Wind Turbine Flicker
what is it?
"Flicker" in the context of wind turbines usually refers to turbine blades casting a rotating shadow across nearby properties. The term can also be used in a different sense by engineers to describe short lived voltage variations in the electrical grid.
Here we are referring to the shadow variation effect: Turning blades of a wind turbine located between an observer and the sun "chop" the sunlight.
A video of it is provided on the following Danish wind energy site.
The phenomenon is amenable to calculations which provide an objective measure of the intensity of the flicker at various distances from the turbine and also give an indication of how many hours per year a particular location is subject to flicker. A simple model yields the following results.
| Approximations For Observer East or West of Tower | ||||||||
|---|---|---|---|---|---|---|---|---|
| Distance from turbine (metres) | 100 | 200 | 300 | 500 | 700 | 1000 | 2000 | 4000 |
| Percent of Sun Eclipsed | 100% | 65% | 45% | 27% | 20% | 14% | 7% | 3% |
| Daily flicker hours for tower E or W of observer | 1.89 | 1.31 | 0.95 | 0.59 | 0.43 | 0.30 | 0.15 | 0.08 |
| Approx days in flicker zone per year | 114 | 39 | 28 | 18 | 13 | 9 | 5 | 2 |
| Annual Accumulated hours of flicker | 215 | 52 | 27 | 11 | 6 | 3 | 1 | 0 |
| Typical flicker frequency: About 1.5 cycles per second | ||||||||
It relies on some simplifying assumptions:
- Blade Geometry: Real blades vary in profile with chords as wide as 4 metres near the root and tapering to a minimum near the tip. If the wider part of the blade comes between the observer and the sun it will block off more sunlight than a narrower section. Since the tip of the blade is travelling faster than the root and is usually narrower, less sunlight is blocked for less time by the tip. So lets keep it simple and just assume a constant one metre chord along a rotating blade of length 40 metres.
- Observer position: If an observer moved out of the turbine's shadow he would not observe flicker. So let's concrete him to the turf. We then note that the turbines shadow moves not only with the time of day, but also with the time of year. The sun rises/sets on different parts of the horizon as the season varies and as it does our observer gets more or less exposure to shadow each day. For the sake of the calculations let's assume our observer is in a fixed position due west of a single turbine. Note that in a real situation over the course of a year the position of flicker shadow will vary over any given property and a person on that property will move around, possibly walking in and out of flicker zones.
- Single Turbine A single turbine is assumed. The shadows are multiplied if more than one is involved.
Interpretation of results
The physical interpretation of the results is simple. The model predicts a certain percentage of the sun's light eclipsed by each blade as it rotates, and the observer is exposed to flicker for the accumulated number of hours shown each year.
But what does it mean?
To reduce flicker to its physical components is to accept an underlying premise that if the physical effects are understood then human effects can be safely ignored. The issue is
What is the effect of flicker on people? Should people be subjected to it? and if so What is a reasonable exposure?
The above Danish Wind industry site refers to a German court case in which the judge tolerated 30 hours of actual shadow flicker per year at a certain neighbour's property. The figure is also promoted in Australia by the industry as a "standard" for allowable flicker. As predicted by our simple model, a static observer more than three hundred metres from a turbine would find it almost impossible to accumulate more than thirty hours of flicker in a year. Yet reports from people living as far away as a kilometre from a turbine report that flicker can be annoying, as noted on this site of wind turbine opponents.
An engineering assessment from the wind industry is also provided on this developer website.
Spring Range View.
Spring Range suspects that the subjective annoyance of flicker is higher than is granted by either the turbine manufacturers or embodied in the oft quoted "standard" of 30 hours of flicker exposure per year. The "standard itself is promulgated by the wind industry based on a single German case and, as noted above, provides almost free rein to developers when siting turbines. If we assign some credence to reports from those who live in the shadow of turbines, flicker is at least a source of annoyance and perhaps more than that. It is quite likely that the "standard" is simply wrong. An impartial study to set a better one is needed.
However, to argue the annoyance level of flicker is to miss an essential question. Is flicker something we wish to designate as acceptable in human living spaces. Is unfettered, unprocessed sunlight a fundamental condition we should strive for in all places where humans live? To accept that some level of high frequency sunlight variation is OK is to cross an environmental rubicon which has long term implications for the quality of life in our society. Rural settings have a peace and tranquility missing from most urban environments. To introduce any level of flicker in such a setting is to fundamentally change its character and to degrade a resource which belongs to us all. The step should not be taken without a greater level of thought than is provided by the stroke of a pen of a single Government Minister