Climate parameters

Extreme wind analysis is important for the wind turbine classification

Figur 1. Gumbel-plot of extreme values

Extreme wind analyses of the wind project area are important for the wind turbine manufacturers. This information is used when the classification of the wind turbine is decided. A correct classification is important to find the proper turbine that can stand up to the wind conditions at the site.  

Extreme wind values are normally given as wind speed with a 50 year return period. This is the highest wind speed that is likely to occur during a period of 50 years.   

The lifetime of a wind turbine depends of the turbulence conditions

Figur 2. Comparison of the measured turbulense intensity at a site and the IEC standard

Turbulence is an important factor that can reduce the lifetime of a wind turbine. International standards give values for turbulence conditions that should be avoided for each turbine classification.

A turbine located at a site with some turbulence is normally not at problem. High turbulence created from detached flow will however be a problem.

It can be difficult to estimate when and where such turbulence will occur. Measurements at the site will after a short period establish the turbulence intensity. Models can also be helpful.

With our experience we can help you locating areas where the turbulence intensity should be evaluated. 

Vertical winds

The flow profile of the wind at the wind turbine blades has a large influence on the level of the wind turbine construction loads. The lifetime of the wind turbine depends both of the frequency and the level of these loads. One of the factors that create such loads is the angle between the wind speed direction and the horizontal plane; the vertical winds.

International standards give values for the vertical winds that should not be exceeded, but wind turbine manufacturers may also use limits that are different from this.

With our models we can evaluate the vertical wind speed conditions and give advise on which areas should be avoided when deciding the wind turbine positions.

Figure 3. Example of calculated and observed number of days with icing for a site.

Icing

For some locations icing can be a problem. We have experience with the conditions in Norway. Based on international research we have developed our own models for calculating number of days with icing and the risk of ice thrown off the wind turbines. We can use these models to evaluate the icing conditions in an area.

At certain combinations of temperature, humidity and wind icing will occur on the wind turbines. Ice may form on the turbine blades and on the instruments that gives the wind farm information about wind speed and wind direction. This can be a problem for two reasons:

• Ice that falls from the wind turbine can create dangerous situations for people being in the wind farm area
• Ice on the turbine blades or on the measuring instruments might reduce the energy production

Different cathegories of ice may accrete on a wind turbine:

• Hroar frost: May accumulate on a surface from the humidity in the air when the surface becomes colder than the surrounding air. Hroar frost is what you can find on your cars windscreen in the morning after a cold night. This frost will normally only form a thin layer.
• Glaze (from raindrops): Glaze will be accumulated on a surface from raindrops if the temperature of the surface is below freezing. In Norway it will usually not get very thick.
• Wet snow: Accretion occurs during snow and positive temperatures close to 0, typically 0-2 degrees C. It is the free water on the snowflakes that make them stick to the surface. The wet snow will normally not stick to the turbine blades, but if the temperature later falls below zero, the wet snow on the surface will freeze to ice.
• Hard rime: Is very similar to glaze. When the air is very cold, and the droplets are small (typically fog), the droplets will freeze immediately to strong clear ice. The accretion of the rime occurs on the windward side of constructions.
• Soft rime: Light (<0.6 kg/dm3) snow that looks almost like rime. It falls easily off the construtions.

For wind turbines it is mainly hard rime that creates problems. To calculate the number of days with hard rime we utilize observations of cloud height, cloud cover and temperature. This method has been developed by the Finnish Meteorological Institute. It has also been tested by Kjeller Vindteknikk, we have found a good correlation between the model and observations.