Wind site assessment

The wind site assessment (or "wind and site" assessment) is one of the most important steps in the development of a wind farm.

Basically is a in depth analysis of the site conditions of the area where a wind farm could be built.

Purpose of this assessment is calculate energy production and suitability of a specific WTG model to the local conditions.

Such study is usually performed by different stakeholders – external consultancies (possibly on behalf of financial institutions), wind turbines manufacturers and even developers (if they are big enough to have a wind & site department in house).

The inputs for the site assessment are:

  • Wind data
  • Topography & Roughness conditions
  • Other environmental conditions

Wind data includes usually raw data from one or more met masts. Measurement period should be sufficiently long, ideally several years. Key data are the wind rose (from where the wind is blowing), the distribution of the wind speeds (it follows a Weibull distribution) and the normal and extreme wind speeds.

Topography & Roughness conditions have an impact on turbulence, flow inclination and local speed up effects that can be key in the selection of the correct wind turbine.

Other environmental conditions include parameters such as temperature (both very high and very low temperature will need a special “package” and usually leads to decreased energy production), air density (will change the loads, and if very high or low could lead to a derating) and seismic actions (will the tower withstand earthquakes?).

All this data is cross checked against standard wind classes. These classes have been defined by the IEC, an international committee of experts, and are often used to categorize a wind turbine model. For instance, wind turbine A could be certified for wind class I (strong wind) while wind turbine II could be certified for wind class III (weak wind).

It’s important to highlight that usually there is uncertainty on one or more parameters. Therefore different assumptions are made by the wind & site engineers. The interesting part of the story is that, depending on where you are working, you will be interested in “twisting a bit” the numbers in a different direction.

For instance, an external consultant will usually be more conservative when analysing energy production (as he doesn’t want to be blamed if the actual production is lower).

Conversely, a WTG manufacturer could possibly give you higher number when calculating energy production. This is good for 2 reasons: because more production means more money for the prospective customer, and because considering higher loads put the engineer on the safe side when assessing expected life of the key components of the wind turbine.

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.

Meteorological towers in a wind farm

In a standard wind farm, 3 different Meteorological Tower (known as Met Mast) are installed.

First of all, a tower is needed to provide several years (normally at least 2) of wind data. This tower is used to collect not only wind data, but also other information such as temperature, humidity, rain and so on.

Normally these towers are lower that the WTGs that will be installed on site, with an average height of 40 meters: the wind speed at the hub height will be interpolated at the correct elevation using one of the available formulas.

If after a deep study of the energy that can be provided by the wind a promising location is found and the wind farm construction start, one or more temporary met mast are erected.

This towers measures the actual wind speed in the positions where a WTG will be erected, at the hub height (80 meters or more) and are used to calibrate the power curve, that is if the produced energy is in line with the expected values.

After a couple of weeks (normally 2 months) this tower is removed and a turbine is installed nearby.

The third tower is the wind farm tower: this tower will be used to transmit to the control centre the meteorological situation in the location and it has a paramount importance for the management of the site. It normally has the same height of the turbines of the wind farm.