Experimental Analysis of Kinetics and Cyclic Performance of Cobalt Oxide Powder As Redox Reactant Agent for High-Temperature Thermochemical Energy Storage

Abstract

For continuous operation of Concentrated Solar Power (CSP) Plants it is necessary to integrate thermal energy storage module. High-density energy storage system at a high temperature is required for the new generation of large scale CSP plants. The Thermochemical Energy Storage (TCES) systems use the enthalpy of formation of a reversible chemical reaction for energy storage and release. Gas/solid reduction-oxidation (redox) reactions of solid metal oxides using air as heat transfer fluid (HTF) can be directly integrated with air operated CSP plants, and there is no need for HTF storage and any intermediate heat exchanger. A new generation of large scale CSP plants uses high-temperature solar collectors to increase power cycle efficiency. Such operating conditions require the development of suitable high-temperature TCES systems. The selection of suitable metal oxide reactant is very critical in the design of such high-temperature storage systems and requires a detailed study of the physics of reaction within the reactor. Cobalt oxide (Co3O4/CoO) has been verified to have a high reaction temperature, high enthalpy of reaction together with reasonable cyclic and thermal stability. Unique features of cobalt oxide require more fundamental study of the physics behind the redox reaction and its cyclic performance. Study of the physics of materials during the storage/release cycle is necessary for the design and improvement of the reactor and can be used as a benchmark for comparison of any implemented changes. A high precision, true differential TGA/DSC instrument is used for simultaneous measurement of weightchange (TGA) and true differential heat flow (DSC) for pure cobalt oxide (Co3O4) powder. Storage cycle (charge/discharge) was conducted for five cycles. Complete re-oxidation was achieved within reasonable times by performing the two reactions at close temperatures and controlling heating/cooling rates. Basic performance parameters were derived as a benchmark for future references. Single-cycle controlling parameters such as heating/cooling rate, dwelling time, and purge gas rate were investigated. System response for few initial cycles was studied. It was shown that pure cobalt oxide could regain weight and complete re-oxidation with reasonable stability. A transition for heat flow was detected after a few initial cycles which reduced discharge heat and decreased overall performance.