Horizontal wind speed is the most important indicator of a site’s wind resource. For this reason, close attention should be paid to choosing appropriate anemometers for your project.
Three general anemometer types are available for measuring horizontal wind speed
- Cup Anemometer: This instrument consists of a cup assembly (three or four cups) connected to a vertical rotating shaft. The wind causes the assembly to turn in one direction. A transducer in the anemometer converts this rotational movement into an electrical signal, which is sent through a wire to the data logger. The logger measures the frequency (or magnitude) of the signal and applies a predetermined multiplier (slope) and offset (intercept) to convert the signal to a wind speed.
- Propeller Anemometer: This instrument consists of a propeller (or prop) mounted on a horizontal shaft which is kept pointing into the wind by a tail vane. Like a cup anemometer, a propeller anemometer generates an electrical signal whose frequency (or magnitude) is proportional to the wind speed. This type of anemometer tends to record slightly lower speeds than cup anemometers under turbulent conditions. This so-called “under-speeding” is caused by the prop- vane’s tendency to oscillate around the central direction or to lag behind sudden wind directional shifts, so that the propeller does not always point directly into the wind. Propeller anemometers are sometimes favoured in icing-prone climates due to their greater resistance to icing.
- Sonic Anemometer: This instrument, which does not have any moving parts, measures the wind speed and direction by detecting variations in the speed of ultrasonic sound. The geometry can be set up to measure the wind in two or three dimensions; the latter allows the measurement of vertical winds. Because it has no rotational inertia, it is more responsive to rapid speed and direction fluctuations than cup or propeller anemometers. Sonic anemometers are usually more expensive than other types, however, and may require significantly more power.
Of these, the cup anemometer is by far the most popular because of its relatively low cost and generally good accuracy, and is the default choice for island wind resource campaigns. However, both propeller and sonic anemometers can be used.
Combination of instruments for your project
- Accuracy: Some anemometers have been classified according to the standards outlined by institutions like MEASNET or the International Electrotechnical Commission (IEC).8 The performance of these anemometers, which are referred to as IEC Class I, complies with specifications for high-accuracy applications such as power curve testing. Class I anemometers are generally more expensive than standard instruments, however. A good compromise is to pair a Class I anemometer with a less expensive model, at one or more heights on the tower.
- Response to turbulence: Another distinguishing characteristic of cup anemometers is their distance constant. This is a measure of how quickly an anemometer responds to an abrupt change in wind speed. A smaller distance constant is preferred in highly turbulent environments, as otherwise the anemometer tends to “overspeed”, i.e. read too high. All cup anemometers are prone to overspeeding, but those with a large distance constant overspeed more. Anemometers commonly used for resource assessment have distance constants ranging from 2.1 m to 3.0 m.
- Response to vertical winds: In relatively steep terrain, the wind often has a significant vertical component. Since turbines respond only to the horizontal wind, it is important in such conditions to measure only the horizontal component. Class I anemometers are generally better in this respect than others, but even their behaviour is not perfect. A complimentary approach is to measure the vertical speed directly (using a vertical propeller anemometer or a 3D sonic anemometer) and then remove its effects from the cup anemometer data, assuming the anemometer’s vertical response characteristics are known.
- Icing: The build-up of ice can cause problems for any anemometer. (The same applies to direction vanes.) For cup anemometers, ice can make the shaft rotate more slowly or freeze entirely. Naturally this is not a problem for most tropical islands, but if your project is in an area that might be prone to icing, it is best to replace some of the planned anemometers with a heated anemometer. You should not use heated anemometers exclusively, however, as they are generally less accurate than unheated ones (except when there is icing), and they also draw much more power. In a typical configuration, a heated anemometer is paired with an unheated one on each of the top two levels of the mast.
- Power consumption: Most instruments intended for wind resource assessment are designed to consume as little power as possible.
However, certain types of anemometers, such as heated anemometers and sonic anemometers, consume much more than others. This is not an issue if you have access to grid power, but if you do not, the power draw needs to be considered both in the wind speed sensor choice and in the design of the power supply.