Improving WAsP predictions in too complex terrain

1
Improving WAsP predictions in(too) complex
terrain

• Niels G. Mortensen and Ioannis Antoniou
• Risø National Laboratory
• Anthony J. Bowen
• University of Canterbury

2006 EWEC 2 March 2006
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Outline

• Case study in northern Portugal
• RIX and ?RIX concepts
• RIX configuration
• Correction procedure
• Improving WAsP predictionsin (too) complex
  terrain?
• Wind farm verification
• Conclusions

3
Case study in northern Portugal
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Cross-correlation of wind speeds
(From Bowen and Mortensen, 1996 EWEC conference)
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Effect of a steep hill flow separation
The flow behaves to some extent as if moving
over a virtual hill with less steep slopes than
the actual hill gt actual speed-up is smaller
than calculated by WAsP N. Wood (1995). The
onset of flow separation in neutral, turbulent
flow over hills, Boundary-Layer Meteorology 76,
137-164.
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Complex terrain analysis

• Ruggedness index, RIX
• fraction of terrain surface which is steeper than
  a critical slope ?c
• Calculation radius 3.5 km
• Critical slope ?c 0.3
• Onset of flow separation
• Performance envelope for WAsP is when RIX 0
• Performance indicator, ?RIX
• ?RIX RIXWTG RIXMET
• ?RIX lt 0 ? under-prediction
• ?RIX gt 0 ? over-prediction

• Terrain steeper than ?c is indicated by the thick
  red (radial) lines

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Prediction error vs. RIX difference
This performance indicator provides encouraging
results(Bowen and Mortensen, 1996 EWEC
conference)
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The Ruggedness Index revisited

• Reanalyses of Portuguese data sets
• Larger and more detailed maps (SRTM 3)
• Improved RIX calculation
• Calculation implemented in WAsP and ME
• More calculation radii 72 rather than 12
• RIX configuration corresponds to BZ-model grid
• Improved predicted wind climate and power
  production
• Emergent wind speed distribution

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Maps for RIX calculation and modelling

• Hand-digitised map
• 8 by 8 km2
• 50- and 10-m cont.
• SRTM-derived map ?
• 20 km diameter
• 50-, 10- and 5-m height contours spot heights

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Wind speed prediction error vs. ?RIX
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ln(Up/Um) vs. ?RIX
Up Um exp(? ?RIX) where ? 1.5 R 3500 m and
?c 0.3
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Influence of radius and critical slope
R2 for different values of the calculation radius
and critical slope.
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Influence of calculation height

• Vertical wind profile in complex terrain with
  RIX 16
• 40-m anemometer used as predictor
• Vertical profile is predicted well because of
  similarity in ruggedness index?RIX 0

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Improving WAsP predictions in complex terrain

• Analysis procedure
• Observed Wind Climate
• sheltering obstacles
• roughness map
• elevation map
• Regional Wind Climate
• Application procedure
• Regional Wind Climate
• sheltering obstacles
• roughness map
• elevation map
• Predicted Wind Climates
• power and thrust curves
• Predicted wind farm AEP

• Post-processing
• Insert WTG at met. stations
• Make cross-predictions _at_ hhub
• Plot ln(Pp/Pm) vs. ?RIX
• Linear fit Pp Pm exp(? ?RIX)
• Slope of trend line ?
• Correct production estimates
• Apply correction factor
• Pm Pp/exp(? ?RIX)
• Corrected gross AEP
• Apply wake model results
• Corrected net AEP

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ln(AEPp/AEPm) vs. ?RIX _at_ 50 m a.g.l.
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Step 1-2 AEP GWh F(WAsP)
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Step 3-4 AEP GWh F(WAsP, ?RIX)
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Case study summary

• WAsP predictions in (too) complex terrain were
  improved
• 69 on average for five sites with 10 lt RIX lt
  33
• 88 on average for sites with ?RIX gt 10
• In addition, we have found
• SRTM 3 data can be applied for wind resource
  assessment
• optimal configuration values for ruggedness index
  calculation
• an empirical relation between WAsP prediction
  error and ?RIX
• Can this be verified elsewhere?

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Wind farm in complex terrain

• Elevation map w/ 20-m contours
• 23-MW wind farm w/ 38 turbines
• Two reference met. stations (?)

• RIX coloured map, range 0-18
• Turbine site RIX range 4-14
• Met. station RIX range 4-5

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Prediction of power production

• Measured power productions, wind speeds and
  directions over one year available for analyses
• Measured wind farm power production overestimated
  by 13 using standard WAsP procedure
• Correction procedure applied
• Correction based on Portuguese data set
  (similarity)
• Percentage applied to each turbine site
• Corrected wind farm power curve applied
• After correction, the power production is
  overestimated by 3
• Prediction of actual AEP improved by 70
• Site is also partly forested

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Conclusions

• Ruggedness index RIX and performance indicator
  ?RIX
• Concepts supported by new data and procedures
• Optimum radius and slope for RIX determined
• (?RIX, ?U) relation not very sensitive to
  calculation radius R, critical slope ?c, or
  prediction height h
• Relation between WAsP prediction errors and ?RIX
• Linear relation between ln(Up/Um) or ln(Pp/Pm)
  and ?RIX
• ?RIX weighted with the wind rose does not improve
  the relation between ln(Up/Um) and ?RIX
• Correction procedure outside WAsP operational
  envelope
• Percentage can be applied to each turbine site
• Note, that all this is purely empirical
• Similarity of sites ridges, escarpments and
  mountain tops
• constant ? should be determined for site and
  height