Abstract

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.

Abstract

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.

Abstract

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.

Abstract

It is a common practice in the development of point focus solar concentrators to use multiple identical reflecting facets, as a practical and economic alternative for the design and construction of large systems. This kind of systems behaves in a different manner than continuous paraboloidal concentrators. A theoretical study is carried out to understand the effect of the size of facets and of their optical errors in multiple facet point focus solar concentrating systems. For this purpose, a ray tracing program was developed based on the convolution technique, in which the brightness distribution of the sun and the optical errors of the reflecting surfaces are considered. The study shows that both the peak of concentration and the optimal focal distance of the system strongly depend on the size of the facets, and on their optical errors. These results are useful to help concentrator developers to have a better understanding of the relationship between manufacturing design restrictions and final optical behavior.

Abstract

Artificial neural networks (ANN) were proposed as a multivariate experimental design tool for monitoring a photo-Fenton treatment of wastewaters containing a synthetic mixture of pesticides. ANN and Nelder-Mead simplex methods were used to find out the optimum operating parameters of a photo-Fenton pilot plant. ANN was developed to predict the most important operating parameters (e.g., the total organic carbon and the initial mineralization kinetic rate constants of the reactions), which determine the photo-catalytic degradation efficiency in photo-Fenton processes. Experimental measurements of temperature, pH, hydrogen peroxide (H2O2) consumption, initial concentration of Fe2+, and the AE were used as input data for the ANN learning. A feed-forward with one hidden layer, a Levenberg–Marquardt learning algorithm, a hyperbolic tangent sigmoidal transfer function and a linear transfer function were used to develop the ANN model. The best fitting of the training database was obtained with an ANN architecture constituted by seven neurons in the hidden layer. The simulated results were validated with experimental measurements, showing an acceptable agreement (R2 > 0.99). The ANN was subsequently coupled with a Nelder–Mead simplex method to obtain the optimum operating parameters of the photo-Fenton pilot plant. The H2O2 consumption was used as key variable for evaluating the optimization procedure. Errors less than 1% between simulated and experimental data were found. The obtained results showed that the use of ANN provides an excellent predictive performance tool with the additional capability to assess the influence of each operating parameter on the removal process of water pollutants.

Abstract

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.

Abstract

A methodology for the evaluation of the specularity error of a polymeric filmoptical coating is presented. The methodology is based on the comparison of images from the sun produced by two high quality spherical mirrors, one covered with a highly specular evaporated aluminumfilm, and the second one with the polymeric film under study. This film is a commercial product known as Reflectech®. To determine the specularity error, both images are reproduced by means of ray tracing optical simulations. Those simulations use the angular brightness distribution from the sun as input, which were recorded by means of a specially developed solar scope. Significant differences are obtained between images of the sun generated by both mirrors. However, the specularity error of the coating under consideration is found to be just 0.71 mrad. This error is quite small making the polymeric coating highly appropriate for point focus concentration systems. This is illustrated by calculations for a parabolic dish concentrator.

Abstract

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.

Abstract

Atrazine is a highly persistent and carcinogenic compound used as herbicide around the world. This compound has been banned in USA and some European countries but in Mexico it is still widely used in the agriculture. In order to achieve a high mineralization of atrazine, present as active compound in the Gesaprim commercial herbicide, detoxification studies in two-compound parabolic solar reactors by means of photo-Fenton process followed by TiO2 photocatalysis was carried out. The atrazine contents in the Gesaprim solutions tested were 35 mg L−1 (19.0 mg L−1 of TOC) and 20 mg L−1 (9.5 mg L−1 of TOC). [H2O2]0/COD0 ratios of 1, 3 and 5 (1.5 × 10−3, 4.5 × 10−3 and 7.5 × 10−3 mol L−1 H2O2, respectively) were evaluated in combination with 5 mg L−1 and 10 mg L−1 Fe2+ at pH 2.8 in the photo-Fenton oxidation; whereas, in the photocatalytic process, the influence of the pH (4.8, 7.0 and 11.0) and type of TiO2 (Degussa P25 and HB) were studied with TiO2 content of 200 mg L−1. The study showed that photo-Fenton process followed by TiO2 photocatalysis produce a 72% of mineralization (for an initial TOC of 19 mg L−1) and decrease above 90% of toxicity in compliance with NMX-AA-110-1995-SCFI1 Mexican Norm. In order to established a minimum amount of chemical reagents these photodegradation processes were carried out with special emphasis on the optimization of experimental parameters such as concentrations of photocatalyst and oxidant. Atrazine mineralization was influenced by the pH of the solution, the initial concentration of hydrogen peroxide and iron ions.

Abstract

En este trabajo se reflexiona sobre la problemática energética mundial; la finitud de las fuentes fósiles y su impacto al medio ambiente. La estructura energética mundial es no sustentable y se requiere de un cambio de paradigma energético basado en la eficiencia energética y el uso de fuentes alternas como las energéas renovables (ER). La demanda energética mundial está en continuo aumento a un ritmo de crecimiento anual del 2.47%. En un escenario al 2030 para la transformación del sistema mundial de energía, elaborado a partir del cumplimiento de nuevas políticas energéticas que consideran una economía baja en carbón para la protección del medio ambiente, todas las fuentes primarias contribuyen a satisfacer la demanda energética. De hecho, las fuentes primarias de energía que más crecen son el gas y las ER. En particular, la taza a la que las ER están creciendo y penetrando los mercados mundiales de la energía tiene una marcada similitud con la aparición de la energía nuclear en los años 1970’s y 1980’s.

México cuenta con abundantes recursos renovables y se han hecho esfuerzos importantes para avanzar en el uso de las tecnolog´ıas que aprovechan las fuentes de ER. El potencial solar del pa´ıs es realmente muy grande y el uso de las tecnologías solares que aprovechan dicho recurso es muy limitado. Se requiere, para garantizar el desarrollo sustentable en el país, que el estado mexicano se comprometa, con una visión a largo plazo, en el aprovechamiento de las ER y en el uso eficiente de la energ´ıa. Se deberán generar las políticas, los marcos legales, los incentivos económicos y los fondos de financiamiento para apoyar a la investigación científica y tecnológica y permitir el desarrollo masivo de las ER y del uso eficiente de la energ´ıa en el país.