Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 49

a surface 2-axis-tracked surface. In winter only little less solar radiation is inci-
dent on the vertical Southern surface.

250

2-axes tracking

200

Horizontal

150

South 45°

Ea st W e st

100

South

50

North

0

Fig. 2.21 Global radiation incident on surfaces with various alignments in Central Europe
(Climate Graz/Austria) (see /2-14/)

2.3 Wind energy

In addition to the global water cycle, solar radiation also maintains the movement
of the air masses within the atmosphere of the earth. Of the total solar radiation
incident on the outer layer of the atmosphere, approximately 2.5 % or 1.4 10 20 J/a
are utilised for the atmospheric movement. This leads to a theoretical overall wind
power of approximately 4.3 10 15 W. The energy contained in the moving air
masses, which for example can be converted into mechanical and electrical energy
by wind mills, is a secondary form of solar energy. The aim of the following dis-
course is to show the main basic principles of the supply in wind energy and to
discuss its supply characteristics.

2.3.1 Principles

Mechanisms. Wind is generated as equalising currents, essentially as a result of
varying temperature levels on the surface of the earth, by which differences in air
pressure have been created. The air masses then flow from higher pressure areas
to lower pressure areas.
The so-called gradient force caused by the pressure gradient between such a
high and a low-pressure zone impacts on an air particle. Additionally, the Coriolis
force impacts on each particle within a rotating reference system. This Coriolis
force is always vertical to the direction of movement and to the rotational axis.
If there is a large pressure difference at great altitudes, an air particle exposed
to this pressure difference starts moving from a point of higher air pressure to a