Environmental Engineering Reference
In-Depth Information
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