characterising wind fluctuations at the horns rev offshore wind farm

Characterising wind fluctuations at the Horns Rev offshore wind farmClaire Vincent and Gregor Giebel (Ris-DTU)Pierre Pinson (DTU-IMM)

17/04/2008Presentation name*Ris DTU, Technical University of Denmark

IntroductionEpisodes of severe wind variability are observed at Horns Rev.

Particularly serious for offshore wind farms, due to the high concentration of turbines within a small area.

Fluctuating power supply is a challenge for TSOs and wind farm operatorsTechnical considerations grid management, security of supplyFinancial considerations fluctuations must be balanced with reserve power

Examples from 2000 and 2001Episodes of severe wind variability are observed at Horns Rev.

Particularly serious for offshore wind farms, due to the high concentration of turbines within a small area.

Fluctuating power supply is a challenge for TSOs and wind farm operatorsTechnical considerations grid management, security of supplyFinancial considerations fluctuations must be balanced with reserve power


Define an objective measure of variabilityAdaptive spectral analysisWhere is there enhanced spectral intensity?

Apply the objective measure to long time series of wind speed from Horns Rev4 year time series of 10 minute observations of wind speedObservations of wind direction, pressure, temperature, rainfall.

Characterise the types of atmospheric conditions in which variability tends to be enhancedConditional spectraCorrelations between variability and other meteorological variables

Methodology


Observational Data62 metre meteorological mast4 years of 10 minute wind speed observations, from top-mounted cup anemometer.10 months of 12Hz wind speed observations from sonic anemometer mounted at 50 metres.Measurements of wind speed and temperature at several heights, pressure, precipitation, wind direction, water temperature.


Method of adaptive spectral analysis first described in 1998 by Huang et al.Using the Hilbert transform, the instantaneous frequency and amplitude of a time series can be calculatedHowever, the idea of instantaneous frequency only makes sense for a mono-component signal.Huangs innovation:1. Decompose the data, so that each component is a mono-component signal.2. Use the Hilbert transform to calculate the instantaneous frequencies and their amplitudes at each time.The Hilbert-Huang Transform


Empirical Model DecompositionOriginal time seriesFastest oscillations in the time seriesQuick response to non-stationarities in the data


Amplitude Modulation


Frequency Modulation


The Hilbert SpectrumEpisode of enhanced variability


Results: Seasonality of wind fluctuationsMore intense fluctuations in winter and autumn


Results: The effect of wind directionLandWakeSea


What does the wind see from different directions?Fetch: Land sector (southeast to northeasterly flow) Very long fetch to Greenland (north westerly flow) Wake of Norwegian mountains (northerly flow) North sea to England (southwest to northwesterly flow)Synoptic conditions: Storm tracks and fronts tend to approach from the west / southwest


Results: Variability and Pressure TendencyRising pressureFalling pressure


Results: Variability and PrecipitationRmax at time t is the maximum observed 10 minute rain amount within the interval [t 90mins, t + 90 mins]

Rain is observed in discrete 0.25 mm quantiles.

Contours show probability density function of variability for each Rmax value.


Results: Variability and Stability12 Hz sonic anemometer data: 10 month time series10 minute cup anemometer data: 4 year time seriesMost unstableMost stable


Define a severe variability event as being an event: with total variability on timescales of 1-3 hours in 95th percentile with duration > 3 hours.

218 events in the years 2000-200383% occurred in the direction sector 180 330 degrees89% occurred in unstable or neutral conditionsIn 2003, 76% recorded some rain during the event53% occurred when the 3 hour pressure tendency was rising67% occurred in the months September - FebruarySummary of Results


A classic event


Climatological patterns have been seen in large amplitude wind fluctuations at Horns Rev

The majority of episodes are associated with onshore flow, in rainy, unstable conditions

The predictability of wind fluctuations is linked to our ability to accurately forecast the wind field in these conditions

Further work modelling the wind field in severely variable conditionsInstallation of rain radar at Horns Rev in 2009 to track precipitation cells approaching the wind farm (Radar@SEA Danish project)Conclusions


Measurement data was supplied by Vattenfall as part of the Danish Public Service Obligation (PSO) fund project HRENSEMBLE High Resolution ENSEMBLEs for Horns Rev (under contract PSO-6382), which is gratefully acknowledged.

The work was partly supported by the PSO project Mesoscale atmospheric variability and the variation of wind and production of offshore wind farms (under contract PSO-7141).

The measurements from the Blvandshuk meteorological station were provided by the Danish Meteorological Institute.

Coordinates of the Horns Rev II wind farm were kindly supplied by Dong Energy.Acknowledgements

characterising wind fluctuations at the horns rev offshore wind farm

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