To examine this more closely, consider the case of a dense tree (porosity = 0.3), 10 m high and 200 m upwind of a possible site. To keep things simple we assume that the wind always blows in the same direction at the wind speed predicted by NOABL for no obstacles (6 m/s at 12 m above ground level). The predicted variation in wind speed with height is given in the figure below for this case and, for comparison, when there is no obstacle.
An annual energy yield of 8.6 MWh from an Iskra AT5 turbine is calculated by assuming a standard wind speed distribution. Compare this against 13.1 MWh if the obstacles were not included. A 52% overestimate of energy! Note the other often overlooked fact that this is for a change in just over 1 m/s of the mean wind speed.
The key point we are trying to demonstrate is that the location you choose for a turbine is critical.
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