Solid-State Reactions for the Storage of Thermal Energy

In this paper, the use of solid-state reactions for the storing of thermal energy at high temperature is proposed. The candidate reactions are eutectoid- and peritectoid-type transitions where all the components (reactants and reaction products) are in the solid state

Spacing effect on pool boiling performance of three triangular pitched and vertically oriented tubes

There is a scarcity of available data on boiling process in vertically oriented tube bundles in accessible sources. Lack of systematic studies is limiting further expansion of this highly efficient process of heat transfer into heat recovery field. In this paper boiling process of three triangular pitched and vertically oriented tubes has been studied in ethanol at 78$^{\circ}$C. The main focus of this work was to study the effect of tube spacings on heat transfer coefficient (HTC) and bubbles behavior (bubble departure diameter in particular) that were visualised with the help of a high speed camera. Experiments were performed in a wide range of tube spacings (from 10.75 to 0.25 mm) and heat flux densities (from 3 to 70 kW/m$^2$). The obtained results show that, long spacings i.e., much longer than bubble departure diameter, have no influence on HTC as well as on bubbles behavior. On the contrary, the spacings on the order of the bubble departure diameter tend to create slug flow in the bundle, that is very beneficial for the heat exchange at low heat fluxes. Finally, narrow spacings that are much shorter than the bubble departure diameter have shown the potential to enhance the HTC in tube bundles with low length to diameter ratios.Comment: 18 pages, 13 figure

Hierarchical macro-nanoporous metals for leakage-free high-thermal conductivity shape-stabilized phase change materials

Impregnation of Phase Change Materials (PCMs) into a porous medium is a promising way to stabilize their shape and improve thermal conductivity which are essential for thermal energy storage and thermal management of small-size applications, such as electronic devices or batteries. However, in these composites a general understanding of how leakage is related to the characteristics of the porous material is still lacking. As a result, the energy density and the antileakage capability are often antagonistically coupled. In this work we overcome the current limitations, showing that a high energy density can be reached together with superior anti-leakage performance by using hierarchical macro-nanoporous metals for PCMs impregnation. By analyzing capillary phenomena and synthesizing a new type of material, it was demonstrated that a hierarchical trimodal macro-nanoporous metal (copper) provides superior antileakage capability (due to strong capillary forces of nanopores), high energy density (90vol% of PCM load due to macropores) and improves the charging/discharging kinetics, due to a three-fold enhancement of thermal conductivity. It was further demonstrated by CFD simulations that such a composite can be used for thermal management of a battery pack and unlike pure PCM it is capable of maintaining the maximum temperature below the safety limit. The present results pave the way for the application of hierarchical macro-nanoporous metals for high-energy density, leakage-free, and shape-stabilized PCMs with enhanced thermal conductivity. These innovative composites can significantly facilitate the thermal management of compact systems such as electronic devices or high-power batteries by improving their efficiency, durability and sustainabilit

Li4(OH)3Br-Based Shape Stabilized Composites for High-Temperature TES Applications: Selection of the Most Convenient Supporting Material

Peritectic compound Li4(OH)3Br has been recently proposed as phase change material (PCM) for thermal energy storage (TES) applications at approx. 300 °C Compared to competitor PCM materials (e.g., sodium nitrate), the main assets of this compound are high volumetric latent heat storage capacity (>140 kWh/m3) and very low volume changes (<3%) during peritectic reaction and melting. The objective of the present work was to find proper supporting materials able to shape stabilize Li4(OH)3Br during the formation of the melt and after its complete melting, avoiding any leakage and thus obtaining a composite apparently always in the solid state during the charge and discharge of the TES material. Micro-nanoparticles of MgO, Fe2O3, CuO, SiO2 and Al2O3 have been considered as candidate supporting materials combined with the cold-compression route for shape-stabilized composites preparation. The work carried out allowed for the identification of the most promising composite based on MgO nanoparticles through a deep experimental analysis and characterization, including chemical compatibility tests, anti-leakage performance evaluation, structural and thermodynamic properties analysis and preliminary cycling stability study.This research was funded by the Basque Government through the project Elkartek CICe2020 KK-2020/00078 and supported by the Polytechnique National Institute of Bordeaux (Bordeaux INP)

Effect of processing on microstructure and mechanical properties of pentaglycerine based solid-solid phase change materials

The present work addresses the lack of reported information about the mechanical properties of solid-solid PCMs, and how these are affected by their processing, considering that they usually incorporate fillers to increase their thermal conductivity. To this end, this work analyzes pentaglycerine (PG) based composites, which are of great interest in TES intended for 80 °C. These composites are also doped with various contents of expanded graphite (EG) with two different particle sizes. With these combinations, the effect of two typical processing methods, pressing and casting, on the microstructure of the composites is evaluated. Furthermore, the mechanical behavior of these composites in both crystal and plastic phases, as well as their thermal expansion during the transition process, is also reported in the current study. Besides demonstrating the important role that processing plays in these properties in PG/EG-based composites, it has been found that the use of EG is also beneficial for mitigating the permanent deformations experienced by these composites during thermal cycling. Finally, the exposed results give the first evidence of the interesting effect these processing methods have on the thermal properties of the composites.This research was funded by the FEDER/Ministerio de Ciencia e Innovaci ́on–Agencia Estatal de Investigacion, SWEET-TES project (RTI2018-099557-B-C21), as well as from The Basque Government (Elkartek CICe2020, KK-2020/00078). The authors also gratefully acknowledge Yagmur Polat and Cristina Luengo for their technical support

Jet-Injection In Situ Production of PVDF/PCM Composite Fibers for Thermal Management

Thermal management protects against external agents and increases the lifetime and performance of the devices in which it is implemented. Because of their ability to store and release a high amount of energy at a nearly constant temperature, phase change materials (PCMs) are promising thermoregulatory materials. Thus, the manufacture of PVDF fibers containing PCMs has advantages since PVDF is already used in elements that are susceptible to thermal management as a binder in batteries or as a base material for fabrics. This work presents a simple, versatile, in situ, cost-effective, and easy-to-scale-up method to produce PVDF-based fibers containing paraffin RT-28HC for thermal management. To achieve that goal, the microfluidic approach of coaxial flows was simplified to gravity-aided laminar jet injection into a bulk fluid, where fibers were produced by the solvent extraction mechanism. With this methodology, hollow PVDF fibers and core-shell PVDF fibers containing paraffin RT-28HC have been produced. The proposed approach resulted in fibers with up to 98 J/g of latent heat, with a hierarchical porous structure. SEM study of the fiber morphology has shown that PCM is in the form of slugs along the fibers. Such PCM distribution is maintained until the first melting cycle, when molten PCM spreads within the fiber under capillary forces, which was observed by an infrared camera. Manufactured composite fibers have shown low thermal conductivity and high elasticity, which suggest their potential application as a thermal insulation material with thermal buffer properties. Leakage tests revealed outstanding retention capacity with only 3.5% mass loss after 1000 melting/crystallization cycles. Finally, tensile tests were carried out to evaluate the mechanical properties of the fibers before and after thermal cycling.The authors are grateful for the financial support from SWEET-TES project (RTI2018-099557-B-C21), funded by FEDER/Ministerio de Ciencia e Innovación - Agencia Estatal de Investigación and Elkartek CICe2020 project (KK-2020/00078) funded by Basque Government. Mikel Duran Lopez would also like to thank the Department of Education, Linguistic Politics and Culture of the Basque Country government for the granted pre-doctoral contract (PRE_2019_1_0154). This article is part of the grant RYC2021-032445-I funded by MICIN/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR

Characterization of Fatty Acids as Biobased Organic Materials for Latent Heat Storage

This work aims to characterize phase change materials (PCM) for thermal energy storage in buildings (thermal comfort). Fatty acids, biobased organic PCM, are attractive candidates for integration into active or passive storage systems for targeted application. Three pure fatty acids (capric, myristic and palmitic acids) and two eutectic mixtures (capric-myristic and capric-palmitic acids) are studied in this paper. Although the main storage properties of pure fatty acids have already been investigated and reported in the literature, the information available on the eutectic mixtures is very limited (only melting temperature and enthalpy). This paper presents a complete experimental characterization of these pure and mixed fatty acids, including measurements of their main thermophysical properties (melting temperature and enthalpy, specific heats and densities in solid and liquid states, thermal conductivity, thermal diffusivity as well as viscosity) and the properties of interest regarding the system integrating the PCM (energy density, volume expansion). The storage performances of the studied mixtures are also compared to those of most commonly used PCM (salt hydrates and paraffins).This research work was developed in the framework of SUDOKET project (Interreg Sudoe SOE2/P1/E0677). The authors are grateful to the European Regional Development Fund (ERDF) to co-fund the project through the Interreg Sudoe Programme and the Region Nouvelle Aquitaine for subsidizing BioMCP project (Project-2017-1R10209-13023). The authors would also like to extend their thanks to CNRS for promoting the I2M Bordeaux-CICenergiGUNE exchanges in the framework of the IEA PHASE-IR project

Réduction et inversion de modèles de conduction thermique avec changement de phase

Ce travail porte sur la réduction et l'inversion de problèmes de conduction avec changement de phase. Nous proposons dans un premier temps différentes méthodes permettant de réduire efficacement ces problèmes fortement non linéaires. Il s'agit, pour la plupart, de méthodes spectrales "a priori" reposant sur la troncature de la base singulière. On distingue dans ce travail trois cas de figure. Le premier implique des conductivités thermiques indentiques en phases solide et liquide. Le second lève cette restriction de conductivités thermiques constantes mais impose des conditions aux limites particulières (Dirichlet et/ou Neumann). Le troisième, enfin, voit la mise en place de deux approches distinctes visant à réduire des problèmes généraux, c'est-à-dire à conductivités thermiques variables et pour tout type de conditions limites. La première est une méthode spectrale "a posteriori" de type P.O.D. et la seconde repose de nouveau sur la base singulière mais pondérée en considérant un modèle de sollicitations de type Markovien. Différents tests numériques ont, à chaque étape, permis de mettre en évidence la qualité des méthodes proposées. La seconde partie de ce travail est consacrée à l'estimation de fonctions enthalpie/température qui résument à elles seules l'essentiel du phénomène de changement de phase. La méthode proposée repose sur l'écriture d'un problème d'estimation de sources par inversion d'un modèle linéaire de conduction. Le problème inverse consiste en l'intégration temporelle d'un simple modèle d'état dont les sollicitudes sont l'observation, complète ou non, de l'évolution thermique du matériau testé. On montre également que l'on peut aisément remonter aux fonctions enthalpie/température recherchées par une simple intégration temporelle des sources estimées. Des exemples numériques et une validation expérimentale ont montré la puissance de la méthode, que ce soit en termes de rapidité, de précision ou de robustesse au bruit de mesure.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

Le secret du lac Karakul

National audienc

Le secret du lac Karakul

National audienc