Unconventional wind turbines are those that differ significantly from the most common types in use. As of 2024, the most common type of wind turbine is the three-bladed upwind horizontal-axis wind turbine (HAWT), where the turbine rotor is at the front of the nacelle and facing the wind upstream of its supporting turbine tower. A second major unit type is the vertical-axis wind turbine (VAWT), with blades extending upwards, supported by a rotating framework. Due to the large growth of the wind power industry, many wind turbine designs exist, are in development, or have been proposed. The variety of designs reflects ongoing commercial, technological, and inventive interests in harvesting wind resources more efficiently and in greater volume. Some unconventional designs have entered commercial use, while others have only been demonstrated or are only theoretical concepts. Unconventional designs cover a wide gamut of innovations, including different rotor types, basic functionalities, supporting structures and form-factors.
Horizontal axis
Twin-bladed rotor Nearly all modern wind turbines use rotors with three blades, but some use only two blades. This was the type used at Kaiser-Wilhelm-Koog, Germany, where a large experimental two-bladed unit—the GROWIAN, or Große Windkraftanlage (big wind turbine)—operated from 1983 to 1987. Other prototypes and wind turbine types were manufactured by NedWind. The Eemmeerdijk Wind Park in Zeewolde, Netherlands uses only two-bladed turbines. Wind turbines with two blades are manufactured by Windflow Technology, Mingyang Wind Power, GC China Turbine Corp and Nordic Windpower. The NASA wind turbines (1975–1996) each had 2-blade rotors, producing the same energy at lower cost than three-blade rotor designs.
Downwind rotor Nearly all wind turbines place the rotor in front of the nacelle when the wind is blowing (upwind design). Some turbines place the rotor behind the nacelle (downwind design). This design has the advantage that the turbine can be made to passively align itself with the wind, reducing cost. The main drawback is that the load on the blades changes as they pass behind the tower, increasing fatigue loading, and potentially exciting resonances in other turbine structures.
Ducted rotor A research project, the ducted rotor consists of a turbine inside a duct that flares at the back. They are also referred as Diffuser-Augmented Wind Turbines (i.e. DAWT). Its main advantage 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, allowing the mechanical portion to be smaller and lighter. A disadvantage is that (apart from the gearbox) it is more complicated than the unducted rotor and the duct's weight increases tower weight. The Éolienne Bollée is an example of a DAWT.
Co-axial, multi-rotor Two or more rotors may be mounted to a single 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 was multiplied several times using co-axial, multiple rotors in testing conducted by inventor and researcher Douglas Selsam 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 two rotors separated by twelve feet (3.7 m). It is the most powerful seven-foot-diameter (2.1 m) turbine available, due to this extra rotor. In 2015, Iowa State University aerospace engineers Hui Hu and Anupam Sharma were optimizing designs of multi-rotor systems, including a horizontal-axis co-axial dual-rotor model. In addition to a conventional three-blade rotor, it has a smaller secondary three-blade rotor, covering the near-axis region usually inefficiently harvested. Preliminary results indicated 10–20% gains, less efficient than is claimed by existing counter-rotating designs.
Counter-rotating horizontal-axis When a system expels or accelerates mass in one direction, the accelerated mass causes a proportional but opposite force on that system. The spinning blade of a single-rotor wind turbine causes a significant amount of tangential, or rotational, air flow. The energy of this tangential air flow is wasted in a single-rotor turbine design. To use this wasted effort, the placement of a second rotor behind the first takes advantage of the disturbed airflow, and can gain up to 40% more energy from a given swept area as compared with a single rotor. Other advantages of contra-rotation include no gear boxes and auto-centering on the wind (no yaw motors/mechanism required). A patent application dated 1992 exists based on work done with the Trimblemill. 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. When land or sea area for a second wind turbine does not come at a premium the 40% gain with a second rotor has to be compared with a 100% gain via the expense of a separate foundation and tower with cabling for the second turbine. The overall power coefficient of a Counter-rotating horizontal-axis wind turbine may depend by the axial and the radial shift of the rotors and by the rotors' size. As of 2005, no large, counter-rotating HAWTs are commercially sold.
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 efficiency and be used in "do-it-yourself" construction.
Wind-mill style De Nolet is a wind turbine in Schiedam disguised as a windmill.
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