Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 52

2.2.2 Cup anemometer
The cup anemometer is a drag device and consists typically of three cups each
mounted on one end of a horizontal arm, which in turn are mounted at equal angles
to each other on a vertical shaft. A cup anemometer turns in the wind because the
drag coeffi cient of the open face cup is greater than the drag coeffi cient of the
smooth surface of the back. The air fl ow past the cups in any horizontal direction
turns the cups in a manner that is proportional to the wind speed. Therefore, count-
ing the turns of the cups over a set time period produces the average wind speed
for a wide range of speeds.
Despite the simple geometry of an anemometer its measurement behaviour
depends on a number of different factors. One of the most dominant factors is the
so-called angular response, which describes what components of the wind vector are
measured [3]. A so-called vector anemometer measures all three components of the
wind vector, the longitudinal, lateral and vertical component. Thus this type of ane-
mometer measures independently of the infl ow angle and is less sensitive to mount-
ing errors, terrain inclination and/or thermal effects. However, for power curve
measurements the instrument must have a cosine response thus measuring only the
horizontal component of the wind [2]. Since for energy yield calculations the mea-
surement behaviour of the anemometer used for the power curve and used for
resource assessment should be as similar as possible, it is advisable to also use an
anemometer with a cosine response for resource assessment. One of the key argu-
ments for using such an instrument for power curve measurements is that the wind
turbine utilises only the horizontal component. This is, however, a very simplifi ed
approach as, particularly for large rotors, three-dimensional effects along the blades
leads to a utilisation of energy from the vertical component. Care has to be taken
when using a cosine response anemometer as it is sensitive to mounting errors.
One of the most relevant dynamic response specifi cations is the so-called over-
speeding. Due mainly to the aerodynamic characteristics of the cups, the anemom-
eter tends to accelerate faster than it decelerates, leading to an over-estimate of
wind speed particularly in the middle wind speed range.
Another dynamic response specifi cation is the response length or distance con-
stant, which is related to the inertia of the cup anemometer. The dynamic response
can be described as a fi rst order equation. When a step change of wind speed from
U to U + u hits the anemometer it will react with some delay of exponential

shape. The distance constant, i.e. the column of air corresponding to 63% recovery
time for a step change in wind speed, should preferably be a few meters or less.
Different methods to determine the response length are described in [3].

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2.2.3 Ultrasonic anemometer
Ultrasonic or sonic anemometers use ultrasonic waves for measuring wind speed
and, depending on the geometry, the wind direction. They measure the wind speed
based on the time of fl ight between pairs of transducers. Depending on the num-
ber of pairs of transducers, either one-, two- or three-dimensional fl ow can be
measured. The travelling time forth and back between the transducers is different
because in one direction the wind speed component along the path is added to the