Thermochemical storage materials are substances that undergo reversible chemical reactions, converting between different chemical states during the charging and discharging phases of the energy storage process. These materials store and release thermal energy through endothermic and exothermic reactions, respectively. Various types of thermochemical storage materials have been studied for their potential in high-density and long-duration energy storage. Some of the different types of thermochemical storage materials include:
1. Metal Hydrides:
- Metal hydrides involve the reversible reaction between hydrogen gas (H2) and metal elements, forming hydrides during charging and releasing hydrogen gas during discharging.
- Common metal hydrides used for thermochemical energy storage include magnesium hydride (MgH2), sodium aluminum hydride (NaAlH4), and lithium hydride (LiH).
2. Metal Oxides and Carbonates:
- Metal oxides and carbonates undergo reversible reactions with gases, such as carbon dioxide (CO2) or water vapor (H2O), to form solid products during charging and release the gases during discharging.
- Examples include the reaction between calcium oxide (CaO) and CO2 to form calcium carbonate (CaCO3) during charging, and the reverse reaction during discharging.
3. Metal Sulfides and Sulfates:
- Metal sulfides and sulfates can undergo reversible reactions with hydrogen sulfide (H2S) gas or other sulfur-containing compounds, storing energy in the form of solid sulfur compounds during charging and releasing the sulfur compounds during discharging.
- For example, the reaction between zinc oxide (ZnO) and H2S can form zinc sulfide (ZnS) during charging, and the reverse reaction during discharging.
4. Metal Halides:
- Metal halides, such as metal chlorides, bromides, or iodides, can participate in reversible reactions with halogen gases, like chlorine (Cl2), bromine (Br2), or iodine (I2).
- These reactions can store energy in the form of metal halides during charging and release the halogen gases during discharging.
5. Ammonia-Based Systems:
- Ammonia (NH3) can be used as a thermochemical storage medium, undergoing reversible reactions with nitrogen (N2) gas and hydrogen (H2) gas.
- The dissociation of ammonia into N2 and H2 is an endothermic reaction during charging, and the reformation of ammonia during discharging releases the stored energy.
6. Redox Couples:
- Redox (reduction-oxidation) couples involve reversible reactions between different oxidation states of a chemical element, often in the form of metal oxides and their reduced metallic forms.
- Examples include iron (Fe) and iron oxide (Fe3O4) or cerium (Ce) and cerium oxide (CeO2) redox pairs.
Thermochemical storage materials offer the advantage of high energy density and long-term storage capabilities, making them attractive for large-scale energy storage and renewable energy integration. However, their implementation often requires careful engineering to address issues such as reaction kinetics, thermal management, and material stability. Research and development efforts continue to explore and optimize these materials for practical and efficient thermochemical energy storage systems.