Application of Whole Sky Imagers for Data Selection for Radiometer Calibration

Abstract

The accurate measurement of shortwave solar irradiance is essential for the planning and operation of photovoltaic plants. The measurement uncertainty caused by the calibration is the most relevant contribution to the overall uncertainty of radiometers. During the outdoor calibration of radiometers the meteorological conditions have to be observed carefully and data points collected under unfavorable conditions must be rejected. The rejection criteria concern the level of the involved irradiances, wind speed and –direction, and also the cloud conditions. The latter are quantified in terms of the minimum angular distance of clouds from the sun and are usually determined visually by an observer. A purely manual visual determination of the cloud’s angular distance from the sun is time consuming and often inaccurate. We developed an algorithm for whole sky imagers (WSIs) that allows the determination of the minimum angular distance of clouds from the sun and the automatic filtering of the calibration data. The high importance of the cloud filtering for pyranometer calibration is shown with an exemplary calibration result for nine pyranometers. The effect of the cloud filtering for pyrheliometer calibration has shown to be negligible for this example. The filtering method increases the calibration quality and noticeably reduces the personnel effort for pyranometer calibration.