Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 128)

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C 2. Annex 1 to Diffuse Geometry W G Report: The effect of diffusom eter shading geom etry
Prepared by G. Major, Z. Nagy and M. Putsay for the BSRN Meeting, May 1-5, 2000, Melbourne
C 2.1 Introduction
The diffuse radiation is the solar radiation received by the horizontal surface from the above 2p solid
angle except the solid angle of solar disk. To exclude the solar disk from the whole sky som e kind
of shading device should be used. Several years ago, when com puter controlled solar trackers were
not available, shading rings were used. These rings covered m uch larger part (varying with the solar
declination) of the sky than the solar disk, therefore so called “ring correction” had to be applied to
the m easured diffuse radiation values. The m odern solar trackers keep both the pyrheliom eters and
the shading disks or shading spheres of diffusom eters in the proper position, that is they m ove altogether
with the apparent m ovem ent of the Sun. However the shading devices cover som ewhat larger solid
angle than that of the solar disk (but m uch less than the shading rings), m oreover this extra coverage
m ight be different for different diffusom eters, this way the scattered solar radiation com ing from the
covered (circum solar) part of the sky m ight also be different.
The purpose of this work is to give estimation of the difference of diffuse radiation values
measured by diffusometers of different geometry.
For this end geom etrical data of som e diffusom eters have been collected. The num ber of involved
diffusom eters is not large, but it is believed that their range covers that of the m ost frequently used
ones.
The irradiance arising from the circum solar radiation can be calculated knowing the geom etry of the
instrum ent and the distribution of circum solar radiance (sky function). Since the m easurem ent of the
latter one is not a com m on practice, therefore m easurem ents were m ade in Budapest by using 3 shading
sphere of different size, to obtain a 18 m onths tim e series that contains large range of possible natural
environm ental conditions.
Geometrical data of diffusometers
The data were provided in 1997 and 1998 by the colleagues operating the instrum ents in the countries
nam ed in Table C 2.1 and by John Hickey of Eppley Co. The used pyranom eters are: KippZonen (10
m m ), Eppley (5.64 m m ) and Star (16 m m ). Larger variability is seen in the case of shaders and arm
lengths. The shaded solid angle is characterized by the slope and lim it angles (the (half)opening angle
is their m ean value) calculated for the “vertical position”, that is when the Sun would be in the zenith.
Their slope angles are larger than that of m ost pyrheliom eters (Major 1995), this is due to that, that
the shaders should be larger than the (outer) glass dom e of the pyranom eters used in the diffusom eter
set. On the other side, the long arm of the disffusom eters results less lim it angles than that of m odern
pyrheliom eters. In m ost cases it is seen that the dim ension of diffusom eters were determ ined so, that
the (half)opening angle would fit to that of one of the standard pyrheliom eters. The two experim ental
shading spheres used in the Hungarian diffusom eters have been designed so that they would cover
the expected range of geom etry of BSRN station diffusom eters. The sam e size shading sphere and
shading disk have alm ost the sam e effect (Major 1994).
C 2.2 The calculation method
To calculate the irradiance m easured by a diffusom eter from the circum solar belt, we have to know
the circum solar sky function and the penum bra function of the instrum ent (Pastiels method). The penum bra
function describes the fraction of the sensing surface seen from a given direction. For the directions
that decline less from the optical axis (pointed to the solar center) than the slope angle, the penum bra
function is equal to 1 (the whole sensor is seen). For the directions that decline m ore from the optical
axis than the lim it angle, the penum bra function is equal to 0 (no part of the sensor is seen). The
circum solar sky function describes the distribution of radiance around the Sun (up to som e degrees
from the solar center).