Building on the results of the FP5 project HYDROSOL, the main objective of the HYDROSOL-II Project was the demonstration of a successful and efficient scale-up of a carbon-dioxide-emissions-free solar hydrogen production process that will establish the basis for mass production of solar hydrogen towards the long-term target of a sustainable hydrogen economy. Such a successful demonstration aimed to narrow the gap between research and market implementation and to prove to stakeholders from society and industry that the combination of Concentrated Solar Power facilities coupled with high temperature processes can be a viable way to produce large amounts of hydrogen at a reasonable cost without any greenhouse gas emissions, paving the way for a future sustainable, purely renewable hydrogen economy.

The concept set forth and advanced in the Project is the production of Solar Hydrogen via a two-step water splitting process performed on monolithic honeycomb reactors, capable of developing high temperatures under concentrated solar irradiation and coated with active redox materials that are capable of water-splitting and of regeneration. This concept was introduced and demonstrated in the aforementioned predecessor Project HYDROSOL. The aim of HYDROSOL-II was to scale-up the technology on a dual reactor with a power level of 100 kW_th /reactor and to demonstrate continuous solar Hydrogen production within an optimized pilot plant developed and built based on this novel reactor concept. The project involved further scale-up of this technology and its effective coupling with solar platform concentration systems, in order to exploit and demonstrate all potential advantages. Specific challenging problems to be solved included:

• the enhancement and optimisation of the metal oxide-ceramic support system with respect to long-time stability under multi-cycle operation (more than 100 cycles)
• the development and construction of a complete pilot dual absorber/receiver/reactor unit in the 100 kW_th scale for solar thermochemical splitting of water
• the effective coupling of this reactor to a solar heliostat field and a solar tower platform for continuous solar Hydrogen production within an optimized pilot plant (100 kW_th).