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Energies 2020, 13, 5859 9 of 23 in the visible range while a full-spectrum absorption of solar energy was achieved with Cr doping. An impact on the cycling stability of the material was also observed, as it was enhanced via Mn and Al doping, but reduced with the addition of Cr. The solar absorption capacity of CaCO3 was also addressed via the doping of the system with Mn-Fe oxides [53]. Porous CaCO3 was synthesized using calcium gluconate (Ca(C6H11O7)2), and it was doped with Mn-Fe using two different methods—wet grinding using MnFe2O4 powder with ethanol, and adding Fe3+ (Fe(NO3)3) and Mn2+ (Mn(NO3)2) to Ca(C6H11O7)2 in solution to produce Ca-Mn-Fe oxides (Ca:Mn:Fe mole ratio: 100:2:4, 100:4:8 and 100:6:12). The best results were obtained with the mixed oxide containing Ca:Mn:Fe = 100:6:12 mole ratio, which demonstrated a solar absorptance of 90.15% against 11.23% for pure CaCO3. This material also retained above 93% of its heat release capacity over 60 cycles, with over 1438 kJ/kg energy storage density. A similar study also recently reported the synthesis of Ca2FeMnO5/CaCO3 to improve the direct solar energy absorption of the material [54]. The material presented excellent cycling stability and high energy density (2.51 MJ/kg after 20 cycles) and improved the optical absorption up to seven times higher than pure CaCO3. Cheaper sources of calcium carbonate are being researched along with ways to improve their efficiency, in order to recycle waste and provide a cheap source of material for TCES based on calcium looping (CaL). For example, limestone was used for the development of new models of fluidized bed reactors for TCES application at high temperatures, reaching a maximum stable temperature state at 1175 ◦C [55]. Other natural CaCO3 minerals were also evaluated for TCES such as chalk and marble [56]. The various materials possess similar composition but present different cycling stability in CaL-CSP conditions, which is attributed to differences in particle size and microstructure. However, pure CaCO3/CaO material suffers from pore-plugging and the addition of an inert material has proved to help reduce the sintering effect. As an example, the cycling stability of CaO derived from limestone and from pure CaCO3 both suffers from pore-plugging mechanism, but the reaction of CaO derived from dolomite is not limited by this mechanism due to the presence of inert MgO which helps with the diffusion of CO2 into the material [57]. The durability of CaO pellets synthesized from CaO powder with MgO added and from the mixture of limestone and dolomite were compared. Porous CaO powder stabilized with MgO was synthesized, using citric acid as sacrificial template, starting from a solution of calcium and magnesium nitrates [58]. The porous CaO powder showed optimum stability over 20 cycles of calcination/carbonation with 10 mol% of MgO added, greatly enhancing the resistance of the material to sintering. The other synthesis approach using dry mixing of citric acid with limestone–dolomite mixtures was used to make MgO-stabilized CaO porous pellets. The pellets demonstrated negligible capacity losses over the course of 20 cycles as compared to pure CaO powder. However, the pellets made from the limestone–dolomite mixture presented a slightly lower initial thermal energy released than MgO-stabilized CaO powder made from the nitrates reagents, and this difference was attributed to the sintering of impurities present in the limestone–dolomite mixtures. Well-dispersed MgO nanoparticle coating CaO/CaCO3 grains were obtained using calcium and magnesium acetate as precursors [59]. The obtained porous material presented an enhanced resistance to pore-plugging and sintering, together with long-term effective conversion after 30 calcination/carbonation cycles. Samples originating from mined dolomite were also studied, and they demonstrated good qualities as energy storage materials, because they contain impurities, such as quartz, which prevents the grain agglomeration during the calcination/carbonation cycles [60]. The mined samples also had a high porosity which favors gas transport within the material. In this study, the dolomite samples, commercial with and without impurities and mined dolomite, were mixed with molten salt, NaCl:MgCl2 mixture, which was considered to serve as catalyst. The studied mixtures of dolomite and molten salt could sustain over 10 cycles at around 50% capacity between 450 and 550 ◦C without further loss in capacity. A different cheap and renewable option presented recently is the use of biomineralized CaCO3 from waste [61]. Eggshell and snailshell from food waste were investigated as potential precursors for CaL applications. The study revealed that the results obtained on the multicyclic conversion of the biomineralized CaCO3 were comparable to the results reported for limestone,PDF Image | Hi Temp Thermochemical Energy Storage via Solid Gas Reactions
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