Research On Solar Thermochemical Cycle And Reactor Design

Converting sunlight to the chemical energy of fuel carries special significance in addressing unfavorable nature of solar energy such as sparsity and intermittency,which helps meet the energy requirement worldwide and reduces the reliance of human on fossil fuel.The solar thermochemical cycle(TC),chemical energy storage technology via endothermic reaction driven by solar energy,has attracted more and more attention.As oxygen carriers of two-step TC,cerium oxide(CeO2)and its derivate can split H2O(CO2)into H2(CO),which are valuable raw materials for the synthesis of ammonia,methanol,and liquid hydrocarbon.Splitting of H2O or CO2 process using CeO2 as oxygen carriers is as follows:oxygen atoms are first released when solar energy is used to reduce the metal oxide at a high temperature,TRe.Then,H2O or CO2 is reduced by reduced metal/metal oxide at a lower temperature,ToX,yielding H2 or CO.Although the theoretical efficiency of solar-to-fuel for solar-driven TC could be high,the actually-achieved efficiency in the literature is low.In this dissertation,supported by Natural Science Foundation of China and National Key Research Progress,a series of theoretical calculations,experiments,and simulations were conducted,focusing on improving the efficiency of fuel production by splitting of H2O and CO2 via solar TC.The main contents and results are as follows:The mechanism research of high-temperature solar TC was conducted,a thermochemistry thermal engine model was established,and isothermal TC and two-temperature TC were compared from the aspect of efficiencies of the first law of thermodynamics and the second law of thermodynamics.The difference and relationship between isothermal TC and two-temperature TC of producing solar-energy fuels were uncovered.By regulating the selected parameters and the thermodynamic properties of materials and based on the object to achieve the optimal solar-to-fuel efficiency,some materials with higher reducibility exhibit better performance than CeO2,which are more suitable for isothermal TC rather than two-temperature TC,and applying appropriate oxygen carriers and cycle have advantages on improving the efficiency of solar TC reactors.Even without any heat recovery,theoretical solar-to-fuel converting efficiency for some materials could reach 28%.Two-step TC is more suitable for materials with low reducibility,while isothermal TC is more suitable for materials with high reducibility.This provides guidance for the selection of solar TC.A new CeO2-based solar TC for fuel production was proposed.A new operating method that cold fluid(H2O/CO2)exchanges heat with hot catalyst was proposed,in which cold fluid contact the hot catalyst directly to recover the heat,effectively improving the efficiency of metal-oxide based solar TC.The Higher Heating Value(HHV)efficiencies of H2 production from H2O splitting were 24.36%(without heat recovery)and 26.84%(with 80%gas phase heat recovery),respectively,while the HHV efficiencies of the conventional two-temperature TC were 19.76%and 22%,respectively.By reducing the temperature of catalyst intentionally,the temperature difference was controlled within a narrow range,which benefited the simplification of reactor design,and improved the fuel production amount and efficiency of the actual TC in the meantime.The limitation of dynamics is the main factor which influences the efficiency of solar TC.The chemical reaction kinetics in the surface of oxide carriers was tested by electrical conductivity relaxation methodology to obtain the surface reaction rate constants of the oxygen carriers in solar TC reactors at relatively low oxygen partial pressures,and to study the effects of H2O and oxygen partial pressures and temperature on the surface reaction rate constants.The electrical conductivity relaxation curve of oxygen carriers of undoped CeO2 at different H2O partial pressures in low oxygen partial pressure range were measured by electrical conductivity relaxation experiments,and the logarithmic linear relation between the surface reaction rate constants and oxygen partial pressures was obtained by fitting.The computational thermodynamics was complemented from the aspect of experiments.The limitation of material kinetics on solar TC reaction process was investigated.To improve the efficiency of the solar TC reactor,the design and manufacture of solar TC reactor were conducted,and simulation and optimization were conducted by COMSOL Multiphysics software in the aspect of finite element method.The light tracing was carried out by using the Monte Carlo method.To acquire stable boundary heat source,Xenon spotlight simulator was used to concentrate the light instead of the stacked spotlight.The outer structure of the reactor was improved,and hemispherical bottom was applied.Heat insulation design of solar TC reactor was applied by reducing heat conduction and convection,reducing the entire energy loss of the reactor,and isothermal and two-temperature TC modes were matched with suitable oxide carriers.Reactor simulation was conducted based on the coupling of light,thermal and chemical reactions,and the stability and transient temperature,flow field and oxygen vacancies distribution in the reactor were acquired.To increase the thermochemical efficiency of isothermal TC,in the aspect of chemical thermal recovery,a polygeneration system for simultaneous methanol and power production with the comprehensive utilization of solar energy high-temperature thermochemistry and methanol was proposed.This method can produce high-quality syngas and reduce the temperature of the production of isothermal TC oxidization reaction to 600-850?,at which temperature the technique of heat exchanger is relatively mature,avoiding the application of extra-high temperature heat exchanger.The solar to syngas efficiencies for CO2 and H2O splitting of the proposed system were 45.7%and 38.1%,respectively.High-temperature solar isothermal TC can split H2O and CO2 into H2 and CO,the waste heat and unreacted gas species can be recovered via the complementation with fossil fuels.This system enables simultaneous production of power and methanol,and reduces the carbon emission per unit calorific value.