• Energy Research

Abstract This paper focus on the study of peritectic compound Li4(OH)3Br for thermal energy storage in solar power applications. A thoroughly characterization of Li4(OH)3Br as storage material has been performed by measuring transition temperatures (280–289 °C), enthalpies of transition (247 J/g), specific heats (c.a. 1.68 J/g/K in solid, 2.52 J/g/K in liquid) and thermal conductivity (0.47 W/m/K at room temperature). The effect of the synthesis conditions on the storage properties has been investigated as well. It is concluded that neither the cooling rate applied during the synthesis stage nor the type of atmosphere used (ambient air and protective argon atmosphere) has an influence on the material's performance. The stability of the material to thermal cycling has also been analysed, showing good cycling stability. Moreover, particular attention is paid to the elucidation of mechanisms of formation of Li4(OH)3Br. It is shown 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 as pure-phase Li4(OH)3Br final microstructure is achieved. An attempt to enhance the storage capacity of the material by addition of different types of carbon nanoparticles has been carried out. Assets of Li4(OH)3Br as storage materials for high-pressure DSG solar power plants have been assessed through comparison with reference material NaNO3. Main advantages of Li4(OH)3Br are higher volumetric latent heat storage capacity (+54%) and lower volume changes during phase transitions (3% vs. 11%), which would translate into smaller storage tanks (−33%), lower size heat exchangers and longer lifetime.

Li4(OH)3Br/Porous-MgO shape stabilized composites were developed in this study as novel high temperature thermal energy storage materials. Li4(OH)3Br, as storage material, owns a large reaction enthalpy (247 J/g) at 288 °C and excellent thermal cycling stability over 600 cycles. Solid MgO nanopowder was selected in a previous study among several metal oxides as the most promising shape stabilizer for Li4(OH)3Br salt satisfying the criteria of wettability, thermochemical compatibility, structural stability and cycling stability. However, this material ensures the structural stability of the composite at a minimum oxide loading of 50 wt%. This relatively high oxide loading will drastically decrease the overall storage capacity of the composite, which is not practical for TES applications. In order to reduce the MgO loading, new mesoporous MgO particles were tested as supporting materials. The idea is to benefit from the mesoporosity in improving the antileakage efficiency of the composite. To do so, three different porous MgO samples were synthesized and tested. Namely, i) Porous MgO (PMgO) synthesized by combustion using Magnesium nitrate, giving a BET surface area of 40 m2/g and a pore volume of 0.217 cm3/g. ii) MgO synthesized by calcination of basic magnesium carbonate (MgO-BMC), giving a high BET surface area of 129 m2/g and a pore volume of 0.294 cm3/g. iii) nanocrystalline MgO (MgO-BM64h) obtained by ball-milling process of commercial MgO micropowder, giving a BET surface area of about 55 m2/g and pore volume of 0.088 cm3/g. The three porous MgO materials exhibit various pore structures. The composites were synthesized following a simple fabrication method by cold compression, mixing and sintering. The results were promising for PMgO based composites where appreciable thermal and structural stability were achieved as 30 wt% oxide loading, whereas MgO-BMC and MgO-BM64h showed poor cycling stability at the same loading. SEM-EDS analyses of PMgO based composite showed an improvement of the homogeneity of the composite structure over 50 melting/solidification cycles. Moreover, the overall thermal conductivity of the composite was enhanced by 33% over pure salt.

LiOH–LiBr binary system is thoroughly investigated by means of DSC and XRD experimental analysis. Observed discrepancies compared to previous existing studies relate to temperature values of phase equilibria as well as stoichiometric compounds present in the system. From our experimental results, a modified LiOH–LiBr phase diagram is proposed which gives satisfactory explanation to all observations carried out. It includes stoichiometric compounds Li2(OH)Br (peritectoid plateau at 250 °C, x ≤ 0.666), Li3(OH)2Br (stable between 230 and 280 °C, melting peritectically for x ≥ 0.5) and Li4(OH)3Br (peritectic plateau at 289 °C, x ≥ 0.5). It also displays a eutectic transition at 254 °C approx., which extends over the composition range x > 0 to x = 0.66–0.67, with eutectic point at x = 0.40. The disagreements with previous studies also concern the enthalpies of transition. Whatever the transition is considered, the enthalpies measured in this work are much lower than those predicted before. However, the peritectic compound Li4(OH)3Br is still an attractive candidate for TES applications around 300 °C such as Direct Steam Generation CSP technology. In particular, when compared to NaNO3, which the reference material at that temperature, the advantages of using Li4(OH)3Br as heat storage material lie in the higher volumetric latent heat storage capacity (+ 54%) and lower volume changes during phase transitions (3% vs. 11%). This would result in smaller storage tanks, lower size heat exchangers, contributing to decrease the cost of the storage system.

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 sustainability

The objective of this experimental study was to develop a method to induce crystallization of sugar alcohols using an electric field for its future implementation in latent heat thermal energy storage systems. To better understand the mechanisms behind this approach, the first step of this work was dedicated to the replication, continuation, and consolidation of promising results on erythritol reported by another research group. In the second step, a second experimental configuration, previously used to electrically control the supercooling of other phase change materials, was tested with the same sugar alcohol. For both configurations, the influence of the type of current (DC and AC at different frequencies), its amplitude, and time of exposure were studied. However, none of these tests allowed influencing the crystallization of erythritol. Even if surprising at first glance, the difficulty in reproducing experiments and interpreting the results is not new in the field of electric-field-induced crystallization, as shown in particular by the abundant literature reviews concerning water. Currently, to the best of our knowledge, we consider that electric fields could be an attractive option to initiate and accelerate the crystallization of erythritol, but this solution must be considered with caution.

Solid-solid phase-change materials have great potential for developing compact and low-cost thermal storage systems. The solid-state nature of these materials enables the design of systems analogous to those based on natural rocks but with an extraordinarily higher energy density. In this scenario, the evaluation and improvement of the mechanical and thermophysical properties of these solid-solid PCMs are key to exploiting their full potential. In this study, LiNaSO4-based composites, comprising porous MgO and expanded graphite (EG) as the dispersed phases and LiNaSO4 as the matrix, have been prepared with the aim of enhancing the thermophysical and mechanical properties of LiNaSO4. The characteristic structure of MgO and the high degree of crystallinity of the EG600 confer on the LiNaSO4 sample mechanical stability, which leads to an increase in the Young’s modulus (almost three times higher) compared to the pure LiNaSO4 sample. These materials are proposed as a suitable candidate for thermal energy storage applications at high temperatures (400–550 °C). The addition of 5 wt.% of MgO or 5% of EG had a minor influence on the solid-solid phase change temperature and enthalpy; however, other thermal properties such as thermal conductivity or specific heat capacity were increased, extending the scope of PCMs use.

L'ajout de différents types de matériaux à changement de phase (PCM) aux matériaux à base de ciment pour le stockage de l'énergie thermique a été largement étudié dans la littérature. De nombreuses études ont étudié l'ajout de PCM organiques et les performances thermiques du système PCM-ciment. Cependant, les inconvénients tels que les fuites et la mauvaise conductivité thermique des PCM ont stimulé les études visant à améliorer les propriétés thermiques au sein du système PCM-ciment. Parmi les différentes solutions, l'ajout de matériaux carbonés (tels que le graphite et les nanotubes de carbone) pour améliorer la conductivité thermique des PCM a été étudié. Dans le travail actuel, un système innovant contenant des PCM microencapsulés (MPCM) et de l'oxyde de graphène réduit (rGO) synthétisé à dessein a été conçu et évalué. L'ajout de rGO a deux objectifs. La première consiste à accélérer la vitesse de stockage/libération de chaleur en améliorant la conductivité thermique de l'ensemble du système. La seconde consiste à améliorer la conductivité électrique du système afin de pouvoir activer activement (en appliquant une tension) la fonction de stockage/libération thermique. À la connaissance des auteurs, il s'agit d'une nouvelle approche pour le développement de systèmes de stockage d'énergie thermique à base de ciment PCM actif. En outre, dans la présente étude, l'utilisation des PCM paraffiniques a été comparée à celle des PCM biosourcés afin de fournir une solution plus durable à la conception d'éléments à base de ciment pour les applications du bâtiment. Une caractérisation thermique complète (capacité de stockage thermique, conductivité thermique et diffusivité) a été réalisée ainsi qu'une caractérisation microstructurale. De plus, la spectroscopie diélectrique à large bande a été utilisée pour caractériser la conductivité électrique du nouveau système MPCM-rGO-cement. La adición de diferentes tipos de materiales de cambio de fase (PCM) a los materiales a base de cemento para el almacenamiento de energía térmica se ha investigado ampliamente en la literatura. Muchos estudios han investigado la adición de PCM orgánicos y el rendimiento térmico del sistema PCM-cemento. Sin embargo, inconvenientes como las fugas y la mala conductividad térmica de los PCM han estimulado estudios para mejorar las propiedades térmicas dentro del sistema PCM-cemento. Entre las diferentes soluciones, se ha investigado la adición de materiales carbonosos (como el grafito y los nanotubos de carbono) para mejorar la conductividad térmica de los PCM. En el trabajo actual, se ha diseñado y evaluado un sistema innovador que contiene PCM microencapsulados (MPCM) y óxido de grafeno reducido (rGO) sintetizado a propósito. La adición de rGO tiene dos objetivos. La primera es acelerar la velocidad de almacenamiento/liberación de calor mejorando la conductividad térmica de todo el sistema. El segundo es mejorar la conductividad eléctrica del sistema para poder activar activamente (aplicando voltaje) la función de almacenamiento/liberación térmica. Hasta donde saben los autores, este es un enfoque novedoso para el desarrollo de sistemas activos de almacenamiento de energía térmica basados en PCM-cemento. Además, en el presente estudio, el uso de PCM parafínicos se comparó con el de PCM de base biológica para proporcionar una solución más sostenible al diseño de elementos a base de cemento para aplicaciones en edificios. Se ha realizado una caracterización térmica integral (capacidad de almacenamiento de calor, conductividad térmica y difusividad) así como una caracterización microestructural. Además, se utilizó la espectroscopia dieléctrica de banda ancha para caracterizar la conductividad eléctrica del nuevo sistema de cemento MPCM-rGO. Addition of different types of phase change materials (PCMs) to cement-based materials for thermal energy storage has been broadly investigated in the literature. Many studies have researched the addition of organic PCMs and the thermal performance of the PCM-cement system. However, drawbacks such as leakage and poor thermal conductivity of the PCMs have stimulated studies to improve thermal properties within the PCM-cement system. Among the different solutions, addition of carbonous materials (such as graphite and carbon nanotubes) to improve thermal conductivity of the PCMs have been investigated. In the current work, an innovative system that contains microencapsulated PCMs (MPCMs) and purposely synthesized reduced graphene oxide (rGO) has been designed and assessed. The addition of rGO has two aims. The first one is to speed up the heat storage/release velocity by improving the thermal conductivity of the whole system. The second one is to improve the electrical conductivity of the system in order to actively (by applying voltage) be able to turn on the thermal storage/release feature. Up to the authors' knowledge, this is a novel approach for the development of active PCM-cement based thermal energy storage systems. Furthermore, in the present study, the use of paraffinic PCMs was compared with that of biobased PCMs in order to provide a more sustainable solution to the design of cement-based elements for buildings applications. A comprehensive thermal characterization (heat storage capacity, thermal conductivity and diffusivity) has been carried out as well as microstructural characterization. Moreover, broadband dielectric spectroscopy was used to characterize the electrical conductivity of the novel MPCM-rGO-cement system. تم التحقيق على نطاق واسع في إضافة أنواع مختلفة من مواد تغيير الطور (PCMS) إلى المواد القائمة على الأسمنت لتخزين الطاقة الحرارية في الأدبيات. وقد بحثت العديد من الدراسات في إضافة PCMs العضوية والأداء الحراري لنظام الأسمنت PCM. ومع ذلك، فإن العيوب مثل التسرب وضعف الموصلية الحرارية لـ PCMs قد حفزت الدراسات لتحسين الخصائص الحرارية داخل نظام الأسمنت PCM. ومن بين الحلول المختلفة، تم التحقيق في إضافة مواد كربونية (مثل الجرافيت والأنابيب النانوية الكربونية) لتحسين الموصلية الحرارية لـ PCMS. في العمل الحالي، تم تصميم وتقييم نظام مبتكر يحتوي على PCMs المغلفة الدقيقة (MPCMs) وأكسيد الجرافين المنخفض المركب عن قصد (rGO). إضافة rGO لها هدفان. الأول هو تسريع سرعة تخزين/إطلاق الحرارة من خلال تحسين الموصلية الحرارية للنظام بأكمله. والثاني هو تحسين الموصلية الكهربائية للنظام من أجل أن يكون قادرًا بنشاط (من خلال تطبيق الجهد) على تشغيل ميزة التخزين/التحرير الحراري. على حد علم المؤلفين، يعد هذا نهجًا جديدًا لتطوير أنظمة تخزين الطاقة الحرارية النشطة القائمة على الأسمنت PCM. علاوة على ذلك، في هذه الدراسة، تمت مقارنة استخدام PCMs البرافينية مع استخدام PCMs الحيوي من أجل توفير حل أكثر استدامة لتصميم العناصر القائمة على الأسمنت لتطبيقات المباني. تم تنفيذ توصيف حراري شامل (سعة تخزين الحرارة والموصلية الحرارية والانتشار) بالإضافة إلى التوصيف الهيكلي الدقيق. علاوة على ذلك، تم استخدام التحليل الطيفي العازل عريض النطاق لتوصيف الموصلية الكهربائية لنظام الأسمنت MPCM - RGO الجديد.