Shadows cast by wind turbines

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When shadows of wind turbines is generated by the rotor and tower shadow that while direct sunshine occurs. In particular, the shadow cast by the rotor is unpleasant for many people because, in contrast to stationary objects, it causes periodic fluctuations in brightness at the immission location. The shadow of a standing wind turbine, on the other hand, cannot be assessed differently than the shadow of a normal building. The appearance of the shadow depends on the location and height of the system, the location of the immission point and the weather .

The shadows cast by wind turbines are often referred to as the disco effect , although, strictly speaking, the disco effect denotes light reflections from rotor blades .

Affected area

Shortly after sunrise , the shadow of the wind turbine hits the ground at a very great distance, in a westerly direction. When the sun rises higher, the shadow moves closer to the wind turbine and moves away again in the east in the afternoon. In summer, when the sun rises in the northeast, the shadow first falls to the southwest. Around noon it is only a few meters away from the base and migrates to the southeast in the evening. In winter, the sun rises in the south-east, only rises to around 15 ° above the horizon at noon (depending on the date and the geographical latitude, applies to Germany at the end of December) and sinks in the south-west. Accordingly, the shadow moves its path coming from the northwest, past the wind turbine at a greater distance, to the northeast. The area affected by the shadow is shown in the adjacent drawing.

Shadow area of ​​a wind turbine

The shape of the area depends on the latitude. Beyond the polar circles, the sun can move all around, so that the shadow cast reaches all directions. In the tropics, the sun can stand vertically above a wind turbine, so that its base point can also be in the shade. The shape shown is created in Germany.

The shadow cast can be neglected at a greater distance from the system. Since the rotor blade only covers a small area of ​​the sun from a distance, the fluctuation in brightness resulting from this cover is no longer perceptible. The range of the shadow depends on the width of the rotor blade and the distance to the projection surface. In the case of large wind turbines, the shadow cast must sometimes be taken into account at a distance of more than 1000 m.

Immission control

According to the so-called "Instructions for determining and assessing the optical immissions of wind turbines" of the light immissions working group of the Federal / State Working Group on Immission Control , the shadows cast by wind turbines must not affect a residential building for more than 30 hours a year and 30 minutes a day. If this period is exceeded, the wind turbines must be switched off as long as their shadow falls on the immission point.

A distinction is made between the theoretically maximum possible and the actual shadow duration. The theoretically maximum possible shadow duration would be reached if the sun was always shining, the rotor was always in motion and the rotor was also always perpendicular to the sun. Since this is not the case, the actual duration of shadow casting is well below the theoretically maximum possible. Experience has shown that at locations with a theoretically maximum possible shadow duration of 30 h / a, the actual shadow duration is approximately 8 h / a. In the building or Federal Immission Control Act approval for wind turbines, it is therefore required that a wind turbine whose shadow can theoretically increase the shadow duration of an immission point to over 30 h / a or 30 min / d, must be equipped with an automatic switch-off. This automatic switch-off must be programmed in such a way that the actual shadow duration is limited to 8 h / a and 30 min / d.

When considering the duration of the shadow cast, the point of immission is assumed: The decisive factor is not how long a specific wind turbine casts shadow, but how long the point of immission is affected. If several wind turbines cast shadows, one of them must be switched off in the afternoon, for example, if another one in the morning has exhausted the permissible shadow duration of 30 min / d.

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