Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 35

The proportion of diffuse and direct radiation within the overall global radia-
tion incident on a particular spot is subject to daily and seasonal fluctuations.
Fig. 2.9 therefore shows, as an example, the annual course of direct, diffuse and
global radiation at one particular site in South Germany. According to that, the
annual mean share of diffuse radiation in Central European regions considerably
exceeds the amount of direct radiation. During the winter months, global radiation
almost exclusively consists of diffuse radiation. In summer, the share of direct
radiation increases significantly, but is on average always smaller than the share
of diffuse radiation. This can be completely different in other places throughout
the world. In the desert, the share of direct radiation is in most cases very high. On
the other hand in regions with a very high precipitation and/or lots of fog, the
contribution of diffuse radiation to the overall global radiation might be well
above 80 %. Also, this might vary dramatically according to different times of the
day and/or different seasons of the year.

210

Global
radiation

Direkt-
strahlung
Diffus-
strahlung

Direct
radiation
Diffuse
radiation

180

150

120

90

60

30

0

Time in months

Fig. 2.9 Annual course of diffuse, direct, and global radiation, exemplary for a site in
South Germany (see /2-3/)

Direct radiation on tilted, aligned surfaces. The direct radiation incident on a
tilted surface is determined by its incident angle ψ (Fig. 2.10). This angle in turn is
dependent on the alignment and the location of the receiving surface and the posi-
tion of the sun (Equation (2.6)).
Here α is the slope or tilt angle of the surface (horizontal 0), β the surface azi-
muth angle (i.e. diversion from the South alignment, south 0, west positive), φ the
latitude (north positive), δ the solar declination and ω h the hourly angle of the sun;
this angle is at 0° when the sun is at its highest position and is negative in the
morning and positive in the afternoon.