Design masterclass 1 Wind power

Urban fallacy

Masterclass

Professor Doug King

In this new series, Professor

Doug King examines

aspects of environmental and sustainable design, addressing common areas of misunderstanding and answering some of the ‘why’ and ‘how’ questions that crop up in design reviews and team meetings. Part one looks at wind power and explains why we are better off designing low energy buildings and systems, and then investing in large-scale wind energy, rather than trying to offset excessive building energy consumption with small-scale, urban wind generation

T

o understand the importance of location and scale for wind power we need to start from first principles. The theoretical maximum power that can be extracted from the wind is given by

the equation:

P = ρAv3

2

Where: ρ = the density of air A = the swept area of the turbine rotor normal to the

wind direction ν = the velocity of the wind Now, since area scales as the square of dimension, it

can easily be seen that the theoretical power of a wind turbine scales as the square of its size and the cube of

www.cibsejournal.com

the wind speed. In simple terms, large turbines are much more efficient at converting the kinetic energy of the wind into usable power than small ones – and turbines in windy locations will generate far, far more energy overall than the same turbines in locations with poor wind resource. However, the size of wind turbines actually has

a much greater impact than the simple scaling of dimension, due to the boundary effect of wind close to the ground. Figure 1 indicates the typical depth of boundary layer in different locations. Within this boundary layer the wind velocity is reduced due to friction with the ground, which dissipates the wind energy, generating turbulence. At around 100m in height, even large-scale wind

turbines are well within the boundary layer and so it is tempting to simply ignore the effect. But we cannot. Due to the cubic scaling of power with velocity, even small differences in velocity can have a significant impact on generation. We need to look at the situation in more detail. In Figure 2, the Suburban and Open Country

boundary layer curves have been enlarged to the point at which we can overlay some typical turbine sizes. This is not 100 per cent accurate and should not be

>

June 2010 CIBSE Journal

45

In order to generate electricity it is necessary to place your turbine in the wind. If it is sheltered by its surroundings it cannot perform.

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