Abstract. Resource assessment for concentrated solar power (CSP) needs accurate direct normal irradiance (DNI) measurements. An option for such measurement campaigns is the use of thoroughly calibrated rotating shadowband irradiometers (RSIs). Calibration of RSIs and Si-sensors is complex because of the inhomogeneous spectral response of these sensors and incorporates the use of several correction functions. One calibration for a given atmospheric condition and air mass might not be suitable under different conditions. This paper covers procedures and requirements of two calibration methods for the calibration of rotating shadowband irradiometers. The necessary duration of acquisition of test measurements is examined with regard to the site-specific conditions at Plataforma Solar de Almería (PSA) in Spain. Seven data sets of long-term test measurements were collected. For each data set, calibration results of varying durations were compared to its respective long-term result. Our findings show that seasonal changes of environmental conditions are causing small but noticeable fluctuation of calibration results. Calibration results within certain periods (i.e. November to January and April to May) show a higher likelihood of deviation. These effects can partially be attenuated by including more measurements from outside these periods.Consequently, the duration of calibrations at PSA can now be selected depending on the time of year in which measurements commence.
ISES Solar World CongressAbu Dhabi • Introduction of subordinate standard spectra • Evaluation of applicability for Photovoltaic (PV), Concentrating Photovoltaic (CPV) and Concentrating Solar Power (CSP) • Recommendation for ISO 9845 update •ISO 9845-1:1992. Solar energy --Reference solar spectral irradiance at the ground at different receiving conditions --Part 1: Direct normal and hemispherical solar irradiance for air mass 1,5 DLR.de • Slide 2 Content Standard spectra exist (IEC 60904 / ASTM G173 / ISO 9845)• IEC/ASTM: GTI 37° tilt and DNI for clear, cloudless atmosphere, air mass 1.5 & sea level derived with SMARTS• IEC/ASTM spectra remain THE standard spectra (will be adapted by ISO 9845)• These main spectra are & stay obligatory e.g. for efficiency rating of PV However, additional subordinate standard spectra would be useful. Expectations:• Subordinate standard spectra can represent sites with average atmospheric conditions different from those of IEC spectra better • Efficiency derived from appropriate subordinate spectra for such sites more accurate than from IEC spectra DLR.de • Slide 3
Two spectra of solar direct normal irradiance (including circumsolar) are estimated based on spatio-temporal averages of the relevant atmospheric parameters extracted from two different databases: MODIS satellite sensor retrievals and AERONET sun photometer network. The satellite database is used to calculate an average spectrum for the area of the Atacama Desert. The AERONET database is used for two purposes: (i) to apply bias-removal linear methods to correct the MODIS parameters over Atacama Desert, and (ii) to calculate an average local spectrum for the Paranal station. The SMARTS radiative transfer model is used to obtain the three representative spectra developed in this study. Both the Atacama Desert and Paranal spectra are compared against each other and also to the world reference, ASTM G173. In one of the cases, significant differences are found for short wavelengths. In order to quantify the relative importance of these spectral differences, the propagation of errors due to the use of each spectrum is evaluated for CSP applications over the Atacama Desert, considering twelve different scenarios involving the reflectance, transmittance or absorptance of various materials.
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