Study On Solar Thermochemical System Based On Selective Membrane Separation

The pressing environmental problems and limited reserves of fossil fuels make it increasingly urgent to find clean and renewable energies.Solar energy is expected to be an ideal alternative to fossil fuels because of its abundance in nature and environmental friendliness.But the drawbacks of solar energy such as the low energy density and unstable energy supply become the bottleneck restricting the rapid development of solar energy.Therefore,the conversion and storage of solar energy into a secondary energy such as hydrogen,which has a high energy density,and is clean and environmentally friendly,has become one of the hot spots in the field of energy researches.The solar thermochemical study based on selective membrane separation is expected to separate the products,which could increase the reactant conversion rate and reduce the temperature of the reaction,leading to a low cost of solar fuel.Supported by the National Natural Science Foundation of China,the National Key Research and Development Program of China and other national research projects,this research explores the efficient conversion method of solar energy to chemical energy at a relative low temperature based on the concept of energy level.The mechanisms of energy conversion,the energy level improvement relationships and thermodynamic efficiencies of solar thermochemical utilization process based on selective membrane separation were studied and verified experimentally.The main contents and conclusions of this paper are as follows:(1)Thermodynamics analysis and simulation researches on the solar thermochemical generation of hydrogen(or carbon monoxide)from water(or carbon dioxide)by selective membrane separation in high temperature have been studied.The energy efficiency analyses of the pump-assisted and methane-assisted oxygen permeation membrane reactor system are proposed.The energy efficiency of the pump-assisted oxygen permeation membrane system is 2.9%;the solar-to-chemical conversion efficiency of the methane-assisted system is 63%.This theoretical analysis provides a basis for the practical application of high temperature solar thermochemistry based on selective membrane separation.(2)The novel alternating hydrogen and oxygen permeation membrane reactor is proposed,which is a great improvement of the traditional membrane reactor.The innovative reactor can increase the conversion rate and energy efficiency of solar thermochemical water splitting dramatically in a relative low temperature.For example,the conversion rate and energy efficiency of water splitting are increasing from 1.26%and 2.9%of isothermal chemical cycling to 99.99%and 42.6%at 1500 ℃,respectively.The new isothermal alternating hydrogen and oxygen permeation membrane system provides a valuable research direction for the hydrogen generation from solar thermochemical water splitting,and it is of great significance to the further application.(3)A new type of mid/low-temperature solar methane reforming system based on hydrogen and carbon dioxide permeation membrane is proposed for the combination of traditional solar methane reforming reaction and selective membrane separation method.The system can achieve complete conversion of methane at 300-400 ℃(the industrial methane steam reforming reaction requires a high temperature in the range of 800-1000 ℃ to reach a great conversion rate of methane),leading to a substantial increase in energy efficiency and significant reduction of solar fuel costs.Finally,the feasibility of the model proposed is verified through the preparation of the catalyst,the design of the reactor,the construction of the test bed and the results of the experiment.The experiment results show the conversion rate of methane can be as high as 88.02%at 400 ℃ by means of arranging catalysts and Ca(OH)2 alternately to model the alternating separation of H2 and CO2,which lays the foundation for combining methane steam reforming with trough solar collectors in the future.