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Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
Offshore Wind Farm Design: offshore wind climate
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Offshore Wind Farm Design: offshore wind climate

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  • 1. Module 4: Offshore Wind Climate OFFSHORE WIND CLIMATE Wim Bierbooms Wind Energy Research Group (Aerospace Eng.) 1/64
  • 2. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 2/64
  • 3. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 3/64
  • 4. Module 4: Offshore Wind Climate Introduction Assessment of wind climate important: • assessment of resources • determination of gross energy yield mean wind speed • site conditions at installation • accessibility for maintenance • loads on structures turbulence extreme wind 4/64
  • 5. Module 4: Offshore Wind Climate What is wind? • caused by pressure differences (resulting from temperature differences) • influenced by earth rotation and terrain 5/64
  • 6. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 6/64
  • 7. Module 4: Offshore Wind Climate Geostrophic windhigh lowaltitude altitude G : pressure gradient force C : Coriolis force W : drag force Ug: geostrophic wind U : wind at the ground 7/64
  • 8. Module 4: Offshore Wind Climate Coriolis effect Fictitious force: just effected by rotating observer With respect to fixed frame With respect to rotating frame 8/64
  • 9. Module 4: Offshore Wind Climate Atmospheric boundary layer Ekman layer variation of wind direction~ 1000 m Surface layer variation of wind speed 9/64
  • 10. Module 4: Offshore Wind Climate Wind profile in surface layer (~100 m) mechanical friction I Wind profile (wind shear):  ln( h / z 0 )  V ( h ) = V ( h ref )   ln( h / z )   ref 0  href is the reference height (10 m) z0 is the roughness length 10/64
  • 11. Module 4: Offshore Wind Climate Wind profile in surface layer (~100 m) mechanical friction II At land • z0 = 0.03- 0.25 m At sea: • z0 ~ 0.0002 m but dependent upon • wave height • fetch • wave age Charnock relation: u*2 z0 = α ; α = 0.0185 g 11/64
  • 12. Module 4: Offshore Wind Climate Potential wind speed correct for local roughness Standard height and roughness: • 10 min. ( or 1 hour) average • 10 m. height (= href) • z0 = 0.03 m 12/64
  • 13. Module 4: Offshore Wind Climate Wind profile in surface layer (~100 m) thermal effects I • Parcel expands (pressure drops with height) → cools down • Adiabatic: no heat exchange Height Temperature 13/64
  • 14. Module 4: Offshore Wind Climate Wind profile in surface layer (~100 m) thermal effects II Unstable atmosphere • Vertical movements of air due to hot surfaceHeight • Vertical velocity exchange • Steeper gradient Temperature 14/64
  • 15. Module 4: Offshore Wind Climate Wind profile in surface layer (~100 m) thermal effects IIIH • stable (cold surface) • neutral • unstable (hot surface) V 15/64
  • 16. Module 4: Offshore Wind Climate Temperature difference Tsea -Tair Average wind - water temperature difference at North Sea location.∆T • Summer: stabilizing ∆T • Winter: destabilizing ∆T Month 16/64
  • 17. Module 4: Offshore Wind Climate Boundary layer stability at sea Danish “offshore“ sites% occurrence Rødsand (DK) % occurrence Wind speed 17/64
  • 18. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 18/64
  • 19. Module 4: Offshore Wind Climate Variability of the wind in time daily patterns (often thermal driven):contribution to wind speed • sea breezevariability • mountain slope winds turbulence, related to • terrain, obstacles • storm fronts (gusts) large scale weather systems • high/low pressure systems • seasonal variations “spectral gap” time scale year month day hour minute secondSpace: 100-1000 km < 1 km < 10 m 19/64
  • 20. Module 4: Offshore Wind Climate Basic wind statisticsV time series of 10-min. (1-hour) time means number pdf Weibull V V histogram probability density function 20/64
  • 21. Module 4: Offshore Wind Climate Weibull distribution V −( ) k probability of wind speed < V: 1− e a with k : shape parameter (k ≈ 2 in NW Europe coast) a : scale parameter Vavg ( year ) a= 1 Γ(1 + ) ∞ Γ(α ) = ∫ β α −1e − β dβ k 0 21/64
  • 22. Module 4: Offshore Wind Climate Fit of Weibull distribution – example I Frequency of occurrence (wind rose) 22/64
  • 23. Module 4: Offshore Wind Climate Fit of Weibull distribution – example II Weibull scale parameter Weibull shape parameter 23/64
  • 24. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 24/64
  • 25. Module 4: Offshore Wind Climate Existing offshore wind map • Evaluation of offshore potential • Extrapolation from land measurements • Not suitable for energy yield calculation • Coastal effects cause main problems 25/64
  • 26. Module 4: Offshore Wind Climate How to obtain wind statistics for a given location ? problem: solution: Windatlas + correction use existing data data not use of correction methods representative methods (fetch, WAsP obstacles on land) Other sources two basic options not possible to correlate with other measure long sites which have “Measure perform local enough to obtain reliable long term Correlate measurements good statistics statistics Predict” MCP 26/64
  • 27. Module 4: Offshore Wind Climate WASP offshore (Risø DK)speed-up Determine Offshore windover hills atlas at 150 m height • Calculate downwardchange in to hub height usingroughness local orography • Account for coastal effectssheltermodel • Not yet fully operational 27/64
  • 28. Module 4: Offshore Wind Climate Available offshore wind data Platform data (since ’80) • KNMI, RWS: Measuring Network North Sea (limited w.r.t. land) • wind and wave data Other sources • light ships • Voluntary Observing Ships • databases (e.g. NESS / NEXT “hindcast” data) • pressure data (weather forecast) • remote sensing / satellite data KNMI wind network (NL) 28/64
  • 29. Module 4: Offshore Wind Climate Remote sensing Satelite sampling • Spatial distribution • Accuracy 2 m/s • Time shift • Satellite passages • Diurnal cycles !? 29/64
  • 30. Module 4: Offshore Wind Climate Dutch Offshore wind map Netherlands Exclusive Economic Zone Derived from pressure data (1997-2002) Height: 120 m Distribution available for 5 locations © ECN 30/64
  • 31. Module 4: Offshore Wind Climate Measure Correlate Predict - example x, +: measurements Background MCP: time series annual wind speeds look similar; high and low values occur at the same years + + xx Long term correction factors 2 years of measurements of a site near Schiphol +: 0.95 x: 1.05 31/64
  • 32. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 32/64
  • 33. Module 4: Offshore Wind Climate MCP: dedicated measurements I Risø/SEAS meteo masts at sea 5 locations 50 m height in Danish seas 33/64
  • 34. Module 4: Offshore Wind Climate MCP: dedicated measurements II “Fino” platform in German part of the North Sea 105 m height; full site assessment: Hydro, meteo and biological surveys www.fino-offshore.de 34/64
  • 35. Module 4: Offshore Wind Climate MCP: dedicated measurements III Platform OWEZ (116 m) • WEOM • 3 levels; 3 sides; booms of 12m • 2003 • also metocean data • “Monitoring- en evaluatie programma Near Shore Windpark” by the Dutch Government 35/64
  • 36. Module 4: Offshore Wind Climate MCP: dedicated measurements IVcup sonic Meteo masts at sea: • Corrections for mast shadow • Well proven but expensiveSodar / Lidar anemometers• Doppler shift of an acoustic pulse / laser• Promising but expertise required 36/64
  • 37. Module 4: Offshore Wind Climate Hourly variation of offshore wind speed 9.5 9.0 10-m wind speed [m/s] 8.5 8.0 7.5 * 7.0 6.5 ‚onshore site‘ 6.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 hour AUK EUR K13 LEG MPN IJM 37/64
  • 38. Module 4: Offshore Wind Climate Monthly variation of offshore wind speed 12 Tsea -Tair 11 ∆T 10-m wind speed [m/s] 10 Month 9 8 7 6 5 Jan Feb Mrt Apr Mei Jun Jul Aug Sep Okt Nov Dec AUK EUR K13 LEG MPN IJM 38/64
  • 39. Module 4: Offshore Wind Climate Annual variation of offshore wind speed 10.5 10.0 Consequences: 9.5 10-m wind speed [m/s] 9.0 • Annual energy yield / income will 8.5 vary from year to 8.0 year !! 7.5 7.0 • Base energy yield 6.5 estimation on long period (20-30y) 6.0 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 AUK EUR K13 LEG MPN IJM 39/64
  • 40. Module 4: Offshore Wind Climate Offshore versus onshore: wind shear • larger V • smaller z0 H • stability effects 021(m) more important • offshore verification 001 needed 08 06 04 02 0 9 5.8 8 5.7 7 5.6 6 5.5 5 V (m/s) 40/64
  • 41. Module 4: Offshore Wind Climate Offshore versus onshore: Weibull distribution 0.14 mean wind speed 6.5 m/s 0.12 (onshore) mean wind speed 8 m/s 0.1 (offshore, Baltic) probability [-] mean wind speed 10.1 m/s 0.08 (remote offshore, North Sea) 0.06 0.04 0.02 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 wind speed at hub height [m/s] 41/64
  • 42. Module 4: Offshore Wind Climate Wind climate offshore versus onshore onshore offshore diurnal pattern daily maximum uniform seasonal pattern less pronounced more pronounced stability diurnal pattern seasonal pattern wind profile “unstable” on average “neutral” on average mean wind speed decreasing inland higher than on land 42/64
  • 43. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 43/64
  • 44. Module 4: Offshore Wind Climate Coastal effects I I II sea up to ~30 km land I “windatlas” methods (zo depends on waves fetch age) II coastal discontinuity zone: • strong variations in air/sea temperature gradient (winter ~ summer onshore ~ offshore wind) • resulting strong effect upon stability • further complicated by internal boundary layer effects model improvement still underway 44/64
  • 45. Module 4: Offshore Wind Climate Coastal effects II change in surface roughness External boundaryExternal boundary layer layer External boundary External boundary layer layer z Internal boundary Internal boundary layer (IBL) layer (IBL) ~150 m ~150 m z01 z02 x 5 km land Land sea Sea 5 km 45/64
  • 46. Module 4: Offshore Wind Climate Overview Wind climate • in general • variation in height, space and time • offshore wind measurements • special offshore effects • offshore turbulence / extreme winds 46/64
  • 47. Module 4: Offshore Wind Climate Basic wind statistics distinction between long and short term long term 20 m/s20 m/sV→ 0 m/s short term 5 m/s0 m/s 30 min. t→ -5 m/s 10 min. 10 min. 10 min. 47/64
  • 48. Module 4: Offshore Wind Climate RECAP: Basic wind statistics – long termV time series of 10-min. (1-hour) time means number pdf Weibull V V histogram probability density function 48/64
  • 49. Module 4: Offshore Wind Climate 5 Basic wind statistics - short term 4 V 3 2 1 0 time series of - 1 - 2 - 3 wind (turbulence) - 4 - 5 0 1 0 2 0 3 0 4 0 5 0 6 0 time (s) number pdf Gauss V V histogram probability density function 49/64
  • 50. Module 4: Offshore Wind Climate Basic wave statistics - short term time series of η→ 0 . 4 0 . 3 0 . 2 0 . 1 0 water elevation - 0 . 1 - 0 . 2 - 0 - 0 . 3 . 4 0 1 0 H 2 0 3 0 4 0 5 0 6 0 Time → narrow small waves banded number pdf pdf Rayleigh number Gauss η H Hs H η 50/64
  • 51. Module 4: Offshore Wind Climate Basic wave statistics - long term654 time series of3 Hs and Tz21 (each 3-hour)0 0 2 4 6 8 1 0 1 2 1 4 time (days) → Number scatter diagram (‘2d histogram’) Tz → Hs → 51/64
  • 52. Module 4: Offshore Wind Climate Turbulence - characteristics• random variations around mean U σ• turbulence intensity TI=σ/U• production: – wind shear (mechanical) – buoyant (convective / thermal)• loss: – dissipation (into heat)• superposition of eddies, swirls (size from 2 km to 1 mm) 52/64
  • 53. Module 4: Offshore Wind Climate Intermezzo: spectrum 2 10 → 1 10Power spectral density function Spectrum 0 10• ‘distribution of energy content -1 10 over frequencies’ -2 10 -3 10• area below curve = σ2 (variance) -4 10 Frequency → -2 -1 0 1 10 10 10 10• largest eddies have most energy(Integral) time scale• measure of time over which wind speed is correlated(Integral) length scales• characteristic size of eddy 53/64
  • 54. Module 4: Offshore Wind Climate Stochastic wind simulation; one-point IGeneration of a 10-min time series → } 12Summation of harmonics: Wind speed 11 K u (t ) = ∑ Sk ∆f cos(ωk t + φk ) 10 9 k =1 8 Sk: spectrum value 7 π φk: random (uniform 0-2π) 6 0 10 20 30 40 50 60 Time → 54/64
  • 55. Module 4: Offshore Wind Climate Stochastic wind simulation; one-point II Choice of spectrum: 10 2 → • von Karman (isotropic) 10 1 Spectrum • Kaimal 0 10 -1 10 Spectrum depends on: 10 -2 • mean wind speed 10 -3 -4 10 • length scales Frequency → -2 -1 0 1 10 10 10 10 • turbulence intensity (offshore ~ 8%) 0.1 0.09 0.08 0.07 pdf → 0.06 Generate wind field for several mean 0.05 0.04 wind speeds 0.03 0.02 0.01 • distribution: Weibull 0 0 5 10 15 20 Mean wind speed → 55/64
  • 56. Module 4: Offshore Wind Climate Stochastic wind field simulation - example Bladed; 3 velocity components: → Wind speed • u; longitudinal • v; lateral • w; vertical 56/64
  • 57. Module 4: Offshore Wind Climate Onshore vs. offshore turbulence 57/64
  • 58. Module 4: Offshore Wind Climate Measured offshore turbulence intensity T.I.T.I. 10 m σ U 30 m Height T.I. = U 50 m [m] Windspeed [m/s] T.I. 58/64
  • 59. Module 4: Offshore Wind Climate Rotational sampling of turbulence eddies rotor 59/64
  • 60. Module 4: Offshore Wind Climate Rotational turbulence spectrumturbulenceV 4 3 2 1 0 - 1 - 2 - 3 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 Time (s) 0 6 0 0 { ⇒ summation of harmonics (random phases) ⇓ spectrum rotating observer fixed observer 60/64
  • 61. Module 4: Offshore Wind Climate Extreme winds • No offshore map (yet) • Extrapolation of data covering several years (extreme value theory) 61/64
  • 62. Module 4: Offshore Wind Climate Extreme value analysis - example • determine yearly maxima Ui • order them to magnitude: 19.8, 20.7, …. 27.2, 27.5 m/s • fit to extreme value distribution (different functions / methods available) • read from graph: 50-years value 62/64
  • 63. Module 4: Offshore Wind Climate Extreme gusts ‘Real’ Wind speed → →Wind speed • maximum amplitude • maximum rise time → Time → Time → IEC (included in Bladed): deterministic gust → uniform over rotor plane 40 Wind speed also: 35 – extreme direction change 30 25 – combination 20 – extreme wind shear (vertical 15 -2 0 2 4 6 8 10 12 and horizontal) Time → 63/64
  • 64. Module 4: Offshore Wind Climate Low Level Jet Wind maximum at relative low levels (~150 m); larger then geostrophic wind Stable night time conditions Common in Baltic Sea; Also in North Sea? Sodar measurements 64/64

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