Shadow Flicker Analysis
A shadow flicker analysis calculates for each point of interest:
- Number of hours per year that the flickering occurs,
- Maximum length (in minutes) that flickering occurs on the worst day in the year, and
- Number of days in the year that shadow flickering appears at all.
All above numbers should be calculated for both the worst case and the so-called "real" case. Worst case is that there are no clouds and that the astronomical shadow is visible. Real case takes into account statistical distribution of clouds and visibility at the location.
Following German regulation,shadow flickering cannot be perceived by the human eye if the angle of the sun over the horizon is less than 3°. Plus, the blades of the turbines must cover at least 20% of the sun.
Results from above analysis help demonstrating that a wind farm proposal complies with regulation. While guidelines differ, the ones in Germany are most widely adopted. Accordingly, the maximum impact allowed by shadow flickering is:
- 30 hours per annum of flickering in the worst case
- 30 minutes maximum on the worst day in the year
- Real shadow impact limited to 8 hours per year.
Shadow Flicker Maps
In the first step, a shadow flicker map illustrates the figures on a geographic map of the vicinity of the wind turbines. In the example below, we ahve assumed a row of turbines, 450m distance from each other.
In a point analysis, the impact on specific windows in the area is computed. For illustration purposes, we have imagined a house (see map above for location) with three windows, one facing north, one north-west and the third one west. All windows are of the same size and height.
The computed results were as follows
|Window||Hours per year (worst case)||Days per year||Length on worst day [mins/day]||Hours per year (real case)|
:Hence, none of our windows passes the German shadow-test!
Shadow Flicker Calendar
The shadow flicker - calendar shows how many minutes of flickering a particular window (area) will receive on each day. For illustration purposes, we have calculated calendars for the north-facing and the west-facing window. In both instances, there are several flicker periods a year, owing to the fact that there are mutliple turbines. In this instance, there is no flickering to be observed in the middle of the year. This is when the sun is from the south, but there are no turbines south of the house in question.