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Wake Measurements of a Multi-MW Wind Turbine with Coherent Long-Range Pulsed Doppler Wind Lidar

September 2010
Journal of Atmospheric and Oceanic Technology 27(2010):1529-1532

DOI:10.1175/2010JTECHA1483.1

Source
DLR

Authors:

Stephan Rahm

German Aerospace Center (DLR)

Martin Kühn

Carl von Ossietzky University of Oldenburg

Long-range Doppler wind light detection and ranging (lidar) measurements at a wind turbine were carried out for the first time. The turbine was of the type Areva M5000 and is located at a site near the coastline in Bremerhaven, in the northern part of Germany. This wind turbine is the prototype for the German offshore test site ‘‘alpha ventus’’ and has a rated power of 5 MW. Information about the ambient wind field before and after this multimegawatt wind turbine was obtained. In this paper the measurement technique is discussed and the results of measurements in the diurnal layer and in the stable nocturnal boundary layer are shown. The main focus of this work is to determine the reduction of the wind speed at certain distances downstream from the rotor.

(a) Map of the area in Bremerhaven with the position of the 2-mm lidar and the wind turbines. A top view of an azimuth scan is given in the layout plan. (b) Side view of an elevation scan.

…

(a) Results for the LOS component of the wind vector for an elevation scan through the rotor blades at night. The wind was blowing from the northeast parallel to the laser beam. Wind direction and position of the wind turbine are indicated in the figure. The white vertical line indicates the rotor disc. (b) Azimuth scan at night covering the ambient wind field of several wind turbines, including the M5000-1 prototype. The vertical white lines indicate the position of the rotor discs of M5000-1 and M5000-2.

…

Velocity deficit downstream from the wind turbine derived from the elevation scan in Fig. 2a.

…

Figures - uploaded by Stephan Rahm

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Content uploaded by Stephan Rahm

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Wake Measurements of a Multi-MW Wind Turbine with Coherent

Long-Range Pulsed Doppler Wind Lidar

YVONNE KA

¨SLER,STEPHAN RAHM,AND RUDOLF SIMMET

Deutsches Zentrum fu

¨r Luft- und Raumfahrt, Institut fu

¨r Physik der Atmospha

¨re, Wessling, Germany

MARTIN KU

¨HN

Research Group Wind Energy Systems—ForWind, Center for Wind Energy Research,

University of Oldenburg, Oldenburg, Germany

(Manuscript received 15 April 2010, in ﬁnal form 8 June 2010)

ABSTRACT

Long-range Doppler wind light detection and ranging (lidar) measurements at a wind turbine were carried

out for the ﬁrst time. The turbine was of the type Areva M5000 and is located at a site near the coastline in

Bremerhaven, in the northern part of Germany. This wind turbine is the prototype for the German offshore

test site ‘‘alpha ventus’’ and has a rated power of 5 MW. Information about the ambient wind ﬁeld before and

after this multimegawatt wind turbine was obtained. In this paper the measurement technique is discussed and

the results of measurements in the diurnal layer and in the stable nocturnal boundary layer are shown. The

main focus of this work is to determine the reduction of the wind speed at certain distances downstream from

the rotor.

1. Measurement technique

Lidar is a remote sensing technique that transmits

alaserbeamintotheatmosphereandthebackscat-

tered light is detected. The pulsed Doppler wind lidar,

which was used for the measurements in Bremerhaven,

Germany, takes advantage of the fact that the center

frequency of the received laser pulses is shifted compared

to the outgoing pulses because of the Doppler effect,

which occurs from backscattering on moving particles.

This shift in frequency provides information about the

line-of-sight (LOS) component (component in beam di-

rection) of the wind vector. The Doppler lidar of the

German Aerospace Center [Deutsches Zentrum fu

¨r

Luft- und Raumfahrt (DLR)] is based on a WindTracer

transceiver unit of Lockheed Martin (which acquired

Coherent Technologies, Inc. and its division CLR

Photonics, Inc.; Ko

¨pp et al. 2004). It has been modi-

ﬁed by the DLR and has been used for wind and tur-

bulence measurements from the ground, as well as from

its research aircraft in the past (Rahm and Smalikho 2008;

Smalikho et al. 2005). The most important aspect was the

addition of a scanner that makes different scan patterns at

variable scan speeds possible. The measurement range on

ground is between 500 m and 10 km. The maximum range

used for the measurements in Bremerhaven was 3 km.

The main system parameters and important information

for data acquisition and processing are summarized in

Table 1. The speciﬁcations of the lidar do not completely

coincide with those discussed in Smalikho et al. (2005)

because of modiﬁcations of the laser system.

Figure 1a shows a map of the area in Bremerhaven

(using the software program OpenStreetMaps). The 2-mm

lidar is located at a distance of about 1820 m north-

east of the wind turbine M5000. Technical data of the

wind turbinecan be found in Table 2. Furthermore, it can

be seen in Fig. 1a that there are other wind turbines in the

measurement area. One of them is of the same type as the

Areva Multibrid M5000. Two different scanning tech-

niques to analyze the ambient wind ﬁeld of the prototype

of the Multibrid M5000 were carried out: azimuth scans

(Fig. 1a) and elevation scans (Fig. 1b). For the azimuth

scans, the elevation of the laser beam was constant and the

azimuth angle was continuously changed. The duration of

an azimuth scan was 15 s and the scan speed 28s

. For

Corresponding author address: Yvonne Ka

¨sler, Deutsches

Zentrum fu

¨r Luft- und Raumfahrt, Mu

¨nchner Str. 20, Oberpfaffen-

hofen, D-82234 Wessling, Germany.

E-mail: yvonne.kaesler@dlr.de

SEPTEMBER 2010 K A

¨SLER ET AL. 1529

DOI: 10.1175/2010JTECHA1483.1

Ó2010 American Meteorological Society

the elevation scans, the azimuth angle was not changed

but the elevation was increased continuously. One single

elevation scan measurement took 10 s. The scan speed

for these types of scans was 18s

. The great advantage

of the measurements is that it is possible to obtain in-

formation about the airﬂow in front and behind the wind

turbine at the same time. This is in contrast to wake

measurements that were carried out previously with

a focused continuous wave lidar system (Bingo

¨l et al.

2010; Trujillo et al. 2010).

2. Results

Figure 2 shows two measurements of the ambient

ﬁeld of the Multibrid M5000 in the nocturnal boundary

layer: one elevation scan (Fig. 2a) and one azimuth scan

(Fig. 2b). The wind was blowing from the northeast di-

rection, parallel to the laser beam. Atmospheric conditions

were very stable when the measurements were taken. The

turbulence intensity at hub height (102 m) upstream

from the wind turbine was only 2.8%. This means that the

wake effects are expected to be very distinct (Barthelmie

et al. 2003). To determine this turbulence intensity, the

LOS velocity at a range of two rotor diameters upstream

from the M5000 at hub height was observed for 10 min.

This corresponds to 46 scans or a time series with

46 values. The turbulence intensity I was then calculated as

I5sLOS

LOS .

In this equation s

LOS

is the standard deviation and LOS

is the mean of the time series of the line-of-sight velocity

(Stull 1988).

It needs to be considered that s

LOS

and I, which were

determined by the lidar data, are signiﬁcantly damped in

comparison to anemometer measurements because of

the long pulse length (cf. Table 1) of the pulsed wind

lidar.

In Fig. 2a, the elevation scan through the rotor blades,

only the wake effects of the prototype of the Multibrid

M5000 can be seen. The range is the distance from the

2-mm lidar, and the height reﬂects the elevation of the

TABLE 1. The 2-mm lidar system parameters and data acquisition.

FWHM is the full width at half maximum.

Slave laser (Tm:LuAG)

Wavelength 2.022 mm

Repetition rate 500 Hz

Pulse energy 1.5 mJ

Pulse length (FWHM) 0.5 ms575 m (0.5 ms3cO2)

c: speed of light

Telescope Off-axis type

Aperture: 108 mm at lidar

with afocal setting

Scanner Double wedge with

variable speed

Measurement range Maximum: 500 m to .10 km

Processed data: 500 m to 3 km

Data acquisition

Sample length 0.3 m

Width of lidar

range gates

512 30.3 m 5154 m

Number of range gates

in 3 km

Sampling rate between

two scans

Elevation scan: 14 s

Azimuth scan: 22 s

FIG. 1. (a) Map of the area in Bremerhaven with the position of

the 2-mm lidar and the wind turbines. A top view of an azimuth scan

is given in the layout plan. (b) Side view of an elevation scan.

1530 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 27

laser beam. The position of the wind turbine and the

wind direction are indicated in the picture. Between

1800 and 2600 m, a decrease of the absolute value of the

LOS component of the wind vector downstream from

the Multibrid M5000 can be clearly observed. One rotor

diameter (116 m 51D) downstream from the LOS ve-

locity at hub height is reduced about 66% (the maximum

of LOS velocity reduction) in comparison to the values

upstream. After 7Ddownstream (18% reduction of the

LOS velocity at hub height), the wind ﬁeld only recovers

very slowly. The velocity deﬁcit downstream from the

wind turbine is shown in Fig. 3. Because of data storage,

the maximum range recorded in this measurement was

9Ddownstream. At this range the reduction of the LOS

velocity was 16% of the initial value. The black and red

horizontal lines at a range between approximately 1700

and 2100 m in Fig. 2a indicate the areas where the laser

beam hits the rotor blades. The backscatter of the blades

is several orders of magnitude stronger than the atmo-

spheric backscatter. The two lines represent two differ-

ent rotor blades. The position of the wind turbine is at

a range of 1823 m. One needs to consider that the pulse

length of the laser (0 to 0) is approximately 400 m. When

the beginning of the laser pulse hits the blades, there are

120 m in front of the turbine that are inﬂuenced by the

blade because of the length of the laser pulse. With a

symmetric laser pulse, one would expect that the black

lines would end approximately 120 m after the turbine

(i.e., at a range of approximately 1940 m) when the end

of the laser pulse hits a part of the blades. The laser pulse

has a small tail containing little energy. Therefore, the

horizontal lines are longer after the turbine than in front

of the turbine. As a result, the shadows are approximately

400 m long and not only 240 m. If not only part of the

laser beam but the whole beam hits a blade, there is a

black line behind the position of the turbine until the end

of the measurement range (cf. Fig. 4). The different colors

of the horizontal lines (black and red) therefore depend

on the LOS velocity of the blades.

In Fig. 2b an azimuth scan through the rotor blades

can be seen. The width indicates the different azimuth

angles. The elevation was 3.38for this measurement.

This resulted in a scan through the rotor blades only

somewhat above hub height. In comparison to Fig. 2a,

this scan also covers the disturbance of the wind ﬁeld by

other wind turbines near the Multibrid M5000 prototype

(M5000-1). The location of the different wind turbines

can be estimated by the horizontal lines where the laser

beam hits the rotor blades. There is another wind tur-

bine (M5000-2) of the same type as M5000-1 and two

turbines of different types. According to the horizontal

lines, the laser beam hits one blade of the M5000-2 and

TABLE 2. Technical data of the M5000.

Rotor diameter 116 m

Hub height 102 m

Length of a rotor blade 62 m

Rated power 5 MW

Rated wind speed 12 m s

Maximum elevation height of the rotor 164 m

Speed range of the rotor 4.5–14.8 min

610%

FIG. 2. (a) Results for the LOS component of the wind vector for

an elevation scan through the rotor blades at night. The wind was

blowing from the northeast parallel to the laser beam. Wind di-

rection and position of the wind turbine are indicated in the ﬁgure.

The white vertical line indicates the rotor disc. (b) Azimuth scan at

night covering the ambient wind ﬁeld of several wind turbines,

including the M5000-1 prototype. The vertical white lines indicate

the position of the rotor discs of M5000-1 and M5000-2.

FIG. 3. Velocity deﬁcit downstream from the wind turbine derived

from the elevation scan in Fig. 2a.

SEPTEMBER 2010 K A

¨SLER ET AL. 1531

two blades of the M5000-1. It needs to be considered that

it is possible to hit one or more blades with one azimuth

scan. It seems as if the turbines different from M5000

have a smaller effect on the wind ﬁeld. But the shorter

wakes are due to the elevation of the azimuth scan. This

means that for these wind turbines, the wake is only cut

by the scan. By analyzing the length of the wake of the

M5000 wind turbines of this measurement, it can be

found that the results correspond to the results of the

elevation scan. Thus, it can be shown that for the mea-

surements of the Multibrid M5000 turbines, the eleva-

tion scan as well as the azimuth scan captured the whole

wake and not only cut it.

Figure 4 shows an azimuth scan during daytime with

a turbulence intensity of more than 15%, two rotor di-

ameters upstream from the wind turbine at hub height.

Notice that this measurement was taken on another day,

and that the wind was blowingfrom a southwest direction

parallel to the laser beam. As mentioned before, wake

effects are supposed to be less distinct in a turbulent at-

mosphere (Barthelmie et al. 2003). This is evident when

Fig. 2b is compared to Fig. 4: the length of the wakes of

the M5000 turbines is shorter in Fig. 4.

To sum up, it can be said that ithas been shown that the

2-mm Doppler lidar is a convenient tool for wind energy

research and could help to improve the modeling of wind

turbine wakes for single turbines, as well as for wind

parks. A method for wake capturing has been developed.

We plan a detailed analysis for these measurements in

the nocturnal boundary layer, and of measurements, were

taken during daytime in a more turbulent atmosphere.

Furthermore, a comparison with the Parallelized Large-

Eddy Simulation (LES) Model (PALM; Steinfeld et al.

2009; Raasch 2010) is already in preparation.

Acknowledgments. This work is done in the framework

of the project ‘‘LIDAR-Development of LIDAR tech-

nologies for the German offshore test ﬁeld’’ of the German

Federal Environment Ministry (BMU; Contract 0327642).

Thanks are given to Areva - Multibrid GmbH for pro-

viding access to the M5000 wind turbine and to the whole

Lidar group of the Endowed Chair of Wind Energy at

the University of Stuttgart, especially Juan Jose

´Trujillo

for his helpful discussions and cooperation.

REFERENCES

Barthelmie, R. J., L. Folkerts, F. T. Ormel, P. Sanderhoff,

P. J. Eecen, O. Stobbe, and N. M. Nielsen, 2003: Offshore wind

turbine wakes measured by sodar. J. Atmos. Oceanic Technol.,

20, 466–477.

Bingo

¨l, F., J. Mann, and G. C. Larsen, 2010: Light detection

and ranging measurements of wake dynamics. Part I: One-

dimensional scanning. Wind Energy, 13, 51–61.

¨pp, F., S. Rahm, and I. Smalikho, 2004: Characterization of

aircraft wake vortices by 2-mm pulsed Doppler lidar. J. Atmos.

Oceanic Technol., 21, 194–206.

Raasch, S., cited 2010: PALM—A parallelized LES model. [Available

online at http://www.muk.uni-hannover.de/~raasch/PALM_group/

doc/app/index.html.]

Rahm, S., and I. Smalikho, 2008: Aircraft wake vortex measure-

ment with airborne coherent Doppler lidar. J. Aircr., 45, 1148–

1155.

Smalikho, I., F. Ko

¨pp, and S. Rahm, 2005: Measurement of at-

mospheric turbulence by 2-mm Doppler lidar. J. Atmos. Oce-

anic Technol., 22, 1733–1747.

Steinfeld, G., S. Raasch, J. Tambke, J. Peinke, and D. Heinemann,

2009: Development of a large-eddy simulation (LES) model

for modelling the far-wake effects of offshore windfarms.

Proc. Euromech Colloquium 508 on Wind Turbine Wakes,

Madrid, Spain, E.T.S.I. Industriales and Universidad Polite

´cnica

de Madrid, in press.

Stull, R. B., 1988: An Introduction to Boundary Layer Meteorology.

Kluwer Academic Publishers, 666 pp.

Trujillo,J.-J.,F.Bingo

¨l, G. C. Larsen, J. Mann, and M. Ku

¨hn,

2010: Light detection and ranging measurements of wake

dynamics. Part II: Two-dimensional scanning. Wind Energy,

in press.

FIG. 4. Azimuth scan during daytime covering the ambient wind

ﬁeld of several wind turbines including the M5000-1 prototype. The

white lines show the position of the rotor discs of M5000-1 and

M5000-2.

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An Introduction to Boundary Layer Meteorology

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Jan 1988

Roland B. Stull

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Nov 2005

Two methods for the estimation of the turbulence energy dissipation rate (TEDR) from data measured by a 2-μm coherent Doppler lidar are described in this paper. Based on data measured at the Tarbes-Lourdes-Pyrénées International Airport in summer 2003, height profiles of TEDR have been retrieved. The results of TEDR estimation both from the Doppler spectrum width and from the velocity structure function are compared. Moreover, the experiment has been treated by numerical simulation and the theoretical results have been used for verification of the described methods.

Characterization of Aircraft Wake Vortices by 2-mum Pulsed Doppler Lidar

Article

Feb 2004

The 2-μm pulsed Doppler lidar, already successfully used for wind and turbulence measurements, has been modified for long-range wake-vortex characterization. In particular, a four-stage data processing algorithm has been developed to achieve precise profiles of tangential velocities from which the vortex parameters such as trajectories, core separation, tilt angle, and circulation can be derived. The main advantage of the pulsed lidar is its long-range capability of more than 1 km. This allows for observations over long periods from the moment of wake generation to a progressed state of vortex decay. With the field experiment at Tarbes airfield the potential of the 2-μm pulsed Doppler lidar for full-scale wake-vortex characterization has been demonstrated. Two examples showing the parameters of wake vortices generated by large transport aircraft (LTA)-type aircraft will be presented.

Light detection and ranging measurements of wake dynamics. Part II: Two-dimensional scanning

Article

Jan 2011
WIND ENERGY

A nacelle-mounted lidar system pointing downstream has been used to measure wind turbine wake dynamics. The new measurement and data analysis techniques allow estimation of quasi-instantaneous wind fields in planes perpendicular to the rotor axis. A newly developed wake tracking procedure delivers the instantaneous transversal wake position which is quantitatively compared with the prediction of the Dynamic Wake Meandering model. The results, shown for two 10-min time series, suggest that the conjecture of the wake behaving as a passive tracer is a fair approximation; this corroborates and expands the results of one-dimensional measurements already presented in the first part of this paper. Consequently, it is now possible to separate the deterministic and turbulent parts of the wake wind field, thus enabling capturing the wake in the meandering frame of reference. The results correspond, qualitatively and to some extent quantitatively, to the expectations from CFD simulations which are compared in the paper. Copyright © 2010 John Wiley & Sons, Ltd.

Light detection and ranging measurements of wake dynamics Part I: One-dimensional Scanning

Article

Jan 2010
WIND ENERGY

The vast majority of wind turbines are today erected in wind farms. As a consequence, wake-generated loads are becoming more and more important. In this first of two parts, we present a new experimental technique to measure the instantaneous wake deficit directly, thus allowing for quantification of the wake meandering, as well as the instantaneous wake expansion expressed in a meandering frame of reference. The experiment was conducted primarily to test the simple hypothesis that the wake deficit is advected passively by the larger-than-rotor-size eddies in the atmospheric flow, and that the wake at the same time widens gradually, primarily because of mixing caused by small-scale atmospheric eddies. In this first paper, we focus on our new measurement technique, and test if the wake meandering follows the wind direction fluctuations, i.e. if it is advected passively in the lateral direction. The experimental results are used as a preliminary verification of a wake meandering model that essentially considers the wake as a passive tracer. Copyright © 2009 John Wiley & Sons, Ltd.

Characterization of Aircraft Wake Vortices by 2mü Pulsed Doppler Lidar

Article

Feb 2004

The 2-mum pulsed Doppler lidar, already successfully used for wind and turbulence measurements, has been modified for long-range wake-vortex characterization. In particular, a four-stage data processing algorithm has been developed to achieve precise profiles of tangential velocities from which the vortex parameters such as trajectories, core separation, tilt angle, and circulation can be derived. The main advantage of the pulsed lidar is its long-range capability of more than 1 km. This allows for observations over long periods from the moment of wake generation to a progressed state of vortex decay. With the field experiment at Tarbes airfield the potential of the 2-mum pulsed Doppler lidar for full-scale wake-vortex characterization has been demonstrated. Two examples showing the parameters of wake vortices generated by large transport aircraft (LTA)-type aircraft will be presented.

An Introduction To Boundary Layer Meteorology

Chapter