Efficient Capture And Absorption Of CaCO₃ Series Based On Solar Thermal Storage

Solar energy is of significance to alleviate the world energy shortage,greenhouse effect at 21th century.However,the current intermittency of solar energy is a drawback for further large-scale applications.The use of energy storage technology can play a"Peak cut"effect,which will effectively solve the negative effects of intermittent of solar energy.Thermochemical heat storage technology based on Calcium-Looping（Ca L）has attracted extensive attention due to its advantages of non-toxic materials,low-cost and high energy density.A traditional topic is that the conversion of CaCO₃ series will be reduced to a very low level after multiple heat storage/release cycles.At present,the problem to be solved is that the CaCO₃ series have a very low optical absorption,which means that they are difficult for the materials to reach the reaction temperature only by absorbing sunlight in the calciner.In this paper,materials with high optical absorption are used to dope to increase the absorption of CaCO₃ series,and to solve the problem of reduced conversion after multiple cycles.Then,CaCO₃series suitable for Ca L-based thermochemical heat storage systems system were studied.The research has been carried out from the following aspects:（1）Pre-calcination experimentThe CaCO₃ series work under the environment of 700-900℃,N₂ and CO₂ gas.Therefore,the absorption of the materials that can be used to improve the absorption of CaCO₃ series must be insensitive to temperature,and it should also be safe and non-toxic.Mn O2,Fe3O4,Ti C and Mn Fe2O4were preliminarily selected for calcination at 850℃.The results of XRD（X-ray Diffraction）proved that Mn O2 generated toxic substances Mn2O3 and Mn3O4 after calcination.The test results of UV-Vis-NIR showed that the absorption of Ti C decreased significantly after calcination,so both Mn O2and Ti C were not suitable for additive materials to improve the absorption of CaCO₃ series.Fe3O4 and Mn Fe2O4 can be used as additive materials to improve the absorption of CaCO₃ series,but Fe3O4 is suitable for doping through simple physical meth ods（such as grinding）,because it is easy to be oxidized into Fe2O3,thus greatly reducing the absorption.Test results of XRD and UV-Vis-NIR have proved that Mn Fe2O4 has the potential to improve the absorption of the CaCO₃ series.The absorption of Mn Fe2O4 remains almost unchanged after calcination in N2 and CO2 environments,and the absorption after calcination in air only decreases by an absolute value of 2.9%.In summary,this paper used Mn Fe2O4 as an additive to improve the absorption of CaCO₃ series.（2）Hollow structure CaCO₃ series thermochemical heat storage materialsHollow structure CaCO₃ series can provide huge space to overcome the adverse effects of shrinkage and collapse caused by volume expansion during the transition from Ca O to CaCO₃.Hollow structure CaCO₃ series are beneficial for long-term storage and release thermal cycles,which were synthesized using carbonaceous microspheres as templates.The effects of precursor concentration,reaction temperature and duration on the part icle size of carbon spheres were studied,and higher concentration,temperature and reaction durations made the size of carbon spheres lager.ICP-OES（Inductively Coupled Plasma Optical Emission Spectrometer）were used to prove the adsorption capacity of carbon spheres for Ca²⁺,Fe³⁺and Mn²⁺,and the ratio of Fe³⁺and Mn²⁺in the precursor solution was determined to be 2:7.The effects of different additions on the hollow CaCO₃series were studied according to the selected ratio.Test results of UV-Vis-NIR and cycles proved that higher additions will lead to higher absorption,but will also reduce the heat release density and conversion.Ca100Fe2Mn7 and Ca200Fe2Mn7 have absorption of 76.8%and 69.7%,respectively,which are 7.11 and 6.45 times that of pure CaCO₃;meanwhile,after 20 cycles,conversions are 72.5%and 77.7%,respectively which are 2.40 and 2.58 times of pure CaCO₃.The heating rate during the process of calcining in air to removing carbon spheres has a great influence on maintaining the hollow structure.The rapid heating rate causes the final CaCO₃ series to shrink and collapse into small particles,which is unfavorable for long-term heat storage and release.（3）Porous structure CaCO₃ series thermochemical heat storage materialsThe porous CaCO₃ series is favorable for the heat release process and cycling stability,which were synthesized by the sol-gel method.The relative proportions of Fe and Mn were optimized when the total amount of Fe and Mn was 10%.It was found that Fe:Mn=8:2 not only has full-spectrum absorption characteristics,but also has high absorption and high heat release density.Taking Fe:Mn=8:2 as an example,the comprehensive effect s of the total addition amount on the CaCO₃series were studied.The results show that the higher the addition amount will lead to a higher absorption,but Ca:Fe:Mn=90:8:2 is the exception,because this group of ratios eventually leads to the formation of Mn Fe2O4.The presence of Mn Fe2O4 can greatly increase the absorption of CaCO₃ in the near infrared.The absorption of Ca90Fe8Mn2 and Ca85Fe12Mn3 are 68.6%and 61.3%,respectively,which are 6.4 times and 5.8 times that of pure CaCO₃.The heat release densities after 20 cycles are2.30MJ/kg and 2.18MJ/kg,respectively,which are 2.4 times and 2.3 times of pure CaCO₃.The amount of citric acid added during the synthesis process also has a great impact on the final CaCO₃series.The absorption of CaCO₃ series based on 0.6 times citric acid synthesis is the highest,reaching70.2%,and the porosity and pore size are the smallest,so the cycle stability is the worst.