Abstract

El presente trabajo presenta el diseño, construcción e instrumentación de un concentrador solar pedagógico del tipo canal parabólico, el cual funciona siguiendo el ángulo horario (w) solar. Además, se describe la caracterización térmica del mismo.

En la primer sección se introduce al tema de la energía con un esbozo de las energías renovables en México y el mundo. Posteriormente se justifica el diseño y construcción de instrumentos capaces de generar calor de proceso en la industria, que paralelamente se relacionan con instituciones de educación. Así mismo, en esta parte se detallan los alcances del trabajo.

Posteriormente, en el capítulo 2, se introduce a los términos usados durante el trabajo, y se explican la relación entre el Sol y la Tierra que hacen posible, el uso de la energía solar para generar energía térmica o eléctrica. Igualmente, se hace una descripción de los precedentes industriales y educativos de proyectos similares.

En el capítulo 3, se detalla el diseño del seguimiento solar. Primeramente, se habla de cómo se controla el sistema, de la electrónica y programación necesaria. Luego, se realiza una evaluación del rastreo del ángulo horario por medio de procesamiento de imágenes y al mismo tiempo se generan estimaciones de la concentración solar en una sección del equipo. Finalmente se obtiene una aproximación del error óptico de la superficie reflectiva colocada con el software Tonatiuh®.

La caracterización térmica del equipo se desarrolla en el capítulo 4. Se explica la instrumentación, la programación realizada y el sistema de adquisición usado. También se explican las normas tomadas como base de la metodología dispuesta. Se detallan las pruebas realizadas y las comparaciones entre un simulador solar, una configuración con orientación norte-sur y otra este-oeste.

Las conclusiones se encuentran en el capítulo 5 por tema y de manera general del proyecto. Finalmente, se colocan observaciones para pruebas y mejoras a futuro.

Abstract

The radiation flux distributions produced by the concentrating solar systems used to produce thermal/electrical power are usually non-homogeneous. This results in non-uniform temperature distributions on the solar receivers, causing adverse effects on the system’s overall performance. An approach to better understand the problem is to study the surfaces around the focal zone where the radiation density is homogeneous (isosurfaces), generating them from experimental data. For this, it is necessary to superimpose built volumes of the different irradiance levels using parallel planes in different directions from the focal point of a concentrator. These volumes are known as effective volumes. This study presents the model used to generate effective volume produced by a point focus concentrator, comparing it with experimental results in a direction perpendicular to the focal axis. The effective volumes were developed considering a global optical error of the system of 2.8 mrad. The set of methods used to generate effective volumes has not been previously presented in the literature. The theoretical-experimental research consisted of the combination of the camera-target method and the simulations by the ray-tracing technique. The results showed effective volumes with the highest value of 10 MW/m2 and the lowest value of 4.5 MW/m2.

Abstract

Hybrid graphemne-fiber systems could present an alternative for various industrial applications in need of large area graphene sheets. One way to produce these carbon-based structures is by subjecting an aqueous polyvinyl alcohol solution containing sodium chloride to centrifugal spinning under high humidity conditions. The developed polymer fibers are then subjected to a dehydration and carbonization process to promote the formation of the hybrid carbon structure. Potential applications of this material are highly dependent upon their conducting properties. In this work we analyzed the effect of the NaCl content and humidity conditions during the spinning process and ultimate thermal conductivity of the resultant hybrid graphene-fiber carbon systems. Results show an optimum NaCl added to the carbon precursor solution and spun at a high relative humidity (around 70%) promote the development of veils of graphene oxide multilayer that interconnect with produced fibers. We applied for the first time a thermographic method to determine the thermal conductivity of carbon mats. The thermal conductivity of the hybrid fibers increases as graphene multilayers veils expand between carbon fibers, to reach values up to 28 W m K−1.

Abstract

Increasing the electrochemical performance of electrode materials in sodium ion batteries (NIBs) remains a major challenge. Here, a combined experimental and theoretical investigation on the modification induced by Sb2S3 embedded in a heteroatom-doped 3D carbon matrix (CM) for efficient anodes in NIBs is presented. The structural and chemical characterization demonstrates the successful doping of 3D CM with S and Sb atoms. When evaluated as anode materials for NIBs, the heteroatom-doped nanocomposites delivered a better cycling stability and superior rate capability than those of undoped Sb2S3/CM anodes. First principle calculations were used at the Density Functional Theory level to systematically study the Sb2S3/CM and Sb2S3/heteroatom doped-CM composites, as NIBs anodes. Doping the carbon substrate by heteroatoms improved the adsorption of Sb2S3 on the matrix and allowed for ionic/covalent attraction with the Sb2S3 nanoparticle, respectively. Such results could be used to model the stabilty of the composite architectures observed in the experiment, for superior cycling stability.

Abstract

In this work, it is proven that a biochar obtained from a commercial gasifier can be used as electrode material for supercapacitors (SC). This biochar was produced at 1000 °C from corn cob wastes (GAS), and was compared to an activated biochar obtained in a traditional lab pyrolysis process (LAB). Both biochars were characterized by different physicochemical techniques, observing their amorphous nature with well-developed microporosity dependent of their pretreatment and production methodology. Furthermore, a computational modeling based on Molecular Dynamics at the ReaxFF level was also performed to elucidate the geometry of the resulting microporous structure after simulated pyrolysis. X-ray structure and pore size distribution are in agreement with those results obtained via computational simulation. Both carbon materials were electrochemically evaluated in acidic electrolyte using 3 and 2 electrode systems, obtaining capacitances of 130 F g−1 (20 mV s−1), and excellent performance compared to commercial activated carbons, with only about 10% of capacitance loss after 5000 cycles. However, GAS performance in SC was higher than activated biochar due to its higher micropore volume. This study provides a novel useful application to use gasifier residues from agricultural biomass waste for energy storage devices.

Abstract

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.

Abstract

As a semiconductor of “earth-abundant” elements, Sn2S3 with a bandgap (Eg) close to 1 eV merits attention, but a method to prepare phase-pure thin film remains elusive. We report the formation of Sn2S3 thin film of 360 nm in thickness by heating chemically deposited tin sulfide thin films at 450 °C during 30–45 min in presence of sulfur at a pressure, 75 Torr of nitrogen. Energy dispersive x-ray emission spectra and grazing incidence x-ray diffraction established a reaction route for this conversion of SnS completely to Sn2S3 via an intermediate phase, SnS2. The optical bandgap of the material is 1.25 eV (indirect) and 1.75 eV (direct, forbidden). The optical absorption suggests a light-generated current density of 30 mA/cm2 for the Sn2S3 film (360 nm) as a solar cell absorber. Thin film Sn2S3 formed in 30 min heating has a p-type electrical conductivity in the dark of 1 × 10−4 Ω−1 cm−1, which increases to 3 × 10−4 Ω−1 cm−1 in 0.2 s under 800 W/m2 tungsten-halogen illumination. An estimate made for its mobility-lifetime product is, 6 × 10−6 cm2 V−1. We discuss the prospects of this material for solar cells.

Abstract

Hybrid perovskite films are prepared via two-step spin coating. The impact of magnetic fields during the spin coating of the PbI2 precursor solution is assessed with atomic force microscopy and scanning electron microscopy of the obtained layers. Deeper and narrower peak–valley–peak formations are obtained in PbI2 films when the magnetic field applied and the spinning direction result in a Lorentz force that pushes the [PbI6]4− ions towards the inner area of the substrate. This produces rougher and more porous PbI2 films with an increased surface area that facilitates the infiltration of the methylammonium iodide and chloride precursor solution, thereby enhancing the formation of perovskite. Increased cell performance and more repeatable results are obtained when the PbI2 film is spin-coated under the influence of a negative magnetic field. Opposite effects are obtained when the direction of the magnetic field, and therefore the Lorentz force, is inverted. This demonstrates that a magnetic field can be used to modify the surface morphology of spin-coated thin films prepared from ionic precursor solutions.