WIND DESCRIPTION/ENERGY CONTENT
Short term variability in the air
The wind speed is always fluctuating and thus the energy content of the wind is also always changing. Exactly how large the variation is depends both on the weather and on local surface conditions and obstacles. Energy output from a wind turbine will vary as the wind varies, although the most rapid variations will to some extent be compensated for by the inertia of the wind turbine rotor.
Trends in wind speed
Operators of windfarms have been concerns about the downward trend in wind availability sinds 1988. In the graph it can be seen the trend in yearly averages of windspeed calculated out of measurements from several weather stations in the Netherlands. The year 2003 delivered the lowest yearly average sinds the beginning of the measurements in 1962.
This declining tendencie can also be seen in several other European countries (http://home.planet.nl/~windsh/windexen.html).
Diurnal (night and day) variations of the wind
In most locations around the globe it is more windy during the daytime than at night. The graph shows how the wind speed at Beldringe, Denmark varies by 3 hour intervals round the clock (information from the European Wind Atlas).
This variation is largely due to the fact that temperature differences e.g. between the sea surface and the land surface tend to be larger during the day than at night. The wind is also more turbulent and tends to change direction more frequently during the day than at night.
From the point of view of wind turbine owners, it is an advantage that most of the wind energy is produced during the daytime, since electricity consumption is higher than at night. Many power companies pay more for the electricity produced during the peak load hours of the day, when there is a shortage of cheap generating capacity.
- Turbulence and variability (A): general explanation
- APSD (van der Hoven) spectrum: explanation, spectral gap
- Interpretation of APSD spectrum: notes and characteristics
- Turbulence and variability (B): more notes
- Climate and turbulence