Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 57

The altitude of the maximal increase in wind speed above the peak for most
hills is between 2.5 and 5.0 m /2-10/, /2-14/.

Wind power. Due to the described context, the air masses within the atmosphere
are permanently moving. The kinetic energy E Wi of these moving air masses de-
pends on the air mass m Wi and the square value of the wind velocity v Wi (Equation
(2.17)).

1

2

E = m v

(2.17)

2

Wi Wi Wi

The air mass flow entering through a particular surface m Wi is defined by the
surface S it flows through, the density of the air ρ Wi and the wind velocity v Wi in
line with Equation (2.18).

dx

m = S ρ = S ρ v (2.18)

Wi Wi Wi Wi

dt

With the Equations (2.17) and (2.18) the power contained in wind P Wi can thus
be calculated (Equation (2.19)). According to that, the wind power is proportional
to the cube of the wind velocity; furthermore it depends on the density of the air
ρ Wi and the cross-sectional area S of the wind flow in question.

1 1

2 3

P = m v = S ρ v

(2.19)

2 2

Wi Wi Wi Wi Wi

2.3.2 Supply characteristics

Measuring wind direction and wind speed. The direction of wind is measured
using wind vanes aligning themselves, under wind pressure, to the respective di-
rection of the wind. The result can be transferred mechanically or electronically to
a registration instrument.
Instruments that measure the wind speed (anemometers) are distinguished as
either measuring the instantaneous or the mean value. Instruments that measure
the instantaneous value of the wind are
− Plate-anemometers, where the wind pressure directs a pendulous plate aligned
vertically to the wind direction;
− Dynamic air speed indicators, where either the pitot pressure (i.e. the pressure
at the front stagnation point of a body flowing against (Pitot-tube)) or the dy-
namic pressure (i.e. the difference between the Pitot-pressure and the static sur-
rounding pressure (Prandtl's Pitot tube)) are measured;
− Thermal anemometers, where the temperature of e.g. heating wires changes as
a result of the air masses flowing past. This change can easily be measured.