Real-time visualization and experimental analysis of stabilized Ca(OH)2 granules for thermal energy storage

Abstract

The reversible reaction Ca(OH)₂ + 104.4 kJ/mol ⇌ CaO + H₂O offers several advantages as energy storage system. For example, it possesses a higher energy density than lead-acid or nickel–cadmium batteries. In addition, it has proven cyclability, low cost and worldwide availability. For this reason, it is suitable for seasonal heat storage applications. However, the raw powder material displays properties that represent a major challenge for the design of reactors that decouple power from capacity e.g. low thermal conductivity, cohesivity and tendency to form agglomerates. In order to overcome these drawbacks, different approaches to stabilize the Ca(OH)₂/CaO particles have been investigated e.g. shaping, micro encapsulation, macroencapsulation, etc. The assessment of the stabilized products, however, has limitations in terms of amount of mass and reaction conditions as it is carried out in TA (thermal analyzer). Furthermore, it does not allow to analyze the structural decay and agglomeration of stabilized particles in-situ. For this reason, a more comprehensive assessment of a bulk of material under reactor conditions is necessary. In this work, a reaction chamber is developed to enable the observation of a bulk of storage material (0.1 L) during thermal cycling. Thus, two samples were subject of 20 cycles of dehydration and rehydration. The experiments were carried out at a temperature range of 350 °C – 500 °C and water vapor pressure of 0 – 1 bar. The analysis of images and post experiment tests (e.g. thermogravimetric analysis (TGA), dynamometry, X-ray diffraction (XRD) analysis) prove that the mechanical strength and structural integrity resulted enhanced for both samples. In addition, it allows to further understand the bulk behavior of Ca(OH)₂ granules and thus important implications for the design of technical scale reactors can be derived.