Unconventional Wind Turbines - Modified Horizontal

Modified Horizontal

Two blades

Nearly all modern wind turbines uses rotors with three blades, but some use only two blades. This was the type used at GROWIAN, some other prototypes and several wind turbine types manufactured by NedWind. Eemmeerdijk Wind Park uses only 2-blade turbines. Turbines with two blades are manufactured by Nordic Windpower (Model N 1000) and by GC China.

Downwind rotor

Nearly all wind turbines are of an upwind design, meaning the rotor is in front of the nacelle when the wind is blowing. Some turbines are of a downwind design, meaning the rotor is behind the nacelle when the wind is blowing.

Ducted rotor

Still something of a research project, the ducted rotor consists of a turbine inside a duct which flares outwards at the back. They are also referred as Diffuser-Augmented Wind Turbines (i.e. DAWT). The main advantage of the ducted rotor is that it can operate in a wide range of winds and generate a higher power per unit of rotor area. Another advantage is that the generator operates at a high rotation rate, so it doesn't require a bulky gearbox, so the mechanical portion can be smaller and lighter. A disadvantage is that (apart from the gearbox) it is more complicated than the unducted rotor and the duct is usually quite heavy, which puts an added load on the tower. The Éolienne Bollée is an example of a DAWT.

Co-axial, multi-rotor

Two or more rotors may be mounted to the same driveshaft, with their combined co-rotation together turning the same generator: fresh wind is brought to each rotor by sufficient spacing between rotors combined with an offset angle (alpha) from the wind direction. Wake vorticity is recovered as the top of a wake hits the bottom of the next rotor. Power has been multiplied several times using co-axial, multiple rotors in testing conducted by inventor and researcher Douglas Selsam, for the California Energy Commission in 2004. The first commercially available co-axial multi-rotor turbine is the patented dual-rotor American Twin Superturbine from Selsam Innovations in California, with 2 propellers separated by 12 feet. It is the most powerful 7-foot-diameter (2.1 m) turbine available, due to this extra rotor.

Counter-rotating horizontal-axis

When a system expels or accelerates mass in one direction, the accelerated mass will cause a proportional but opposite force on that system. A single rotor wind turbine causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single-rotor propeller design. To use this wasted effort, the placement of a second rotor behind the first takes advantage of the disturbed airflow. Contra-rotation wind energy collection with two rotors, one behind the other, can gain up to 40% more energy from a given swept area as compared with a single rotor. Much work has been done recently on this in the USA. A patent application dated 1992 exists based on work done with the Trimblemill. Other advantages of contra-rotation include no gear boxes and auto-centering on the wind (no yaw motors/mechanism required). Ability to be point suspended, thus reducing support structure overturning moments theoretical ability to be "grid linked" without electronics, thus giving the possibility of "arrays". Counter-rotating turbines can be used to increase the rotation speed of the electrical generator. As of 2005, no large practical counter-rotating HAWTs are commercially sold. When the counter-rotating turbines are on the same side of the tower, the blades in front are angled forwards slightly so as to avoid hitting the rear ones. If the turbine blades are on opposite sides of the tower, it is best that the blades at the back be smaller than the blades at the front and set to stall at a higher wind speed. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower. To reduce sympathetic vibrations, the two turbines should turn at speeds with few common multiples, for example 7:3 speed ratio. Overall, this is a more complicated design than the single-turbine wind generator, but it taps more of the wind's energy at a wider range of wind speeds.

Furling tail and twisting blades

In addition to variable pitch blades, furling tails and twisting blades are other improvements on wind turbines. Similar to the variable pitch blades, they may also greatly increase the efficiency of the turbine and be used in "do-it-yourself" construction

Wind-mill style

De Nolet is a wind turbine in Rotterdam disguised as a windmill.

Ducted 2-Blade HAWT

Looking similar to the standard 2-Blade three-bladed horizontal-axis wind turbine (HAWT), which is the most used type of wind turbine in the world today, this type which the British experimented with in the early 1950s, as the wind cause the blades to turn, draws air in from near the bottom of the turbine's large hollow mast, through turbines that spin an electrical generator and expels the air at the tip of the wind turbine blades. The engineers of this type of wind turbine believed the advantage of this system saved cost in not having to have a linkage and transmission for the generator and the lighter weight as the generator was located near the bottom of the mast and anchoring foundation and not at the top near the blade. One was built and tested near St Albans, Hertfordshire, England.