Figure 1: The effect of surface friction in reducing wind velocity near the ground is pronounced. Over a city centre, the boundary layer extends to twice the height that it does over open countryside, due to the increased surface roughness.

Figure 2: Within the boundary layer the wind power available is critically affected by both height and location.

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used in calculations, but for now we simply want to gain an understanding of the principles. This approach is more than adequate to demonstrate the sensitivity to height and location. The large-scale turbine (A) has a 70m diameter

rotor and is typically mounted on masts around 80m high. This machine will generate 2MW at peak output. The small scale turbine (B) is 15m to the hub, has a 10m diameter rotor and is rated at 15kWp. A typical 1kW domestic micro turbine (C) has a rotor diameter of 1.75m, and mounted above the ridge on a typical house is at a similar height to the small-scale turbine. However, by definition, a building-mounted micro- turbine will only ever be found in the equivalent of an urban or suburban location. This is because the turbulence created by the building itself is typically of a scale equivalent to or larger than the turbine. Now, the large turbine is just over seven times the

size of the small scale one. Based on size alone, we would expect the output to be 50 times greater than the small turbine. However, in open country the velocity is about 25 per cent higher at 80m than it is at 15m, and therefore the power available is nearly twice as much due to the cubic scaling. Our scaling estimate therefore works out to 1.5MW – close to the 2MW peak output actually achieved. The remaining increase in peak power output is due to the higher maximum wind velocity for the larger machine. Wind turbines do not always generate at their peak capacity, as this only occurs at the maximum rated wind velocity. We should therefore examine the situation in terms of the annual energy generation, to complete the picture. To estimate the annual generation capacity of a wind

turbine, we apply a capacity factor to account for the annual distribution of wind velocity. For open country locations with good wind resources, we can expect a

46

CIBSE Journal June 2010

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