Best Practices for Solar Irradiance Measurements with Rotating Shadowband Irradiometers

Abstract

Large-scale solar plant projects require diligent solar resource assessments. For concentrating solar technologies the focus of the resource assessment lies on direct beam irradiation. Unfortunately, high accuracy irradiance data are scarcely available in regions which are attractive for solar energy applications. Satellite data can only be used in combination with ground data to estimate inter-annual variability and long-term mean values. Hence, new ground measurements have to be collected for solar plant projects. Ground measurement data usually show significantly higher accuracies than satellite derived irradiance data, when general guidelines regarding site selection and preparation, instrument selection and maintenance and data quality monitoring are respected. These best practices for Rotating Shadowband Irradiometers (RSIs) are presented in this document. Appropriate irradiance sensors for ground measurements must be selected in consideration of general surrounding conditions for equipment and maintenance to gain and maintain the necessary accuracy over the entire operation period. Thermopile instruments like pyrheliometers as specified in ISO standard 9060 [ISO9060 1990] are severely affected by soiling [Pape2009] and also require expensive and maintenance- intensive support devices such as solar trackers and power supply. Thus, the uncertainty of resource assessment with pyrheliometers depends heavily on the maintenance personnel and cannot be determined accurately in many cases. Due to their low soiling susceptibility, low power demand, and comparatively lower cost, Rotating Shadowband Irradiometers (RSI) show significant dvantages over the thermopile sensors when operated under the measurement conditions of remote weather stations. RSIs are also known as RSP (Rotating Shadowband Pyranometers) or RSR™ (Rotating Shadowband Radiometers). Here we use the notation RSI to refer to either instrument measuring irradiance by use of a rotating shadowband following the decision of the International expert group in IEA Solar Heating and Cooling Task 46, subtask B. The initially lower accuracy of RSIs, which can yield deviations of 5 to 10 % and more, is notably improved with proper calibration of the sensors and corrections of the systematic deviations of its response. Main causes of the systematic deviations are the limited spectral sensitivity and temperature dependence of the Si-photodiode commonly used in most RSIs. Besides the systematic deviations of the sensor response, a significant contribution to the measurement inaccuracy originates from the sensor calibration at the manufacturer, where no corrections are applied. For proper calibration however, the proposed corrections need yet to be considered in the calibration procedure. While well documented standards exist for the calibration of pyrheliometers and pyranometers ([ISO9059 1990], [ISO9846 1993], [ISO9847, 1992]) they cannot be applied to RSIs and no corresponding standards exist for RSIs This document contains RSI specific best practices for the following tasks: - Requirements on the selection of a location for a measurement station - Installation, operation and maintenance of a measurement station, including the case of remote sites - Documentation and quality control of the measurements - Correction of systematic errors & instrument calibration: procedure and frequency Also the performance and accuracy of RSIs are described.