C.A. Arancibia-Bulnes

Solar photocatalytic degradation of Aldrin

Abstract Photocatalytic degradation of the pesticide Aldrin dissolved in water was carried out, in one case, using concentrated solar radiation and, in another case, using non-concentrated solar radiation. In these experiments, the effects of catalyst concentration, oxidant agent concentration, and solar irradiation were tested. In experiments without irradiation, strong adsorption of the pesticide over titanium dioxide was observed in the first few minutes of contact in the presence of titanium dioxide (TiO 2 ). These results can be explained by means of Coulombic interactions between the catalyst surface and the pesticide molecules. During the photodegradation process, results show a residual degradation (photolysis) in both the cases, when no catalyst was added. In the case of the non-concentrated solar system, the achieved results suggest that the use of H 2 O 2 increased the degradation rate. For concentrated sunlight, an increase of the Aldrin concentration was observed during the first few minutes of irradiation. This can be explained as a desorption process that is triggered by a change in surface charge of the catalyst in the presence of hydrogen peroxide (H 2 O 2 ) during irradiation. When photocatalysis was performed with TiO 2 alone, no Aldrin was detected in the water solutions throughout the entire experiment. This result was unexpected; however, it might be explained by the adsorption of the pesticide on the catalyst surface and by the absence of the oxidant’s effect. Three transformation products (TPs) of the degradation process were identified: dieldrin, chlordene and 12-hydroxy-dieldrin. The results presented here are in agreement with previously reported results for photocatalytic degradation of other chlorinated pesticides using lamps as radiation sources.

Radiation absorption and rate constants for carbaryl photocatalytic degradation in a solar collector

Abstract We discuss an analytical model for the evaluation of radiation absorption in a tubular photocatalytic reactor. The model has no adjustable parameters and takes into account scattering in all directions. We compare the results of this model with those of Monte Carlo (MC) simulations and of a Lambert–Beer (LB) approximation, for a reactor illuminated by a parabolic solar concentrator. A good correspondence is found with the MC simulations. In particular, the model displays the correct saturation behavior of absorption for large catalyst particle concentrations, which is not obtained with the LB approximation. We have carried out experiments for the degradation of carbaryl in a solar parabolic collector (PC). The model is used to calculate the rate constant for this degradation from the experimental data. The theoretical model predictions reproduce well, the trends observed in the experiments.

Solar absorptance and thermal emittance of cermets with large particles

We evaluate, theoretically, the solar absorptance and thermal emittance of cermets with large particles. In particular, we consider Co and Ni particles with radii from 0.05 to 0.13 µm, embedded in binders of alumina and silica, respectively. For these particle sizes, it is necessary to consider the effect of the multiple scattering of light in the material for the visible and near infrared parts of the spectrum. The response of the particles is calculated using the Lorenz-Mie theory, and the multiple scattering effects are taken into account through a four-flux radiative transfer model. In the medium infrared, the Lorenz-Mie theory cannot be used because of the high absorption of the binder. The reflectance of the cermets in this regime is calculated using the Maxwell-Garnett effective medium approximation. In general, moderate values of the absorptance-to-emittance ratio are found.

Average path-length parameter of diffuse light in scattering media

We use Monte Carlo simulations to study in detail the propagation of light in a plane-parallel medium containing scattering particles. In particular, we compute the forward and backward average path-length parameters (FAPP and BAPP, respectively) of four-flux radiative transfer models as functions of the optical depth. Strong dependence on the single scattering albedo and phase function asymmetry is found for both quantities. In general the values of the FAPP decrease with increasing absorption, whereas the opposite occurs for the BAPP. A similar effect is produced when changing from isotropic phase functions to phase functions with a large asymmetry in the forward direction. We present analytical results for the asymptotic values of the FAPP and BAPP as functions of albedo for the particular case of isotropic scattering. Our results differ markedly from the predictions obtained recently with two multiple-scattering models by Vargas and Niklasson [J. Opt. Soc. Am. A 14, 2243 (1997); Appl. Opt. 36, 3735 (1997)]. The differences found point out the intrinsic limitations of these models.

Radiative heat transfer analysis of a directly irradiated cavity-type solar thermochemical reactor by Monte-Carlo ray tracing

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 thermoc...

Development and Modeling of Solar Photocatalytic Reactors

Abstract The design of solar photocatalytic reactors is aimed at making the best use of the available solar radiation. This contribution addresses the development and modeling of solar photocatalytic reactors. Particular emphasis is placed on the work carried out by our research group and on recent contributions by researchers at other institutions. Due to the kinetics of photocatalytic reactions, nonconcentrating solar reactors are energetically more efficient, even though concentrating reactors are faster. Recent studies have focused mostly on the former, but reactors with low concentration factors are also interesting, particularly those based on compound parabolic collectors (CPC). The evaluation of radiation absorption in photoreactors is a very important matter in order to correctly model reactions and helping in making appropriate design decisions. Both heuristics and first principles methods based on the radiative transfer equation have been considered for slurry solar photoreactors. These works are reviewed, with the P1 approximation model being discussed thoroughly.

Modeling the Solar Photocatalytic Degradation of Dyes

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.

Spectral selectivity of cermets with large metallic inclusions

In order to address the problem of the optical reflectance of cermets beyond the quasistatic dipolar limit, we consider Monte Carlo multiple scattering simulations as well as a radiative transfer study. Slabs of alumina with inclusions of Cu, Au, Co and Cr are studied; reflectance coefficients are calculated as a function of wavelength, considering metal inclusion diameters up to 2.0 μm. We show that despite the incoherent diffuse propagation of light inside the slab, strong selectivity is achieved by particle diameters around 0.2 μm, even for very low metal concentrations.

Heliostat image drift behavior for different error sources

Drift is ubiquitous in heliostat fields, and may be caused by diverse geometrical inaccuracies during heliostat installation and operation. This phenomenon is studied for three important primary errors in the present paper: Angular offset in the drive mechanism, pedestal tilt, and canting error. Each error produces characteristic signatures, but there is a diversity of behavior depending on the error parameters and location of the heliostat. The variation of the extent of drift curves is studied as a function of distance, for fixed error parameters. It is found that, in general, this extent is not proportional to distance, except for far heliostats, and depends on a complicated manner on the different parameters involved. Moreover, even though the extent of drift curves becomes proportional to distance for far heliostats, the convergence is very slow, and very variable with the error parameters.

Improving parabolic trough mirror module qualification by FOCuS tool

An improved tool for the shape qualification of parabolic trough mirror modules used in concentrated solar power plants was developed. The tool is based on the fringe reflection theory, in which sinusoidal fringe patterns are projected on a screen and their reflection over a specular surface is recorded by a camera. The observed distortions in the image are related directly to surface deviations from ideal geometry. Relevant aspects of the technique are its high spatial resolution (more than 1 × 106 points per mirror facet), short measurement time and easy setup. The developed tool (called FOCuS) is capable of calculating the local mirror slope deviations from its ideal design and the RMS value as a quality factor. Furthermore, the tool generates a file which can be loaded into CENERs TONATIUH ray tracing software, through a specially developed plug-in, for mirror modeling and intercept factor calculation with several tube absorber geometries.