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Within the general framework of investigating novel routes for solar hydrogen production, the idea of combining a solar centrifugal particle receiver with sulphur thermochemical cycles, involving SO3 dissociation to SO2 and O2 as key step, is pursued. In this perspective, the present work concerns the synthesis, development, evaluation and characterisation of particles suitable to operate as media for direct solar irradiation absorption, transfer and storage as well as catalysts for the SO3 dissociation reaction. Commercial bauxite-based proppants were modified to incorporate raw materials with elements known for their catalytic activity with respect to the SO3 dissociation, namely iron, copper, manganese and their combinations. The catalytic activity of such modified proppants was tested in fixed bed reactor test rigs at 850 °C and ambient pressure with concentrated liquid sulphuric acid as feedstock. Extensive screening tests complemented by physicochemical properties measurements before and after catalytic testing, identified systems that at 850 °C, 1 atm and Gas Hourly Space Velocity of 11,800 h−1 could achieve high SO3 conversions (60%, corresponding to 68% of equilibrium value) for over 125 h of continuous operation. This performance was achieved without degradation of their mechanical strength which, in fact exhibited a slight increase from 53 N in the fresh state to 55 N after long-term exposure to reaction conditions. However such systems were susceptible to colour alteration, affecting adversely their absorptance in the 1000–2500 nm wavelength range. Compositions with the best combination of properties are scheduled for large-scale synthesis and on-site testing in a pilot-scale solar receiver.

Structured bodies from redox materials are a key element for the implementation of thermochemical cycles on suitable reactors for the solar H2O splitting. In the current work different configurations of nickel ferrite were investigated with respect to their performance in H2O splitting: i) powder, ii) disk, iii) honeycomb flow-through monoliths. The structured bodies were prepared via pressing and extrusion techniques. The performance of the different structures was affected significantly by differences in the structural characteristics. Alternative approaches involving casting techniques for the structuring of nickel ferrite porous bodies were also investigated. This work constitutes a preliminary attempt towards tuning such characteristics to achieve enhanced and cycle-to-cycle stable production yields.

Production of fuel-cell grade hydrogen via reforming of gaseous hydrocarbon fuels requires the design and implementation of efficient formulations, which incorporate high performance materials for the removal of undesired impurities; notably H2S. To this respect, the present study relates to the preparation and parametric evaluation of small-scale honeycomb structured flow-through reactors, coated with an in-house synthesized ZnO/CuO, 50%mol/50%mol, sorbent. Experimental tests involved quantification of the H2S trapping capacity and efficiency of the coated monoliths by monitoring the evolution of H2S concentration at the reactor outlet for known H2S feeds at the reactor inlet. The de-H2S performance of the coated monoliths was examined at reactor temperatures ranging from 160 °C to 400 °C, under two different synthetic mixtures containing a) 30%vol H2, 32%vol H2O, N2 balance and b) 30%vol H2, 32%vol H2O, 6.5%vol CO2, 4.5%vol CO, N2 balance. Space velocities at the reactor ranged from 20,000 to 60,000 h−1 for the experiments designed to examine trapping capacity and from 10,000 to 90,000 h−1 for the experiments designed to examine trapping efficiency. The breakthrough curves obtained proved that the particular formulation can provide a very efficient solution, particularly for the case of decentralized applications, for which system compactness is of prime importance. Total capture capacities up to a targeted H2S breakthrough value of 0.1 ppmv at the reactor outlet varied between 5 and 25 (gsulphur/100 × gsorbent). This value depended on reactor temperature and flow conditions, gas composition, monolith loading and monolith cell density.

The present study relates to the preparation and evaluation of small-scale honeycomb structures as compact reactors/heat exchangers via exploitation of the cobalt/cobaltous oxide (Co3O4/CoO) cyclic reduction–oxidation (redox) heat storage scheme. The structures considered included in-house extruded monoliths (pure cobalt oxide and cobalt oxide/alumina composites) and commercial cordierite substrates coated with Co3O4. The samples were subjected to multi-cyclic redox operation under air flow, in the temperature range of 700–1000 °C. Reduction occurred during heating up to 1000 °C, while oxidation took place during cooling. Redox performance was evaluated on the basis of on-line oxygen release/consumption measurements, while continuous monitoring of imposed air flow reactor inlet/outlet temperatures facilitated the preliminary estimation of heat dissipation in the duration and after completion of the exothermic reaction (oxidation). For all samples, redox performance remained stable in the course of multi-cyclic exposure. In terms of heat transfer, there is strong indication that both composition and the geometry of the honeycomb are important. The pure Co3O4 extruded honeycomb exhibited the highest heat dissipation efficiency but suffered from severe deformation upon multi-cyclic operation. The addition of a small amount of alumina in the aforementioned composition (10% on the basis of total initial mass of oxides), particularly when combined with an increase of the honeycomb wall thickness, substantially improved macro-structural stability upon thermal/redox cycling. The Co3O4-coated cordierite monoliths showed essentially the same normalised redox performance with the pure Co3O4 extruded honeycomb, however the overall heat dissipation achieved was lower. Regarding the effect of redox cycling on the structural stability of studied formulations, pure Co3O4 samples exhibited notable swelling. In the case of the extruded body, this resulted to structural collapse while for the coated cordierite honeycomb, expansion of the coating layer led to partial channels blocking. Based on relevant morphological and structural post-analysis, it was concluded that formation of cobalt aluminate largely reduced swelling intensity.

The present work provides an overview of activities performed in the framework of the EU-funded collaborative project RESTRUCTURE, the main goal of which was to develop and validate a compact structured reactor/heat exchanger for thermochemical storage driven by 2-step high temperature redox metal oxide cycles. The starting point of development path included redox materials qualification via both theoretical and lab-scale experimental studies. Most favorable compositions were cobalt oxide/alumina composites. Preparation of small-scale structured bodies included various approaches, ranging from perforated pellets to more sophisticated honeycomb geometries, fabricated by extrusion and coating. Proof-of-concept of the proposed novel reactor/heat exchanger was successfully validated in small-scale structures and the next step included scaling up of redox honeycombs production. Significant challenges were identified for the case of extruded full-size bodies and the final qualified approach related to preparation of cordierite substrates coated with cobalt oxide. The successful experimental evaluation of the pilot reactor/heat exchanger system constructed motivated the preliminary techno-economic evaluation of the proposed novel thermochemical energy storage concept. Taking into account experimental results, available technologies and standard design aspects a model for a 70.5 MWe CSP plant was defined. Estimated LCOE costs were calculated to be in the range of reference values for Combined Cycle Power Plants operated by natural gas. One of main cost contributors was the storage system itself, partially due to relatively high cost of cobalt oxide. This highlighted the need to identify less costly and equally efficient to cobalt oxide redox materials.

Redox-pair-based thermochemical cycles are considered as a very promising option for the production of hydrogen via renewable energy sources like concentrated solar energy and raw materials like water. This work concerns the synthesis of various spinel materials of the iron and aluminum families via combustion reactions in the solid and in the liquid-phase and the testing of their suitability as redox-pair materials for hydrogen production by water splitting via thermochemical cycles. The effects of reactants' stoichiometry (fuel/oxidizer) on the combustion synthesis reaction characteristics and on the products' phase composition and properties were studied. By fine-tuning the synthesis parameters, a wide variety of single-phase, pure and well crystallized spinels could be controllably synthesized. Post-synthesis, high-temperature calcination studies under air and nitrogen at the temperature levels encountered during solar-aided thermochemical cyclic operation have eliminated several material families due to phase composition instabilities and identified among the various compositions synthesized NiFe 2 O 4 and CoFe 2 O 4 as the two most suitable for cyclic water splitting – thermal reduction operation. First such thermochemical cyclic tests between 800 and 1400 °C with NiFe 2 O 4 and CoFe 2 O 4 in powder form in a fixed bed laboratory reactor have demonstrated capability for cyclic operation and alternate hydrogen/oxygen production at the respective cycle steps for both materials. Under the particular testing conditions the two materials exhibited hydrogen/oxygen yields of the same magnitude and similar temperatures of oxygen release during thermal reduction.