OWEMES 2006, Civitavecchia, Italy

Jens Tambke, Lueder von Bremen, *Jörg-Olaf WolffForWind and *ICBM, Carl von Ossietzky University Oldenburg

John A. T. ByePhysical Oceanography, The University of Melbourne, Australia

Bernhard LangeISET, Universität Kassel, Germany

Lorenzo ClaveriFinnish Meteorological Institute, Helsinki, Finland

Francesco Durante,DEWI German Wind Energy Institute, Wilhelmshaven, Germany

Modelling ofWind Fields abovethe North Sea

Horns Reef

FiNO1

Jens Tambke, University of Oldenburg / Slide 2

Overview

Offshore Wind Speed Profiles

Influence of Thermal Stratification

at Horns Rev and FiNO1

Accuracy of Numerical Weather Analysis at FiNO1

New Model: Inertially Coupled Wind Profiles (ICWP)

Jens Tambke, University of Oldenburg / Slide 3

log(Height z)

Speed u(z)

neutralunstable stableClassical Profiles:

Logarithmic Profiles + Monin-Obukhov-Formula

L

z

z

zuzu

0

* ln)(

2*2*0 ,max

0123.0wu

gz

Charnock Relation for variable Roughness

© E

lsa

m A

/S

Jens Tambke, University of Oldenburg / Slide 4

Measurements at Horns Rev

• Cup anemometers at 4 heights

• Temperature at 3 heights (T)

• Investigated period: 10/2001– 4/2002

• German Weather Service DWD

• at 10m, 34m, 110m

- 15 m

- 62 m

- 45 m

- 30 m

(T) 55 m -

(T) -4 m -

(T) 13 m -

Numerical Weather Analysis

© E

lsam

A/S

Jens Tambke, University of Oldenburg / Slide 5

Wind Profiles and Predictions at Horns Rev

Different wind speed gradients!

DWD-Model

Observation

RMSE = 15%

Jens Tambke, University of Oldenburg / Slide 6

Influence of Thermal Stratification at Horns Rev

unstable stable

Binned wind speed ratios

Jens Tambke, University of Oldenburg / Slide 7

Influence of Thermal Stratification

U

z

T

g

Measure for atmospheric stability:

Bulk-Richardson-Number Rib

2

zU

zT

gRib

- Acceleration of Gravity

- Temperature [K]

- Measurement-Height

- Difference of virtual potential temperature between sea-surface and height z

- Wind speed at height z

Calculation of Monin-Obukhov (MO) Length L: Grachev and Fairall (1997)

)( bRifuncL

z allows calculation of Businger-Dyer stability

functions at Horns Rev and FiNO1

Jens Tambke, University of Oldenburg / Slide 8

Influence of Thermal Stratification at Horns Rev

unstable

stable

u(62m)u(15m)

= 1.5

u(62m)u(15m)

< 1.1

Jens Tambke, University of Oldenburg / Slide 9

Observations at FiNO1

Located in the German Bight 45km north of Borkum

Investigated period: 2004

Wind speed measurements at 33, 41, 51, 61, 71, 81, 91 and 103m height

Massive lattice mast causes strong flow distortion: Corrections are very important

Jens Tambke, University of Oldenburg / Slide 10

Binned Wind-Speed Ratios

Influence of Thermal Stratification at FiNO1

unstable stable

Jens Tambke, University of Oldenburg / Slide 11

Influence of Thermal Stratification at FiNO1

unstable

stable

u(51m)u(33m)

= 1.15

u(51m)u(33m)

< 1.05

Jens Tambke, University of Oldenburg / Slide 12

Influence of Thermal Stratification at FiNO1

unstable

stableu(103m)u(33m)

= 1.4

u(103m)u(33m)

< 1.1

Jens Tambke, University of Oldenburg / Slide 13

Comparison of Modelled Profiles at FiNO1

RMSE(103m) = 1.4m/s

MM5 (NCEP)

Observation

DWD analysis

wind directions between 190° and 250°

Jens Tambke, University of Oldenburg / Slide 14

Accuracy of DWD Analysis at FiNO1

Observation

Analysis

Wind Speed Potential Power Output

Mean Values vs. Hour of the Day, Average of 12 months, 2004

RMSE: 1.4 m/s RMSE: 13% of P(inst)

Jens Tambke, University of Oldenburg / Slide 15

2004: Mean 103m Wind Speeds in the DWD Analysis

Mean Potential Power Production

in the German Bight 2004:

51% of the Installed Capacity

Jens Tambke, University of Oldenburg / Slide 16

New Air-Sea-Interaction Model:Inertially Coupled Wind Profiles (ICWP)

1.) Coupling of Ekman- and Log-Profile2.) Coupling of wind and wave fieldHeight

Speed0

zB< 20m :

Matching height for

speed, stress and

eddy viscosity

Ekman Layer:

(z) = A u*2/f = const.

(z) = ρ ∂u/∂zWave Boundary Layer:

(z) = u* z Φ (MO-Log.)

(z) = (wave)

Jens Tambke, University of Oldenburg / Slide 17

Inertially Coupled Wind Profiles (ICWP)

Similarity Assumptions:

2.)

Ratio of drift velocities close to the air-sea interface

2)()()( BwaterBairI zuzuKwave

3.)

InertialCouplingRelation:

a drag law with respect to the matching height zB in air and sea

1.) Ratio of eddyviscosities

Ekman Spiral:

Onset at zB

~ 1/29

Jens Tambke, University of Oldenburg / Slide 18

Comparison of theoretical and observed Profiles at Horns Rev

ICWP Model

ObservationWAsP

Model Input: time series of wind speed at 30m height

WAsP bias = - 0.4 m/sICWP bias = -0.1 m/sRMSE(62m) = 6% (3%)

for wind directions between 135° and 360°

Jens Tambke, University of Oldenburg / Slide 19

Comparison of Mean Profiles at FiNO1

ICWP

Observation

WAsP

for wind directions between 190° and 250°

Model Input: time series of wind speed at 33m height

WAsP bias = - 0.3 m/sICWP bias = +0.1 m/s

RMSE(103m) = 10% (5.5%)

Jens Tambke, University of Oldenburg / Slide 20

Comparison of Mean Profiles at FiNO1

Observation

for wind directions between 190° and 250°

Model Input: time series of wind speed at 33m height

z0=0.2mm IEC-3

Jens Tambke, University of Oldenburg / Slide 21

Current Research: Analysis of Turbulence Intensities at FiNO1

Turbulence Intensity (σu/u) vs. Wind Speed (u) at 103m, Jan-Dec 2004

for wind directions between 190° and 250°

Jens Tambke, University of Oldenburg / Slide 22

Conclusions

2. Observed Wind Profiles show higher wind shears above 45m height than expected

3. The ICWP-Model reproduces these higher wind shears with an Ekman-Approach

1. Thermal stratification has a crucial impact on offshore wind profiles

Thank You for Your Attention!

This work was funded by the EU within the Projects ANEMOS and POW’WOW.