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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 study deals with the mechanical properties of structured reactors/heat exchangers, for high temperature heat storage via the cobalt oxide cyclic redox scheme. Two different structures (i.e. honeycomb and perforated block) and two different compositions (i.e. 100% cobalt oxide and 90 wt% cobalt oxide – 10 wt% aluminium oxide) were evaluated. During thermal cycling in the range of 800-1000 o C, different loads were applied to the sampleswhile monitoring their length variation. The integrity of the samples was assessed after every cycle. It was found that mechanical strength was substantially improvedupon addition of 10 wt% aluminium oxide. The cobalt oxide/alumina composite presented lower maximal expansion during cycling and exhibited higher integrity, already after one thermal cycle. Another important result is that, for both the honeycomb and the perforated block, the load decreases the over-all sample net expansion. Moreover, the perforated block exhibited lower expansion and better mechanical strength as compared to the honeycomb. Due to the better chemical performance expected to be achieved by the honeycomb structure, a compromise between these two structureshas to be chosen (e.g. honeycomb structure with thicker walls). The results are used for building a thermochemical storage system prototype, implemented for the first time in an existing concentrated solar power facility (STJ).

The Co 3 O 4 /CoO redox system has been recently proposed and is currently under consideration by several research groups as a promising thermochemical heat storage (THS) scheme to be coupled with high temperature Concentrated Solar Power plants. The present work is an investigation of cobalt oxide based honeycomb structures as candidate reactors/heat exchangers in relevant compact and efficient THS systems. The formulations studied included extruded bodies from pure cobalt oxide and two different cobalt oxide/alumina composites (i.e. 95/5 wt% and 90/10 wt%) as well as cobalt oxide-coated cordierite honeycombs with two different loadings (i.e. 28% and 65%). The structures were evaluated with respect to their redox performance in the course of 5 successive cycles in the temperature window of 800-1000 o C and under air flow. Based on measured oxygen evolution profiles, honeycombs from pure cobalt oxide and cobalt oxide-coated cordierites exhibited very similar normalized (i.e. μmol O 2 /g Co 3 O 4 ) redox performance. On the other hand, the addition of alumina had a moderately negative effect on normalized redox performance versus the two aforementioned formulations but contributed to the substantial increase of the honeycombs structural stability when compared to the extruded pure cobalt oxide monolith. The particular finding was based on performing, in a separate experimental setup, 10 redox cycles under idealized (i.e. no imposed loads) conditions. Pre- (i.e. fresh/calcined) and post-characterization (i.e. after 10 redox cycles) of extruded structures via mercury porosimetry revealed measurable decrease of bulk density and increase of mean pore size, indicating a net structure expansion/‘swelling’ effect.

The “holy grail” of solar chemistry and solar engineering, is the technical storage of solar energy into a more easily transformable and transportable form, namely an energy carrier such as H2. The two-step redox-based solar thermochemical H2O splitting cycle is considered to be among the most promising approaches for the production of H2 from entirely renewable sources (solar energy and water). In this redox cycle an active material is initially reduced thermally under inert atmosphere and at the next step it is oxidized from H2O producing H2. The materials that have been in the core of solar chemistry research are metal oxides such as ferrites, cerium oxides, perovskites, etc. The reactor types that are being investigated for the redox thermochemical splitting of H2O are either powder-particle reactors or structured reactors. In the current work Ni-ferrite and Ce-oxide structured into different monolithic bodies (honeycombs, foams) were evaluated w.r.t. their redox activity. Based on this investigation, the most promising structure was further scaled-up for the construction of the full-scale reactors of the HYDROSOL-PLANT solar plant installation.The “holy grail” of solar chemistry and solar engineering, is the technical storage of solar energy into a more easily transformable and transportable form, namely an energy carrier such as H2. The two-step redox-based solar thermochemical H2O splitting cycle is considered to be among the most promising approaches for the production of H2 from entirely renewable sources (solar energy and water). In this redox cycle an active material is initially reduced thermally under inert atmosphere and at the next step it is oxidized from H2O producing H2. The materials that have been in the core of solar chemistry research are metal oxides such as ferrites, cerium oxides, perovskites, etc. The reactor types that are being investigated for the redox thermochemical splitting of H2O are either powder-particle reactors or structured reactors. In the current work Ni-ferrite and Ce-oxide structured into different monolithic bodies (honeycombs, foams) were evaluated w.r.t. their redox activity. Based on this investigation,...

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.

This work is a follow-up of previous efforts reported on the synthesis of various single and mixed oxide materials and their evaluation as catalysts for the sulfuric acid dissociation reaction for the production of SO 2 and O 2 . The current work concerns the comparative assessment of Fe 2 O 3 , CuO, Cu–Fe, Fe–Cr, Cu–Al and Cu–Fe–Al mixed oxides coated as catalysts on silicon carbide monolithic honeycomb structures, with respect to sulfuric acid decomposition reaction conditions for 100 h at 850 °C and ambient pressure, as well as their ex-situ characterization after such operation. The exposure conditions are representative to a potential future real application. The exposure time, although of relatively short-term, is adequate to extract safe conclusions on the stability and therefore to a large extent also on the suitability of the candidate oxide-based catalysts. All catalytic systems tested exhibited high SO 3 conversions reaching or exceeding 70%, for space velocities in the range of 5–35 h −1 . For some of the samples, the relatively high initial activity decreased by about 5–10 percentage points in the course of the 100 h testing, reaching stable mean values. It was concluded that Fe 2 O 3 , CuO and Fe–Cr mixed oxide retained their chemical and structural stability after exposure to reaction conditions, while the other three mixed oxides studied suffered from significant phase decomposition phenomena. Based on the fact that the initial catalytic activity of the Fe–Cr mixed oxide, as identified in a previous comparative study among several materials, was found higher than the ones of Fe 2 O 3 and CuO and relatively close to the one of the highly active but costly Pt/Al 2 O 3 catalyst, the particular mixed oxide is considered a promising catalyst for the SO 3 dissociation reaction.

The present work concerns the synthesis of various single and mixed oxide materials and their study as catalysts for the sulfuric acid dissociation reaction via which the production of SO2 and O2 is achieved. This is the most energy intensive step of sulfur-based thermochemical cycles for the production of hydrogen. Commercial (i.e. FeO, Fe3O4 ,F e2O3, CuO, Cr2O3, g-Al2O3, Pt/g-Al2O3) and in-house binary and ternary compositions of the CueFeeAl system as well as FeeCr mixed oxide materials prepared by the Solution Combustion Synthesis (SCS) technique were comparatively tested. The materials were studied in

Over the last years, several research groups have focused on developing efficient thermochemical heat storage (THS) systems, in-principle capable of being coupled with next generation high temperature Concentrated Solar Power plants. Among systems studied, the Co3O4/CoO redox system is a promising candidate. Currently, research efforts extend beyond basic level identification of promising materials to more application-oriented approaches aiming at validation of THS performance at pilot scale reactors. The present work focuses on the investigation of cobalt oxide based honeycomb structures as candidate reactors/heat exchangers to be employed for such purposes. In the evaluation conducted and presented here, cobalt oxide-based structures with different composition and geometrical characteristics were subjected to redox cycles in the temperature window between 800 and 1000°C under air flow. Basic aspects related to redox performance of each system are briefly discussed but the main focus lies on the evaluation of the segments structural stability after multi-cyclic operation. The latter is based on macroscopic visual observation and also supplemented by pre– (i.e. fresh samples) and post–characterization (i.e. after long term exposure) of extruded honeycombs via combined mercury porosimetry and SEM analysis.

The present work summarizes the recent activities of our laboratory in the field of solar-aided hydrogen production with structured monolithic solar reactors. This reactor concept, “transferred” from the well-known automobile exhaust catalytic after-treatment systems, employs ceramic supports optimized to absorb effectively solar radiation and develop sufficiently high temperatures, that are coated with active materials capable to perform/catalyze a variety of “solar-aided” reactions for the production of hydrogen such as water splitting or natural gas reforming. Our work evolves in an integrated approach starting from the synthesis of active powders tailored to particular hydrogen production reactions, their deposition upon porous absorbers, testing of relevant properties of merit such as thermomechanical stability and hydrogen yield and finally to the design, operation simulation and performance optimization of structured monolithic solar hydrogen production reactors. This approach, among other things, has culminated to the world's first closed, solar-thermochemical cycle in operation that is capable of continuous hydrogen production employing entirely renewable and abundant energy sources and raw materials – solar energy and water, respectively – without any CO 2 emissions and holds, thus, a significant potential for large-scale, emissions-free hydrogen production, particularly for regions of the world that lack indigenous resources but are endowed with ample solar energy.

The sulfuric acid dissociation reaction, via which the production of SO2 and O2 is achieved, is the most energy intensive step of the so-called sulfur-based thermochemical cycles for the production of hydrogen. Efforts are focused on the feasibility and effectiveness of performing this reaction with the aid of a high temperature energy/heat source like the sun. Such coupling can be achieved either directly in a solar reactor by concentrated solar radiation, or indirectly by means of a heat exchanger/decomposer reactor using a suitable heat transfer fluid. Since a very limited amount of work regarding the potential formulations and sizing of such suitable reactors has been performed so far, the present work addresses further steps necessary for the efficient design, manufacture and operation of such reactors for sulfuric acid decomposition. In this respect, parametric studies on the SO3 decomposition with iron (III) oxide based catalysts were performed investigating the effect of temperature, pressure and space velocity on SO3 conversion. Based on these results, an empirical kinetic law suitable for the reactor design was developed. In parallel, structured laboratory-scale reactors employing siliconised silicon carbide honeycombs coated with iron (III) oxide were prepared and testes in structured laboratory-scale reactors employing to test evaluate their durability (i.e. activity vs. time) during SO3 decomposition, demonstrating with the result of satisfactory and stable performance for up to 100 hours of operation. The results in combination with characterization results of “aged” materials can provide valuable input for the design of prototype reactors for sulfuric acid decomposition.

The present work relates to the investigation of cobalt and manganese oxide based compositions as candidate materials for the storage of surplus energy, available in the form of heat, generated from high temperature concentrated solar power plants (e.g. solar tower, solar dish) via a two-step thermochemical cyclic redox process under air flow. Emphasis is given on the utilization of small structured monolithic bodies (flow-through pellets) made entirely from the two aforementioned oxides. As compared to the respective powders, and in addition to the natural advantage of substantially lower pressure drop that monolithic structures can offer, this study demonstrated that structured bodies can also improve redox kinetics to a measurable extent. Cobalt oxide was found to be superior to manganese oxide both from an estimated energy density and from a redox reactions kinetics point-of-view. Among the redox conditions studied, the optimum reduction-oxidation operating window for the former oxide was determined to be in the range of 1000-800 °C, while for the latter material no clear conclusion was drawn with reduction reaching its maximum extent at 1000 °C and oxidation occurring in the range of 500-650 °C. In both cases, no significant degradation of redox performance was observed upon cyclic operation (up to 10 cycles), however manganese oxide showed notably slower oxidation kinetics.