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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.

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.

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.

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.

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.

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.

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...

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.

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...

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.