Title-Wind as alternative energy source
|The measurement of wind speeds -Anemometres|
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 1.1 3 - 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.
|Wind Speed Measurement in Practice|
|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. By fitting the anemometer to the top of the mast one minimises the disturbances of airflows from the mast itself. 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.
Anemometer, pole and data logger (mentioned below) will usually cost somewhere around 5,000 USD.
|NRG data logger Photograph by Soren Krohn © 1998 DWIA|
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 to the left. Once a month or so you may need to go to the logger to collect the chips and replace them with blank chips for the next month's data. (Be warned: The most common mistake by people doing wind measurements is to mix up the chips and bring the blank ones back!)
|If there is much freezing rain in the area, or frost from clouds in mountains, you may need a heated anemometer, which requires an electrical grid connection to run the heater.|
|10 Minute Averages|
|Wind speeds are usually measured as 10 minute averages, in order to be compatible with most standard software (and literature on the subject). The result for wind speeds are different, if you use different periods for averaging .|
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