Thermophysics, Radiation and Flow for Solar ... - PROMES · PDF file59C AXIS 2: ONVERSION, STORAGE AND TRANSPORT OF ENERGY. |Thermophysics, Radiation and Flow for Solar Facilities

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AXIS 2: CONVERSION, STORAGE AND TRANSPORT OF ENERGY. | Thermophysics, Radiation and Flow for Solar Facilities

Thermophysics, Radiation and Flow for Solar Facilities

Identity Composition of the team (or participants) Team leader: C. Caliot (CR CNRS), A. Toutant (MCF UPVD) Permanent personnel: W. Baltus (T CNRS), F. Bataille (PR UPVD), C. Caliot (CR HDR CNRS), B. Claudet (PR UPVD), M. Daumas (PR UPVD), Q. Falcoz (MCF UPVD), O. Faugeroux (MCF HDR UPVD), A. Ferrière (CR CNRS), G. Flamant (DR CNRS), D. Gauthier (IR CNRS), P. Neveu (PR UPVD), G. Olalde (DR CNRS), A. Perez (AI CNRS), J.-Y. Peroy (AI CNRS), A. Soum-Glaude (IR CNRS), A. Toutant (MCF HDR UPVD) Not permanent personnel: Post Doc: A. Amokrane (07/2014-10/2015), F. Aulery (10/2013-10/2014) PhD students: (1) F. Aulery (14/12/2013), H. Benoit (16/12/2015), F. Larrouturou (16/11/2015), F. Ordonez-Malla (10/10/2014), M. Sanchez (9/12/2015), D. Verdier (29/01/2016); (2) thesis in progress: K. Zeng (02/2013-04/2016), M. Bellec (10/2013), M. Belot (10/2015), M. Coquand (10/2014), D. Dupuy (10/2015), T. Fasquelle (10/2014), K. Zheng (05/2013), Y. Lalau (11/2014), C. Leray (10/2013), S. Mey (01/2013), Rui Li (10/2015). Staff under contract: Y. Lalau (02/2013-08/2014), T. Lebel (09/2014-02/2015), A. Le Gal (01/2015-06/2016), Y. Volut (09/2014-09/2018) Keywords Modelling and experiment of high temperature receivers; Material ageing under high radiative flux; Transfer intensification in turbulent regime; Combined transfers; thermal radiation. Topics Optics for solar power plants: Simulation and control of performances Aging of components and materials for HT solar receivers Radiative transfer in HT solar receivers Turbulent convection: Flow optimization and transfer intensification in HT solar receivers Performance measurements of HT solar receivers and new heat transfer fluids Collaborations National - L. Dubost (HEF-IREIS, Saint-Etienne), F. Siros (RDF R&D) J. Stolarz (SMS, Mines Saint-Etienne), O. Raccurt (CEA,

Grenoble), T. Chotard (SPCTS CNRS, Limoges), M. Huger (SPCTS CNRS, Limoges), A. Nzihou, M. El Hafi, J.-J Bezian (RAPSODEE, Mines d’Albi), R. Fournier (LAPLACE, Toulouse), S. Blanco (LAPLACE, Toulouse), J. Dauchet (Institut Pascal, Aubière), J.-F. Cornet (Institut Pascal, Aubière), B. Rousseau & L. Luo (LTN, Nantes), J. Vicente (IUSTI, Marseille), G. Vignolles (LCTS, Pessac), P. Lenormand (CIRIMAT, Toulouse), M. Roger (CETHIL, Lyon), G. Bois (CEA, Saclay), J.-M. Foucaut (LML, Lille), N. Tregoures (IRSN, Cadarache), G. Vinay (IFPEN, Paris), M Hémati et R Ansard (LGC, Toulouse).

International - Abraham Kribus (TAU, Tel-Aviv, Israël), Maria Isabel Roldan Serrano (PSA CIEMAT, Almeria, Espagne), J. Gonzales

(IMDEA Energia, Madrid, Espagne), Y. Zhou (LLNL, USA), Peter Heller (DLR, Almeria, Espagne), Florian Sutter (DLR, Almeria, Espagne), Fernando Oliveira (LNEG, Lisboa Portugal), German Mazza (PROBIEN, CONICET, Argentine), Nikolay Gorbunov (St Petersbourg, Russie), Zhifeng Wang (Key Laboratory of Solar Thermal Energy and Photovoltaic System of Chinese Academy of Sciences, Beijing), Jan Baeyens (KU Leuven, Belgique).

Contracts - CSP2 (European project FP7), coordinator (2011-2015) - SOLPART (European project, H2020), coordinator (2016-2019) - PEGASE project and storing receiver, EDF (2012-2015) - LFR500 (ADEME AMI Solaire), partner (2012 –2016)

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- ASTORIX (ANR 2014), partner (2014 –2018) - CARTUS (ADEME), partner (2012 –2015) - STAGE-STE (European project, FP7, 2015-2019), partner - SFERA (European project FP7-« Infrastructures », 2008-2012), partner - SFERA 2 (European project, FP7, « Infrastructures », 2014-2017), partner - OPTISOL (ANR SEED 2011), coordinator (2012-2016) - Franco-Israeli contract (Hubert Curien Program, MAE) - SICSOL (Collaborative research contract with TOTAL), coordinator (2010- 2015) - Collaborative research contract VTI-aéraulique, coordinator (2013- 2016) - DENOPI (ANR RSNR), partner (2013- 2017) References 10, 12, 17, 19, 20, 26, 33, 36, 44, 45, 46, 59, 60, 62, 70, 73, 75, 84, 85, 90, 91, 94, 106, 117, 127, 128, 135, 149, 156, 157, 158, 159, 161, 162, 164, 165, 166, 170, 171, 174, 175, 176, 177, 178, 179, 180, 181, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194,196,197, 198, 199, 207, 208, 212, 217, 219, 220, 221, 222, 225, 227, 228, 229, 231, 232, 233, 234, 240, 241, 245, 248.

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Scientific report INTRODUCTION

National and international context The world capacity for the production of electricity by the concentrated solar power plants (Concentrated Solar Power, CSP) has reached 5 GW in 2016 with a strong contribution of Spain and the United States of America (USA). The technology used is primarily that of the parabolic troughs heating a thermal oil (between 250 and 390 °C) allowing the use of a Rankine thermodynamic cycle. The projections for 2025 predict between 10 GW and 22 GW of global capacity depending on the scenarios. The countries of north and south Africa, the Middle East, India and China will be playing major roles in the deployment of the CSP. The attractiveness of the CSP requires a reduction of the costs of investment and a better profitability. In France, two industrials have won French project calls to develop their technology of solar power plants using Fresnel mirrors to conquer new international markets (Solar Euromed and CNIM). Since 2012, commercial solar tower power plants have been installed which use either a receiver producing water vapor (575 °C) or molten-salts (NaNO2-KNO3, 565°C) as the heat transfer fluid (HTF). The gains in investment and performances are expected by the use of these two new technologies. In addition, the international research focuses on the new concepts of collector, receiver, thermal storage, heat transfer fluid and thermodynamic cycle to exceed the current performances. Other uses of CSP technologies have been developed to produce industrial heat or to introduce solar energy in power plants using fossil fuels (Integrated Solar Combined Cycle, ISCC, Solar boost). Issues and challenges The challenge of exceeding the current performances of the CSP plants requires resolving the challenges related to (1) the receiver temperature increase and (2) the integration of high efficiency thermodynamic cycles, by removing material temperature and cycle limitations. The team Thermophysical, Radiation and Flow for Solar Power Plants (TRECS in French) has included its research strategy in the field of high efficiency solar power plants implementing very high temperature solar receivers and thermodynamic cycles. The optimization of high temperature (HT) receivers requires modeling works on the conversion of solar radiation into heat transported by a heat transfer fluid, on the aging of materials and on the intensification of transfers. These modeling types constitute scientific challenges by the complexity of the treated geometries and by the extreme conditions of operation which result in strong coupling effects between the high temperatures and the process of combined transfers where the thermophysical, thermoradiatives and thermomechanical properties drive the phenomena. In addition, thanks to the facilities especially solar available at the PROMES laboratory (solar concentrators of 1.5 kW to 1 MW and the solar tower Thémis) it is possible to exceed the challenges related to the experimental validation of models or concepts at the prototype scale and those related to the in-situ characterization of material behavior. Scientific objectives The scientific objectives are the transfer intensification in the conversion step of the solar radiation in heat (while limiting thermal losses and pressure drops), the aging characterization of materials constituting the receivers, the search of conversion processes by innovating components and architectures, and the performance predictions of the systems integrating these components. The development of high flux and HT air solar receivers constitutes a major challenge in the emergence of new high efficiency CSP plants and it focuses the efforts of the TRECS team. It combines the problematics related to high optical concentration ratios, of radiative transfers in complex geometries and/or semi-transparent media, of material aging submitted to solar fluxes, the convective heat transfer intensification in turbulent regime and the fluid distribution complexity.

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Summary 1. Optics for solar power plants

2 Thermophysics: Study of the aging of components and materials for HT receivers

2.1 Aging of TiAlN and W/SiCH selective coatings

2.2 Aging of SiC and SiC/C

2.3 Thermomechanical study of HT solar receivers

3 Radiation: Influence of radiative transfers in HT solar receivers

3.1 Influence of selectivity in surface receivers

3.2 Influence of selective optical properties in volumetric receivers

4 Fluid mechanic: flow optimization and heat transfer intensification in HT solar receiver

4.1 Turbulent kinetic energy equation

4.2 Spatial development of thermal boundary layers

4.3 Innovative geometry and optimization of a complete receiver

4.4 Other studies in fluid mechanics

5 Solar power plants: Tests of HT solar receivers and new heat transfer fluid

5.1 Experiments of ceramic module at THEMIS

5.2 Fluidized dense particle suspension

5.3.Conclusion and perspectives

6. Solar thermochemistry

6.1 Solar flash pyrolysis of biomass

6.2 Solar calcination processes

6.3 Conclusion and perspectives

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1. OPTICS FOR SOLAR POWER PLANTS

To study the components and optimize the whole solar power plant the prediction of concentrated solar fluxes is required. These fluxes are the boundary conditions in numerical predictive models of solar receiver performances. The angular and spatial distributions of irradiation fluxes on receivers are driven by the reflectors from which the size, the shape and the position on the ground should be optimized from an economical point of view. With the need to use a fast and flexible numerical tool to compute the solar fluxes in large CSP plants, the SOLFAST-4D software was developed in partnership with a French company HPC-SA. This software uses a Monte-Carlo Ray Tracing algorithm which results from theoretical research lead by the LAPLACE (UPS Toulouse), RAPSODEE (Mines d’Albi) and PROMES laboratories. The SOLFAST-4D software and the model were validated experimentally (see Figure 1) and the results were published.

(a) Experiment

(b) Simulation

Figure 1 : Experimental validation of SOLFAST-4D; Concentrated solar flux density on a defocalized target at the EuroDish parabola of PROMES from (a) measurement and (b) computation with SOLFAST-4D SOLFAST-4D is used to study the receiver and heliostat field performances. In the European project STAGE-STE, where different software is compared, SOLFAST-4D is used to optimize the location of heliostats for different solar tower power plants (north or circular fields). During the European project CSP2, SOLFAST-4D was used to predict and simulate the solar irradiation (see Figure 2) in the receiver cavity located at the focal zone of the 1MW-CNRS Solar Furnace of Odeillo. Among the optical methods used at PROMES laboratory and especially at Thémis, an additional method is under study and aims at evaluating the canting and waviness defects of heliostats. This method was proposed as a patent (DI 6288-01) and collaboration between the laboratories IPAG and PROMES with a joint Ph.D. thesis (M. Coquand). This multiple camera Backward-Gazing method allows to quickly characterize the mirrors to evaluate, control and correct repetitively the heliostat tracking errors. This optical method will be experimentally validated at the Themis solar facility.

Figure 2 : Vertical profiles of concentrated solar irradiation at the middle of the cavity, obtained experimentally (Exp) or by a SOLFAST-4D simulation (Sim)

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2. THERMOPHYSICS: STUDY OF THE AGING OF COMPONENTS AND MATERIALS FOR HIGH TEMPERATURE SOLAR RECEIVERS

The materials used for high temperature solar receivers are submitted to extreme operating conditions (high flux, long or short thermal cycles) which could lead to a premature degradation of their thermal and mechanical properties. The durability of these materials is a crucial stake for the design of competitive solar power plants with low maintenance costs. Works on the “Aging” topic aim to answer the following questions: will the material be able to withstand the stresses? What will be its lifetime? The study of the aging of materials first requires a modelling of its behavior, in order to understand the thermophysical and thermomechanical phenomena involved. Then experimental devices generating an accelerated aging are needed. Finally it is necessary to be able to follow the material properties and its evolution of which is representative of its aging. 2.1. Aging of TiAIN and W/SiCH selective coatings The aging of TiAlN and W/SiCH spectrally selective coatings (high solar absorption, low IR emissivity) was studied for solar receivers below 700°C. Samples were annealed in an electrical furnace up to 600°C in ambient atmosphere, for several tens to several hundreds of hours. The evolution with time of their thermo-optical properties was followed, through measurements before/after annealing of their spectral reflectance in the 250 nm - 25 µm range, in temperature up to 500°C for the IR range. These measurements allowed the calculation of solar absorptance and thermal emittance, and their evolution during aging (see Figure 3). The variation of these properties with temperature follows an Arrhenius law, indicating oxidation and diffusion degradation phenomena, the activation energy of which was calculated. This value gives access to an aging acceleration factor at a higher temperature than the aimed operation temperature, to apply accelerated aging tests that are representative of several years of operation of the coated solar receiver. In a first approach, aging tests revealed that samples were sensitive to oxidation at the aimed operation temperature, which helped redirect design and fabrication strategies of the selective coatings. Based on previous studies, thermal cycling is now envisaged, to better take into account the strongly cyclic nature of solar irradiation to which the coatings will be submitted when operated. Day/night-like slow cycles will be carried out in an electrical furnace in various atmospheres (vacuum, dry air or with controlled humidity), while cloudy spell-type fast cycling will be carried out under concentrated solar flux (SAAF). Coated receiver thermomechanical properties will also be studied (in particular, their resistance to coupled fatigue-creep phenomena), in collaboration with experts in the field from Saint-Etienne.

Figure 3 : Evolution of solar absorptivity (calculated from spectral reflectance measurements) of TiAlN tandem selective coatings with annealing duration at different temperatures

2.2. Aging of SiC and SiC/C Aging in air of candidate materials for high temperature receivers has been studied. Two materials have been selected: the first one is a bulk SiC from the Boostec Company, already used in surface solar receivers; the second one is a C-SiC two-layer material (graphite substrate with a SiC coating) from the CEA, a new material for solar applications under high radiative flux. In a first step, it was necessary to find the configuration of boundary conditions and the parameters of the thermal aggressions allowing to generate the most severe stresses in the material and so to accelerate its aging.

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The thermal behavior of the two materials under study has been modelled and several indicators have been defined. The sensitivity coefficients of these indicators to various parameters characterizing the boundary conditions and the thermal aggressions have been computed. This phase of optimal design allowed to determine the most suitable experimental conditions to accelerate the aging of the samples. Thanks to the SAAF device (Solar Accelerated Aging Facility), these samples have been submitted to the thermal aggressions previously defined during the theoretical study. The characteristic properties of the materials after thermal treatment have been monitored and compared to those of unexposed materials, to evaluate their evolution and thus assess the impact of the aggression on the aging. Several characterization devices have been used, in order to get information on optical, thermal, physicochemical and microstructural surface properties : optic fiber reflectometer (REFFO) to measure bidirectional and normal hemispherical monochromatic reflectivity ; optical characterization device (DISCO) to measure bidirectional and normal hemispherical reflectivity in the “solar band” ; scanning electron microscope to examine the surface state and determine the chemical composition ; photothermal bench to estimate the thermal diffusivity of the samples. This study showed that the surface temperature was a crucial factor. A high temperature (above 600°C) in the center of the sample will promote the oxidation of SiC, which leads to the formation of a silicon dioxide layer on the surface and substantially modifies the thermoradiative properties, particularly the absorptivity. Regarding the thermal diffusivity, for the bulk SiC, we observed relatively steady values for the samples submitted to different thermal treatments, which could be interpreted as a high resistance of the material versus the thermal aggression and in particular the high thermal gradients it had to withstand. Apart from the surface modifications, the material kept its crystallographic structure. For the C-SiC two-layer material, we noted a decrease of the thickness of the SiC coating after 7.5 hours of irradiation in steady state, which weakens the material because the graphite substrate will burn when the coating will be totally oxidized. This phenomenon did not occur for shorter thermal treatments. The thermal diffusivity of this material showed a slight decrease, which suggests that it could diffuse the heat less effectively over time. The activity around the SAAF device continues with a Ph.D. thesis started in November 2014 in collaboration with the SPCTS in Limoges. The aim is to develop a thermoacoustic diagnostic method of the thermomechanical behavior of materials (metals and ceramics) at high temperature. The potential and the sensitivity of the method are currently estimated. Furthermore, SAAF will be also used as part of the European project RAISE LIFE which will start in 2016.

Figure 4 : Principle of accelerated aging 2.3. Thermomechanical study of HT solar receiver Based both on numerical simulations and experiments under high solar flux, we finely evaluated the thermal gradients between the irradiated face and the non-irradiated face of a receiver in SiC. This thermal behavior induced high thermomechanical constraints on the absorber/exchanger that can cause its deterioration particularly when ceramic materials are used. Indeed, these materials are very brittle and sensitive to thermomechanical constraints. In this context, we have defined geometries of absorbers associated to operating strategies (level of irradiation, mass flow rate, conditions of startup and shutdown, etc.) that allow minimizing the thermomechanical stresses suffered by the receiver while preserving

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the efficiency of heat transfer. In particular, we tested different distributions of irradiation on a large module (1280X180mm) (Gaussian, plateau, etc.). Among the tested distributions, we have identified that a decreasing distribution along the module allows minor differences of temperature. Numerical simulations in 2D and 3D of SiC and metal modules solve the solid and fluid temperature fields in the modules. The boundary conditions used in these simulations are derived from measured radiative fluxes. The simulations take into account radiative exchanges between the environment, the cavity and the solar absorber, transfers by conduction and thermomechanical deformations within the absorber. The exchange between the working fluid and the absorber/exchanger is calculated from correlations. Because of the nature of their crystallographic structure, ceramics do not have plastic deformation. The elastic deformation is therefore followed by rupture. This break depends on the imperfections found within the material, which appear during the manufacturing process. As it is not possible to determine exactly the number and the position of defects, these characteristics of defects are evaluated statistically using a Weibull law. Sensitivity studies realized by numerical simulations enabled to move from a probability of failure of the order of 50% to less than 1%. In addition, the first comparisons between experiments under high solar flux and the numerical results show that the deformations and the thermal constraints of the solar receiver at HT are relatively well predicted by simulations. This analysis of the thermomechanical stresses to which this type of receiver is subjected will ultimately make its industrialization more reliable in order to develop the sector of HT solar power plants. 2.4. Conclusions and future work Our research team has developed efficient numerical and experimental tools that predict the evolution of optical and thermal properties of bulk and composite materials (substrate-coating). These materials are the components of the future high temperature solar receivers. In the case of ceramics, knowledge and control of the thermomechanical behavior are a major issue. It is particularly necessary to monitor the tensile stresses, which are mainly responsible for the materials breaking. It is also important to take into account the impact of the heat transfer fluid for the temperature distribution (particularly in unsteady regime). Most of the researches in the field of thermomechanical behavior of ceramic and metallic materials at high temperature deal with the analysis of the evolution or the modification of the properties (particularly thermoradiative) in real operating conditions, but at the scale of the sample. Very few works exist on the behavior of real parts at the scale of industrial pilot. This new approach is however essential to validate and secure the sector. This is the way our team has started to study with some very promising first results. So, the works on thermal and thermomechanical simulation of real parts and on development of diagnostic methods will continue, in order to validate the models and to dispose of prediction tools necessary to optimally design the receivers.

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3. RADIATION : INFLUENCE OF RADIATIVE TRANSFERS IN HT SOLAR RECEIVERS

For HT solar receivers, the radiative transfers (absorption, emission, reflection, transmission) drive the material temperatures directly irradiated by the concentrated solar fluxes. The heated materials at high temperature (800-1000°C) are responsible of large thermal losses by emission toward the environment decreasing the efficiency of receivers and CSP plants. Then, the problematic is to keep the same HTF temperature level at the receiver outlet while limiting the radiative losses to increase the receiver efficiency. Modelling the radiative losses, when the geometry and the optical properties (spectral and directional) are complex, becomes a major challenge useful to predict and control the losses. Thus, we have developed surface and volumetric models computing their efficiency to lead parametric studies, optimizations and especially a study of the spectral selectivity influence on the receiver efficiency. Ideal spectral selective properties should result in a maximum absorption in the solar spectrum and beyond a cutoff wavelength, in a minimum infrared emissivity. The influence of these selective optical properties was studied in 3 Ph.D. thesis (F. Ordonez-Malla, F. Larrouturou, S. Mey-Cloutier) for different types of HT solar receivers. 3.1. Influence of selectivity in surface receivers For HT surface receivers, the increase of efficiency needs to control the surface optical properties and the receiver geometry. However, this control induces a cost, which should be quantified relatively to the expected gains in efficiency. Thus, we have studied and quantified by modelling (Monte Carlo Ray Tracing algorithms) the potential gains provided by different geometries and by the use of spectral and directional selectivity of the surface optical properties. The solar tower power plant efficiency was studied for plane or cavity receivers having selective or gray optical properties (see Figure 5). The cavity leads to multiple reflections that increase the effective absorptivity of the receiver. For this case, it was shown the directional reflectivity (e.g. specular or diffuse) shows the bottom wall reflectivity drives the effective absorptivity. For gray walls (ε=0.8), the cavity receiver has the best thermal efficiency resulting in maximum theoretical solar-to-electric plant efficiency of 22.2 % for wall temperatures of 1100 K. At higher temperature, the plane receivers having spectrally selective optical properties could reach higher plant efficiency (e.g. 28.6 %) for higher wall temperature (e.g. 1325 K). A solution to obtain the spectral selectivity was studied and consists to use a low emissive material in the infrared and to microstructure its surface to artificially increase its solar absorptivity up to a cutoff wavelength of about 2 µm. The modification of spectral selectivity was computed with respect to the microstructure shape by solving the Maxwell equations by the Rigorous Coupled wave Analysis (RCWA). Bi-periodic pyramid structures (and perhaps slots) resulted in the best selective properties while limiting the increase of the infrared emissivity (εIR<0.4). This work will be completed, with the partnership of material scientists, by the identification of refractory materials presenting a low infrared emissivity and high oxidation resistance behavior.

(a) η, plane

(b) η, cavity Figure 5 : electro-solar performance of plants according to the selective wall temperatures and their cutoff wavelength for plans receptors (a) and cavity (b), with αsol = 0.8 et εIR = 0.2

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3.2. Influence of selective optical properties in volumetric receivers Our contribution to the study of solar volumetric absorbers is included in the study combined transfer mechanisms driving the heat conversion in HT receivers. Two types of volumetric receivers were studied, the entrained particle receivers and the porous receivers which constituents are directly irradiated by the concentrated solar radiation. The research objectives are: (1) the development of combined transfer models of volumetric receivers, (2) their numerical and/or experimental validation; (3) the study of coupled processes; (4) the test of prototypes, and (5) the proposal of solutions to improve the receiver efficiency thanks to numerical prediction. New models of HT volumetric solar receivers were proposed and approximate radiative transfer models (Discrete Ordinates, P1, Rosseland, 2-Streams) were compared and numerically validated par the reference Monte-Carlo method. For porous receivers, a test bench was used to compare and validate the combined heat transfer models developed for this application (see Figure 6). The absorber model uses homogenized balance equations (local volume average) and effective properties valid at the scale of the representative elementary volume. The composition and the ideal optical properties leading to the best receiver efficiency were studied for volumetric receivers. This study showed high volume fractions of ideally selective materials are required to limit the losses by reflection and emission. However, when an entrained particle receivers was considered (transverse direct irradiation), an optimum temperature was found that homogenizes the temperature field and limits the thermal losses. For volumetric receivers with a co-current configuration (the main solar flux direction is the same as the one for the HTF), the best efficiency was predicted for large pore and high porosity foams allowing the radiation to be absorbed within the depth and limiting the emission. In addition to the aging of porous materials, the study of the approximate models should continue to better describe the processes occurring below the representative elementary volume scale used today to compute the effective properties.

(1) CaF2 window (2) Glass balloon (3) Focal point (orange star) (4) Homogenizer (inside cooling) (5) Lower part

(a) Experimental test-bench

(b) Blackbody equivalent temperature (1100-1400 K) for foam samples ZrB2, α-SiC, α-SiC+SiO2+Al2O3, and α-SiC honeycomb

Figure 6 : Test-bench to characterize porous volumetric absorbers (a) and (b) equivalent blackbody temperatures for different material and geometry absorbers

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4. FLUID MECHANIC : FLOW OPTIMIZATION AND HEAT TRANSFER INTENSIFICATION IN HT SOLAR RECEIVER

High temperature solar receivers are the place of complex flows. Indeed, the high powers involved lead to use velocity such as one is in the presence of turbulent phenomena. In addition, only one face of the receiver is irradiated. This asymmetric heating creates important gradients of temperature which modifies the properties of the flow. Furthermore, the actual geometries are complex. We study locally and finely these flows to improve knowledge and to propose strategies of heat transfer intensification. The main scientific issue concerns the understanding of the coupling between the velocity and the temperature for highly non-isothermal turbulent flow. From a technological point of view, we propose internal geometries in order to intensify the heat transfer while minimizing pressure drop. Each of these issues corresponds to a level of physical description, a numerical strategy and the means of experimental diagnostics. Thus, a better understanding of the complex interactions between turbulence and thermal gradients requires experiments or fine simulations where all the scales of the flow are explicitly resolved (direct numerical simulation, DNS, or large eddy simulation, LES). On the other hand, the search of an optimized geometry requires a parametric study and consequently many simulations. To do that, we use statistical models (RANS for Reynolds Average Navier-Stokes). Finally, at the industrial scale, the fluid temperatures at the outlet of the receiver as well as global pressure drop are evaluated by correlations. Finest simulations (DNS and LES) provide information and serve as validation for more macroscopic models (RANS) as well as the correlations. 4.1. Turbulent kinetic energy equation In order to improve the understanding of the coupling between velocity and temperature, we studied theoretically and numerically the turbulent kinetic energy equation. Physically, we have specified the exchange of energy between the mean motion, the turbulent agitation and the microscopic agitation (temperature). We have thus theoretically highlighted the terms specific to the highly non-isothermal case, i.e. the terms that reflect a transfer mechanism that does not exist in the isothermal case. These terms were expressed in the spectral domain and in the physical domain. The relationship between these spectral and physical domains has also been explained. From direct numerical simulations in a configuration of a bi-periodic channel flow, we have shown that terms specific to the highly anisothermal case represent approximately ten per cent of transfers. Figure 7 shows that the temperature gradient leads to destroy kinetic energy at the hot side and to create kinetic energy at the cold side. This study was conducted in a bi-periodic channel flow. In addition, to get closer to a real solar receiver, it is important to study the spatial development of thermal boundary layers.

Figure 7 : Terms of the turbulent kinetic energy transfer specific to the highly non-isothermal case in function of the number of wave (horizontal axis) and the non-dimensional distance to wall (vertical axis). Left: hot side and right : cold side. The amplitude is non-dimensionalized by the maximum of production.

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4.2. Spatial development of thermal boundary layers The spatial development of thermal boundary layers is studied experimentally in the wind tunnel MEETIC and numerically using large eddy simulations. The MEETIC wind tunnel allows testing turbulent flows for heat transfer intensification. This experimental platform, located on the site of Odeillo, consists of a plane wind tunnel with an open vein instrumented by multiple sensors of pressure and speed, and by means of optical diagnostic.). The S-PIV (Stereo Particle Image Velocimetry) method allows to measure the three local and instantaneous components of velocity in a plane (see figure 8). To compare numerical and experimental results, the thermal boundary conditions of simulations are determined from measurements of temperatures at the top and bottom walls of the wind tunnel. Two types of boundary conditions were tested: imposed temperature and imposed flux. Initial results indicate that the impact of the temperature gradient on the flow is strengthened in this configuration compared to the bi-periodic channel. Indeed, the thickness of the thermal boundary layer is lower in this configuration.

Figure 8 : The wind tunnel MEETIC (left) and an example of velocity field measurement (right) 4.3. Innovative geometry and optimization of a complete receiver We have proposed an innovative geometry that combines vortex generators and riblets. Generated eddies allow the decrease of the thickness of the boundary layer, the increase of the turbulent intensity close to the wall and intensify fluid/solid heat transfers. The riblets increase the exchange surface and drive the vortices. The combination of these textures little obstructs the fluid passage section and therefore generates low pressure drop. The dimensions and forms of texturing were the subject of a parametric study by RANS simulation in order to retain the most powerful geometry. We have proven that this geometry, originally developed for modules in silicon carbide, could be used for receivers in Inconel. This is especially interesting because the manufacturing cost and the fragility of Inconel are much lower than those of silicon carbide. Finally, experimental tests in the wind tunnel and under concentrated solar flux confirmed the strong potential of this geometry. These tests also showed some limits of the averaged simulations underscoring the interest of fundamental studies. On the other hand, collaboration with 2iE (Ouagadougou, Burkina Faso) focuses on the development of micro-CSP for rural electrification in the sub-Sahelian West Africa. This project aims at building low cost and robust plants with a power of tens to hundreds of MWel and that can be built on the basis of local material and labour. A cavity receiver that satisfies these criteria has thus been designed, optimized, built and tested locally. It consists of a steel tube, wound in several contiguous turns forming the cavity. Sizing and optimization of this receiver are carried out on the basis of a simple model (the wall of each coil is assumed insulated), taking into account the radiative exchange of each coil between external (environmental and incident flux) and the other turns, the convective exchange between the coolant and the wall, heat loss (wall/environment) and pressure drop. Heat transfer and pressure drop coefficients are estimated from correlation coming from literature. Radiative exchanges are modeled with radiosity method. The opening of the cavity is replaced by an equivalent black surface, taking into account both concentrated solar flux and the external radiation (room temperature). This concept was developed in cooperation with the DLR-Köln. The system optimization is realized by minimizing the destroyed Exergy (see integration) under a constraint of inlet/outlet temperature. The optimization variables are the absorber tube diameter and the number of turns (or the length of the cavity). This receiver is being tested in real conditions in Ouagadougou. The model, developed in steady conditions for the sizing and optimization phases, has also been extended to unsteady conditions to be integrated into a comprehensive model of the plant.

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4.4. Other studies in fluid mechanics The fluid mechanic skills of the team are also used for technologies other than the solar receiver at HT. In particular, we have worked with the PPCM team on the influence of the flow on anti-reflective layer deposition velocity for photovoltaic cells. Collaboration with the company VTI-aeraulic allowed us to test the potential of the Open Foam software for the design of air Extractor. With IRSN, we have studied the numerical modelling of turbulent two-phase flows with mixed convection in an ANR project on nuclear safety. Finally, we started collaboration with the IFPEN for heat storage. The main goal is the simulation of particle suspension made of phase change materials. 4.5. Conclusions and outlook In order to improve the knowledge and the understanding of the mechanisms of couplings between the velocity and the temperature of strongly non-isothermal turbulent flows, we studied the turbulent kinetic energy equation and the spatial development of the thermal boundary layer. We have thus highlighted the terms of transfer specific to the non-isothermal cases, which are responsible for destruction of energy at the hot side and creation at the cold side. In the spatial development of thermal boundary layer, the impact of the temperature on the velocity is reinforced because of the thermal gradient increase. Building on this progress, we have proposed an innovative geometry that combines vortex generators and riblets. This geometry allows heat transfer enhancement with low pressure drop. We now initiate the modelling of these thermal/dynamic couplings for large eddy simulations. From a technological point of view, the model of the helical receiver is currently extended to the heating of pressurized air to high-temperature in the context of a European contract being signed. The developed model will be used for the design of a pressurized air receiver associated with a parabola. This application is part of a larger project of generating electricity through solar, in which two systems are associated in a cascade: a parabolic trough collector for medium temperatures and parabolic concentrator for high temperatures.

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5. SOLAR POWER PLANTS: TESTS OF HT SOLAR RECEIVERS AND NEW HEAT TRANSFER FLUID

The research works have focused on solar receivers and heat transfer fluids suited for solar tower systems with very high efficiency, featuring Brayton cycle (i.e. gas-turbine) and combined cycle as power blocks. The objective is to overcome the limit of 560°C which is presently observed in the state-of-the-art molten salt or direct steam generation solar receivers implemented with steam Rankine cycles. The solar heat transferred to the air in the receiver is directed into the Brayton cycle, which uses the air as working fluid. A thermal storage unit is optional. The gas turbine runs in hybrid mode with a modified combustor. The solar share varies according to the temperature achieved in the solar receiver. A test-bed featuring a pressurized air loop has been implemented in the Themis solar tower facility for testing solar receivers in various operating conditions representative of the hybrid solar gas-turbine technology. The test-bed also features a cavity, which is actively cooled by the HTF (air). Two technologies of pressurized air absorbers with advanced design are tested, in continuation of previous tests done in the solar furnace facility with primary design. A pilot scale pressurized air solar receiver at high temperature has been designed. This 450 kWth receiver includes a modular and multi-stage absorber located at the bottom of a cavity. A single absorber module of 30-50 kWth is tested for each technology. A dual copper/Inconel 600 metallic absorber with parallel straight tubes equipped with twisted tapes has been modified in order to decrease the pressure drop and to accommodate manifolds on the rear side. Another absorber, made of HT ceramic material (bulk SiC) with textured channels, has been modified to address the critical issue of thermo-mechanical stress and strain. Last, a tubular receiver cooled by an upward dense particles suspension has been successfully tested in the solar furnace facility. Fine SiC particles transported in air have been used as heat transfer fluid in a single tube prototype receiver and in a 16-tubes pilot-scale receiver of 150 kWth. The outlet temperature of 750°C has been achieved and a global heat transfer coefficient of 1000 W/m2.K was observed. The advantage of this concept is the direct thermal storage through the transportation of particles between two tanks. The charging mode corresponds to the heating of the particles which flow through the receiver, the discharging mode is obtained by transferring the heat to the gas of the Brayton cycle in a heat exchanger A fluidized bed heat exchanger dedicated to this application is currently studied. 5.1. Experiments of ceramic module at THEMIS The use of combined thermodynamic cycles is planned to increase the current yields of the solar tower power plants. These cycles require a high-temperature and high-pressure working fluid (1000 °C and 10 bar at minimum). Because these working conditions are more stringent than for current solar power plants conditions, new solar receivers must be designed to achieve these objectives. The experimentation of solar receivers under real operating conditions is an important and indispensable step, both for the improvement of simulation codes as for the validation of their concept and performance. We chose to work with an elementary module component of a large-scale receiver. The experimental results of the basic module are used mainly to validate the simulation codes, but also to study and analyze: - The general concept of the module architecture - The materials used to build it, - Thermal and thermo-mechanical behavior, - The effectiveness or efficiency, - The various technological implemented solutions, particularly for the aeraulics ceramic-metal connections. In this context and following 3D modeling to assess the thermal and thermo-mechanical behavior of this module, we did experiments at the site of THEMIS, a surface receptor module pilot size made of SiC. The chosen geometry is a parallelepiped (1280 mm long, 167 mm wide and 28 mm thick). The module has four channels (10 mm height, 28 mm wide) through which air under pressure circulates at high temperature under pressure. To experimentally reproduce the incident solar flux representative of a high-temperature operation, a solar flux campaign at focus of "Mini Pegasus" volume at THEMIS was conducted. Distribution and intensity of the concentrated solar flux defined by simulation could be obtained by the judicious choice of the number and location of heliostats of the plant. A scrutiny of heliostats strategy was simulated and validated during experiments. The contribution of the heliostats are placed one by one on the cavity that receives the module, to rise gradually in temperature and thereby avoid a significant thermal shock, which can cause damage.

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A specific experimental setup was developed. It allows to align the module in the cavity and to ensure freedom of movement thereof during its complex expansion in presence of solar irradiation. The measurements concern: the deformations created in the module (elongation and spire), the SiC temperatures rear panel, the air temperature inside the module, the mass airflow and the various air pressures at different points of the device. The technology of ceramic-metal connections developed in the laboratory has been validated. Experiments are underway. In all the experiments conducted to date, the module performs very satisfactory, given the different phases of incremental heating and cooling to which it was submitted. To illustrate this type of experiments, the solar incidents fluxes are a maximum of around 250 kW/m². The air outlet temperature is about 600 °C, for a module temperature back face of about 700 °C. The last experimental campaign that will take place in 2016 should achieve the most important incidents solar fluxes to obtain higher output gas temperatures. It will also evaluate the general behavior of the module exposed to extreme solar fluxes.

(a)

(b) Figure 9 : (a) Implementation of the module in the cavity; (b) whole experimental volume during experiments 5.2. Fluidized dense particle suspension The molten salt central receiver (solar tower) technology is on the way to become the reference for solar thermal electricity (STE) production since it allows implementing Rankine type (Hirn) thermodynamic cycles with efficiency higher than 40%, while permitting thermal storage (up to 15h). Nowadays, these plants operate around 550°C. The objective is to propose fluids that do have the same properties (thus combining both capacities of heat transfer fluid and storage medium), but able to operate at higher temperature, typically in the range 700°C-800°C in order to feed higher efficiency thermodynamic cycles, in solar only or hybrid mode. Dense particle suspensions flowing in tubes meet the above-defined specifications, but so far they were never implemented for such an application. So, the first challenge was to demonstrate that the concept patented in 2010 jointly with INPT-LGC (an opaque vertical tube exposed to concentrated solar radiation transmits the absorbed energy to the particles in dense suspension flowing inside the tube) was feasible at lab scale. Other challenges were to measure the wall-to-suspension heat transfer coefficient, and to develop a solar receiver at pilot scale operating with closed loop solid circulation. During the study developed in the frame of CSP2 European project –coordinated by PROMES-, a 1-tube receiver was successfully tested at the focus of the 1 MW Solar Furnace in Odeillo, in the particle flow rate range 10-50 kg/m2.s (SiC, 64

) e le pa le e pe a e a a a °C, which proved the concept validity and permitted to calculate the wall-to-suspension heat transfer coefficient between 400 and 1100 W.m-2.K-1 depending on the particle mass flux (Figure 10), under tested conditions. The flow hydrodynamics and the heat transfer mechanisms were described in collaboration with the LGC laboratory (Toulouse). A 16-tube pilot solar receiver was tested at the focus of the 1 MW-CNRS Solar Furnace with particles circulating in closed loop. Operation lasted around 100h, in both transient and steady regimes, with incident solar power and solid flow rate ranging between 60 kWth to 142 kWth and 660 to 1760 kg/h, respectively (Figure 11). The measured thermal efficiency was as high as 90% (Figure 12).

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Figure 10 : Fluidized bed-wall (tube) heat transfer coefficient versus pa le a l , )

Figure 11: CSP2 pilot test in court in solar oven 1000kW

This receiver was integrated in a complete loop including a hot storage bin, a heat exchanger and a cold storage bin, thus permitting the complete concept validation at significant scale (Figure 11). In addition, we have been developing reactive fluidized bed models for several years, in collaboration with PROBIEN laboratory (Neuquén, Argentina). These models were validated in the case of heavy metal vaporization during waste combustion, in front of experimental results obtained at PROMES. They were also applied to the heat transfer in the tubular solar receivers described before.

Figure 12 : Pilot solar receiver efficiency versus particle mass flux 5.3. Conclusion and perspectives We intend to continue the process analysis to improve our knowledge of the hydrodynamics and of the influence of the temperature and of the tube height on heat transfers. Moreover, we must work on the ways to increase the wall-to-suspension heat transfer coefficient. The successful CSP2 project opened the way to other developments in the fields of thermochemistry (reactive particles) and solar thermal electricity (industrial pilot). A H2020 European project was submitted; it involves a 4 MWth receiver to be installed at Themis solar tower, with storage and electricity production thanks to a gas turbine operating in hybrid regime.

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6. SOLAR THERMOCHEMISTRY

6.1. Solar pyrolysis of biomass Solar pyrolysis of carbonaceous materials such as biomass produces gaseous, liquid and solid fuels, whose mass distribution varies strongly with operating conditions, especially the heating rate and the maximal temperature (plateau). Moreover, this solar treatment increases the heating value (low heating value, LHV) of the initial biomass feedstock; the energy increase corresponds to the solar energy fraction stored in the pyrolysis products. The study developed in the frame of SOLSTICE Labex intends to: (1) determine the pyrolysis products’ distribution as a function of operating conditions; (2) measure the compositions of the pyrolysis gas, of the tar fraction, and of the char fraction; (3) model solar pyrolysis; and (4) determine the products’ LHV and the energy upgrading of solar pyrolysis (never published according to literature survey). Experiments were run at the focus of a lab-scale vertical solar furnace, pyrolyzing 400 mg beech wood pellets in a solar reactor (Figure 13). The unique setup permitted to vary and master the heating rate from 5°C/s to 450°C/s and the maximal temperature (5 min plateau) between 600°C and 2000°C.

Figure 13: Experimental setup for solar pyrolysis Figure 14 : Evolution of the pyrolysis products as a function of the maximal temperature (50°C/s)

Products’ distribution and pyrolysis gas composition. Temperature is the most important parameter that controls the pyrolysis products’ distribution, whereas heating rate is a significant but secondary parameter. As shown on figure 14, the gaseous fraction is tripled when the temperature increases from 600°C to 2000°C. The produced gas is mainly composed of H2, CO, CH4 and CO2, and the H2/CO ratio tends to 1 when the temperature increases (Figure 15). Products’ heating value and energy upgrading of solar pyrolysis. The products’ low heating value was calculated from the atomic composition of the compounds (Figure 16). The liquid fraction LHV is strongly dominant at low temperature, and the gas LHV increases with the pyrolysis maximal temperature. Finally, the gas and liquid LHVs are equivalent at temperature higher than 1200°C, whereas the

Figure 15 : Pyrolysis gas composition versus temperature

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char LHV represents 10% only of the total heating value. The energy upgrading due to solar pyrolysis of biomass is nearly 50% and it varies with temperature, it is maximal (53%) at 900°C and decreases to 45% at 1200°C. This original result clearly indicates that solar pyrolysis allows around 50% energy upgrading of the initial biomass. Therefore, this thermochemical process appears to be an efficient mean to chemically store solar energy.

Figure 16: Pyrolysis products’ LHVs versus temperature 6.2. Solar calcinations processes Solar thermochemistry consists in processing endothermic chemical reactions at various temperature levels using concentrated solar energy. Examples of such reactions are as follows: reduction reactions occurring in thermochemical cycles for producing hydrogen and carbon monoxide or for storing energy; pyrolysis and gasification; and solid-gas decomposition reactions. In the latter domain, calcination is a very endothermic reaction occurring in the production of hydraulic binders that emits about 10% of anthropogenic emission of greenhouse gases, of which 40 % comes from the combustion required for providing the reaction energy in current classical processes. As a continuation to the successful studies dealing with particulate tubular receivers, we examined the ways to –partially- turn into solar these industrial processes, and we analyzed the potential reduction of fossil fuel consumption and greenhouse gas emission with industrial partners. This work finally results in new European project named SOLPART and coordinated by PROMES started in January 2016. Its objective is the development of a solar calcination process to be used in cement industry, and more generally in ore thermal processing. The principle of integration of a solar calcination process into a hybrid cement factory is schemed in Figure 17.

Figure 17: Solar calcination process integrated into a hybrid cement factory (SOLPART EU project) 6.3. Conclusions and perspectives The use of solar heat in industry, and particularly in solid processing, is an open field of research that links our research in the field of new heat transfer fluids and solar thermochemistry. Testing of pilot size solar reactors/receivers is planned during next years.

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