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Abstract In the present work carbonatization of selected metal oxides was investigated at temperatures below 100 °C and CO 2 pressures of 1-55 bar (25 °C) and 110 bar (60 °C) by in-situ powder diffraction, as well as experiments in a pressure reactor. Moisture was confirmed to be a crucial requirement for any successful carbonatization, as under dry conditions always the initial metal oxide was recovered. Under the applied conditions a successful carbonatization of CaO, MnO, ZnO, CoO and PbO was achieved, highlighting, that although commonly studied at high temperatures, carbonatization of metal oxides can also be successfully operated in the presence of moisture with an elevated CO 2 pressure as driving force. [ABSTRACT FROM AUTHOR]

Abstract Thermochemical energy storage based on the Mg(OH) 2 / MgO cycle is considered as attractive process for recycling of industrial waste heat between 350-400 °C. Based on a recent study, revealing MgCO 3 -derived MgO as highly attractive starting material for such a storage cycle, three different natural magnesites were investigated to analyze the process-performance using industrially available raw-materials. Whereas, the varying amounts of Ca2+ and Fe2+ as major impurities did not notably affect the reactivity / cycle stability within the series, compared to the analytically pure reference material a notable decrease of performance was evidenced. [ABSTRACT FROM AUTHOR]

The thermochemical energy-storage material couple CuSO4/[Cu(NH3)4]SO4 combines full reversibility, application in a medium temperature interval (<350 °C), and fast liberation of stored heat. During reaction with ammonia, a large change in the sulfate solid-state structure occurs, resulting in a 2.6-fold expansion of the bulk material due to NH3 uptake. In order to limit this volume work, as well as enhance the thermal conductivity of the solid material, several composites of anhydrous CuSO4 with inorganic inert support materials were prepared and characterized with regard to their energy storage density, reversibility of the storage reaction, thermal conductivity, and particle morphology. The best thermochemical energy storage properties were obtained for a 10:1 CuSO4-sepiolite composite, combining an attractive energy storage density with slightly improved thermal conductivity and decreased bulk volume work compared to the pure salt. [ABSTRACT FROM AUTHOR]

Metal oxides are promising potential candidates for thermochemical energy storage in concentrated solar power plants. In particular, the Cu2O/CuO system is suitable because of its high energy density, applied temperature interval, and reduced cost compared to the CoO/Co3O4 system. In heterogenous gas-solid reactions, the pressure affects the kinetics significantly. To quantify this effect for oxidation of Cu2O to CuO, isothermal runs between 800 °C and 930 °C at different oxygen partial pressures (0.1, 0.2, 0.5, and 1.0 bar) were conducted with thermogravimetric analysis (TGA). Defined fractions of CuO samples (1–100 µm) were analyzed with X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, and scanning electron microscopy (SEM) analysis. The kinetic analyses were performed with extended non-parametric kinetics (NPK), which is applied for the first time to consider the pressure term in the general kinetic equation in addition to the conversion and the temperature term. The results show how the oxygen partial pressure impacts the kinetics and how reparameterization of the pressure term affects the kinetic analysis of the oxidation reaction of Cu2O to CuO. The best conversion model is a two-dimensional Avrami-Erofeev model with an activation energy of 233 kJ/mol. The kinetic models for conversion, temperature, and pressure presented in this work provide one of the most important requirements for reactor designs. [ABSTRACT FROM AUTHOR]

Thermochemical energy storage is considered as an auspicious method for the recycling of medium-temperature waste heat. The reaction couple Mg(OH)2–MgO is intensely investigated for this purpose, suffering so far from limited cycle stability. To overcome this issue, Mg(OH)2, MgCO3, and MgC2O4·2H2O were compared as precursor materials for MgO production. Depending on the precursor, the particle morphology of the resulting MgO changes, resulting in different hydration behavior and cycle stability. Agglomeration of the material during cyclization was identified as main reason for the decreased reactivity. Immersion of the spent material in liquid H2O decomposes the agglomerates restoring the initial reactivity of the material, thus serving as a regeneration step. [ABSTRACT FROM AUTHOR]

The viability of thermochemical energy storage for a given application is often determined by the reaction kinetics under process conditions. For high exergetic efficiency the process needs to operate in close proximity to the reaction equilibrium. Thus, accurate kinetic models that include the effect of the reaction equilibrium are required. In the present work, different parametrization methods for the equilibrium term in the General Kinetic Equation are evaluated by modeling the kinetics of two reaction systems relevant for thermochemical energy storage ( CaC 2 O 4 and CuO) from experimental data. A non-parametric modeling method based on tensor decompositions is used that allows for a purely data driven assessment of different parametrization methods. Our analysis shows that including a suitable equilibrium term is crucial. Omitting the equilibrium term when modeling formation reactions can lead to seemingly negative activation energies. Our tests also show that for formation reactions, the reaction rate decreases much faster towards the equilibrium than theory predicts. We present an empirical modeling approach that can predict the reaction rate of gas-solid reactions, regardless of the shortcomings of theory. In this way, non-parametric modeling offers a powerful tool for applied research and may contribute to the advancement of the thermochemical energy storage technology. • Different forms of the pressure/equilibrium term in the General Kinetic Equation are assessed. • Omitting the equilibrium term may be the cause for negative activation energies. • A data-driven modeling method is presented for predicting the reaction rate of gas-solid reactions. [ABSTRACT FROM AUTHOR]

• Salt hydration is viewed as two steps: water adsorption and hydrate formation. • Hydration is limited by nucleation at low supersaturations of water vapour. • Hydration is limited by diffusion at high supersaturations of water vapour. • The vapour pressure of the wetted salt is fixed at the metastable zone boundary p *. • The pressure difference with respect to the wetting film, p − p * drives the hydration. K 2 CO 3 is a promising salt for thermochemical heat storage. For a high performance, the thermochemical reaction must take place as close as possible to the equilibrium, while ensuring sufficient reaction rates. In this work, we studied the near-equilibrium hydration kinetics of K 2 CO 3 and other salts (CuCl 2 , MgCl 2 and LiCl). We proposed a generic two-step mechanism for the hydration of salts, consisting of (1) adsorption of water vapour and dissolution of ions from the initial phase (a wetting film) and (2) formation of the hydrate crystal (crystallisation from the wetting film). The two steps are assumed to be in momentarious balance during the hydration process. As a result, nucleation is rate limiting at low supersaturations of water vapour (inside the metastable zone), and water diffusion to the wetting film is rate limiting at high supersaturations (outside the metastable zone). We have seen that the vapour pressure of the wetting film stabilises at the metastable zone boundary p *. The driving force for hydration outside the metastable zone (MZ) is therefore the pressure difference between the atmospheric vapour pressure and the vapour pressure of the wetting film, p − p *. Non-Parametric Kinetic analysis of the hydration of K 2 CO 3 indicates that nucleation plays a central role inside the metastable zone (at low supersaturations) as expected. Outside the MZ, the analysis suggests a steady conversion rate, in agreement with a water vapour diffusion limitation. The diffusion limited process at high supersaturations hardly depends on the temperature, but mainly on the pressure difference, as expected. It is further shown that the diffusion limited process can be characterised with an apparent activation energy. However, this apparent activation energy is in fact the hydration enthalpy and does not refer to a real energy barrier. [ABSTRACT FROM AUTHOR]

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 Co 3 O 4 , MnO 2 and PbO 2 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 Co 3 O 4 and Mn 2 O 3 /Mn 3 O 4 (originating from MnO 2 ) were found fully reversible. In the case of PbO 2 for none of the numerous intermediate phases any kind of reversibility was observed. The effect of the O 2 -concentration in the reactive atmosphere was most distinct for the Mn 2 O 3 system, notably affecting the reduction/oxidation temperatures, whereas for the Co 3 O 4 system only a moderate influence of the O 2 concentration was found. Based on the stability of the intermediate phases under various atmospheres, an isothermal TCES-cycle for Co 3 O 4 and Mn 2 O 3 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. [ABSTRACT FROM AUTHOR]

The dehydration and subsequent rehydration of calcium oxalate monohydrate has yet to find application in thermochemical energy storage. Unlike for many other salt hydrates, complete reversibility of the dehydration-rehydration reaction was observed. Additionally, it was found that the rehydration temperature is strongly affected by the water vapour concentration: Full reversibility is not only achieved at room-temperature, but, depending on the water vapour concentration, at up to 200 °C. This allows isothermal switching of the material between charging and discharging by a change of the H 2 O-partial pressure. Cycle stability of the material was tested by a long-term stress experiment involving 100 charging and discharging cycles. No signs of material fatigue or reactivity loss were found. In-situ powder X-ray diffraction showed complete rehydration of the material within 300 s. The experimental findings indicate that the CaC 2 O 4 ·H 2 O/CaC 2 O 4 system is perfectly suited for technical application as a thermochemical energy storage medium. [ABSTRACT FROM AUTHOR]

Thermochemical energy storage (TCES) is considered as an emerging green technology for increased energy utilization efficiency, thereby achieving a reduction of greenhouse gases. Various reaction systems based on different substance classes (e.g. hydrates, hydroxides, oxides) were suggested and investigated so far. Nevertheless, the number of know reactions which are suitable is still limited, as the main focus concentrates on the investigation of a handful known substances, their further improvement or applicability. To find novel promising candidates for thermochemical energy storage and also to allow for a broader view on the topic, this work present a systematic search approach for thermochemical storage reactions based on chemical databases. A mathematical search algorithm identifies potential reactions categorized by the reactant necessary for the reaction cycle and ranked by storage density. These candidates are listed in the online available VIENNA TCES-database, combined with experimental results, assessing the suitability of these reactions regarding of e.g. decomposition/recombination temperature, reversibility, cycle stability, etc. [ABSTRACT FROM AUTHOR]

• Dehydration conditions yielding highly reactive MgO for energy storage were found. • Controlling dehydration avoids the need for chemical modification by doping. • Controlled dehydration prevents premature material fatigue. • Impurities in MgO from natural magnesite significantly affect rehydration performance. Systematic variation of the dehydration temperature and time enables the preparation of highly reactive magnesium oxide for thermochemical energy storage purposes. The reactivity of the MgO, resulting from varying dehydration conditions has been studied by a comparative approach, including reactive surface area, particle morphology and reactivity towards rehydration. For the rehydration an in-situ powder X-Ray diffraction setup is used, allowing for continuous monitoring of Mg(OH) 2 formation. The outcome of this investigation was subsequently applied to MgO from natural magnesites to assess the impact of impurities in the material on rehydration reactivity. [ABSTRACT FROM AUTHOR]