13 Décembre – Thesis defense - Imane Mahroug

11 h30 In videoconferencing

Development and study of new materials for high temperature compact thermal storage applications.

Peritectic compounds have been investigated recently as promising HT-TES materials. Peritectic refers to a reaction in which a liquid phase reacts with at least one solid phase, at a defined temperature, to form one new solid phase. These reactions show to afford outstanding theoretical energy densities. The charge/discharge processes of the peritectic compounds take place at constant temperature and atmospheric pressure through the reversible peritectic reaction. As a result, a simple thermal storage system is provided with, potentially, much higher energy density than that of currently used phase change materials and comparable to that of best gas-solid reactions under development.
This thesis aims at developing new materials based on the stoichiometric peritectic compound Li4(OH)3Br for thermal energy storage in solar power applications with high performances in terms of energy density, compactness, stability, and cost-effectiveness. Li4(OH)3Br was selected, as a candidate TES material, based on the outstanding theoretical energy density (800 J/g) it displays around 300 °C. In this study, an experimental validation of the choice of Li4(OH)3Br, as thermal storage material, was carried out through an exhaustive investigation of the LiOH-LiBr system. This validation is driven by the large discrepancies observed in the literature related to the temperature and enthalpy values of the peritectic reaction, as well as the stoichiometric compounds present in LiOH-LiBr phase diagram. The results allowed to propose a modified phase diagram for LiOH-LiBr that better adapts to the experimental results. Additionally, a comprehensive characterization of the thermo-physical properties of Li4(OH)3Br was carried out. It allowed to elucidate the mechanisms of the formation of Li4(OH)3Br. It was demonstrated that Li4(OH)3Br needs neither the presence nor contact with the pro-peritectic phase to form. It nucleates and grows directly from the melt so a pure phase Li4(OH)3Br final microstructure is achieved. The effect of the synthesis conditions on the storage properties has been investigated, allowing to develop and optimize a suitable synthesis process of Li4(OH)3Br. Overall, thermal storage properties of Li4(OH)3Br showed to be suitable for application in high-pressure (c.a. 100 bar) Direct Steam Generation (DSG) solar thermal power plants, showing higher performances over the currently employed material for this application (NaNO3). Moreover, Li4(OH)3Br/Carbon composites were investigated with the aim of enhancing the thermal storage performances of the peritectic. Slight storage capacity enhancement was achieved at low carbon doping. Furthermore, the compatibility between Li4(OH)3Br and potential storage tank materials was investigated under various corrosion conditions. Stainless steel 316 seems to be a very attractive option to be used as a container material for Li4(OH)3Br. Carbon steel, on the contrary, showed to react with the salt causing its degradation. Finally, Li4(OH)3Br based shape stabilized composites were developed to mitigate the corrosion limitations of container materials by the salt, which will allow reconsideration of the use of carbon steel as a container structure material given its low cost compared to the stainless steel. A methodology was established to select suitable supporting materials for shape stabilization of Li4(OH)3Br. It included the study of chemical compatibility between the salt and the supporting materials, anti-leakage performance evaluation, structural and thermodynamic properties analysis of the composite and cycling stability study. MgO was selected as the most convenient shape stabilizer for Li4(OH)3Br. Porous MgO materials exhibiting various textural properties were synthesized and tested as shape stabilizers. These materials allowed decreasing the MgO content by 40 %. Besides, the overall homogeneity and thermal conductivity of the composite was improved.

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