Mostrando artículos por etiqueta: 2018 publication

Abstract

The numerical evaluation of the radiative heat transfer in a multichanneled solar reactor coated with ZnFe2O4 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 ZnFe2O4 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 ZnFe2O4 layer thickness, incoming radiation profile, diameter and length of pores and position of the monolith in the focal zone of the solar furnace.

Abstract

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.

Abstract

In this work, the seasonal and yearly optical performance of supported catalyst CPC solar photocatalytic reactors has been theoretically analyzed. A detailed model for the optical response of the anatase catalyst films is utilized, based on the characteristic matrix method, together with Monte Carlo ray tracing simulations. The catalyst is supported over glass tubes contained inside a larger glass tube that functions as receiver of the CPC reflector. Arrangements with four, five, and six tubes are considered. Overall, the four-tube scenario presents the worst performance of all, followed by the five-tube case. In general, the six-tube configuration is better. Nevertheless, important differences can be observed depending on the specific arrangement of tubes. The six-tube case surpasses the absorption rate of all the other configurations when the distance between tubes is extended. This configuration exhibits 27% increased yearly energy absorption with respect to the reference case and 47% with respect to the worst case scenario.

Abstract

This study investigates the phase transformations that can occur in an austenitic stainless steel (AISI 316) by demonstrating the appearance of δ-ferrite that is obtained in the initial heating cycles using Concentrated Solar Irradiation (CSI) at magnitudes needed to obtain the operational temperatures of central tower systems. Four AISI 316 stainless steel specimens cut from one single initial piece, were exposed to CSI in the Horno Solar de Alto Flujo Radiativo at the Universidad Nacional Autónoma de México to perform the thermal cycles. AISI 316 stainless steel is fully austenitic and is selected because it is reportedly one of the cheaper material used in CSI receivers. Monotonic tensile strength tests were performed, and it is assumed that there is no relevant effect on the mechanical behavior for the reported experiment. Phase transformations were characterized using optical microscopy, X-ray diffraction and by scanning electron microscopy analysis with an energy-dispersive X-ray spectroscopy. The appearance of δ-ferrite phase was the principal difference between CSI treated specimens, a non-treated specimen and one specimen heated by conventional method. Concentrated UV irradiation from the solar spectrum on Earth surface demonstrated to have the potential to obtain the phase transformation at a temperature near 630 °C.

Abstract

The optical characteristics of solar concentrators are key factors influencing the overall efficiency of solar power plants. For instance, heliostats need to be evaluated prior to installation and during its operation lifetime. This guarantees that the optical and thermal performance of these systems is close to design. One methodology that has gained importance due to its potential capabilities has been the Fringe Reflection Technique. This technique uses the reflection of a series of regular stripes to obtain the local slope deviations from a specular surface. Coupled to a ray tracing analysis, these slopes can be used to identify the distortion in concentrated solar spots. The enormous amount of data needed to carry out this analysis difficult its implementation at large scale. In this work, a study for determining the optimal number of sample points for heliostat surface characterization is realized. It has been found that, depending on the level of errors, the number SPFS required to reach convergence in the flux distribution profiles and intercept factors is variable. However, for the wide range of parameters considered in all cases 48 SPFS where enough to reach convergences to 1%. This is equivalent to one point per every 2.5cm of facet side length. For values of slope and canting errors up to 2mrad, half this density is sufficient.

Abstract

In this paper, we present two statistical methods to quantify the heterogeneity of the irradiance flux distribution, in a Concentrator Photovoltaic (CPV) dense-array, based on its operation and the optimization of current-matching. Preventing non-uniform flux distribution from design avoids the generation of hot spots, current mismatch and increases the overall efficiency of the system. This new approach considers the effects of the lowest irradiance values in the performance of the complete array, and its performance was corroborated by the simulations of a CPV array modelled in Matlab/Simulink; the irradiance distribution data as an input parameter was obtained from the images taken in a homogenization experiment, in the HoSIER, an 18,000 X solar furnace. The results are interpreted through the new concept of photovoltaic homogeneity, proven that the methodology successfully predicts the flux distributions, which enhances the efficiency of a series connected CPV array. Additionally, we found that the proposed methodology can also be used to optimize the electrical performance of dense-array CPV systems, working under the effects of non-uniformity illumination by rewiring the series connections into series-parallel configurations.