• Questions
  • Mitigating measures to reduce bird and bat mortality (June 30, 2021)

Mitigating measures to reduce bird and bat mortality (June 30, 2021)

    Dia de Wijs
    • Public
    Asked by Dia de Wijs 120 days ago Answers (1)

    What will be the energy loss caused by mitigating measures to reduce bird and bat mortality for offshore windfarms?

    1 answer

      • Beheerder RVO
        • Public

        Beheerder RVO 120 days ago

        Offshore wind farms (OWFs) can be lethal to birds and bats from a variety of species, as rotating turbines pose a collision risk. All bird and bat species are protected by law, which obligates the wind farm developer to take mitigating measures to reduce the amount of victims (1). Mitigating measures include stopping turbines during periods with high presence of birds and/or bats. However, such measures result in lower energy yield. It is difficult to quantify the resulting energy loss. However, studies do offer some estimates.

        To reduce the amount of collision victims among migratory birds (2) in particular, contractors are obligated in the site decision (‘kavelbesluit’) to stop their turbines during large bird migrations. However, stopping (in practice: reducing the speed to less than 2 rpm/minute of) a significant number of offshore wind turbines at the same time would cause instability on the grid, therefore the Universiteit van Amsterdam is creating a prediction model, which should be able to predict large migrations at rotor height two days in advance (3).
        This mitigating measure may result in lower energy yields, as turbines are not functioning for certain periods. On average, this adds up to around 0.3 percent of the time annually.

        The presence of bats on the North Sea depends highly on meteorological conditions, such as wind speed- and direction and temperature. Therefore, the site decisions contain regulations concerning cut-in wind speeds which depend on these parameters (4) (see Table 1). 

          Adjusted cut-in wind speed [m/s] at different wind speeds and temperatures
        Wind direction
        Temperature (degrees Celsius) N NNE NOR O SE SSE Z SSW SWW H NWW NNW
        11 3 3 3 3 3 3 3 3 3 3 3 3
        11-15 3.5 4.5 5.5 6 5.5 5.5 3.5 3.5 3.5 3 3 3
        15 3.5 4.5 5.5 6 5.5 5.5 4.0 3.5 3.5 3 3 3

        Table 1: Adjusted cut-in wind speed as mitigating measure for bats mortality.

        Implementing this rule would result in an estimated energy loss of about 13 MWh/turbine/year (determined with turbines with an axis height of 100m, and rotor diameter of 164m)4.

        1: Wozep ecological programme - Noordzeeloket UK
        2: Birds - Noordzeeloket UK
        3: Kamerstuk 35092, nr. E | Overheid.nl > Officiële bekendmakingen (officielebekendmakingen.nl) (only available in Dutch)