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Blanc P.,MINES ParisTech | Espinar B.,MINES ParisTech | Geuder N.,Stuttgart University of Applied Sciences | Gueymard C.,Solar Consulting Services | And 8 more authors.
Solar Energy | Year: 2014

The direct irradiance received on a plane normal to the sun, called direct normal irradiance (DNI), is of particular relevance to concentrated solar technologies, including concentrating solar thermal plants and concentrated photovoltaic systems. Following various standards from the International Organization for Standardization (ISO), the DNI definition is related to the irradiance from a small solid angle of the sky, centered on the position of the sun. Half-angle apertures of pyrheliometers measuring DNI have varied over time, up to ≈10°. The current recommendation of the World Meteorological Organization (WMO) for this half-angle is 2.5°. Solar concentrating collectors have an angular acceptance function that can be significantly narrower, especially for technologies with high concentration ratios. The disagreement between the various interpretations of DNI, from the theoretical definition used in atmospheric physics and radiative transfer modeling to practical definitions corresponding to specific measurements or conversion technologies is significant, especially in the presence of cirrus clouds or large concentration of aerosols. Under such sky conditions, the circumsolar radiation- i.e. the diffuse radiation coming from the vicinity of the sun-contributes significantly to the DNI ground measurement, although some concentrating collectors cannot utilize the bulk of it. These issues have been identified in the EU-funded projects MACC-II (Monitoring Atmospheric Composition and Climate-Interim Implementation) and SFERA (Solar Facilities for the European Research Area), and have been discussed within a panel of international experts in the framework of the Solar Heating and Cooling (SHC) program of the International Energy Agency's (IEA's) Task 46 ". Solar Resource Assessment and Forecasting". In accordance with these discussions, the terms of reference related to DNI are specified here. The important role of circumsolar radiation is evidenced, and its potential contribution is evaluated for typical atmospheric conditions. For thorough analysis of performance of concentrating solar systems, it is recommended that, in addition to the conventional DNI related to 2.5° half-angle of today's pyrheliometers, solar resource data sets also report the sunshape, the circumsolar contribution or the circumsolar ratio (CSR). © 2014.

Reinhardt B.,German Aerospace Center | Buras R.,Ludwig Maximilians University of Munich | Bugliaro L.,German Aerospace Center | Wilbert S.,Institute of Solar Research | Mayer B.,Ludwig Maximilians University of Munich
Atmospheric Measurement Techniques | Year: 2014

Reliable data on circumsolar radiation, which is caused by scattering of sunlight by cloud or aerosol particles, is becoming more and more important for the resource assessment and design of concentrating solar technologies (CSTs). However, measuring circumsolar radiation is demanding and only very limited data sets are available. As a step to bridge this gap, a method was developed which allows for determination of circumsolar radiation from cirrus cloud properties retrieved by the geostationary satellites of the Meteosat Second Generation (MSG) family. The method takes output from the COCS algorithm to generate a cirrus mask from MSG data and then uses the retrieval algorithm APICS to obtain the optical thickness and the effective radius of the detected cirrus, which in turn are used to determine the circumsolar radiation from a pre-calculated look-up table. The look-up table was generated from extensive calculations using a specifically adjusted version of the Monte Carlo radiative transfer model MYSTIC and by developing a fast yet precise parameterization. APICS was also improved such that it determines the surface albedo, which is needed for the cloud property retrieval, in a self-consistent way instead of using external data. Furthermore, it was extended to consider new ice particle shapes to allow for an uncertainty analysis concerning this parameter. We found that the nescience of the ice particle shape leads to an uncertainty of up to 50%. A validation with 1 yr of ground-based measurements shows, however, that the frequency distribution of the circumsolar radiation can be well characterized with typical ice particle shape mixtures, which feature either smooth or severely roughened particle surfaces. However, when comparing instantaneous values, timing and amplitude errors become evident. For the circumsolar ratio (CSR) this is reflected in a mean absolute deviation (MAD) of 0.11 for both employed particle shape mixtures, and a bias of 4 and 11%, for the mixture with smooth and roughend particles, respectively. If measurements with sub-scale cumulus clouds within the relevant satellite pixels are manually excluded, the instantaneous agreement between satellite and ground measurements improves. For a 2-monthly time series, for which a manual screening of all-sky images was performed, MAD values of 0.08 and 0.07 were obtained for the two employed ice particle mixtures, respectively. © 2014 Author(s).

Wilbert S.,German Aerospace Center | Reinhardt B.,German Aerospace Center | DeVore J.,Visidyne, Inc. | Roger M.,Institute of Solar Research | And 3 more authors.
Journal of Solar Energy Engineering, Transactions of the ASME | Year: 2013

Due to forward scattering of direct sunlight in the atmosphere, the circumsolar region closely surrounding the solar disk looks very bright. The radiation coming from this region, the circumsolar radiation, is in large part included in common direct normal irradiance (DNI) measurements, but only partially intercepted by the receivers of focusing collectors. This effect has to be considered in the performance analysis of concentrating collectors in order to avoid overestimation of the intercepted irradiance. At times, the overestimation reaches more than 10% for highly concentrating systems even for sky conditions with relevant DNI above 200 W/m2. The amount of circumsolar radiation varies strongly with time, location and sky conditions. However, no representative sunshape measurements exist for locations that are now of particular interest for concentrating solar power (CSP) or concentrating photovoltaics (CPV). A new sunshape measurement system is developed and analyzed in this study. The system consists of the sun and aureole measurement instrument (SAM), an AERONET sun photometer and postprocessing software. A measurement network is being created with the presented system. The uncertainty of this system is significantly lower than what was obtained with previous devices. In addition, the spectral optical properties of circumsolar radiation are determined. As a result, the necessary information for CSP and CPV systems, and a basis for the development of modeling methods for circumsolar radiation, can now be achieved. Copyright © 2013 by ASME.

Polo J.,CIEMAT | Wilbert S.,Institute of Solar Research | Ruiz-Arias J.A.,University of Jaen | Meyer R.,Suntrace GmbH | And 13 more authors.
Solar Energy | Year: 2016

At any site, the bankability of a projected solar power plant largely depends on the accuracy and general quality of the solar radiation data generated during the solar resource assessment phase. The term "site adaptation" has recently started to be used in the framework of solar energy projects to refer to the improvement that can be achieved in satellite-derived solar irradiance and model data when short-term local ground measurements are used to correct systematic errors and bias in the original dataset. This contribution presents a preliminary survey of different possible techniques that can improve long-term satellite-derived and model-derived solar radiation data through the use of short-term on-site ground measurements. The possible approaches that are reported here may be applied in different ways, depending on the origin and characteristics of the uncertainties in the modeled data. This work, which is the first step of a forthcoming in-depth assessment of methodologies for site adaptation, has been done within the framework of the International Energy Agency Solar Heating and Cooling Programme Task 46 "Solar Resource Assessment and Forecasting". © 2016 Elsevier Ltd.

Montecchi M.,ENEA | Delord C.,French National Solar Energy Institute | Raccurt O.,French National Solar Energy Institute | Disdier A.,French National Solar Energy Institute | And 7 more authors.
Energy Procedia | Year: 2015

Mirrors are the first link in the energy-conversion chain from Sun to electricity-delivery in the grid. Shape and solar reflectance are the key-parameters of mirrors, respectively affecting how solar radiation is concentrated around the focus, and how much of the impinging solar power is reflected. In SolarPACES Task III, an expert group is drafting the solar reflectance guidelines; in order to speed up the discussion the SRRR round robin was launched at the beginning of 2013. Identical kits, each one consisting of ten specimens collected from eight cooperating producers, were distributed and measured at six research institutes, acting as evaluators. The kit includes both traditional (glass based) and innovative (first-surface) solar mirrors. The paper only reports on the simplest task among those of SRRR: the solar hemispherical reflectance measurement. Near-specular solar reflectance was also measured and compared but the results are still under investigation and are not part of this paper. The measurements were accomplished according to the guidelines. The differences among the achieved results are within the typical accuracy of spectrophotometers, demonstrating the reliability of the reflectance guidelines. The statistic of the deviations from the true value is analysed separately for each evaluator, and allows us to infer information abut the gauging-status of the adopted reference mirror, as well as the measurement reproducibility. © 2015 The Authors. Published by Elsevier Ltd.

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