Turbulence intensity

Turbulence intensity is the standard deviation of wind speed divided by the mean over the same interval, usually computed on the ten-minute records in SCADA data. Low values around five percent are typical offshore; onshore sites in complex terrain or downwind of obstacles routinely see fifteen to twenty percent, and wake-affected positions higher still.

It affects performance in two directions at once. Because available power varies with the cube of wind speed, a gusty ten-meter-per-second average carries more energy than a steady one, which can lift measured output above a smooth power curve in the partial-load region. Above rated wind speed the effect reverses, because the surplus is pitched away rather than captured.

For loads, the effect runs one direction only. Turbulence is the dominant driver of fatigue on blades, the main shaft, and the tower, and the design class a turbine was certified to under IEC 61400 is defined largely by the turbulence it is expected to see. A machine operating above its design class accumulates damage faster than its certification assumed, which is a central question in any lifetime extension assessment.

In analysis, turbulence intensity is one of the conditioning variables that makes a comparison fair. Two turbines producing differently at the same mean wind speed may simply be standing in different flow, and a model that ignores turbulence will read the terrain as a machine problem.