Hydrogen production from water utilizing solar heat at high temperatures

Abstract Possibilities of producing hydrogen and oxygen from water utilizing solar heat at high temperatures are investigated. The process of direct thermal decomposition of water is studied using a conceptual model. It is shown that the thermodynamic requirements for the direct thermal decomposer are difficult to realize from the structural viewpoint and that existing separation methods are not applicable for such a decomposition process if it is to attain sufficiently high thermal efficiencies. Feasibilities of realizing simple two-step thermochemical decomposition processes are investigated based on existing thermochemical data. It is predicted, as the results of thermochemical as well as thermodynamic analyses, that a two-step thermochemical decomposition process using iron oxide operates efficiently at relatively low temperatures attainable with solar heat and compatible with structural materials.