Shucks! My wind turbine just froze over... and finally, Part III (5 min read)
Great title! ;-) Unfortunately, I don't have a wind turbine...?It's awesome if you're reading this, and also read part 1 (here) and part 2 (here) you'd have gotten the full gist regarding the reason behind this article. In part I, I harped on about how complicated wind is and the magnificence of wind turbines, went into more detail on wind turbine classes and the ice formation processes. It's super interesting, no kidding ;-)
Part 3: The law and other things…
Part 3 concludes with mitigations for ice formation.
Regulations
RenewableUK onshore wind health and safety guidelines mentions ice loads as a site-specific risk to be considered and lists ice throws as a key health and safety risk to be considered. It also states that developers should ensure that risks to public safety including ice throw, if the wind turbine is operated with ice build-up on the blades, are considered and managed effectively over the project lifecycle and they should be prepared to share their plans for managing these risks with stakeholders and regulators.?
Recently (in 2020), the French authorities published new regulations (Modifications édictées dans l’Article 25) to ensure that:?
Other jurisdictions have regulations in place dealing with icing of wind turbines. This makes it pertinent to have in place a strategy to predict and prepare for inclement weather prior to its occurrence to avoid / reduce potential losses.?
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Operational Strategy for Cold Weather
The amount of ice accretion on the turbine strongly depends on its operating regime.?
Iced wind turbine (https://www.eirosproject.com/project/case-study-applications/)
Wind turbine operational strategies in use, may be the initially determined operating strategy from the manufacturer, that was designed for “clean blade” conditions. This typically calculates the generator torque controller and blade pitch controller as a function of generator speed.
In this scenario, uncontrolled ice accretion (leading to performance degradation) can occur on the blade if the operating strategy doesn’t account for “iced blade” conditions.?
A viable “iced blade” operational strategy could include decreasing the turbine rotational speed and accepting a slight energy conversion decrease during the icing event. This can lead to a performance improvement when full operation is restored compared to the baseline operational strategy (due to reduced icing), the impact of blade velocity on icing in discussed later. Decreasing the rotational speed of the turbine during the early phase of ice formation reduces the water capture / amount of ice accreted on the blade.
On the flip side, sustaining the rotational speed during the icing event could lead to a higher performance loss afterwards compared to the same baseline (due to increased icing).?
Powering down the turbine during a meteorological icing event may also help to reduce ice accumulation and the subsequent restart period. This downtime potentially means taking an earnings hit.
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Ice mitigation
Ice mitigation could include reducing or stopping turbine rotation to prevent unsafe ice build-up or de-icing using internal / external heating treatments. These operational limitations to reduce buildup / heating cycles for de-icing can lead to high peak energy production losses. Full winterization of wind turbines can come at a steep cost of up to 10% of the cost of the turbine.??
Passive solutions include the use of special coatings including:?
Active anti-icing / de-icing technologies are typically more effective but have higher energy consumption compared to passive technologies, examples include:?
WICETEC Ice Prevention System (WIPS)
Implanted Carbon Fibre Heating Element in Wind Turbine Blade - Alireza Maheri
A blower, heater and duct system targeting heat to the tip of the blade (https://www.borealiswind.com/).?
Super famous picture of a helicopter deicing a wind turbine with hot water in Northern Sweden
领英推荐
These could be used individually or in combination, such as heating components within the nacelle, partially heating the blades, then using coatings to reduce ice adhesion / accumulation.
Ice formation on turbine blades
When the blade of a wind turbine strikes supercooled water droplets, the leading edge becomes covered with a film of supercooled water that freezes from inside out. This was discussed in greater detail in part 2.
Water Collection Efficiency on Blades
This matters when considering what sections of the blades are integral to energy production.?
The wind turbine power coefficient signifies how efficiently a turbine converts the energy in the wind to electricity. Mathematically, it is the ratio of actual electric power produced by a wind turbine, divided by the total wind power flowing into the turbine blades at specific wind speed. This wind turbine power coefficient increases starting from the root of the blade to the tip.?
Approximately 85% of power production in a wind turbine is generated from the section on the blade that is located in the last 60% of the blade towards the tip, with the last 20% (towards the tip) of the blade generating the highest power (36%).
The water collection efficiency along the blade also varies, especially as the water droplet size drops. The examples are shown below with the smallest droplet sizes representing supercooled stratus clouds (fog), followed by supercooled drizzle and freezing rain with the largest droplet sizes.
Water Droplet Classification
The largest water droplet size has high collection efficiency throughout the blade whilst the smallest water droplet size has its highest collection efficiency towards the blade tip.
Collection efficiency along the blade for different droplet diameters (fog, drizzle, rain)
In summary, the highest velocity sections of the blades have the highest collection efficiency for the smallest water droplet size.
Ice formation on the blades
Using the following liquid water content values with associated severity classes, the ice build-up along the rotor blade is displayed below.?
Ice thickness rate along the span for different liquid water contents
This shows the area on the blade that is most affected aerodynamically due to ice accumulation, is located (approximately) on the last 20% of the outer section of the blade.
There’s a huge amount of interest in putting more wind turbines in hard to reach areas where icing is an issue. A lot of work is going in to reduce the impact of icing and reduce the need for maintenance on even bigger turbines. It’s a really interesting field with super challenges, such as the wind changing from season to season. However, there are also solutions being delivered as we speak.?
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References:
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