The influence of sky conditions on the standardized calibration of pyranometers and on the measurement of global solar irradiation

Global solar irradiance measurement is used for applications such as verifying satellite measurements, studying the distribution and variation of the radiation received or estimating the energy yield of solar plants. This measurement is directly influenced by the reference calibration coefficient and uncertainty of the field pyranometer used. The radiometer calibration laboratory of CENER is accredited by ENAC (Spanish National Accreditation Body) since 2010 according to ISO 9847:1992 for the calibration of field pyranometers via comparison to a reference pyranometer. The accreditation's scope covers calibrations “type Ia” (outdoor horizontal calibrations for meteorological and resource measurements, part 5.2.2, annex B.2). For the outdoor calibration, the methodology takes three different sky conditions into account: stable cloudless sky conditions, unstable sky conditions with some clouds or cloudy sky conditions. The classification criteria described by the standard have been enhanced for an easier application, establishing additional radiometric conditions, especially for unstable sky conditions with some clouds. Detailed criteria are based on CENER's experience in solar collector testing and on the experimental CENER BSRN station (Baseline Surface Radiation Network) measurements and total sky camera images. In this study, several calibration coefficient results have been analyzed in order to validate the three different methodologies regarding the sky conditions mentioned before. The calibration coefficient can be slightly different depending on the sky conditions and the calibration methodology; therefore global solar irradiance measurements can differ too. The influence of this dependency has been evaluated in order to estimate the monthly and yearly global solar irradiation.