Active Thermochemical Barrier Coatings Using Metal Oxides: First Experimental Results.

A new concept of active thermal coating based on the use of reversible thermochemical reactions is presented in this paper. The new active thermal barrier coating uses redox reactions to buffer the temperature changes that a metallic component may suffer at high temperatures. The heat is stored when the temperature is equal/above the reduction temperature of the active coating (endothermic reaction) and the heat is released when the temperature is equal/below the oxidation temperature (exothermic reaction). The paper describes the development and testing of a reactive thermal barrier coating based on the redox reaction of Co3O4 and its cyclability. Co3O4 was chosen as a reference material due to the high enthalpy of reaction (844 kJ/kg) and redox reversibility. The activity of coatings with 1, 2, and 3 Co3O4 layers was demonstrated by simultaneous thermal analysis, showing good stability for 5 five cycles.

Sodium Manganese Ferrite Water Splitting Cycle: Unravelling the Effect of Solid-Liquid Interfaces in Molten Alkali Carbonates.

In this work, the Na2CO3 of the sodium manganese ferrite thermochemical cycle was substituted by different eutectic or eutectoid alkali carbonate mixtures. Substituting Na2CO3 with the eutectoid (Li0.07Na0.93)2CO3 mixture resulted in faster hydrogen production after the first cycle, shifting the hydrogen production maximum toward shorter reaction times. Thermodynamic calculations and in situ optical microscopy attributed this fact to the partial melting of the eutectoid carbonate, which helps the diffusion of the ions. Unfortunately, all the mixtures exhibit a significant loss of reversibility in terms of hydrogen production upon cycling. Among them, the nonsubstituted Na mixture exhibits the highest reversibility in terms of hydrogen production followed by the 7%Li-Na mixture, while the 50%Li-Na and Li-K-Na mixtures do not produce any hydrogen after the first cycle. The loss of reversibility is attributed to both the formation of undesired phases and sintering, the latter being more pronounced in the eutectic and eutectoid alkali carbonate mixtures, where the melting of the carbonate is predicted by thermodynamics.

Improved Thermophysical and Mechanical Properties in LiNaSO4 Composites for Thermal Energy Storage.

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.

Development of a Kinetic Model for the Redox Reactions of Co2.4Ni0.6O4 and SiO2/Co2.4Ni0.6O4 Oxides for Thermochemical Energy Storage.

One of the possible solutions for the transition of the actual energetic model is the use of thermal energy storage technologies. Among them, thermochemical energy storage based on redox reactions involving metal oxides is very promising due to its high energy density. This paper deals with the development of the kinetic study based on data extracted from the thermogravimetric analysis of a cobalt-nickel mixed oxide (Co2.4Ni0.6O4) without and with the addition of SiO2 particles to improve the cyclability. The results show that in the reduction reaction the activation energy is not affected by the addition of SiO2 particles while in the oxidation reaction an increase in the activation energy is observed. The theoretical models fitting with the experimental data are different for each material in the reduction reaction. The mixed oxide is controlled by a nucleation and growth mechanism for conversion ratios higher than 0.5, while the added material is controlled by diffusion mechanisms. In the oxidation reaction, the two materials are controlled by a nucleation and growth mechanism for conversion ratios higher than 0.5.

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.

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.

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).

Study of the Phase Transitions in the Binary System NPG-TRIS for Thermal Energy Storage Applications.

Neopentylglycol (NPG) and tris(hydroxymethyl)aminomethane (TRIS) are promising phase change materials (PCMs) for thermal energy storage (TES) applications. These molecules undergo reversible solid-solid phase transitions that are characterized by high enthalpy changes. This work investigates the NPG-TRIS binary system as a way to extend the use of both compounds in TES, looking for mixtures that cover transition temperatures different from those of pure compounds. The phase diagram of NPG-TRIS system has been established by thermal analysis. It reveals the existence of two eutectoids and one peritectic invariants, whose main properties as PCMs (transition temperature, enthalpy of phase transition, specific heat and density) have been determined. Of all transitions, only the eutectoid at 392 K shows sufficiently high enthalpy of solid-solid phase transition (150-227 J/g) and transition temperature significantly different from that of the solid-state transitions of pure compounds (NPG: 313 K; TRIS: 407 K). Special attention has also been paid to the analysis of metastability issues that could limit the usability of NPG, TRIS and their mixtures as PCMs. It is proven that the addition of small amounts of expanded graphite microparticles contributes to reduce the subcooling phenomena that characterizes NPG and TRIS and solve the reversibility problems observed in NPG/TRIS mixtures.

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 (300⁻800 °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).

The Li2SO4-Na2SO4 System for Thermal Energy Storage.

In this paper, the system Li2SO4-Na2SO4 is proposed as a candidate material for thermal energy storage applications at high temperatures (450-550 °C). Depending on the composition, the thermal energy can be stored by using a eutectoid reaction and solid-solid phase transition. In these types of systems, all the components (reagent and products) are in the solid state. This work includes the theoretical analysis (based on the Calphad method) of the system selected obtaining all the theoretical parameters (for example, enthalpies of reaction, transition temperatures, volume expansion, and the heat capacities) necessary to determine the theoretical performance in terms of thermal energy storage. The theoretical analysis allowed to identify two compositions (Li2SO4/Na2SO4 79/21 and 50/50) in the phase diagram with the most promising theoretical enthalpy of transformation (270 J/g and 318 J/g, respectively) corresponding to a eutectoid reaction and a solid-solid phase transition (stoichiometric compound LiNaSO4). The experimental analysis carried out allowed to confirm the great potential of this system for TES application even if some discrepancies with the theoretical calculation have been observed experimentally (energy densities lower than expected). For the two compositions studied, 79/21 and 50/50, the enthalpies of reaction are 185 J/g and 160 J/g, respectively. The reactivity of the system was tested under different experimental conditions preparing materials with a different degree of nanocrystallization to favor the diffusion in the solid state, testing the reactivity of the materials under controlled atmosphere and under air, and performing preliminary durability analysis (cycling behavior up to 20 cycles) to test the stability and reversibility.

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.

Determination of the heat of hydride formation/decomposition by high-pressure differential scanning calorimetry (HP-DSC).

Among the thermodynamic properties of novel materials for solid-state hydrogen storage, the heat of formation/decomposition of hydrides is the most important parameter to evaluate the stability of the compound and its temperature and pressure of operation. In this work, the desorption and absorption behaviors of three different classes of hydrides are investigated under different hydrogen pressures using high-pressure differential scanning calorimetry (HP-DSC). The HP-DSC technique is used to estimate the equilibrium pressures as a function of temperature, from which the heat of formation is derived. The relevance of this procedure is demonstrated for (i) magnesium-based compounds (Ni-doped MgH2), (ii) Mg-Co-based ternary hydrides (Mg-CoHx) and (iii) Alanate complex hydrides (Ti-doped NaAlH4). From these results, it can be concluded that HP-DSC is a powerful tool to obtain a good approximation of the thermodynamic properties of hydride compounds by a simple and fast study of desorption and absorption properties under different pressures.