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Solar Hydrogen via Water Splitting in Advanced Monolithic Reactors for Future Solar Power plants.

Final Report Summary - HYDROSOL II (Solar Hydrogen via Water Splitting in Advanced Monolithic Reactors for Future Solar Power plants)

The overall objective of HYDROSOL-II project was to develop a successful and efficient scale-up of a solar hydrogen production process free from carbon dioxide emissions in order to establish the basis for mass production of solar hydrogen towards the long-term target of a sustainable hydrogen economy. As part of that target HYDROSOL-II aimed to prove that it is feasible to promote a purely renewable hydrogen economy, without gas emissions at a viable cost.

The concept of the project was the production of solar hydrogen via a two-step water spilling process on monolithic honeycomb reactors, capable of developing high water temperatures under solar irradiation and coated with active redox materials that are capable of water splitting and of regeneration. HYDROSOL-II, which was a follow up project of HYDROSOL, aimed to scale-up the employed novel technology using a dual reactor with a power level of 100 kWth per reactor and to demonstrate continuous solar hydrogen production within an optimised pilot plant developed and built based on the novel concept. Among the challenged to be addressed were:
1. the enhancement and optimisation of the metal oxide-ceramic support system with respect to long time stability under multi-cycle operation.
2. the development and construction of a complete pilot dual absorber-receiver-reactor unit in the proposed scale for solar thermochemical water splitting and
3. the effective coupling of this reactor to a solar heliostat field and a solar tower platform for continuous solar hydrogen production within an optimised pilot plant.

Hydrogen production through water splitting using solar energy is of great interest worldwide for solar fuels production since it combines reduced hydrogen cost and virtually zero CO2 emissions. Interest is shifted towards thermochemical water splitting cycles, which decompose water into hydrogen and oxygen via two or more chemical reactions at temperatures lower than those required for direct water thermolysis. All intermediate reactants and products are recycled. HYDROSOL technology employs a two step cycle. During the first step of water splitting the reduced material is oxidised to the higher one by taking oxygen from water and producing hydrogen. During the second step the oxygen trapping material releases oxygen by increasing the amount of solar heat absorbed by the reactor and is thus regenerated establishing a cyclic operation.

HYDROSOL-II solar reactor is constructed from special refractory ceramic thin wall, with multi-channelled monoliths optimised to absorb solar radiation, coated with highly reactive redox materials based on mixed iron oxides. The monoliths are capable of preserving high temperatures when irradiated with concentrated solar irradiation. The complete reaction is carried out in a single solar energy converter such as a direct absorbing volumetric receiver. The need for different temperature levels with different heat demands for the completion of the chemical cycle is met using a modular twin chamber fixed honeycomb absorber, so that continuous hydrogen generation is enabled. After the reaction completion the regenerated modules are switched to the splitting process by switching the feed gas. During the project execution the pilot scale dual reactor consisted of an absorber surface that enabled 100 kwth per reactor. The different heat demands for the two process stages were realised by the provision of periodically switching solar flux through partitioning of the heliostat field into a fixed part and a flexible part, refocused at regular intervals simultaneously to the switch over of modules from one process step to the other.

Various materials employed for water splitting have been synthesised during the initial phase of the project, and different coating techniques have been employed. The produced coating and support assembly was proven to be stable enough, with more than fifty cycles of hydrogen generation and metal oxide reduction being performed using one sample. During the second research period, parametric cycles of different redox materials were performed to determine hydrogen production as a function of synthesis route, redox material stoichiometry and water splitting temperature and the design of the pilot dual reactor was finalised. Parametric studies to determine the reactions intrinsic kinetics, necessary for modelling needs, were then performed; optimisation of temperatures and mass flows was implemented and the first hydrogen production tests were carried out. The final project phase included completion of the experimental tests, evaluation and assessment of the results. A design study for a production plan in commercial scale was carried out, along with cost targets establishment.

HYDROSOL-II was the largest pilot scale project of its kind and had the following achievements:
1. a monolithic honeycomb reactor for solar hydrogen production was scaled up to 100 kW level;
2. the reactor contained neither moving parts nor recirculating solid particles, in contrast with other competitive solar reactors;
3. intelligent control strategies to match the solar platform facility with the specific heat requirements of the two step process were developed and implemented;
4. Cyclic hydrogen production and redox material regeneration were demonstrated for the first time on a solar tower facility under realistic conditions enabling the technology to reach a pilot plant demonstration.

HYDROSOL technology provided solutions to concerns regarding energy supply, environmental pollution and climate change, since renewable energy sources were employed for fuel production with virtually zero carbon dioxide emissions. Thus the future challenge is the technology implementation into a complete plant, at a commercial scale, in order to establish it as a mature and reliable route for sustainable hydrogen generation.

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