Thermodynamic Analysis And Performance Enhancement Of High-temperature Thermochemical Cycle Based On Non-stoichiometric Metal Oxides

Solar-driven high temperature thermochemical energy storage technology,based on the redox of metal oxides,can store,transform and utilize solar thermal energy in the form of chemical energy.This technology can effectively overcome the intermittent,fluctuating and low energy grade of solar energy,which is of great significance to the development of renewable energy utilization technology under the strategic goal of "emission peak and carbon neutrality" in China.Non-stoichiometric metal oxides,represented by perovskite and ceria,have great potential for application in the field of thermochemical energy storage technology due to their unique redox reaction mechanism.However,the current thermochemical energy storage technology based on the non-stoichiometric metal oxides still faces some key bottlenecks,such as poor reaction performance of materials,low reaction and energy conversion efficiency.This thesis focuses on two important technologies:high-temperature thermochemical heat storage and thermochemical CO₂/H₂O splitting,aiming at dev eloping efficientt thermochemical reaction systems and enhancing the performance of thermochemical cycle.The main contents and conclusions are summarized asfollows:（1）Based on the closed system and the open system,the thermodynamic analysis model of the thermochemical H₂O-splitting cycle has been established.The main thermodynamic driving forces of the reduction and the H₂O-splitting reaction based on the non-stoichiometric metal oxide were clarified,and the enhancement method of the cycle performance was also proposed.The concept of"temperature-swing factor" has been proposed to evaluate the variation of cyclic temperature,and the quantitative relationship among the core parameters such as the reaction temperature,temperature-swing factor,reaction conversion rate was established.A thermodynamic criterion for evaluating the redox characteristics of metal oxides has been proposed.The reduction enthalpy and entropy of metal oxides are taken as the main parameters to distinguish the energy storage application scenarios of metal oxides with different redox properties The research has laid a theoretical foundation for the subsequent research of thermochemical energy storage technology in this paper.（2）The doped-CaMnO₃-δ perovskite has been investigated to meet the technical requirem ents of developing non-stoichiomietric metal oxides for thermochemical heat storage.By doping active element Co and inert element Zr,the mechanism of element doping on the redox characteristics and thermochemical heat storage properties of perovskite oxides were studied.Based on thermo gravimetric analysis,the effect of Co and Zr doping on the reduction oxygen release capacity of CaMnO3-δ was clarified.In addition,the effects of the solid solubility of Co and Zr in CaMnO3-δ perovskite and the formation of impurity phase on the redox characteristics of the materials were determined through XRD,SEM,EDS,and other characterization method.The pure-phase solid solutions CaCo0.05Mn0.95O3-δ and CaZr0.1Mn0.9O3-δ have been developed for thermochemical thermal storage at 1000℃,in which CaCo0.05Mn0.95O3-δ)could achieve the highest thermochemical thermal storage density of 571 ± 64 kJ/kg in the current class of materials.Finally,the different mechanisms of Co and Zr doping on the redox properties of CaMnO3-δ perovskite were revealed by density functional theory calculation,providing a basis for the development of perovskite for thermochemical heat storage.（3）The Sr and Mn co-doped SmCrO₃ perovskite has been proposed for isothermal and near-isothermal thermochemical CO₂ splitting,and the influence of Sr and Mn contents and synthesis methods on the material phase structure was investigated.The cycle performance of Sm1-xSrxCr1-yMnyO3 for thermochemical CO₂ splitting was tested based on the thermogravimetric analyzer and self-made thermochemical cycle performance test platform.The results show that the reduction of Sr content and the increase of Cr content could enhance the isothermal and near-isothermal thermochemical cycle performance.The "φ=peak/full width at half maxima" based on the curves of CO production rate has been proposed as an evaluation index for the performance of the CO₂-splitting reaction.Taking account of both thermodynamic and kinetic characteristics,the index is expected to evaluate the reaction performance under different reaction conditions with high sensitivity.Based on this method,SSCM7373 perovskite with potential application of the isothermal and near-isothermal thermochemical CO₂-splitting cycle was obtained.Finally,the reaction mechanism of thermochemical CO₂ splitting based on Sm1-xSrxCr1-yMnyO3 was analyzed by the density functional theory calculation,revealing that the oxygen migration process is the decisive step in the thermochemical CO₂-splitting process.The results provide an important basis for improving the performance of isothermal and near-isothermal thermochemical CO₂-splitting cycles.（4）In order to address the thermodynamics limitation of oxidation reaction in the isothermal and near isothermal CO₂/H₂O-splitting cycles,thermodynamic analysis of isothermal H₂O-splitting cycle with methane-assisted reduction was carried out,and thermodynamic screening criteria for reaction systems were also established.On this basis,novel thermochemical H₂O-splitting cycles based on TiO₂/TiO₂-δ with methane-assisted reduction and carbon assisted reduction have been proposed.Based on the Gibbs minimization principle and thermodynamic analysis,the influence of reaction temperature and reactant ratio on the reaction conversion rate and product selectivity was clarified.Based on the sensitivity analysis of ηsolar-fuel,a strategy has been proposed to determine the optimal thermodynamic conditions,maximizing the conversion efficiency of solar energy.Without considering the heat recovery,the maximum efficiency of the optimized thermochemical H₂O-splitting cycles with methane-assisted reduction and carbon-assisted reduction could be up to 40.0%and 35.7%,respectively.While considering 90%heat recovery and 15%heat dissipation,the efficiency could further reach 43.4%and 36.6%,respectively,showing great advantages compared with other isothermal thermochemical cycles.The research can also provide a new idea for the development of high-efficiency isothermal thermochemical H₂O/CO₂-splitting cycle.