A preliminary assessment of perforated shadow band performance is given under partly cloudy and overcast conditions.
Decomposition of daily 1-minute pyranometer data into diffuse and global values and subsequent reconstitution of the
separate curves is achieved using an interpolating algorithm and a ray trace model of pyranometer exposure. Accuracy of
the reconstituted curves relative to reference data is a strong function of clearness index, K<sub>T</sub>, with a reduction in relative
error for K<sub>T</sub> < 0.3 and K<sub>T</sub> > 0.7. As expected, algorithm performance degrades under highly variable conditions, when
interpolation fails to capture the instantaneous fluctuations in irradiance associated with clouds drifting into the sun path.
A radiometric monitoring program for solar irradiance has been initiated on the Howard College campus of the
University of KwaZulu-Natal (UKZN) in Durban, South Africa. This paper describes the establishment of the broadband
ground station which employs conventional radiometers and a new type of pyranometer shadow band. The ZEBRA
(Zonal Exposure to Broadband RAdiation) band consists of a perforated metal strip that permits the separation of direct
normal and diffuse irradiance from global data with a single pyranometer. In this paper a time-based model of the new
band's shading mask is described. The model is derived from a ray tracing exercise that accounts for ZEBRA geometry
and solar position throughout a generic 365-day year. The UKZN facility lies at 29.9° South latitude and is part of a
larger test initiative for the new shadow band that includes the NREL Solar Radiation Research Laboratory in Colorado.
Data from northern and southern hemisphere test sites are to be used to characterize performance of the band under a
range of conditions and for comparison with output from the newly developed model.
A point-focus solar concentrator is described for use in high temperature terrestrial and space applications. The system
comprises a nonimaging reflective lens in the form of a ring array concentrator coupled to a fiber optic bundle through
which the concentrated light is transmitted. Unlike mirror-based systems, the concentrator is a ring array composed of
concentrically nested elements focusing energy to the rear of the structure. Energy is transported from the concentrator
by optical fibers and directed at a target where high heat flux is needed. The fiber optic bundle is tailored for solar energy
transmission and includes a fused end to minimize inter-fiber losses at the injection point. The integrated system is
intended to produce concentrated thermal energy without the need for electricity intensive heating elements. Optical
efficiency is investigated through a ray trace analysis.
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.