How to measure wind resource

Met mast, weather vane and anemometer

I have to  start with a disclaimer – I’m not a specialist in wind resource analysis.

However, through the year I’ve seen several time this process so I think I can summarize it with a reasonable level of accuracy (and obviously if you spot a mistake, please let me know).

The wind resource assessment is done for several reason: to define the most suitable wind turbine given the local meteorology, to define the layout of the wind farm and, above all, to calculate the expected energy production of the wind farm – which is obviously a key input in the calculation of the profitability of the project.

To calculate the wind resource you will need to measure several variables:

  • Wind speed
  • Wind direction
  • Wind shear
  • Wind turbulence
  • Air density
  • Roughness of the area

This variables are usually measured installing one or more meteorological mast (“met mast”) in the area where the wind farm is planned  - obviously with the exception of the roughness, which is assessed by a specialist keeping into account the topography and the vegetation of the area.

This activity is called “site measurement campaign”.

The met mast is a tower made of steel (or, more unusually, in concrete) where the measuring equipment is installed. Ideally the met mast should have the same high of the wind turbines that are going to be installed in the area – however, to save money sometimes shorter masts are used.

The equipment installed on the met mast include usually the following:

  • Anemometers (usually there are several anemometers at different heights)
  • Weather vane (to record the direction of the wind)
  • Barometer
  • Thermometer

All the information collected is safely stored in an element called “data logger”. Auxiliary elements in a met mast are solar panels, a protective lightning rod on top, anti-vandalism fence and obviously the foundation.

Ideally, at least 1 year of data should be recorded. However longer measurements (2 to 5 years) have less uncertainties and capture better the seasonal and intraday variability of the wind in the area.

After the site measurement campaign the wind resource assessment start.

The first step is to “clean” all records before processing, removing errors that can occur due to malfunctioning of the instruments.

After, several key parameters are defined:

  • Mean speed
  • Wind rose
  • Wind speed distribution
  • Wind shear
  • Wind turbulence
  • Air density, pressure, temperature

The last step is to use this parameters to estimate electrical power production. There are quite a lot of commercial software in the industry, being some of the most widely used WAsP, WindPRO and OpenWind .

This software will try to optimize the wind farm layout to maximize energy production considering certain limitations – for instance, they will leave a distance of at least 6 rotors from one wind turbine to the other in the direction parallel to the wind.

Finally, when the layout is defined, the software will combine the power curveof the WTG with the wind speed distribution of the site to have the power output.


Guy-wire supported mast for a permanent met mast

Here you have some interesting pictures that we’ve received trying to define the meteorological tower (also known as “met mast”) foundation for one of our wind farms.

Strangely, our customer is going to use a wired met mast for both towers (the permanent mast of the wind farm and the temporary mast used to calibrate the power curve).

As you can see from the pictures the mast has an interesting hinged base joined to the foundation of the WTG with 4 screws - looks like an effective technique.

The mast is tower model is KT470 from Kintech engineering.

Power curve: what is it and how to measure it

A power curve is a relationship between free wind speed at the WT location and the associated expected power being produced. Power curve warranties are often included in contracts, to assure the wind turbine performs according to specification.

Here you have an example of how it looks like:

It is measured following international standards to demonstrate compliance with warranted values.

Following a standard not only reduces variance due to data analysis, but it also provides solutions to different problems, for example:

  • How to judge if an anemometer is good enough.
  • How to deal with complex terrain and obstacles.
  • How to correct for low air pressure.
  • How to measure the production and with which devices.

Basically, it provides an accepted way of estimating the result’s uncertainty. Any deviations from the standard (in procedure or interpretation) will cause severe deviations and uncertainties that will make conclusions very hard to take.

The standards in use to measure the power curves are made by the International Electromechanical Commission (IEC):

IEC 61400-12, in use since 1998, has been replaced by IEC 61400-12-1 (December 2005), which includes MEASNET requirements and has additional requirements for anemometer.

The following version is IEC 61400-12-2, addressing nacelle anemometry.

IEC 61400-12-1 without site calibration is the method used on flat terrain.

Basically, a meteorological tower (met mast) is positioned near a turbine to measure the wind, and a uniform flow at mast and turbine is assumed. The normal uncertainty is 5-8 %, depending on annual wind.

The best position for the mast is in the middle of sector, and it has to be between 2 and 4 diameters in front of the turbine (for instance for a V90 it would be between 180 and 360 meters). It is often erected at a 2.5 diameters distance.

The met mast can be erected before or after turbines are on site.


IEC 61400-12-1 with site calibration is the method used on complex terrain.

In this case, a temporary met mast is placed in the future position of the selected turbine, and at the same time the wind farm definitive met mast is erected.

The 2 masts works together for a certain amount of time, until a relationship between the wind measured by the definitive met mast and the temporary met mast can be found.

After, the temporary met mast is disassembled, the WTG is erected in his position and it can be defined if the energy produced is in line with the expected generation with the wind speed (extrapolated using the data of the wind farm met mast and the previously defined correlation).