Draft National Wind Farm Development Guidelines - July 2010

Figure E-2
Blockage of sunlight for various distances (typical 2 MW turbine blade)
FigureE-3
Shadow intensity regions and the geometric relationship between the Sun, turbine blade, and
shadow intensity
Turbine blade
cross section
Sun
Penumbra
Umbra
Since turbine blades narrow towards the tip, the actual amount of sunlight blocked
depends on which part of the blade is blocking the Sun. The shadow from the root of the
blade will extend further than that from the tip. The German regulations (Ministry for
Environment and Climate Change, 2002) use the mean chord (calculated from the mean
of the maximum chord and the chord at 90% of the blade length) to determine blockage.
They set a limit of 20% of the solar surface blocked before regulations come into effect,
while acknowledging that differences in light of as little as 2.5% can be detected by
humans. The limit of 20% appears somewhat arbitrary, designed to achieve a balance
between what is detectable and what may be practical. 2 km is offered as an alternative
limit if the calculation of 20% blade chord is not undertaken.
Determining the distance at which the mean chord causes 20% blockage of the solar disk
is not straightforward because the necessary information on blade profile is not always
easily accessible. An alternative approach, recommended here, is just to use the
maximum chord (which is easily accessible) with a higher threshold for blockage. The
relationship between these two approaches is shown in Figure 4. It is worth noting that this
figure also shows that to achieve no more than 2.5% blockage, the receptor would need
to be at least 7.6 km from the turbine.
Page 158 Draft National Wind Farm Development Guidelines – 2 July 2010