Wind speed measurements
The measurement of wind speeds is usually done using a cup anemometer, such as the one in the picture. 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 where there is much freezing rain in the area, or frost from clouds in mountains. The heated anemometer requires an electrical grid connection to run the heater.
Quality anemometers are a necessity for wind energy measurements
Often you get what you pay for when you buy something and this also applies to anemometers. Cheap anemometers may be suitable for meteorology and they are suitable to mount on a wind turbine, where a large accuracy is not really important (the anemometer on a wind turbine is only used to determine whether there is enough wind to make it worthwhile to yaw the turbine rotor against the wind and start it). 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% or even 10%.
For the estimation of the energy output from a planned wind farm it may be an economic disaster if an anemometer which measures wind speeds with a 10% error will be used. In that case, there is a risk on counting on an energy content of the wind which is 1.13 - 1 = 33% higher than than it is in reality. If the measurements are recalculated for a different wind turbine hub height, say from 10 to 50 m height, then that error may be multiplied with a factor of 1.3, thus finally there is a 75% error on the energy calculation.
It is possible to buy a professional, well calibrated anemometer with a measurement error around 1%. That is quite plainly peanuts compared to the risk of making a potentially disastrous economic error. However, it should be noted that price may not always be a reliable indicator of quality.
Measurements in practice and towers for anemometers
The best way of measuring wind speeds at a prospective wind turbine site is to fit an anemometer to the top of a mast which has the same height as the expected hub height of the wind turbine to be used. This way one avoids the uncertainty involved in recalculating the wind speeds to a different height. Moreover, by fitting the anemometer to the top of the mast the disturbances of airflows from the mast itself are minimised. If anemometers are placed on the side of the mast it is essential to place them in the prevailing wind direction in order to minimise the wind shade from the tower.
Guyed, thin cylindrical poles are normally preferred over lattice towers for fitting wind measurement devices in order to limit the wind shade from the tower. The poles come as kits which are easily assembled, and you can install such a mast for wind measurements at (future) turbine hub height without a crane.
Data Logging and 10 min averages
The data on both wind speeds and wind directions from the anemometer(s) are collected on electronic chips on a small computer, a data logger, which may be battery operated for a long period. An example of such a data logger is shown in the picture. Once a month or so it is necessary to go to the logger to collect the chips and replace them with blank chips for the next month's data. Note that the most common mistake by people doing wind measurements is to mix up the chips and bring the blank ones back!
Wind speeds are usually measured as 10 min averages, in order to be compatible with most standard software and literature on the subject. The result for wind speeds are different, if different periods for averaging are used, as will be sawn later.
- Wind measurements (A): instrumentation
- Wind measurements (B): quantities to be recorded