• 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.

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.

Abstract Heat storage technologies are subject of great research efforts aimed at improving the energy efficiency of power plants and heat recovery processes. In this context, the development of highly efficient and low-cost materials for thermal energy storage is imperative for a large use of this technology. The storage of thermal energy using reversible thermo-chemical reactions can provide large storage capacities especially at high temperatures. Within this class of materials, the red-ox reactions have particular interest due to the low cost of the materials involved (metal oxides) and the use of air both as reacting gas and heat transfer fluid. Therefore, many efforts are doing to improve the efficiency and reversibility of this type of reactions. In this work the synthesis and thermal performances of a novel mixed metal oxide based on cobalt/nickel metals with spinel structure Co3-xNixO4 is reported. A deep study was carried out in order to find the best synthesis conditions and optimum relative metal content in the structure with the objective of decreasing as much as possible the red-ox temperature. The study allowed to determine the optimum Ni content in the oxide structure in order to minimize the reaction temperature. In particular, a linear relationship of the red-ox temperature as a function of the Ni content was observed, enabling reaching red-ox temperature below 700 °C. These results are very promising and open the perspective of using of these types of materials to a wider field of application.

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.

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).

International audience ; determined in appropriate conditions for space heating (adsorption at 30 °C under a water vapor pressure of 12.5 mbar, and desorption at 100 °C). The composites were ranked as follows: γ-alumina-10LiCl (711 kWh.m -3 ), γ-alumina-10CaCl 2 (700 kWh.m -3 ) and γ-alumina-7MgCl 2 (334 kWh.m -3 ). The second type of composites is made of a silica-PEG matrix and of MgCl 2 or CaCl 2 . Experiments in an open lab-scale reactor demonstrated that the silica-PEG matrix effectively stabilized hydrated 25 wt.% MgCl 2 and 36 wt.% CaCl 2 . Among these, the composite with 25 wt.% of MgCl 2 showed the best performance, with an average heat storage density of 216 kWh.m -3 (adsorption at 30 °C under a water vapor pressure of 12.1 mbar, and desorption at 130 °C).

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.