Research On Physical And Chemical Characteristics Of Solar Energy Storage Materials Based On Mn₂O₃/Mn₃O₄ System

Solar energy is widely distributed.Solar photothermal power generation system can store heat on a large scale and provide stable power supply,then it may participate in the grid peak adjustment and is expected to bear basic load,which has great potential for development.The thermochemical energy storage system based on redox reaction of metal oxides has high energy storage temperature?>500??and high energy density?200-900 kJ/kg?,especially it can use air as heat transfer fluid and reactant,thus redox energy storage system has broad application prospects.In this paper,the doping modification,reaction kinetics,thermal shock resistance and reaction characteristics of porous energy storage modules are systematically studied for the thermochemical energy storage reaction system based on Mn₂O₃/Mn₃O₄.Against at the slow re-oxidation rate and low ratio of re-oxidation in Mn₂O₃/Mn₃O₄ system,doping Fe₂O₃ in Mn₂O₃ by sol-gel method has improved the reoxidation performance of Mn₂O₃/Mn₃O₄ system,the effects of Fe₂O₃ on the re-oxidation properties of the materials is analyzed,the reasons for the improvement of the reaction properties and the influencing factors of the reaction process are explored,the reaction kinetics of the materials is also studied.The results show that:After doping Fe₂O₃,the reoxidation ratio can increase from 3%to98-100%.The doped material has higher porosity and larger specific surface area,especially the reduction products finding the presence of MnFe₂O₄,which chemical stability is low,these factors promote the reoxidation reaction together.The best doping ratio is 20%Fe₂O₃,the redox reaction time of samples decreases almost proportionally with the increase of the heating/cooling rate,when the partial pressure of oxygen is up to 70%,complete redox reaction can also be achieved.80%Mn₂O₃+20%Fe₂O₃ can maintain good cycle stability in multiple heating and cooling cycles,its activation energy of reductive reaction is 1876.35-2032.54 kJ/mol,and the activation energy of oxidation reaction is 794.22-855.63 kJ/mol,the most probable mechanism function of redox reaction in air is first order reaction?f?a??28?1-a?.Because the actual application of energy storage requires molding materials wih high strength,so that modules can withstand large temperature fluctuations to maintain the integrity of the material,therefore 80%Mn₂O₃+20%Fe₂O₃ powders with Al₂O₃,ZrO₂ and SiO₂ as additives are pressed into rectangular porous module structure by self-designed pressing mold,then adopting the electric furnace heating-cooling method to test the thermal shock resistance of the modules.The results show that:Adding 10%or more SiO₂,20%or more ZrO₂ can significantly improve the thermal shock resistance of 80%Mn₂O₃+20%Fe₂O₃ module,increasing the number of thermal cycle shocks from 45 times to more than 250 times.The negative expansion coefficient of SiO₂ and the composite metal oxides generated in the calcining process are important reasons for the improvement of the thermal shock resistance of modules.The phase-change toughening mechanism of ZrO₂ can against crack propagation and hinder the diffusion of large cracks,thus improving the strength and toughness of the modules.In addition to good thermal shock properties,the moulded energy storage materials should also have good reaction characteristics,therefore the self-designed experimental system of electric heating energy storage is used to explore the changes of unit oxygen release/absorption,unit reaction heat release,module reduction reaction ratio and module redox ratio of 80%Mn₂O₃+20%Fe₂O₃ modules after undoped or doped 10%SiO₂ in different experimental temperature zones,exploring the influence of SiO₂ on reaction rate of modules.The results show that:When SiO₂ is not doped,the oxygen transfer ability of the modules are better,the reaction performance of the modules are not affected by temperature when the maximum temperature of the tubular furnace is during 1000-1100?,and the modules response performance is the best under 700-1060?,while the unit oxygen release/absorption of modules is 773/763?mol/g,the unit reaction heat release reaches the highest value?167.27 J/g?,the reoxidation ratio is as high as98.81%,which is close to complete reoxidation;After doping 10%SiO₂,reaction performance of the modules are greatly affected by temperature,the degree of reduction and reoxidation reaction significantly reduced when the maximum temperature of the tubular furnace decreases.The modules response performance is the best under 700-1060?,while the unit oxygen release/absorption of modules is 747/710?mol/g,the unit reaction heat release of the module is 165.75 J/g,the reoxidation ratio is 97.55%.The reaction time of modules decreases from 30 min to 20 min after doping 10%SiO₂,the rapid oxidation zone of modules shifts to higher temperature zone,and the grade of reaction exothermic will increase.