Inter-comparison of commercially available SODARs for
wind energy applications
Ioannis Antoniou1, Hans E. Jørgensen1, Sabine Von Hunerbein2, Stuart G. Bradley2, Detlef
Kindler3,
1
Risoe National Laboratory, Wind Energy Dpt., P.O. Box 49, 4000 Roskilde, Denmark
(ioannis.antoniou@risoe.dk, hans.e.joergensen@risoe.dk)
2
School of Computing Science and Engineering, University of Salford, Salford M5 4WT,
UK (s.vonhunerbein@salford.ac.uk, s.g.bradley@salford.ac.uk)
3
WINDTEST Kaiser-Wilhelm-Koog GmbH, Sommerdeich 14b, D-25709 Kaiser-WilhelmKoog, Germany (kd@windtest.de)
Abstract
The evolution of the wind energy and the development of the multi-MW wind
turbines has created the need for measuring the wind climate at increasingly larger
distances above the ground. Wind energy applications include measurement of the
wind potential as well as measurement of the power curve or the wind-induced loads
on a wind turbine. Traditionally the wind speed is measured with the use of topmounted cup anemometers on a met tower at turbine hub height. The increase in the
met tower costs and the demand for a more detailed knowledge of the wind profile
has made the use of the sodar an attractive alternative. However before the sodar can
be used in such applications, a number of issues such as the accuracy of the
measurement itself and the calibration of the instrument have to be resolved, as wind
energy applications pose large demands on these matters. As a part of the work done
within the EU-funded WISE project (WInd energy Sodar Evaluation), the PIE task
(Profiler Inter-comparison Experiment) aims in comparing the measurements from
three closely situated, commercially available, phased array sodars, to the
measurements from a nearby heavily instrumented met tower.
1. Introduction
To the present date, the Doppler sodars have
extensively been used in the field of meteorology as well
as air pollution improving our understanding of the
atmospheric boundary layer. A lot of interest has been
given therefore in comparison studies between sodar and
met tower data during the last three decades, in order to
confirm reliability of the sodar data, as accurate remote
sensing could effectively replace high met towers and
offer at the same time lower costs and higher versatility.
In the field of wind energy a typical application for a
sodar would be the measurement of the wind speed in
front of a turbine or the measurement of a site’s energy
potential. In wind energy applications, strict demands
are applied to the sensors (cup anemometers) used,
regarding their calibration and mounting in order to
minimise uncertainties. An increase in the uncertainty of
the wind speed measurement may influence both the
energy potential and the turbine’s production ability and
make difficult the financing and the realization of a
project. For the same reason, if sodars are to be used in
wind energy, rules defining their use have to be
introduced. Sodars have sporadically been used in wind
energy applications (Helmis et al 1993, Dam et al 1999,
Beyrich et all 1994, Högström et al 1988). In the
reported work sodars have been used to measure the
wind field inside a wind turbine park or in the wake of it
and in some occasions these measurements found place
in complex terrain. Since sodars are known to measure
the wind field over a large volume and not at the same
position, their application in situations where large
spatial variations are expected, like the ones described
above, can only be considered of more qualitative than
quantitative nature.
Although acoustic sounding is, in principle an
absolute method, and therefore no reference to other
measurements is needed, experience has shown that
large differences occur in praxis between different
measurement systems. Crescenti (1997), gives account
of the work done on Doppler sodar comparison studies.
The paper points out that the major comparison tests
have taken place using three-axis monostatic systems,
while little and not very systematic work has been done
using the phased array systems. Even less work has
taken place in directly inter-comparing phased sodars
against each other and at the same time against a met
tower.
In the present paper, three phased array sodars have
been installed in a flat test site and in the vicinity of an
instrumented met tower. The aim of this test is to gain
more experience on the behaviour of the sodars under
similar meteorological conditions and compare their
response. The tests started primo April 2004 and are
expected to finish ultimo June 2004 (still ongoing while
this paper is written).
2. Description of the site and the
experimental setup
3.
the Scintec SFAS (Windtest), (Small Flat Array
Sodar) sodar, consisting of a 64 element array, and a
choice of 10 out of a total of 64 selectable frequencies in
the range between 2540 to 4850 Hz, height resolution of
5m.
2.1. Description of the site
The test site is the National Danish Test Station for
Large Wind Turbines and is situated in the northwest of
Denmark close to the North Sea. The test site is flat
surrounded by grassland with no major obstacles in the
immediate neighbourhood and at a distance of 1.7 km
from the west coast of Denmark. The prevailing wind
direction is from the west. It consists of five turbine test
stands, Figure 1, where five wind turbines are presently
installed. The stands are placed in the north-south
direction at a distance from each other of 300m with
stand 5 the southernmost one. In front of every test stand
and at a distance of 240m in the prevailing wind
direction, a met mast is situated, with a hub height equal
to the turbine height at the corresponding stand.
Figure 1 The test site, the met tower and the sodars
(blowing from the east)
Figure 2 The three sodars (in row from upper left: 1st
Scintec, 2nd AV4000, 3rd Metek RASS)
Figure 3 Description of the met tower instrumentation
(looking to the tower from the west)
Table 1 The met mast instrumentation
At the south of the turbine row, a met tower (stand 6)
is located at 200m from stand 5, Figure 1. A layout of
the met tower and its instrumentation is presented in
Figure 3 and Table 1. The rain sensor was not installed
from the start of the test period and shortly after its
installation it failed. Later on a tip bucket rain sensor
was installed. Likewise the 100m wind direction sensor
was not available from the beginning of the
measurement period due to a lightning strike.
The three phased array sodars used in the present tests
are located to the southwest of the met tower, Figure 2.
The three sodars are the following:
1.
the AeroVironment 4000 sodar (Risoe),
consisting of a 50 element array, acoustic operating
frequency 4500Hz, height resolution of 10m.
2.
the Metek-Rass (University of Salford),
consisting of a 64 element array, acoustic operating
frequencies: 1674 Hz for Sodar, (2950 +-50) Hz for
RASS. The RASS type is: "Metek PCS2000-64 Sodar
with RASS Extension 1290 MHz", height resolution of
15m.
Sensor
Position
Cup anemometer
Cup anemometer, wind vane, sonic anemometer,
temperature, differential temperature, relative
humidity, air pressure
Cup anemometer, sonic anemometer, differential
temperature
Cup anemometer, sonic anemometer, differential
temperature, wind vane
Cup anemometer, sonic anemometer, differential
temperature
Sonic anemometer
Cup anemometer, sonic anemometer, differential
temperature, wind vane
Cup anemometer, temperature, differential
temperature, relative humidity, air pressure, rain
116.5m
100m
80m
60m
40m
20m
10m
2m
The statistical results are sampled as 10-minute
averages and all four measurement systems (met tower
plus one per sodar) were kept synchronised during the
tests with the help of an Internet clock. Care has been
taken to avoid interactions and fixed echoes from the
near by met tower and between the sodar system
themselves. The sodar interactions were avoided by
3. Presentation of the results
3.1. The climatology at the test site
The wind speed and turbulence intensity at the test site
during the results are presented in Figure 4 and Figure 5.
The settings for the Scintec SFAS were modified some
days after the start of the measurement period, so wind
speeds above 17ms-1 are not reported for it.
Wind speed at 116.5m (m/s)
Met tower wind speed vs wind direction at 60m
25
20
The data sets from the Scintec SFAS and the AV4000
systems, apart from the measurements which were
marked as 99,99 included a number of erroneous data
points, in the form of very high wind speeds, which were
identified to coincide with rain periods and which were
removed. The data set from the Metek RASS sodar at
100m height was not analysed with the rest of the Metek
RASS data from other heights, as it contained a large
number of measurements at low wind speeds which were
did not seem adequately filtered. For all three sodars
mean wind speed results fitted with are presented for the
heights from 40m to 116.5m as a function of the met
tower measurements.
Sodar vs. met tower wind direction
Sodar wind direction (°)
adapting the sodar frequencies. The participating
institutes have been responsible each for their sodars and
have delivered to Risoe mean wind speed and wind
direction values at or close to the corresponding met
tower heights. At a later stage all the results will be
introduced in a data base.
340
y = 0.9797x + 1.3361 (AV4000)
y = 0.9804x - 6.4924(Scintec)
275
210
145
y = 0.9727x + 7.4874(Metek Rass)
80
15
80
145
10
210
275
340
Met tower direction 60m (°)
Metek_Rass-58 m
AV4000--60m
Linear (Scintec-60m)
5
0
80
115
150
185
220
255
290
325
Wind direction at 60m(°)
Scintec-60m
Linear (AV4000--60m)
Linear (Metek_Rass-58 m)
Figure 6 The wind direction as depicted by the three
sodars
Figure 4 The wind speed vs. the wind direction
Met tower vs. Metek-RASS wind data
20
Tower wind speed (m/s)
Met tower TI at 116m height vs. wind direction at 60m
60
TI at 116.5m (%)
50
40
y(116.5m) = 0.9666x + 0.6241
y(80m) = 0.9961x + 0.2719
y(60m) = 0.9874x + 0.3205
y(40m) = 0.9642x + 0.3894
15
10
5
30
20
0
0
10
5
10
15
20
Metek-RASS wind speed (m/s)
0
80
115
150
185
220
255
290
Wind direction at 60m(°)
Tower 60m
Linear (Tower 80m)
Tower 80m
Linear (Tower 116_5m)
Linear (Tower 40m)
Tower 116_5m
Tower 40m
Linear (Tower 60m)
325
Figure 7 The Metek RASS wind speed data
Figure 5 the turbulence intensity vs. the wind direction
3.2. The sodar results
The results from the comparison to the mast data are
presented in Figure 6 to Figure 9. The data have been
delivered filtered, yet in the case of the Scintec SFAS
and AeroVironmet AV4000; it was necessary to remove
a number of points where erroneously large wind speeds
(others than 99,99) were recorded. These points were
identified with periods of rain weather and were
excluded during the analysis.
The results of the wind direction comparison, Figure
6 shown that all three sodars measure the wind direction
accurately. Points with large deviations from the main
data body are due to low wind speed data (less than 4ms1
) where the direction is expected to be of more random
character between the two positions.
Met tower wind speed (m/s)
Met tower vs. Scintec SFAS wind speed
16
y(116.5m) = 1.0282x + 0.306
y(100m) = 1.0341x - 0.0071
12
y(80m) = 1.033x - 0.1194
8
4
y(60m) = 1.0134x - 0.0735
y(40m) = 0.9798x + 0.0339
0
0
4
8
12
16
Sodar wind speed (m/s)
Tower-40m
Tower-100m
Linear (Tower-60m)
Linear (Tower-80m)
Tower-60m
Tower-116_5m
Linear (Tower-116_5m)
Tower-80m
Linear (Tower-40m)
Linear (Tower-100m)
Figure 8 The Scintec SFAS wind speed data
Met tower vs. SODAR AV4000 wind data, 7<SNRU,V,W<35
Tower wind speed (m/s)
16
y(116.5m) = 0.8645x + 0.4623
y(100m) = 0.8732x + 0.3115
12
y(80m) = 0.8761x + 0.1957
8
4
y(60m) = 0.8705x + 0.1778
y(40m) = 0.8578x + 0.2962
0
0
4
8
12
Sodar AV4000 wind speed (m/s)
Tower 116_5m
Tower 80m
Linear (Tower 40m)
Linear (Tower 100m)
Tower 40m
Tower 100m
Linear (Tower 60m)
16
Tower 60m
Linear (Tower 116_5m)
Linear (Tower 80m)
Figure 9 The AeroVironment AV4000 wind speed data
4. Discussion
The results presented above have been filtered per sodar,
by removing all sodar data for which data at any height
were missing. In this way the number of data points is
the same for all heights and equal to the minimum
number of observations, which coincides mostly with the
number of points at the highest measured height. This
has led to small changes in the gain and offset of the
fitted curves.
One of the largest problems of the sodar systems is
that they fail to measure the higher end of the wind
speeds and in this case, the Metek RASS has measured
the highest wind speeds. In many occasions high wind
speeds are measured correctly but are removed from the
filtering criteria impose in order to remove outlying
measurements.
The slope of the fitted curves varies between the
sodars with the Metek RASS and the Scintec SFAS
being closer to unity. It is not yet clear what is the reason
for the deviation of the AV4000 slope, as in previous
tests it has been considerably closer to unity (Antoniou
and Jørgensen, 2003). In terms of the deviations of the
slopes with the distance from the ground, the AV4000
shows the most consistent behaviour, whereas the largest
differences are observed for the Scintec SFAS sodar.
Certainly the reasons for the deviations between
different heights remain to be further investigated. Based
on corresponding results from the AV4000 sodar
(Antoniou and Jørgensen, 2003), it was suggested that
the calibration of the sodar could take place at a lower
height (e.g. 40m) with the help of a met tower and a top
mounted cup anemometer. Then this relation could be
transferred at hub height by applying the same relation.
5. Conclusions
A field experiment has taken place where three phased
array sodars of different type and make have been tested
against a heavily instrumented met tower. The filtering
of the sodar results and a calibration method is a central
issue before the sodars can be used in wind energy
applications.
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Appendix
The authors would like to thank the European
Commission for making possible this work by funding
the WISE project.