Fabrizio Magni magni.fabrizio@gmail.com

Mattia Boccolini

mattia.boccolini@garradhassan.com

Christof Stork christof.stork@garradhassan.com

Study of extreme wind speedfor italian wind farm sites

EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork

Study of extreme wind speedfor italian wind farm sites

• To assess the validity of three different codes to estimate extreme wind speed;

Targets:

• To identify factors that improve extreme wind speed predictions;

• To compare directional roughness length and extreme wind speed values;

• To compare the whole data set and the data subset analysis results

Slide : 02 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Study of extreme wind speedfor italian wind farm sites

• Analysis of anemometer data;

Wind shear evaluation:

• Roughness length calculation;

1 10

i i i i iv v

1 2i i i id d d d

0 0ln ln ln lnz zV V z z z z

z zV V z z

• Time average calculation;

3600 0( ) ( )* , ,t aveU z U z T z

3600 600 60 31,05 0,84 0,67ref ref ref refV V V V

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1 10 100 1000 10000

t(sec)

U / t

36

00

U

Slide : 03 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Study of extreme wind speedfor italian wind farm sites

Wind shear evaluation:

Difference between global and sector results

Some errors in extreme wind speed evaluation

Slide : 04 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Sitesz0 [m] z0 [m] z0 [m]

Campania 0.004 0.126 0.017 0.127 0.0002 0.089

Puglia 60 0.085 0.269 0.083 0.265 0.0648 0.249

Puglia 10 0.069 0.251 0.065 0.239 0.0423 0.220

Sardegna 0.252 0.231 0.532 0.217 0.1845 0.199

Global Sector Extreme

Study of extreme wind speedfor italian wind farm sites

• Standard Gumbel (SG);

Extreme wind speeds evaluation:

• Modified Gumbel (MG);

• Improved Independent Storm (IMIS);

• Gumbel-Bergström (GB);

Site location

Length of data set

N. of data

Campania 5.43 years 47,652 10.0 30.0 - - -Sardegna 4.28 years 37,510 20.0 30.0 - - -Puglia 10 1.05 years 58,458 9.0 18.0 25.3 36.6 37.1 Puglia 60 3.86 years 33,761 9.0 25.0 37.0 - -

Height of measurements

CodesMethods

• Eurocode 1 (EC1)

• Testo Unitario (TU)

• Old Italian Code (DM’96)

Slide : 05 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

[m]

Too short observation period

Study of extreme wind speedfor italian wind farm sites

Aspects involved in result accuracy:

Slide : 06 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

;

• Amplitude of data sets;

• Accuracy of Roughness length evaluation

• Extreme wind speed evaluation techniques;

• Number, height and precision of anemometers;

Study of extreme wind speedfor italian wind farm sites

Slide : 07 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Err [%]

methodscodes

36.6 28.0 30.0 31.8 - - - -31.4 22.3 24.4 26.3 28.2 28.7 28.5 25.3 32.2 23.2 25.3 27.1 29.0 29.5 29.3 26.2

11.3 2.0 2.8 3.5 - - - -10.1 0.8 1.5 2.3 7.0 3.2 3.0 3.9

2.3 7.9- 7.1- 6.3- 1.1- 5.2- 5.5- 4.5-

21.4- 11.8 6.7 7.6 - - - -21.4- 11.8 6.7 7.6 19.5- 7.2 5.9 3.8 32.4- 3.6 2.0- 1.0- 30.3- 1.4- 2.8- 5.2-

23.4- - - 2.7 - - - -23.4- - - 2.7 26.9- - - 0.9- 34.1- - - 5.6- 37.8- - - 9.6-

DM'96TU

EC1

Site

Puglia 10

GB SG MG

DM'96TU

EC1

DM'96TU

EC1

Puglia 60

Sardegna

GB SG

DM'96TU

EC1

Type of Data Ve50 [m/s] Ve1 [m/s]

Campania

MG IMIS IMIS

Extreme wind speed evaluation techniques:Percentage difference between code predictions and data results

[Err %] [Err %]

Study of extreme wind speedfor italian wind farm sites

Amplitude of data sets:

Slide : 08 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Invo

lved

da

ta

• SG, MG : 1 value/year

• IMIS : ≈ 100 values/year

• GB : All data recordedLess accuracyErrors sensitivityGood accuracy

Years Vref [Err%] Years Vref [Err%] Years Vref [Err%]

Year 1 1.6 Years 1-2 11.2- Years 1-2-3 3.2- Year 2 13.8- Years 3-4 10.6 Years 4-5-6 4.2 Year 3 16.8 Years 5-6 2.1- Year 4 19.4 Year 5 3.3 Year 6 25.9-

Extreme wind speed in Campania using data subset

Study of extreme wind speedfor italian wind farm sites

Number, height and precision of anemometers:

Few anemometers, too close to each other and to the ground

Slide : 09 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Large errors in wind shear evaluation

z

V

Campania Data set:

3030

1010

V V k

V V k

k

100 97 567 1,427

100 100 1,954 5,720

0.15

0.3

-1.64 -1.00 +1.00 +1.64

Err [%]

Roughness lenght

evaluation

Study of extreme wind speedfor italian wind farm sites

Accuracy of Roughness length evaluation:

Slide : 10 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

2 0

1 2 0

12 0

ln ln

ln lnk z z

k z k zV k z V

z k z

2 0 0

0

1 1

1

% 100k z z

z

V k z V k zErr

V k z

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

1 1.2 1.4 1.6 1.8 2 2.2

K2

Err

[%

]Accurate

evaluation of extreme wind speed

anem hubz z

Study of extreme wind speedfor italian wind farm sites

Wind speed amplification coefficient:

Slide : 11 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

1 2

00

( ), , 1

2.5 lnave

ave

c TT z

z z

0z Exact roughness length

2 0k z Measured roughness length

It is important:

• to measure roughness length carefully

• to choose accurate time averaging value

Study of extreme wind speedfor italian wind farm sites

Slide : 03Slide : 12 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Return Period dependence:

1 0,2 ln ln 1 1* *

1 0,2 ln ln 0,98

RR R ref ref R

TV T V V T

ln ln 1 1* *

ln ln 0.98R

R R ref ref R

b a TV T V V T

b a

Tr

• TR dependence formula (EC1, TU, DM’96);

• TR dependence formula (SG, MG, IMIS, GB);

Err. % TR = 1 TR = 500

SG 9.16 4.96MG 5.97 2.37IMIS 8.25 3.33GB 5.46 3.89

TR

ACCURACY OF THE EXTRAPOLATION

Study of extreme wind speedfor italian wind farm sites

Slide : 03Slide : 13 EWEC 2007, Milan, 8 May 2007 - Boccolini, Magni, Stork -

Concluding remarks:

0 0(global) (sector)z z(anemometer) (hub)z z

600 secaveT n. anemometer 3

ref refV IMIS V TU

• More extensive study in terms of number and spread of masts as well as longer observation periods should be performed to draw robust conclusions