Page 44 - North American Clean Energy September/October 2019 Issue

wind power
Ready for Winter Ice?
by Marty McKewon
Ice accumulation from freezing fog event in Kansas
AFTER LAST WINTER, BRINGING UP A TOPIC SUCH AS ICE ACCUMULATION
to turbines may draw a few groans. If anything, it confirms just how significant the impact of extreme cold and icing can be on wind turbine performance and production. Of course, there is nothing you can do to actually stop the meteorological icing event from happening, but there are steps you can take to recognize - and prepare for - icing. There are additional steps you can take to mitigate the total impact of an icing event, and even position your site for a faster return to full production.
Understanding the Impacts
The impacts of icing to wind turbines are both safety- and production- based. Ice accumulation on blades can lead to ice throw, which can be dangerous to onsite employees and, potentially, the general public. Even stationary wind turbines can shed large chucks of ice. This not only creates a further safety hazard, but the accumulation of ice on blades increases component fatigue from load, and can increase noise levels. Perhaps most importantly, the production of the turbine will be reduced due to loss of aerodynamic properties, resulting in less power output. Continued ice accumulation and rising safety concerns may lead to total shutdown of the turbines, perhaps for days at a time. When that happens, the financial losses can be huge.
Recognizing the Threat
The threat of ice accumulation is not always obvious. The most well understood icing event is freezing rain. That may be because it is widely advertised by local weather sources, and rarely
sneaks up on anyone. However, it’s often the less recognized events of freezing fog and heavy wet snow that cause bigger headaches. A wind driven, heavy, wet snow can stick to almost anything; due to high moisture content, the weight of accumulating snow piles up fast. A worst-case scenario is a heavy, wet snowfall lasting 3-6 hours, followed by a sharp drop in temperature
in a short period of time. This can create a “flash freeze” that essentially bonds the accumulation to whatever it has fallen on - even a turbine blade. That ice will stay on the blades until the sun returns and temperatures warm.
Freezing fog, on the other hand, can form in several ways. One method that often sneaks up on renewable energy professionals is when a cold, shallow arctic airmass undercuts an existing moist airmass in place. Temperatures at the wind site might be in the upper 30s to near 40, and a couple of hours later it can be 25 and windy, with freezing mist and fog coating your blades, and you’re suddenly blindsided by a major drop in production.
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