In cold climate regions, wind energy harvesting is complicated by the risk of ice build-up on the different structures of a wind farm. In Eastern Canada, there are 1,500 wind turbines affected by icing, responsible for a loss of approximately 50M$ in energy sales. A better adaptation of wind turbines to cold climate depends on an appropriate site instrumentation. Indeed, ice detection instrumentation is needed for the planning and operation of wind turbines: it serves to detect incipient icing as quickly as possible for controlling anti-icing functions on wind turbines and ultimately allow a better energy production.
In 2004, the world meteorological organization (WMO) stated the need to improve the quality of meteorological measurements under cold climate conditions by first promoting a classification for sensors taking into account accuracy, climatic conditions and reliability of data and second, by setting up recommendations for testing and approving ice detectors and ice-free sensors. As a result, the second phase of COST Action 727 “Measuring and forecasting atmospheric icing on structures” (COST 727) was created in 2006. The goal of COST 727 was to promote the development of different types of instruments to be used in an icing-prone wind farm. The main conclusion of the study was that to that day, no existing instrument worked in an acceptable manner in an icing climate.
To further the work of COST 727, the TechnoCentre éolien(TCE) evaluated the performance of different anemometers in icing conditions. The goal of this evaluation was to quantify the performance of anemometers to give Tugliq S.A.R.F recommendations about which ones are best suited to operate in cold climate.
To evaluate anemometers, a performance index was developed by the TCE. The index is based on the recovery rate, the meteorological, instrumental and extreme icing durations and the severity of icing events. The recovery rate is the ratio between the recovered data during an event and the total data that should be generated by the instrument. The meteorological icing is the duration of ice accumulation on the sensor, the instrumental icing is the time where the sensor is affected by icing, and the extreme icing is the time where the sensor no longer works due to icing. Finally, the severity of the event is the measured accumulated ice.
A total of 10 icing events that occurred in winter 2015-2016 were selected for the study: five at a wind farm located in Rivière-au-Renard, and five at a meteorological station located in Murdochville, both located in Eastern Québec, Canada. The two sites were considered because of their different intensity of icing classes: Murdochville is exposed to severe icing conditions while Rivière-au-Renard is exposed to medium icing conditions. The events themselves were selected on the basis of their different characteristics: severity, intensity, duration and type of icing. That way, the anemometers were tested in the largest icing conditions available to the TCE, allowing the broadest possible results.
Different types of anemometers were studied: non-heated cup anemometers, heated cup anemometers and heated ultrasonic anemometers. The following models were compared: Thies Compact, Thies 2D, Thies First Class Advanced, Vaisala WAA252, NRG RT240, Metek USA1, Lufft 8371-UMT, NRG IceFree3A and Kriwan INT10.
Heated ultrasonic anemometers were found to be the most performant in icing climate. Indeed, they had the best recovery rates and were the less affected by icing. The models that had the best performance index were the Thies 2D, NRG RT240, Metek USA1 and Lufft 8371-UMT. The NRG IceFree3A, a heated cup anemometer, also had a good performance.
In general, the worst performances were with non-heated cup anemometers. Considering all anemometers for the two sites and all icing events, it was not possible to identify a tendency where the performance index was related to the icing time period or to the severity. That is probably caused by the limited number of icing event studied.
Limitations and conclusions:
A study with a greater number of events would provide a stronger confirmation about the performance of anemometers. Some other parameters could also be included in the performance index such as the intensity of icing events, the type of ice, the IEA class of the sites, etc.
Still, the performance index developed by the TCE allows Tugliq to choose the anemometer best suited to perform in cold climate. Considering the quality price ratio of the tested anemometers, the NRG RT240 and the Lufft 8371-UMT are the best suited to be used in an icing-prone wind farm.