• Energy Research

The urgent need for sustainable energy supply requires maximum exploitation of renewable energy sources. The latter, being of intermittent nature, need to be coupled with efficient energy storage. Solar-thermal power-plants are inherently compatible with thermal storage, which is a cost-efficient method of storing energy for later use but the field is currently dominated by sensible heat molten salts used as heat storage media but with a maximum operating temperature of about 560oC. Certain ceramic materials, able to induce reversible reduction-oxidation reactions under air flow, are promising alternatives to molten salts because they can withstand much higher temperatures (>1000oC) and thus can be integrated with high-efficiency air-Brayton thermodynamic cycles. At the same time the chemical energy stored/released during such reduction-oxidation reactions can boost energy storage density by up to 10 times cf. sensible only concepts. In this framework, Ca-Mn-based perovskite compositions were demonstrated to function effectively as energy storage materials. The current work offers insights on material synthesis parameters to achieve relatively high purity Ca-Mn-based compositions and subsequently optimize their redox performance in the course of a preliminary 5-cycle campaign. Moreover, the occurring structural transitions and their corresponding heat effects are also discussed and elaborated upon. This study is the first step towards the, currently in progress, process of synthesising – at multi kg scale – and shaping these compositions into extruded honeycomb-like monolithic structures for subsequent future application in lab- and pilot-scale high temperature thermochemical energy storage systems.

AbstractThe Co3O4/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 800-1000oC 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 O2/g Co3O4) 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.

Being a relatively new but quite promising field in CSP, thermochemical energy storage could be the next generation solution towards the implementation of baseload cost-effective solar thermal plants. The present work, taking place within the framework of EU-funded project PEGASUS, aims at synthesizing, shaping and experimentally evaluating the catalytic activity and main physicochemical attributes of Fe2O3−based particles. These particles can be exploited as both heat transfer fluid of the CSP plant and highly active catalysts for the key step of the SO3 dissociation reaction to produce SO2 and O2 in the framework of a solid sulfur thermochemical energy storage cycle. The experimental campaign takes place in a purpose-built setup used to evaluate the materials in a fixed bed reactor formulation at a temperature of 850°C and ambient pressure while concentrated liquid sulfuric acid (H2SO4) was used as feedstock. Based on the findings, pure Fe2O3 particles sintered at 950°C offered the most promising compromise between high SO3 conversion on the one hand and crushing strength value (i.e. a preliminary measure of thermomechanical stability) and particles’ color before and after exposure at reaction conditions on the other hand.

AbstractCe-based materials, that have been extensively investigated for catalytic applications, such as in automotive emission control and more specifically in the oxidation of diesel soot particles, were evaluated with respect to their performance on CO2 splitting. DFT calculations assisted towards the synthesis of two Zr-doped Ce-oxide formulations (Zr content: 20% and 40% respectively) via the LPSHS method. The materials were evaluated with respect to their CO2 splitting activity and compared with other Ce-Zr-based reference oxides. The most active materials were further evaluated under different CO2 splitting temperatures in the range of 800-1480oC in the course of successive redox cycles. The effect of thermal reduction temperature (T: 1480oC and 1600oC) on the most active material was also assessed. The Ce-based oxide formulation with the best performance (Ce0.8Zr0.2O2) was further investigated with respect to its activity towards H2O splitting. In addition, a porous flow-through structure consisting entirely of Ce0.8Zr0.2O2 was manufactured and the resulted body was also evaluated and compared to the respective powder in terms of its CO2 and H2O splitting ability.

AbstractWithin the framework of the recent trend to identify efficient ways of producing solar syngas (CO/H2), the two-step redox based solar thermochemical water dissociation cycle, already employed at a semi-pilot scale for the renewable production of H2, can be modified to include CO2 and/or combined CO2/H2O splitting. The present work relates to Ni-ferrite, as candidate redox material to be employed in Concentrated Solar Power (CSP)-aided thermochemical processes for CO2 and CO2/H2O splitting for the renewable production of CO and syngas respectively. The mixed oxide was synthesized via the Self-propagating High temperature Synthesis (SHS) method and subsequently calcined under air at 1400oC for 1h. Upon calcination, the material obtained the single phase spinel structure. The material was tested, in the form of powder and as a small cylinder-shaped porous structured body, in a lab-scale fixed bed reactor. The experimental protocol involved a thermal activation step of the material for 1h at 1400oC under N2 flow, the CO2 or CO2/H2O splitting step at 1100oC for 30min, which resulted in the production of CO or CO/H2 respectively and the thermal reduction step under N2 at 1400oC for 30min. The effect of CO2 concentration in the feed gas (4%-100%) was investigated in two-cycle experimental runs. In addition, a preliminary durability test was conducted under pure CO2 flow for 8 consecutive splitting and thermal reduction steps. Co-feeding of H2O and CO2 was also conducted for two different compositions; 8%H2O/4% CO2/N2 and 16%H2O/8% CO2/N2). The porous structured body showed somewhat lower yield, in terms of CO2 splitting, compared to the powder.

Abstract The present work describes the realisation and successful test operation of a 100 kW pilot plant for two-step solar thermo-chemical water splitting on a solar tower at the Plataforma Solar de Almeria, which aims at the demonstration of the feasibility of the process on a solar tower platform under real conditions. The process applies multi-valent iron based mixed metal oxides as reactive species which are coated on honeycomb absorbers inside a receiver–reactor. By the introduction of a two-chamber reactor it is possible to run both process concepts in parallel and thus, the hydrogen production process in a quasi-continuous mode. In summer 2008 an exhaustive thermal qualification of the pilot plant took place, using uncoated ceramic honeycombs as absorbers. Some main aspects of these tests were the development and validation of operational and measurement strategy, the gaining of knowledge on the dynamics of the system, in particular during thermal cycling, the determination of the controllability of the whole system, and the validation of practicability of the control concept. The thermal tests enabled to improve, to refine and finally to prove the process strategy and showed the feasibility of the control concept implemented. It could be shown that rapid changeover between the modules is a central benefit for the performance of the process. In November of 2008 the absorber was replaced and honeycombs coated with redox material were inserted. This allowed carrying out tests of hydrogen production by water splitting. Several hydrogen production cycles and metal oxide reduction cycles could be run without problems. Significant concentrations of hydrogen were produced with a conversion of steam of up to 30%.

AbstractThe 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.