How the Wind Winds.

An anemometer is used to measure wind speed. It is a common weather station instrument, and is frequently used in locating prime sites for constructing wind turbines or entire wind farms. Anemometers can measure both wind speed and pressure, but since the two are proportional, measuring one will give the other. Anemometers have increased in efficiency and accuracy over the years, and come in many shapes and sizes, including the original cup anemometers, as well as windmill, hot-wire, Laser Doppler, sonic, plate, and tube anemometers. The following is a comprehensive look at the anemometer and its usefulness in detecting prime locations for the construction of wind farms.

4 Cup Anemometer
Figure 1: 4 Cup Anemometer
The earliest written reference to an anemometer was in 1450 by Italian art architect Leon Battista Alberti, and was simply a disk placed perpendicular to the wind (Bellis 2011). It wasn’t until recently, however, that they were used on a large scale to locate prime areas to construct wind farms. The simplest and most well-known anemometer is the hemispherical cup anemometer, and was invented by Dr. John Thomas Romney Robinson in 1846 (Bellis 2011). There was a fair amount of error in this original design, however, and the readings changed frequently. Eventually a three cup anemometer with far less error was invented and improved.
Sonic Anemometer
Figure 2: Sonic Anemometer

 

A much more modern device for measuring wind speed is the sonic anemometer. This version measures sound waves traveling between a pair of transducers and how much they are sped up or slowed down by the effect of the wind. “The sonic anemometer was invented by geologist Dr. Andreas Pflitsch in 1994” (Bellis 2011). Sonic anemometers don’t work well in all conditions however, as rain drops can affect the speed of sound. Overall, there are many different anemometers, but there are none as cheap and easy to use as the three cup anemometer. Although it may not be the most high tech and accurate, it gives good enough readings for analysis of a potential site for harnessing energy.

The original anemometer was made of four hemispherical cups each mounted on one end of four horizontal arms, which in turn were mounted at equal angles to each other on a vertical shaft. The three cup anemometer used a similar system. Three cup anemometers are being used as the current industry standard for wind resource assessment studies, so it can be inferred that they are simple designs and can take a lot of punishment from the elements without breaking down (Wikipedia 2011).

 

3 cup anemometer diagram
Figure 2: 3 Cup Anemometer http://www.jayashree.co.in/images/anemometer_clip_image002.gif

To calculate wind speed, one finds the radius of the distance between the center of a cup and the center of the machine. The circumference is then calculated, and wind speed is then calculated by multiplying it by the number of revolutions, and dividing by the time it took to do that number of revolutions. For a more accurate calculation, a coefficient of friction for the materials used on the joint that rotates can be factored into the equation. In most cases, it is impractical to observe the system manually, so a tachometer is used to calculate revolutions. Think about a bicycle computer. A magnet is attached to the spokes, and a reader is connected to the fork. Whenever the magnet passes the receiver, it counts a revolution. The same principle is involved when a tachometer calculates the revolutions per unit of time on an anemometer.

The anemometer is the foremost tool in analyzing a wind resource. “Modern wind resource assessments have been conducted since the first wind farms were developed in the late 1970s. The methods used were pioneered by developers and researchers in Denmark, where the modern wind power industry first developed” (Wikipedia (1) 2011). The best improvements to the cup anemometer were by Brevoort & Joiner in 1935 and led to a cupwheel design which was linear and had an error of less than 3% up to 60 mph (Wikipedia (1) 2011). These improvements have led to a more efficient system and with GIS software and other mapping technology, more areas can be patterned and the measuring tools can be moved to unmapped areas (Wikipedia (1) 2011).

Typically, the following steps are taken when analyzing a potential wind farm site.
If the site is large, there will be multiple meteorological towers installed. Towers are located a few km apart for accurate readings. Least squares linear regressions are usually used along with other methods to calculate long term variances by borrowing data from the closest weather stations. At this point, vertical shear is calculated to extrapolate measured wind speeds to turbine hub height. The next step is to use mapping software. "Wind speeds can vary considerably across a wind farm site if the terrain is complex (hilly) or there are changes in roughness (the height of vegetation or buildings). Wind flow modeling software, based on either the traditional WAsP linear approach or the newer CFD approach, is used to calculate these variations in wind speed" (Wikipedia (2) 2011). The gross energy production is then calculated using the wind turbine manufacturer's power curve (Wikipedia (2) 2011)

The anemometer is used everywhere a potential source of wind energy is being assessed. They can frequently be seen on meteorological towers also equipped with wind vanes, and often along with temperature, pressure, and relative humidity sensors. They are small and can be transported, assembled, and disassembled with relative ease, making them easy to use in “the field.” This technology has been improved greatly, and has had a lot of time to progress, but there is always room for improvement. At the moment, it’s the cheapest and most efficient way to measure wind speed, and will consistently be used as such for future projects.

For further study, try building one! Follow this link to directions on how to construct a simple four-cup anemometer: <http://inventors.about.com/od/lessonplans/ht/anemometer.htm>

To calculate wind speed, follow these directions: <http://www.ehow.com/how_6571137_calculate-wind-speed-rotor-rpm.html>

References

1. (2011) “Anemometer.” Wikipedia: The Free Encyclopedia, 7 November 2011 <http://en.wikipedia.org/wiki/Anemometer>

2. (2011) “Wind resource assessment.” Wikipedia: The Free Encyclopedia, 28 October 2011 <http://en.wikipedia.org/wiki/Wind_resource_assessment>

3. Bellis, Mary (2011). “History of the Anemometer.” About.com, Inventors <http://inventors.about.com/od/astartinventions/a/Anemometer.htm>

4. Bellis, Mary (2011). “How to Make an Anemometer - Weather Vane” About.com, Inventors <http://inventors.about.com/od/lessonplans/ht/anemometer.htm>

5. Figure 1: Image ID: wea00920, NOAA's National Weather Service (NWS) Collection Photographer: Archival Photograph by Mr. Sean Linehan, NOS, NGS <http://www.photolib.noaa.gov/htmls/wea00920.htm>

6. Figure 2: 2007-02-13 08:23 Gillinstruments 200×259×??? (17618 bytes) WindMaster 3D Anemometer<http://en.wikipedia.org/wiki/File:WindMaster.jpg>

7. Figure 3: (2008) anemometer_clip_image002.gif, Jayashree Electron: Others/Anemometer <http://www.jayashree.co.in/images/anemometer_clip_image002.gif>