Fitch, Anna
C., University of Bergen, Norway, Julie K. Lundquist, University of Colorado at
Boulder and NREL, USA, and Joseph B. Olson, NOAA/ESRL and CIRES, USA
Few observations are
available to give insight into the interaction between large wind farms and the
boundary layer. As wind farm
deployment increases, questions are arising on the potential impact on
meteorology within and downwind of large wind farms. While large-eddy simulation can provide insight into the
detailed interaction between individual turbines and the boundary layer, to
date it has been too computationally expensive to simulate wind farms with
large numbers of turbines and the resulting wake far downstream. Mesoscale numerical weather prediction
models provide the opportunity to investigate the flow in and around large wind
farms as a whole, and the resulting impact on meteorology. To this end, we have implemented a wind
farm parameterization in the Weather Research and Forecasting (WRF) model,
which represents wind turbines by imposing a momentum sink on the mean flow;
transferring kinetic energy into electricity and turbulent kinetic energy
(TKE). The parameterization
improves upon previous models, basing the atmospheric drag of turbines on the
thrust coefficient of a modern commercial turbine. In addition, the source of TKE varies with wind speed,
reflecting the amount of energy extracted from the atmosphere by the turbines
that does not produce electrical energy.
We simulate a wind farm
covering 10x10 km over land, consisting of 100 turbines each of nominal power
output of 5 MW. Results will be
presented showing how the wake structure varies dramatically over a diurnal
cycle characteristic of a region in the Great Plains of the US, where wind farm
deployment is planned. At night, a
low-level jet forms within the rotor area, which is completely eliminated by
energy extraction within the wind farm.
The deep stable layer and lack of higher momentum air aloft at this time
maximises the wind deficit and the length of the wake. The presentation will discuss the
maximum reduction of wind speed within and downwind from the farm, and the wake
e-folding distance is quantified.
Accelerations beneath the turbines, close to the ground, are also
seen. The enhanced turbulent
mixing within the rotor area induces a temperature perturbation. The near surface temperature change is
found to be smaller than in previous studies, with a maximum increase of 0.5 K
in the evening. The temperature
change is negligible within the farm during the day, and is negligible
downstream at all times.