by PETER H. van EMDEN
The truth about
wind speed & wind power measurements
calculation of wind power from wind speed data:
Remember from the lessons in
the energy of a moving particle with mass m [kg] and speed v
Energy = 1/2 * m * v2
(this is called the kinetic energy, or
"energy of movement").
Wind is moving
air, which has a mass and a speed (and a direction). The mass
of wind can be calculated when we realize that during T seconds
a volume of air has passed the surface A (e.g. the surface of
a wind turbine), which is equal to
A * L = A * v * T (m3)
(v = wind speed in m/s, T is time in seconds)
When r is the density of air (kg/m3) the passed mass
during T (seconds) is:
Mass m [kg] = A * v * T * r
so the kinetic energy of the wind during T seconds is:
Energy [Joule] = 1/2 * m * v2
= 1/2 * A * v * T * r * v2 = 1/2 * A * T * r * v3
As power [Watts] is Energy [Joule]
per second, the available wind power is equal to: (divide by
Wind power [Watts] = 1/2 * A
* v * r * v2 = 1/2 * A * r * v3
So wind power is proportional to the cube of the wind speed!
A small deviation in measured windspeed result in a big deviation
in calculated wind power, refer fig. 2 and fig. 3. We call v3
the wind power factor, see fig. 2:
Note that r depends on the atmospheric pressure and temperature:
proportionial to atmospheric pressure(hP) / Temperature (Kelvin).
So for wind power measurements the pressure and temperature
should also be measured!
Depending on the quality and efficiency of a wind turbine the
wind power (power in the wind) can be converted into mechanical
or electrical energy.
For windpower feasibility (or evaluation) studies it is important
to determine the potential wind power, before high investments
are made. In order to obtain an accurate calculation of the wind
power (within appr. 10 %), the wind speed should be measured
accurately, so with a small deviation!
As above 4 m/s a wind turbine is producing electric energy an
acccuracy between 0,2 m/s and 0,1 m/s is recommended! Note that
the resolution (smallest "step" in which a value is
expressed) of the wind measurement should be small compared to
the required accuracy (e.g resolution 0,05 m/s when accuracy
is 0,2 m/s).
Above information is important
during selection of wind monitoring equipment.
EKOPOWER supplies wind monitoring equipment (EKO21B)
with standard accuracy of 0,2 m/s;
with optional individual MEASNET*) calibration
certificate of even 0,1 m/s, so high accuracy is guaranteed!
This is required when high investments are planned or for technical-scientific
measurements. Also the air-temperature and atmospheric pressure
should be measured! For the calculation of the energy production
of a wind turbine is required:
- accurate wind speed data: wind
speed frequency distribution, wind rose, turbulence intensity,
- the characterictic of the planned
wind turbine (power curve)
EKOPOWER supplies all the required
hardware and software tools to carry out such calculations.
It is also important to carry out the measurements according
to the international standards, like IEC 61400-12 and WMO, AWEA,
*) MEASNET: the new European
accuracy standard for wind measurements
If you are interested in Ekopower's products please contact
P.O. Box 4904, 5604 CC Eindhoven, the
Tel: +31.40.2814458 Fax: +31.40.2814119