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Theses

Modélisation du stockage de chaleur par changement de phase d'alliages à composition binaire soumis à un refroidissement contrôlé

Abstract : Latent Heat Thermal Energy Storage (LHTES) shows high storage density compared to sensible thermal systems. For high temperature applications, the use of alloys as phase change materials presents many advantages. Principally, varying alloy composition allows controlling the storage\discharge of thermal energy through an expected temperature range (defined by the heat source), and the high thermal conductivity givessuitable heat transfer properties to the system that receives/supplies the energy. However, some systems need a specific temperature range to correctly operate. In such conditions, subcooling (also known as undercooling) and segregation are undesirable phenomena in alloys when they are used as PCM. In thepresent work, we propose a method to predict the latent heat release during phase transformation of a binaryalloy submitted to a controlled cooling rate, including subcooling, segregation and variation of composition.This thesis describes the physical models that apply when heat is released from such a material. We takeinto consideration the cooling rate applied to the PCM, the solidification velocity, convective phenomena,melting temperature and subcooling. In the present work, phase diagrams and the CALPHAD methodologyare used to determine the temperature range for phase change (or constant temperature value for isothermal transformation) by minimizing the Gibbs equilibrium energy. The Gibbs free energy minimization has been implemented in a homemade numerical code. The material can be screened with different compositions for equilibrium or off-equilibrium solidification allowing quick selection of the optimal material for the specific heatsource. In the proposed method, the solidification velocity is obtained from the cooling rate. Then, variationin microstructure is driven by the solidification velocity using the local non-equilibrium diffusion model. Based on the local nonequilibrium model that depends on the partition coefficient variation, the subcooling degree, wich is derived from the applied cooling rate is predicted. A bibliographic study has been carried out and anumerical comparison has been undertaken to ensure the capacity of our code to reproduce the phase change of various materials that include phenomena such as subcooling and recalescence. The results highlight that the cooling rate is one of the most important parameters in the performance of the thermal storage, having a large effect on segregation and subcooling degree. Moreover, we show the influence ofpartition coefficient on the time evolution of solid fraction, considering a constant or a composition-dependent value. We can conclude that the latent heat release can be correctly predicted provided that the method correctly predicts the phase diagram and the variable partition coefficient. This work helps to accelerate the design and development of thermal storage systems and lays the foundation to continue exploring other kinds of materials (e.g. paraffins).
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https://tel.archives-ouvertes.fr/tel-02102859
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Submitted on : Wednesday, April 17, 2019 - 5:05:12 PM
Last modification on : Monday, August 31, 2020 - 9:36:05 AM

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MORENO_REYNA_ABRAHAM_2018.pdf
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  • HAL Id : tel-02102859, version 1

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Abraham Moreno Reyna. Modélisation du stockage de chaleur par changement de phase d'alliages à composition binaire soumis à un refroidissement contrôlé. Mécanique [physics]. Université de Bordeaux, 2018. Français. ⟨NNT : 2018BORD0012⟩. ⟨tel-02102859⟩

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