk/items/131ba7f1-7771-49db-b724-d0ba9ea4b689/full)
 Calcium looping, residual CaO conversion (model input) 7.5%^[4](https://zaguan.unizar.es/record/120012/files/texto_completo.pdf) Mn2O3 doped with TiO2, loss after 30 cycles 53-63% attenuation^[6](https://pubs.rsc.org/en/content/articlehtml/2021/ra/d1ra05472d) K2CO3 (CREATE) cycles simulating 20 years, no degradation seen^[7](https://iea-shc.org/article?NewsID=304)

#### Variants compared^[2](https://www.mdpi.com/1996-1073/15/2/647)^[3](https://pure.tue.nl/ws/portalfiles/portal/65069212/1_s2.0_S0306261917304750_main.pdf)^[4](https://zaguan.unizar.es/record/120012/files/texto_completo.pdf)^[5](https://www.energy.gov/eere/solar/articles/lessons-learned-devolping-thermochemical-cycles-solar-heat-storage-applications)^[7](https://iea-shc.org/article?NewsID=304)

| Variant | Working temperature | Energy density | Status and example |
|---|---|---|---|
| **Salt hydrates (K2CO3, MgCl2, SrBr2)** | heat out above about 50-65 C; recharge below about 100-120 C | about 1.0-1.9 GJ/m3 material (K2CO3 1.3) | Pilot: CREATE, 182 kWh in Warsaw (2020); SaltX 0.5 MW / 10 MWh in Berlin |
| **Metal oxides (Co3O4, Mn2O3, BaO2, CuO)** | about 700-1,030 C | Co3O4 about 850 kJ/kg; Mn2O3 202 kJ/kg | Laboratory and small reactors |
| **Calcium looping (CaO/CaCO3)** | calcine 920-950 C, carbonate 600-850 C | 390-490 kWh/t; about 1.3 GJ/m3 | Laboratory and pilot scale |
| **Metal hydrides (related)** | high temperature | not given | TEXEL, about USD 15 per kWh heat (company claim) |

#### Sources

1. [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
2. [A review of salt hydrates for seasonal heat storage in domestic applications (Donkers et al., Applied Energy 199, 45-68)](https://pure.tue.nl/ws/portalfiles/portal/65069212/1_s2.0_S0306261917304750_main.pdf) Eindhoven University of Technology repository, 2017
3. [Operation maps in calcium looping thermochemical energy storage for concentrating solar power plants (Pascual, Lisbona and Romeo, Journal of Energy Storage 55, 105771)](https://zaguan.unizar.es/record/120012/files/texto_completo.pdf) University of Zaragoza repository, 2022
4. [Lessons Learned: Developing Thermochemical Cycles for Solar Heat Storage Applications (Wong, General Atomics; conference slides, DOE Workshop on TES for CSP, January 2013)](https://www.energy.gov/eere/solar/articles/lessons-learned-devolping-thermochemical-cycles-solar-heat-storage-applications) US Department of Energy, 2013
5. [Effects of TiO2 doping on the performance of thermochemical energy storage based on Mn2O3/Mn3O4 redox materials (Wang, Wang, Dou, Ma and Liang, RSC Advances 2021)](https://pubs.rsc.org/en/content/articlehtml/2021/ra/d1ra05472d) Royal Society of Chemistry, 2021
6. [Compact thermochemical heat storage prototype (CREATE project, IEA SHC Task 58)](https://iea-shc.org/article?NewsID=304) IEA Solar Heating and Cooling Programme, 2020
7. [Developments in calcium/chemical looping and metal oxide redox cycles for high-temperature thermochemical energy storage: A review (Yan et al., Fuel Processing Technology 199, 106280)](https://dspace.lib.cranfield.ac.uk/items/131ba7f1-7771-49db-b724-d0ba9ea4b689/full) Cranfield University repository, 2020

## Other sections

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