Draft National Wind Farm Development Guidelines - July 2010

E.7 Technical discussion of key concepts/issues
E.7.1
Other international standards
The regulations in most other European and British countries refer to the German standard
with some minor modifications. For instance the Danish / Swedish regulations allow an
actual number of shadow flicker hours per year of ten (10) compared to eight (8) in the
German regulations. In the USA, regulations and standards vary widely between
jurisdictions. Some apply no requirements, assuming the issue is implicitly satisfied by the site
location and noise requirements, while others apply extensive requirements, over and
above those used in Europe and Britain.
Some examples of the various conditions are described here. Some regulations require
shadow flicker to be calculated for a single location representing each dwelling, others
require calculation for each window of a dwelling, or for an area extending some distance
outside each dwelling. Two instances were identified where the regulations were applied
equally to dwellings and roads (with the exception of roads where average annual daily
traffic (AADT) is less than 500 vehicles). These jurisdictions are Shawano County (2008) and
Eveline Township (Weed, 2006) in the USA.
The basis for each of the above regulations was not specified or easily determined. In the
context of a road, annoyance caused by shadow flicker could conceivably by elevated
to a safety issue because of its distraction value and may have been included in
regulations purely on this basis (i.e. without objective assessment). BERR (2008) also
introduces the assessment of turbine frequency, because of its potential impact on
sufferers of epilepsy. It is noted that this is usually not a problem because wind turbines
operate at around 1 Hz, well below the minimum level of 2.5 Hz that may trigger a seizure.
One commonality in regulations is the 30 hr/yr limit. This appears to be based on a German
court ruling (propagated through various sources such as the Danish Wind Industry
Association, 2003) which set this as the acceptability limit for a particular wind farm.
Subsequent studies have supported its use as a reasonable determinant for acceptable
levels of annoyance (Dudleston, 2000). However, one study concludes that setting a limit
based on hours per year is overly simplistic, as survey results show that the time of day and
year at which shadow flicker occurs is important in determining its annoyance value
(Widing et al, 2004). In the above court ruling and in the subsequent studies, the 30 hr/yr
limit is taken as the modelled limit, not the actual amount of shadow flicker, which may be
considerably less because of cloud cover. This interpretation is not, however, universal and
some regulations (notably in Australia) use 30 hr/yr as the actual limit (not the modelled
limit).
E.7.2
Explanation of modelling parameters
Zone of influence of shadows
The intensity of the shadow associated with shadow flicker is directly related to the amount
of sunlight blocked by the blade. When the receptor is close to the turbine, 100% of the
sunlight is blocked as the blade passes in front of the Sun, and the receptor experiences
alternate full sunlight and full shade. As the receptor moves away from the turbine, the
apparent size (angle) of the Sun remains effectively constant but the apparent size of the
blade decreases (because the distance between turbine and receptor is large relative to
the size of the blade but insignificant compared to the size of the Sun). At some point, the
blade will no longer fully cover the Sun as it passes in front of it, and the receptor will
experience alternate full sunlight and partial shade. The variation in blocking effect with
distance is illustrated in Figures F-2 and F-3 for a typical blade (2 MW turbine).
Draft National Wind Farm Development Guidelines – 2 July 2010 Page 157