117
Country R
mm
r
mm
L
mm
Slope
angle
Limit
angle
Opening
angle
Remark
Australia I 34.8 10 795 1.79 3.23 2.51 Sky solar
Australia II 34.8 5.64 795 2.10 2.91 2.51 Sky infrared
Austria 45 16 500 3.32 6.96 5.14 To calibrate star
pyranometers
Germany I 25 10 298 2.88 6.70 4.79 To fit to Linke-Feussner
pyrheliometer
Germany II 30 10 603 1.90 3.80 2.85 To fit to NIP
pyrheliometer
Germany III 30 10 687 1.67 3.33 2.50 BSRN station
diffusometer
Hungary I 25.4 10 577 1.53 3.51 2.52 SciTec-KIPP
diffusometer
Hungary II 30 10 577 1.74 3.72 2.73 Experimental 1
Hungary III 34 10 577 2.23 4.21 3.22 Experimental 2
USA 30.17 5.64 603.3 2.33 3.40 2.86 NOAA diffusometers
Table C 2.1. Geometrical data of diffusometers.
R: radius of the shading disk or sphere; r: radius of the pyranometer sensing area;
L: distance between shader and sensor. The slope, limit and opening angles are valid when the shader
is above the pyranometer (Sun at zenith)
C 2.2.1 The penumbra functions.
In the case of circular pyrheliometers and diffusometers (when rotational symmetry exists around the
optical axis of the instrument) the penumbra functions can be calculated by the relatively simple formula
of Pastiels (see Major 1994). This holds for diffusometers only then, when the Sun is in the zenith.
At lower solar elevations the rotational symmetry of diffusometers is not valid, since the radiation receiving
surface is not perpendicular to the optical axis. W ith deacreasing solar elevation the slope and limit
angles tend to the (half)opening angle (Major 1992, 1994). In this case the geometrical penumbra depends
also on the azimuth angle measured around the optical axis in the plane perpendicular to it. To calculate
the penumbra function for any position of shader numerical simulation should be used (Major 1994).
In Figures C 2.1a and C 2.1b the penumbra functions (averaged in azimuth) are shown for the
diffusometers listed in Table C 2.1. As it is seen from Figure C 2.1 the sm allest diffuse radiation is
measured by the Austrian diffusometer (largest shaded area around the Sun), while the largest one
might belong to any of a group of instrum ents (Australian I, Australian II, Germ an III, Hungarian I).
C 2.2.2 Sky functions
It is a century long effort to derive reliable (spectrally integrated) circumsolar sky functions. For clear
atmosphere calculations can be made using aerosol models. In this respect Deirmendjian (1959),
Frohlich and Quenzel (1974), Thomalla et al. (1983) and Putsay (1995) published results usable for
pyrheliometric/diffusometric purposes. Their results are identical for identical aerosol conditions. The
measurement of circumsolar radiation is more complicated than that of diffuse or direct solar radiation.
Statistically significant amount of such measurements has only been made available in 1991 by the
National Renewable Energy Laboratory (NREL), USA (Noring, Grether and Hunt). This database contains
170000 sky functions collected in eleven sites in the United States and cover a wide range of solar
elevations and atm ospheric conditions. The NREL kindly provided us with the data measured in April
and May of 1977 at Boardman (Oregon). This dataset contains about 2000 functions of radiance distribution
along the solar disc and the aureola up to 3.2 from the solar center.
0
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Komentáře k této Příručce