## 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).