Complaints about wind turbines are nothing new – the noise of the machines and their impact on picturesque landscapes are common irritants. But there is another hated side effect: shadow flicker.
“Shadow flicker drives people mad, even if they keep their curtains shut or their blinds down,” says Angela Kelly, head of Country Guardian, a British anti-wind-turbine pressure group.
As each turbine blade sweeps around, repetitive, strobing shadows can be cast on houses when the sun is low in the sky. Outdoors, it is distracting, but inside it is even worse – some people who experience flicker liken it to having the lights continually switching on and off (see video below).
A possible solution is at hand. Vestas Wind Systems, a turbine maker in Randers, Denmark, has developed a predictive system that works out when shadow flicker is about to happen and stops the turbine rotating until the sun moves the shadows onto uninhabited land.
The measure is designed to head off an oft-cited objection in local authority wind farm planning enquiries, says Bruno Lund Mathiasen, a Vestas engineer.
Called the Vestas Shadow Detection System, the technology is based on software that computes four risk factors: the angle and position of the sun, the distance of the wind turbine to any potentially affected properties, the radius of the rotor blades and the height of the turbine hub from the ground.
Light levels are assessed using two light meters placed on the east and west-facing sides of a wind turbine’s support tower. If one is reading very high light levels while the other is low, it means the weather is set for very strong shadow production shortly after sunrise or before sunset, says Vestas spokesman Michael Holm.
Once the risk of shadow flicker has been calculated, the software decides whether the turbine should be temporarily shut down.
“The turbine will be put in idle mode, which means we twist the blades so the rotor turns very slowly. This stops any shadow flickering,” says Mathiasen. “Based on its calculation of the shadow effect, the system will decide by itself whether to put the turbine in idle mode or not.”
The system has been shown to be effective in tests at 50 Vestas sites around the world, Holm claims.
Mathiasen will not say how much Vestas will charge the firms that operate turbines for the technology, but Kelly thinks any price would be too much. “If Vestas is serious about this they ought to contact anyone with wind turbines causing flicker problems and offer them this technology for free,” she says.
Paul Marks, New Scientist