Why do wind turbines sometimes stand still?

When driving on a windy day past a row of wind turbines, you may notice that a few suddenly stop or, even worse, all of them. It happens regularly and raises eyebrows. Why does it happen?

Reason 1: Storm

Let’s start with the most well-known: storm. Too much wind for a windmill: that can happen. The limit is at wind force 10, as we found out earlier. How they shut down in such cases? They can do it themselves. The speed limiter in the turbine stops it by “pitching” the blades 90 degrees, making them cross the wind, in a feathered position, causing the mill to stop automatically. This information is available on the Pure Energy website.

If you let the windmills continue running in stormy conditions, it can have significant consequences. Fire in the mill is not ruled out, and cracks can also occur in the material. Some windmills can handle such wind speeds. These storm-resistant mills are heavier and can withstand more powerful winds. But placing them everywhere? No. “They are much more expensive,” says energy expert Martien Visser from Hanze University of Applied Sciences in Groningen. Another drawback: they rotate less easily in light winds, which are more common in the UK than storms.

So, if it’s stormy, windmills will shut down. But it doesn’t often storm across the entire country. Visser says, “If it’s storming in Bristol, it’s probably just windy in the northern part of the UK. And that still yields a lot of energy”.

Reason 2: Shadow Flicker

Alright, that covers storms. There are more reasons why the mills sometimes keep their blades still. Common but less known is shadow flicker. Here’s a video to illustrate how annoying it can be:

Quite annoying, isn’t it? That’s why there are rules for it. “A façade with windows may not receive more shadow flicker than 17 days per year. And not more than 20 minutes per day,” states the government’s website.

Calculations are made to determine the days when shadow flicker may occur. This is related to the sun’s position and, consequently, the angle of the shadow on houses and buildings. “The mill regularly shuts down due to this, and when the sun has rotated, it starts up again,” says Visser.

Wind turbines causing shadow flicker are equipped with a so-called shutdown provision. According to the government: “This allows the turbine to only shut down when shadow flicker actually occurs.” Because it could be that due to cloud cover or a different wind direction, there is no shadow flicker.

Reason 3: Birds

Animals can also be the reason for windmills standing still. It’s a good reason; otherwise, the harm would be significantly greater than it is now. According to estimates, ‘about 100,000 birds die’ each year, says BCC Science focus. Not too bad, says MIT Climate: “In traffic, 2 million birds die annually.”

Nevertheless, it’s a reason to stop the devices in some locations, and that’s done with smart software. An example from Zeeland. There is the Krammer wind farm, close to the habitat of the sea eagle. The turbines in the park have cameras and microphones. If the sea eagle, spoonbill, crane, or great egret is spotted, the turbines shut down, as reported by the BBC earlier. This way, the birds can safely continue hunting and breeding.

There are more solutions to the problem. There are suggestions to paint the blades black, making them more visible.

Reason 4: Oversupply

It doesn’t happen that often yet, but I think it will happen more and more,” says energy expert Visser about the oversupply of electricity. Nowadays, it occasionally occurs that more energy is produced than used in the UK. “About 100 hours so far this year. But we’re producing more and more. So, that will increase.”

During the hours of overproduction, windmills are sometimes turned off. This way, there is no overproduction that could overload the power grid. You might say: store that excess production, but we’re not there yet. Visser says, “Those batteries are really terribly expensive.”

As consumers, we can also do something about this. Use as much electricity as possible during the day instead of at night. Because much less electricity is generated at night.

Reason 5: Maintenance

If you see a lot of windmills in a row turning, but one isn’t, the chances are that it’s broken. Short-circuit, wear and tear, maintenance, these can all be reasons to temporarily stop the windmill. It remains a device that can simply break down. The average windmill lasts about 20 years before it needs to be replaced.

Reason 6: Ice

Snow, ice, it can cause ice on the blades of the mill. You can imagine that this is quite dangerous when the blades start spinning. If that’s the case, it is detected by the windmill. So, they wait before they start turning.

And the last one: too little wind

There’s one more reason for windmills not turning: no or too little wind. Below wind force 2, they simply don’t rotate. They are in the free position and can start rotating when there is enough wind force.