Thermochemical Heat Storage Materials Research Articles

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...

Diffusive transport of water in magnesium chloride dihydrate under various external conditions for long term heat storage: A ReaxFF-MD study

Pure Magnesium chloride hydrates are exceptional materials for long term thermochemical heat storage. In a heat storage cycle, the material is charged by dehydration, taking up heat, and discharged by hydration, generating heat. On charging, the H2O molecules released by the dehydration reaction have to diffuse through the solid material. Gas–solid interactions play an important role in mass and heat transport throughout the volume of the storage materials. Under certain experimental conditions, the mass transport may become a rate-determining step. In order to simulate the complex reaction coupled diffusion process, a reliable MgCl2–H2O reactive forcefield (ReaxFF) is parameterized using a single parameter search algorithm. The parameters of ReaxFF are trained against a set of data obtained from density functional theory (DFT). ReaxFF-MD simulations are carried out on a 2D periodic slab of MgCl2⋅2H2O to simulate various possible operating conditions of temperature, external water vapor pressure, incomplete dehydration layers and various vacancy defects during a charging–discharging cycle. The diffusion coefficient of H2O through the 2D periodic slab of dihydrate is observed as 1.24±0.37×10−10 m2/sec at 300 K. The diffusivity increases with temperature and follows the Arrhenius equation. External water molecules impede the dehydration and promote the diffusion of water. The vacancies of MgCl2⋅2H2O molecules are characterized using vibrational density of states obtained from ReaxFF-MD simulations. The vacancy concentrations for MgCl2⋅2H2O molecules have been varied from 1.38% to 4.16%, which result in a diffusivity enhancement from 32.9% to 107.7% when compared with perfect slab. The incomplete hydration at the surface and the mid-layer increases diffusivity by 76.7% and 75.0%.

A Facile Method to Construct Graphene Oxide–Based Magnesium Hydroxide for Chemical Heat Storage

ABSTRACTMg(OH)2 nanoparticles anchored on graphene oxide (GO) were facilely prepared by a hydrothermal method. The main diameter scale of nanoparticles on the graphene sheet was about 25–50 nm shown by transmission electron microscopy characterization results. X-ray diffraction results indicated that the nanoparticles are in accordance with the data on magnesium hydroxide. This material exhibited significantly improved heat storage capacity and a higher hydration rate than pure magnesium oxide, and the introduction of GO leads to greatly increased thermal conductivity of the nanocomposites. As a novel thermochemical heat storage material, Mg(OH)2/GO has huge potential for high-efficiency energy systems.

Thermochemical heat storage materials – Performance of mixed salt hydrates

Thermochemical heat storage is highly promising, in particularly with a view to long-term heat storage. For the implementation of heat storage in households, thermochemical reactions in the low temperature range below 120°C are important. Especially salt hydrates such as MgCl2, CaCl2 or MgSO4 were tested with micro gravimetric methods for their suitability. However, the cycle stability of consecutive charging (dehydration) and discharging (hydration) reactions of these materials was low and could be improved only by control of the water uptake (i.e. discharging time) to prevent overhydration. In contrast, mixtures of these salt hydrates showed significant improvements in cycle stability, mass and enthalpy balances. The experiments also showed that the cycleability of all investigated materials increased if hydration and dehydration reactions were performed under constant vapor pressure of 21mbar. Contrary to other materials, the mixture of CaCl2 and MgCl2 showed good cycleability under all tested conditions. In addition, the mixture showed superior kinetic properties. Additionally, there is evidence of tachyhydrite (CaMg2Cl6⋅12H2O) formation during cycling of the mixture by the use of XRD after the thermal analysis. Further investigations will be performed to identify further synergies, ideal mixing ratios and formed phases.

Manganese oxide-based thermochemical energy storage: Modulating temperatures of redox cycles by Fe–Cu co-doping

Manganese oxides are promising materials for thermochemical heat storage at high temperatures due to their low price and toxicity. However, doping with other transition metals is needed for stabilizing the oxidation kinetics, enhancing the material durability over prolonged charge-discharge cycling. As well, this system presents a high thermal hysteresis if compared with other redox couples. Namely, metal oxide reduction takes place at higher temperatures (ca. 200°C) than re-oxidation of the reduced phase. Narrowing the hysteresis loop for the manganese oxide system means that heat is stored and released in a closer range of temperatures, which could boost the energy storage efficiency.In this work, it was studied the effect that co-doping of Mn oxides with Fe and Cu has on the redox temperatures of both forward and reverse reactions. Materials were prepared by a variation of Pechini method and characterized by XRD and SEM. The capacity to withstand several redox cycles was analyzed by thermogravimetric analyses. It was found that addition of certain amount of both dopants narrowed the thermal hysteresis of such redox couple, at the same time that stable reversibility was achieved.

Calcium oxide based materials for thermochemical heat storage in concentrated solar power plants

The present study relates to the preparation of mixed calcium oxide–alumina compositions as candidate materials for a cyclic thermochemical hydration–dehydration scheme at moderate to high temperatures (e.g. 400–600°C) that can offer the possibility of short and long term energy storage, particularly suitable for concentrated solar power installations. The synthesized materials were assessed in terms of their cyclic hydration–dehydration performance in the temperature range of 200–550°C. Acknowledging the fact that the particular thermochemical scheme has been identified to result in substantial cycle-to-cycle fragmentation of pure CaO particles which is detrimental to particle reactor bed concepts, one of the main purposes of using Al as additive is related to materials structural enhancement. To this respect, preliminary studies related to macro-structural integrity assessment were also conducted. In addition, the performance of synthesized material is compared to the one of natural lime (benchmark material). The additive content spanned over a wide range of Ca/Al molar ratios, namely from 95/5 to 52/48, while two different calcium oxide precursors, i.e. calcium nitrate and calcium acetate, were employed. Fresh and hydrated compositions were characterized in-detail with respect to their physicochemical properties in order to correlate different behaviors with certain attributes of the materials. Synthetic materials, both calcium nitrate and calcium acetate derived, favored the formation of Ca/Al mixed phases. The latter led to materials with higher surface areas and, for a given Ca/Al ratio, resulted to higher hydration/dehydration performance. Mixed oxides, although capable of being hydrated at ambient temperature, did not participate in the reaction scheme at temperatures ⩾200°C and thus presence of such phases resulted in considerable decrease of hydration/dehydration capacity versus the one of natural lime. On the other hand, the presence of such mixed compositions improved, albeit not dramatically, macro-structural integrity. A relatively good combination of hydration–dehydration performance with better-than-natural lime structural integrity was achieved for the mixed materials with a Ca/Al molar ratio equal to 89/11 and 81/19 molar ratio. The Ca-precursor used in these materials slightly affected their cyclic performance with the ex-CaN ones presenting better behavior.

Thermodynamic and kinetic study of the dehydration process of CaO/Ca(OH)2 thermochemical heat storage system with Li doping

Thermochemical heat storage material is very promising to realize long-term heat storage. In this paper, Li-doped Ca(OH)2 prepared by ball grinding is compared with pure Ca(OH)2 to study the influence of Li doping amount on heat storage speed by thermogravimetric (TG) experiment. Furthermore, the heat storage capacity (including the reaction enthalpy and specific heat capacity) with Li doping is studied, and the related kinetic control equation is derived. The results show that the heat storage capacity remains almost unchanged with Li doping, but the kinetic process of Ca(OH)2 heat storage with Li doping is divided into two stages which can be analyzed from the molecular structure. There is an interesting coincidence in the reduction of energy between the apparent activation energy and the energy barrier of Ca(OH)2 dehydration process with Li doping. Finally, the theoretical results using the kinetic control equations agree well with the experimental results.

Zeolite–MgCl2 composites as potential long-term heat storage materials: Influence of zeolite properties on heats of water sorption

Thermochemical heat storage materials such as MgSO4 and MgCl2 offer high energy storage densities and an inexpensive and clean means of long-term solar energy storage. The aim of this paper is to investigate zeolite–MgCl2 composites as potential heat storage materials, studying the link between the composites physico-chemical properties and the heat of dehydration/hydration obtained. Four series of wide pore zeolite composites containing 5–15wt% MgCl2 have been prepared and characterized. Zeolites Na-X, Mordenite and Y (Na and H) were selected with the goal of impregnating the highest quantity of MgCl2, while achieving the highest heat release/store possible. Heats of water sorption/desorption studies of MgCl2 composites using thermogravimetry coupled to differential scanning calorimetry (TG–DSC) were recorded and compared with those of analogous MgSO4 composites. Both Na-Y and H-Y containing 15wt% MgCl2 achieved the highest heats of water sorption (1173 and 970Jg−1, respectively). BET surface area and pore volume measurements lead to clarification that a retention of the host zeolites large pore volume upon impregnation is essential to allow accessibility of water molecules to the dehydrated salt particles. It was shown that MgCl2 composites offered a far superior heat release upon hydration in comparison to that of MgSO4 composites, due to the higher energy density of MgCl2, but also due to the blocking of zeolite pores with MgSO4 particles.

Experimental Studies of the Mechanism and Kinetics of Hydration Reactions

The mechanism and the kinetics of hydration reactions are important for the application of a salt hydrate as a thermochemical heat storage material. MgSO4·H2O and Na2SO4 were chosen in this study because they are both promising candidates for such an application. Considering that the hydration of these salts yields MgSO4·7H2O and Na2SO4·10H2O as the reaction products, the maximum overall heat effect can be calculated from the heat of condensation of water vapor (44kJ mol–1) and the heats of hydration of 75 kJ·mol-1 (for MgSO4·H2O) and 81kJ mol-1 (for Na2SO4). Based on the densities of the two hydrated phases, this results in the very high theoretical energy densities of 2.3 GJ·m-3 and 2.4 GJ·m-3, respectively, for MgSO4·7H2O and Na2SO4·10H2O. Not only the energy density is important for the dimensioning of a storage system, but also the kinetics of hydration reactions play a major role for the application as storage material. In the present study, hydration reactions under varying climatic conditions were investigated by using water vapor sorption measurements and in-situ Raman microscopy. Using the phase diagrams, it can be clearly shown that the mechanism and the kinetics depend on the climatic conditions. Below the deliquescence humidity of the lower hydrated phase the hydration proceeds as solid state reaction, whilst above the deliquescence humidity a through solution mechanism takes place.

Heats of water sorption studies on zeolite–MgSO4 composites as potential thermochemical heat storage materials

The use of magnesium sulphate as a means for long term heat storage, offers a compact, clean, and cheap way of storing solar energy during the summer season, and due to its high energy storage density, is a superior candidate for chemical heat storage. The impregnation of MgSO4 in a highly porous host provides a variety of enhancements, particularly offering a large absorption surface area available for MgSO4 and H2O molecules. Four series of wide pore zeolite composite materials containing varying quantities of MgSO4 (from 5 to 15wt%) have been prepared and characterized. Heats of sorption/desorption cycles were recorded using thermogravimetry coupled to differential scanning calorimetry (TG-DSC). Zeolites Na–X, Mordenite and Y (Na and H) were selected, with the aim of impregnating the highest quantity of MgSO4, whilst achieving the highest heat release/store possible. Zeolites Na–Y and H–Y composites containing 15wt% MgSO4 achieved the highest heats of hydration (1090 and 867Jg−1 sample, respectively) compared to zeolite Na–X and Mordenite composites containing 15wt% MgSO4 (731 and 507Jg−1, respectively) and was attributed to the retention of the high surface area and pore volume of the host zeolite.

Calorimetric studies of thermochemical heat storage materials based on mixtures of MgSO 4 and MgCl 2

Attapulgite granulate impregnated with mixtures of MgSO 4 and MgCl 2 hydrates was investigated for suitability as a composite thermal energy storage material. These mixtures were chosen because of the very different deliquescence relative humidities of the salts. The thermochemical heat storage of these materials was characterized by measurements of isothermal heat of sorption and thermogravimetry (TG). The salt mixtures showed a different sorption behavior compared to pure MgSO 4. The reduction of the deliquescence relative humidity of the mixture by the partial substitution of MgSO 4 by MgCl 2 increases the capacity of condensation and therefore the released heat. The energy density of the composite heat storage material containing a salt mixture of 20 wt% MgSO 4 and 80 wt% MgCl 2 was 1590 kJ/kg measured by calorimetry (at 30 °C/85% RH) with a desorption temperature of 130 °C.