The HYDROSOL-Plant project was the continuation of a series of successful research projects (HYDROSOL, HYDROSOL-II and HYDROSOL-3D) focusing on the production of H₂ from the two-step solar thermochemical water splitting based on redox structured reactors. The principal objective of HYDROSOL‐PLANT was the development and operation of a plant for solar thermo‐chemical hydrogen production from water in a 0.750 MW_th scale on a solar tower, based on the HYDROSOL technology.

The predecessor projects HYDROSOL and HYDROSOL-II have introduced the concept of redox structured monolithic solar reactors for the production of solar H₂ from water. The proof of concept at lab-scale was presented in the first HYDROSOL project, while the feasibility of the main principles of the process for longer-term cyclic operation at a pilot reactor set-up in a scale of 100 kW_th at the Plataforma Solar de Almeria (PSA) was proven in HYDROSOL-II project. In the third project of the series, HYDROSOL-3D, the relevant design aspects necessary to prepare the erection of a solar demonstration plant at the 1 MWth scale were addressed. However, the actual realization of a solar demonstration plant near the MW scale (0.75MW_th) was achieved in the HYDROSOL-Plant project. In the HYDROSOL-Plant project several advancements were introduced related to large scale redox materials’ synthesis, shaping into porous structures, reactor design, etc. The ultimate objective was to achieve the production of H₂ at a level of 3 kg/week. The HYDROSOL-Plant is the largest demonstration facility of its kind, worldwide.

The specific Scientific and Technical Objectives of HYDROSOL-3D were:

• Define all key components and aspects necessary for the erection and operation of a 750 kW_th solar plant for H₂O splitting (heliostat field, solar reactors, overall process monitoring and control, feedstock conditioning, etc.)
• Develop tailored heliostat field technology (field layout, aiming strategies, monitoring and control software) that enables accurate temperature control of the solar reactors.
• Scale-up the HYDROSOL reactor while advancing the state-of-the-art (redox materials, monolithic honeycomb fabrication and functionalization) for optimum hydrogen yield.
• Design the overall chemical process, covering reactants and products conditioning, heat exchange/recovery, use of excess/waste heat, monitoring and control.
• Construct a solar hydrogen production demonstration plant in the 750 kW_th range to verify the developed technologies for solar H₂O splitting.
• Operate the plant and demonstrate hydrogen production and storage on site (at levels > 3 kg/week).