Experimental characterization of marine atmospheric boundary layer in the Havsul

area, Norway

10th Deep Sea Offshore Wind R&D Conference , 24./25.01.2013, Trondheim10th Deep Sea Offshore Wind R&D Conference , 24./25.01.2013, Trondheim

Konstantinos Christakosa, Joachim Reudera, Birgitte R. Furevika,b

a Geophysical Institute, University of Bergen, Norwayb Norwegian Meteorological Institute, Norway

Outline

• Introduction

• Data overview

• Results

• Outlook

Source: Norcowe

Marine Atmospheric Boundary Layer (MABL)

• Average wind profiles• Wind shear over the rotor disk• Turbulence• Atmospheric stability• Wind-waves interactions

the main problem:• the lack of observational data

in the relevant altitude range (sea surface to 200m)

Source: http://www.ieawind.org/GWEC_PDF/GWEC%20Annex23.pdf

Marine Atmospheric Boundary Layer (MABL)

• Average wind profiles• Wind shear over the rotor disk• Turbulence• Atmospheric stability• Wind-waves interactions

the main problem:• the lack of observational data

in the relevant altitude range (sea surface to 200m)

Remote sensing instruments

(i.e LIDAR)

Source: http://www.ieawind.org/GWEC_PDF/GWEC%20Annex23.pdf

LIDAR (LIght Detection And Ranging)

Advantages:Advantages:• Simultaneous measurements in

several heights (up to 200 m)• 3D wind velocity vector

(u, v, w)

Disadvantage :Disadvantage :• absence of temperature

measurements (vertical gradient).

Atmospheric stability?Atmospheric stability?

Stability and turbulence

affect wind energy production [1], [2]

Source: VestaVind Offshore

How can atmospheric stability be estimated? Wharton and Lundquist (2012) suggested different turbulence parameters for

classifying wind profiles by stability [2] ,[3], based on onshore data (in western North America)

Turbulence parameters

• The horizontal turbulent intensity is dimensionless parameter which is defined as the standard deviation of horizontal velocity fluctuation divided by the mean horizontal wind speed:

• The TKE is defined as the sum of the velocities variances in latitudinal (u), longitudinal (v) and vertical (w) direction divided by 2 :

I U=σU

U

TKE=12(σ u

2+σ v

2+σ w

2)

Data overview• 4 years(2008-2012) wind profile data were collected at the small island of

Storholmen which is located 8 km northwest of the island of Vigra on the west coast of Norway.

Fig.1. Location of Storholmen island (black square) in Ålesund, Norway. Source: Google Maps

Data overview• The wind speed was measured

by WindCube v.1 LIDAR at 8 height levels between 60 m and 200 m a.s.l. (above sea level)

• For higher levels the data availability was reduced due to low aerosol concentration in the air which leads to a low SNR.

Only complete 10 min. average wind profiles (75249) between 60 m and 150 m a.s.l. have been used for the presented analysis.

Source: Vestavind Offshore

Investigation of Turbulence Intensity and Wind Speed

• Log-normal distribution is applied to describe the turbulence intensity distribution for different classes of wind speed at 100 m a.s.l.

Results:

For increasing wind speed:

1. the center of distribution moves towards to lower turbulence intensities

2. The probability density for the peak value increases

Turbulence Intensity and Wind Profiles

• Average wind profiles for different classes of horizontal turbulence intensity (at 100 m a.s.l.).

• The number of profiles for each class is given in parenthesis

Results :• Clear dependency between

turbulence intensity and wind profiles

• For turbulence intensities greater than 6%, increase of U is related to decrease of turbulence intensity.

• For turbulence intensities below 9% the average profiles are closely grouped between 10m/s and 12m/s

• The wind profiles have been normalized to 1 at 100m a.s.l.

Result:• A general increase in

wind shear for decreasing turbulence intensities.

Turbulence Intensity and Wind Shear

TKE and Wind Profiles• Average wind profiles for

different classes of TKE (at 100 m a.s.l.).

• The number of profiles for each class is given in parenthesis

Results :• Clear dependency of TKE on

wind profiles• The higher the TKE, the

higher the wind speed • TKE is mainly generated by

wind shear in MABL

• The wind profiles have been normalized to 1 at 100m a.s.l.

Result:• For lower levels:

for increasing TKE, the wind shear decreasing

• For higher levels:

very little variation between TKE and wind shear

TKE and Wind Shear

Summary and outlook

• Measurements of offshore wind conditions are essential for the accurate characterization of MABL• Remote sensing instruments can provide a rich of source

data for a better understanding of turbulence of the wind field

• Turbulence parameters such as turbulence intensity and TKE are strongly related to the wind profiles

• For offshore conditions turbulence intensity seems more promising for the classification of stability

• Need for simultaneous measurements of temperature gradient and turbulence parameters for the classification of stability

Thanks for your attention !

Source: Norcowe

References

[1] B J Vanderwende and J K Lundquist (2012) The modification of wind turbine performance by statistically distinct atmospheric regimes, Environ. Res. Lett. 7

[2] Wharton S and Lundquist J K (2012) Atmospheric stability affects wind turbine power collection, Environ. Res. Lett. 7

[3] Wharton S and Lundquist J K (2012) Assessing atmospheric stability and its impacts on rotor-disk wind characteristics at an onshore wind farm, Wind Energy 2012 15:525–546

The authors express appreciation to Vestavind Offshore AS for sharing the wind data. The leader author expresses his gratitude to NORCOWE and Statoil ASA for received travel grant.

Acknowledgements