page_title: Unconventional wind turbines
section_title: Co-axial, multi-rotor
hierarchical_section_title: Unconventional wind turbines / Horizontal axis / Co-axial, multi-rotor
caption_attribution_description: English: Computer image of a wind turbine with contra-rotating rotors
context_page_description: Unconventional wind turbines are those that differ significantly from the most common types in use.
  As of 2012, the most common type of wind turbine is the three-bladed upwind horizontal-axis wind turbine, 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, 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.
context_section_description: 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 2 propellers separated by 12 feet. It is the most powerful 7-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.
caption: Counter-rotating wind turbine (dual rotor)
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