KEYWORDS: Solar radiation models, Data modeling, Solar radiation, Databases, Performance modeling, Atmospheric modeling, Systems modeling, Solar energy, Statistical modeling, Optical filters
Solar global and diffuse radiation intensities are, in general, measured on horizontal surfaces, whereas stationary solar
conversion systems (both flat plate solar collector and PV) are tilted towards the sun in order to maximize the amount of
solar radiation incident on the collector surface. Consequently, the solar radiation incident on a surface tilted to the south
(northern hemisphere) must be determined by converting the solar radiation intensities measured on a horizontal surface
to that incident on the tilted surface of interest. There exist a large number of models designed to perform such a
conversion. Eleven such models have been tested utilizing data measured in Beer Sheva, Israel. The data consist of
hourly solar global and diffuse radiation on a horizontal surface, normal incidence beam and global radiation on a south-oriented
surface tilted at 40°. The individual model performance is assessed by an inter-comparison between the
calculated and measured solar global radiation on the south-oriented surface tilted at 40° using both graphical and
statistical methods. The relative performance of the different models under different sky conditions, i.e., clear, partially
cloudy and cloudy as defined by the hourly clearness index value, has been studied.
KEYWORDS: Systems modeling, Solar radiation models, Data modeling, Databases, Sun, Sensors, Atmospheric modeling, Solar energy, Performance modeling, Meteorology
The measurement of the horizontal diffuse radiation, a priori a straightforward task, is fraught with difficulties. It is
possible to measure the diffuse radiation by both direct and indirect methods. The most accurate method is probably the
indirect one, which utilizes concurrent measurements of the horizontal global and the normal incidence beam radiation.
The disadvantage of this method is the relatively expensive tracking system required for measuring the latter. The
diffuse radiation can be measured directly with a pyranometer outfitted with either an occulting disk or shadow ring,
which prevent the beam radiation from impinging on the pyranometer sensor. The former method can provide accurate
measurements of the diffuse radiation but requires a relatively expensive sun tracking system in the east-west axis. The
shadow ring is a stationary device with regard to the east-west axis and blocks the beam radiation component by creating
a permanent shadow on the pyranometer sensor. The disadvantage of the shadow ring is that it also blocks a portion of
the sky, which necessitates a geometrical correction factor. There is also a need to correct for anisotropic sky conditions.
Four correction models have been applied to the data and the results evaluated and ranked.
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