ACURACY
OF WIND SPEED DATA:
a key-factor in the economic analysis of wind energy projects
by PETER H. van EMDEN
- Introduction
- The structure
of a wind measuring system
- The accuracy of
a wind measuring system
- The accuracy of
the calculated energy production
- The accuracy of
the period for return on investment
- Conclusions
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1.
Introduction
Before an investment in wind
turbines takes place a feasibility study will be carried out,
which gives information to potential investors about costs and
benefits (energy production) of a planned wind energy project.
The Return On Investment (ROI) has been reached when the total
profits are equal to the investment. The period in which the
Return On Investment has been reached is a very important figure
for investors, so it should be determined accurately. In this
article it is clarified that the accuracy of wind speed data
is a very important key-factor for the accurate determination
of the period for Return On Investment.
Accurate wind speed data can
save a lot of money, as wrong decisions about investment in wind
turbines can be avoided. Only accurate wind speed data (accuracy:
0.1 - 0.2 m/s) can be used for this purpose. Many suppliers of
wind measuring equipment do not supply equipment with the required
accuracy. A scientific calibration procedure is necessary to
obtain the required accuracy of the wind speed.
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2.
The structure of a wind measuring system
Anemometer - signal conditioning
and sampling - pre-processing - data collection - data processing
- output
The anemometer converts the wind speed into a physical
signal, usual an electrical pulse signal.
The signal conditioning converts the signal from the anemometer
into an appropriate signal for sampling.
During the pre-processing the average, maximum and standard
deviation of the wind speed are calculated.
During the data collection all the recorded data is stored
in a memory, with sufficient resolution (at least 10 bits).
Then the data processing can take place using a PC and
software which gives the results of the measurements.
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3.
The accuracy of a wind measuring system
All system parts are contributing
to the total system accuracy! The most critical system parts
are the calibration of the anemometer and the way of signal conditioning
and sampling.
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3.1
The anemometer calibration
The calibration of an anemometer
takes place in a wind tunnel. As there is an interaction between
anemometer and wind tunnel an accurate anemometer calibration
is not just putting an anemometer in a wind tunnel: this can
easily cause errors of about 0.5 m/s or more. Only a scientific
calibration procedure, in which the interaction between wind
tunnel and anemometer is known, can result in a calibration accuracy
of up to 0.1 m/s (if carried out in a very accurate windtunnel).
An individual calibration certificate
is preferable, not only before a monitoring period but it is
also recommended to check the calibration after this period.
For accurate measurements it
is required that the complete calibration curve is taken into
account. There are suppliers of wind monitoring equipment who
simplify the calibration curve to a "one point calibration",
or in other words: a straight line approximation, which will
result in unacceptable calibration errors. For example: for the
well-known MAX40 anemometer this simple approximation will result
in calibration errors up to 0.5 m/s!
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3.2
The signal conditioning and sampling
The signal conditioning and
sampling technique of the wind speed is also very important for
the accuracy. When the pulses from a MAX40 anemometer are counted
during a 1 second interval, the resolution of the sample values
will be up to appr. 0.8 m/s! When the signal conditioning and
sampling are carried out correctly (by using a special technique),
the resolution of the sample values can be reduced to 0.05 m/s
and for the average values 0.01 m/s.
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3.3
The total system accuracy
The best total system accuracy
that can be realized with "low-cost" equipment is 0.1
m/s. This can be achieved by an excellent (scientific) anemometer
calibration and the right way of signal conditioning and sampling.
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4.
The accuracy of the calculated energy production
As the energy production of
a wind turbine is proportional to the cube of the long-term average
wind speed (v3), a small deviation in wind speed will highly
affect the calculation of the energy production. The relative
deviation of the calculated energy production is equal to: 3*total
system accuracy / average wind speed.
For example: when the yearly average wind speed is 6 m/s, measured
with a system accuracy of +/- 0.5 m/s, the error in the calculation
of the annual energy production is appr. +/- 25%.
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5.
The accuracy of the period for return on investment
The period for Return On Investment
(ROI) can be calculated by: ROI (years)=INVESTMENT/PROFIT (yearly
average) The profit is proportional to the energy production.
So the relative deviation of the calculated ROI is equal to the
relative deviation of the energy production! The following examples
will clarify the significant influence of the accuracy of wind
speed data (refer also to the graph in section 4). In the examples
the accuracy of the ROI is determined for some different cases.
The values for the calculated ROI are assumed values; in reality
these figures can be different! The costs for maintenance etc.
are ignored. The difference between the low and high ROI strongly
depends on the accuracy of the wind speed !
Total accuracy
wind speed (m/s) |
Yearly average wind speed (m/s) |
Rel. deviation of annual energy prod (+/-
%) |
when ROI is: (Years) |
Accuracy of ROI (Years) |
|
0.5 |
6 |
25 |
12 |
9 - 15 |
|
0.2 |
6 |
10 |
12 |
10.8 - 13.2 |
|
0.1 |
6 |
5 |
12 |
11.4 - 12.6 |
|
0.5 |
8 |
18.7 |
8 |
6.5 - 9.5 |
|
0.2 |
8 |
7.5 |
8 |
7.4 - 8.6 |
|
0.1 |
8 |
3.7 |
8 |
7.7 - 8.3 |
|
0.5 |
10 |
15 |
6 |
4.1 - 5.9 |
|
0.2 |
10 |
6 |
6 |
5.6 - 6.4 |
|
0.1 |
10 |
3 |
6 |
5.8 - 6.2 |
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6.
Conclusions
In order to obtain high accuracy
of the period for Return On Investment (ROI), accurate wind speed
data with total system accuracy of 0.1 to 0.2 m/s is required.
Especially for research in areas with moderate average wind speed
it is important to measure the wind accurately as the economic
feasibility of wind energy systems may be critical in that case.
The extra costs for accurate wind monitoring are relative very
small compared to a high investment in a wind energy project!
In this way more security about the investment can be "bought"
inexpensively by using an accurate wind monitoring system. Accurate
determination of the period for Return On Investment can save
a lot of money as it can prevent wrong decisions for investments
in wind energy projects and may prevent damage of the imago of
wind energy. In order to obtain the high accuracy of the wind
speed data, the anemometer calibration must be carried out with
a scientific calibration procedure. Most of the available low-cost
wind measuring systems do not meet the mentioned accuracy requirements.
However EKOPOWER supplies a new system (EKO21B)
with an optional (scientific) calibration
certificate, certified by The Royal Dutch Meteorological
Institute KNMI, with calibration accuracy of 0.1 or 0.2 m/s
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If you are interested in Ekopower's products please contact
us.
P.O.
Box 4904, 5604 CC Eindhoven, the Netherlands.
Tel: +31.40.2814458 Fax: +31.40.2814119
E-mail:
info@ekopower.nl
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