It should be noted that the above are reportedly 40%+ more efficient in energy collection from the wind than single rotor “turbines” (which they are not, a true turbine involves at least two rotors) see:-
Kari Appa Contra-Rotating Wind Turbines, US Patents 6,127,739, 6,278,197, and California Energy Commission report on www.eisg.sdsu.edu/Far/00-09%20Far.pdf

This paper confirms work done and/or reported on under references 2, 3, 4, 5, 6 & 7 below and unpublished work by Simon Bromet in 1976. In addition there are structural benefits, and the possibility of linking turbines together in a grid controlled network without the use of electronics .[Ref 6], Ref 8 makes use of this possibility. This also offers interesting possibilities for mounting an array of smaller wind turbines on large buildings.

It should be noted that in basic mechanical principles today’s three bladed “wind turbines” are very similar to the machine rented by Nicholas from the Knights Templars (molendium venti) in E. Yorks in 1185, and possibly similarly mechanically to the machines made by “the slave skilled in the manufacture of windmills” who slew Sultan Omar of Baghdad in 645 AD. John Smeaton used five blades for maximum energy collection. Now only to be seen on the Boston windmill.
References
1/. Betz A, Die Windmuhlen im Lichte neuerer Forschung, Die Naturwissenschaften, 18 November 1927

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2/. Honneff H, Windcraftwerke, F.Vieweg, Braunschweig 1932, (copy in Science Museum Library). Hohenwindkraftwerke, Elektrotechnik und Maschinenbau, Vol 7, No. 41 & 42, 13 October 1939, pp 501-6, German Patent No. 871580 1953,

3/ Osborne,W C and Turner, C G, Woods Practical Guide to Fan Engineering, 2nd edition, July 1960, pages 134-5.

4/. Stobart A F, Heating from Wind, Building Technology and Management, July/August 1981.

5/. de Paor A M, O’Malley M J, & Ollivier D, Identification and passive control of a wind driven permanent magnet alternator system for heating water. Wind Engineering Vol 7 No. 4 pp 193-207, 1983

6/. Kloss M, Der direkte Antreib von Synchrongeneratoren durch Grosse-Windkraftwerke in Parrallelbetreib mit einem taktgebenden Netz, Elektrotechnische Zeitsschrift 63 Jahrg Heft 31/32 August 1942

7/. Gaenger B, Foettinger, Robitsch, Teubert, and Witte, FIAT report No. 1111 on the work of the Reicharbeitsgemainschaft Windkraft (12 reports/papers) (Believed to be in a USA archive)

8/. Sachs T, Christmas Tree Turbine, Electrical Review Vol 221 No 23, page 20, 30 November-13 December 1988.

9/. Bockris J O’M, Possible means of Large-scale use of Wind as a source of Energy, Environmental Conservation, Vol 2 No. 4 Winter 1975 [25 references].

10/. Van der Linden, K, Sixmaster Turbine, Holland (address known). Unit reportedly bought by The Municipal Energy Authority of Rotterdam about 1990.

A contra rotating eight sail wing bladed wind turbine, based on a permanent magnet alternator which also acted as the blade support. No gear box.
The supporting structure was made of ductile iron pipe, for low cost, no corrosion, and ease of erection.
Main application was heating, hot water or hot air, with a tap off for battery charging.
Rated 5kW @ 10 mps wind, max output .