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International audience; Three main components of biomass were pyrolyzed individually in a closed reaction system at 500–700 °C for 60s and 90s. Then bio-oil heavy compounds were further analyzed with Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) and Kendrick mass defect (KMD) analysis. The evolution paths of heavy compounds for the different pyrolysis stages were proposed. It was found that the sugars and phenolic-like species in heavy compounds were the most active substances during secondary reactions. Moreover, the rising temperature promoted this secondary reaction of phenolic-like species as the decrease in their abundances growing from 13% to 54%, while contrarily inhibited it for hemicellulose as the decrease in their abundances changing from 44% to −2%. The lignin-derived lipids and unsaturated hydrocarbons that generated in the secondary reactions increased with rising temperature. KMD analysis showed that the heavy compounds of cellulose and hemicellulose prefer homologous evolution during pyrolysis, while those of lignin had more complex evolution paths like cracking and recombination. more...

A fluidized bed of Geldart-A particles is promoted as heat transfer fluid in the tubular solar receivers of solar towers. A pressure-driven upward particle flow affects the hydrodynamic flow structure and properties of the fluidized bed. Experiments involved a tube of 0.05 m internal diameter but of very high height/diameter ratio (>120), representative of the future solar receiver and of numerous chemical reactors. Solid circulation fluxes and aeration velocities were varied. Configurations of a bare tube and a tube with bubble rupture promoters were compared. In the bare tube, freely bubbling is transformed into axi-symmetric slugging at a bed level of ~1 m. With bubble rupture promoters, freely bubbling prevails to about a bed level of 3 m, and a turbulent fluidization mode develops higher up the tube (a more chaotic two-phase system with elongated and unstable “gas voids” and “dense solid clusters”), without axi-symmetric slugging detected. Experimental results for both tube configurations were assessed and compared with CFD predictions by the Euler n-fluid code, NEPTUNE_CFD. A good agreement of bed properties was obtained for slug/void frequencies and solids volume fraction in both tube configurations. BRPs moreover enhance the bubble through flow of the fluidizing gas, thus limiting the visible bubble flow rate and bubble sizes while increasing the gas/particle contact, and hence important in designing multi-tube chemical reactors. Whereas slugging limits the heat transfer from the tube wall to the suspension at ~200 W/m2K, the presence of BRPs maintains a heat transfer coefficient in excess of 600 W/m2K. more...

Direct Numerical Simulations are performed to analyze the influence of key boundary conditions and flow characteristics on entropy generation rate statistics in a highly anisothermal forced convective turbulent channel flow of a thermo-dependent fluid submitted to asymmetrical heating. Each parameter is varied separately to quantify its influence compared to a reference simulation, the hot to cold wall temperature ratio from 1.5 to 3, the mean friction Reynolds number from 150 to 210, the mean thermodynamic pressure from 1 b a r to 3 b a r . Two thermal boundary condition types are also compared (fixed temperatures vs. fixed heat flux density at the walls). For each parameter that is varied, the evolution of the entropy generation rate mean, root-mean-square and correlations with the temperature and the streamwise or wall-normal velocity components are presented and discussed. All simulations have in common the thermal asymmetry and the highly non-isothermal nature of the flow: beyond the changes induced by the variation of each parameter, the common characteristics of the statistical quantity profiles are also analyzed. This work establishes how the spatial distribution of the local entropy generation rate is modified by the parameters studied, in which direction and with which sensitivity. It demonstrates the significant influence of the thermal boundary condition type on the entropy generation rate fluctuations near the walls. more...

International audience; Do compact Photovoltaic-Concentrated Solar Power hybrid systems offer the opportunity for the expansion of the global solar market? Despite the inherent potential of this technology to provide both affordable and dispatchable solar electricity, it is still unclear how the combination of Photovoltaic and Concentrated Solar Power may offer a net advantage over conventional solar plants. Herein, large-scale compact Photovoltaic-Concentrated Solar Power hybrid plants are modelled considering two different hybrid approaches, over a whole year operation, and compared with a conventional Concentrated Solar Power plant. A detailed optical, electrical and thermal model is developed to analyze the temporal output characteristics of the hybrid plants, based on realistic input parameters and representative meteorological data in Targassonne, France. Results show that both hybrid systems may deliver significantly higher energy output over the conventional Concentrated Solar Power plant, provided that several key parameters are optimized. Furthermore, the sensitivity of the two-hybrid strategies investigated to the operating conditions, and to the characteristics of the solar cells used, are analyzed. In the light of these results, the main scientific and technological issues one should address to ensure optimal operation of these compact hybrid plants are finally discussed. more...

The design and the performance of a medium-scale modular solar power plant (~20 MW) integrating a gas turbine combined cycle with a fluidized particle-in-tube receiver and direct thermal storage are investigated in this paper. A practical technique is used to design each part of the solar power plant. The complete design starts with the solar gas turbine (SGT) since it defines the necessary power to run it; then, the other parts are designed upstream. Three different cases are investigated under different operation strategies corresponding to two particle temperatures 750 °C and 880 °C, and hybrid and solar-only operation modes respectively. The results show that the nominal efficiency of the components including the heliostat field, the solar receiver, the gas turbine, and the steam turbine can reach 67%, 80%, 32%, and 34.5% respectively. As a result, the nominal thermal efficiency and the annual capacity factor of the complete solar power plant achieve 46% and 33.47% respectively. The overall nominal efficiency (solar-to-electric efficiency) of the plant for hybrid operation mode is 25.80%. It varies from 21.16% to 24.7% for the solar-only operation mode. Special interest is shown to the part-load operations. more...

Calcination of limestone for lime production was successfully performed in the continuous flow mode on a daily basis in a fluidized bed indirectly heated by concentrated solar radiation. Industrial calcium carbonate feedstock was decomposed at the focus of the CNRS 1 MW solar furnace in a pilot-scale solar reactor operating at an average power of 55 kW. The reactor was a four-stage horizontal fluidized bed, irradiated on a front metallic wall of 1 m long and 0.4 m high. A novel aiming strategy was applied to reduce the hot spots on the irradiated wall. The conversion degree was analyzed as a function of the fluidization conditions (air mass flow rate) and the particle mass flow rate. This latter parameter varied in range (14.5 – 25 kg/h), and the highest conversion degrees were obtained at high fluidization velocity. The best result was obtained for a calcite mass flow rate of 20 kg/h, resulting in a degree of conversion of 95.2%, a BET surface area of the lime of 5.39 m2/g, and 17% and 29% thermochemical and thermal efficiencies of the reactor, respectively. This achievement corresponds to a particle mass flow rate three times higher than the current state of the art for solar calcination of lime. more...

High temperature solar receivers are developed in the context of the Gen3 solar thermal power plants, in order to power high efficiency heat-to-electricity cycles. Since particle technology collects and stores high temperature solar heat, CNRS (French National Center for Scientific Research) develops an original technology using fluidized particles as HTF (heat transfer fluid). The targeted particle temperature is around 750 °C, and the walls of the receiver tubes, reach high working temperatures, which impose the design of a cavity receiver to limit the radiative losses. Therefore, the objective of this work is to explore the cavity shape effect on the absorber performances. Geometrical parameters are defined to parametrize the design. The size and shape of the cavity, the aperture-to-absorber distance and its tilt angle. A thermal model of a 50 MW hemi-cylindrical tubular receiver, closed by refractory panels, is developed, which accounts for radiation and convection losses. Parameter ranges that reach a thermal efficiency of at least 85% are explored. This sensitivity analysis allows the definition of cavity shape and dimensions to reach the targeted efficiency. For an aperture-to-absorber distance of 9 m, the 85% efficiency is obtained for aperture areas equal or less than 20 m2and 25 m2for high, and low convection losses, respectively. more...

Variational methods are used to optimize convective heat transfer in a channel gas flow. The minimization of a functional objective combining the rate of total entropy generation in the channel on the one hand and the total viscous dissipation on the other hand results in velocity and temperature fields. A weighting factor allows varying the relative importance of these two terms and a virtual body force field is applied to vary the velocity field pattern via the momentum equation source term. The result is velocity configurations that minimize the objective functional. This study shows that the velocity fields determined by the optimization process effectively lead to a reduction in the entropy generation rate in the flow as well as a reduction in the temperatures of the heated plate. In addition, the value of the weighting factor triggers the transition from slightly to highly perturbed velocity and temperature fields when compared to a non-optimized flow. The heat transfer enhancement is assessed and the increase of the Nusselt number is put in perspective with the reduction of the entropy generation rate and the increase of viscous friction. The results could be used to design passive technologies for enhancing wall-to-fluid heat transfer. more...

International audience; This work presents the first use of Cofalit® (vitrified asbestos-containing waste) as a solid filler in pilot-scale thermocline thermal energy storage (TES). The thermocline size is 4 m³ connected to the MicroSol-R installation at the PROMES research facility in Odeillo, France. The study compares the thermal performance of the thermocline filled with Cofalit® to the reference case of alumina spheres for typical charge and discharge operations. It evaluates the actual thermal behavior of thermocline considering three leading performance indicators: process duration, thermocline thickness, and process efficiency. The investigation shows a 22% shorter charge duration in Cofalit® compared to alumina and 16% shorter discharge duration. Cofalit® exhibits about 16% lower thermocline thickness during both charge and discharge compared to alumina. The charge efficiency is slightly better in Cofalit® than alumina with an efficiency of 82% and 78%, respectively. Also, Cofalit® has a better discharge efficiency, 90% with respect to 84% for alumina. These results confim a good thermal performance of Cofalit® as filler material in thermocline TES. Considering the cost-saving and positive environmental impact of using Cofalit® as well as the good thermal performance of the thermocline filled with it, Cofalit® appears a very good filler material in TES. The obtained temperatures from radial positions indicate no significant variation during charge and discharge, and this confirms the one-dimensional thermal behavior of this setup. A parametric analysis is performed using a 1D continuous solid (C-S) model to investigate the influence of particle diameter, porosity, thermal conductivity, and volumetric heat capacity on the thermal performance of the thermocline. The analysis confirms the experimental findings, and it indicates that about a 10.9% longer process duration is associated with a 10% larger volumetric heat capacity and less affected with other parameters. Thermocline thickness is mainly affected by the diameter as well as the volumetric heat capacity of the solid filler; it grows 2.2% for each 10% diameter increase and around 3.23% for doubling the volumetric heat capacity. Charge efficiency demonstrates independency from evaluated properties. While discharge efficiency increases sharply at a tiny particle diameter before an optimum diameter value is reached, then it starts to decrease by 1.4% to each 10% bigger diameter. more...

Heat and mass transfer enhancement in a convective flow is studied using a variational optimization technique. Entropy generation rate is minimized while allowing to vary the relative weight of the total viscous dissipation in the objective functional to determine optimized velocity and scalar field patterns. The resulting velocity, thermal and mass concentration fields are analyzed as well as the influence of boundary conditions. The relative improvement of the optimized solutions is assessed and heat transfer vs. mass diffusion results are compared. The optimization approach leads to improved entropy generation rates. The viscous dissipation weighting factor influences the entropy generation rates and the velocity and scalar field patterns. A transition between two levels of perturbation by comparison to non-optimized flows occurs at a critical value of the weighting factor that depends on the boundary conditions. The flow patterns obtained by variational optimization can be the basis for enhanced exchanger design. more...

International audience; The study presented an innovative way to dispose and upgrade heavy metal (HM) contaminated biomass by solar pyrolysis. Pyrolysis of raw and HM (Cu and Ni) impregnated willow wood was performed in a solar reactor under different temperatures (600, 800, 1000, 1200, 1400 and 1600 °C) and heating rates (10 and 50 °C/s). The combined effects of HM and heating parameters (temperature and heating rates) on solar pyrolysis products were investigated. The results indicated that a threshold temperature of 1000 °C was required in impregnated willow pyrolysis to make sure copper and nickel catalytic effects on promoting the cracking and reforming reactions of tar. It led to the gas yields from copper or nickel impregnated willow pyrolysis at 1200 °C increased by 14.76% and 34.47%, respectively, compared with that from raw willow. In particular, the H2 and CO production from nickel impregnated willow were higher than those from raw willow (10.30 and 12.16 mol/kg of wood versus 6.59 and 8.20 mol/kg of wood) in case of fast pyrolysis (50 °C/s). Under heating rate of 10 °C/s, H2 and CO yields from only nickel impregnated willow increased compared with that from raw willow. Solar pyrolysis of HM contaminated biomass is a promising way to produce valuable syngas. more...

International audience; Solar pyrolysis of cotton stalk was carried out in a molten salt reactor heated by 4 kW solar simulator. The effects of pyrolysis temperature and mass ratio of molten salt to biomass on pyrolysis products properties were investigated. The use of molten salt as pyrolysis media increased gas yield. At 850 °C, the gas yield (mainly of CO and H2) continued to rise from 41.35 wt% to 82.57 wt% when mass ratio of molten salt to cotton stalk increased from 0 to 10. Pyrolysis in molten salt significantly decreased bio-oil acids and phenols, while increased aromatics among pyrolysis temperature range of 450–850 °C. There was positive correlation between the increased content of aromatics and mass ratio of molten salt to cotton stalk (from 0.5 to 10). The bio-char carbon content showed a general decreasing trend while oxygen, BET surface area and pore volume increased with using molten salt as pyrolysis media. Bio-char obtained from CS1MS5 pyrolysis at 850 °C had the highest BET surface area of 972.57 m2/g and the biggest total pore volume of 0.6203 cm3/g. High quality pyrolysis products with more uniform chemistry suggest catalytic reactions occur inside the solar reactor due to the intermediates degradation with molten salt. more...

International audience; Hybrid solar photovoltaic/thermal power systems offer the possibility of dispatchable, low-cost, efficient and reliable solar electricity production. A key design strategy capable of fully exploiting the heat generation stemming from both solar cell thermalization and sub-bandgap photons involves an integrated photovoltaic/thermal absorber operated under concentrated sunlight, at temperatures conducive to efficient turbine operation, to wit, hundreds of degrees C. A pivotal aim is attaining the highest efficiency possible while ensuring a substantial fraction of the total power derives from the turbines, with gas-fired backup heating and/or thermal storage mitigating the ephemeral character of solar availability. However, the performance of solar cells at unprecedented elevated temperatures remains an open question. Key issues include (a) whether the efficiency loss stemming from high-temperature solar cell operation can be maintained acceptably small, as well as how optical concentration affects it, and (b) whether the solar thermal contribution can constitute a significant fraction of total electricity production. Here, we try to establish upper bounds on photovoltaic and system performance, covering a broad range of cell temperature and concentration levels, for single- and multi-junction cells operating at the radiative limit. We demonstrate that (1) the use of highly concentrated sunlight markedly diminishes photovoltaic - as well as thermal - efficiency losses at high temperature, and (2) the extent to which high operating temperature affects cell efficiency strongly depends on cell architecture. The implications for future generations of high-temperature/high-concentration solar cells are also addressed. more...

International audience; Solid particles can be used as a heat transfer medium in concentrated solar power plants to operate at higher temperature and achieve higher heat conversion efficiency than using the current solar heat transfer fluids that only work below 600°C. Among various particle circulation concepts, the dense particle suspension (DPS) flow in tubes, also called upflow bubbling fluidized bed (UBFB), was studied in the frame of the CSP2 FP7 European project. The DPS capacity to extract heat from a tube absorber exposed to concentrated solar radiation was demonstrated and the first values of the tube wall-to-DPS heat transfer coefficient were measured. A stable outlet temperature of 750°C was reached with a metallic tube, and a particle reflux in the near tube wall region was evidenced. In this article, the UBFB behavior is studied using the multiphase flow code NEPTUNE_CFD. Hydrodynamics of SiC Geldart A-type particles and heat transfer imposed by a thermal flux at the wall are coupled in two-dimensional unsteady numerical simulations. The convective/diffusive heat transfer between the gas and dispersed phase, and the inter-particle radiative transfer (Rosseland approximation) are accounted for. Simulations and experiments are compared here and the temperature influence on the DPS flow is analyzed. more...

International audience; Hybrid solar photovoltaic (PV)/thermal power systems offer the possibility of dispatchable, affordable and efficient solar electricity production – the type of transformative innovation needed for solar cell devices to realize high grid penetration. The PV sub-system enjoys high efficiency, and the thermal sub-system can ensure uninterrupted power delivery via backup gas heating and/or multi-hour thermal storage. However, elucidation of the basic performance bounds, and the quantitative perspective required for judging the leading hybrid strategies relative to one another, as well as relative to the existing alternative of autonomous photovoltaic and solar thermal power systems, have remained incomplete. A more thorough and basic evaluation of the performance of the assorted combinations of PV and solar thermal sub-systems over a wider range of possible operating conditions than regarded previously is presented here. This involves analysis of the most fundamental processes limiting system efficiency, tempered by the realities of current and foreseeable PV and thermal technologies. The 3 leading hybrid strategies are: (1) concentrated solar beam radiation irradiating an integrated PV–thermal receiver, with the unique advantage of recuperating PV thermalization losses as heat delivered to the thermal receiver, thereby contributing to driving the turbine, (2) the spectral splitting of concentrated solar beam radiation, with sub-bandgap photons directed to a thermal receiver and the rest to concentrator PV cells, and (3) nominally 1 sun PV cells performing double duty as both a direct converter and as a spectrum-splitting reflector that concentrates sub-bandgap photons onto a thermal receiver. The two figures of merit appraised are: (a) the solar-to-electricity conversion efficiency, and (b) the share between thermal and PV electricity production more...

International audience; Hybridizing solar energy with a coal-fired power plant has proven to be an efficient way of reducing coal consumption and discharged pollutants. In this study, solar energy was employed to heat boiler inlet feedwater through a solar high-pressure feedwater heater H0 to increase its temperature. Solar-aided feedwater heating of a N600-24.2/566/566 supercritical coal-fired power plant is discussed as a case study. An all-condition mechanism model (ACMM) of SACFPP was proposed and simulated by MATLAB/Simulink. The maximum solar energy input for the safe operation of a boiler was determined as 66,544 kW based on ACMM simulation results. Moreover, the boiler efficiency and solar-to-electricity efficiency plummeted as solar energy input increased. The solar-to-electricity efficiency decreased from 23.33% to 20.33% when the solar energy input increased from 16,636 kW to 66,544 kW (in 100%THA). The solar-to-electricity efficiency decreased from 16.76% to 13.29% when the solar energy input increased from 16,636 kW to 66,544 kW (in 35%BMCR). A high unit load corresponds to high solar-to-electricity efficiency. SACFPP had a high coal saving rate when it operated in a lower load condition. more...

This paper presents an original study on the design and optimization of baffled fluid distributor for the realization of optimal fluid flow distribution in a tubular solar receiver. The basic idea is to install a perforated baffle in the inlet fluid distributor and to optimize the configuration of orifices on the baffle so as to approach the target flow distribution among downstream parallel tubes. A pressurized-air solar receiver comprising of 45 parallel tubes is used for study, with copper or Inconel 600 used as the filling material. Results show that the final fluid flow distributions realized by the geometrically optimized baffles are in good agreement with the target curves. The peak temperature of the receiver wall can be minimized accordingly with moderate increase in total pressure drop of the receiver system. It is shown that the insertion of a geometrically optimized baffle is generally a practical solution with various features: capable of realizing non-uniform target distribution; small pressure drop increase; compact geometry; flexible and adaptive; easy fabrication with a reasonable cost, etc. more...

International audience; Solar thermal electricity generated by concentrated solar power (CSP) plants is increasingly implemented. CSP plants can supply electricity on a fully matched supply-demand basis if equipped with a thermal energy storage. To increase the efficiency and reduce both capital and operating costs, a next generation of CSP concepts is required. Particle suspensions can be applied to meet these targets and can be used throughout the CSP conversion process, as high temperature heat transfer medium in the receiver, for heat storage, and in the power block of the plant. This work presents the novel concept of using particle suspensions as heat carriers, mostly further to initial testing at the CNRS 1 MW solar furnace of Odeillo Font-Romeu (F). Values of the heat transfer coefficient up to 1100 W/m²K (bare tubes) and 2200 W/m²K (finned tubes) were obtained for operation at low superficial gas velocities of 0.04–0.19 m/s, thus limiting heat losses by the exhaust air. Despite additional costs for particle handling and for an appropriate boiler, the required overall investment and operating costs are expected to be significantly lower than for common equivalent molten salt CSPs, leading to a reduction in Levelized Cost of Electricity (LCOE) from approximately 125 €/MWh to below 100 €/MWh. High-efficiency solar power towers using particle suspensions as heat carrier in the receiver and in the thermal energy storage | Request PDF. Available from: https://www.researchgate.net/publication/316343932_High-efficiency_solar_power_towers_using_particle_suspensions_as_heat_carrier_in_the_receiver_and_in_the_thermal_energy_storage [accessed Oct 04 2018]. more...

International audience; Compatibility tests were conducted between synthetic oil and potential solid filler materials in a thermocline tank built at the PROMES-CNRS laboratory. These experiments consisted of placing the oil and the solid materials together in small crucibles (2.3 L) and performing compatibility tests at 300 °C and accelerated ageing tests at 330 °C for 500 h. The mutual influence of the oil and the solid materials was assessed through different measurements: X-ray diffraction and scanning electron microscopy. The influence of the solid on the oil was assessed by measuring thermo-physical and chemical properties. While no significant degradation of the oil is measured at 300 °C, at 330 °C, an increase in the oil degradation rate was observed. Its density, thermal conductivity and specific heat did not change, but its viscosity showed a significant increase. However, after accelerated ageing tests at 330 °C, the oil still shows good heat transfer performance. The solid crystalline structure did not change during the test but some of the samples seemed to absorb oil. more...

International audience; Compatibility tests were conducted between synthetic oil and potential solid filler materials in a thermocline tank built at the PROMES-CNRS laboratory. These experiments consisted of placing the oil and the solid materials together in small crucibles (2.3 L) and performing compatibility tests at 300 °C and accelerated ageing tests at 330 °C for 500 h. The mutual influence of the oil and the solid materials was assessed through different measurements: X-ray diffraction and scanning electron microscopy. The influence of the solid on the oil was assessed by measuring thermo-physical and chemical properties. While no significant degradation of the oil is measured at 300 °C, at 330 °C, an increase in the oil degradation rate was observed. Its density, thermal conductivity and specific heat did not change, but its viscosity showed a significant increase. However, after accelerated ageing tests at 330 °C, the oil still shows good heat transfer performance. The solid crystalline structure did not change during the test but some of the samples seemed to absorb oil. more...

International audience; This paper presents an analytical model of a multistage fluidised bed heat exchanger for particle-based solar power plants. This model was developed as an applicable design tool for similar devices. It enables a parametric analysis of the heat exchanger performance to be conducted as a function of the operating specifications of the plant power block, the heat exchanger geometry and the fluidised bed properties, among other parameters. A 50MWe solar plant with a two-stage Rankine cycle operating at 535°C was used to analyse the heat exchanger design. The results indicate that for the proposed application, improvements in the thermal behaviour mostly depend on the addition of preheating and superheating stages. The most efficient configuration includes seven fluid bed stages with a thermal efficiency of 99.3% and a global heat exchange efficiency of 49.7%. With such a configuration, a maximum solid temperature difference of 387°C may be achieved between the heat exchanger entrance and its exit for particle inlet temperature of 650°C, thus enabling the best utilization of the thermal energy stored in the solid particles. more...

International audience; Solar pyrolysis of biomass is a smart way to upgrade biomass and, thus, store intermittent solar energy as solar fuels (gas, bio-char and bio-oil). Distribution and energy content of gas, char and oil depend on experimental conditions. In order to determine these characteristics, experiments have been performed at temperatures of 600, 900, 1200 and 2000 degrees C, heating rate of 50 degrees C/s and argon flow rate of 6 NL/min. The gas product was analyzed by micro-GC. The char product was characterized by CHNS, whereas the oil product was subjected to CHNS, Karl-Fischer titration and GC-MS analysis. The LHVs (lower heating values) for gas, char and oil were determined from empirical equations. The gas product yield and LHV significantly increase with temperature, which is mainly due to more H-2 and CO formation by the enhanced secondary tar reactions. The char and oil characteristics highly depend on the temperature. Their high energy contents show that the obtained char and oil can be utilized as valuable solid and liquid fuels. The biomass energy upgrading due to solar processing is discussed. At optimum temperature 900 degrees C, it ranges from 38% to 53% accounting for the uncertainty of bio-oil water content. more...

International audience; This work aims to predict the general performances of a pilot parabolic trough collector during transient periods. To do so, a one-dimensional thermal model has been developed. It has been validated with experimental results from two different experimental setups, in steady-state conditions, with a transmitted power maximum error of 3.4%. Since the model only predicts the collector's thermal behavior, the parabolic trough collector has been first optically qualified. Then, optical efficiencies were used as input for the model. Experimental results were obtained in steady-state conditions and compared to the model. Then, experimental and numerical results were compared during two period of time with varying inlet conditions (i.e. dynamic condition tests): the first one with stable conditions, and the other one with harsh conditions. The developed model showed a good capability of predicting the thermal behavior of the parabolic trough collector with unstable environment (DNI, mass flow, inlet temperature), with a 9.6% relative standard error in the worst case. As a conclusion, while previous studies only focused on steady-state conditions, it has been showed that this kind of model can be used to precisely predict the dynamic behavior of large power plants. more...

The interest for hybrid solar gas-turbine systems (HSGT) in solar power tower plant technologies is growing. This is due to the high conversion efficiency and to the low water consumption that are achieved when a combined cycle is implemented. This paper presents a simulation tool which is dedicated to the performance analysis a hybrid solar gas turbine solar plant featuring a thermal energy storage (TES) unit. This study is conducted within the framework of the French PEGASE project (Production of Electricity from Gas and Solar Energy) which aims at setting up and testing at THEMIS site (France) a demonstration plant based on HSGT technology. The TES unit called packed bed consists in a cylindrical tank filled with spherical rocks. For a given gas-turbine, the designs of the storage and the heliostat field are carried out. Depending on the operating strategy, this work establishes that the integration of a TES unit offers a higher and more stable electrical generation. The TES unit increases, in some case drastically, the daily average solar share of the power plant. Finally it leads to a higher electrical production per unit of fuel consumed. more...

International audience; The spectral selectivity results in increasing the absorption of the incident solar flux and decreasing the radiative losses due to emission of heated walls. This study investigates the influence of the spectral selectivity on power generation efficiency of a central receiver solar concentrating system (solar power tower). A parametric study is conducted to quantify the potential efficiency gain that may result from spectral selectivity with the solar absorption, the cutoff wavelength, the infrared emissivity, the wall temperature and the receiver geometry (plane or cavity) as parameters. The model used to compute the receiver efficiency is based on a Monte Carlo ray-tracing algorithm for the radiative losses, the Clausing model for the convective losses and the Chambadal–Novikov–Curzon–Ahlborn approach for thermodynamic efficiency. The two tested receiver geometries are a plane receiver and a cubic cavity having the same cross section area of the aperture. State-of-the-art (metal alloys) and promising ceramic materials were studied such as SiC, ZrB2, ZrC and TaC. In the latter cases the native measured optical properties of carbides are considered. Finally, an intrinsic spectrally selective material such as TaC demonstrates promising results with global facility efficiency close to the maximum when solar absorptivity is enhanced by microstructuration, for example. As a conclusion, it is shown that spectral selectivity may result in an increase of overall solar power plant efficiency by about 6% (or 28% in relative value) with current state-of-the-art. more...

International audience; PurposeFluidized bed combustion is currently intensively developed throughout the world to produce energy from several types of solid fuels, while significantly reducing pollutant emissions with respect to conventional combustion units. Accurate models must be formulated at both bed and particle levels to operate efficiently such units, since local phenomena such as particle temperature and combustion rate are crucial aspects for process improvement and control. In this sense, this article proposes a classification of local scale models to represent the evolution of char heterogeneous combustion of any carbonaceous particles.MethodsExisting models are described and classified based on the characteristics of the governing equations, the thermal behavior of the gas and solid phases and the description of both the burning particle and the surrounding gas, under a heterogeneous or pseudo-continuous assumption. Criteria for choosing one model instead of others are also considered, depending on the case. The so-called Intrinsic Reactivity Models are described in detail for evaluating the pertinence of their simulated results. The use of CFD to build a simulation scheme of the solid combustion process at local scale is also presented and discussed.ResultsA complete description of the solid fuel burning process is given, along with useful information concerning the evolution of different variables, such as particle internal temperature that governs the reaction rate and gas composition.ConclusionsThis comparative analysis gives a strong basis to select the appropriate modeling approach. Finally, recommendations are proposed for model application and future development. more...

International audience; A novel application of powders relies on their use as heat transfer medium for heat capture, conveying and storage. The use of powders as heat transfer fluid in concentrated solar systems is discussed with respect to current technologies. The specific application reported upon is the use of powder loops in Solar Power Tower plants. In the proposed receiver technology, SiC powder is conveyed as a dense particle suspension through a multi-tube solar receiver in a bubbling fluidization mode, the upwards flow being established by pressurizing the powder feed. Tests were conducted with a single-tube receiver unit at the 1 MW solar furnace of CNRS (Odeillo Font-Romeu, F). The measured wall-to-suspension heat transfer coefficient is a function of operating temperature, applied air velocity and imposed solid circulation flux: values increased with increasing solids flux from ∼430 to 1120 W/m2 K. Empirical approaches and a heat transfer model were applied to compare experimental and predicted values of the heat transfer coefficient, with a fair agreement obtained. The research moreover provides initial data concerning the overall economy of the system. The high temperature of the circulating powder leads to an increased power cycle efficiency, an increased storage density, reduced thermal power requirements, reduced heliostat field size, reduced parasitic power consumption and increased plant capacity factor. more...

High temperature solar receiver is a core component of solar thermal power plants. However, non-uniform solar irradiation on the receiver walls and flow maldistribution of heat transfer fluid inside the tubes may cause the excessive peak temperature, consequently leading to the reduced lifetime. This paper presents an original CFD (computational fluid dynamics)-based evolutionary algorithm to determine the optimal fluid distribution in a tubular solar receiver for the minimization of its peak temperature. A pressurized-air solar receiver comprising of 45 parallel tubes subjected to a Gaussian-shape net heat flux absorbed by the receiver is used for study. Two optimality criteria are used for the algorithm: identical outlet fluid temperatures and identical temperatures on the centerline of the heated surface. The influences of different filling materials and thermal contact resistances on the optimal fluid distribution and on the peak temperature reduction are also evaluated and discussed. Results show that the fluid distribution optimization using the algorithm could minimize the peak temperature of the receiver under the optimality criterion of identical temperatures on the centerline. Different shapes of optimal fluid distribution are determined for various filling materials. Cheap material with low thermal conductivity can also meet the peak temperature threshold through optimizing the fluid distribution. more...

International audience; Char samples were produced from pyrolysis in a lab-scale solar reactor. The pyrolysis of beech wood was carried out at temperatures ranging from 600 to 2000 °C, with heating rates from 5 to 450 °C/s. CHNS, scanning electron microscopy analysis, X-ray diffractometry, Brunauer–Emmett–Teller adsorption were employed to investigate the effect of temperature and heating rate on char composition and structure. The results indicated that char structure was more and more ordered with temperature increase and heating rate decrease (higher than 50 °C/s). The surface area and pore volume firstly increased with temperature and reached maximum at 1200 °C then reduced significantly at 2000 °C. Besides, they firstly increased with heating rate and then decreased slightly at heating rate of 450 °C/s when final temperature was no lower than 1200 °C. Char reactivity measured by TGA analysis was found to correlate with the evolution of char surface area and pore volume with temperature and heating rate. more...

International audience; The design of solar concentrating systems and receivers requires the spatial distribution of the solar flux on the receiver. This article presents an integral formulation of the optical model for the multiple reflections involved in solar concentrating facilities, which is solved by a Monte Carlo ray-tracing (MCRT) algorithm that handles complex geometries. An experimental validation of this model is obtained with published results for a dish configuration. The convergence of the proposed algorithm is studied and found faster than collision-based algorithms. In addition, an example of the use of the sensitivity of the power on a target to the mirror rms-slope is given by treating an inverse-problem consisting in finding the equivalent rms-slope of mirrors that best match the flux map measurements. more...

International audience; This paper presents a CFD-based, heuristic evolutionary algorithm for shape design and optimization of baffled fluid distributor. In this algorithm, the baffle surface is firstly divided into numerous identical control areas (volumes), each control area having an orifice in the middle. Under the constraint of constant global porosity of the baffle, the algorithm adjusts the size distribution of orifices so as to approach identical mass flowrate passing through every control area. An automatic program is processed iteratively so that the baffle configuration evolves toward the optimized shape, providing a uniform flow distribution among parallel outlet channels.To illustrate the principles and procedure of this algorithm, a 2D example of baffled fluid distributor is introduced and tested. Numerical results show that this algorithm can successfully reach uniform flow distribution with a small pressure drop increase. Sensitivity analysis also shows that this algorithm is robust, effective, general and flexible compared to traditional arbitrary or empirical propositions. A parametric study of influencing design parameters on the performance of the algorithm is carried out in order to provide some design guidelines. Finally, the theoretical basis and essential steps for the extension of the current algorithm to tackle 3D problem are established. The easy implementation of this simply solution for the general fluid maldistribution problem demonstrates its promising application in real engineering field. more...

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International audience; Managing the optical properties of a cavity solar receiver to create spectral and directional selectivities is a solution to improve receiver efficiencies. A reduction in the incident solar power lost by reflection and by emission in a solar receiver allows the absorption of the solar flux to be maximized. This report investigates the influence of the cavity walls directional reflectivity on the thermal radiative efficiency of a cubic cavity solar receiver. A Monte Carlo ray-tracing method is used to calculate the power lost by reflections and by emission with respect to the incident radiation angular distribution and the bidirectional reflectance distribution function of the cavity walls. To study the influence of the directional dependency of the incident flux on the radiative efficiency, four patterns are considered: collimated, diffuse, focused, and Themis incidences. The directional-hemispherical reflectivity for the bottom wall (face to aperture) and lateral walls are distinguished. For diffuse walls, the absorption efficiency is primarily affected by the lateral walls reflectivity because of the back reflection losses. For specular walls, the driving parameter is the bottom wall reflectivity. In addition, the radiative efficiency with thermal emission was studied for the Themis configuration and a slightly weakest dependency of the efficiency on the lateral walls reflectivity was found. more...

The influence of radiative heat transfer in a CO2 pipe flow is numerically investigated at different pressures.Coupled heat and mass transfer, including radiation transport, are modeled. The physical models and the high temperature and high pressure radiative properties method of computation are presented. Simulations are conducted for pure CO2 flows in a high temperature pipe at 1100 K (with radius 2 cm) with a fixed velocity (1 m·s-1) and for di erent operating pressures, 0:1, 1, 5 and 20 MPa (supercritical CO2). The coupling between the temperature and velocity fields is discussed and it is found that the in uence of radiation absorption is important at low pressure and as the operating pressure increases above 5 MPa the influence of radiation becomes weaker due to an increase of CO2 optical thickness. more...

International audience; This paper demonstrates the capacity of dense suspensions of solid particles to transfer concentrated solar power from a tubular receiver to an energy conversion process by acting as a heat transfer fluid. Contrary to a circulating fluidized bed, the dense suspension of particles' flows operates at low gas velocity and large solid fraction. A single-tube solar receiver was tested with 64 µm mean diameter silicon carbide particles for solar flux densities in the range 200-250 kW/m2, resulting in a solid particle temperature increase ranging between 50 °C and 150 °C. The mean wall-to-suspension heat transfer coefficient was calculated from experimental data. It is very sensitive to the particle volume fraction of the suspension, which was varied from 26 to 35%, and to the mean particle velocity. Heat transfer coefficients ranging from 140 W/m2 K to 500 W/m2 K have been obtained, thus corresponding to a 400 W/m2 K mean value for standard operating conditions (high solid fraction) at low temperature. A higher heat transfer coefficient may be expected at high temperatures because the wall-to-suspension heat transfer coefficient increases drastically with temperature. The suspension has a heat capacity similar to a liquid heat transfer fluid, with no temperature limitation but the working temperature limit of the receiver tube. Suspension temperatures of up to 750 °C are expected for metallic tubes, thus opening new opportunities for high efficiency thermodynamic cycles such as supercritical steam and supercritical carbon dioxide. more...

International audience; Life time of components is one of the technological bottlenecks in the development of solar tower power plant technology. The receiver, which is subjected to high and variable concentrated solar flux density is particularly affected: High, variable and non-homogeneous solar flux on the solar receiver walls results in strong stresses because of high temperatures, thermal shocks and temperature gradient that contribute to the reduction of the life time of this key component. This work aims to present an open loop approach to control the flux density distribution delivered on a flat plate receiver for a solar power tower. Various distributions of aiming points on the aperture of the receiver are considered. The flux density distribution on the aperture is simulated by a computer code. A specific neighborhood is defined for the TABU optimization meta-heuristic according to the size of the image of each individual heliostat. This modified algorithm is implemented to select the best aiming point for each heliostat. This approach has been validated using the example of THEMIS solar power tower in Targasonne, France. more...

Issu de : WESC 2010 - World Energy System Conference, Targoviste, Romania, July 1-3, 2010; International audience; Biomass represents a renewable source for transport fuels when processed by gasification, followed by catalytic conversion of the syngas to liquids. The efficiency of biomass gasification can be improved by supplying process heat from concentrated solar systems, which can attain the required temperature of 900 degrees C. Various chemical routes and contacting configurations are reviewed. The challenges related to biomass-based processes are discussed. Heat and material balances are then deduced. The area of land required for growing biomass can be reduced using the application of thermal solar to one half of that needed for a standard gasification system. If hydrogen is generated by solar means in order to reduce carbon dioxide emissions to zero, the figure becomes one third. Examples of the land requirements for three different biomass materials are presented. more...

Eurotherm Conference 95 - Computational Thermal Radiation in Participating Media IV, Nancy, FRANCE, APR 18-20, 2012; International audience; In this communication, the software SOLFAST is presented. It is a simulation tool based on the Monte-Carlo method and accelerated Ray-Tracing techniques to evaluate efficiently the energy flux in concentrated solar installations. more...

Solar methane dissociation appears as a possible route toward a hydrogen-based economy. The competitiveness of the process strongly depends on the carbon black properties and economic value. At CNRS-PROMES, a 50 kW tubular solar reactor was developed to produce carbon black and hydrogen from methane. The reaction was carried out in a graphite receiver crossed by seven graphite tubes heated up by concentrated solar radiations at the 1 MW CNRS solar furnace. The temperatures ranged between 1608 K and 1928 K and the methane flow-rates varied from 10.5 to 21 NL/min. Total methane dissociation was reached with hydrogen yield higher than 80% and the carbon yield was drastically affected by the acetylene by-product. The carbon samples were analysed in detail and their properties were compared to a commercial conductive grade carbon black. Transmission electron microscopy showed primary particles of 10–70 nm diameter. The crystallinity of the samples was characterised by Raman spectroscopy. It was also possible to correlate the specific surface area with the reaction temperature and with the concentrations of residual methane and of the acetylene by-product. The resistivity and the structure of the agglomerates were determined by simultaneous measurements of the conductivity and the density under compression. more...

International audience; The conception and optimization of high temperature plasma process for carbon blacks synthesis from hydrocarbons by pyrolysis is the objective of a European industrial association. To control the process by correlating the operating conditions and the product properties, it is necessary to examine the internal behavior of the reactor and more particularly the temperature field in the plasma flow and along the reactor wall. Typical emission spectroscopy and optical pyrometry results are presented in this paper, including: evolution and distribution of plasma temperature inside the reactor obtained by analysis of C2 Swan Band and of wall temperature. Experimental temperature distribution is then compared with numerical simulation data including electromagnetic forces, fluid dynamics and heat transfer. more...

International audience; A pilot-scale solar reactor was designed and operated at the 1 MW solar furnace of CNRS for H 2 and carbon black production from methane splitting. This constitutes the final objective of the SOLHYCARB EC project. The reaction of CH 4 dissociation produces H 2 and carbon nanoparticles without CO 2 emissions and with a solar upgrade of 8% of the high heating value of the products. The reactor was composed of 7 tubular reaction zones and of a graphite cavity-type solar receiver behaving as a black-body cavity. Temperature measurements around the cavity showed a homogeneous temperature distribution. The influence of temperature (1608Ke1928K) and residence time (37e71 ms) on methane conversion, hydrogen yield, and carbon yield was especially stressed. For 900 g/h of CH 4 injected (50% molar, the rest being argon) at 1800K, this reactor produced 200 g/h H 2 (88% H 2 yield), 330 g/h CB (49% C yield) and 340 g/h C 2 H 2 with a thermal efficiency of 15%. C 2 H 2 was the most important by-product and its amount decreased by increasing the residence time. A 2D thermal model of the reactor was developed. It showed that the design of the reactor front face could be drastically improved to lower thermal losses. The optimised design could reach 77% of the ideal black-body absorption efficiency (86% at 1800K), i.e. 66%. more...

This study focuses on the use of cerium-based mixed oxides for hydrogen production by solar-driven thermochemical two-step water-splitting. Mixed cerium oxides are proposed in order to decrease the reduction temperature of ceria and to avoid material sublimation occurring above 2,000 °C during the high-temperature solar step. Ceria-based nanopowders were synthesized by soft chemistry methods including the modified Pechini method. The influence of the synthesis method, the type of cationic element mixed with cerium, and the content of this added element was investigated by comparing the reduction temperatures of the derived materials. The synthesized powders were characterized by X-ray diffraction, thermogravimetric analysis, SEM, and Raman spectroscopy. Results showed that the synthesized pure cerium oxide is more reactive toward reduction than a commercial powder. Among the different elements added to ceria that were screened, the addition of zirconium significantly improved the reduction of ceria at temperatures below 1,500 °C. Increasing zirconium content further favored cerium reduction yield up to 70%. Water-splitting tests were performed to demonstrate the reactivity of the developed materials for H2 production. The amount of H2 evolved was enhanced with a temperature increase, the maximum H2 production from Ce0.75Zr0.25O2−δ was 0.24 mmol/g at 1,045 °C, and the powder reactivity upon cycling was demonstrated via thermogravimetry through two successive reduction–hydrolysis reactions. more...

A high-temperature lab-scale solar reactor prototype was designed, constructed and operated, allowing continuous ZnO thermal dissociation under controlled atmosphere at reduced pressure. It is based on a cavity-type rotating receiver absorbing solar radiation and composed of standard refractory materials. The reactant oxide powder is injected continuously inside the cavity and the produced particles (Zn) are recovered in a downstream ceramic filter. Dilution/quenching of the product gases with a neutral gas yields Zn nanoparticles by condensation. The solar thermal dissociation of ZnO was experimentally achieved, the reaction yields were quantified, and a first concept of solar reactor was qualified. The maximum yield of particles recovery in the filter was 21% and the dissociation yield was up to 87% (Zn weight content in the final powder) for a 5 NL/min neutral gas flow-rate (typical dilution ratio of 300). more...

International audience; Metal emissions from waste incineration plants become a great environmental concern because of their toxicity for both human health and environment. Metals are not destroyed by high-temperature thermal treatment and some vaporized metals may be emitted. It is thus essential to understand the release mechanism of metals during high-temperature waste treatment, in order to improve the understanding of their behavior and the control of their emission. The objective of this study is to identify the kinetic law for metal release from realistic artificial waste. The vaporization of three metals of most concern (Cd, Pb and Zn) during municipal waste incineration was studied. The vaporization rate at the particle level was determined from the experimental concentration profile in the outlet gas of a fluidized bed reactor, by using the inverse method that was previously developed and validated by our team. As a first step, the kinetic parameters were determined thanks to experiments carried out at several temperatures. Specific laws, for each studied metal, were thus obtained as a function of temperature. Nevertheless, it is very useful to identify all the experimental kinetic curves (all studied metals at all temperatures) to one single mathematical law. Therefore, a general kinetic law, expressing the solid matrix influence on the metal vaporization dynamic, was determined as a second step. It permits to predict the vaporization characteristic time and the time course of the metal concentration in the waste. more...

International audience; Methane cracking is a possible route for the co-production of hydrogen and carbon black avoiding CO 2 emission. An indirect heating solar reactor prototype, allowing thermal dissociation of methane without catalyst, is tested at high temperatures. It is composed of a cubic graphite cavity receiver (20 cm side) equipped with four independent vertical tubular reaction zones. The concentrated solar radiation (2-4 MW/m²) is absorbed through an hemispherical quartz window by a 9 cm-diameter aperture. A set of experimental results on solar methane dissociation is presented for the temperature range 1823-2073 K. The effects of temperature, residence time from 10 ms and beyond 100 ms, gas flow-rates up to 12 NL/min (2x10-4 Nm 3 s-1) of methane, and methane concentration up to 100% are investigated. Moreover, an experimental run was also carried out with natural gas instead of pure methane. Typical hydrogen and carbon mass balances show that methane is efficiently converted into hydrogen whereas it is not as well converted into solid carbon due to the production of acetylene. A two-step mechanism for methane dissociation is presented in order to define accurately the reactor efficiencies (thermal efficiency and solar-to-hydrogen thermo-chemical conversion). more...

International audience; The effects of the non-gray thermal radiation on the heating of a methane/argon laminar flow at high temperature are investigated. The preheating zone of a tubular reactor is studied, where the thermal decomposition of methane does not occur. The laminar flow is simulated with the commercial Fluent code in an axisymmetric geometry. The discrete ordinates method is applied to the numerical simulation of radiative heat transfer. The non-gray gas radiative model used is the absorption distribution function (ADF) using high temperature methane radiative properties which were recently published. Several thermal entrance regions of tubular reactors are compared and the influence of methane participation in radiative heat transfer is studied. The results show that the temperature field is significantly influenced by radiation due to methane absorption. Furthermore, the average flow temperature increases when the wall temperature, the tube diameter or the methane mole fraction increases. Due to intense absorption bands of methane, it is shown that the influence of methane non-gray thermal radiation should be included in simulations of such reactors more...

The thermal pyrolysis of natural gas as a clean hydrogen production route is examined. The concept of a double-walled reactor tube is proposed and implemented. Preliminary experiments using an external plasma heating source are carried out to validate this concept. The results point out the efficient CH4 dissociation above 1850 K (CH4 conversion over 90%) and the key influence of the gas residence time. Simulations are performed to predict the conversion rate of CH4 at the reactor outlet, and are consistent with experimental tendencies. A solar reactor prototype featuring four independent double-walled tubes is then developed. The heat in high temperature process required for the endothermic reaction of natural gas pyrolysis is supplied by concentrated solar energy. The tubes are heated uniformly by radiation using the blackbody effect of a cavity-receiver absorbing the concentrated solar irradiation through a quartz window. The gas composition at the reactor outlet, the chemical conversion of CH4, and the yield to H2 are determined with respect to reaction temperature, inlet gas flow-rates, and feed gas composition. The longer the gas residence time, the higher the CH4 conversion and H2 yield, whereas the lower the amount of acetylene. A CH4 conversion of 99% and H2 yield of about 85% are measured at 1880 K with 30% CH4 in the feed gas (6 L/min injected and residence time of 18 ms). A temperature increase from 1870 K to 1970 K does not improve the H2 yield. more...

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International audience; Solar methane cracking is a promising pathway to produce hydrogen and carbon black with the bonus of zero CO 2 emission. A kinetic simulation of the methane decomposition in a tubular solar chemical reactor prototype is presented. This reactor is composed of four independent tubular reaction zones inserted in a graphite cavity receiver. Chemical reaction modelling is carried out thanks to the Dsmoke software, using a detailed kinetic scheme for the wide range modelling of alkane transformation. First, a kinetic analysis of the chemical system is presented to determine the sequence of methane cracking and a sensitivity analysis of the results on temperature (in the range 1500-2300 K) and on natural gas composition is performed. Then, a kinetic simulation of the solar reactor is proposed and implemented, in which each tubular reaction zone is modelled by three plug-flow reactors in series representing the pre-heating, isothermal, and cooling zones of the reactor. It predicts the evolution of gas species concentrations as a function of residence time. Comparisons with experimental results between 1670 K and 1770 K show good agreement for CH 4 conversion, and CH 4 and H 2 off-gas compositions. more...