International audienceThe 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 sig-nificant, especially in the presence of cirrus clouds or large concentration of aerosols. Under such sky conditions, the circumsolar radi-ation—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 sys-tems, it is recommended that, in addition to the conventional DNI related to 2.5° half-angle of today's pyrheliometers, solar resource ScienceDirect Solar Energy 110 (2014) 561–577 data sets also report the sunshape, the circumsolar contribution or the circumsolar ratio (CSR)

This paper presents some first results of the effect of dust on different solar mirror materials in Morocco. For this purpose, thirteen glass and aluminum mirrors were exposed at the University of Oujda (Eastern Morocco) for a period of three months (from April to June). For each type of the mirrors, one sample mirror was installed horizontally and twelve mirrors were installed in the so called “mirrors sphere” in four different directions (North, South, East and West) with three tilt angles relative to the vertical plane: +45°(facing the sky), 0° (vertical) and -45° (facing the ground). The drops in specular reflectance were measured after each month of exposition for both mirror materials. Furthermore, X-ray diffraction analyses were performed for the dust collected from the mirrors to identify the mineralogical components of the dirt settled on the mirror surfaces. For both mirror materials, the drop on cleanliness per time interval was quite the same for all the mirrors and over all the test periods. The highest average cleanliness drop per month for the horizontal mirrors was 45 % and 33 % for the glass and aluminum mirrors respectively. The +45° mirrors come in the second position with a cleanliness drop of about 14 % for both reflectors. However, the mirrors installed on the 0° and -45° angles remained cleaner with a cleanliness average of about 97 % for both mirrors. These first results can be of high importance to motivate scientists and actors in the field of solar energy to further investigate the soiling of CSP mirrors and add it as a parameter in solar resource assessment

The solar field is the major cost component of a solar thermal power plant and the optical quality of the concentrators has a significant impact on the field efficiency and thus on the performance of the power plant. Measuring slope deviations in the parabolic shape of the mirror panels in the accuracy and resolution required for these applications is a challenge as it is not required with the same characteristics in other industries. Photogrammetry and deflectometry are the two optical measurement methods that are typically used to measure this shape accuracy of mirror panels used in CSP applications. They have been compared and validated by measuring a typical mirror panel under optimal conditions. Additionally, a flat water surface has been measured as an absolute reference object using deflectometry. The remaining deviations between the results of both methods and to the reference object are discussed and possible sources of errors during the measurement are identified. A detailed error analysis is conducted for both methods and compared to the experimental findings. The results show that both methods allow for surface slope measurement with the necessary accuracy for present CSP applications and that among the two, deflectometry exhibits advantages in speed, measurement accuracy, and spatial resolution. However, for obtaining correct results several sources of errors have to be addressed appropriately during measurement and postprocessing.

Losses of reflected Direct Normal Irradiance due to atmospheric extinction in concentrated solar tower plants can vary significantly with site and time. The losses of the direct normal irradiance between the heliostat field and receiver in a solar tower plant are mainly caused by atmospheric scattering and absorption by aerosol and water vapor concentration in the atmospheric boundary layer. Due to a high aerosol particle number, radiation losses can be significantly larger in desert environments compared to the standard atmospheric conditions which are usually considered in ray-tracing or plant optimization tools. Information about on-site atmospheric extinction is only rarely available. To measure these radiation losses, two different commercially available instruments were tested, and more than 19 months of measurements were collected and compared at the Plataforma Solar de Almería. Both instruments are primarily used to determine the meteorological optical range (MOR). The Vaisala FS11 scatterometer is based on a monochromatic near-infrared light source emission and measures the strength of scattering processes in a small air volume mainly caused by aerosol particles. The Optec LPV4 long-path visibility transmissometer determines the monochromatic attenuation between a light-emitting diode (LED) light source at 532 nm and a receiver and therefore also accounts for absorption processes. As the broadband solar attenuation is of interest for solar resource assessment for concentrated solar power (CSP), a correction procedure for these two instruments is developed and tested. This procedure includes a spectral correction of both instruments from monochromatic to broadband attenuation. That means the attenuation is corrected for the time-dependent solar spectrum which is reflected by the collector. Further, an absorption correction for the Vaisala FS11 scatterometer is implemented. To optimize the absorption and broadband correction (ABC) procedure, additional measurement input of a nearby sun photometer is used to enhance on-site atmospheric assumptions for description of the atmosphere in the algorithm. Comparing both uncorrected and spectral- and absorption-corrected extinction data from 1-year measurements at the Plataforma Solar de Almería, the mean difference between the scatterometer and the transmissometer is reduced from 4.4 to 0.57 %. Applying the ABC procedure without the usage of additional input data from a sun photometer still reduces the difference between both sensors to about 0.8 %. Applying an expert guess assuming a standard aerosol profile for continental regions instead of additional sun photometer input results in a mean difference of 0.8 %. Additionally, a simulation approach which just uses sun photometer and common meteorological data to determine the on-site atmospheric extinction at surface is presented and corrected FS11 and LPV4 measurements are validated with the simulation results. For T1 km equal to 0.9 and a 10 min time resolution, an uncertainty analysis showed that an absolute uncertainty of about 0.038 is expected for the FS11 and about 0.057 for the LPV4. Combining both uncertainties results in an overall absolute uncertainty of 0.068 which justifies quite well the mean RMSE between both corrected data sets. For yearly averages several error influences average out and absolute uncertainties of 0.020 and 0.054 can be expected for the FS11 and the LPV4, respectively. Therefore, applying this new correction method, both instruments can now be utilized to sufficiently accurately determine the solar broadband extinction in tower plants.