offshore wind farm design: offshore wind climate

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  • 1. Module 4: Offshore Wind ClimateOFFSHORE WIND CLIMATEWim BierboomsWind Energy Research Group (Aerospace Eng.)1/64

2. Module 4: Offshore Wind Climate OverviewWind 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 OverviewWind 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 ClimateIntroduction Assessment of wind climate important: assessment of resources determination of gross energy yield mean windspeed site conditions at installation accessibility for maintenance loads on structures turbulenceextreme wind 4/64 5. Module 4: Offshore Wind Climate What is wind? caused by pressuredifferences(resulting fromtemperature differences) influenced by earthrotation and terrain 5/64 6. Module 4: Offshore Wind ClimateOverviewWind 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 ClimateGeostrophic windhigh lowaltitude altitudeG : pressure gradient forceC : Coriolis forceW : drag forceUg: geostrophic windU : wind at the ground7/64 8. Module 4: Offshore Wind ClimateCoriolis effectFictitious force: just effected by rotating observerWith respect tofixed frameWith respect torotating frame8/64 9. Module 4: Offshore Wind ClimateAtmospheric 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 IWind profile (wind shear): ln( h / z 0 ) V ( h ) = V ( h ref ) ln( h / z ) ref0 href is the reference height (10 m)z0 is the roughness length10/64 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 ageCharnock relation:u*2z0 = ; = 0.0185 g 11/64 12. Module 4: Offshore Wind ClimatePotential wind speedcorrect for local roughnessStandard height androughness: 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 exchangeHeightTemperature 13/64 14. Module 4: Offshore Wind ClimateWind profile in surface layer (~100 m)thermal effects II Unstable atmosphere Vertical movements of air due to hot surfaceHeight Vertical velocity exchange Steeper gradientTemperature14/64 15. Module 4: Offshore Wind Climate Wind profile in surface layer (~100 m)thermal effects IIIH stable (cold surface) neutral unstable (hot surface)V15/64 16. Module 4: Offshore Wind ClimateTemperature difference Tsea -Tair Average wind - watertemperature differenceat 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% occurrenceRdsand (DK)% occurrenceWind speed 17/64 18. Module 4: Offshore Wind ClimateOverviewWind climate in general variation in height, space and time offshore wind measurements special offshore effects offshore turbulence / extreme winds18/64 19. Module 4: Offshore Wind ClimateVariability 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 gaptime scale year month day hourminute secondSpace:100-1000 km< 1 km < 10 m 19/64 20. Module 4: Offshore Wind Climate Basic wind statisticsVtime series of10-min. (1-hour)timemeans number pdf Weibull V Vhistogram probability density function 20/64 21. Module 4: Offshore Wind ClimateWeibull distributionV( ) k probability of wind speed < V: 1 ea withk : shape parameter (k 2 in NW Europe coast)a : scale parameterVavg ( year ) a= 1(1 + ) ( ) = 1e d k 0 21/64 22. Module 4: Offshore Wind Climate Fit of Weibull distribution example IFrequency of occurrence (wind rose)22/64 23. Module 4: Offshore Wind ClimateFit of Weibull distribution example II Weibull scale parameter Weibull shape parameter 23/64 24. Module 4: Offshore Wind ClimateOverviewWind climate in general variation in height, space and time offshore wind measurements special offshore effects offshore turbulence / extreme winds24/64 25. Module 4: Offshore Wind Climate Existing offshore wind map Evaluation ofoffshore potential Extrapolation fromland measurements Not suitable forenergy yieldcalculation Coastal effectscause main problems25/64 26. Module 4: Offshore Wind ClimateHow to obtain wind statistics for a given location ?problem:solution:Windatlas+ correction use existing data data notuse of correction methodsrepresentative methods (fetch, WAsPobstacles on land) Other sources two basic optionsnot possible to correlate with othermeasure longsites which have Measure perform local enough to obtain reliable long term Correlate measurements good statistics statisticsPredict MCP 26/64 27. Module 4: Offshore Wind Climate WASP offshore (Ris DK)speed-upDetermine Offshore windover hillsatlas at 150 m height Calculate downwardchange in to hub height usingroughness local orography Account for coastaleffectssheltermodel Not yet fully operational27/64 28. Module 4: Offshore Wind ClimateAvailable offshore wind dataPlatform data (since 80) KNMI, RWS: Measuring NetworkNorth Sea (limited w.r.t. land) wind and wave dataOther sources light ships Voluntary Observing Ships databases (e.g. NESS / NEXThindcast data) pressure data (weather forecast) remote sensing / satellite dataKNMI wind network (NL) 28/64 29. Module 4: Offshore Wind ClimateRemote sensingSatelite sampling Spatial distribution Accuracy 2 m/s Time shift Satellite passages Diurnal cycles !?29/64 30. Module 4: Offshore Wind ClimateDutch Offshore wind map Netherlands Exclusive Economic Zone Derived from pressure data (1997-2002) Height: 120 m Distribution available for 5 locations ECN30/64 31. Module 4: Offshore Wind Climate Measure Correlate Predict - example x, +: measurements Background MCP:time series annual wind speedslook similar; high and low valuesoccur at the same years+ +xxLong term correction factors2 years of measurements of asite near Schiphol+: 0.95x: 1.05 31/64 32. Module 4: Offshore Wind Climate OverviewWind climate in general variation in height, space and time offshore wind measurements special offshore effects offshore turbulence / extreme winds32/64 33. Module 4: Offshore Wind Climate MCP: dedicated measurements IRis/SEAS meteo masts at sea5 locations 50 m height in Danish seas 33/64 34. Module 4: Offshore Wind ClimateMCP: dedicated measurements IIFino platform in German part of the North Sea105 m height; full site assessment:Hydro, meteo and biological surveys www.fino-offshore.de34/64 35. Module 4: Offshore Wind ClimateMCP: dedicated measurements IIIPlatform OWEZ (116 m) WEOM 3 levels; 3 sides; booms of 12m 2003 also metocean data Monitoring- en evaluatieprogramma Near ShoreWindpark by the DutchGovernment35/64 36. Module 4: Offshore Wind ClimateMCP: dedicated measurements IVcupsonic Meteo masts at sea: Corrections for mast shadow Well proven but expensiveSodar / Lidar anemometers Doppler shift of an acousticpulse / laser Promising but expertiserequired 36/64 37. Module 4: Offshore Wind ClimateHourly variation of offshore wind speed 9.5 9.0 10-m wind speed [m/s] 8.5 8.0 7.5 * 7.0 6.5onshore 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 EURK13 LEG MPN IJM 37/64 38. Module 4: Offshore Wind Climate Monthly variation of offshore wind speed12 Tsea -Tair 11T 10-m wind speed [m/s] 10Month 9 8 7 6 5Jan Feb MrtApr Mei JunJul AugSepOktNovDecAUKEURK13 LEG MPN IJM38/64 39. Module 4: Offshore Wind ClimateAnnual variation of offshore wind speed 10.5 10.0Consequences:9.5 10-m wind speed [m/s]9.0 Annual energy yield / income will8.5 vary from year to8.0year !!7.57.0 Base energy yield6.5 estimation on long period (20-30y)6.01987 1988 1989 1990 19911992 1993 1994 1995 1996 1997AUK EURK13 LEGMPN IJM 39/64 40. Module 4: Offshore Wind ClimateOffshore versus onshore: wind shear larger V smaller z0 H stability effects021(m)more important offshore verification001needed08060402 09 5.8 8 5.7 7 5.6 6 5.5 5V (m/s)40/64 41. Module 4: Offshore Wind ClimateOffshore versus onshore: Weibull distribution0.14 mean wind speed 6.5 m/s0.12 (onshore) mean wind speed 8 m/s 0.1 (offshore, Baltic)probability [-] mean wind speed 10.1 m/s0.08 (remote offshore, North Sea)0.060.040.020 0 2 4 6 810 12 14 1618 2022 24 26 28 30 wind speed at hub height [m/s]41/64 42. Module 4: Offshore Wind Climate Wind climate offshore versus onshoreonshoreoffshore diurnal patterndaily maximumuniform seasonal pattern less pronouncedmore pronounced stabilitydiurnal patternseasonal pattern wind profile unstable on averageneutral on average mean wind speed decreasing inland higher than on land42/64 43. Module 4: Offshore Wind Climate OverviewWind climate in general variation in height, space and time offshore wind measurements special offshore effects offshore turbulence / extreme winds43/64 44. Module 4: Offshore Wind Climate Coastal effects IIIIsea up to ~30 km land I windatlas methods (zo depends on waves fetch age) IIcoastal 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 effectsmodel improvement still underway

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