Author: Alicia Webb
Shadow flicker occurs when the sun is low on the horizon and behind a rotating wind turbine casting a flickering shadow onto a house. Depending on the season, the locaton of the house and the location of surrounding turbines — this effect may last for several minutes in a day. The best practise guidelines limit shadow flicker to a maximum of 30 hours per year on any surrounding residences.
In order to prove that your turbine layout does not create more than 30 hours a year of shadow flicker at any surrounding houses, you'll need to get an expert report written. Two shadow flicker reports are linked in the 'More Information' section of this article.
Calculating shadow flicker
Consultants use specialised software to calculate shadow flicker for specific locations. Calculations are based on the turbine locations, topography of the surrounding land, and the known path of the sun through the sky. Due to diffraction of sunlight in the atmosphere, shadow flicker can not be detected more than 2 km from a wind turbine.
The theoretical or worst case amount of shadow flicker occurs when:
- there is no cloud cover
- the window at the residence is not covered
- the turbine is oriented exactly perpendicular to the line between the house and the sun
- the house is orientated such that the windows are facing the sun
- there are no obstacles like trees between the house and the turbine
This theoretical or worst case calculation is then converted into a more realistic amount of shadow flicker using data on local weather patterns, and the actual size and orientations of windows at a residence.
If shadow flicker is found to be more than 30 hours per year at a residence, simple measures can be taken to mitigate this such as planting screening trees.
Of course, the outcome of shadow flicker assessment, like every other type of assessment, may result in adjusting the turbine layout and wind farm design in order to reduce the impact of this effect.
Scoping the job for a consultant
When engaging a consultant to write a shadow flicker report it is best to follow the tips and tricks in the article Engaging specialists for environmental & technical studies. Essentially that article suggests writing a detailed scope and getting multiple quotes for the job.
The scope should generally include:
- A detailed description of your wind farm including turbine types or dimensions and the locations of residences (See suggested inputs below)
- The reason you are having the report written (e.g. to satisfy planning conditions)
- Who will need to read the report
- What you expect too see in the report (See "what to expect" below).
Information a consultant will need
The inputs to a shadow flicker report are typically:
- GPS coordinates of all wind turbines
- GPS coordinates of all residences within 2 kms of wind turbines
- A digital topographical map with height contours at intervals of not less than 10 m
- The type of wind turbines to be used, or at least the dimensions (hub height and blade length)
- The maximum size of the residence window that you expect to calculate for
Notes:
- All inputs should be in the same coordinate system
- If you don't know the maximum window size, recommend a conservatively large one to the consultant. An example could be a window 2 m high x 10 m wide and 0.5 m off the ground.
- If the turbine model has not yet been selected, estimate the hub height and blade length that will be used. It's best to overestimate these and thus calculate the worst case scenario. If the turbine eventually selected is smaller then the shadow flicker report will not need to be redone.
What to expect in the consultant report
A good consultant report will be a stand-alone document with plenty of description such that it can be interpreted by somebody who is not a professional in the field. It will also list in appendices all of the input data including locations of residences and turbines.
A few things to look for are:
- A description of shadow flicker
- A description of all inputs and assumptions
- A description of the applicable guidelines and limits
- A description of the methodology used to change 'theoretical worst case' shadow flicker to 'realistic' shadow flicker
- Output tables showing shadow flicker results for each residence within 2 kms of wind turbines. These results should express
- Total hours per year
- Maximum minutes per day
- The distance of the residence to the nearest turbine
- Recommendations for mitigating shadow flicker for any residences experiencing over the maximum amount of shadow flicker
- If the consultant recommends moving one or more wind turbines, they should show the effect this has by recalculating the shadow flicker for all houses using the new suggested layout