Calibration Procedures for Rotating Shadowband Irradiometers

Kurzfassung

Concentrated Solar Power (CSP) projects require accurate assessment and monitoring of the available direct beam resource. To this end, ground measurements on site are inexpendable. Pyrheliometers with thermopile sensors as specified in [ISO 9060] are the standard instruments for field measurements of direct beam irradiance. During long-term deployment however, their comparatively high accuracy can only be maintained if the window through which the direct beam enters the instrument is cleaned frequently [Geuder and Quaschning, 2006]. In the case of long-term solar resource assessments on site this can pose a problem in terms of logistics and maintenance costs. This is particularly relevant, if the measuring site is in a remote location. Rotating Shadowband Irradiometers (RSI) on the other hand have a lower accuracy to start with due to systematic errors of its photodiode sensor. However, they are less affected by soiling [Geuder and Quaschning, 2006] and provide higher accuracy in long-term measuring campaigns at sites where cleaning is not possible on a daily basis. Furthermore, they are comparatively inexpensive and do not require as much additional equipment i.e. a tracking system. A solar panel is sufficient for power supply. The systematic errors of the sensor used in RSIs are caused by cosine and temperature effects and its spectral non-uniform responsivity. A number of correction functions can be employed to reduce these errors significantly. Additionally, a thorough calibration of the sensor with application of the correction functions can further improve the quality of measurements. The calibration procedures of thermopile pyranometers and pyrheliometers are well documented in standards such as [ISO 9059], [ISO 9846] and [ISO 9847]. Because these standards are not applicable to RSIs due to their inherent characteristics specific calibration procedures for this type of instrument were developed. This work shall discuss the differences between calibration procedures for thermopile irradiometers and RSI and aspects which can be transferred from the existing standards to RSI calibration. Some of the existing calibration methods have been successfully employed for a number of years. However, a thorough assessment of the necessary calibration duration with multiple measurements from several instruments has not been carried out before this project. Therefore, besides the comparison of calibration methods this work examines to which extend the RSI calibration results fluctuate in dependence on the duration of the calibration measuring period and furthermore provide a basis on which to choose the most suitable duration.