The extending market of the concentration solar power plants requires the use of high-temperature materials for two critical elements. The first element is the solar receiver that has to convert the incoming concentrated solar flux into heat transferred to a coolant. The main bottleneck is to develop a surface that would absorb the largest possible spectrum of the incoming concentrated solar flux and present the lowest possible radiative losses. As this surface is directly exposed to the atmospheric air, it may therefore oxidize, which could affect its optical properties. An anticorrosive and spectral selective coating could optimize the performance of this surface. The second element is the heat exchanger that enables to transfer the heat from a first liquid or fluidized coolant (molten salts, ceramic particles...) to a gas (ideally air). This heat exchanger may support corrosion at high temperature, and an advanced coating could optimize its resistance to it. The research philosophy adopted in this research program is the design and development of an optimal "coating system" that optimizes the surface properties (spectral selectivity, resistance to oxidation and abrasion) of a high temperature metallic alloy chosen for its good thermal conductivity. Using the High Temperature Chemical Vapour Deposition technique that was developed at SIMaP laboratory with the support of the SIL’TRONIX ST company, we expect to conceive multi-layers materials, with a ceramic coating on a metallic alloy, that would be able to support a working point beyond 1000°C, which would increase the yield of a solar power plant by 15 %. The first objective will be the selection of candidates that fulfil (a) the requirements of stability in operating conditions and (b) the ability to be deposited by high temperature CVD above 1000 °C. The second objective is to determine what really happens in temperature-induced ageing phenomena in solar absorbers and heat exchangers. This investigation can be lead using the unique solar facilities available at PROMES-CNRS such as the REHPTS and MEDIASE facilities. The first originality of this research program is to develop a multi-layered and multi-functional system by (a) the addition of the coating (AlN) on a known high temperature metallic alloy (TZM) in the temperature range of operating conditions of the solar elements, (b) the management of mechanical and chemical compatibility of the coating with the others layers by the design of thin films barriers. The second originality of this project leads on the use of these specific and unique tools to evaluate the performance of the proposed stack. We expect within 48 months to jump from a TRL 2 (the production of ceramic coating is possible, but no analysis of the performance of the material in a critical environment was ever done) to a TRL 4 (validation of the potential of this material on a laboratory scale with a significant working environment). The first difficulties to face concern the coating and will be (a) its production at high temperature and (b) the control of its optimal microstructure and properties to protect the core material from oxidation, without provoking a thermal insulation. The surface topography may also be important, as it will affect its emissivity, and therefore the absorption of heat fluxes and/or the radiative losses. We are planning to investigate firstly the AlN coating that is currently the most advanced through HT-CVD process, but other compositions such as SiC (good spectral selectivity with a high absorptivity in the solar spectrum) or WC could be investigated. The second difficulty will be to reproduce, in a short-time laboratory experience, oxidation conditions in extreme environment to predict the long-term (several months) ageing of the processed multi-materials. The REHPTS facility can enable to perform such an analysis in reproducible conditions.

The concentrated solar power (CSP) will take a significant part in the diverse scenarios for reduction of greenhouse gas emissions. The CSP sector presents the considerable advantage, in comparison with the other intermittent energies, to be able to integrate a storage function, which allows to shift the electricity production to periods of strong environmental or economic interest. For it, a high-temperature thermal storage has to be integrated between the solar receiver and the power block. Compared to existing systems (sensible or latent heat), the thermochemical storage processes constitute innovative and promising solutions, still in the state of research. The thermochemical processes based on reversible solid /gas reactions are particularly relevant by their high effective energy density (until 400 kWh /m3) and operating temperatures (until 1000°C) depending on the reactants. The aim of this project is to investigate the integration of such a thermochemical process in a CSP installation by an approach which aims at the global optimization of the performances of the power plant integrating this storage. For that purpose, the project focuses on the following key points : 1) The concepts of coupling the thermal storage and the power plant: the objective is to enhanced transfers between the themochemical reactor and the Rankine cycle. The coupling modes will be analyzed in an exhaustive way, then simulated. The most pushed integration of the storage to the Rankine cycle will be looked for. We shall analyze the impact on the sizing, the functioning and the performances of every component, but also globally of the solar plant. 2) The thermochemical reactor: it will achieve a thermal storage adapted to the CSP sector according to several criteria: operating temperature, energy density, stored energy, restored power... A strongly compact reactor (a fixed bed) will be chosen. By means of a local model, we shall model and optimize the gas and heat diffusion network, as well as the kinetics and transfer characteristics of the composite (combining the reactive salt with a binder), according to the above criteria. 3) An experimentation on a pilot: the pilot will couple a reactor of optimized configuration and a complete Rankine cycle, according to the high performance concept selected at point 1. We shall analyze the functioning and the performances of components, and the global performances of the whole system. A particular attention will be paid to the dynamic aspects depending on several causes: variability of the solar source, the intermediate phases of the thermochemical reactor functioning, the variation of the reaction kinetics. 4) The scenarios of storage/production for CSP: they cover from the peak production (a few hours a day) to the base production (over 24 hours) and strongly influence sizing, investments, saling price of the produced electricity, environmental criteria … A global model of solar installation integrating a thermochemical storage will allow to realize an economic optimization by a global approach coupling sizing of the system and choice of the scenarios of storage/production. Several applications are in scope: large-sized power plant, solar installation coupled with a smart-grid, isolated installation. 5) The impact of the thermochemical storage on the CSP sector: in parallel with the scenarios, we shall study the extrapolation to a large-scale reactor, by investigating the size of modules and an adequate manufacturing process optimizing the cost of the reactor. Simulations of these configurations will be done thanks to the tool developed in point 2. Furthermore, a life cycle analysis will allow to direct the choice of the configuration in order to reduce the environmental impacts.

A common industrial challenge to improve the efficiency of the solar-to-electricity conversion for concentrating solar power (CSP) is to operate at high temperatures (900-1000°C). Research and development efforts on over recent years have therefore focused on the materials that compose the solar absorber which plays the key role in the overall CSP system performance. Silicon carbide (SiC) exhibits a chemical inertness, a high temperature oxidation resistance and a robustness compatible with the operating conditions of further CSP systems. However, despite a good sunlight absorption, SiC has a high thermal emittance, leading to a poor optical selectivity. Promising properties for absorber materials can be found in transition metal carbides and nitrides of column IV according to their refractivity, their inherent spectral selectivity and a lower thermal emittance compared to SiC. However, their major limitation is their tendency to oxidize in the targeted temperature range. By entering the scope of the Challenge 3 “Stimuler le renouveau industriel” (theme “Matériaux et procédés” and more particularly the priority 14), the CARAPASS project proposes to prepare nanocomposites of the type MX/SiC (M = Ti, Zr, Hf; X = CxN1-x, 0 = x = 1) by combining SiC and transition metal carbide and/or nitride in the same materials with the goal to combine optical selectivity, thermomechanical properties and oxidation resistance to fit the requirements of the next generation of high temperature absorber materials. These materials are prepared as dense monoliths to maintain their mechanical strength and robustness at high temperature. The four year CARAPASS collaborative research project brings together specialists in materials synthesis, materials characterization, and computational approaches. It is built from five French research institutes, IEM, ICSM, SPCTS, PROMES and CRM2, with complementary expertises in chemistry, in processing, in characterization of materials - especially for CSP - and in modeling which have already collaborated in the past. To reach our objectives, the project is based on the promising results obtained by IEM and ICSM with TiC/SiC nanocomposites. CARAPASS is subdivided into five interconnected scientific tasks. The first task is focused on the preparation of nanocomposite powders using two chemical routes investigated by IEM and ICSM. The second task consists to prepare dense materials following three strategies based on pressing, casting and Spark Plasma Sintering processes to be characterized in tasks 3 and 4. Physical and chemical characterization of nanocomposites is the topic of the task 3. In addition to standard material science techniques available in each institute, the thermostructural, mechanical and thermal properties of the nanocomposite monoliths will be evaluated before and after thermal ageing. The task 4 studies the optical characterization of the nanocomposites to demonstrate the selective behavior of nanocomposites The optical properties will also be measured after accelerated ageing. A theoretical work will be done in task 5 using density functional theory and the GW approximation. The present project is built to elaborate materials that are expected to lead to benefits for the advancement of science, industry and society and should allow France to be in place on this growing thematic at international scale.