The measurement of wind speeds is usually done using a cup anemometer,
such as the one in the picture to the left. The cup anemometer
has a vertical axis and three cups which capture the wind. The
number of revolutions per minute is registered electronically.
Normally, the anemometer is fitted with a
wind vane to detect the wind direction.
Instead of cups, anemometers may be fitted
with propellers, although this is not common.
Other anemometer types include ultrasonic
or laser anemometers which detect the phase shifting of sound
or coherent light reflected from the air molecules. Hot wire
anemometers detect the wind speed through minute temperature
differences between wires placed in the wind and in the wind
shade (the lee side).
The advantage of non-mechanical anemometers
may be that they are less sensitive to icing. In practice, however,
cup anemometers tend to be used everywhere, and special models
with electrically heated shafts and cups may be used in arctic
Anemometers are a Necessity
for Wind Energy Measurement
You often get what you pay for, when you buy something. That
also applies to anemometers. You can buy surprisingly cheap anemometers
from some of the major vendors in the business. They may be OK
for meteorology, and they are OK to mount on a wind turbine,
where a large accuracy is not really important.*)
But cheap anemometers are not
usable for wind speed measurement in the wind energy industry,
since they may be very inaccurate and calibrated poorly, with
measurement errors of maybe 5 per cent or even 10 per cent.
If you are planning to build a wind farm
it may be an economic disaster if you have an anemometer which
measures wind speeds with a 10% error. In that case, you may
risk counting on an energy content of the wind which is 1.13
- 1 = 33% higher than than it is in reality. If you have to recalculate
your measurements to a different wind turbine hub height (say,
from 10 to 50 m height), you may even multiply that error with
a factor of 1.3, thus you end up with a 75% error on your energy
It is possible to buy a professional, well
calibrated anemometer with a measurement error around 1% for
about 700-900 USD. That is quite plainly peanuts compared to
the risk of making a potentially disastrous economic error. Naturally,
price may not always be a reliable indicator of quality, so ask
someone from a well reputed wind energy research institution
for advice on purchasing anemometers.
*) The anemometer on a wind turbine
is really only used to determine whether there is enough wind
to make it worthwhile to yaw the turbine rotor against the wind
and start it.