Environmental Engineering Reference
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100%
80-100%
60-80%
40-60%
20-40%
0
80
60
40
90
70
50
γ
°
30
Sun height S
20
°
10
°
0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
k T
Figure 2.9 Diffuse Irradiance Component as a Function of k T and
S
E G,hor
E 0 sin
k T =
(2.12)
γ S
With this factor, the diffuse irradiance E diff,hor can be calculated easily using
the global irradiance E G,hor and the sun height
S :
E diff,hor = E G,hor
(1.020 - 0.254 k T + 0.0123 sin
S )
for k T
0.3
E diff,hor = E G,hor
(1.400 - 1.749 k T + 0.177 sin
S )
for 0.3 < k T < 0.78
E diff,hor = E G,hor
(0.486 k T - 0.182 sin
S )
for k T
0.78.
(2.13)
Figure 2.9 shows this correlation graphically. It is obvious that the diffuse
irradiation component is very low if the global irradiance values are high on
clear days ( k T
1); however, the diffuse irradiation component is rarely below
20 per cent. If it is very cloudy and the global irradiance is low ( k T
0), the
diffuse irradiance component can reach 100 per cent. The following section
describes methods to calculate the solar altitude angle, or sun height
S .
C ALCULATION OF THE S UN ' S P OSITION
The position of the sun is essential for many further calculations for solar
energy systems. The two angles sun height (solar altitude or elevation)
S and
solar or sun azimuth
S define the position of the sun. However, definitions for
these angles and the symbols used vary in the literature. The convention used
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