A shadow-ring device for measuring diffuse solar radiation on a vertical surface in a tropical zone

Abstract

A shadow-ring device is developed for use with a pyranometer to measure diffuse solar radiation incident on a vertical surface. The shadow-ring device is operated by affixing it and moving the pyranometer horizontally to account for the seasonal variation of the sun’s path. The correction factor for the measured diffuse solar radiation incident on a vertical surface facing different directions under isotropic sky conditions is developed to account for the amount of diffuse solar radiation blocked by the shadow-ring. The effect of the installation location and ground reflectance on the shadow-ring device’s performance are also investigated. The shadow-ring device is specially designed for use at a location in a tropical zone (i.e. Bangkok (latitude 13.73°N)), and is capable of measuring diffuse solar radiation incident on a vertical surface facing in any direction. The constructed shadow-ring device is tested to verify its accuracy. It was found that the shadow-ring device’s performance is quite satisfactory and the difference between the corrected diffuse solar radiation and the referenced diffuse solar radiation (measured global solar radiation (in the morning)) was in the range around 0.8–8.8% on the tested days which were well within the range of the uncertainty of the measured data.