Wind Farm parametrization in the mesoscale model WRF

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    Abstract

    The project’s objective is to investigate and develop methods for prediction
    of mesoscale climate, wake effects and atmospheric feedbacks, for scenarios
    where large portions of the sea are covered with wind farms. The atmospheric
    flow is simulated with the WRF mesoscale model, since it has significantly lower
    computational costs compared to high resolution models. Due to the fact that its
    typical horizontal grid spacing is on the order of 2km, the energy extracted by the turbine, as well as the wake development inside the turbine- containing grid cells, are not described explicitly, but are parametrized as another sub-grid scale process.
    In order to appropriately capture the wind farm wake recovery and its direction,
    two properties are important, among others, the total energy extracted by the
    wind farm and its velocity deficit distribution. In the considered parametrization
    the individual turbines produce a thrust dependent on the background velocity.
    For the sub-grid scale velocity deficit, the entrainment from the free atmospheric
    flow into the wake region, which is responsible for the expansion, is taken into
    account. Furthermore, since the model horizontal distance is several times larger
    then the turbine diameter, it has been assumed that the generated turbulence and dissipation are balanced.
    From version 3.2.1 onwards, the WRF (Weather Research and Forecast) model
    includes a wind farm parametrization option (Fitch Scheme). In contrary to the
    above described parametrization where the wind turbines are positioned explicitly,
    the wind farms in the default scheme are treated as a density distribution, which
    limits the description of the internal wind farm velocity deficit development and
    its related efficiency. In the Fitch Scheme the wind turbines act as drag devices,
    where the extracted force is proportional to the turbine area interfacing a grid
    cell. The sub-grid scale wake expansion is achieved by adding turbulence kinetic
    energy (proportional to the extracted power) to the flow. The validity of both
    wind farm parametrizations has been verified against observational data. We use
    Synthetic Aperture Radar (SAR) satellite data, as well as mast measurements
    from meteorological masts and power measurements from wind turbines, at Horns Rev and Nysted. From the SAR satellite data the wake extension can be derived.
    The wind farm measurements have been used to compare the total thrust produced by both types of parametrization. In case studies the wake deficit has been estimated by the deflection of the wake due to the slowing down of the wind speed.
    The results of the wind farm parametrization will be used to investigate eventual
    climate impacts of large wind farms. Furthermore it will develop techniques
    for up-scaling the effects simulated by wind farm wake models into mesoscale atmospheric planetary boundary layer (PBL) parameterisations and perform simulations using these parameterisations to understand the feedbacks between the wind farms and the regional wind climate. The work will extend the current knowledge about wake effects from observations and small-scale models to potential feedbacks in the PBL atmosphere.
    Original languageEnglish
    Title of host publicationProceedings
    Number of pages8
    PublisherEuropean Wind Energy Association (EWEA)
    Publication date2012
    Publication statusPublished - 2012
    EventEWEC 2012 - European Wind Energy Conference & Exhibition - Copenhagen, Denmark
    Duration: 16 Apr 201219 Apr 2012
    http://events.ewea.org/annual2012/

    Conference

    ConferenceEWEC 2012 - European Wind Energy Conference & Exhibition
    Country/TerritoryDenmark
    CityCopenhagen
    Period16/04/201219/04/2012
    Internet address

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