Case Study

A common rule of thumb shown in the right hand figure is that turbines should be sited at a distance from an obstacle which exceeds 20 times the obstacle height. Other factors to account for when considering an obstacle are its porosity (how easily the wind flow through it) and whether it is upwind of the prevailing wind direction.

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 wind speed at the wind turbine is given by the red line.

For comparison, the green line shows the wind speed when there is no obstacle. An annual energy yield of 8.6 MWh from a 12 m high 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 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.