Some sites are windier than others. A lowland site in the middle of southern England might have an average wind speed of 6 m/s, whereas an exposed site on the top of a hill on the west coast of Wales or Scotland might have an average wind speed of 9 m/s.
Because the ‘power in the wind’ is proportional to the cube of the velocity, this means that the wind turbine on the 9 m/s site would on average be exposed to well over three-times the loads compared to the 6 m/s site. Clearly this means that the more exposed wind turbine will have a tougher life and will be subjected to greater wear and tear.
To avoid having to make over-engineered wind turbines that could all operate reliably on all sites, no matter how windy they were, manufacturers design their wind turbines for a specific Wind Class. A Wind Class 3 turbine is designed for an easy life with average wind speeds up to 7.5 m/s, and these turbines typically have extra-large rotors to allow them to capture as much energy as possible from the lower wind speeds they are subjected to.
Wind Class 2 turbines are for windier sites up to 8.5 m/s average, and are the most common class of wind turbines available.
Wind Class Class 1 turbines are designed to cope with the tough operating conditions experienced at sites with average wind speeds above 8.5 m/s. Typically these turbines have smaller rotors (i.e. shorter blades) and are on shorter towers to minimise structural loads. They are also heavier-duty in design, which makes them more expensive.
There is also a second dimension that affects wind class which is ‘turbulence intensity’, which is basically a measure of how turbulent the wind is at a site. This is important because complex topography can cause turbulence, and turbulence can cause varying loads on wind turbines which causes them to wear more quickly. In extreme cases, if a site is just too turbulent the wind turbine manufacturer will refuse to supply a wind turbine because they will know that the turbine will not operate reliably for the full design life at such a site.
Basically, for wind class 2 and wind class 1 sites on-site wind monitoring is essential to determine the exact annual average wind speed and the turbulence intensity, so that the optimum turbine can be specified to ensure long term, reliable operation. Class 3 sites can sometimes get away without having to do wind monitoring because the wind turbine manufacturer will be confident that the loads on the turbine will be acceptable (though site owners often still want wind monitoring so they can be sure of the income the wind turbine will generate).
The table here shows the IEC Wind Classes and the wind speeds that the turbine must be designed to withstand.
|IEC Wind Class|
|1 (High Wind)||2 (Med. Wind)||3 (Low Wind)|
|Reference Wind Speed||50 m/s||42.5 m/s||7.5 m/s|
|Annual Average Wind Speed (Max)||10 m/s||8.5 m/s||7.5 m/s|
|50-year Return Gust||70 m/s||59.5 m/s||52.5 m/s|
|1-year Return Gust||52.5 m/s||44.6 m/s||39.4 m/s|
Want to install a wind turbine?
If you are in the UK then take our Wind Site Self-Assessment - Step 1 of our modular Windpower Feasibility Study. It takes about 20 minutes to work through the basic checks to see if your site might be suitable, including:
- Estimating wind speed
- Checking proximity of nearby properties
- Checking site access and approach roads
- Investigating connection with the grid