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Energies 2020, 13, 5859 6 of 23 composite material was reduced (71% of the value obtained for pure Ca(OH)2), while the volumetric storage density was higher than that of Ca(OH)2 powder. The enhancement of the reaction properties of the Ca(OH)2/CaO system was studied via KNO3 addition to Ca(OH)2, which reduced the dehydration reaction duration and decreased the dehydration temperature of the system due to a nitrate–hydroxide interaction as KNO3 melts during the dehydration step [23]. The influence of the amount of added KNO3 on the reaction temperature was then investigated and showed that the minimum dehydration onset temperature (459 ◦C, instead of 494 ◦C for pure Ca(OH)2) was reached with 5 wt% of KNO3 added, with the material losing only 7% of its energy storage capacity, down to 1280 kJ/kg. Additional information on the system is available, as a kinetic study of the effect of KNO3 addition to Ca(OH)2/CaO was conducted [24]. The optimum amount of KNO3 was determined to be around 10 wt% to reduce the charging temperature to 428.49 ◦C and accelerate the reaction with little loss in energy storage density. The cycling stability of the mixture was tested under air and under nitrogen atmosphere. The KNO3-doped calcium hydroxide fared poorly under air, due to the presence of CO2 and carbonation of the sample, but results revealed a good cycling stability under nitrogen atmosphere. SEM observations showed that within the Ca(OH)2/10 wt% KNO3 mixture, the once dehydrated material changes to a flower-like structure when rehydrated. When dehydrated again, the flower-like structure shrinks back into a blocky structure. This modification of the material morphology is attributed to the addition of KNO3 and contributes to enhancing the mass transfer during the storage cycles. Another example of reaction enhancement via doping is the improvement of reduction kinetics of Ca(OH)2 via Li doping [25]. The enhancement of the Ca(OH)2/CaO system was also approached via a modification of the structure of the material, for example with the synthesis of hexagonal boron nitride (HBN)-doped calcium hydroxide composite [26]. The material was tested in TGA/DSC (thermogravimetry analysis/differential scanning calorimetry) and showed better cycling stability than pure calcium hydroxide, showing 67% rehydration after ten dehydration/hydration cycles, with an optimum amount of HBN added at 15 wt%, and the dehydration kinetics were also enhanced. In addition, the composite material exhibited higher thermal conductivity and reaction enthalpy as compared to pure calcium hydroxide. Another studied approach to modify the properties of the system is the modification of the structure of the pure material. Ca(OH)2 nanomaterials with spindle and hexagonal structure were synthesized by a deposition-precipitation method and compared to commercial nanoparticles [27]. The spindle-shaped Ca(OH)2 demonstrated the highest specific surface area in BET and the energy storage density among the tested nanomaterials. In addition, the dehydration/hydration kinetics were improved with the spindle-shaped material and it presented the best cycling stability over ten cycles, as it retained a conversion rate above 70%. Mg(OH)2/MgO is another potential system for TCES which is currently getting attention. Mg(OH)2 was considered at reactor scale, and an economical study was conducted [28]. However, the material suffers from slow and incomplete rehydration, as stated by Müller et al. (2019) [29]. The authors recently studied the rehydration mechanism of MgO and of natural magnesite in order to assess the effect of impurities on the reaction. The enhancement of the TCES system consisting of MgO/Mg(OH)2 was studied via the addition of LiNO3 with 1, 3, 6 and 10 wt% added [30]. The dehydration temperature of the LiNO3-Mg(OH)2 composites was lower, from 289 down to 269 ◦C for 1 wt% and 10 wt% doping, respectively, than that of pure Mg(OH)2 which was measured at 325 ◦C. The dehydration temperature of the LiNO3-Mg(OH)2 composite may then be tuned via the addition of an adequate amount of LiNO3, and the composite materials could sustain more than ten dehydration/rehydration cycles without losing thermal efficiency. In addition, the calculated dehydration rate constant was higher with LiNO3 doping, but the composite material presented lower released heat from the reaction. The mixture LiNO3/Mg(OH)2 was also studied explicitly for TCES at a lower temperature (<300 ◦C) since the addition of LiNO3 to Mg(OH)2 decreases the dehydration temperature of Mg-based system (76 ◦C difference) [31,32].PDF Image | Hi Temp Thermochemical Energy Storage via Solid Gas Reactions
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