The “wake effect” is the reduction of wind speed and increase of turbulence down wind of a turbine.
[source: 3D-Simulation of the turbulent wake behind a wind turbine, Weßow, Sitzki, & Hahm, Journal of Physics: Conference Series 75 (2007) 012033 [The Science of Making Torque from Wind], doi:10.1088/1742-6596/75/1/012033]
If you have a wind farm of two or more HAWTs then, inevitably in some wind directions, the disturbance from one turbine will affect another.
The combined effect of a two-dimensional matrix of turbines has been measured at Horns Rev. This is currently the largest farm, with a 10 by 8 matrix spaced by 560m (7 rotor diameters).
On average, the Park Efficiency of Horns Rev is about 87%. This means that the farm is generating 13% less energy than if there was no interaction between individual turbines.
The figure below shows that the park efficiency is affected by wind speed.
[source: Recalibrating wind turbine wake model parameters – validating the wake model performance for large offshore wind farms, Sørensen, Nielsen, & Thøgersen, presented at European Wind Energy Conference, 2006, Athens ]
The figure below shows there are some very nasty wind directions.
This should be expected with a regular grid when the wind lines up with the grid.
[source: RANS-modelling of wind flow through large offshore wind farms, Riedel & Neumann, presented at European Wind Energy Conference, 2007, Athens ]
There has been some work on the best way to enlarge a farm or creating a “farm of farms”. The figure below shows the wind profile before and after Horns Rev. The farm is placed at around 15 to 20 km. It can be seen that it takes about 15 km downwind of the farm before the wind speed returns to near the upstream speed.
The figure below shows some possible arrangements of “farm of farms”.
[source: Summary report: The shadow effect of large wind farms: measurements, data analysis and modelling, Frandsen, Barthelmi, Rathmann, Jøgensen, Badger, Hansen, Ott, Rethore, Larsen & Jensen, Risø National Laboratory, Technical University of Denmark, Roskilde, Denmark, October 2007]