Study On Hybrid Thermochemical-electrochemical System For Efficient Solar Fuel Production

With the increasingly severe energy environment problems faced by mankind,solar energy,as the cleanest and most abundant renewable energy source available,is expected to replace fossil fuels and accounts for the majority of energy consumption,but its intermittence hinders its further development and application.Converting solar energy into solar fuels,such as solar splitting H2O and CO2,is beneficial to energy store and transportation.The solar fuel hydrogen is the most basic and the best clean energy.However,the solar-to-fuel efficiency is limited to a relatively low value,such as solar-driven electrochemical and photochemical processes of 6.5%using a PV-electrolyzer,2%using a PV photoelectrochemical cell and less than 1%for photochemical cells,5.25%of solar-driven thermochemical cycle.From a strategic viewpoint,solar-fossil fuel hybridization can be considered a more practical means of sustainable development of renewable energy utilization from the present to midterm.H2O or CO2 can be converted to H2 and CO with much reduced reaction energy barrier by reacting with fossil fuels.So a hybrid thermochemical-electrochemical system for efficient solar fuel production and a full-spectrum solar energy utilization system integrating spectral splitting,photovoltaics and SOEC for hydrogen production system are proposed.(1)To realize the purpose of efficient solar fuel production,a hybrid thermochemical-electrochemical system for efficient solar fuel production is proposed in this paper.A new photovoltaic-thermal(PVT)conceptual system integrating photon-enhanced thermionic which turn solar energy into electricity and high temperature heat,where electricity can be used in solid oxide electrolysis cell(SOEC)for hydrogen production,high-temperature heat for chemical looping reforming of methane(CLRM).The three-step CLRM consisted of reduction process with syngas(H2:CO molar ratio is 2)production,oxidation process with hydrogen production,and reoxidation process of the oxygen carrier,respectively.By CLRM providing the reaction heat,the reactant of high-temperature steam and the reducing atmosphere required by cathode for SOEC,and simultaneously,SOEC providing high-temperature O2 for re-oxidation process of CLRM.The system was simulated by ASPEN plus software,and the characteristics of the oxygen carrier cerium oxide of CLRM cycle were analyzed based on experimental data by means of Python software.Under the condition of considering optical loss,thermodynamic analysis shows that compared with 44.7%solar-fuel conversion efficiency and 28.5%solar net exergy efficiency of the reference system,the solar-fuel conversion efficiency and solar net exergy efficiency of the hybrid thermochemical-electrochemical system can be up to 61.8%and 44.3%respectively.The hybrid system has advantages over the reference system in various indicators and can produce high-quality solar fuel efficiently.(2)According to the characteristics of SOEC,a full-spectrum solar energy utilization system integrating spectral splitting,photovoltaics and SOEC for hydrogen production system is proposed.SOEC is a device for hydrogen production by electrolysis of water.The total enthalpy change(?H)equired for electrolysis of water to produce hydrogen is the sum of the Gibbs free energy change(?G)and high-temperature heat(T?S).Sunlight with wavelengths shorter than 870 nm is assigned to photovoltaic cells for power,the rest of the solar spectrum is converted into high-temperature heat,which can provide the Gibbs free energy change(?G)and the high-temperature heat(TAS)of electrolysis of water,respectively.The SOEC is modeling with EES software and the system was simulated by ASPEN plus software.When the operating temperature of the SOEC stack is 1123K and the current density is 5000A/m2,the solar-H2 conversion efficiency and the net exergy efficiency of the combined hydrogen production system are 30%and 31.2%,respectively,which are better than the reference system.Since a large amount of high-temperature heat cannot be utilized in the combined hydrogen production system,to realize high-efficiency utilization of solar energy,methanol cracking unit is added to further recover heat energy in the system,the solar-fuel conversion efficiency and solar net exergy efficiency are improved to 47.1%and 35.4%,respectively.