Alfonso J. Carrillo

Thermochemical heat storage based on the Mn2O3/Mn3O4 redox couple: influence of the initial particle size on the morphological evolution and cyclability

Thermochemical energy storage (TCS) based on reduction–oxidation cycles of multivalent metal oxides is of great interest for concentrating solar facilities, as it can allow enhancing the global plant efficiency and improving the energy generation dispatchability. However, to guarantee the feasibility of the process, selected materials should present long term durability, which requires the evaluation of the redox couple cyclability. In this work we have demonstrated, for the first time, the suitability of the Mn2O3/Mn3O4 pair for this application during 30 cycles, performed in thermobalance. Nevertheless, an appropriate design of such materials is crucial since it has been found that initial particle size influences the redox behaviour of these oxides. Results showed a 2-fold influence of particle size of the as-prepared materials on the redox reversibility. Firstly, this parameter affects to the thermodynamics and kinetics of the redox reactions. Namely a decrease of the particle size shifted the oxidation temperature to lower values and produced slower reduction and oxidation reactions. Secondly, depending on the particles size, samples followed different sintering mechanisms. This fact influenced dramatically the behaviour of the materials with lower particle size, which suffered from a higher degree of densification that eventually caused a total loss of cyclability.

Improving the Thermochemical Energy Storage Performance of the Mn2O3/Mn3O4 Redox Couple by the Incorporation of Iron

Redox cycles of manganese oxides (Mn2 O3 /Mn3 O4 ) are a promising alternative for thermochemical heat storage systems coupled to concentrated solar power plants as manganese oxides are abundant and inexpensive materials. Although their cyclability for such a purpose has been proved, sintering processes, related to the high-temperature conditions at which charge-discharge cycles are performed, generally cause a cycle-to-cycle decrease in the oxidation rate of Mn3 O4 . To guarantee proper operation, both reactions should present stable reaction rates. In this study, it has been demonstrated that the incorporation of Fe, which is also an abundant material, into the manganese oxides improves the redox performance of this system by increasing the heat storage density, narrowing the redox thermal hysteresis, and, above all, stabilizing and enhancing the oxidation rate over long-term operation, which counteracts the negative effects caused by sintering, although its presence is not avoided.

Design of efficient Mn-based redox materials for thermochemical heat storage at high temperatures

Mn-based oxides are promising materials for thermochemical heat storage based on redox cycles, since they are abundant materials whose reduction and oxidation reactions take place in the temperature range at which future CSP plants will work. However, sintering processes related to high temperature cycling can lead to a complete material deactivation that eventually will suppose the loss of cyclability. In this work we present two approaches that have been proposed as to overcome such deactivation. In this respect morphological and chemical modifications were studied. Results showed that even if the first cycle oxidation is enhanced by the presence of macroporosity, sintering also affects to that structures causing a decrease on the oxidation rate. Conversely, chemical modifications, namely addition of cations of Cr and Fe can stabilize the oxidation rate over long term cycling. Specially, by incorporating Fe to the Mn oxide structure the oxidation reaction is remarkably stabilized and improved.