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

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)

On the Use of Infrared Thermography for the Estimation of Melting Enthalpy

A calorimetry method based on infrared thermography is showing promise for material screening, allowing the simultaneous detection of phase transitions of multiple samples at a time, hence enabling the establishment of phase diagrams in a record time. The working principle of this method is similar to the one of Differential Thermal Analysis. Therefore, this work aims at identifying if the melting enthalpy of materials could be estimated on the same basis using infrared thermography. In this work, the melting of six eutectic mixtures of fatty acids is estimated under three considerations. The results are compared to Differential Scanning Calorimetry measurements and literature data. The accuracy of the method is discussed and improvements are proposed

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

Effect of atomic substitution on the sodium manganese ferrite thermochemical cycle for hydrogen production

This work presents the effect of atomic substitution on the MnFe2O4-Na2CO3 thermochemical cycle for H-2 production. The non-oxidative decarbonation/carbonation reaction of the MnFe2O4-Na2CO3 mixture is investigated as the starting reference. Repeated cycling results in a 30% loss of reversibility due to an overall reduction of the reactive interfaces. The substitution of Na2CO3 for Li2CO3 decreases the decarbonation onset temperature by about 100 degrees C, but almost no reversibility is observed during the cycles due to the irreversible Li+ intercalation. The effect of partial Mn substitution for Ca, Ni, and Zn is presented. The 5% Zn mixture shows the best decarbonation/carbonation reversibility and is tested for H-2 production together with MnFe2O4-Na2CO3. The reference mixture produces more H-2 during the first cycle (asymptotic to 1.1 vs. 0.7 mmol/g), but its production drastically drops by two orders of magnitude upon cycling and becomes negligible after 5 cycles. By contrast, the Zn-doped mixture exhibits a stable H-2 production of 0.22 mmol/g with no decreasing trend observed from cycle 2 to cycle 5. As result, in the fifth cycle, the Zn-doped mixture produces 23 times more H-2 than MnFe2O4-Na2CO3. Thermogravimetry and X-ray diffraction confirm that doping with Zn significantly improves the regeneration of the reactants.Acknowledgment This Project is funded by the Department of Economic Devel-opment, Sustainability and Environment of the Basque Govern-ment (CICe 2019-KK-2019/00097-and H2BASQUE-KK-2021/00054) . The authors express their sincere gratitude to Cristina Luengo and Mikel Intxaurtieta 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

NPG–TRIS Thermal Storage System. Quantification of the Limiting Processes: Sublimation and Water’s Adsorption

The NPG–TRIS binary system (NPG = (CH3)2C(CH2OH)2 = 2,2-dimetyl-1,3-propanodiol; TRIS = NH2C(CH2OH)3 = 2-Amino-2-(hydroxymethyl)-1,3-propanediol) was intensively investigated as a thermal energy storage system, due to the reversibility of its phase transitions and their associated energy. An adapted methodology was applied to precisely quantify its sublimation tendency. Relevant thermochemical data were revisited and evaluated using some specific experimental procedures. We also determined that the widely accepted requirement of working in an inert atmosphere to avoid deviations due to hygroscopicity is not necessary. Nevertheless, to take advantage of the energetic properties of the NPG–TRIS system, closed containers will be required to avoid NPG losses, due to its quantitatively determined high sublimation tendency.This study was financially supported by the Basque Government (IT1301-19, IT1364-19) and through the Elkartek18 R&D program, by the University of the Basque Country UPV/EHU (GIU19/019), and by the Ministry of Science and Innovation of Spain (PID2019-106644GB-I00

Vers un usage du Xylitol surfondu pour le stockage d’énergie thermique

L’objectif de ce travail est de trouver une solution pour surmonter les difficultés en lien avec la décharge d’énergie d’un système de stockage saisonnier intégrant du xylitol surfondu dans le cadre d’une application auxbâtiments(problème d’activation et puissances insuffisantes). Le bullage est une technique simple, peu intrusive, peu coûteuse et prometteuse pour activer et accélérer la décharge d’énergie. Cette technique a permis la décharge d’un récipientde 16,1 dm3contenant une colonne de 18 cm de xylitol surfonduà une température proche de l’ambianteen moins d’une heureThis work aims at finding ways to overcome the difficulties related to the energy discharge of a seasonal storage system integrating undercooled xylitol for building applications (problem of activation and insufficient powers).Stirring by bubbling is a simple, low intrusive, low cost and hence a promising technique to activate and accelerate the energy discharge. It allows discharginga 16,1 dm3container with an undercooled xylitol column of 18 cmat a temperature close to ambient onein less than one hour