Shrouded wind turbine

A 'shrouded', 'cowled', 'ducted' or 'wind lens' turbine is one that is housed in a ring-shaped aerofoil that increases airflow through the turbine blades' swept area by generating a localised ring vortex.

Such systems can increase the power of a Horizontal-Axis Wind Turbine by 2-5 times, for a given blade diameter and windspeed. While the use of an engine duct in aircraft design is considered a trade-off of additional weight for efficiency, the weight that it adds to ground-based wind turbine systems is of negligible importance, and a carefully designed diffuser can not only increase power output, but also cut out downstream turbulence and noise by inhibiting or removing blade-tip vortices and by inducing rapid mixing of the high and low-speed air flows behind the turbine

Design Considerations

A wind lens must be supported around the turbine somehow, which implies a highly rigid (in order to prevent blade-tip collision) framework within the shroud, held from below, or radial members spanning from the turbine's axis to the inner surface of the shroud. Using fixed radial support members necessitates that they be streamlined with aerofoils/fairings in order to reduce drag and turbulence that could otherwise interfere with the turbine's operation.

Once a system is installed with radial aerofoils around the rotor shaft, it becomes possible to use them to induce an initial rotation in the airflow, as do stator blades in gas turbine engines, which can improve turbine blade performance characteristics, however as the turbine blades cut through regions of higher & lower pressure, created by stator camber and/or angle of attack, at high speeds this can result in generation of additional noise, and so inclining stator blades may not always be desirable.

It is typically preferable to use differing numbers of stator & rotor blades, especially prime numbers, in order to reduce resonant vibrations that could otherwise decrease service life of parts.