Solar hydrogen production via two-step thermochemical redox cycles a promising candidate for green fuel production in the future energy market. In a European consortium, the Hydrosol concept has been developed to split water in such a thermochemical cycle. We developed a transient reactor model in order to predict the upper-limit efficiency of a large-scale process based on the Hydrosol reactor concept. The analysis shows that the state-of-the-art ferrite-based redox material properties need to be improved considerably in order to reach competitive efficiency. The two main properties that need improvement are the reaction kinetics on the one hand and the amount of oxygen that can be incorporated into and released from the redox material on the other hand. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.