Weather condition and different types of icing
Weather condition and different types of icing
There are different types of ice. Atmospheric icing is classified based on two different formation processes (S. Fikke et al., 2007). These are precipitation icing and in-cloud icing (S. Fikke et al., 2007). Precipitation icing is ice that form due to wet snow or freezing rain (Cattin, 2012). “In-cloud icing” occurs when super cooled liquid droplets (SLD) like clouds collide with a structure or object and freezes on the turbine blade. The physical properties and appearance of the ice accretion will vary on the variations in meteorological conditions during the ice growth. Parameters such as compression and shear strength for instance would be used to describe the nature of accreted ice. Other important factors would be for example humidity, temperature and the duration of the ice accretion (Ethiopian Standards Agency, 2001). The main preconditions for significant ice accretion are the dimensions of the object exposed and its orientation in relation to the direction of the icing wind (Cattin, 2012). Figure 6 gives an indication of the parameters controlling the major types of ice formation (Cattin, 2012). The density of accreted ice varies widely from low (soft rime) over medium (hard rime) to high (glaze).
Figure 1. Type of ice as function of wind speed and air temperature (Ethiopian Standards Agency, 2001).
Glaze is the type of icing that has the highest density, and is caused by freezing rain, freezing drizzle or in- cloud icing (Energy & Systems, 2012). It causes smooth evenly distributed ice accretion. The surface temperature of accreting ice is near freezing, and as a result, liquid water may due to wind and gravity flow around the object and freeze on the leeward side (L Tallhaug et al., 2009). The main factors in determining the accretion rate for glaze is rate of precipitation, wind speed and air temperature (Ethiopian Standards Agency, 2001).
Wet snow is able to stick to the surface of an object because of the occurrence of free water in the partly melted snow crystals. The accretion occurs when the air temperature is below the freezing point (Rindeskär, 2010). When the temperature decreases the build-up of wet snow will freeze (Ethiopian Standards Agency, 2001). The density and adhesive strength vary widely with the fraction of melted water, wind speed and other factors (Ethiopian Standards Agency, 2001).
Rime is the most common type of in-cloud icing and often vanes on the windward side of linear, non-rotatable objects. Icing on small linear objects is the cross section of the rime vane triangle with the WOP angle pointing windward nut as the width (diameter) of the object increase the ice vane changes its form (Ethiopian Standards Agency, 2001). Distributed ice can be formed by in-cloud icing when the object is a nearly horizontal “string” which is rotatable around its axis. The accreted ice on the windward side of the “string” will force it to rotate when the weight of ice is sufficient. The mechanism will continue as long as the ice accretion is going on. This may result a cylindrical ice accretion around the string. The most severe rime icing accrue on freely exposed mountains(coastal or inland), or where mountain valets force moist air through passes and consequently both lifts the air and increase the wind speed over the pass (Baring-Gould et al, 2009). The rime mainly varies with the dimensions of the objects exposed, wind speed, liquid water content in the air, drop size distribution and air temperature (Ethiopian Standards Agency, 2001).
Hoar frost is caused by direct phase transition from water vapor into ice, and is common at low temperatures (Rindeskär, 2010). Hoar frost has low density, low strength and normally does not result in significant load in structures.
Effect of icing
Cold climate site affects the design of a wind turbine. Ice, rime and high air density at low temperatures will affect the aerodynamics. Thus the loads and power will further impact on the construction of the turbine (Seifert, 2003a). The control system can be affected if temperature and high masses of ice on the structure change the natural frequencies by high amplitude vibrations (Seifert, 2003a). Resonance and mass imbalance between the blades of wind turbine components may change the dynamics behavior of the whole turbine (Parent & Ilinca, 2011). Frozen and iced control instruments give faulty information to the supervisory system of the turbine. Extremely low temperatures will require special materials; for example, normal steel will become brittle at those temperatures (Seifert, 2003a).
There are health and safety restrictions in each cold climate operations and they have to be taken under consideration, for example large ice pieces falling down and ice fragments thrown over large distances may cause injury to humans and animals or damage objects. The turbine may also be affected by heavy unbalance due to unsymmetrical icing, because of changed natural frequencies of components exceeding the designed fatigue loads (Seifert, 2003a).
Low air density can increase the loads and maximum power output (Seifert, 2003b). If the turbine does not automatically react, the windings or transformers can burn, and gearboxes may be overloaded. Overloading may reduce lifetime of components and further damage the turbine, if it does not automatically react (Seifert, 2003a). Higher air density related to low temperatures and airfoil modification can lead up to 16 % overproduction in the wind turbine (Parent & Ilinca, 2011). In icing conditions the measurement errors of the wind speed can be up to 30%, maximum error of 40% for an ice-free anemometer and 60% for a standard anemometer during icing event (Parent & Ilinca, 2011). There are chances for lower production of energy due to increased vibration (higher load) and too low temperatures around the wind turbines. If the turbines are lightly iced, you will experience production loss even while the turbines are in operation. The reason for low production is that the ice changes the airflow across the air foil and creates turbulence, resulting in lower rotation, caused by a loss in aerodynamic lift and an increase in drag (Andersen, Börjesson, Vainionpaää; Silje Undem, 2011). The biggest production loss is caused by ice accretion on the tip of the rotor blade (Seifert, 2003a). The effect on power production will be approximately the same if the outermost 5% of the rotor blade is iced up as when about 75-95 % of the rotor blade is covered in ice (Andersen et al., 2011). Electrical failure may be caused by snow infiltration in the nacelle, and extreme temperatures may also lead to condensation in the electronics (Laakso et al., 2003a).