This PhD thesis presents an in-depth characterization of the magnesium manganese oxide redox system for energy storage applications. The study is divided into three main parts. Each one of them explores the features of the energy storage material at increasing length scales: starting from the pellet-scale (on the order of millimeters), then moving to the packed-bed scale (on the order of centimeters), and finally reaching the reactor-scale (on the order of meters), in which the energy storage concept is demonstrated in an experimental reactor.The first part of the study deals with the experimental characterization and modeling of the magnesium manganese oxide redox system thermodynamics. The test sample is an individual cylindrical pellet in the 1:1 magnesium-to-manganese molar ratio composition. Its extent of oxidation is measured via a series of thermogravimetric experiments conducted at temperatures between 1000 and 1500 °C and oxygen partial pressures between 0.01 and 0.9 bar(a). The experimental results are used to develop two thermodynamic models that accurately predict the behavior of the redox system within these temperature and oxygen partial pressure ranges. Furthermore, these models allow to improve the material characterization by providing estimates on the average enthalpy and entropy of reaction. This study provides the minimum theoretical knowledge needed to develop computational models to predict and optimize the operation of such energy storage material whenintegrated in a large-scale reactor. The second part of the study deals with the measurement of the effective electrical conductivity of a packed bed of magnesium manganese oxide pellets. During this experimental campaign, different pellet form factors (cylindrical and spherical) and compositions (1:1 and 3:2 magnesium-to-manganese molar ratios) have been examined. These measurements are performed using a four wire technique at temperatures ranging between 1000 and 1500 °C under atmospheric pressure. This study demonstrates that, under such conditions, the energy storage material is electrically conductive. This result plays a crucial role in the development of fast charging strategies for energy storage systems based on the magnesium manganese oxide redox system. In fact, given its electrical properties, the packed bed can be thermally charged by directly passing an electrical current through it (Joule heating) instead of relying on external heating elements. This study provides valuable insights into the design and operation of such energy storage systems, and the findings have important implications for the development of more efficient and cost-effective energy storage products.The third part of the study deals with the modeling and experimental validation of a thermochemical energy storage reactor based on the magnesium manganese oxide redox system. The model combines transient lumped (0D) species and energy governing equations for both the solid and gas phases within the packed bed, with 1D axial and radial transient heat conduction equations within the reactor insulating layers. The model is validated using the experimental data collected during a redox cycling campaign of a nominally 1 kW/5 kWhth reactor based on the magnesium manganese oxide redox system. The redox material chemical kinetics is modeled using an equilibrium kinetics approach. Experimental correlations are also used to validate the pressure drop measured across the packed bed upon system discharge. This model provides a starting point forthe design and optimization of commercial-scale energy storage systems based on the magnesium manganese oxide redox system.Overall, this PhD thesis provides a foundational understanding of the magnesium manganese oxide redox system behavior at different length scales, starting from the pellet-scale, moving to the packed bed scale, and finally reaching the reactor-scale. The results of this study have significant implications for the development of efficient and scalable thermochemical energy storage systems.
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