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Characterization and electrochemical evaluation of novel metal-free electrocatalysts obtained by solar pyrolysis is reported. Carbon-based electrocatalysts were prepared from agave bagasse waste, using a sustainable process based on concentrated solar energy as heat source. Agave was processed in a spherical borosilicate glass solar reactor using a heating rate of 30 °C min−1 to a target of 500, 700, or 900 °C, and maintaining temperature for 1 h under inert atmosphere. The structure and composition of the prepared electrocatalysts were influenced by pyrolysis temperature. In addition, electrocatalytic activity towards the oxygen reduction reaction in 0.1 M KOH solution was explored. The electrocatalyst obtained at 500 °C showed the highest activity among all pyrolyzed samples due to its moderate surface area, but mostly due to its higher oxygen content. The metal-free electrocatalysts reported in this work are promising eco-friendly alternative as cathode materials for anion-exchange membrane fuel cells. This study provides a sustainable approach to use agricultural biomass waste to produce valuable materials for electrochemical energy devices.

Este estudio presenta el diseno, el modelado y el comportamiento de un destilador solar utilizando Computational Fluid Dynamics (CFD). Los parametros medidos son: 1) el comportamiento de la temperatura interna, 2) la cantidad de cobertura y volumen generada despues del proceso de destilacion solar, 3) los coeficientes de transferencia obtenidos durante los meses de operacion. Los principales resultados indican que el mes de mayo tiene la temperatura promedio mas alta del agua en el interior con 44oC y la mayor produccion de agua destilada con un promedio de 1000 ml por dia. El mismo mes tuvo un promedio de radiacion global incidente por dia de 5,8 kWh / m2, que es el mas alto entre los meses de operacion. Se aplica el modelo de Kumar y Tiwari para estudiar el comportamiento termico del destilador solar. Para determinar la capacidad de beber del agua destilada, se realizaron los analisis de laboratorio y se encontro que el agua cumple con la norma NOM-127-SSA1-1994. Por lo tanto, el agua limpia se obtuvo al mismo tiempo del sistema disenado en condiciones climaticas reales.

Activated carbons are key electrode materials for supercapacitors (SCs) widely used in commercial devices. Here we investigate the preparation of activated solar carbons (ASCs) from pecan nutshell waste for their use as electrode materials for SCs. The ASCs are prepared using a sustainable process where concentrated solar energy is used as heat source for the one-step carbonization/chemical activation using H3PO4. From physical characterization we observed that our prepared ASCs are amorphous carbons, which have well-developed microporous surface of different features related with a pretreatment of pecan nutshell. The ASCs are electrochemically evaluated by cyclic voltammetry in four aqueous electrolytes using a three-electrode system, obtaining capacitances at 5 mVs−1 of 150 and 129 Fg-1 in acidic (0.5 M H2SO4) and environmentally-friendly neutral (1 M CH3COONa) electrolytes, respectively. Finally, we assembled asymmetric carbon-carbon SC cells showing excellent performance compared to a commercial activated carbon, with capacitances around 30 Fg-1 at 0.5 A g−1 with only ∼10% of capacitance loss after 5000 cycles, reveling a high stability, and a widest voltage window with the 1 M CH3COONa electrolyte. Our study provides a novel sustainable approach to use agricultural waste biomass to produce useful electrode materials for energy storage devices using concentrated solar energy.

Carbon materials can be produced by different methods, one of them is solar pyrolysis, which is attaining renewed interest due to simplicity to transform biomass with environmental advantages. However, pyrolysis operation parameters are decisive factors that determine the morphological and chemical properties of carbon. This work aims to study the influence of temperature and heating rate of solar pyrolysis in the properties of carbon. Pyrolysis experiments were carried out in the 25 kW IER-UNAM solar furnace. Agave biomass was processed in a glass spherical solar reactor. Temperatures between 500 and 900 °C were utilized, as well as heating ramps between 4 and 30 °C/min. Samples were analyzed with several techniques. The results of the experiments indicate that solar pyrolysis temperature and heating rate scarcely impact the carbon composition, however the structure, surface area and electrochemical response are highly affected by both. The control of the operation conditions allows to produce carbon materials that exhibit promising properties to be used as catalyst supports for polymer electrolyte membrane fuel cells and electrolyzer cells applications. Therefore the use of concentrated solar energy represents an attractive method to obtain these valuable materials.

Reticulate porous ceramic reactors use foam-type absorbers in their operation which must fulfill two essential functions: favoring the volumetric effect and increasing the mass and heat transfer by acting as a support for the reactive materials. Heating these absorbers with highly inhomogeneous concentrate irradiation induces high thermal gradients that affect their thermal performance. Owing to the critical function of these component in the reactor, it is necessary to define a selection criterion for the foam-type absorbers. In this work, we performed an experimental and numerical thermal analysis of three partially stabilized zirconia (PSZ) foam-type absorbers with pore density of 10, 20, and 30 PPI (pores per inch) used as a volumetric absorber. A numerical model and an analytical approximation were developed to reproduce experimental results, and calculate the thermal conductivity, as well as volumetric heat transfer coefficient. The results show that an increase in pore density leads to an increase in the temperature difference between the irradiated face and the rear face of the absorber, this occurs because when pore density increases the concentrated energy no longer penetrates in the deepest space of the absorber and energy is absorbed in areas close to the surface, therefore, temperature gradients are created within the porous medium. The opposite effect occurs when the airflow rate increases, the temperature gradient between the irradiated face and the rear face is reduced. This behavior is more noticeable at low pore densities, but at high pore densities, the effect is less relevant because the internal structure of porous absorbers with high pore density is more complex, which offers obstructions or physical barriers to airflow and thermal barriers to heat transfer. When the steady state is reached, the temperature difference between the two faces of the absorber remains constant if the concentrate irradiation changes slightly, even changing the airflow rate. The results obtained in this work allow us to establish a selection criterion for porous absorbers that operate within solar reactors, this criterion is based on knowledge of the physical properties of the porous absorber, the environment, the working conditions, and the results expected.

High-crystalline tungsten trioxides (WO3) have been synthesized by an environmentally friendly method using concentrated solar energy. The obtained tungsten trioxides (WO3) at three different temperatures and two oxygen mole fractions used for the highest synthesis temperature were characterized by XRD, SEM, and XPS. Higher crystallinity and concentration of W5+ was observed in tungsten trioxides as the synthesis temperature increased. Nevertheless, despite of the different synthetic conditions used, a mixture of two different crystalline structures was observed in all solar-prepared tungsten trioxides: monoclinic and triclinic. Comparing oxides obtained at 1000 °C, higher concentration of W5+ and more defects were found when using lower oxygen molar fraction (WO3-1000-2). Their electrochemical performance was evaluated using cyclic voltammetry (CV) in a conventional three-electrode cell in the following three aqueous electrolytes: acidic, alkaline, and neutral media. In the acidic medium, all the tungsten trioxides showed a capacitive behavior, which was enhanced for oxides obtained at 1000 °C due to a mixed valence of W. On the other hand, in the alkaline medium, a catalytic behavior was detected with higher activity towards hydrogen evolution reaction for the oxide with more defects, higher crystallinity, and monoclinic phase, obtained at 1000 °C and a lower oxygen molar fraction in the synthesis. Finally, in the neutral medium, the oxides synthesized at 1000 °C presented a capacitive behavior whereas the oxides prepared at the lowest temperatures (600 and 800 °C) presented electrochemical processes related to a catalytic behavior for water reduction, which must correspond to their minor concentration of defects, as confirmed by XPS.

The numerical evaluation of the radiative heat transfer in a multichanneled solar reactor coated with ZnFe 2 O 4 thin-film is performed by using a channel-level simulation. A ray-tracing simulation of a 25 kW solar furnace allows obtaining the radiation distribution at each channel aperture. Then a Monte Carlo ray tracing is performed to analyze the radiative heat transfer on the monolith to optimize the channel-level geometry and film thickness for maximum absorptance and more homogeneous temperature distributions. The model considers the optical properties of ZnFe 2 O 4 films deposited on zirconia substrate, obtained through the characteristic matrix method. This approach allows accounting for important reactor design parameters and operational conditions, such as ZnFe 2 O 4 layer thickness, incoming radiation profile, diameter and length of pores and position of the monolith in the focal zone of the solar furnace.

Hydrogen is a promising energy carrier for transportation, domestic and industrial applications. Nowadays hydrogen is consumed basically by the chemical industry, but in long term its demand is expected to grow significantly due to emerging markets. Hence production of hydrogen with sustainable methods is a relevant issue. This work presents a review of the different CSP- aided thermochemical processes for hydrogen and syngas production. For each process, some relevant solar-tested reactor prototypes are described. In a second part, the developed solar furnaces for investigation of thermochemical process are also discussed. In addition, relevant research on hydrogen or syngas production in solar tower installations is presented. Finally the current challenges of the technology and the process for its future commercialization are also analyzed.

A multiscale model is presented to describe the radiation absorption field in photocatalytic reactors with supported catalyst. The characteristic matrix method is applied at the photocatalyst layer scale, and is embedded within a Monte Carlo ray tracing method, applied at the photoreactor scale. This approach allows to account for important design parameters, such as photocatalyst layer thickness, location of supporting surfaces, and incoming radiation profiles, among others. To resolve the validity of the characteristic matrix method for the description of the optical properties of the catalyst, modeled transmittance and reflectance of the supported films is compared to experimental data. This comparison is carried out for different wavelengths and film thicknesses. Afterwards, the model is applied to a solar reactor with anatase TiO 2 catalyst films supported on multiple surfaces. The reactor consists of a compound parabolic concentrator with a tubular borosilicate glass receiver. Smaller glass tubes coated with the catalyst are located inside this receiver. With the developed model, a study is conducted to analyze the reactor optical performance as a function of two important design variables: TiO 2 film thickness and radius of the absorber tubes. The results of the model indicate directions for the improvement of the current design.

Hydrothermal processes are attractive options for the transformation of mixtures of biomass with large amounts of water, i.e. above 20wt % . At hydrothermal conditions, the special properties of water makes it an attractive reaction medium to obtain several bio-based platform chemicals or fuel gases, such as hydroxymethilfurfural or fufurals, syngas, hydrogen, methane, etc. However, one of the main challenges is that a large amount of energy is required to heat reactants (mixture of water and biomass), which is usually achieved by combustion of a fraction of the bio-oil product. Therefore, to reduce this consumption, their integration with an external renewable energy source, such as concentrated solar radiation has been proposed. This approach has been recently analyzed by several research groups as an option to have sustainable and economically attractive processes. This work provides an overview of the different experimental and theoretical strategies to incorporate concentrated solar technologies into hydrothermal processing of biomass, including the main challenges of such integration for process technical feasibility.

The various lignin isolation methods and pretreatments are continuously developing and thermochemical conversion of lignocellulosic biomass and tuning of the activation parameters are vital to obtaining high energy density materials. In this review, different lignin extraction methods, pretreatments, and influence of the extraction conditions on the yield and properties are presented. The thermochemical conversion of lignin-based biomass and application in supercapacitors and hydrogen storage were investigated. The study revealed that chemical extraction via the organosolv process presents higher purity and partly preserved lignin structure compared to sulfur processes. Different parameters such as the method of extraction, the temperature, pH, resident time, and pressure greatly influences the Kappa value and yield of lignin. The potassium hydroxide (KOH) dosage as an activating agent and the activating temperature is vital to obtaining high surface area and microporosity, which enhances the lignin-based activated carbon performance towards high hydrogen storage and capacitance. Metals doping on activated carbon marginally enhance the hydrogen storage capacity and capacitance, however, reversible desorption of the adsorbed hydrogen requires a higher temperature for hydrogen storage. Besides, high metal doping reduces available surface area, collapses the cage-like structures of fullerenes, and results in lower hydrogen storage capacity of activated carbon. The presence of heteroatoms on activated carbons enhances the performance towards high hydrogen storage and capacitance. Moreover, techno-economic and exergy-based sustainability analysis of the different lignin isolation techniques must be explored to provide valuable insights on energy and associated operational costs.

In the present study, the optimization of a multi-tubular solar thermochemical cavity reactor is carried out. The reactor consists of a cubic cavity made of woven graphite, housing nine 2.54 cm diameter tungsten tubes. A heat transfer model is developed and implemented considering high-temperature radiative transfer at steady state. The temperature distribution on the receiver tubes is determined by using a hybrid Monte Carlo-finite volume approach. The optimization aims at maximizing average tube temperature by varying tube locations. Optimal tube distributions are explored by using a custom-made stochastic, multi-parameter, global optimization algorithm. A considerable increase in average temperature as well as improvement on temperature uniformity is found in the optimized tube arrays. Patterns among the different optimal distributions are found, and general features are discussed.

The use of concentrated solar energy in processing ceramic materials is an attractive route to obtain these materials with low CO2 emissions. In this work, nanostructures of monoclinic zirconia (m-ZrO2) were obtained using concentrated solar energy provided by the IER-UNAM solar furnace as a heat source. In the first stage of the process, a Zr/O/C complex was obtained by sol-gel method at a temperature of 120 °C using zirconium n-propoxide and sorbitol as precursors reagents. This complex was used in a second stage to obtain m-ZrO2 by heating it at a temperature of 1200 °C for one hour in air atmosphere. This last stage was performed in a solar furnace. Samples were analyzed by characterization techniques: FT-IR, TGA/DSC, XRD, TEM, and SEM confirming the formation of nanostructures of zirconia in monoclinic phase.

Combined heat transfer mechanisms in open cavities is a relevant research topic because of its direct application in the design and thermal characterization of solar receivers'prototypes. These devices have several applications, for example, solar furnaces and solar tower systems where a central receiver is used to increase the temperature of a working fluid, which is then employed to produce electricity. So that can reach temperatures above 500 °C because they are designed to trap concentrated solar radiation, and due to high temperatures, the predominant heat transfer phenomena are natural convection and radiative heat transfer. This paper presents a numerical and experimental heat transfer analysis of an open square-cavity solar receiver. The numerical study was carried out considering laminar flow in an open square-cavity with Rayleigh numbers (Ra) of 104, 105 and 106. For the radiative heat transfer, the study considered both transparent and radiatively participating media. The results show difference in both cases and exhibit the importance of taking into account a radiatively participating media. An acceptable comparison was obtained with the experimental results.

This work aims to propose a sustainable green process to obtain bio-derived carbons (BDCs) for utilization in supercapacitors. The process consists in carrying out solar pyrolysis to produce BDCs from abundant lignocellulosic wastes, Agave Angustifolia leaves and pruned tomato plant. Concentrated solar radiation from a high flux solar furnace was utilized to reach sample temperatures between 450 and 1564 °C in a spherical reactor. Before pyrolysis, both wastes were characterized by thermogravimetric analysis to semi-quantify cellulose and hemicellulose as well as ash content. XRD was used to determine the ash composition in both wastes, and the effect of solar pyrolysis temperature on the obtained BDCs. Additional structural properties of BDCs were analyzed by SEM, Raman spectroscopy, and physisorption. Elemental analysis and EDAX were used to determine the chemical composition of wastes, and the effect of this on BDCs. Electrochemical properties of BDCs were analyzed by cyclic voltammetry in half cells, and those showing better performance were also tested in supercapacitor cells. Results show that BDCs from tomato plant waste have higher surface areas, with well-developed microporosity, without needing an additional activation process. This is attributed to self-activation during pyrolysis, produced by the high K and Ca content of the tomato plant pruning. Ragone plots indicate that the assembled supercapacitor cells employing the best BDCs from solar pyrolysis have specific energies and power values similar to a commercial carbon designed for supercapacitors. These results indicate that the proposed green procedure is suitable for obtaining BDCs with properties suitable for supercapacitors.

International audience; Carbon fiber-reinforced carbon (C/C) composites consist in a carbon matrix holding carbon or graphite fibers together, whose physical properties are determined not only by those of their individual components, but also by the layer buildup and the material preparation and processing. The complex structure of C/C composites along with the fiber orientation provide an effective means for tailoring their mechanical, electrical, and thermal properties. In this work, we use the Laser Flash Technique to measure the thermal diffusivity and thermal conductivity of C/C composites made up of laminates of weaved bundles of carbon fibers, forming a regular and repeated orthogonal pattern, embedded in a graphite matrix. Our experimental data show that: i) the cross-plane thermal conductivity remains practically constant around (5.3 ± 0.4) W*m−1 K−1, within the temperature range from 370 K to 1700 K. ii) The thermal diffusivity and thermal conductivity along the cross-plane direction to the fibers axis is about five times smaller than the corresponding ones in the laminates plane. iii) The measured cross-plane thermal conductivity is well described by a theoretical model that considers both the conductive and radiative thermal contributions of the effective thermal conductivity.

Agave angustifolia leaves and tomato pruning biomasses were processed into carbon materials by solar pyrolysis. The influence of temperature and heating rate was studied in the physicochemical properties of the obtained biochars. The characterization techniques include elemental analyses (CHONS), physisorption to determine surface area by BET and DFT models, and capacitance values were determined by cyclic voltammetry as the electrochemical technique. It was found that for both biomasses, temperatures below 900°C are beneficial because more homogeneous and porous structures are obtained. The highest values of capacitance and surface area were obtained for tomato carbons at 450 and 600 °C, respectively.

Silicon carbide (SiC) has been prepared successfully using concentrated solar energy provided by the IER-UNAM solar furnace. This has led to the development of a low CO2 emissions process for the production of this material via carbothermic reduction of a silica/carbon (SiO2/C) nanocomposite, which has shown a more reactive carbon for formation of composite, being more thermally stable. Silica (obtained by a sol–gel process) and sucrose were used as precursors of silicon and carbon, respectively, at a temperature of 700 °C in controlled atmosphere (nitrogen) for the formation of the SiO2/C composite. This composite was used in a second step to obtain SiC at a temperature of 1500 °C, in argon atmosphere. The experimental setup used a Pyrex® glass spherical vessel designed to work with concentrated solar power and controlled atmospheres. The structure and morphology of the solar obtained SiC were analyzed with FTIR, XRD, TGA/DSC, SEM and TEM techniques. Results show that it is feasible to use concentrated solar energy for the synthesis of SiC. The solar SiC obtained is nanostructured and is mainly β-SiC.

In this work a theoretical and experimental study of heat transfer by natural convection and thermal radiation on a solar open cubic cavity-type receiver is presented. The theoretical study consists on solving the laminar natural convection and the surface thermal radiation on a square open cavity at one end. The overall continuity, momentum, and energy equations in primitive variables are solved numerically by using the finite-volume method and the SIMPLEC algorithm. The thermophysical properties of the fluid are considered, for the first case, as temperature dependent in all the governing equations, and for the second case, constant, except for the density at the buoyancy term (Boussinesq approximation), with the purpose of comparing the results of both theoretical models with experimentally obtained results. Numerical calculations are conducted for Rayleigh number ( Ra ) values in the range of 10 4 –10 6 . The temperature difference between the hot wall and the bulk fluid (Δ T ) is varied between 10 and 400 K, and is represented as a dimensionless temperature difference ( φ ) for the purpose of generalization of the trends observed. Experimental results include air temperature measurements inside the receiver. These results are compared with theoretically obtained air temperatures, and the average deviation between both results is around 3.0%, when using the model with variable thermophysical properties, and is around 5.4% when using the Boussinesq approximation.

A 1 kWth cavity-type solar reactor devoted to the thermal reduction of volatile oxides as part of a two-step thermochemical cycle is analyzed numerically. The thermochemical reactor consists of a vertical-axis cavity-type receiver in which the reactant is injected from the bottom in the form of an ascending rod made of a stack of zinc oxide compressed pellets undergoing thermal dissociation. A transient heat transfer model allows the simulation of the thermal behavior under real conditions for the rod of reacting particles exposed to concentrated solar radiation. The developed numerical model couples radiation, conduction and convection heat transfers to the kinetic of the reaction. The incident solar irradiation on the reactant surface is obtained using the Monte-Carlo ray tracing technique applied first to the solar concentrator and second to the reactor cavity. The model is used to predict the temperature profile from the irradiated front surface of the compressed reactant, the evolution of outlet oxygen molar flow-rate during the reduction reaction and the instantaneous thermochemical efficiency, as a function of time. The calculated results are compared with the experimentally obtained data. The agreement between experimental data and simulation related to both the temperature and the oxygen progress is fairly good with E a = 380 kJ mol −1 and k 0 = 246 × 10 6 mol m −2 s −1 for the kinetics of the ZnO dissociation reaction.

Concentrating solar power (CSP) systems use solar absorbers to convert sunlight into thermal electric power. In CSP systems, a high reflective surface focuses sunlight onto a receiver that captures the solar energy and converts it into heat. The operation of high efficiency CSP systems involves improvements in the performance of the coatings of the solar absorption materials. To accomplish this, novel, more efficient selective coatings are being developed with high solar absorptance and low thermal losses at their operation temperature. Heat losses in a CSP system occur by three mechanisms: conduction, convection and radiation. It has been widely documented that energy losses increase with increasing operating temperature of CSP systems, and the precise knowledge of the thermophysical properties of the materials involved in CSP systems may allow us to increase the efficiency of systems. In this work, we applied the pulsed photoradiometry technique (PPTR) to evaluate the changes in the thermophysical prope...

Radiative transfer inside a slurry photocatalytic reactor with hybrid illumination from both solar radiation and lamps is examined. The local volumetric rate of photon absorption is evaluated. For this purpose, the P1 and the modified differential approximations (MDAs) are used, and results compared to a solution by the Monte Carlo method. It is found that significant differences may arise between the predictions of the above approximations and the exact results provided by the Monte Carlo simulations. The P1 approximation is very inaccurate near to the radiation entrance for the partially collimated solar radiation, although it improves, as optical depth increases. As expected, the MDA improves the results near to the boundary. Surprisingly, it turns out to be much worse than the P1 approximation at medium and large optical depths. In the case of lamp irradiation, the behavior of the MDA is the opposite; it works better at small optical depths. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3256–3265, 2012

This study addresses the solar thermal dissociation of CO2 and H2O by two-step thermochemical looping using Fe3O4/FeO redox system. The reactions produce clean H2 and/or CO and the only process inputs are abundant and low-cost H2O and CO2 feedstock with no heating value combined with high-temperature solar heat to drive the endothermic reactions. The produced syngas can serve as the precursor to a renewable synthetic liquid fuel. Solar concentrated energy is thus used to reenergize H2O and captured CO2 for ultimately reversing combustion. The two-step process consists of (1) the solar thermal reduction of iron(II,III) oxide to FeO and O2 in a high-temperature solar chemical reactor heated by concentrated solar energy; and (2) the H2O/CO2 splitting with the solar-produced FeO to generate H2/CO, respectively; the resulting Fe3O4 being indefinitely recycled to the first step. The CO2 and H2O splitting reactions with the synthesized FeO-rich material were investigated in the range 600–800 °C with a thermobalance and with a packed bed of reactive particles enclosed in a tubular reactor, which showed that nearly complete FeO conversion could be reached. The powder reactivity was analyzed as a function of the reactant (CO2 and/or H2O), the temperature, and the material composition. A concept of the solar reactor technology was finally developed and experimentally tested in batch mode operation for the thermal reduction of Fe3O4. The rotary cavity-type reactor was operated under controlled atmosphere at about 1600 °C with a liquid phase of iron oxide subjected to thermal reduction.

Solar technology operating at elevated temperature conditions demands accurate knowledge of the optical and thermal properties of the materials involved in the construction and operation of solar collectors, reactors, and energy storages, among many others. Thermal energy storage (TES) devices involve successive melting and crystallization processes, which result in high complexity materials where the morphology, composition, and porosity could be highly non-homogeneous. In these cases, contact techniques for determining the thermal properties are highly susceptible and do not provide reliable measurements. It is under these conditions that non-contact photothermal techniques can provide superior performance, because in this case, the heat inducing source is a laser beam and the detector is usually a photodiode or a thermographic camera which are in non-contact with samples. The materials applied as storage medium in a TES unit can be divided into four groups: metals and alloys, ceramics and glasses, polymers and elastomers, and composites that include natural materials. Soda lime silicate glass recyclable waste is a very promising material for storage medium due to its inexpensive and wide availability. In this paper, we examined soda lime silicate glass-graphite composites for use as storage medium in a TES unit. A simple one-dimensional model for thermal conductivity was developed based on equivalent thermal circuits for series-parallel composite walls, and we found that thermal conductivity values depend on the amount of graphite dispersed into the samples, the porous media, and their structure.

Radiative heat transfer in a solar thermochemical reactor for the thermal reduction of cerium oxide is simulated with the Monte Carlo method. The directional characteristics and the power distribution of the concentrated solar radiation that enters the cavity is obtained by carrying out a Monte Carlo ray tracing of a paraboloidal concentrator. It is considered that the reactor contains a gas/particle suspension directly exposed to concentrated solar radiation. The suspension is treated as a non-isothermal, non-gray, absorbing, emitting, and anisotropically scattering medium. The transport coefficients of the particles are obtained from Mie-scattering theory by using the optical properties of cerium oxide. From the simulations, the aperture radius and the particle concentration were optimized to match the characteristics of the considered concentrator.

A multi-tubular solar thermochemical cavity reactor is proposed and the tubular array optimized. The optimized reactor design aims at operating under different temperatures and carrying out different kinds of thermochemical reactions. The radiation entering the receptacle comes from a solar concentrating system and the reactor consists of a cubic receptacle made of woven graphite, housing nine 2.54 cm diameter tungsten tubes. A model is developed and implemented considering high-temperature radiative transfer at steady state. The temperature distribution within the cavity surfaces is determined by employing a hybrid Monte Carlo-Finite Volume approach. Optimal tube distributions are explored by using a custom-made stochastic, multi-parameter, optimization algorithm. In this way, multiple global maxima are determined. Patterns among all possible optimal tube distributions within the cavity are obtained for different scenarios, by maximizing average tube temperature. From this study, practical guidelines are...

Background. The calculation of radiation absorption by the catalyst in solar photocatalytic reactors has been addressed by some authors, because it is a necessary step for the modeling of the detoxification of polluted water in these systems. Generally transparent pollutants have been considered, which somewhat simplifies the calculations. However, there has been an increasing interest in the study of solar photocatalytic degradation of dyes. These substances are not transparent to the radiation that the catalyst is able to absorb, and therefore their optical properties must be taken into account in the radiative modeling. Method of Approach. Absorption of radiation by the catalyst suspended in colored water is modeled by using the P1 approximation of radiative transfer theory. The absorption coefficient of the dye is taken into account in these calculations. A kinetic model is used to model degradation rates, based on the results of the radiative calculations. This has to be done through an Euler type method, because the reduction of dye concentration constantly modifies the optical conditions on the reactor, requiring a recalculation of radiation absorption at each step. Also, photocatalytic degradation experiments were carried out in a CPC solar photocatalytic reactor with tubular reaction space. Degradation of the Acid Orange 24 Azo dye was studied. The experimental degradation rates are compared with theoretical predictions. Results. An important influence of dye concentration is observed in the distribution of absorbed radiation, and also this parameter has a notorious effect on the predicted degradation rates. As a function of catalyst concentration, the degradation rate first increases rapidly and then at a smaller pace with an apparent linear trend. The experimental results can be reproduced well by the model. Conclusions. The proposed methodology allows modeling the solar photocatalytic degradation of dyes. The method should be applicable as long as the dye absorption coefficient is not too high in the wavelength region where the catalyst absorbs.

A directly irradiated cavity solar reactor devoted to the thermal reduction of SnO2 particle-cloud is studied numerically by using the Monte Carlo method. The steady-state model solves the radiation and convection heat transfers in the semitransparent particle suspension and the chemical reaction. It was used to predict the temperature distribution and the reaction extent inside the cavity, as well as the theoretical thermochemical efficiency for different operational conditions. The simulations assume that the reactor contains a nonuniform size suspension of radiatively participating reacting SnO2 particles. The model takes into account the radiative characteristics of the particles, as well as the directional characteristics of the power distribution of the incoming concentrated solar energy. The particle concentration, the particle size, and the length of the reactor are varied. Results show that the particle temperature and the yield of the endothermic reaction are higher when the reactor is fed with a cloud of particles with average diameter of 20 μm. The maximal thermochemical efficiency reached is 10%, which corresponds to an optimal optical thickness of around 2.

International audience; A 1 kW thermochemical solar reactor/receiver fitted with a porous ceramic foam structure is studied numerically to predict the thermal transfers inside the volumetric solar receiver. This reactor is devoted to the production of hydrogen from two-step thermochemical cycles based on mixed metal oxides, and it features a porous media coated with the reactive ferrite material (MFeO) that is directly irradiated by concentrated solar energy. The developed numerical model couples the fluid flow, heat and mass transfer, and the chemical reactions. The FLUENT code solves the transport equations of the fluid phase. The source terms of the solid phase, radiative heat transfer and chemical reactions are then computed using user-defined functions. The complete model was used to predict the thermal behaviour of the receiver under different operational conditions, which concerns the inert gas flow rate, the incident solar flux, the porosity, the mean cell size of the foam, the length of the volumetric solar receiver, and the influence of the chemical reactions. Results show that the maximal temperature of the foam is around 1710K with 6 L/min N for a mean solar concentration of 1040 suns. The higher the gas flow rate, the lower the temperature of the foam. The suitable operating conditions were defined for carrying out the reduction and hydrolysis reactions in the whole reactor volume at 1400K and 1200K, respectively. A model validation was performed with experimental data obtained from the reactor testing at the focus of a solar furnace.

Radiative heat transfer in a 1 kW cavity-type solar reactor devoted to the thermal reduction of compressed ZnO and SnO2 powders is analyzed by a Monte Carlo ray tracing simulation. The developed model takes into account the radiative properties of the reactant particles and of the ceramic cavity walls, as well as the angular intensity distribution of the incoming concentrated solar irradiation. The model also includes the conduction heat losses through the lateral walls and the energy consumed by the endothermic chemical reaction. It is used to predict the temperature and the absorbed flux density profiles on the inner cavity walls for different main features of the reactor, concerning the dimensions of the cavity and the type of reactant. Results show that the absorbed flux density profile and the theoretical thermochemical efficiency change with the cavity aspect ratio and with the oxide reactant. The cavity with an aspect ratio of 3 and a SnO2 pellet undergoing dissociation presents the highest thermochemical efficiency. Additionally, a different configuration of the reactor design is considered, which consists in implementing a layer of reactive particles on the inner lateral cavity wall. The model highlights that this type of reactor operation with a reactant layer on the lateral cavity wall results in an improved thermochemical efficiency.