Cobalt oxide based structured bodies as redox thermochemical heat storage medium for future CSP plants

Abstract

The present work is an investigation of the redox performance of several cobalt oxide based compositions, as candidate materials for energy storage in future concentrated solar power plants. To this respect, various commercial and in-house synthesized grades were evaluated in the form of small structured perforated monolithic bodies (flow-through pellets) and assessed in terms of their capability to perform reversible cyclic reduction–oxidation reactions under air flow in the temperature range of 800–1000°C. The compositions studied involved pure cobalt oxide as well as composites of cobalt oxide with ceria, zirconia, alumina, iron oxide, silicon carbide and manganese oxide. The main criterion for the evaluation of compositions considered was a combination of high redox reaction extent with good thermo-mechanical stability of fabricated structured bodies. Among the materials studied and based on this criterion, the most promising ones were the cobalt oxide–alumina and cobalt oxide–iron oxide composites. Although pure cobalt oxide, and especially one grade synthesized in the lab, exhibited the highest redox performance, the respective shaped structures did not manage to retain their macro-structural integrity in the course of 10 redox cycles. Moreover, it was found that, under certain conditions, the addition of ceria improved redox reaction kinetics, while total performance of cobalt oxide was not affected. However, the structural stability of cobalt oxide–ceria pellets was also problematic. It was also demonstrated that by varying the second oxide, the start-of-reduction/oxidation temperatures of cobalt oxide can be significantly altered. A preliminary simplified kinetic model was developed and its good agreement with pure cobalt oxide redox experimental data was also demonstrated. Post-characterization of used structured bodies confirmed the experimental findings of redox performance measurements and, to some extent, provided explanations regarding the main phenomena involved upon cyclic operation of different compositions employed.