R.E. Cabanillas

Dust accumulation effect on efficiency of Si photovoltaic modules

We experimentally studied the electrical efficiency effects of naturally forming atmospheric dust deposits on commercial photovoltaic panels. The variable considered for measurements was the electric potential for three commercial silicon modules: monocrystalline, polycrystalline, and amorphous. A mathematical model was developed to determine maximum potential as a function of temperature and of total incident radiation. The study presents two essential parts: the naturally deposited dust particles and the variation in maximum electric potential between clean and dirty modules. The results indicate that the maximum reduction in potential is around of 6% for monocrystalline and polycrystalline modules and of 12% for the amorphous silicon.

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.

Modelación y Simulación de la Transferencia de Calor en Muros de Bloque de Concreto Hueco

Resumen Se presenta el estudio de transferencia de calor en un muro de bloques de concreto con cavidades, para conocer su resistencia termica. Se plantea un modelo teorico unidimensional en estado estacionario, considerando conduccion, radiacion y conveccion. El problema se resuelve numericamente utilizando el metodo iterativo de Gauss-Seidel. La simulacion se efectua en forma horaria para dos dias en condiciones extremas de temperatura ambiente. Se obtiene un valor promedio de la resistencia termica de 0.18 °Cm 2 /W, con variaciones de 2.3% a 23%. Las aportaciones al flujo total de calor por cada mecanismo son de 25%, 19% y 56%, para conduccion, conveccion y radiacion respectivamente. El muro presenta valores de resistencia termica que tienden a disminuir con el aumento de los diferenciales de temperatura, aspecto que desfavorece su uso para climas calidos, siendo la radiacion el mecanismo que mas favorece la transferencia de calor. Palabras clave: transferencia de calor, resistencia termica, muros de bloque, concreto hueco

Determinación Experimental y Contraste Numérico de la Resistencia Térmica de un Muro de Bloques de Concreto Hueco

This paper presents design and construction of an experimental test to determine the thermal resistance of a hollow concrete blocks wall, according to the international standard known as guarded hot plate. The construction procedure and implementation of the device are described, reproducing operations conditions for buildings located in warm dry climate, and the thermal resistance was experimentally and numerically determined. The results were found by applying controlled temperature differences on both sides of the wall between heating and cooling temperature zones, in steady state conditions. Experimental and numerical results show variations in the value of thermal resistance, ranging from 0.15 to 0.19 °C m2/W. Values of the thermal resistance were fitted to an empirical expression as function of the temperature difference between the walls and the average temperature of the wall.

Analysis of Drift Phenomena in Heliostat Images

Heliostat image drift is defined as the wandering of the irradiance spot produced by a heliostat on a receiver or observation screen. Two factors producing heliostat image drift have been analyzed theoretically in the present paper: errors in reference position, and time delay. Some regular behavior is found in the drift curves. Maximum deviations from target tend to be linearly dependent with either angular error, or time delay. In particular, the curves produced by this later effect are circular or elliptical. Heliostats at different distances from a tower have been analyzed. For heliostats far from the tower the drift curves due to errors in reference position also have a more or less elliptical shape. Results are presented as dimensionless quantities. Some practical implications of these results are discussed.© 2012 ASME

Estudio Numérico de la Resistencia Térmica en Muros de Bloques de Concreto Hueco con Aislamiento Térmico

Resumen Se presenta el estudio numerico de la resistencia termica en dos tipos de muros de bloques de concreto hueco para diferentes temperaturas en la superficie exterior. Se analiza el efecto de colocar aislamiento termico por la cara exterior y en sus cavidades. El modelo teorico para estado permanente, fue resuelto numericamente con diferencias finitas y validado con resultados experimentales de la literatura. Las resistencias termicas promedio para los bloques de 0.12 m y 0.15 m son de 0.159 m 2. K/W y 0.171 m 2. K/W respectivamente. Cuando se rellenan las cavidades con aislamiento termico son de 0.407 m 2. K/W y de 0.530 m 2. K/W, respectivamente. Al aplicar el mismo volumen de aislante de la cavidad rellena, sobre la cara exterior, se obtienen valores de 1.555 m 2. K/W y 2.147 m 2. K/W, para los bloques de 0.12 m y 0.15 m respectivamente. Palabras clave: resistencia termica, bloque concreto hueco, aislamiento termico, diferencias finitas Numerical Study of the Thermal Resistance of Hollow Concrete Block Walls with Thermal Insulation

Heliostat Characterization by Optical Techniques and Image Processing

In the framework of the National Laboratory of Solar Concentrating and Solar Chemistry Systems (LACYQS, for its Spanish acronym), a Heliostat Test Field (HTF) was built in Mexico. This research facility is located 10 km away from the city of Hermosillo, in the state of Sonora. The main purpose of the HTF, at the present stage, is to serve as platform for the development and testing of heliostat technology.In order to evaluate the performance of heliostats, various optical tests have been implemented. In the sun tracking test, the heliostat is operated as a solar tracker. A camera is attached to the heliostat, which is pointed directly to the sun. Images are captured throughout the day to quantify the wandering of the solar disc in the image. In the reflected spot test, the image produced on the Lambertian target by the concentrating heliostat, due to the reflection of the sun, is recorded by a CCD camera throughout the day. Image processing algorithms calculate the centroid of energy of the image and evaluate the position and wandering across the white screen at all times. After this information is gathered, and the influence of wind and external factors eliminated, data are interpreted to characterize the behavior of solar projection algorithms and mechanical components.In the fringe projection analysis, also known as deflectometry, fringe patters are projected at night on a Lambertian target. The image of the pattern reflected by the heliostat is recorded with a camera. Distortions in the fringes, due to mirror stress and canting, allows the characterization of the surface error of the facets.Copyright