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 shadow band correction for diffuse irradiation based on a two-component sky radiance model

When diffuse radiation is measured with a pyranometer and the access of beam radiation to the measuring instrument is obstructed by using a shadow band, a band correction has to be made to obtain the correct result of measurement. The magnitude of the correction depends on the size and location of the shadow band and the sky radiance distribution. A two-component sky radiance model can be utilized when making the band correction. Measurements indicate that the band correction based on the isotropic approximation of the radiance distribution leads to an underestimation of the diffuse irradiation. By using the radiance model, this systematic error can be eliminated. In measurements of high quality, the effect of the radiation reflected from the inside of the band to the pyranometer should also be taken into account.     

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.     

Solar radiation modeling and measurements for renewable energy applications: data and model quality

Measurement and modeling of broadband and spectral terrestrial solar radiation is important for the evaluation and deployment of solar renewable energy systems. We discuss recent developments in the calibration of broadband solar radiometric instrumentation and improving broadband solar radiation measurement accuracy. An improved diffuse sky reference and radiometer calibration and characterization software for outdoor pyranometer calibrations are outlined. Several broadband solar radiation model approaches, including some developed at the National Renewable Energy Laboratory, for estimating direct beam, total hemispherical and diffuse sky radiation are briefly reviewed. The latter include the Bird clear sky model for global, direct beam, and diffuse terrestrial solar radiation; the Direct Insolation Simulation Code (DISC) for estimating direct beam radiation from global measurements; and the METSTAT (Meteorological and Statistical) and Climatological Solar Radiation (CSR) models that estimate solar radiation from meteorological data. We conclude that currently the best model uncertainties are representative of the uncertainty in measured data.     

MEO shadowring method for measuring diffuse solar irradiance: Corrections based on sky cover

The present paper deals with atmospheric corrections factors proposed as a function of the atmospheric transmissivity in order to correct the diffuse solar irradiance measured with the Melo-Escobedo-Oliveira Shadowring Measuring Method (MEO shadowring Method). Global irradiance was measured by an Eppley-PSP pyranometer; direct normal irradiance by an Eppley-NIP pyrheliometer fitted to a ST-3 sun tracking device and diffuse irradiance by an Eppley-PSP pyranometer fitted to a MEO shadowring. The Solar Radiometric Laboratory at Sao Paulo State University provided the measurements during the years 1996–2005. Two correction models for diffuse solar irradiance were proposed: All Sky Correction Model (ASC Model) and Sky Cover Correction Model (SCC Model). The MBE and RMSE statistical indicators performed the validations. The correction models showed results in the same order of magnitude: ASC Model showed 0.81% deviation, while SCC Model showed 0.66% deviation. Therefore, the correction models proposed as a function of the sky covering (atmospheric transmissivity) were efficient to correct the isotropic diffuse irradiance, approaching the measured and reference diffuse irradiance less than 1%. Corrections show dependence on sky coverage and seasonality. The results presented that the sky cover corrections improve the MEO shadowring method, allowing the generation of a reliable global, direct and diffuse radiation database without high financial investments.     

On shadowband correction methods for diffuse irradiance measurements

Diffuse irradiance, G_d, is an important variable in solar resource assessment. The diffuse irradiance can be worked out from global, G, and direct, G_b, irradiance measurements, but this method involves the use of relatively expensive tracking mechanisms. Alternatively, a widely accepted technique uses a pyranometer with a shadowband. Because the shadowband screens the sensor from part of the diffuse radiation coming in from the sky, a correction must be made to the measurements. However, because of the anisotropy of diffuse radiation it is difficult to compute an exact theoretical correction. In this study we use two data sets registered in two locations in Spain. The first one consists in coincident hourly values of global, direct, and diffuse irradiance; the latter by means of shadowband. The other data set includes the same variables but as 5-minute values. Our goal is to study the necessary correction factor for diffuse irradiance measurements obtained by means of shadowband. After testing several well-known correction methods, we have developed two different correction models, using two-thirds of the hourly data set, while the remaining one-third and the whole 5-minute data set have been used for validation purposes. The last validation test suggests that our anisotropic models provide reliable corrections for conditions different than the ones where they have been developed. The results obtained by the developed models show a negligible mean bias deviation. Approximately 55% of cases present deviations lower than 5% over the mean value of diffuse irradiance.     

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.     

Experimental testing of models for the estimation of hourly solar radiation on vertical surfaces at Arcavacata di Rende

More than 55,000 data of hourly solar radiation on a horizontal surface and on vertical surfaces exposed to the south, west, north and east, measured at Arcavacata di Rende (CS), were compared with hourly radiation data calculated by various calculation models.Erbs, Reindl et al. and Skartveit et al. correlations for the split of hourly global radiation in the diffuse and beam components were used together with the isotropic sky model and three anisotropic sky models.The agreement between experimental and calculated data is generally good.     

Solar radiation on sloping surfaces with different orientations in granada, Spain

Daily radiation on slopes in usually computed by the Liu and Jordan method. Measurements of the global and diffuse radiation fluxes at various tilt and azimuth angles have been made in clear days with a pyranometer equatorially mounted upon a modified theodolite system. The resulting data have been used to evaluate the validity of the Liu and Jordan model with the assumption of an isotropic distribution of diffuse radiation in the sky. It is concluded that the model yields significantly better results when the reception surface is oriented in the azimuth of the Sun, but not for other azimuthal orientations of the slope. Although an evident anisotropy has been found in the diffuse radiation distribution, the results obtained by this modeling approach are considered sufficient for flat-plate collector applications.     

Atmospheric transparency, atmospheric turbidity and climatic parameters

Global solar and diffuse sky radiation data, measured with a Moll–Gorczynski pyranometer in a Mediterranean location, are used to investigate the diurnal and seasonal variations of the atmospheric turbidity using Linke’s factor. The analysis of the solar data, which were appropriately selected at constant solar elevations, proves that the atmospheric transparency coefficient decreases with decreasing relative atmospheric mass. This fact leads to a virtual increase of Linke’s turbidity factor with increasing solar elevation. Real diurnal and annual variations of the atmospheric turbidity are found, with a summer afternoon maximum and a winter morning minimum. The correlation between atmospheric turbidity and specific humidity shows that the summer maximum is due to the heavy water vapour content of maritime air masses, carried by the west–southwestern winds prevalent during this season. Continental dust particles, carried by the east–northeastern winds, growing due to water vapours result in high turbidity at the end of summer. The winter minimum is caused by a considerable decrease of the humidity and dust content of the continental air masses, carried by strong east–northeastern winds, prevalent during the cold period. Correlations of atmospheric turbidity with specific humidity and of diffuse radiation with atmospheric turbidity for maritime and continental air masses are derived.     

Estimation of solar radiation from Australian meteorological observations

A carefully prepared set of Australian radiation and meteorological data was used to develop a system for estimating hourly or instantaneous broad band direct, diffuse and global radiation from meteorological observations. For clear sky conditions relationships developed elsewhere were adapted to Australian data. For cloudy conditions the clouds were divided into two groups, high clouds and opaque (middle and low) clouds, and corrections were made to compensate for the bias due to reporting practices for almost clear and almost overcast skies. Careful consideration was given to the decrease of visible sky toward the horizon caused by the vertical extent of opaque clouds. Equations relating cloud and other meteorological observations to the direct and diffuse radiation contained four unknown quatities, functions of cloud amount and of solar elevation, which were estimated from the data. These were the proportions of incident solar radiation passed on as direct and as diffuse radiation by high clouds, and as diffuse radiation by opaque clouds, and a factor to describe the elevation dependance of the fraction of sky not obscured by opaque clouds. When the resulting relationships were used to estimate global, direct and diffuse radiation on a horizontal surface, the results were good, especially for global radiation. Some discrepancies between estimates and measurements of diffuse and direct radiation were probably due to erroneously high measurements of diffuse radiation.     

An assessment of a revised Olmo et al. model to predict solar global radiation on a tilted surface at Beer Sheva,Israel

A model to convert horizontal solar global radiation to that on a tilted surface is presented. It is based upon a relatively simple model proposed by [Olmo FJ, Vida J, Foyo I, Castro-Diez Y, Alados-Arboledas L. Prediction of global irradiance on inclined surfaces from horizontal global irradiance. Energy 24 (1999) 689–704]., which requires only measurements of horizontal solar radiation but was found to produce significant errors when tested with data from another site. The present model assumes the availability of databases for at least two of the three solar radiation types, viz., global, beam and diffuse. The horizontal global radiation is converted to that on a tilted surface by applying the Olmo model to the diffuse component, whereas the beam component is converted by using the geometrical relationship between the two surfaces. The original Olmo anisotropic radiation correction factor is now assumed to be a function of sky conditions. The solar radiation databases were converted to subsets corresponding to clear, partially cloudy and cloudy sky based upon clearness index values. The three anisotropic correction factors were determined by fitting to a 12-months database. The present model was then tested by applying it to a second database consisting of 24-months not involved in the model development. It was found to give better results than three highly regarded more complex models.     

Spectral and temperature correction of silicon photovoltaic solar radiation detectors

Silicon photovoltaic sensors are an inexpensive alternative to standard thermopile sensors for the measurement of solar radiation. However, their temperature and spectral response render them less accurate for global horizontal irradiance and unsuitable for direct beam and diffuse horizontal irradiance unless they can be reliably corrected. A correction procedure for the rotating shadowband radiometer, which measures all three components, based on a three-way parameterization of the solar position and sky conditions is proposed. After correction, root-mean-square errors for the global and diffuse horizontal irradiance and the direct normal irradiance are about 10, 12, and 13 W/m² in comparison with coincident, 5-minute thermopile measurements. While the numerical results are specific to the rotating shadowband instrument, the correction algorithm should apply universally.     

Evaluation of the estimation of diffuse irradiance from global and direct normal irradiance measurements

As part of a radiometer intercomparison experiment, different combinations of pyranometer and pyrheliometers were used to estimate diffuse solar irradiance on a horizontal surface. Sixteen combinations were possible with four pyranometers and four pyrheliometers. The values were intercompared and then compared to the measured values, obtained with a solar tracking/occulting disc system. The difference in estimated values using different radiometer combinations varied from 1 to 21 W/m². It was found that uncertainty in measuring the global irradiance accounted for most of the difference in estimated values. In the worst case, a 2.1 per cent difference in the global irradiance as measured by two different pyranometers caused a 12 per cent difference in estimates of the diffuse irradiance. It is shown that, if the estimated and measured values are analyzed statistically, agreement to within 1 per cent is possible.     

Performance characteristics of a perforated shadow band under clear sky conditions

A perforated, non-rotating shadow band is described for separating global solar irradiance into its diffuse and direct normal components using a single pyranometer. Whereas shadow bands are normally solid so as to occult the sensor of a pyranometer throughout the day, the proposed band has apertures cut from its circumference to intermittently expose the instrument sensor at preset intervals. Under clear sky conditions the device produces a saw tooth waveform of irradiance data from which it is possible to reconstruct separate global and diffuse curves. The direct normal irradiance may then be calculated giving a complete breakdown of the irradiance curves without need of a second instrument or rotating shadow band. This paper describes the principle of operation of the band and gives a mathematical model of its shading mask based on the results of an optical ray tracing study. An algorithm for processing the data from the perforated band system is described and evaluated. In an extended trial conducted at NREL’s Solar Radiation Research Laboratory, the band coupled with a thermally corrected Eppley PSP produced independent curves for diffuse, global and direct normal irradiance with low mean bias errors of 5.6 W/m², 0.3 W/m² and −2.6 W/m² respectively, relative to collocated reference instruments. Random uncertainties were 9.7 W/m² (diffuse), 17.3 W/m² (global) and 19.0 W/m² (direct). When the data processing algorithm was modified to include the ray trace model of sensor exposure, uncertainties increased only marginally, confirming the effectiveness of the model. Deployment of the perforated band system can potentially increase the accuracy of data from ground stations in predominantly sunny areas where instrumentation is limited to a single pyranometer.     

Evaluation of conventional and high-performance routine solar radiation measurements for improved solar resource, climatological trends, and radiative modeling

The solar renewable energy community depends on radiometric measurements and instrumentation for data to design and monitor solar energy systems, and develop and validate solar radiation models. This contribution evaluates the impact of instrument uncertainties contributing to data inaccuracies and their effect on short-term and long-term measurement series, and on radiation model validation studies. For the latter part, transposition (horizontal-to-tilt) models are used as an example. Confirming previous studies, it is found that a widely used pyranometer strongly underestimates diffuse and global radiation, particularly in winter, unless appropriate corrective measures are taken. Other types of measurement problems are also discussed, such as those involved in the indirect determination of direct or diffuse irradiance, and in shadowband correction methods. The sensitivity of the predictions from transposition models to inaccuracies in input radiation data is demonstrated. Caution is therefore issued to the whole community regarding drawing detailed conclusions about solar radiation data without due attention to the data quality issues only recently identified.     

Study of the corrective factor involved when measuring the diffuse solar radiation by use of the ring method

By comparing the diffuse solar radiation on a horizontal plane as continuously measured in Geneva over a full year by using both a fixed ring and a moving disk, we conclude that the ring corrective factor can be evaluated on the basis of simple models, but with a daily uncertainty of the order of 5 per cent. In this study, the isotropy of the diffuse radiation can be used as a reasonable approximation, even if this assumption is not fully verified. More precise measurements would require the use of a disk.     

Study of the corrective factor involved when measuring the diffuse solar radiation by use of the ring method

By comparing the diffuse solar radiation on a horizontal plane as continuously measured in Geneva over a full year by using both a fixed ring and a moving disk, we conclude that the ring corrective factor can be evaluated on the basis of simple models, but with a daily uncertainty of the order of 5 per cent. In this study, the isotropy of the diffuse radiation can be used as a reasonable approximation, even if this assumption is not fully verified. More precise measurements would require the use of a disk.     

带有平面反射板的全玻璃真空集热管表面能流分析

本文用解析方法对带平面反射板的单根太阳能真空集热管表面辐射流分布进行了分析，并得出了相应的结果。太阳直射辐射和散射辐射直接或通过反光板间接散布在真空管吸热体表面。为能更清楚地理解辐射的物理过程，在文中对不同辐射成分，不同反射方式分别进行了分析计算。