Modelling of ice throws from wind turbines

Abstract: As the wind energy sector expands into areas with colder climate, the problem with ice throw will increase. Due to a rotor diameter of more than 120 meters for a typical modern turbine with an effect of 3.3 MW, the separated ice fragment will get a high initial velocity, and therefore, they will also be thrown a long distance. Ice throw might therefore be a large safety risk for the people, who are staying in surrounding areas to wind turbines. A ballistic ice throw model has been developed to be able to investigate how far the ice fragments can be thrown from a wind turbine. The work was divided into two parts, one sensitivity analysis and one real case study. In the sensitivity analysis, the influence of eight important parameters was investigated. The results from this part show that changes in the parameters initial radius and angle position, and mass and shape of the ice fragments have a significant influence on the throwing distance both lateral and downwind. The wind speed has only a significant influence on the downwind throwing distance, but this is quite large. A maximum throwing distance of 239 meters downwind the wind turbine was achieved with U=20 m/s, r=55 m and θ=45°. While including the lift force, a maximum downwind distance of 350 meter was achieved. However, the uncertainties about the shape of the ice fragment make these results quite uncertain. In the real case study, ice throws were simulated by letting the ice throw model run with modeled meteorological data for a wind farm in northern Sweden. The wind farm consists of 60 wind turbines, and the probability for that an ice fragment will land in a square of 1'1m was calculated around each turbine. To be able to calculate this probability, a Monte Carlo analysis was necessary in which a large number of ice fragments were separated. The result shows a large correlation between the landing positions of the ice fragments and the wind direction. Due to the fact that the wind farm is located in a complex terrain, the shape and density of the probability field vary among different parts of the farm. Especially in the southern part of the wind farm, the probability field will have the highest density and largest extension to the northeast of the turbines due to a prevailing wind direction during ice throw events from southwest.