Hoar frost, rime ice and glaze ice in the operation of wind turbines and their effects

Hoarfrost, rime ice and glaze ice occur at temperatures just below the freezing point and are collectively referred as “Nebelfrostablagerungen”.

These fog-frost deposits are characterised, among other things, as follows:

  • usually form when fog droplets freeze on predominantly vertical surfaces whose surface temperature is below the freezing point
  • show their typical growth against the wind
  • represent a major source of danger in terms of material exposure, as fog-frost deposits can grow into heavy “meteorological loads”

You can find out how icing occurs, which phases of icing there are and what specifically matters in our white paper Ice detection to optimise your wind turbine.

Hoarfrost

Hoarfrost is a form of fog-frost deposition, usually thin, fragile ice needles or scales that adhere only loosely to objects and are formed almost exclusively by sublimation. The prerequisites for the formation of frost are high humidity (around 90 % or more), weak wind and temperature values generally below minus 8 °C.

Rime ice

RIme ice is a solid precipitation that forms mainly at high wind speeds and an air temperature of typically -2 to -10 degrees Celcius. This ice forms from supercooled fog water droplets and forms on surfaces, typically against the wind direction. The formed ice layer of grey-white granular particles have a sponge-like appearance and are quite loose compared to solid glaze ice.

Rime ice is characterised by the fact that it 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, depending on the prevailing temperature conditions.

Glaze ice

Smooth, compact, transparent and very firmly adhering ice deposit of indeterminate shape and irregular surface. Glaze ice is formed at air temperatures between 0 and minus 3 °C by slow freezing of supercooled mist droplets on objects. Glaze ice can grow into extremely heavy ice loads.

Hoarfrost

high humidity (>90%)

below -8°C

light wind

Rime ice

typ. -2 to -10°C

especially with high wind speeds

Glaze ice

between 0 and -3°C

clear, transparent, solid

Nebulous frost depositions

Collective term for the deposited precipitation hoar frost, rime ice and glaze ice. (Deutscher Wetterdienst)

Hoarfrost

Before 1986, “Raureif” was the collective term used at the “Deutscher Wetterdienst” for glaze ice, rime ice and frost.

Learn how to minimise the risks of ice accumulation and optimise your wind turbine with the help of ice detection in our 21-page white paper

Preview whitepaper

Types of icing on rotor blades and their dangers

In principle, all three types of icing, namely hoarfrost, rime ice and glaze ice can occur on rotor blades.

DSC00808

Rime ice

In low-lying countries, ice layers are usually very thin and light and therefore do not represent any danger. At high altitudes, however, wind turbines are operated within the cloud cover, so that layers of rime ice can very often become very dense and hard and can be 10 cm or more thick at the leading edge of the rotor blades. Such deposits of rime ice pose a danger during operation of the wind turbine.

Electric City 2021

Hoarfrost on rotor blades

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 like snowfall.

DSC00121

Glaze ice

Due to the very high density and the possible layer thickness of several centimetres, ice accumulations in the form of glaze ice pose a danger during operation of the wind turbine.

Dropped ice fragments from iced rotor blades have already been found at a distance of more than 100m from wind turbines. Therefore, appropriate safety measures must be taken against ice throw and ice fall.

Ice accumulation on rotor blades represents a potential risk for the operation of a wind turbine

  • Falling fragments of ice from the rotor blades can pose a safety risk
  • Danger from icing depends, among other things, on the type of icing and the thickness of the ice
  • Iced rotor blades can significantly minimise the performance of the wind turbine and cause increased vibrations

 

eologix systems regarding types of icing

  • eologix systems measure ice and surface temperature at each sensor directly on the rotor blade surface
  • Differentiation between types of icing possible – enables shut-down only in case of safety-relevant icing (in independent tests, on average 2.5 times fewer downtimes compared to other rotor blade-based ice detection systems)

eologix sensors in use in the Climate Wind Tunnel at the FH Joanneum

The best way to make good decisions is to base them on solid knowledge. That’s why our products undergo a wide range of tests before we introduce them to the market.

  • eologix ice detection enables the classification of the surface condition in five different levels, that means that we also measure the ice thickness on your rotor blades. The following figure shows an example of this for glaze ice.
  • Compared to other blade-based systems, eologix ranks, among others, as the safest system (highest availability in independent tests), with the lowest downtimes and at the same time the highest sensitivity of these four tested blade-based systems available on the market – read more here

Increase the efficiency of your wind turbine. We will help you to do so:

References
  1. Deutscher Wetterdienst (Publisher), Nebelfrostablagerungen, accessed via: https://www.dwd.de/DE/service/lexikon/Functions/glossar.html?lv2=101812&lv3=101872
  2. Wikipedia (Publisher): Raueis, accessed via: https://de.wikipedia.org/wiki/Raueis
  3. Hohenlohekreis.de (Publisher): Angaben zum Schutz vor Eisabwurf Vereisungsgefahren und Enteisungssysteme, 2017
  4. Deutscher Wetterdienst (Publisher), Raureif, Raueis, Raufrost – Was denn nun?!, accessed via: https://www.dwd.de/DE/wetter/thema_des_tages/2014/12/6.html