Thermochemical Energy Storage Material Research Articles

Experimental Study of MgCl2 ⋅ 6 H2O as Thermochemical Energy Storage Material

AbstractHydrates of magnesium chloride are interesting as a thermochemical energy storage material. An open storage system was set‐up, which enables the reaction of water vapor with the salt hydrate at a given relative humidity. It is demonstrated that hydrates of magnesium chloride are only suited if the relative humidity is below 30 %. Otherwise the hygroscopic salt will cause the formation of an aqueous, liquid phase. This liquid inhibits mass transport and leads to the agglomeration of particles. As a consequence of the low partial pressure of the water, the reaction is very slow. Therefore, rather low effective energy densities of around 300 kJ kg−1 were measured. A way to enable higher humidities is to use a solid support like zeolite 13X. However, the adsorption of water vapor on the zeolite, without magnesium chloride, reached higher energy densities even at low relative humidity. Thus, it must be concluded that hydration of magnesium chloride for thermochemical energy storage cannot be recommended for most technical applications.

Investigation of metal oxides, mixed oxides, perovskites and alkaline earth carbonates/hydroxides as suitable candidate materials for high-temperature thermochemical energy storage using reversible solid-gas reactions

Thermochemical storage of solar heat involves the heat effects of reversible chemical reactions and can be coupled with a solar thermal power plant for continuous electricity generation. A comprehensive study of chemical materials for thermochemical heat storage based on reversible redox reactions was conducted. The evaluation of the selected systems was based on their suitable transition temperatures, chemical conversion rates, reversibility, energy storage density as well as general criteria such as toxicity and cost, and resulted in the selection of metal oxides (based on Co and Mn), perovskites and carbonates/hydroxides (of Ca, Sr, Ba).For metal oxides processed in air, Co3O4/CoO was cycled as powder bed and porous foam without reactivity loss, whereas Mn2O3/Mn3O4 showed poor reversibility, which can be enhanced via the synthesis of porous microstructure. The potential improvement of their performance was further studied using transition metal addition. The addition of Fe, Cu or Mn to Co3O4/CoO was found to adversely decrease the redox activity and energy storage capacity. In contrast, the reaction rate, oxygen exchange capacity, reversibility and stability of Mn2O3/Mn3O4 were significantly enhanced with added Fe, Co, or Cu amounts above ∼15, 40 and 30 mol% respectively, while the energy storage capacity was improved accordingly.Perovskites represent another attractive class of thermally-stable redox materials (e.g., energy storage density above 200 kJ/kg for Ca0.5Sr0.5MnO3), but the continuous lattice oxygen transfer may be a barrier for the recovery of the absorbed energy, thus requiring pressure-swing operation with inert gas utilization. Finally, carbonates and hydroxides were identified as promising candidates chiefly because they exhibit the highest energy storage capacities among the considered materials, but they require closed-loop operation. Among these systems, both SrCO3/SrO and Sr(OH)2/SrO stabilized with MgO as well as Ca(OH)2/CaO showed remarkable cycling stability and energy storage densities.

Exploring the thermochemical heat storage capacity of AMn2O4 (A = Li or Cu) spinels

At present, there is a clear interest in developing redox materials with improved properties for high temperature thermochemical energy storage. Chemical modification of manganese oxides with cations such Li and Cu can produce, among other phases, LiMn2O4 and CuMn2O4 spinels, which are feasible candidates for heat storage due to their redox capacity. In this work, these materials were synthesized by Pechini method, and the characterization results confirmed the formation of the targeted phases with some minor contribution of Mn3O4. Thermogravimetrical redox tests in air established that both materials experience fully reversible redox transformations when the temperature is varied between 900 and 1000 °C. These assays showed the stability of both Cu and Li mixed oxides after five consecutive redox cycles and, in accordance, the XRD confirmed that the two samples retaining their spinel crystal structure after the treatment. However, in these conditions reduction temperatures are higher than 940 °C for both oxides and the enthalpies of these transformations are modest, with a maximum value of 36 kJ/kg for LiMn2O4. Alternatively, if the reduction is performed in argon and the oxidation in air, it is possible to increase the amount of oxygen exchanged in the gas-solids reactions and, accordingly, the heat storage capacity. Therefore, the heat recovered in the re-oxidation of CuMn2O4 at 700 °C was 144 kJ/kg (34 kJ/mol), while LiMn2O4 showed an enthalpy of 209 kJ/kg (37 kJ/mol). These changes in the composition of the atmosphere do not affect to the stability of the system and the conversion is maintained after five consecutive cycles. In all cases, the initial spinel phase is recovered after reoxidation, which takes place at remarkably fast rates. Analysis of the intermediate reduced materials reveals a significant complexity of the redox transformations, which imply the formation of LiMnO2 and CuMnO2, among other phases. Accordingly, considering the stability of these systems, as well as the relatively high enthalpies CuMn2O4 and LiMn2O4 appear to be promising materials for thermochemical energy storage.

Development of low-cost inorganic salt hydrate as a thermochemical energy storage material

Thermochemical storage is based on a reversible chemical reaction; energy can be stored when an endothermic chemical reaction occurs and then, energy is released when it is reversed in an exothermic reaction. According to literature and based on the energy storage density (esd), MgCl2·6H2O is a promising candidate material for thermochemical energy storage. Bischofite is an inorganic salt obtained as a by-product material from extraction processes of non-metallic minerals, from Salar de Atacama in Chile, containing approximately 95% of MgCl2·6H2O. Thus, the purpose of this study was to characterize the dehydration reaction of bischofite ore, studied as a low-cost thermochemical storage material. Thermogravimetric data for bischofite were obtained using a TGA instrument coupled to a DSC, at four different isotherms 70°C, 80°C, 90°C and 100°C. The results of conversion reaction (α-t) from the thermal dehydration experiments, demonstrated the first phase of dehydration with the loss of two water molecules. The study showed a typical sigmoid curve with a significant acceleration in the conversion at the beginning of the reaction until it reaches a maximum rate, where the curve keeps constant. The same behavior was observed for all the temperatures used. The kinetics of bischofite dehydration model was determined using the isothermal kinetics method. For this, the thermogravimetric data were fitted to the most used kinetic models (D, F, R, A) and then their respective correlation coefficients R were evaluated. The results indicated that the dehydration reaction of bischofite was described by the kinetics of chemical reaction of cylindrical particles R2. The rate of dehydration reaction and esd of bischofite are lower as compared to synthetic MgCl2·6H2O, at temperatures higher than 80°C. However, the cost of materials to store 1MJ of energy is three times lower for bischofite, which is an evident advantage to promote the reuse of this material left as waste by the non-metallic industry.

High-temperature high-pressure calorimeter for studying gram-scale heterogeneous chemical reactions.

We present an instrument for measuring pressure changes and heat flows of physical and chemical processes occurring in gram-scale solid samples under high pressures of reactive gases. Operation is demonstrated at 1232 °C under 33 bars of pure hydrogen. Calorimetric heat flow is inferred using a grey-box non-linear lumped-element heat transfer model of the instrument. Using an electrical calibration heater to deliver 900 J/1 W pulses at the sample position, we demonstrate a dynamic calorimetric power resolution of 50 mW when an 80-s moving average is applied to the signal. Integration of the power signal showed that the 900 J pulse energy could be measured with an average accuracy of 6.35% or better over the temperature range 150-1100 °C. This instrument is appropriate for the study of high-temperature metal hydride materials for thermochemical energy storage.

Combining in-situ X-ray diffraction with thermogravimetry and differential scanning calorimetry – An investigation of Co3O4, MnO2 and PbO2 for thermochemical energy storage

Metal oxides with multiple accessible oxidation states are considered as promising candidates for high-temperature thermochemical energy storage materials. To shed light on the chemical processes involved in redox thermochemical energy storage materials, in-situ powder diffraction was used in combination with atmospheric control to investigate the redox-reactions of Co3O4, MnO2 and PbO2 under various conditions. Thermogravimetry and differential scanning calorimetry under the same conditions provided information on heat-flows and mass-changes.In contrast to theoretical thermodynamic considerations, only Co3O4 and Mn2O3/Mn3O4 (originating from MnO2) were found fully reversible. In the case of PbO2 for none of the numerous intermediate phases any kind of reversibility was observed. The effect of the O2-concentration in the reactive atmosphere was most distinct for the Mn2O3 system, notably affecting the reduction/oxidation temperatures, whereas for the Co3O4 system only a moderate influence of the O2 concentration was found.Based on the stability of the intermediate phases under various atmospheres, an isothermal TCES-cycle for Co3O4 and Mn2O3 was investigated, triggering the redox-process by an abrupt change of reactive-gas atmosphere. The fast reaction rate combined with a significant down-shift of the reaction temperatures compared to an isokinetic redox reaction suggests application as a chemical heat pump, as well towards a broadened operational profile not only in combination with concentrating solar power plants, but also with e.g. recycling of industrial flue gas heat.

Application of lithium orthosilicate for high-temperature thermochemical energy storage

A lithium orthosilicate/carbon dioxide (Li4SiO4/CO2) reaction system is proposed for use in thermochemical energy storage (TcES) and chemical heat pump (CHP) systems at around 700°C. Carbonation of Li4SiO4 exothermically produces lithium carbonate (Li2CO3) and lithium metasilicate (Li2SiO3). Decarbonation of these products is used for heat storage, and carbonation is used for heat output in a TcES system. A Li4SiO4 sample around 20μm in diameter was prepared from Li2CO3 and SiO2 using a solid-state reaction method. To determine the reactivity of the sample, Li4SiO4 carbonation and decarbonation experiments were conducted under CO2 at several pressures in a closed reactor using thermogravimetric analysis. The Li4SiO4 sample’s carbonation and decarbonation performance was sufficient for use as a TcES material at around 700°C. In addition, both reaction temperatures of Li4SiO4 varied with the CO2 pressure. The durability under repeated Li4SiO4 carbonation and decarbonation was tested using temperature swing and pressure swing methods. Both methods showed that the Li4SiO4 sample has sufficient durability. These results indicate that the temperature for heat storage and heat output by carbonation and decarbonation, respectively, could be controlled by controlling the CO2 pressure. Li4SiO4/CO2 can be used not only for TcES but also in CHPs. The volumetric and gravimetric thermal energy densities of Li4SiO4 for TcES were found to be 750kJL−1 and 780kJkg−1, where the porosity of Li4SiO4 was assumed to be 59%. When the reaction system was used as a CHP, and heat stored at 650°C was warmed and output at 700°C, 14% of the heat supplied by carbonation was needed for self-heating of the material from 650 to 700°C, and the volumetric and gravimetric thermal energy densities for heat output were calculated as 650kJL−1 and 670kJkg−1, respectively.

Agglomeration Behavior of Calcium Hydroxide/Calcium Oxide as Thermochemical Heat Storage Material: A Reactive Molecular Dynamics Study

Thermochemical energy storage is a promising alternation in heat recovery application compared to phase change energy storage. However, cycling instability caused by agglomeration of the reactant particles is the main problem that hinders the application of this system. The present paper focuses on the agglomeration behavior of the calcium hydroxide/calcium oxide particles as a thermochemical energy storage material at the molecular level. Molecular dynamics simulations with the reactive force field were carried out to investigate the agglomeration of two nano-CaO/Ca(OH)2 particles. The results indicated that the agglomeration rate of two Ca(OH)2 particles was faster than that of two CaO particles in the presence of H2O, which was attributed to the greater spatial displacements of atoms in the reactant particles when thermochemical reaction occurred. The present of H2O could accelerate the agglomeration of the CaO particles. Moreover, the hydration of the CaO agglomeration lump was more difficult than tha...

Perovskite materials for hydrogen production by thermochemical water splitting

The performance of perovskites as redox materials for solar thermochemical hydrogen production and energy storage have been studied theoretically by several authors but there are only a few experimental studies about them. In this work, an evaluation of commercial perovskite materials La1−xSrxMeO3 (Me = Mn, Co and Fe) for thermochemical water splitting is presented. The studied perovskites showed suitable redox properties for energy storage in thermogravimetric analysis (TGA) in presence of air, although only the Co-perovskite material (LSC) exhibited cyclability capacity. Experiments of thermochemical water splitting revealed hydrogen production, with increasing yields for Mn-, Fe- and Co-substituted perovskites, respectively. La/Sr ratio in the range of x = 0.2 to 0.4 showed only a slight influence on the amount of hydrogen produced and on the temperature required for the processes. On the other hand, metal substitution type seems to be a critical factor for the thermal reduction of these perovskites, taking place at temperatures above 1000 °C for the Mn-perovskite, 800 °C for Co-material and 900 °C for Fe-material. These results experimentally demonstrate the suitability of solar hydrogen production based on La1−xSrxMeO3 thermochemical cycles. Moreover, the required temperatures for hydrogen production (230 °C) are lower than those commonly reported in literature for “pure” MenOy oxide cycles (500 °C), making perovskite-based cycles a promising alternative. The cyclability studies with the LSC showed a slight decrease in the hydrogen production, derived from the segregation of metallic Co during the thermochemical cycle. This study confirmed the LSC perovskite as a promising material for hydrogen production by solar-driven thermochemical water splitting, although a further insight in the optimization of the operation under consecutive cycles is necessary in order to assess the material as alternative as redox material for a full-scale application.

Technical challenges and future direction for high-efficiency metal hydride thermal energy storage systems

Recently, there has been increasing interest in thermal energy storage (TES) systems for concentrated solar power (CSP) plants, which allow for continuous operation when sunlight is unavailable. Thermochemical energy storage materials have the advantage of much higher energy densities than latent or sensible heat materials. Furthermore, thermochemical energy storage systems based on metal hydrides have been gaining great interest for having the advantage of higher energy densities, better reversibility, and high enthalpies. However, in order to achieve higher efficiencies desired of a thermal storage system by the US Department of Energy, the system is required to operate at temperatures >600 °C. Operation at temperatures >600 °C presents challenges including material selection, hydrogen embrittlement and permeation of containment vessels, appropriate selection of heat transfer fluids, and cost. Herein, the technical difficulties and proposed solutions associated with the use of metal hydrides as TES materials in CSP applications are discussed and evaluated.

Combination of Thermochemical Energy Storage and Small Pressurized Water Reactor for Cogeneration System

Abstract In recent decades, small nuclear reactors for cogeneration systems have been studied. Generally, nuclear plants are operated at steady state, but the heat demand load is not steady. Load leveling using thermochemical energy storage (TcES) is a potential method for overcoming this problem. In this work, the heat load of a town was assumed, and the required amount of TcES material for load leveling was estimated. The amount of material required, relative to the volume of the nuclear reactor vessel, was acceptable for installing a TcES system at the nuclear plant.

The cobalt-oxide/iron-oxide binary system for use as high temperature thermochemical energy storage material

The use of thermochemical reactions is a promising approach for heat storage applications. Redox-reactions involving multivalent cations are recently envisaged for high temperature applications. In temperature range of 900–1000°C, however, where heat storage required for concentrated solar power (CSP) processes only few metal oxides with sufficient heat storage capabilities do exist. Binary systems, on the other hand, could provide a wider range of suitable materials. In the present experimental study the cobalt-oxide/iron-oxide binary system is investigated. For pure iron-oxide the transformation of Fe2O3/Fe3O4 occurs at 1392°C with a reaction enthalpy of 599J/g. The reaction temperature, however, is far too high for CSP applications. Cobalt-oxide, on the other hand, reacts from Co3O4/CoO at 915°C with an enthalpy of 576J/g. Iron-doped cobalt-oxides transform at similar temperature as pure cobalt-oxide but the reaction enthalpy gradually decreases with increasing iron content. Microstructural stability and related long-term reversibility of the chemical reaction, however, is higher with respect to pure cobalt-oxide. Compositions of around 10% iron-oxide were identified having appropriate enthalpies and being beneficial in terms of microstructural stability.

DFT Study on Characterization of Hydrogen Bonds in the Hydrates of MgSO4

Magnesium salt hydrates are potential thermo-chemical energy storage materials considering their high energy storage density and their availability. However, in practical applications, these materials suffer from low efficiency due to their sluggish kinetics and significant structural changes during hydration and dehydration. A DFT PW91-TZ2P level optimization is performed on the various hydrates of magnesium sulfate molecules to study their structural properties. The study identifies a wide network of hydrogen bonds that is significantly influencing the chemical structure of the molecules. These hydrogen bonds appear to cause distortions in the hydrated structures and even hinder the coordination of water with magnesium, resulting in lower-energy isomers. In the case of hexahydrated isomers, the hydrogen bond stabilizes a conformation that has only four coordinated water molecules and is energetically more stable than the conformation with six coordinated water molecules. The sluggish hydration kinetics ...

Thermodynamics and Dynamics of the Mg—Fe—H System and Its Potential for Thermochemical Thermal Energy Storage.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thermodynamics and dynamics of the Mg–Fe–H system and its potential for thermochemical thermal energy storage

The reversible Mg 2FeH 6 and the mixed Mg 2FeH 6–MgH 2 hydride systems [ Eqs. (1) and (2)] turned out to be highly suitable materials for thermochemical thermal energy storage at around 500 °C, for example as the storage of solar or excess industrial heat. The systems are characterized by a high gravimetric and volumetric thermal energy density in comparison to sensible or latent heat storage materials ( Table 1) and have excellent stability in cycle tests. Further advantages include low price of the starting materials, a free choice and constancy of the heat delivery temperature by controlling the applied hydrogen pressure, and the absence of heat losses with time. By means of combined TEM–EDX investigations essential features of the initial formation and the subsequent de- and rehydrogenation processes of Mg 2FeH 6 could be elucidated on a micro- or nanoscale level.

Energy storage using the reversible oxidation of barium oxide

Co-based oxides have been considered as one of the most promising materials for thermochemical energy storage (TCES) systems, however, the high operation temperature limits their applications. Specially, when Co3O4-based materials are used in concentrated solar power (CSP) system, a large mirror field area is required to meet the high endothermic temperature, which significantly increases the cost of a CSP system. In addition, the high cost of Co-based materials also limits its application. Improving the energy density can effectively reduce the use of the materials. Thus, in this work, to reduce the reduction onset temperature and increase the energy density of Co-based oxides, the TCES performance of Co3-xMgxO4 oxides is investigated. Co2.8Mg0.2O4 shows the best TCES performance. The reduction onset temperature of Co2.8Mg0.2O4 is ∼775.6 °C, which is 133.7 °C lower than that of the pure Co3O4 (∼909.3 °C). Mg-decoration can also improve the energy density of Co3O4 by 8.02 %. Furthermore, the in situ XRD spectra of the oxides indicate that precipitation of MgO from the Co3O4 lattice during the reduction reaction result in a metastable structure of the oxide, which is conducive to its reduction reaction. Density functional theory calculations indicate that Co2.8Mg0.2O4 possessed a higher defect formation energy and shorter chemical bonds than the pure cobalt oxide, which result in its lower reduction onset temperature and higher energy density. The cost-benefit analysis results showed that Mg-decoration could significantly reduce the area of mirror field and the use of energy storage material for a CSP system.