Matériau à changement de phase (thermique)

On appelle matériau à changement de phase, ou MCP, tout matériau capable de changer d'état physique dans une plage de température restreinte. Cette plage est située entre et environ. Dans cet intervalle de température, le changement de phase prépondérant reste la fusion/solidification. Ces températures sont accessibles naturellement et sont omniprésentes dans la vie quotidienne (température d'ambiance d'une maison, température d'un corps humain, de l'eau chaude sanitaire). Cet article n'aborde que les MCP dont le changement d'état se situe entre les phases liquides et solides. Principe du changement de phase : chaleur sensible et chaleur latente Tout matériau, solide, liquide ou gazeux possède une capacité à absorber, stocker ou céder de l'énergie sous forme de chaleur. On distingue deux types de transfert de chaleur (ou transfert thermique) :

le transfert thermique par chaleur sensible (CS) : dans ce cas, le matériau en question peut absorber ou céder de l'énergie en

Multi-Scale Study of High-Temperature Latent Heat Storage With Metallic Phase Change Materials

Nithin Mallya

Energy storage is a central issue in the green economy with half the global end energy usage being heat. Latent heat thermal energy storage (LHTES) provides an energy dense heat storage solution with a well-defined discharge temperature. Utilizing low-cost, high-temperature, high-conducting metal alloys as phase change materials (PCMs) enables high energy and power density LHTES systems. Detailed characterization, reliable design tools and guidelines for modelling and building such systems are missing. This thesis presents design-related and operation-oriented innovations for such systems at different scales: (a) application-scale experimental-numerical demonstration of a LHTES test-bed, (b) non-dimensional numerical study of low and high-conducting PCMs at storage-unit scale, (c) novel power dense macro-porous PCM structures at encapsulation scale, and (d) interface scale experimental melt interface tracking by advanced tomography techniques for high-temperature PCMs.A megajoule scale high-temperature LHTES modular LHTES test-bed was built and tested, for charging-discharging at temperatures up to 950 K with aluminium alloy PCM encapsulated in stainless steel and air as the heat transfer fluid (HTF). Engineered open-cell cellular ceramic lattice were used as porous fins for enhancing the HTF-PCM heat transfer. Successful first operation was demonstrated with near-isothermal discharge for 2.7 hours in a 1.31 MJ storage. A quasi-1D lumped parameter model was developed and validated to quantify its thermal stratification, heat losses, and phase change characteristics. Heat storage characteristics of scaled configurations of the test-bed were predicted using the model.A 2D transient phase change model was used to compare a high-conducting, high melting point aluminium alloy PCM with a commercially available low-conducting, low melting point paraffin wax to quantify the effect of PCM properties, encapsulation shape, load conditions, and buoyancy driven internal convection on the power density. Parametric study and non-dimensional analysis quantified the influence of thermal and geometrical parameters on phase change and contribution of natural convection inside the encapsulations linking the melt fraction and heat transfer rates to combination of Fourier, Stefan, Rayleigh and Nusselt numbers.With high-conducting PCMs, the LHTES power density is limited by HTF-PCM convective heat transfer which can be enhanced by using novel macro-porous encapsulation designs. A comparison of several random and ordered macro-porous cells was performed with a 3D coupled CFD-phase change model. A mesh-based convolutional neural network was trained to emulate the 3D model and predict phase change time and pressure drop characteristics. Finally, a synchrotron source X-ray tomography experimental campaign to create a validation case for phase change models of high-temperature PCMs is detailed. Transient 3D melt interface tracking was conducted for aluminium alloy in cylindrical encapsulations heated using lasers. As the beamline was limited to small samples, a furnace heating setup was also built for radiography experiments of larger cubic samples at a tabletop CT facility.This thesis presents a combination of modelling frameworks and experimental setups to provide general understanding of transient heat transfer in PCMs at different scales, provide novel approaches for heat transfer enhancement, and tools for designing high-temperature LHTES systems.

EPFL2022

Analysis of energy requirements versus comfort levels for the integration of phase change materials in buildings

François Maréchal, Martin Vautherot

This paper investigates the importance of the design parameters when looking at possible energy savings and comfort enhancement in a building using Phase Change Materials (PCMs). Computer based simulations are performed using a simulation software for modelling a house and its thermal behaviour over a year. It is found that by varying the heating set point and the phase change (melting) temperature range of the PCM, significant changes can be observed. Some poor scenarios show that the integration of PCM can increase both the discomfort (up to 6% more discomfort hours) and the energy requirements (up to 25% more energy needed). On the other hand, appropriate scenarios bring significant energy savings (up to 33% less energy needed) and comfort enhancement (up to 31% less discomfort hours). This highlights the strong need for a clever design when integrating PCM into buildings. The goal is to find a trade-off between energy savings and comfort enhancement. The PCM with a phase change temperature range between 21 °C and 26 °C shows the best results. The study is based on climate conditions for Auckland City in New Zealand but most of the conclusions drawn can be applied to any climate. © 2015 Elsevier Ltd.

2015

Aqueous Redox Flow Batteries Enhanced by Solid Boosters

Elena Zanzola

One of the main challenges for certain electricity production technologies (e.g. renewables) is the capability to supply energy according to the demand. It follows that storing energy is a possible solution to the inadequacy between production and demand. Among various technologies, electrochemical energy storage is a particularly good solution. Redox flow batteries, as electrochemical energy storage technology are scalable, durable, robust and quite safe, but as compared to other technologies, they have a quite poor volumetric capacity (10-40 Wh/L) determined by the solubility limit of the redox-active species dissolved in the electrolytes. In this thesis, an approach to overcome the low storage capacity in aqueous redox flow batteries is presented. The approach consists of adding solid-phase materials (herein called solid boosters) into the liquid electrolytes contained in the battery reservoirs. These boosters can receive charge from the redox-active species reacting in the flow cell, if the respective Fermi levels are aligned to those of the liquid electrolyte materials. This concept is denoted as âredox-mediatedâ charge transfer, where solid boosters become the primary storage media, bypassing the solubility limit issue that restraints the storage capacity. This work opens a series of future studies to enable bulk-phase charge storage in the reservoirs of redox flow batteries and it looks promising for industrial demonstrators.

EPFL2020

Multi-Scale Study of High-Temperature Latent Heat Storage With Metallic Phase Change Materials

Nithin Mallya

EPFL2022