Investigations on thermochemical energy storage based on technical grade manganese-iron oxide in a lab-scale packed bed reactor

- Thermochemical energy storage using granular manganese-iron oxide of technical grade.
- Demonstration of storage concept feasibility in open-loop operation with air as HTF.
- Development of characteristic temperature profiles in a lab-scale packed bed reactor.
- Experimental parametric study of influencing operating factors.
- Discovery of strong redox reaction sensitivity to changes of local O2 concentration and temperature.

Thermochemical energy storage (TCS) based on gas-solid reactions constitutes a promising concept to develop efficient storage solutions with higher energy densities compared to widely investigated sensible and latent thermal energy storage systems. Specifically for high temperature applications multivalent metal oxides represent an interesting storage material, undergoing a reversible redox reaction with oxygen. Due to the inherently high working temperatures such a TCS system could potentially be implemented in future generation concentrated solar power (CSP) plants with central receiver technology, in order to increase the total plant efficiency and ensure the dispatchability of power generation.In this work an experimental test rig with a lab-scale tube reactor has been developed to analyze a packed bed of granular manganese-iron oxide storage material regarding heat and mass transport effects coupled with the chemical reaction. For this purpose manganese-iron oxide with a Fe/Mn molar ratio of 1:3 has been selected as a suitable reference material, which can be prepared from abundant, economical and nonhazardous raw materials. Consequently, in the context of this work the TCS technology is systematically approached based on the reference metal oxide in the temperature range between 800 °C and 1040 °C in order to derive the main influencing aspects of this storage concept.Experimental results showed the development of characteristic temperature profiles along the bed height, which proved to be dependent on the thermodynamic properties as well as kinetic behavior of the redox reaction. It was demonstrated that bed temperatures could be stabilized due to the proceeding redox reaction in dynamic charging and discharging operation modes. Parametric studies have been carried out to examine the influence of different operating parameters on thermal charging and discharging and to analyze the main limitations affecting the reaction progress. Finally, cycling experiments of the material in the lab-scale reactor exhibited no reactivity degradation over 17 cycles, verifying the comparability of the experimental results obtained from the conducted parametric studies. Analysis and comparison of the raw and cycled material, however, indicated signs of material alterations due to sintering processes.