Modification of CaO/Ca(OH)2 for Thermochemical Energy Storage

Efficient storage of thermal energy can contribute significantly to a sustainable and cost-effective energy solution. Thermochemical storage, where heat is stored as reaction enthalpy of a reversible chemical reaction, has great potential as an economic option for thermal energy store. This is especially due to the high specific storage densities associated with the chemical reaction. Suitable thermochemical storage materials are characterized by high reaction enthalpies, fast dynamics as well as cycling stability. Furthermore, sufficient heat and mass transfer through the reaction bed and within the particles is crucial. A currently investigated reaction system for application at temperatures between 400 and 600 °C is the CaO/Ca(OH)2 system, where calcium oxide (CaO) reacts in an exothermic reaction with water vapour (H2O) to calcium hydroxide (Ca(OH)2). This system has already been studied in stationary bench- and pilot-scale reactors. In order to improve the storage’s efficiency a moving bed reactor is proposed. However, due to the very fine particle size, the material is a Geldart C powder and therefore difficult to move. The aim of this work is therefore to increase the particle size in order to enable the movement of the storage material without negatively affecting the material properties. To this end, different material modifications such as pelletisation, granulation of the storage material within a matrix and encapsulation with porous materials are conducted. In this contribution, the effects on the reaction behaviour are discussed. Based on important criteria for the thermochemical heat storage such as cycling stability, potential side reactions as well as impacts on the reactivity, a practical approach for further material optimization is derived.