Environmental Engineering Reference
In-Depth Information
conclusions and discuss our ongoing and future work in developing a closed-loop
de-icing strategy.
8.2 Atmospheric Icing
There are traditionally three main types of atmospheric ice accumulation on wind
turbine blades: in-cloud icing, precipitation icing, and frost [
4
-
8
]. In-cloud icing
consists of soft rime, hard rime, and glaze ice, while precipitation icing includes
freezing rain and wet snow.
In-cloud icing occurs when small, supercooled, airborne water droplets, which
make up clouds and fog, freeze upon impacting a surface. These water droplets can
remain liquid in the air at temperatures down to -35 C due to their small size, but
will freeze upon striking a surface [
9
]. Different types of rime and glaze ice are
formed depending on the droplet sizes and the energy balance of the surface. For
small droplets with almost instantaneous freezing, soft rime forms. With medium
sized droplets and slightly slower freezing, hard rime forms. If the buildup of rime
is such that a layer of liquid water is present on the surface during freezing, glaze
ice forms [
10
]. Accretions are different in size, shape, and property, depending on
the number of droplets in the air (liquid water content—LWC) and their size
(median volume diameter—MVD), the temperature, the wind speed, the duration
of icing event, the chord length of the blade, and the collection efficiency. Rime ice
forms at colder temperatures of accretion, while glaze forms at warmer temper-
atures [
8
]. Rime and glaze have different thermodynamic characteristics and
therefore require different heat fluxes for melting. Hard rime ice is denser than soft
rime and is more difficult to remove. The ability to not only detect the presence of
ice, but the specific character, could enable significant energy savings by tailoring
the response of a de-icing system.
8.3 Sensing and Actuation Background: Existing Methods
8.3.1 Ice Sensing
There are two different types of ice detection: indirect and direct. Typical meth-
odologies for detecting ice in current wind turbine systems are called indirect
(passive) sensing. They use weather stations on the tower or nacelle of the turbine.
Humidity and temperature measurements are correlated with measured wind
speeds to determine whether icing is likely to occur [
11
]. Another example of
indirect ice sensing is monitoring the power output during the wind turbine
operation to detect an icing event. However, on many occasions, indirect ice
sensing methods are neither accurate nor have enough spatial resolution for active
de-icing [
12
]. Furthermore, icing is most likely to occur on the far reach leading
