Shucks! My wind turbine just froze over... Part II (5 min read)
Great title! ;-) Unfortunately, I don't have a wind turbine...?If you'd love to know the reason behind this article, preview the part I, where I harped on about how complicated wind is and the magnificence of wind turbines ;-) have a look here or here for more…
Wind Classes and distribution
Wind turbines typically have a lifespan of 20/30 years and are designed to withstand varying wind categories (classes from low to high), dependent on the average annual wind speed in the area, turbulence and 50-year gusts. This is super important to ensure the right specification wind turbines are installed, nameplate capacities are realized and that structural integrity is maintained. Temperature extremes are also important (more on that later). The IEC classification of wind into classes I, II, III and IV is displayed below. This shows that a minimum average 6 m/s wind speed at hub height is desirable for wind turbine siting and installation.
IEC Classification of wind turbines
Note: The extreme wind speeds are based on the 3 second average wind speed. Turbulence is measured at 15 m/s wind speed – IEC 61400-1 edition 2. (Reference:https://en.wikipedia.org/wiki/IEC_61400#cite_%20note-woeb-1)
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Though, the IEC classification is helpful, within the last 2 decades, several hurricanes have actually exceeded wind class I speeds including Katrina, Rita, Wilma, Irma, Dorian etc. So, some currently installed class I wind turbines may actually be under-specified for wind conditions they may experience given the increased frequency of more extreme weather / climatic conditions.
The figure below shows that generally the most attractive areas for electricity generation via wind lie through the middle of the US (assuming a hub height of 80m). Generally, the higher above ground you go, the higher the wind speed.?
The below graph buttresses this point showing the largest utility scale projects running through the aforementioned corridor via Iowa and Texas, with wind turbine installed capacities of 11,660MW and 33,133MW respectively. The state of Texas is a beast when it comes to wind power generation capacity, with around?60 percent of the state experiencing wind speeds over 7 m/s, which is suitable for wind turbine installation.?
The next graph displays the distribution of installed and operating wind turbines over the past 2 decades in the US. There is a mix of class I through IV wind turbines in the US. More class III wind turbines have been installed in the US in the last 5 years in comparison to the previous 5 years, when class 2 wind turbines dominated.
Icy wind turbines
Let’s revert to the famous picture of a helicopter spraying hot water on a wind turbine.?
Ice can and does form on wind turbine blades and other equipment within the nacelle. This can be a big issue especially as more wind farms are sited in colder, sparsely populated areas with good wind speeds (perfect for icy conditions).?
The below picture shows a wind turbine on Skellefte? Kraft's wind farm (in the?Blaiken area of Northern?Sweden) being de-iced?during a demonstration project for the technology. Blaiken is one of the largest land-based #wind farms in Europe.
Super famous picture of a helicopter de-icing a wind turbine with hot water in Northern Sweden
On?Skellefte? Kraft’s, Uljabuouda wind farm in Sweden, the wind turbines are designed to withstand arctic conditions of up to -40?oC (-40?oF)?temperatures. Some wind turbines in Canada also operate in regions that experience temperatures down to -30?oC (-22?oF). Typical wind turbine manufacturer specifications state?minimum operating temperatures of -20?oC (-4?oF), although they can be designed to operate in extreme temperature areas.
Blaiken wind farm (DYWIDAG-Sverige AB, Sweden)
Impacts of icing on wind turbines can be financial, technical and safety-related. These include:
Measures that can be taken to mitigate against cold weather disruptions include the use of cold-weather / de-icing packages. This is discussed in more depth later.?
On the Blaiken wind farm which was installed in stages, several methodologies were used to mitigate against cold weather impacts including heated carbon fibre coatings, ice sensors (to detect when there is a risk of ice formation which triggers the heating prior to ice formation), a decision was also taken to use?wind turbines with fewer moving parts (from Dongfang Electric Corporation) in one of the stages.?
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Ice formation
This may seem mind-numbingly boring, I assure you it is anything but ;-)
Prior to ice formation is the incubation stage. This requires the right environmental conditions for ice formation, if conditions persist ice formation increases with the final stage being when the conditions are no longer suitable for ice formation / persistence.
Phases of ice formation
Ice formation phases (Reference eologix.com)
A variety of ice detection techniques exist and include:
Do you remember train delay announcements due to the wrong type of leaves or snow on the tracks, well I used to think that was a horrible excuse… I do have a bit more sympathy now though, especially relating to the fact you can have different types of ice that the system may or may not be designed to handle.
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Relating to wind turbine operation in cold weather conditions, different types of “ice” that can form / accumulate on the structure include hoarfrost, rime ice and glaze ice.
Hoarfrost
Hoarfrost formation requires high humidity (greater than 90%), weak wind and temperature values generally below minus 8 °C. It is a form of fog-frost deposition and usually consists of thin, fragile ice needles / scales that loosely adhere to objects and are formed almost exclusively by sublimation. It really only poses a safety hazard from a very high density (several cm). Due to its low density and large surface area, it detaches from the rotor blades fairly easily like snowfall.?
Hoarfrost (https://www.britannica.com/science/hoarfrost#/media/1/268403/118048)
It doesn’t normally result in significant load on structures, except if it manages to form a high density, whereby it can also lead to safety hazards.?
Rime ice
Rime ice is of 2 types (hard rime and soft rime). It typically forms at high wind speeds from supercooled fog water droplets (freezing drizzle) between -2?oC (28?oF)?to -10?oC?(14?oF). Factors favouring the formation of rime ice include small droplet size, slow accretion, a high degree of supercooling and rapid dissipation of latent heat of fusion. The ice layers formed have a sponge-like appearance (like freezer frost) and are quite loose compared to solid glaze ice.
Rime ice has no crystalline structures and includes a large number of air bubbles in its structure. The partial melting and refreezing of the particles causes them to stick together to varying degrees.?Rime ice first appears near the root, follows the shape of the airfoil, gradually spreading towards the tip of the blade.
Within high altitude areas, where wind turbines are operated within the cloud cover, layers of rime ice can become very dense and hard at the leading edge of the rotor blades. Such deposits of rime ice pose a danger during operation of the wind turbine.?A TüV study, assuming a typical wind turbine of 141 m hub height and 117 m rotor diameter, identified rime ice on wind turbines with masses above 240g as a scenario that can cause fatalities.?In low-lying areas, ice layers are usually very thin and light and therefore do not represent much danger.
Rime is the most common type of in-cloud icing, the most severe of which occur at freely exposed mountains (coastal or inland), or where mountain valleys force moist air through passes, consequently lifting the air and increasing wind speed over the pass.?
Hard rime on a tree (Marcin Sochacki - Own work, CC BY-SA 4.0,?https://commons.wikimedia.org/w/index.php?curid=18513)
Soft Rime?(softrime2006 - Own work, CC BY 2.5,?https://commons.wikimedia.org/w/index.php?curid=1699361)
The accretion rate for rime varies with:?
Glaze ice
Glaze ice is formed at air temperatures between 0?oC (32?oF)?and -3?oC (26.6?oF)?via freezing of supercooled mist droplets on objects into hard, high density (900 kg/m3), firmly adhering ice deposits (similar to an ice cube). These can grow into extremely heavy ice loads several centimetres thick (can also form icicles) and pose a danger during operation of the wind turbine.?A TüV study, assuming a typical wind turbine of 141 m hub height and 117 m rotor diameter, identified glaze ice on wind turbines with masses above 180g as a scenario that can cause fatalities.
Glaze ice forms rapidly and can take on irregular shapes depending upon the freezing fraction.?Factors favouring the formation of glaze are large to medium droplet size, rapid accretion, slight supercooling, and slow dissipation of the latent heat of fusion. Glaze ice is denser, harder and more transparent than rime ice.
Glaze Ice on a blade of grass
The accretion rate for glaze varies with:?
Properties and Meteorological Parameters controlling atmospheric ice accretion
Icing events on a wind turbine might typically involve rime ice as well as glaze ice formation dependent on upstream and other environmental conditions, glaze ice tending to occur on the tip while rime ice tends to occur near the root of the rotors.
Wind speed and temperature conditions favouring rime / glaze ice formation are displayed below
Summary
As previously mentioned, the degree / amount of accreted ice depends largely on humidity, temperature, duration of the ice accretion, exposure of wind turbine blades and its orientation to the direction of the icing wind. In general, dry icing results in different types of rime (containing air bubbles), while wet icing always forms glaze (solid and clear).
Part 3 coming soon...
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References:
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