This section of the calculator lets you specify the rated power
of the main generator, the rotor diameter, the cut
in wind speed, and the cut
out wind speed, and the hub height of your machine. At the
bottom of the page you may then specify the power curve of your
It is much easier, however, to use the first
pop up menu which allows you to set all turbine specifications
using a built-in table of data for typical Danish wind turbines.
We have already put data for a typical 600 kW machine in the
form for you, but you may experiment by looking at other machines.
The second pop up menu will allow you to
choose from the available hub heights for the machine you have
chosen. You may also enter a hub height of your own, if you wish.
Try experimenting a bit with different hub
heights, and see how energy output varies. The effect is particularly
noticeable if the machine is located in terrain with a high roughness
class. (You can modify the roughness class in the wind distribution
data to see for yourself).
If you modify the standard machine specifications,
the text on the first pop up menu changes to User example,
to show that you are not dealing with a standard machine. It
is safe to play with all of the variables, but it does not
make much sense to change the generator size or rotor diameter
for a standard machine, unless you also change the power curve.
We only use the rotor diameter to show the power input, and to
compute the efficiency of the machine (in terms of the power
coefficient). We only use the rated power of the generator
to compute the capacity factor.
Turbine Power Curve
For practical reasons (keeping your input data and your results
in view at the same time) we have placed the listing of the turbine
power curve at the bottom of the page.
You can use this area to specify a turbine which is not listed
in the built-in table. The only requirement is that wind speeds
be ordered sequentially in ascending (increasing) order.
The programme approximates the power curve
with a straight line between each two successive points which
have non zero values for the power output.
Note: The programme only uses wind
speeds up to 40 m/s in its calculations of the wind climate,
so do not bother about fantasy machines that work beyond 30 m/s.
Calculate recalculates the results on the form. You
may also click anywhere else or use the tab key after you have
entered data to activate the calculator. Note that if you change
the power curve, the machine will not recalculate your data until
you click calculate, or change other data.
Reset Data sets the data back to the
user example you first encountered on your screen.
Power Density plots the power
density graph for this site and machine in a separate window.
Power Curve plots the power
curve for the machine you have selected in a separate window.
Power Coefficient plots the power
coefficient, i.e. the efficiency of the machine at different
Power Input Results
Power input per square metre rotor area shows the
amount of energy in the wind which theoretically would flow through
the circle containing the rotor area, if the rotor were not present.
(In reality, part of the airflow will be diverted outside the
rotor area due to the high pressure area in front of the rotor).
Maximum power input at x m/s shows
at what wind speed we achieve the highest contribution to total
power output. The figure is usually much higher than average
wind speed, cf. the page on the power
Mean hub height wind speed shows how
the programme recalculates your wind data to the proper hub height.
If you have specified a hub height which is different from the
height at which wind measurements were taken, the programme automatically
recalculates all wind speeds in the Weibull distribution in accordance
with the roughness class (or roughness length) you have specified.
Power Output Results
Power output per square metre of rotor area tells
us how much of the power input per square metre the machine will
convert to electricity. Generally, you will find that it is cost
effective to build the machine to use about 30 per cent of the
power available. (Please note, that the figure for site power
input includes the power for wind speeds outside the cut
in/cut out wind speed range, so you cannot divide by that figure
to obtain the average power coefficient).
Energy output per square metre rotor area
per year, is simply the mean power output per square metre
rotor area multiplied by the number of hours in a year.
in kWh per year, tells us how much electrical energy the
wind turbine will produce in an average year. That is probably
the figure the owner cares more about than the rest. When the
owner considers that figure, however, he will also have to take
the price of the machine, its reliability, and the cost of operation
and maintenance. We return to those subjects in the section on
the economics of wind energy.
The annual energy output calculated here
may be slightly different from the real figures from the manufacturer.
This is particularly the case if you vary the density of air.
In that case the manufacturer will calculate different power
curves for each density of air. The reason is, that with a pitch controlled turbine the
pitching mechanism will automatically change the pitch angle
of the blade with the change of air density, while for a stall controlled
turbine, the manufacturer will set the angle of the blade slightly
differently depending on the local average air density. This
programme may be up to 3.6% below the correct figure from the
manufacturer for low air densities, and up to 1.6% above the
manufacturers' figures for high air densities.
tells us how much the turbine uses the rated capacity of its
(main) generator. You may read more on the page on annual
energy output from a wind turbine.
Note 1: Make sure that you use the same hub height,
if you wish to compare how two machines with the same rotor diameter
Note 2: If you wish to compare machines
with different rotor diameters you should look at the energy
output per square metre of rotor area instead (you should still
use the same hub height).
Note 3: Low wind machines (large rotor
diameter relative to generator size) will generally perform badly
at high wind sites and vice versa. Most low wind machines are
not designed for use in high wind areas with strong gusts.