USING A COLLISION MODEL TO DESIGN SAFER WIND TURBINE ROTORS FOR BIRDS

A mathematical model for collisions between birds and propeller-type t urbine rotors identifies the variables that can be manipulated to redu ce the probability that birds will collide with the rotor. This study defines a safety index-the ''clearance power density ''-that allows ro tors of different sizes and designs to be compared in terms of the amo unt of wind energy converted to electrical energy per bird collision. The collision model accounts for variations in wind speed during the y ear and shows that for model rotors with simple, one-dimensional blade s, the safety index increases in proportion to rotor diameter, and var iable speed rotors have higher safety indexes than constant speed roto rs. The safety index can also be increased by enlarging the region nea r the center of the rotor hub where the blades move slowly enough for birds to avoid them. Painting the blades to make them more visible mig ht have this effect. Model rotors with practical designs can have safe ty indexes an order of magnitude higher that those for model rotors ty pical of the constant speed rotors in common use today. This finding s uggests that redesigned rotors could have collision rates with birds p erhaps an order of magnitude lower than today's rotors, with no reduct ion in the production of wind power. The empirical data that exist for collisions between raptors, such as hawks and eagles, and rotors are consistent with the model: the numbers of raptor carcasses found benea th large variable speed rotors, relative to the numbers found under sm all constant speed rotors, are in the proportions predicted by the col lision model rather than in proportion to the areas swept by the rotor blades. However, uncontrolled variables associated with these data pr event a stronger claim of support for the model.