Shadow Ring Research Articles

Low-cost, open-hardware shadow band system for diffuse irradiance measurements

In this work, we present the design, implementation, and test of a low-cost, open-hardware shadow band system that enables a pyranometer to measure global and diffuse irradiance. This system was implemented in Lima, Peru, and experimentally tested by contrasting it with a commercial reference system consisting of two pyranometers, one blocked by a shadow ring. The developed shadow band system automatically opens and closes to measure the global and diffuse irradiance consecutively, respectively, without requiring regular, manual adjustments of the shadow ring position as in the reference system. Thus, it potentially saves costs on equipment to characterize de irradiance components. Initial measurements in Lima indicate 100% and at least 23% of diffuse irradiance under cloudy and clear sky conditions, respectively.

Radiação global e difusa diária na região de transição Cerrado-Amazônia brasileira

This study aimed to analyze the seasonal variations in atmospheric transmissivity and solar radiation (global and diffuse) on the horizontal surface in Sinop, Mato Grosso (MT) (11.865°S, 55.485°W, and altitude of 371 m) from 06/02/2011 to 12/31/2014. The values of diffuse radiation were measured using the Melo-Escobedo-Oliveira (MEO) shadow ring, with application of astronomical, geometric, and anisotropic correction factors. The analysis of atmospheric transmissivity was based on the classification of sky cover as cloudy, partly cloudy, partially clear, or clear. The diffuse radiation showed similar behavior to the radiation at the top of the atmosphere, reaching a maximum between October and April (rainy season), while the global radiation displayed higher levels during the dry season (May to September). The average daily global radiation ranged from 22.75±0.61 MJ m−2 d−1 in August to 16.44±1.45 MJ m−2 d−1 in January. In Sinop, cloudy and partly cloudy skies occurred on 45.6% of days and atmospheric transmissivity of global radiation was greater than 55% on 54.6% of days. The variations in diffuse radiation in the region were influenced by cloudiness and the concentration of biomass burning aerosol particles. The diffuse radiation can represent 8.02%–99.12% of the global radiation and 5.33%–29.01% of solar energy incident at the top of the atmosphere.

Evaluation of anisotropic correction models for diffuse solar irradiance measured by the MEO shadow ring method

Diffuse solar radiation is measured using a pyranometer positioned under the shadow of a ring. The function of the ring is to block direct solar radiation. However, the ring also blocks a portion of the diffuse solar radiation, and therefore, the diffuse radiation needs corrections. The most well-known assembly in the literature on the shading ring method is the Drumond assembly. This study aims to determine the effectiveness of three methods of correction of diffuse irradiance measured with shadow bands: the LeBaron method, the Battles method, and the Dal Pai method. The Laboratory of Solar Radiometry of Botucatu provided global, direct, and diffuse solar irradiances. The laboratory measures diffuse solar radiation using the Melo–Escobedo–Oliveira (MEO) shading ring method, a different setup compared to Drummond's setup. The period used for the study comprised the years 1996 to 2005. The results showed that LeBaron and Battles correction models with the original coefficients are not suitable for correction of diffuse irradiance in Botucatu. According to MBE (Mean Bias Error) and RMSE (Root Mean Square Error), the statistical indicators, LeBaron and Battles with adjusted coefficients, and Dal Pai correction models are indicated to correct the diffuse irradiance in Botucatu measured under an MEO shading ring. For monthly applications, the LeBaron model with adjusted coefficients is recommended. Although this model is more complex from the operational point of view, it better explains anisotropic effects, especially for the winter months.

Design of a free-form diffuser for the entrance optic to correct the cosine error in the photometer.

The entrance optic is an important part of the photometer head. It can match the directional response of the photometer to the cosine function. In this paper, an entrance optic consisting of a free-form diffuser, a shadow ring, and an integrating cavity is introduced. An iterative optimization algorithm is presented to design a free-form diffuser that exhibits better cosine response characteristics. Diffusers of different materials and sizes are designed in a simulation experiment. After a finite number of iterations, in the absence of the shadow ring, the integral cosine error of the free-form diffuser is less than 1%. The directional cosine error is less than 3% for incidence angles between 0° and 70°. After adding a shadow ring to correct the directional response of the incident angle greater than 80°, for incident angles between 0° and 85°, the cosine errors are typically less than 3%, except that the cosine errors of very few large incident angles are close to 5%. The experimental results show the effectiveness and robustness of the proposed method. In addition, the influence of an important percentage constant σ on iterative optimization is studied. It is found that the larger the parameter σ, the fewer the number of iterations, and the directional cosine error may be slightly larger but still acceptable. The wide range of values of σ further embodies the versatility and flexibility of the proposed method.

An HDRi-based data acquisition system for the exterior luminous environment in the daylight simulation model

This paper proposes an HDRi-based data acquisition system to improve the quality of the Sky Component (SC) and the Exterior Reflected Component (ERC) in the daylight simulation model. The proposed system consists of an HDRi device equipped with shadow ring for high-frequency sky luminance capture and the HDRi-based gray card referencing procedure to measure the reflectances of surrounding structures. This system can provide timely updates of the exterior luminous environment for model-based real-time daylight control. Physical experiments with a test chamber were conducted to evaluate the performance of the prototypical system. In the experiments, HDRi techniques were applied to capture luminance distributions of the sky, window view and interior surfaces. Simulations of the indoor luminance distributions of the test chamber were conducted under two sets of daylight descriptions. The first simulation, derived from the concept of highly accurate image-based rendering, uses the HDRi daylight scene of the window view. The second simulation, the proposed method, uses a combination of the Tregenza sky derived from HDRi sky view and the model of the surrounding structures. Comparisons were made between both sets of simulations and the measured interior luminance levels. The RMSE and MBE were calculated using cells from the sub-divisions of the interior surface. The statistical analysis demonstrates that the proposed system in continuous operation can achieve a level of accuracy comparable to that of the image-based daylighting description from the window view.

A method for optimizing the cosine response of solar UV diffusers

Instruments measuring global solar ultraviolet (UV) irradiance at the surface of the Earth need to collect radiation from the entire hemisphere. Entrance optics with angular response as close as possible to the ideal cosine response are necessary to perform these measurements accurately. Typically, the cosine response is obtained using a transmitting diffuser. We have developed an efficient method based on a Monte Carlo algorithm to simulate radiation transport in the solar UV diffuser assembly. The algorithm takes into account propagation, absorption, and scattering of the radiation inside the diffuser material. The effects of the inner sidewalls of the diffuser housing, the shadow ring, and the protective weather dome are also accounted for. The software implementation of the algorithm is highly optimized: a simulation of 109 photons takes approximately 10 to 15 min to complete on a typical high‐end PC. The results of the simulations agree well with the measured angular responses, indicating that the algorithm can be used to guide the diffuser design process. Cost savings can be obtained when simulations are carried out before diffuser fabrication as compared to a purely trial‐and‐error‐based diffuser optimization. The algorithm was used to optimize two types of detectors, one with a planar diffuser and the other with a spherically shaped diffuser. The integrated cosine errors—which indicate the relative measurement error caused by the nonideal angular response under isotropic sky radiance—of these two detectors were calculated to be f2=1.4% and 0.66%, respectively.

Comparison of shadow‐ring correction models for diffuse solar irradiance

Reliable and accurate measurements of diffuse solar irradiance are needed in order to partition global irradiance into its direct and diffuse components. Diffuse irradiance is commonly measured using sun tracking systems or shadow rings. Data obtained using a shadow ring must be corrected for the portion of diffuse irradiance blocked by the ring. In this paper we have examined and evaluated six of the most widely used correction models. Approaches that account for radiation anisotropy perform notably better than those using only geometric corrections. Our results also argue for the need to adjust empirical models to local conditions. Empirical approaches developed by LeBaron et al. (1990) and by Batlles et al. (1995) perform best when compared with the more theoretical models.

Estudo da radiação solar global e do índice de transmissividade (kt), externo e interno, em uma floresta de mangue em Alagoas - Brasil

As florestas de manguezais são de extrema importância social, econômica e ambiental, pois as mesmas são detentoras de uma rica fauna e flora. O Brasil possui uma das maiores áreas de manguezais do mundo que se estende desde o extremo Norte até o Sul do país, e devido a isso, os efeitos da alteração de manguezal em área degradada influenciam diretamente no microclima da região. No aspecto meteorológico, é necessário que seja limitada a forma de exploração desse ecossistema, pois a conversão de florestas de mangues em áreas degradadas expõe a superfície à radiação solar direta alterando o balanço radiativo. Então, conhecer o comportamento da Radiação Solar Global (Rg) e suas componentes, Radiação Difusa (Rd) e Radiação Direta (R D), dentro das florestas, são fatores primordiais para o entendimento da disponibilidade de energia para os diversos processos desse sistema. O principal objetivo deste trabalho foi avaliar e comparar o comportamento da Rg e suas componentes R D e Rd, dentro e acima de uma floresta de mangue. As medidas de radiação solar foram provenientes de uma torre micrometeorológica, instalada dentro de uma floresta de mangue, localizada no município de Marechal Deodoro (distante 15 km da cidade de Maceió-AL), em uma Área de Proteção Ambiental (APA), denominada APA de Santa Rita. A torre estava situada em 9° 42' 18'' S e 35° 48' 32'' W, e ficou em operação no período de outubro de 2004 a setembro de 2005. Os dados de Rd foram coletados com piranômetros CM5 da Kipp e Zonnen, os quais eram equipados com um anel de sombreamento de 10 cm de largura por 80 cm de diâmetro. Já para os dados de Rg utilizou-se o piranômetro SP - LITE da Campbell Scientific. Obteve-se também, por método empírico, o índice de transmissividade atmosférica k t (Rg/Ro), onde Ro é a radiação no topo da atmosfera. Os resultados mostraram uma grande variação entre as estações seca e chuvosa no tocante aos valores máximos alcançados pelo Rg, devido às variações das condições de transmissividade atmosférica. Internamente observou-se a grande dependência que a penetração dos raios solares tem do ângulo zenital solar (Z) com as estações do ano, onde se constatou uma redução de até 66% entre os períodos seco e chuvoso. Verificou-se também, que até 35% da radiação incidente na copa atingiu a superfície interna do mangue, e que esse valor reduziu-se para apenas 15% no período chuvoso.

Soiling and other optical losses in solar‐tracking PV plants in navarra

AbstractField data of soiling energy losses on PV plants are scarce. Furthermore, since dirt type and accumulation vary with the location characteristics (climate, surroundings, etc.), the available data on optical losses are, necessarily, site dependent. This paper presents field measurements of dirt energy losses (dust) and irradiance incidence angle losses along 2005 on a solar‐tracking PV plant located south of Navarre (Spain). The paper proposes a method to calculate these losses based on the difference between irradiance measured by calibrated cells on several trackers of the PV plant and irradiance calculated from measurements by two pyranometers (one of them incorporating a shadow ring) regularly cleaned. The equivalent optical energy losses of an installation incorporating fixed horizontal modules at the same location have been calculated as well. The effect of dirt on both types of installations will accordingly be compared. Copyright © 2010 John Wiley & Sons, Ltd.

The assessment of different models to predict the global solar radiation on a surface tilted to the south

Global and diffuse solar radiation intensities are, in general, measured on horizontal surfaces, whereas stationary solar conversion systems (both flat plate solar collector and PV) are tilted towards the sun in order to maximize the amount of solar radiation incident on the collector surface. Consequently, the solar radiation incident on a tilted surface must be determined by converting the solar radiation intensities measured on a horizontal surface to that incident on the tilted surface of interest. There exist a large number of models designed to perform such a conversion. 11 such models have been tested utilizing data measured in Beer Sheva, Israel. The data consist of hourly global and diffuse solar radiation on a horizontal surface, normal incidence beam and global radiation on a south-oriented surface tilted at 40°. The horizontal diffuse radiation measured using a shadow ring was corrected using four different correction models. This resulted in 44 model permutations. The individual model performance is assessed by an inter-comparison between the calculated and measured solar global radiation on the south-oriented surface tilted at 40° using both graphical and statistical methods. The relative performance of the different models under different sky conditions has been studied. Different grading systems have been applied in an attempt to score the relative performance of the models.

The assessment of four different correction models applied to the diffuse radiation measured with a shadow ring using global and normal beam radiation measurements for Beer Sheva, Israel

The measurement of the diffuse radiation incident on a horizontal surface, a priori a straightforward task, is fraught with difficulties. It is possible to measure the diffuse radiation by three different techniques: two of which measure it directly and the third indirectly. The most accurate is the indirect one, which is based upon the concurrent measurements of the horizontal global and the normal incidence beam radiation. The disadvantage of this being the relatively expensive tracking system required for measuring the latter. The diffuse radiation can be measured directly with a pyranometer outfitted with either an occulting disk or shadow ring, which prevent the beam radiation from impinging on the pyranometer sensor. The occulting disk can provide accurate measurements of the diffuse radiation but it requires a relatively expensive sun tracking system in the east–west axis. The shadow ring is a stationary device with regard to the east–west axis and blocks the beam radiation component by creating a permanent shadow on the pyranometer sensor. The major disadvantage of the shadow ring is that it also blocks that portion of the diffuse radiation obscured by the shadow ring. This introduces a measurement error that must be corrected to account for that portion of the sky obscured by the shadow band. In addition to this geometric correction factor there is a need to correct for anisotropic sky conditions. Four correction models have been applied to the data for Beer Sheva, Israel and the results have been evaluated both graphically and statistically. An attempt has been made to score the relative performance of the models under different sky conditions.

An anisotropic shadow ring correction method for the horizontal diffuse irradiance measurements

Diffuse irradiance is usually measured with a shadow ring which prevents beam radiation from entering the measuring instrument. The shadow ring also obscures part of the sky, and therefore, it is necessary to correct the measured diffuse irradiance. This correction is often assumed to be constant at a certain time of the year and at a certain latitude. However, the radiance distribution of the sky is not isotropic, and the anisotropy must be taken into account in the correction. In this paper, a new numerical method for calculating the shadow ring correction is presented. The correction factor is calculated directly from the radiance distribution of the sky. For Helsinki latitude (60°N), the yearly average isotropic correction factor is 1.10 and the additional anisotropic correction factor is about 1.05. Realistic sky models, e.g. Perez all-weather model, were used in the calculation.

Analysis of diffuse radiation data for Beer Sheva: Measured (shadow ring) versus calculated (global-horizontal beam) values

We have utilized concurrently measured global, normal incidence beam, and diffuse radiation data, the latter measured by means of a shadow ring pyranometer to study the relative magnitude of the anisotropic contribution (circumsolar region and nonuniform sky conditions) to the diffuse radiation. In the case of Beer Sheva, the monthly average hourly anisotropic correction factor varies from 2.9 to 20.9%, whereas the “standard” geometric correction factor varies from 5.6 to 14.0%. The monthly average hourly overall correction factor (combined anisotropic and geometric factors) varies from 8.9 to 37.7%. The data have also been analyzed using a simple model of sky radiance developed by Steven in 1984. His anisotropic correction factor is a function of the relative strength and angular width of the circumsolar radiation region. The results of this analysis are in agreement with those previously reported for Quidron on the Dead Sea, viz, the anisotropy and relative strength of the circumsolar radiation are significantly greater than at any of the sites analyzed by Steven. In addition, the data have been utilized to validate a model developed by LeBaron et al. in 1990 for correcting shadow ring diffuse radiation data. The monthly average deviation between the corrected and true diffuse radiation values varies from 4.55 to 7.92%.

The measurement and prediction of global and diffuse components of solar radiation for Kano in northern nigeria

Abstract Two Kipp and Zonen solarimeters, one of which incorporated a shadow ring, were used for the measurement of the global and diffuse components of solar radiation for Kano. The readings from the solarimeters, for a time period covering April 1985 to May 1986, were recorded by a Fluke datalogger. A calculation model, which was originally based on cloud amount but modified to be in terms of sunshine duration hours, was employed for the computation of the monthly average daily global and diffuse radiation components for Kano in northern Nigeria. The calculation model incorporates effects of multiple reflection between ground surface and the atmosphere, as well as the new value of the solar constant of 1367 W m −2 . The comparisons of the measured with the calculated radiation components showed satisfactory agreements. The calculation procedure can thus be used by practitioners of solar energy technology in designing appliances for localities that have no solar radiation measuremennts but where the sunshine duration hours exist.

Intramembrane Particles (IMPs) in Halobacterium Halobium

Whole organisms of Halobacterium halobium were frozen in liquid propane and fractured in a Balzers 360 apparatus at ~−140ÅC at ~10−7 Torr and replicated without etching. Fig. 1 is a reverse print of a micrograph of two fractured organisms. The one above shows two purple patches in the EF face and the one below one in the PF face. The area labelled “1” was used for the filtered image in Fig. 2 and area “2” for the one in Fig. 3. The arrows in Figs. 2-3 indicate the centers of adjacent repeating units of the hexagonal lattice (d ~60Å). The repeating unit in Fig. 2 is a glob of metal representing three bacteriorhodopsin (BR) molecules. Each repeating unit in Fig. 3 consists of a ring of metal surrounding a dark shadow in the center of which there is a bright spot of metal. Each dark shadow ring is the complement of each ~50Å circular glob of metal in Fig. 2. Hence it is the impression of a set of three BR molecules around a three-fold axis of the molecular lattice. Note that there is a slight depression perpendicular to the shadowing direction at the edge of the purple membrane patch (black arrow). The area labelled “3” is another purple membrane patch turned up ~45°. The regular pattern is prominent here because of the shadowing angle. Note the many large intramembrane particles that at first sight appear globular. Many of these cast rectangular shadows (inset enlargements above of particles labelled “4-10)”. Some particles, eg. #7, are linear. The angularity of the shadows shows that these particles represent water crystals (Gross, H., O. Kuebler, P. Bass and H. Moor, 1978, J. Cell Biol. , 79:646). Fig. 4 is an enlargement of a PF face showing triplets of metallic aggregates ~50Å in diameter spaced ~100Å apart. Several are encircled and the spacing indicated by arrows. Fig. 5 is an EF face in which complementary triplets can be seen as sets of three dark circular shadows (encircled to the lower left). Note the very small (<10Å) spots of metal approximately in the center of some triplet components. The center-to-center spacing of ~100Å of three adjacent triplets is indicated to the upper center. The triplet structures in the EF and PF faces are complementary. The features of the filtered image of the EF face in Fig. 3 indicate that the spots of metal in the center of the triplet impressions in Fig. 4 are significant. Fig. 6 is a diagram showing the BR lattice above with three-fold axes separated by 62Å. In the lower part of the diagram the appearance of the PF faces is shown at the same scale. Comparison shows that each triplet consists of three sets of three BR molecules. Each ~50Å metal glob in the triplet is therefore a replica of three bacteriorhodopsin molecules. We interpret the <10Å spots in the dark ring shadows in Fig. 5 and the globs of metal around the ring shadows in the EF face as lipid molecules. They are revealed by decoration and shadowing.