Improvement of a semipermeable shell for encapsulation of calcium hydroxide for thermochemical heat storage solutions: Material design and evaluation in laboratory and reactor scale

Abstract

Thermochemical heat storage concepts offer a promising contribution to an economic, efficient and sustainable future energy supply. The reaction system CaO/Ca(OH)2 is amongst the most considered systems for Concentrated Solar Power (CSP) applications, but as the cost efficiency and good availability of the material are accompanied by poor powder properties, complex and costly reactor solutions are required. Hence, modifications of the storage material promise a option for a reliable long-term operation. The present work discusses considerations on inter- and intraparticular changes of the storage material and options for particle size stabilization. With the background of a previous work, the experimental section presents a successful approach for the enhancement of the mechanical stability of a ceramic shell material by admixing selected additives to the powdery precursor. To assign for possible complex, mutual interactions of the additives, the approach is based on a full factorial design of experiments. Investigations of the microstructure of the material are performed by gas-adsorption and mercury intrusion measurements and enriched by combining light- and scanning electron microscopy on respective samples. The obtained mechanical stability of the selected material is accompanied by a significantly enhanced thermal conductivity. Cycling stability is proven over ten reaction cycles in laboratory and reactor scale. Due to a minimization of the contact area between the storage material core and its stabilizing shell by utilizing spherical particles, it is demonstrated that only a minor formation of thermochemically inert side product is detectable.