Icing

In the wind business, icing is not a sweet spread that you put on cake.  It is a coat of ice that can affect production, and there is nothing sweet about it.

We had a major ice storm in Ontario last week.  I was at a meeting in Grand Bend in the morning, and got on the road shortly after 1 PM, for what is normally a 3 hour drive.  I made it home at 7PM.  There was freezing rain, ice pellets, snow, blowing snow, a transport truck on its side blocking the road, drifts on the road.  It was a taste of every kind of winter weather imaginable.

One of the defining features of the storm was its breadth.  There was freezing rain all the way from Essex County to Tobermory.  That’s a big swath.  The Bruce Peninsula seems to be hit by freezing rain a lot less than the southern part of the province.  We are more likely to just get snow.  But last Thursday we had freezing rain.
Freezing rain can affect turbines in two ways.  First, if you have a cup based anemometer, it can freeze up.  Wind turbines have logic that tells them to shut down if there is a mismatch between the measured wind speed, and the production level.  When the anemometer freezes, the turbine sees the wind speed as 0.0, and so it will prevent any production.  This is done to protect the equipment, as clearly there is a problem if the wind speed is 0.0, and there is production.  It is of course quite frustrating to have a multi million dollar piece of equipment stop due to the failure of a part worth a couple of hundred dollars.
The V80 has heated cup based anemometers.  And Thursday night at about 4:00 AM, they froze, so the turbine shut down.  It warmed up on Friday, and this, combined with the heating elements in the anemometers, melted the ice, and the turbine resumed operation around 11:00 AM.  This is only the second time the turbine has stopped due to an ice storm in 4 years, with the other outage lasting about an hour.  The V82’s have ultrasonic anemometers, with no moving parts.  One would think that these would be inherently more reliable than cup based solutions, but that has not been my experience.  While they performed wonderfully through the ice storm, they seem to have mysterious failures much more frequently than the old fashioned, but long tested, cup technology.  In heavy icing areas, ultrasonic certainly makes the most sense.

Second, ice can accumulate on the blades.  In extreme cases, this ice can fall from the blade, potentially causing injury (although I know of no documented cases of ice throw from a blade causing injury).   If the turbine is not spinning due to low winds, the ice will fall directly under the turbine blade.  Turbine technicians report that the farthest ice is typically seen from the turbine is 2 rotor diameters.  So with proper setbacks from buildings, ice throw need not be a safety issue.  Indeed, ice fall from a barn roof, or building, presents more danger, as people are more likely to be around.  After all, icy fields where wind turbines are located after an ice storm are rather inhospitable, and unlikely to attract a crowd.  And of course sheet ice flaking off in small chunks would not be a safety issue anyways.  It would take a very long and intense ice storm like the one in 1998 in Eastern Ontario and Quebec, to create any substantive danger from ice.  Of course, if there is wind, the flexing of the blades in the wind will tend to cause ice to fall off before it has a chance to accumulate much.
In Southern Ontario, sheet ice is the main challenge.  But in other places, like the Yukon, and Newfoundland, rime ice can be a challenge.  We call it hoar frost, and in Ontario, it will appear on trees a couple of times a year, and usually only for a few hours, and is quite beautiful.  But in certain places, where there is the right combination of temperature and moisture, it can accumulate to several inches thick.  And it is usually in places that don’t warm up much to let it melt.  There are two 25 MW projects that are being installed in Newfoundland.  It will be interesting to see if they have this problem.

Structurally, ice is not a problem.  A wind turbine blade weighs 6 or 7 tonnes – the addition of ice does not substantially affect its weight, and of course the turbine is designed to withstand icing.  I was assured by Vestas that an ice storm like 1998 would be no problem from a structural viewpoint, although production would likely be affected.

Of course the other problem with ice is its impact on the electrical grid.  I heard that over 50 poles were down along Highway 21, from a combination of wind and ice.  That must have happened after I drove through.  Then again, people don’t need power when the grid is down.

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