Operational strategy of a two-step thermochemical process for solar hydrogen production

A two-step thermochemical cycle process for solar hydrogen production from water has been developed using ferrite-based redox systems at moderate temperatures. The cycle offers promising properties concerning thermodynamics and efficiency and produces pure hydrogen without need for product separation. The process works by cycling stationary ferrite-coated monoliths through sequential oxidation and reduction steps at 1073 and 1473 K, respectively. By using two such monoliths in parallel, it is possible to quasi-continuously produce hydrogen. A prototype solar reactor and peripherals suitable to proof the process concept were developed and have been successfully tested in DLR's solar furnace in Cologne. Repeated cycling operation is possible with a reproducible amount of hydrogen produced. In most cases, a distinct decay of the amount of the evolved hydrogen is observed from cycle to cycle due to inhomogeneous heating of the monolithic absorber in the very first cycles and due to disappearance of the porosity and the associated loss of surface area in later cycles. Results from experimental campaigns with the prototype reactor and simulations with a corresponding reactor model support the development and verification of a process strategy for the continuous production of hydrogen in a larger scale. The tasks include the enhancement of long-term stability of the redox system, the development of an operational strategy and finally the design and development of a 100 kW(th) pilot reactor to demonstrate the feasibility of the process on top of a solar tower under real conditions. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.