Icing characteristics and mitigation strategies for wind turbines in cold climates

Optimized power generation from available wind resources has gained substantial interest by many leaders in the wind power industry worldwide. One issue facing wind power generation in cold climates is ice accumulation on turbine blades, which is not only an energy production and efficiency concern, but also a safety issue. This paper explores various mitigation techniques for delaying and preventing ice accumulation on wind turbines blades. Firstly, a surface mitigation strategy has been applied to blade models involving hydrophobic and ice-phobic coatings. Secondly, a thermal mitigation strategy involving a new composite material has been developed. Subsequently these mitigation techniques are combined to form a third strategy, termed thermface mitigation. Stationary blade configurations have been experimentally tested in the University of Manitoba Icing Tunnel Facility. The results of the mitigation techniques depict icing profiles and aerodynamic changes along the blade leading edges during the icing event and quantify ice adhesion force, accumulation amount and rate of accumulation over a set period under simulated climatological Glaze and Rime icing conditions. Results are extended to wind turbine performance for estimation of energy production. Introduction Wind, as an energy resource, contains the potential to become an important contributor to utility and non-utility power generation in many regions in Canada and in cold climates throughout the world. However, due to extreme climatological factors, many Canadian wind turbine sites are affected by icing problems that impact power generation. Thus, the potential to harness an abundant source of wind for power generation is greatly disadvantaged. Icing is an issue of interest to many groups—from private energy companies, to public utilities and landowners. This paper discusses the experiments performed in the University of Manitoba’s Icing Tunnel Facility, (UMITF), for the investigation of icing mitigation strategies for wind turbines in cold climates. Experiments are performed on aerofoils representative of blades currently used in the wind turbine industry, under simulated climatological conditions for severe icing events found in cold climates where wind turbines are commonly employed. Of particular interest is the 99.9 MW wind farm in St. Leon, Manitoba, which has been developed in a region very well-known for icing events. Wind turbines are subjected to rime, glaze or mixed ice accretion conditions [1]. In the UMITF, glaze ice forms at temperatures just below freezing in air with high liquid water content, characteristically between 0oC and -6oC. Rime ice forms in colder environmental conditions, traditionally below -10oC, in air of low liquid water content, but also forms when surface temperatures are below -6oC. In rime icing, supercooled water droplets freeze immediately upon impact and form a low-density ice, white and feathery in appearance. In glaze icing, part of the water droplets freeze upon impact and the remaining water runs along the surface before freezing, forming a smooth lumpy profile shape of highdensity clear ice. Wind Energy-Technological Advances Oral Presentation Icing Mitigation Strategy Current techniques for ice shedding from airfoil blades are commonly found in literature related to aerospace and aircraft applications. As Fitt and Pope [2] indicate these methods include antiicing and de-icing techniques ranging from freezing point depressants and surface deformation, to thermal melting. Reducing the force of ice adhesion from a surface is a key point to improving the ability to shed ice with ease. As explained by Loughborough [3] of the B.F. Goodrich Company, the force of adhesion is approximated to be linear with temperature, increasing 8.5 lbs per sq. in (5976.09 kg/m 2 ) for each degree centigrade decrease in temperature. The significance of this value indicates that a reduction in the adhesion force of the ice would greatly improve ice shedding and the resulting performance of the turbine. Therefore, a method that reduces the adhesion force of ice on the surface would ideally aid in preventing and delaying the onset of ice accretion. Since environmental conscientiousness is a key factor in the wind turbines systems, hot oil, chemicals and their derivative are not viable mitigation strategies. Furthermore, the use of electrical energy may contradict the purpose of generating electricity, and must be cautiously implemented. Therefore, the initial preference for an icing mitigation strategy is to implement a passive mitigation technique, such as a surface mitigation to improve the resistance of ice accretion and adhesion to the aerofoil surface. The correlation between a high contact angle for droplets on a surface and their enhanced ability to shed from the surface is highly influenced by the ability of a coating to be hydrophobic or ice-phobic [4]. Thus, the following have been selected for experimentation, for their favourable characteristics in icing conditions: Wearlon Super-Icephobic and SuperHydrophobic. The icephobic coating is designed to perform best in icing conditions, whereas the hydrophobic coating is catered more towards repelling water droplets [5, 6]. Subsequently, a less-passive mitigation strategy can be explored. For this experiment, a thermal technique is selected. A highly controllable, lightweight system, compatible with modern composite materials has been adapted as an electro-thermal ice protection system where heat is generated at the point of use. Thermion® heaters are made from finely dispersed metal coated carbon fibre elements that may be integrated into composite or polymer material structures and are ideally suited for leading edge blade ice protection applications due to their lightweight and uniform heat distribution capability, contrary to traditional wire heaters [7]. The heat generated by this method is conducted into the ice-substrate interface, where a thin film of water is formed. This breaks the adhesive bond between the ice and the outer surface so that the ice can be swept away by aerodynamic forces. Furthermore, cycling of the heater power helps to control and reduce energy requirements [8], suggesting the use of various thermal regimes to further enhance the effectiveness of the mitigation strategy. By controlling the duration and timing of heat application both anti-icing and various de-icing regimes can be explored. Finally, the combination of the surface mitigation with the thermal mitigation, termed thermface, is explored as means of more controlledpassive deicing technique, providing insight to the combined effectiveness of these mitigation strategies. The cycling of power for the electro-thermal heaters allows integration with other techniques to optimize mitigation efforts. Wind Energy-Technological Advances Oral Presentation Experiment Description Icing experimentation is performed in the Icing Tunnel Facility located in the Engineering Complex at the University of Manitoba. The UMITF consists of a spray system to emit droplets into the flow and a refrigeration system for cooling of the air as shown in Figure 1 [9]. Figure 1: Top view of spray flow and icing