Comparison and Terrain Influence on Predictions with Linear and CFD Models

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Comparison between linear (WAsP) and CFD model (Meteodyn) on a potential project
RANS equation with one-equation closure scheme (k-L turbulence model)
Project covers an area of 11km x 8km
Equipped with 12 meteorological masts (recording from 6 months to 6 years of data)
Relatively complex (deep valleys, ridges, rolling mountains)
Mix of coastal and inland areas

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Comparison and Terrain Influence on Predictions with Linear and CFD Models

  1. 1. Comparison and Terrain Influence on Predictions with Linear and CFD Models CANWEA Annual Conference, Vancouver, BC October 04, 2011 GILLES BOESCH, M.Eng, Wind Project Analyst Hatch (Montreal), Canada
  2. 2. Overview• Introduction• Presentation of a test case• Model comparison, terrain influence• Conclusions and investigations 2
  3. 3. Introduction• CFD is now well established in the wind industry• Need to quantify the uncertainty associated to these models• Compare the errors with linear models• Influence of the errors with topography complexity – And how to deal with it 3
  4. 4. Test case• Comparison between linear (WAsP) and CFD model (Meteodyn) on a potential project• RANS equation with one-equation closure scheme (k-L turbulence model)• Project covers an area of 11km x 8km• Equipped with 12 meteorological masts (recording from 6 months to 6 years of data)• Relatively complex (deep valleys, ridges, rolling mountains)• Mix of coastal and inland areas 4
  5. 5. Test case Altitude RIXMasts (m) (%) • Forest diversity: M1 540 10.1 – Logged area M2 560 11.0 M3 421 22.4 – 15m high trees M4 420 17.9 – Regrowth M5 448 15.1 M6 521 16.6 • RIX (Ruggedness Index) M7 560 8.0 – % of slopes >30% in a 3500m radius M8 433 22.1 – RIX Variations: M9 440 11.8 • 2 to 25 over the entire projectM10 665 14.3M11 567 2.7 • 2.7 to 22.4 at the meteorological mastsM12 540 12.1 Variety of conditions to evaluate the behavior of the models 5
  6. 6. Test Case• Meteodyn settings : – Structured Mesh (30m cell size within the project area) – Use of a forest model (windflow over canopy) – Neutral stability class assumed (can induce errors for sea shore sites) – Resulting shear verified for some masts• Data : – Measured and Quality controlled – At 50m or 60m high (to avoid extrapolation errors) – Adjusted to long term with standard MCP method (to have the same reference) 6
  7. 7. Results – Methodology• Cross-Prediction Matrix – Predictors : Mast that predicts the others – Predicted : Wind Speed at the « Predicted Mast » Predicted M1 M2 M3 … M12 M1 M1 measured M1 predicts M2 M2 M2 predicts M2 measured Predictor M1 M3 M3 measured … … M12 M12 measured 7
  8. 8. Results - Methodology• Cross-Prediction Matrix – 12 x 12 matrix 132 cross predictions – For both WAsP and Meteodyn – No correction is applied to both models output – Correction often applied with WAsP because of wind speed inconsistencies in complex terrain• Converted into a Relative Error Matrix : V predicted Vmeasured %E Vmeasured• Resulting in 132 relative error values for each cross-prediction 8
  9. 9. Altitude Masts RIX (%) (m) M1 540 10.1 M2 560 11.0Results - Comparison M3 M4 M5 M6 421 420 448 521 22.4 17.9 15.1 16.6 M7 560 8.0 M8 433 22.1 M9 440 11.8 M10 665 14.3• Mean absolute errors M11 567 2.7 M12 540 12.1 Prediction Errors 25.0% 20.0% 15.0%Error (%) WAsP Error 10.0% Meteodyn Error 5.0% 0.0% M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 < 2km from water 9
  10. 10. Results - Comparison• Absolute errors (direct output from models) WAsP Meteodyn Min Error 0.0% 0.0% Max Error 34.0% 14.1% Average 7.7% 4.6%• On average, reduction of the error by 40%.• Some exceptions : 33 cases out of 132 show better results with WAsP 10
  11. 11. Results - Comparison• Generally, errors from both models have the same sign (positive/negative) 40.0% 30.0% 20.0%Relative Error (%) WAsP 10.0% Meteodyn 0.0% -10.0% -20.0%• The difference is in the magnitude 11
  12. 12. Results – RIX Analysis• RIX dependency: – WAsP : Error increase sharply when RIX > 15% – Meteodyn : Error is more constant RIX influence on cross-prediction errors 25.0% 20.0% Average Error (%) 15.0% Wasp Meteodyn 10.0% 5.0% 0.0% 0.0 5.0 10.0 15.0 20.0 25.0 RIX (%) 12
  13. 13. Results – RIX Analysis • RIX dependency: – Possibility to correct WAsP with ΔRIX (between 2 masts) – Correction based on a correlation between logarithmic error and ΔRIX for each cross- prediction : E(%) = A* ΔRIX + B – Can we correct Meteodyn based on the RIX ? Error vs dRIX - Meteodyn Error vs dRIX - Wasp 40.0% 40.0% 30.0% y = 0.5552x 30.0% y = 1.0632x R² = 0.6345 R² = 0.7025 20.0% 20.0% Error (%)Error (%) 10.0% 10.0% 0.0% 0.0% -30.0% -20.0% -10.0% 0.0% 10.0% 20.0% 30.0% -30.0% -20.0% -10.0% 0.0% 10.0% 20.0% 30.0% -10.0% -10.0% -20.0% -20.0% -30.0% -30.0% ΔRIX (%) ΔRIX (%) 13
  14. 14. Results – RIX Analysis • CFD RIX dependency: – Error increases when ΔRIX increases – Error and ΔRIX seem to be correlating (not as good than Wasp however) – The slope is lower for Meteodyn  Influence of site topography differences is lower Error vs dRIX - Meteodyn Error vs dRIX - Wasp 40.0% 40.0% 30.0% y = 0.5552x 30.0% y = 1.0632x R² = 0.6345 R² = 0.7025 20.0% 20.0% Error (%)Error (%) 10.0% 10.0% 0.0% 0.0% -30.0% -20.0% -10.0% 0.0% 10.0% 20.0% 30.0% -30.0% -20.0% -10.0% 0.0% 10.0% 20.0% 30.0% -10.0% -10.0% -20.0% -20.0% -30.0% -30.0% ΔRIX (%) ΔRIX (%) 14
  15. 15. Results – RIX Analysis• Wasp RIX Correction: – 12 towers available – Equation based on 11 towers and evaluate how it corrects the 12th tower Prediction Errors 25.0% 20.0% 15.0% Error (%) WAsP Error Meteodyn Error 10.0% WAsP RIX Corrected Error 5.0% 0.0% M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 15
  16. 16. Results – RIX Analysis• Meteodyn RIX Correction: – Same methodology with updated correction equation Prediction Errors 25.0% 20.0% 15.0% WAsP ErrorError (%) WAsP RIX Corrected Error Meteodyn Error 10.0% Meteodyn RIX Corrected Error 5.0% 0.0% M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 16
  17. 17. Results – RIX Analysis• Summary of average error: Wasp 7.7 % Wasp RIX Corrected 4.3 % Meteodyn 4.6 % Meteodyn RIX Corrected 3.1 % – RIX correction with Meteodyn produces promising results – Reduction by 44% of the error after correcting Wasp with the RIX. – Reduction by 33% of the error after correcting Meteodyn with the RIX. – RIX correction with Wasp compared to Meteodyn direct output shows similar errors. 17
  18. 18. Conclusions• In general, a project in complex terrain requires lots of masts• An alternative is the use of a CFD model but linear corrected models can give good results too• Only few litterature over relation between RIX and CFD models• But quantification of CFD errors is more complex (topography / volume discretisation, forest model etc.)  In some cases error is bigger 18
  19. 19. Conclusions• To go further : – Try with concurrent data (when possible) to avoid MCP related errors – How does RIX correction with CFD performs for other sites ? – Introduction of new complexity index (takes into account RIX, distance, vegetation, stability…) 19
  20. 20. Thank you for your attention Gilles Boesch, M.Eng Wind Project Analyst Hatch Ltd GBoesch@hatch.ca 20

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