Draft National Wind Farm Development Guidelines - July 2010

Shape of the Sun
The Sun can either be modelled as a point source or a disk. In some sources (Garrad
Hassan, 2006) the disk is considered the more conservative and in others (Garrad Hassan,
2007), the point is considered more conservative. The basis on which the point source can
be considered more conservative is that shadow intensity does not decrease with distance
using this method (whereas it does with the disk). Since the shadow intensity is considered
separately as a means to determine the region of influence in all approaches to modelling
shadow flicker (including that proposed here), this argument is mute. The disk can be
considered more conservative from the point of view that it leads to a slightly larger area
of coverage, and is more realistic. On this basis, the Sun should be modelled as a disk.
Time and duration of modelling
It is common practice to produce a model for a single year (arbitrarily selected), to
determine the shadow flicker that occurs in that year and assume this will apply to all years
of the wind farm’s operation. This is slightly flawed, as the orbit and orientation of the Earth
change marginally from year to year. Also, there could be some increase in the amount of
shadow flicker during a leap year relative to any other year.
The assumptions made here have little influence on the result; however, the annual limit is
based on a non-leap year, so this should be used in the assessment. To account for
variation in the Earth orbit, a year at approximately the middle of the lifespan of the wind
farm should be chosen. Given a typical 25 year lifetime, calculation is recommended for a
year between 12 and 15 years after the year in which the planning application is assessed.
Orientation of the rotor
The rotor can (typically) be modelled in one of the following ways:
• As a disk lying in a plane whose normal points to the Sun (also referred to as a sphere).
• As a disk lying in a plane whose normal is horizontal and points towards the azimuth of
the Sun.
• As a disk whose direction is determined probabilistically by the seasonal and time of
day wind distribution.
Of these models, the first is the most conservative as it generates the largest area of shade
at any time. It is most inaccurate when the Sun is overhead and actually passes through a
narrowed rotor plane (as opposed to the full rotor plane as assumed); however, this is
when shadow flicker is of least concern and thus the assumption is of little consequence.
Modelling the rotor as a disk whose normal is horizontal and points towards the Sun is also
conservative. It is essentially the same as the sphere model but includes the effective
narrowing of the rotor plane when the Sun is overhead.
The third approach is theoretically most accurate but is difficult to implement in practice
and is a statistical approach that is hard to test. It is non-conservative. Thus, the first or
second approaches are recommended.
Offset between tower and rotor
Garrad Hassan (2007) notes that it is important to include the offset between the tower
and rotor because the modelling is quite sensitive to the rotor position. While it is
considered prudent to include this information in the model if available, such a high level
of sensitivity is considered best addressed by other means, particularly given that
micrositing of turbines during construction could result in far greater errors in the modelled
position. Further consideration of sensitivity is included under Sensitivity (page 153).
Time step for calculations
When modelling shadow flicker, shadows are typically calculated at some time of day, the
time of day is then incremented by a fixed amount and the process repeated. The
increment in time can impact on the accuracy of the calculations. The increment used in
Page 160 Draft National Wind Farm Development Guidelines – 2 July 2010