Guide to the Wind Turbine Power Calculator
If you have room on your screen, you may open another browser window with the calculator,
in order to look at it while you look at this guide. If
you do not want to read all of these instructions, please read
the advice at the bottom of the page in any case.
Using
the Power Curve and the Weibull distribution to Estimate Power
and Energy Output
In order to use the power curve properly, you have to combine
your knowledge of the Weibull distribution with the power curve.
This is what we will be doing using the power density calculator
on the next page:
For each tiny 0.1 metre interval of wind
speeds we multiply the probability of that wind speed interval
(from the Weibull curve) with the value from the power curve
of the wind turbine.
We then take the sum of all these multiplications
to get the mean (or average) power output.
If we multiply the power by 365.25
by 24 (the number of hours in a year) we get the total energy
output for an average year.
Site
Data
Use the pop up menu to fill out European wind distribution data
automatically. The data calculated for roughness classes 0, 1,
2, and 3 was taken from the European wind atlas. If you use roughness
class 1.5, we interpolate to find the data. If you have data
for other parts of the world you would like to have included,
please e-mail us.
Air
Density Data
As we learned on a previous page, the
energy in the wind varies in proportion to the density of
air. Try changing the air temperature from, say 40 degrees Celsius,
to -20 degrees Celsius. There are almost 25 per cent more air
molecules in a cubic metre of the cold air than in a cubic metre
of the warm air, so watch what happens to the energy output...
If you wish to change the altitude above
sea level, then start setting the temperature at sea level first.
The programme will then automatically compute the likely temperature
and pressure at the altitude you set.
You may set the air density directly, if
you know what you are doing. The programme then computes a likely
set of data for the other variables. (You may also change the
air pressure, but you'd better leave it alone. Your air pressure
obviously has to fit to the local altitude and temperature).
Wind
Distribution Data
The Weibull shape parameter is generally
around 2 in Northern Europe, but situations vary, so you may
really need a wind atlas to set this more accurately. You can
either enter the mean wind speed, or the Weibull scale parameter
(the programme then automatically computes) the other.
The measurement height for your wind speed
is very important, because wind speeds increase with heights
above ground level, cf. the page on wind
shear. Meteorology observations are generally made at 10
m height, but anemometer studies are often made at hub height
of the wind turbine (in our example 50 metres).
The average roughness
of the surrounding terrain is important to determine the wind
speed at turbine hub height, if it differs from the height at
which wind speed measurements were made. You may either set the
roughness length or the roughness class, depending on the local
landscape type. (See the Reference
Manual for guidelines on roughness classes).
