High-temperature Thermochemical Energy Storage Research Articles

Calcium Manganite Based Materials for Thermochemical Energy Storage in High Temperature Solar Thermal Plants: Materials Screening

The urgent need for sustainable energy supply requires maximum exploitation of renewable energy sources. The latter, being of intermittent nature, need to be coupled with efficient energy storage. Solar-thermal power-plants are inherently compatible with thermal storage, which is a cost-efficient method of storing energy for later use but the field is currently dominated by sensible heat molten salts used as heat storage media but with a maximum operating temperature of about 560oC. Certain ceramic materials, able to induce reversible reduction-oxidation reactions under air flow, are promising alternatives to molten salts because they can withstand much higher temperatures (>1000oC) and thus can be integrated with high-efficiency air-Brayton thermodynamic cycles. At the same time the chemical energy stored/released during such reduction-oxidation reactions can boost energy storage density by up to 10 times cf. sensible only concepts. In this framework, Ca-Mn-based perovskite compositions were demonstrated to function effectively as energy storage materials. The current work offers insights on material synthesis parameters to achieve relatively high purity Ca-Mn-based compositions and subsequently optimize their redox performance in the course of a preliminary 5-cycle campaign. Moreover, the occurring structural transitions and their corresponding heat effects are also discussed and elaborated upon. This study is the first step towards the, currently in progress, process of synthesising – at multi kg scale – and shaping these compositions into extruded honeycomb-like monolithic structures for subsequent future application in lab- and pilot-scale high temperature thermochemical energy storage systems.

Pilot-scale investigation of high-temperature thermochemical energy storage based on the material system CaO/Ca(OH)2 in a bubbling fluidized bed

Utilizing fluidized bed technology for thermochemical energy storage with CaO/Ca(OH)2 is promising for temperatures from 400 °C to 600 °C in industrial-scale applications. Much progress has been made on laboratory scale to develop this technology in recent years. Continuously changing material properties throughout storage cycles, mainly due to particle breakage, are identified to be challenging for scale-up. Transferability to large-scale systems needs to be examined.Investigations on a pilot scale (di = 257 mm, h = 600 mm, steam for fluidization, 700 °C, 6 barg) are done. A temperature-induced (456 °C discharging and 586 °C charging) and a pressure-induced (1.5 barg discharging and 3.5 barg charging) cyclization is performed. The applicability of standard scaling rules (scaling factor of 17) is possible. Fluidization regimes and breakage rates are transferrable. A detailed experimental characterization of the storage materials fluidization properties that are crucial for scale-up is done using a fluidization test rig (di = 140 mm, h = 658 mm, nitrogen for fluidization, ambient temperature and pressure).

Redox and melting characteristics of Mn-based ores for high-temperature thermochemical energy storage

Redox and melting characteristics of Mn-based ores were investigated to test their potential use in thermochemical energy storage (TCES). Two Mn-based materials (FJ and LY) were natural ores with the Mn content higher than 35 wt%, and one Mn-based material was prepared by adding an MgO–kaolin inert support into LY ores to increase its melting temperature. Cyclic reduction and oxidation reactivity of these Mn-based materials was studied via thermogravimetric analysis (TGA), and the melting behaviors of these materials were investigated by using a melting test setup with an optical camera–image system. It was found that the oxygen capacity of the FJ Mn ore can approach ∼1.50 wt%, while the LY Mn ore had only 0.42 wt%–0.69 wt% oxygen capacity The deformation temperature of the FJ Mn ore is higher than that of the LY Mn ore, and the melting temperatures of the LY Mn ore can be significantly improved with the addition of an MgO–kaolin inert support, while the reactivity is decreased due to the addition of the MgO–kaolin inert material. This study proves that manganese ores with high oxygen capacity and deformation temperature have potential as TCES materials. For some manganese ores with low deformation temperatures, it is necessary to improve their melting temperatures and ensure oxygen capacity for high-temperature TCES applications.

Experimental and simulation study of Mn–Fe particles in a controllable-flow particle solar receiver for high-temperature thermochemical energy storage

CSP systems hold tremendous potential for large-scale and long-duration energy storage. However, their extensive adoption has been impeded by cost constraints. To overcome this hurdle, the development of the next generation CSP systems, focusing on high-temperature heat absorption and storage, is paramount for reducing the LCOE. The integration of TCES technologies shows promise in bolstering energy storage density, stability at high temperatures, and decreasing the LCOE. This study investigates the performance of Mn–Fe particles in a controllable-flow particle receiver operating at high temperatures. Experimental data demonstrate that the particles reach a maximum temperature of 1041.8 °C, resulting in an outstanding outlet particle thermochemical reaction conversion rate of 96.35%. Critical parameters influencing receiver performance were also examined meticulously. It was observed that increased incident flux density significantly enhanced receiver efficiency of 88%. Moreover, effective management of local overheating through the integration of TCES proved indispensable. The outlet particle temperature differential was merely 9.53 °C, and the sudden fluctuations in incident flux led to a limited temperature rise of 4.84 °C, attributing to intensified reaction enthalpy and reaction rate. By optimizing operational stability, continuous operation of CSP systems under high temperatures can be achieved, maximizing efficiency and allowing for greater system flexibility.

Directional oxygen diffusion in Cu1.5Mn1.5O4 crystals for long reversible thermochemistry

Redox activity of metal oxides are the key to the development of high-temperature thermochemical energy storage. However, few studies have reported that the reactivity of metal oxides as thermochemical energy storage is significantly affected by the changes of crystal structure and morphology during redox, caused by ion diffusion and vacancy formation and migration. In this study, we find a previously unreported high-performance thermochemical energy storage material Cu1.5Mn1.5O4 with special structure. Cu1.5Mn1.5O4 shows high energy densities during reduction and oxidation (−219.852 and 207.618 kJ/kg), and keeps 97.06% and 92.98% reactivity after 600 cycles. Voids on specific surfaces are observed after cycling, but not in the fresh sample. We find the voids provide abundant Cu2+, Mn4+ species and lattice oxygen to promote its reaction performance. Cu ions migrate obviously after oxidation and molecular dynamics simulations reveal that DCu (7.4 × 10−14 cm2 s−1) is higher than DMn (5.1 × 10−14 cm2 s−1) and DO (1.7 × 10−14 cm2 s−1). Cu1.5Mn1.5O4 exhibits voids during oxidation due to ion transport described by the Kirkendall effect. In addition, DFT calculations verify that O2− is more easily to adsorb and diffuse on (111) surface, revealing the optimal transport path and directional diffusion mechanism of O2−. These mechanisms can provide ideas for the reasonable design of materials to improve thermochemical reaction performance.

Long-term replenishment strategy of SiC-doped Mn-Fe particles for high-temperature thermochemical energy storage

Thermal chemical energy storage (TCES) is a promising technology for large-scale energy storage, but long-term use of TCES materials can lead to attrition and reaction performance deterioration, compromising heat storage capacity and system continuity. To tackle this challenge, it is imperative to develop effective replenishment strategies and investigate the evolution of TCES materials. In this study, SiC-doped Mn-Fe particles exhibited superior performance, with only 1.87% mass loss ratio after 4.32×105 rotations attrition testing and a reaction conversion of 91.7% even after 800 reaction cycles. However, assessing material suitability for TCES systems necessitates analyzing both attrition resistance and reaction performance. A replenishment strategy was proposed, based on the attrition conversion factor and analysis of dissipated energy of particles in concentrated solar power (CSP) plants. Compared to undoped Mn-Fe particles, the annual replenishment ratio was reduced by one order of magnitude for SiC-doped particles, to only 1.26% (0.5 wt% SiC) and 0.994% (1 wt% SiC). The replenishment strategy and strengthened particles result in curtailed particle fines and particle flow interruption, and enhanced economics. Continuous replenishment strategy can undoubtedly cater to the requisite heat storage capacity demands for protracted system operation and extend system stability, promoting the advancement of TCES technology.

A novel cascade resorption system for high temperature thermochemical energy storage and large temperature lift energy upgradation

The discrepancy between energy supply and demand can be overcome by sorption based thermochemical energy storage system. Thermochemical energy storage system based on metal hydrides is an attractive option due to cyclic stability and high energy storage density. In the present study a novel cascade resorption system is proposed and analyzed for high temperature energy storage. The cascade resorption system uses Mg-50 wt% LaNi5 for energy storage due to its high reaction enthalpy and hydrogen absorption capacity whereas LaNi5 and LaNi4.7Al0.3 are used for hydrogen storage. The temperature at which energy is recovered is increased by two stage cascading desorption method. Therefore, a large magnitude of energy upgradation is possible. An energy upgradation of 52 °C is obtained using the cascade resorption system when the energy source and regeneration temperatures are at 300 and 80 °C, respectively. Thermodynamic analysis shows that the COP and energy storage density of 0.419 and 141.323 kJ/kg, respectively are obtained. In the end, cascade resorption system is compared with the simple resorption system and discussed the relative merits of cascade system. The energy upgradation of 18 and 23 °C is obtained using two simple resorption systems.

Self-assembly CuO surface decorated with NiAl2O4 for high-temperature thermochemical energy storage: Excellent performance and strong interaction mechanism

High-temperature thermochemical energy storage materials with high energy density is the key technical support for the third generation concentrated solar power plants equipped with advanced energy storage system. CuO/Cu2O redox couple is one of the most promising systems due to its low cost and high energy density, but its application is limited by poor reaction performance. Herein, the self-assembly CuO surface decorated with 15 wt% NiAl2O4 is prepared with excellent redox reversibility and ultralong lifespan. Its reaction rate is increased by 4.2 times with re-oxidation degree increasing from 46 % to 99.9 %. The thermochemical energy density of storage and release reach −772.726 kJ/kg and 764.655 kJ/kg, respectively. After 1000 cycles, the degree of reduction and oxidation reaction can keep 99.9 % and 98 %. The material characterization shows that NiAl2O4 are uniformly and firmly loaded on the surface of CuO/Cu2O, which significantly increases the sintering temperature and improves the reaction performance. DFT studies further confirm that NiAl2O4 has a high binding energy with CuO/Cu2O and can increase the formation energy of copper vacancy. The study reveals the anti-sintering modification mechanism of CuO/Cu2O by introducing NiAl2O4, which provides a solution toward sintering resistance of high-temperature thermochemical energy storage materials.

Experimental assessment of Fe-doped CaMnO3 porous pellets in a bench-scale packed-bed reactor for thermochemical energy storage

Thermodynamic cycles requiring high operating temperatures (≥750 °C up to 1200 °C) are currently being explored to improve the sun-to-electricity conversion efficiency of Concentrating Solar Power (CSP) plants. This is calling for the design of new efficient high-temperature (≥750 °C) Thermochemical Energy Storage (TCES) systems, which are fundamental for supplying power on demand during off-sun periods. Recently, Fe-doped CaMnO3 oxides have been proposed as TCES candidate materials, and the determination of their thermodynamics properties via thermogravimetric (TG) analysis allowed evaluation of their heat storage capacity at a very small scale (mg scale). A 10 % Fe-doped CaMnO3 composition (CaMn0.9Fe0.1O3-δ – CMF91) emerged as optimum candidate material for TCES application due to its large heat storage capacity complemented by enhanced thermal stability over multiple oxidation/reduction cycles. To advance in the thermal characterization of these materials at a multigram scale, here we carried out bench-scale reactor tests using CMF91 under conditions considered representative of future CSP plants. The redox-active material has been extruded in the form of porous pellets through a simple production method that required the use of carboxymethylcellulose as a removable binder and water. With the bench-scale reactor tests, the CMF91 pellets showed fully reversible reduction-oxidation in cycles between 500 and 1100 °C under relevant operating pO2 conditions without any deterioration of the pellet's structural integrity. Remarkably, the material exhibited the same δ(T, pO2) profile at this significantly larger scale (~40 g) than the one derived from thermodynamics. Nevertheless, slight differences in oxygen release/uptake profiles between cooling and heating branches can be tracked down to an excess heat generation in the perovskite bed not efficiently extracted by the carrier gas. These results demonstrate that CMF91 oxide is ideally suited for thermal energy storage applications with a large total (thermochemical and sensible) heat storage capacity (~ 916 kJ/kgABO3 or ~ 400 kWh/m3) and good scalability.

Decorating CaO with dark Ca2MnO4 for direct solar thermal conversion and stable thermochemical energy storage

Calcium looping (CaL) process, a thermochemical energy storage technique, probably offers a mid-term if not near-term solution to the next generation Concentrated Solar Power (CSP) plants featured with high temperature over 700 °C. However, CaCO 3 used in the conventional CaL-CSP system is usually white with little absorption of sun light, hindering its application in efficient volumetric solar energy conversion scheme. This work decorates CaO with dark Ca 2 MnO 4 derived from natural manganocalcite, achieving direct solar absorption and simultaneously enhanced anti-sintering ability due to the presence of dark Ca 2 MnO 4 . As a thermal stabilizer, Ca 2 MnO 4 minimized the sintering-induced decay of the materials' energy storage density and ensured high and stable energy storage performance over prolonged operation cycles. Detailed electron microscopy-based analysis confirmed the compositional homogeneity which is identified, together with the porous structures, as the crucial feature to guarantee a stable performance. Over 20 cycles, CaO/Ca 2 MnO 4 nanoparticles still remained energy storage density of 1932 kJ/kg, exceeding that of the limestone-derived reference material by nearly 100%. Additionally, the average solar absorptance of Ca 2 MnO 4 -stabilized calcium composite is remarkably enhanced in comparison with that of conventional limestone. This work paves the way for designing materials with high energy storage density and excellent cyclic stability for simultaneous solar thermal conversion and high-temperature thermochemical energy storage. • CaO was decorated with dark Ca 2 MnO 4 by calcining natural manganocalcite. • Dark Ca 2 MnO 4 enhanced the direct solar absorption ability of CaCO 3 . • The sintering resistance of calcium composites was improved by inert Ca 2 MnO 4 support. • Dark CaO/Ca 2 MnO 4 composite achieved stable thermochemical energy storage.

Designing calcium-looping composites based on lattice energy and phase combination/separation for thermochemical energy storage

The cyclic stability enhancement of the CaO/CaCO 3 thermochemical energy storage system is crucial for high-temperature thermal utilization of solar energy. In this study, we successfully enhance the long-term durability of calcium-looping composites by co-doping TiO 2 and MgCl 2 based on lattice energy and phase combination/separation mechanism. According to molecular dynamics simulations of metallic oxides, TiO 2 shows higher thermal stability and better doping effects in calcium-looping materials. Further experiments indicate that the phase combination mechanism acting on high-valance metallic oxides like TiO 2 can promote cyclic stability of calcium-looping materials. The calcium-based composite doped with 10 mol% TiO 2 prepared by wet-mixing method exhibits high energy storage performance in 15 cycles. MgCl 2 is also used as a dopant to improve cyclic stability of the calcium-based composite in long-term cycles further. The energy storage performance of the composite doped with 5 mol% TiO 2 and 5 mol% MgCl 2 is superior to single-doped composites. The average effective conversion ratio is 0.54 in 100 cycles and the energy storage densities are close to 1000 kJ/kg in most of the calcination/carbonation cycles. Microstructure characterizations indicate that the co-doped composite has abundant micropores and smaller particles with the more uniform size distribution. The modification mechanism at atomic scale will serve as guidelines for dopant selection preliminarily, and the novel TiO 2 /MgCl 2 co-doped calcium-based composite is promising for long-term industrial applications, such as high-temperature thermal utilization of solar energy. • TiO 2 /MgCl 2 co-doped calcium-looping composites were explored for high-temperature thermochemical energy storage. • Doping metallic oxides with higher absolute values of lattice energy show higher cyclic stabilities. • Co-doping TiO 2 and MgCl 2 increases the average energy storage density from 480.6 to 961.2 kJ/kg in 100 cycles. • Calcium-based precursors, types and ratios of dopants affect energy storage performances.

Compressor operated sorption system for high temperature thermochemical energy storage using Mg based hydrides

Thermal energy storage based on sorption technology has been flourishing due to its high storage density, wide operating temperatures, and long-term storage. In this work, an innovative compressor operated sorption thermochemical energy storage (CSTES) system is proposed and investigated for high temperature (≥250 °C) thermal energy storage with a large heat upgradation. The system uses magnesium-based metal hydrides as energy storage material due to their high hydrogen absorption capacity and good reversibility. The proposed CSTES system stores energy in the temperature range between 228 and 587 °C with a maximum heat upgradation of 78 °C. The highest coefficient of performance and energy storage density of 0.91 and 1991.843 kJ/kg is obtained using Mg–Al–Ti and Mg–Al–Ni alloys, respectively. Variation of compression ratio and heat source temperature on the performance of the CSTES system is studied.

Screening of optimal dopants on cobalt-based ceramics for high-temperature thermochemical energy storage

To date, numerous dopants have been investigated to promote the initial heat storage performance or sintering resistance of Co3O4 for thermochemical energy storage. Owing to the different synthesis methods and heat storage test conditions, the performances of these materials are not comparable. In this work, nine dopants, including four unreported dopants, were systematically compared and screened under the same conditions. Results demonstrated that for five redox cycles, the Si- and Mg- doped sample had adverse effects on the conversion rate, and the others maintained the conversion rate above 0.95. Cu and Zr doping exhibited the highest re-oxidation rate values, and those of the others were lower than that of pure cobalt oxide. Moreover, except for Cu and Zr, all dopants adversely affected energy density. According to aforementioned heat storage properties, Cu-, Zr-doped samples, and pure cobalt oxide were subjected to 100 cycles for further comparison. The Cu-doped sample exhibited a higher re-oxidation rate and energy density, and the Zr-doped sample exhibited more stable cyclability. Finally, by doping Cu and Zr, the micromorphology evolution was more stable than that of pure oxide for multiple cycles, and X-ray photoelectron spectroscopy results revealed that the higher surface oxygen was the primary reason for the superior performance facilitated by Cu and Zr. The Cu- and Zr-modified cobalt oxides were the potential candidates for thermochemical energy storage.

MgCr2O4-Modified CuO/Cu2O for High-Temperature Thermochemical Energy Storage with High Redox Activity and Sintering Resistance.

Metal oxides as high-temperature thermochemical energy storage systems with high energy density based on the gas-solid reaction are a critical demand for the future development of concentrated solar power plants. A copper-based system has high enthalpy change and low cost, but its serious sintering leads to poor reactivity. In this study, MgCr2O4 is decorated on the CuO/Cu2O surface to effectively increase the sintering temperature and alleviate the sintering problem. The re-oxidation degree is increased from 46 to 99.9%, and the reaction time is shortened by 3.7 times. The thermochemical energy density of storage and release reach -818.23 and 812.90 kJ/kg, respectively. After 600 cycles, the oxidation activity remains 98.77%. Material characterization elucidates that nanosized MgCr2O4 is uniformly loaded on the surface of CuO/Cu2O during the reversible reaction, and there is a strong interaction between metal oxides and prompter. Density functional theory (DFT) calculation further confirms that CuO/Cu2O-MgCr2O4 has large binding energy and the formation energy of copper vacancy increases, which can effectively inhibit sintering. The modification mechanism of CuO/Cu2O by MgCr2O4 is revealed, which can provide guidance for the reasonable design of thermochemical energy storage materials with sintering resistance and redox activity.

New insights into Mn2O3 based metal oxide granulation technique with enhanced chemical and mechanical stability for thermochemical energy storage in packed bed reactors

High temperature thermochemical energy storage still requires a significant research effort. Most of the research has been carried out with materials at lab-scale, and proper material fabrication techniques need to be developed in order to make feasible the upscaling of the technology. Agglomeration, abrasion, or low volumetric energy density are some negative consequences observed when trying to pass from the powder state to the material shape and amount required for a thermochemical reactor. In this work, an established granulation technique is investigated, using a Si-doped manganese oxide as active material to determine the critical parameters that provide the best chemical and mechanical stability of the granules. The granulation process uses a polymeric binder to give consistency to the granules and later, it is removed to create a porous structure to facilitate the oxygen diffusion in and out of the granule. We identified the positive effect of decreasing the bath temperature to increase the volumetric energy density of the granules. Furthermore, it was observed that increasing the mechanical stability through a high temperature treatment did not decrease the chemical stability of the material. In order to provide the first insights into the scalability of the solution, the chemical and mechanical stability of the granules have been satisfactorily checked during 100 redox cycles, out of which 50 were carried out in a home-made lab-scale packed bed reactor with an inner diameter of 13 mm and another 50 redox cycles in a simultaneous thermal analyzer.

Modified Calcium Manganites for Thermochemical Energy Storage Applications

CaAl0.2Mn0.8O3-δ (CAM28) and CaTi0.2Mn0.8O3-δ (CTM28) are perovskite metal oxides developed for high-temperature thermochemical energy storage (TCES) applications, e.g., in support of air Brayton power generation. Previous reports for these compounds focus on the equilibrium non-stoichiometry (δ) as a function of temperature and oxygen partial pressure (pO2) and the endotherm (or exotherm) accompanying changes in δ resulting from thermal reduction (or re-oxidation). Herein, we report results for elemental substitution and doping (Al, Co, Fe, La, Sr, Ti, Y, Zn, and Zr) of calcium manganites (CM) that establish the preference for CAM28 and CTM28. Techniques employed include conventional (screening and equilibrium) and ballistically heated multi-cycle thermogravimetric analysis (TGA), conventional and high temperature (in-situ) X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Forward-looking results for A-site Y-doped materials, e.g., Ca0.9Y0.1MnO3-δ (CYM910), establish a route to increasing the reduction enthalpy relative to CAM28 and CTM28, albeit at the expense of increased reduction temperatures and raw materials costs. A thermodynamic model presented for CAM28, but extendable to related materials, provides values for the reaction enthalpy and extent of reduction as a function of temperature and oxygen partial pressure for use in design efforts. Taken as a whole, the results support the choice of Al-doped CaMnO3-δ as a low-cost material for TCES in a high temperature air Brayton application, but point the way to achieving higher stored energy densities that could lead to overall cost savings.

First-principles-based multiscale modelling of heterogeneous CoO oxidation kinetics in high-temperature thermochemical energy storage

Metal oxides experiencing reversible oxygen release and uptake reactions involve the scale-span (surface→grain→particle→reactor) problem in a thermochemical energy storage reactor. This study developed a first-principles-based multiscale modelling to calculate the heterogeneous CoO oxidation kinetics. At the surface scale, density function theory (DFT) is used to compute the reaction mechanism, and the data from DFT calculation are integrated into the transition state theory (TST) to calculate the reaction rate constants. At the grain scale, a rate equation theory is proposed to couple the surface reaction with the lattice oxygen in bulk phase. At the particle scale, intraparticle diffusion and external diffusion of gas molecules are calculated by the effective factor and mass transfer coefficient. At the reactor scale, a two-phase fluidization model is adopted to describe the mass transfer of oxygen from bubble phase to emulsion phase. The model was used to simulate the oxidation kinetic characteristics of a newly prepared kaolin-supported Co-based oxide. The reaction mechanism and kinetic rates constants obtained from DFT and TST are directly used to predict the CoO oxidation kinetics instead of by fitting the experimental data. The oxidation kinetics were validated by comparing with the experimental data obtained using a micro-fluidized bed thermogravimetric analysis technology. Results show the oxidation will be fully finished within ~10 s in the temperature of 700–750 °C, and the reaction is first-order when the O2 partial pressure is within 21 vol%. The effect of particle size shows three-zone characteristics of chemical reaction and gas diffusion combined controlling mechanism. It is demonstrated the first-principle-based multiscale modelling can provide an accurate prediction of the CoO oxidation kinetics in thermochemical energy storage.

Experimental Investigation of a Thermochemical Reactor for High-Temperature Heat Storage via Carbonation-Calcination Based Cycles

We report on the design of a modular, high-temperature thermochemical energy storage system based on endothermic-exothermic reversible gas-solid reactions for application in concentrated solar power and industrial thermal processes. It consists of an array of tubular reactors, each containing an annular packed bed subjected to radial flow, and integrated in series with a thermocline-based sensible thermal energy storage. The calcination-carbonation of limestone, CaCO3 ↔ CaO + CO2, is selected as the reversible thermochemical reaction for the experimental demonstration. Synthetized 4.2 mm-mean size agglomerates and 2 mm-mean size granules of CaO with 42 %wt sintering-inhibitor MgO support attained reaction extents of up to 84.0% for agglomerates and 31.9% for granules, and good cycling stability in pressure-swing and temperature-swing thermogravimetric runs. A lab-scale reactor prototype is fabricated and tested with both formulations for 80 consecutive carbonation-calcination cycles at ambient pressure using a temperature-swing mode between 830°C and 930°C. The reactor exhibited stable cyclic operation and low pressure drop, and yielded specific gravimetric and volumetric heat storage capacities of 866 kJ/kg and 322 MJ/m3 for agglomerates, respectively, and 450 kJ/kg and 134 MJ/m3 for granules, respectively.

Ultra-High Temperature Thermal Conductivity Measurements of a Reactive Magnesium Manganese Oxide Porous Bed Using a Transient Hot Wire Method

Abstract Pelletized magnesium manganese oxide shows promise for high temperature thermochemical energy storage. It can be thermally reduced in the temperature range between 1250 °C and 1500 °C and re-oxidized with air at typical gas-turbine inlet pressures (1–25 bar) in the temperature range between 600 °C and 1500 °C. The combined thermal and chemical volumetric energy density is approximately 2300 MJ/m3. The rate at which a thermochemical storage module can be charged is limited by heat transfer inside the solid packed bed. Hence, the effective thermal conductivity of packed beds of magnesium-manganese oxide pellets is a crucial parameter for engineering Mg-Mn-O redox storage devices. We have measured the effective thermal conductivity of a packed bed of 3.66 ± 0.516 mm sized magnesium manganese oxide (Mn to Mg molar ratio of 1:1) pellets in the temperature range of 300–1400 °C. Since the material is electrically conductive at temperatures above 600 °C, the sheathed transient hot wire method is used for measurements. Raw data is analyzed using the Blackwell solution to extract the bed thermal conductivity. The effective thermal conductivity standard deviation is less than 10% for a minimum of three repeat measurements at each temperature. Experimental results show an increase in the effective thermal conductivity with temperature from 0.50 W/m °C around 300 °C to 1.81 W/m °C close to 1400 °C. We propose a dual porosity model to express the effective thermal conductivity as a function of temperature. This model also considers the effect of radiation within the bed, as this is the dominant heat transfer mode at high temperatures. The proposed model accounts for microscale pellet porosity, macroscale bed porosity, pellet size, solid thermal conductivity (phonon transport), and radiation (photon transport). The coefficient of determination between the proposed model and the experimental results is greater than 0.90.

Self-Assembled Structure Evolution of MnFe Oxides for High Temperature Thermochemical Energy Storage.

Thermochemical energy storage (TCES) materials have emerged as a promising alternative to meet the high-temperature energy storage requirements of concentrated solar power plants. However, most of the energy storage materials are facing challenges in redox kinetics and cyclic stability. Iron-doped manganese oxide attracts raising attention due to its non-toxicity, low cost, and high energy capacity over 800°C. However, there are few investigations on the reversibility enhancement of the redox reaction from the microstructural-evolution-mechanism point of view. Herein, bixbyite-type (Mn0.8 Fe0.2 )2 O3 is synthesized and extruded into honeycomb units, which can maintain an 85% initial capacity after 100 redox cycles. It is also found that a self-assembled core-shell MnFe2 O4 @Mn2.7 Fe0.3 O4 structure forms during the reduction step, and then transforms into a homogeneous solid solution of (Mn0.8 Fe0.2 )2 O3 in the following oxidation step. During the reduction step, shells are formed spontaneously from the Mn2.7 Fe0.3 O4 with the MnFe2 O4 as cores due to the lower surface energy, which facilitates the oxygen adsorption and dissociation during subsequent oxidation step. Through the density functional theory calculation, it is revealed that the lower formation energy of oxygen vacancies in the shell contributes to the improvement of oxygen diffusion rate. This study can provide a guideline to design prospective materials for high-temperature TCES.