Wind gets generally stronger and more persistent with increasing altitude. In the first 1.5 to 2 km above ground level, the planetary boundary layer, this is mainly due to the effect of viscous friction. As an example, the figures show the wind velocity and power density as functions of altitude at the meteorological station of De Bilt, the Netherlands. This averaged wind data covers a period from 1961 until 1980 and clearly indicates that the wind velocity increases significantly up to 1 km altitude.

Because of the increase of wind power with altitude, tower height is an important factor in the design of wind turbines and greatly affects their power output and capacity factor. However, even the largest turbines in the megawatt-range can not exceed altitudes much beyond 200 metre due to the structural limits of tower-based designs. For offshore and particularly for deep-water deployment of such large turbines, the additional investments in foundations or mooring platforms emerge as decisive cost factors.

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