Mesoscale modelling for an offshore wind farm

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Mesoscale modelling for an offshore wind farm

• J. Badger, R. Barthelmie, S. Frandsen, M. B.
  Christiansen
• Risø National Laboratory, Denmark

EWEC, Athens 2006
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Outline

• Motivation
• Introduction
• Offshore impact of onshore orography
• Internal boundary layer growth
• KAMM simulations for Nysted wind farm
• Conclusions

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Motivation

• Offshore wind farms are large, consider variation
  of
• wind conditions over farm area
• wind speed at hub height
• wind profile
• turbulence
• Offshore in situ wind measurements
• accurate
• limited point measurements
• SAR images
• spatial fields
• can be limited in number

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Motivation

• Mesoscale modelling provides
• spatial fields
• vertical profiles
• and can be verified against and complement
• point measurements
• SAR images

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Introduction KAMM
Karlsruhe Atmospheric Mesoscale
Model non-hydrostatic, regular horizontal grid,
stretched vertical coordinate (terrain following)
Typical resolution ?x 2km size 200 x 200 x 5.5
km
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Introduction gap wind
SAR image from Radarsat. Colour shows wind
speed. Wind vectors from NOGAPS model, operated
by U.S. Navy.
http//fermi.jhuapl.edu/sar/stormwatch/user_guide/
Source Young and Winstead, section II of Beal et
al see web link.
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Introduction barrier wind
SAR image from Radarsat. Colour shows wind
speed. Wind vectors from NOGAPS model, operated
by U.S. Navy.
Source Young and Winstead, section II of Beal et
al see web link.
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Introduction offshore effect of onshore orography
Mesoscale modelled climatology (KAMM/WAsP) for
Morocco. Shows gap flow and barrier wind features
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Introduction offshore effect of onshore orography

• The flow around/over an obstacle is dependent on
  thermal stratification and wind speed.
• Froude Number U / (h N)
• U velocity scale
• h height scale of obstacle
• N Brunt-Väisälä frequency
• N2 g/?0(d?/dz)

Froude Number gt 1
Froude Number lt1
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Introduction offshore effect of onshore orography

• For higher stability condition, higher N
• upstream influence of obstacle can increase
• lower obstacles can block flow
• higher wind speed is blocked
• Possible influence of terrain 100 km, could
  play large role in offshore wind resource.

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Introduction effect of change of surface type
surface roughness change ? balance of forces
disturbed.
u
u
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Introduction effect of change of surface type

• Internal boundary layer
• roughness change (coastal jets)
• surface fluxes changes (nocturnal jets, maritime
  jets)
• inflow stratification
• influence of shape of coastline
• detachment and interaction of coastal jets (Orr
  et al, 2004)
• Scales LR (N h0) / f 100 km

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Nysted flow modelling set-up

• Domain
• centred on Nysted
• dx 2 km
• size 200 x 200 km
• Orography for modelling domain contour interval
  25 m

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Nysted flow modelling set-up

• KAMM has been integrated with the
• wind speed profile
• temperature profile
• 12 direction sectors, 30 degree interval

mean NCEP/NCAR 1965-1998
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Nysted flow modelling set-up
Land Offset Sea Offset Set A 0K 0K Set
B -5K 5K Set C 5K -5K What impact does
the surface temperature offset have on wind
characteristics at the site?
control
warm sea cold land
cold sea warm land
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Nysted flow modelling results, example

• Example wind field
• 70 m winds
• geostrophic forcing 240 degrees

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Nysted flow modelling results, comparison
Johns Hopkins University SAR image
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Nysted flow modelling results, surface
temperature impact
warm sea cold land
cold sea warm land
control
Set A
Set B
Set C
wind speed at 70 m for 240 degrees geostrophic
forcing
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Nysted flow direction, speed characteristics
across farm
warm sea cold land
cold sea warm land
Set A
Set B
Set C
wind vectors at 70 m for 12 forcing direction
sectors
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Nysted flow speed profile characteristics across
farm
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Summary from Nysted

• Set B
• warm sea cold land
• smallest wind speed shear from 25 m to 70 m above
  sea level
• largest wind speed gradients across the wind
  farm, when there is a very short sea fetch
• Set C
• cold sea warm land
• largest wind speed gradients across the wind
  farm, when there is a medium length or complex
  sea fetch

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Conclusions

• Offshore wind resource inhomogeneous due to
• topographical effects
• internal boundary layer development effects
  (jets)
• influences can be felt far from coastlines 100
  km
• wind conditions vary within a wind farm
• Mesoscale modelling offers a useful tool to
• understand and predict these flow features
• estimate resource
• Acknowledgements Danish PSO project Large Wind
  Farms Shadow Effects (PSO FU 4103)