Wind characteristics, wind speed and energy

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Wind Characteristics

As the wind power is proportional to the cubic wind speed, it is crucial to have detailed knowledge of the site-specific wind characteristics. Evensmall errors in estimation of wind speed can have large effects on the energy yield, but also lead to poor choices for turbine and site. An averagewind speed is not sufficient. Site-specific wind characteristics pertinent to wind turbines include:

mean wind speed: Only interesting as a headline figure, but does not tell how often high wind speeds occur.wind speed distribution : diurnal, seasonal, annual patternsturbulence: short-term fluctuationslong-term fluctuationsdistribution of wind directionwind shear (profile)

We are providing information on those dimensions and tools for basic yield calculations. However, due to the sensitivities, no calculation canreplace on-site wind measuring campaigns.

Wind Speed Patterns

Wind speed patterns can be depicted asa wind speed spectrum. A high valueindicates a significant change in windspeed over the corresponding timeperiod. Although this graph is obviouslysite-specific, there are distinctivesimilarities. A typical graph is shown onthe right.

The peaks in the wind speed spectrumaccount for annual, seasonal and dailypatterns as well as short-termturbulences. A striking phenomenon isthe spectral gap between time periods of10 minutes to 2 hours.

These patterns are important not only foryield estimations, but also for forecastingof wind power output.

Wind Speed Distribution: The Macrometeorological Range

Large-scale movements of air masses account for 3 peaks on the macrometeorological side of the spectrum.

Diurnal Pattern caused by different temperatures at day and night. This effect is more distinct at coastal sites than off-shore.1. Depressions and Anti-cyclones usually occur with periods of about 4 days. Tthis phenomenon is more distinct in oceanic than continentalregions.

2.

Annual Pattern varies with the degree of latitude and vanishes in close proximity to the equator.3.

The distribution of hourly average wind speeds (i.e. excluding turbulence) can be described by a so-called Weibull distribution:

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Wind Characteristics, Wind Speed and Energy http://www.greenrhinoenergy.com/renewable/wind/wind_characteristics.php

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with a shape factor k and a scaling factorA. The dimensionless shape factorreflects the influence of the topography onwind speeds and ranges between 1.2(mountains) to 4.0 (monsoon regions).The scaling factor A is roughly 125% ofthe average annual wind speed.Alternatively, the following relationshipbetween average wind speed, shapefactor and scaling factor can be used:

where Γ(x) is the gamma-function. Inpractice, the wind distribution is measuredfirst, and then the parameters areadapted and used for further calculations.

Micro-meteorological Range: Turbulence

One of the main characteristics of wind its high temporal variations. Wind speeds can double or triple within seconds, meaning power increased 8or 27 times! Turbulence intensity increases with obstacles such as buildings, tress or steep mountain tops. Sites with high average wind speedstend to suffer less from turbulence.

Why is turbulence bad for wind turbines?

Reduced production of energyIncreased wear and tear shorten lifetime of theturbineIncreased dynamic loads on the blades

What are sure signs of high turbulence?

Inhomogenouse landscapesSteep cliffs or mountain topsRegions with many obstacles - buildings and other

Distribution of Wind Direction


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Although not of interest for the site selection, the distribution of the wind direction is important for the layout of a wind farm. This is done in threesteps:

Measure the time wind blows in each direction - sector. One sector may cover 10° - 30°. In the diagram, wind blows south more than 20% ofthe time, whereas south-east only 5%.

1.

Measure the mean wind speed in each direction.2. Combine both measurements by multiplying the time with the cubic speed for each sector individually to get the distribution of energy acrossall directions, as the energy content per sector is Time x v³.

3.

Wind Shear Profile

Typically, winds increase further off the ground, a microscale phenomenon called wind shear. How much the wind speeds increase with heightdepends not only on prevailing wind speeds at other heights, but also on the type of surface. Given a wind speed (v1) at one height (h1), the windspeed at another height (h2) can be calculated as follows:


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where z0 is an index that describes the roughness of the surface. Values for theroughness index range from 0.01 for flat landscapes to 2.0 in town centres. Twoimportant insights follow from this:

In rough areas, especially built-up areas, the height of the turbine hub is much moreimportant than off-shore, as wind speed change slower along the distance from thesurface.For large turbines, the difference between the wind speed experienced by blade tipsat top and bottom vary much more in rough areas - forces that cause additionalwear and tear.

Long-term Fluctuations

The annual energy yield from wind can also vary from year to year, caused by many factors including variations in solar intensity and otherlarge-scale effects. Empirical evidence shows that these annual variations are much more distinct than for solar irradiance and can vary as muchas 30%.

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Wind characteristics, wind speed and energy