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Archive for the ‘#hurricanebawbag’ Category

Today saw the return of some extremely windy weather across Scotland. The met office has already reported some of the headline wind speeds on their website as a comment dated 03/01/12 17:55 (links to a live website which will be updated and overwritten). I thought it might be good to see the locations of these reported wind speeds on a map:

Some of the windiest locations in Scotland

Data from the Met Office, image produced using Google Earth. Image may be reproduced freely if credited to @turbinetastic.

“Gusts” are short term wind speeds, usually measured over about three seconds. They can be very damaging, as we saw today in Scotland.

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Last Thursday, December 8th, was a particularly windy day across most of Central Scotland. You might have heard about it, it trended on twitter as #hurricanebawbag. It was the windiest day across Scotland at least since 2006 with gusts of over 100mph.

It’s often said rather disparagingly by people who don’t like wind turbines that they need to be switched off when it’s windy. And of course that’s true, and for a very good reason. The amount of energy available in the wind which a turbine could potentially extract is proportional to the cube of the wind speed — so when the wind speed doubles, the energy available is eight times as much. Theoretically we could design wind turbines which operated in the sort of wind speeds that we saw in Scotland, but most of the time they’d be useless. We need to choose the energy range we want to work in, and we choose the one, unsurprisingly, which lets us harvest the most energy.

It was extensively reported, in Scotland at least, that a turbine caught fire during the storm. Dramatic pictures captured by amateurs went viral on the web; you can see them at the stv news site linked above. Infinis have released a statement (pdf), and New Scientist attempted an explanation on their blog, but it’s clearly been tackled by someone who is a scientist rather than someone who works with wind turbines. So I thought it was worth looking at it in more detail.

There are a few comments that can be made about this. Firstly, wind turbines very rarely catch fire. There are currently over 2.5GW of wind turbines generating in Scotland. Since the current onshore wind turbines only really go up to about 2.5 MW that’s at least 1000 turbines. That means that in the wind conditions experienced last week, only 1 turbine in a thousand caught fire. So it’s not a high risk event; if you’re near a turbine and it’s very windy, chances are you’re pretty safe. That’s what those unnamed “experts” quoted in the media as calling it a “freak event” were probably getting at. Although I would still recommend that you don’t stand near a turbine if it’s very windy.

So what went wrong with this one? Well, Infinis, who own the site in question, don’t yet know what happened but shut down the whole site and notified the grid operator, Scottish Power, immediately. They will be slow to comment as they go through the data from the turbine. There are a few things we can think about though:

  • What components are there in a wind turbine that involve naked flame?
    Well, none, as a matter of fact. Turbines don’t use fire.
  • What components are there in a wind turbine that can generate heat?
    Now we’re on to something. Just about any moving component can generate heat if there’s excess friction involved. Brakes slow down moving parts with friction, which releases most of the energy as heat. So there’s the brakes. Any other moving components could be a risk if there’s not enough lubrication on the joints. And then there’s the electricity itself in the generator; there could be a fault or a short circuit.
  • What was happening to the turbine immediately before it caught fire?
    In this case we’re rather lucky because someone video’d it. Without that, we would have no way to narrow down the list of things which can cause heat and potentially start a fire.

It’s this third point that I want to look into in a bit more depth. Notice in the photographs, how this turbine is pointing a different way to it’s neighbours? That’s not a good sign. Smoke trailing out in front of it, horizontal in the wind, is also a bad sign. Wind turbines (of this 3-blade design, at least) are only designed to harvest energy from the wind if they’re pointing at it. Unless they’re completely switched off (rather than simply under shut down through the automatic control system) the turbine should be tracking the wind direction. From the video you can see that the turbine in question is turning its head, so to speak — the technical term is yawing — away from the wind. That means something crucial has failed.

I’m not going to say that I know what caused the fire. But you’ve got a turbine that can no longer control its direction, and a hurricane. You’ve got gyroscopic forces from the rotor spinning at relatively high speed (because it’s windy), you’ve got a turbine facing the wrong way which means the blades are very likely trying to spin in the wrong direction, putting forces on the wrong components in the wrong way. The brakes, if they haven’t failed already, are under tremendous forces. So you’ve got an event which is the culmination of a number of unusual circumstances contributing to a fire.

How dangerous is it if wind turbines can catch fire?

This is an important question. But actually it’s not as bad as it might seem. Generally hot air from a fire spreads upwards more easily than downwards and most of the moving parts of a wind turbine are already at the top. Turbines need to be placed in clear air, so generally they’re well spaced out. Unless a turbine is in an urban area or a forest, both of which are comparatively rare for a turbine of this size, the chances of the fire spreading beyond the one turbine is very small. There are also generally no people inside a turbine except for maintenance.

Scotland’s Right to Roam laws mean that the general public are allowed to walk within wind farm grounds. So although wind farms are generally remote that doesn’t mean that general members of the public are completely safe. However, risk from falling debris is very small if the distance is further than tip height (the total height of the turbine including the top blade).

There are warning signs that dangerous failure is imminent, and to be honest they’re fairly obvious. If you’re near a large turbine, you can generally hear a sort of low-pitched “whoosh” as the blades go by. Even in a high wind, the rate of the blades passing is generally less than a fast heart beat. If a turbine is freewheeling, something critical has failed. Get away from the turbine as fast as you safely can, and if you can get in touch with the operator to let them know (although by that point there’s not much that can be done — the safety systems have already failed and it’s far too dangerous to approach the turbine until the wind drops. Still they can at least close access). A turbine that is turning and isn’t facing the wind direction is also a danger, as we saw in this case.

The thing is that catastrophic failure is incredibly unlikely to ever happen when the wind speed is low. What risk exists is at its maximum when the turbine is being bombarded with lots of energy from gale force winds. It bears saying that no general member of the public should be within the height of a turbine from its base when the winds are that extreme. If you are, then keep away from any turbine which is behaving differently from its companions. Do this and you’re pretty much as safe as you can be.

If you live near a turbine, the risk of debris is very small if you’re further away than the total height of the turbine, generally somewhere between 100 and 150m. In the conditions seen last week, obviously the biggest risk is downwind, and in any other direction you will most likely be fine. There is no harm in leaving your property if you feel at risk, however. Nor in confirming with your insurance company that you’re covered in the event of a problem with your nearest wind turbine.

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