Passive separation of global irradiance into direct normal and diffuse components

Common methods for ground-based measurement of direct normal and diffuse solar irradiance include the simultaneous use of two instruments, usually a pyrheliometer and pyranometer or two pyranometers one of which is fitted with a shading ring. This article describes a passive method of obtaining the direct and diffuse components using a single pyranometer and an innovative shading band containing regularly spaced perforations to allow for alternate shading and exposure of the instrument's sensor as the sun transits the sky. Under clear sky conditions a saw tooth curve is generated that may be reformed into two distinct curves, one each for global and diffuse irradiance. The unknown direct normal values are then readily calculated. The approach potentially offers a cost advantage over dual-instrument and rotating band systems and an accuracy advantage over the single-instrument approach. In conjunction with a reference pyrheliometer under clear sky conditions, the device can be used in shade-unshade calibrations of pyranometers without need of manual operations. Design of the shading band is described and preliminary experimental results are presented. Results show that good accuracy is obtainable, on the order of ± 40 Watts per square meter for global, diffuse and direct estimates, under clear sky conditions, when compared with independent reference data.