Definition of an Ice Index for Wind Turbines in Cold Climate

Icing losses encountered in a wind farm are very hard to assess as they can vary by a factor of three from year to year. This inter-annual variation is a critical issue from the assessment phase to the operation of wind turbines in cold climate. To mitigate the effects of this issue, a proposed solution is to define an ice index to yield a reference for icing conditions based on historical values. Knowing that icing frequency has an inter-annual variation 5 times greater than the wind speed, the definition of an ice index is long overdue. This paper proposes a simple and accessible method for defining an ice index based on cloud base height measured at a weather station. A similar strategy is already used in the industry for other purposes such as estimating the long-term average icing frequency in icing maps. The novelty of the proposed method lies in the description of the variation in icing frequency rather than its long-term average. The ice index proposed in this paper is shown to be representative of the icing losses in 6 wind farms located in eastern Canada. Moreover, it was also shown that the ice index is strongly correlated to the icing measurements made on wind turbine nacelle. In this paper, the yearly and monthly ice indexes, from September 2007 to December 2021, are shared. These indexes are based on the meteorological data from Valcartier weather station near Quebec City. This definition of an ice index directly responds to a critical need of the wind industry in cold climate, a common reference for icing frequency. Ice indexes could be publicly shared each month in cold climate areas to guide wind farm owners and operators in their decisions and analysis of performances. Such an index could also be used to asses the long-term variation of the ice index and the effect of climate change on icing frequency and impacts on the wind energy production in cold climate.