Environmental Engineering Reference
In-Depth Information
Table 2.5 Monthly Average Values in kWh/(m 2 day) of the Daily Global
Irradiation
Bergen Berlin London Rome LA Cairo Bombay Upington Sydney
Norway Germany UK Italy US Egypt India RSA Australia
Latitude 60.40°N 52.47°N 51.52°N 41.80°N 33.93°N 30.08°N 19.12°N 28.40°S 33.95°S
Jan
0.20
0.61
0.56
1.70
2.88
3.09
4.74
8.08
6.41
Feb
0.72
1.14
1.10
2.54
3.97
4.00
5.56
7.45
5.57
Mar
1.71
2.44
2.07
3.78
5.14
5.15
6.29
6.26
4.72
Apr
3.27
3.49
3.04
4.99
6.47
6.27
6.72
5.19
3.47
May
4.13
4.77
4.12
6.03
6.55
7.03
6.77
4.26
2.63
June
4.85
5.44
4.99
6.59
6.57
7.56
4.99
3.72
2.38
July
4.15
5.26
4.38
6.86
7.38
7.34
3.84
4.04
2.52
Aug
3.49
4.58
3.62
6.16
6.82
6.76
3.86
4.95
3.47
Sep
1.86
3.05
2.71
4.69
5.26
5.87
4.65
6.09
4.66
Oct
0.94
1.59
1.56
3.29
4.24
4.69
5.11
7.21
5.63
Nov
0.30
0.76
0.81
2.02
3.22
3.45
4.73
8.27
6.40
Dec
0.12
0.45
0.47
1.51
2.72
2.86
4.46
8.49
6.69
Average 2.15
2.81
2.46
4.19
5.10
5.34
5.14
6.17
4.55
Source: data from Palz and Greif, 1996; NASA, 2003
I RRADIANCE ON A H ORIZONTAL P LANE
As described above, solar radiation is scattered and reflected when passing
through the atmosphere. Rays of extraterrestrial sunlight are virtually parallel.
Terrestrial sunlight, on the other hand, consists of direct and diffuse
components (see Figure 2.6). Direct solar radiation casts shadows, because it
is directional, coming directly from the sun; diffuse irradiation, on the other
hand, has no defined direction. The total irradiance on a horizontal surface on
Earth is also called global irradiance E G,hor . It is the sum of the direct irradiance
E dir,hor and the diffuse irradiance E diff,hor on the horizontal surface:
E G,hor = E dir,hor + E diff,hor
(2.11)
Table 2.6 shows the monthly average daily direct and diffuse irradiation in
Berlin and Cairo. In Berlin the diffuse irradiation dominates, whereas the direct
irradiation is much higher in Cairo, even in winter.
The annual diffuse irradiation does not have to vary greatly between
locations despite high differences in the annual global irradiation (see Table
2.7). Upington in South Africa and London in the UK have annually about the
same amount of diffuse irradiation even though the annual global irradiation
in Upington is more than twice that of London. Regions with high air pollution
or tropical regions have a significantly elevated contribution of diffuse
irradiation. However, differences in annual direct irradiations are much higher.
For example, the annual direct irradiation in Upington is almost five times
that of London.
 
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