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Modélisation multi-échelles d’un système de stockage thermique de vapeur par Matériau à Changement de Phase (MCP)

Abstract : In an industrial process where steam is employed as a heat carrier, the integration of a steam storage solution allows to make the production of steam independent of its usage. Steam storage technologies can be used to decrease the energy consumption of the process, or to valorize waste heat from steam flows. Steam storage is also crucial for thermal solar power plants with direct steam generation. This work presents a model of a shell-and-tube steam storage system using Phase Change Material (PCM). These systems store the thermal energy of steam through the latent heat of the solid-liquid phase change transition of a material called PCM. The heat transfers in a storage module are often influenced by the natural convection flow of the liquid PCM during fusion and solidification. Predicting the thermal performances of a module can only be done by simulating this flow with a Computational Fluid Dynamics (CFD) numerical model with a fine mesh, whose computational times are too high for engineering needs. The goal of this work is to develop a model for the design and the performance prediction of a storage module, which takes into account the fine physical phenomena while having reasonable computational times. A multi-scale modelling approach is adopted: both a fine CFD model of the PCM and a system model of a storage module with a coarser mesh are employed.The CFD model is based on the enthalpy-porosity approach, which allows 3D simulation of solid-liquid phase change, and takes into account the movements of the liquid PCM. The sensitivity of the model to several parameters which characterize the phase change is studied, on two case studies where the natural convection flows has different amplitudes. The crossed influences of the parameters are identified. The comparison to experimental results allows to emit good practices for the use of the model. The values of the latent heat and of the temperature interval where the phase change takes place appear to be fundamental for both cases; this shows that the precision of the PCM characterization is very important for the numerical simulation of solid-liquid phase change. The mushy zone constant, which governs the damping of the liquid flow in the vicinity of the fusion or solidification front, has a different effect on the results and a different optimal value depending on the case study. Therefore, it is recommended to fit the value of this constant on experimental data, whenever such data are available.The system model represents the 1D liquid water / steam two-phase flow in the tubes of a module, and the heat transfers and the phase change in the PCM outside the tubes. The PCM is represented by a purely conductive model with an equivalent conductivity. A prototype storage module with segmented fins, installed at CEA Grenoble, is simulated with the CFD model; sodium nitrate is used as PCM (phase change temperature: 305°C). A 1D law for the heat transfer between the tube and the PCM is obtained from the CFD results; this law takes into account the convective heat transfer, and the heat transfer enhancement by the fins and the conductive inserts that are disposed in the PCM. An equivalent conductivity of the PCM in the system model is computed from the law. The modelling methodology is validated on charge tests from the prototype module (PCM fusion and steam condensation). The system model correctly reproduces the transient heat transfer rate to the PCM that the CFD predicts and the one measured experimentally, while allowing 10 to 90 times shorter computational times.
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Clément Beust. Modélisation multi-échelles d’un système de stockage thermique de vapeur par Matériau à Changement de Phase (MCP). Génie des procédés. Université de Pau et des Pays de l'Adour, 2019. Français. ⟨NNT : 2019PAUU3029⟩. ⟨tel-02501319⟩

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