Thermochemical heat storage at high temperature

Implementation of cost-effective thermal energy storage systems is one of the signature advantages of concentrating solar power (CSP) plants. Currently these components are based on sensible heat storage in molten salts, but those compounds start to decompose below 600 °C. Accordingly, more stable storage media are required for future more efficient CSP plants, which are expected to operate at temperatures exceeding 1000 °C. This has prompted an active investigation on materials and reactors for thermochemical storage because, those systems can achieve higher energy densities than sensible heat media and they able to operate under harsher conditions. Alkaline-earth carbonates and redox oxides are the most relevant types of materials currently under development. Carbonates, such those based in Ca or natural mineral like dolomites, have been used for CO2 capture and they present relatively high energy densities. In the case of oxides, one important advantage is that they can use atmospheric air. Besides, these materials present a great variety of compositions that can be adapted to different operation conditions. Stoichiometric oxides such Co3O4 and Mn2O3 show promising thermodynamic properties. More recently nonstoichiometric oxides, particularly perovskites with general formula ABO3, are gaining interest because their good reversibility and quick response in a broad interval of conditions. Reactor design for thermochemical storage depends on whether the configuration is intended for direct or indirect solar heating, and it must be adapted to the specific solid-gas reaction, enabling an efficient heat exchange. The latest developments in materials and reactors for thermochemical energy storage are reviewed in detail in this chapter.