Conceptual Design of a Two-step Solar Hydrogen Thermochemical Cycle with Thermal Storage in a Reaction Intermediate

This paper presents the conceptual design for a two-step thermochemical cycle producing hydrogen continuously, even off-sun, with the concentrated solar energy as the heat source. For a case study, the two-step iron oxide cycle (Fe3O4/FeO) is selected to illustrate the design concept. Two reactors, one storage tank and the solar collector comprise the system. Molten wustite (FeO) is accumulated in the storage tank on-sun. The FeO is not only involved in the reactions but also acts as the heat transfer medium, obtaining the energy from the solar insolation and delivering energy to support the thermal decomposition of magnetite (Fe3O4). In this way, the temperature limitation (<800 K) of molten salt is solved, and the intermittency problem of variable insolation is circumvented. A simple feedback scheme is used to control the flow rate between the storage tank and the reactors in order to minimize the temperature fluctuations. For the wustite hydrolysis reaction, the volumetric flow rat