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Improving WAsP predictions in (too) complex terrain

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. Outline. Case study in northern Portugal RIX and RIX concepts RIX configuration Correction procedure

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Improving WAsP predictions in (too) complex terrain

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  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

  2. 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

  4. Cross-correlation of wind speeds (From Bowen and Mortensen, 1996 EWEC conference)

  5. 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 => 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 Meteorology76, 137-164.

  6. Terrain steeper than c is indicated by the thick red (radial) lines 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 ΔRIX = RIXWTG – RIXMET ΔRIX < 0  under-prediction ΔRIX > 0  over-prediction Complex terrain analysis

  7. Prediction error vs. RIX difference “This performance indicator provides encouraging results…”(Bowen and Mortensen, 1996 EWEC conference)

  8. 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

  9. 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 Maps for RIX calculation and modelling

  10. Wind speed prediction error vs. RIX

  11. ln(Up/Um) vs. RIX Up = Um exp( RIX) where  = 1.5 R = 3500 m and c = 0.3

  12. Influence of radius and critical slope R2 for different values of the calculation radius and critical slope.

  13. 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:ΔRIX = 0

  14. 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 @ 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 Improving WAsP predictions in complex terrain

  15. ln(AEPp/AEPm) vs. RIX @ 50 m a.g.l.

  16. Step 1-2: AEP [GWh] = F(WAsP)

  17. Step 3-4: AEP [GWh] = F(WAsP, RIX)

  18. Case study summary WAsP predictions in (too) complex terrain were improved • 69% on average for five sites with 10% < RIX < 33% • 88% on average for sites with ∆RIX > 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?

  19. 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% Wind farm in complex terrain

  20. 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…

  21. 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

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