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

Exchange bias effects in Fe nanoparticles embedded in an antiferromagnetic Cr2O3 matrix

Powders consisting of ferromagnetic (FM) Fe nanoparticles, of about 7 nm in size, embedded in an antiferromagnetic (AFM) Cr2O3 matrix have been obtained by high-temperature reduction under a hydrogen atmosphere of a mixed Cr–Fe oxide. This FM–AFM system exhibits exchange bias effects, i.e. a loop shift (HE) and coercivity enhancement (ΔHC), when field-cooled through the N´eel temperature, TN, of Cr2O3. The exchange bias properties were measured as a function of temperature. HE and ΔHC are found to vanish at about TN(Cr2O3), indicating a good quality AFM matrix. Hence, high-temperature reduction of mixed oxides is demonstrated to be a suitable technique to develop new types of FM–AFM exchange-biased nanoparticles, from which novel applications of this phenomenon may be developed

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 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

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

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. To the best of our knowledge, these classes of reactions have not been considered so far for application in thermal energy storage. This study includes the theoretical investigation, based on the Calphad method, of binary metals and salts systems that allowed to determine the thermodynamic properties of interest such as the enthalpy, the free energy, the temperature of transition, the volume expansion and the heat capacity, giving guidelines for the selection of the most promising materials in view of their use for thermal energy storage applications. The theoretical investigation carried out allowed the selection of several promising candidates, in a wide range of temperatures (300&#8315;800 &#176;C). Moreover, the preliminary experimental study and results of the binary Mn-Ni metallic system are reported. This system showed a complex reacting behavior with several discrepancies between the theoretical phase diagram and the experimental results regarding the type of reaction, the transition temperatures and enthalpies and the final products. The discrepancies observed could be due both to the synthesis method applied and to the high sensitivity of the material leading to partial or total oxidation upon heating even if in presence of small amount of oxygen (at the ppm level)

Journal of Energy Storage

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

Neopentyl Glycol as Active Supporting Media in Shape-Stabilized PCMs

The present work explores the feasibility of using polyalcohols with solid-solid phase transition as active supporting matrix of n-alkanes in shape-stabilized phase change materials (SSPCMs). It is well-established that the use of SSPCM avoids leakage and increases stability and easy handling of solid-liquid PCMs. Nevertheless, the resulting composite exhibits a loss of heat storage capacity due to the volume occupied by the supporting material, which does not contribute to latent heat storage. Therefore, the objective of this work is to combine solid-liquid PCMs (alkanes) with solid-solid PCMs (polyalcohols), both exhibiting a phase transition in the same range of temperature, to obtain high energy density SSPCMs. Towards that goal, the performance of Neopentyl Glycol (NPG) and Docosane as a new energetic SSPCM has been proved. The NPG-Docosane chemical compatibility and its outstanding wettability facilitate the propitious association of both materials. The higher capillary forces obtained by decreasing the NPG crystal size together with the addition of expanded graphite (EG) allowed to obtain a maximum Docosane content of 60 wt%. The addition of EG improves the shape stability at the time that increases the heat transfer properties of the composites. The analysis showed that the components of the obtained SSPCMs are able to combine their latent heats, achieving a maximum value of 210.74 J/g for the highest Docosane content. This value is much higher than those latent heats exhibited by existing SSPCMs in the same working temperature range

Experimental Investigations on Electric-Field-Induced Crystallization in Erythritol

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