Ice aggregation and ice throw from small wind turbines

Abstract

Small wind turbines are an attractive technology for decentralized electricity generation. However, there are some risks, which need to be evaluated to enable safe operation of the turbines. In cold climates, risk can arise from icing and subsequent ice throw from the moving turbine blades. This study aims to clarify the risk from ice throw from small wind turbines using observations and an experimental approach. Icing of small wind turbines was monitored and icing intensities evaluated. Ice fragments were collected, 3D-scanned and used to generate realistic specimens using additive manufacturing. The specimens were subsequently thrown using a device simulating a small wind turbine. A three-dimensional reconstruction of the throw trajectories allowed to measure the impact velocity and comparison with ballistic models. The monitoring campaign showed that geometry of ice aggregation is similar to large turbines, while the relative icing intensity is substantially higher. Experimental throw distances were found to be in the range of twice the height from which the fragments were thrown. The impact velocity was found to be below 20m/s, resulting in a minimum weight of potentially deadly ice fragments of 200g. A comparison of the trajectories with the model of a throw parabola shows that drag forces have a substantial impact on velocity and maximum distance and thus cannot be ignored. Comparison with established ballistic models shows significant differences between simulation and experiment. It can be concluded, that ice throw is a relevant risk factor for small wind turbines in cold climates, which needs to be assessed further. Especially considering urban use, additional safety measures are required.