Wind Speed
Measurement:
Anemometers
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 areas.
Quality
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 calculation.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.
© Copyright 2000 Soren Krohn. All rights reserved.
Updated 6 August 2000
wndspeed.htm
