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Effects of Upwind Roughness Changes and Impacts on Hub-Height Winds

Effects of Upwind Roughness Changes and Impacts on Hub-Height Winds. Peter A. Taylor 1,2 , Wensong Weng 1 and James R. Salmon 2 1 Centre for Research in Earth and Space Science, York University, Toronto and 2 Zephyr North Canada, Burlington, Ontario. CANWEA, September 2009.

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Effects of Upwind Roughness Changes and Impacts on Hub-Height Winds

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  1. Effects of Upwind RoughnessChanges and Impacts on Hub-Height Winds Peter A. Taylor1,2, Wensong Weng1 and James R. Salmon2 1 Centre for Research in Earth and Space Science, York University, Toronto and 2 Zephyr North Canada, Burlington, Ontario CANWEA, September 2009

  2. A 2-D numerical model is developed to study atmospheric boundary-layer flow over single or multiple changes in surface conditions. These changes include surface roughness, thermal and moisture properties. • A simple, analytic model dealing with the surface roughness change effects in neutral stratification has also been developed using the concept of an Internal Boundary Layer. • Model results are discussed and compared with other approaches and with published field data. • Impacts of roughness changes on wind energy potential and siting issues will be discussed. • Financial support for York University participants in this work has been provided by CFCAS and MITACS

  3. Internal Boundary Layers From poster by Hong Liu

  4. Idealized velocity profiles – based on modelling and observation: Single roughness change, surface layer (logarithmic profile)

  5. GLW – Guidelines for Windows. GLW – simple roughness change Upstream + Reference z0 = 0.0002m Downstream z0 = 0.035m Prediction site is 2 km from roughness change. GLW includes topographic speed-up Now developed for multiple roughness changes

  6. Longer fetches, greater heights. PBL model, neutral or stratified With Boundary-layer approximations, and in 2D, we solve the RANS and Continuity equations, together with closure hypotheses for Reynolds stresses and fluxes. Details in Weng et al. –subm to JWEIA

  7. Closure hypotheses, TKE and length scale equations Stable Stratification Neutral or Unstable Stratification lb = 40m – 100m ζ = z/L

  8. A typical Southern Ontario onshore flow situation. Neutral stratification. Significant reductions at 80m start about 1 km inland.

  9. Offshore flow: from Toronto over Lake Ontario? Neutral stratification. Note increase at 80m requires fetch > 1 km

  10. Non-Neutral cases City to warm lake. A little less shear in 50-120m height range Lake to cool land surface. 80m winds stay a little stronger than in neutral case.

  11. Comparisons with Riso tower data, direction groups 10 and 5; data from Petersen and Taylor (1973)

  12. Comparisons with the Riso JYLEX data (z = 24m), flow from N Sea over Jutland with several roughness changes. Green points are observations. Fetch varies because of different flow directions.

  13. Concluding remarks • Surface roughness (water, grass, crops, trees or cities) and atmospheric stability (neutral, stable, convective) have a big impact on winds near the surface and up to hub height. • It is not just the roughness at the site that matters but upwind areas need to be taken into consideration. • Wind speed changes taking place within Internal Boundary Layers downwind of roughness and thermal property changes can be modeled effectively. • How must we do Resource Assessment for wind farms in the Great Lakes. Is on-shore monitoring plus careful flow modelling sufficient?

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