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
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.
The development of static aerofoils, for use in a shroud and stator blades, lends itself well to the use of rapid-prototyping or additive manufacturing methods such as 3D-Printing. One design exploiting this fact is the Open Wind Lens.
High-rotational-velocity HAWTs with their rotor blades mounted in upwind of their post/tower run a risk of the blade tips colliding with their tower at high wind-speeds, and so are typically constructed with a moving tail vane that furls upwards in high winds, causing such a turbine to yaw to one side of the approaching wind, which limits the danger that blades pose to the tower and decreases their top speed, while increasing wear on the blades as some of them run backwards as they turn away from the approaching wind. Downwind turbines do not run any risk of colliding with their tower, but due to being pulled into line with the wind (especially with a duct), they are more subject to extremely high rotational velocities during storms, and so their blades must be constructed thick enough to withstand the resulting stresses, and alternators must have a way of keeping cool enough so that they are not damaged by overheating. While powering a dump load in order to slow blades down can help, some form of air-cooled heat sink next to stator coils may be needed to achieve enough cooling.
Longer ducts have shown to induce a stronger vortex effect, and so increase power, but are more difficult to construct and mount.
Downwind radially-mounted wind lens turbines at Kyushu University Ito campus - Image on Wikipedia. Kyushu is a coastal town and so likely gets strong winds; it would be good to know how these turbines manage heat dissipation in such weather.
- A Shrouded Wind Turbine Generating High Output Power with Wind-lens Technology - Yuji Ohya and Takashi Karasudani, Energies, volume 3 issue 4 (open access, PDF, has diagrams of duct profiles tested)
- Elements of Gas Turbine Propulsion - Jack D. Mattingly, McGraw Hill International Editions, 1996, pp.804, Figure 10-56.
- FloDesign wind turbine - might as well mute the video, since the narration is full of 'marketing'.
- Wind turbine with mixers and ejectors - Walter M. Presz, Jr. et al, US Patent number: 8021100, Filing date: Mar 24, 2008, Issue date: Sep 20, 2011, Application number: 12/054,050
- A Wind Turbine Recipe Book - The Axial Flux Windmill Plans - Hugh Piggott, January 2009, Metric Edition, page 9, where he says "In very turbulent and wild conditions the gyroscopic forces on the blades have been known to push them back into contact with the tower so that they break. This is a very rare but persistent problem. In this 2008 book I have changed the direetion of furliag of the turbines so that the gyro forces push the blade tips out from the tower as the machine moves into furl. This is the yaw movement where the blades tend to be racing fastest. This change should reduce or even prevent the contact of blades with tower from now on" Incidentally, on the very same page he makes a sweeping statement dismissing ducted turbines "It simply isn't worth all the extra material involved in building a duct like that. The wind tends to divert around it so you don't gain as much as you would think. It is actually more effective to build a conventional blade rotor with larger diameter, than to make a duct. Some big companies have spent a lot of their investors' money finding this out"; statement 1 is his conclusion, 2 is a mix of incorrect and meaningless depending on what "you would think", 3 depends on your specification, and 4 reflects some failings prior to japanese research.