One of the key considerations when assessing the quality of an anemometer is accuracy. If workforce safety is of crucial importance to your day-to-day work, it’s important to know that wind measurement will give you the accurate and reliable data you need to make critical decisions.
Anemometers measure wind speed, and the three-cup anemometer is currently the most popular configuration. You can see more on how to measure wind speed here. Three-cup anemometers offer a combination of constant torque, fast response, ruggedness and accuracy, making them a reliable tool.
Wind Forecast or Wind Measurement?
Before shortlisting devices, it’s important to make sure its accuracy is suitable for the application at hand. Wind speed itself is not always the main concern in measurement, but rather the forces it can generate.
- Wind forces are proportional to speed squared. If the wind becomes twice as fast, drag forces are multiplied by four. Wind loads are a very important consideration in structural design for example.
- The power extracted by a wind turbine is proportional to speed cubed. In this case, the effect is even more drastic, since a wind turbine extracts eight times as much energy if the wind becomes twice a fast.
Any errors in wind speed measurement are amplified to the power of 2 or 3. This is dependent on the application, making accuracy very important.
If wind force is overestimated when designing a building, the structure can end up being much more expensive than necessary. In the case of wind turbines, overestimating the energy output can result in a much lower financial return than originally calculated.
Measuring the Accuracy of a Cup Anemometer
Anemometer accuracy can vary greatly from device to device. Under normal conditions, a cup anemometer offers an accuracy ranging from 2 to 4%. But even lower values are possible with highly calibrated instruments.
Another measure of quality is the sampling rate. A cup anemometer counts the number of turns over a specified time period. This interval is described by the sampling rate, which is measured in Hertz.
- An anemometer with a sampling rate of 0.1 Hz calculates wind speed based on 10-second intervals ( 1 / 0.1 Hz = 10 seconds). Which means the measured value is actually an average for the time period.
On the other hand, a sampling rate of 1 Hz means the anemometer calculates wind speed once per second. This makes it possible to appreciate shorter-term variations.
For example, if the wind speed increases significantly during 4 seconds, it will be evident in the 1Hz anemometer but probably not in the 0.1Hz one. The average wind speed will appear higher but there would be no indication of its specific behaviour by the second.
The Impact of Accuracy on Real-Life Projects
Anemometer accuracy requirements for projects are in great part defined by the application. For example, the wind power industry tends to be the most demanding due to the drastic effect on wind speed over turbine output.
Overestimating wind speed by only 10% results in a 33% increase in the projected energy output. This makes the payback period appear shorter while the return on investment seems better. A project with a 6-year payback period and a 17% ROI would appear to have a 4.5-year payback period and a 22% ROI if the wind speed is overestimated by 10%.
The University of Leeds carried out a study of wind speed estimation errors for small-scale wind turbines below 50 kW in the UK. They analysed 91 sites and determined that overestimation was common. They found that in 73 sites wind speed had been overestimated and it had only be underestimated in 18 sites. In addition, overestimation had a higher standard error of 23%, in contrast to only 10% on the sites where wind speed was underestimated.
From a financial standpoint, overestimated wind speed causes an extended payback period and ROI decreases. However, underestimation results in even better than expected results, since the wind turbine produces more energy than expected.
How Accurate is WINDCRANE?
The anemometer on a WINDCRANE unit offers 2% accuracy for wind speeds in the range of 2-55 m/s (8-200 km/h, 5-125 mph). The data sampling rate is 1Hz. This makes the unit compliant with the IEC 61400 standard for wind turbines, one of the most demanding in the industry.
WINDCRANE has been successfully deployed in various heavy duty applications like construction, transportation and aviation.