Evolution of Modern Wind Turbines Part A: 1940 to 1994 - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 125

Early Unconventional Systems
Panemones
The earliest practical windmills (see Chapter 1) were of the vertical-axis type driven
by drag forces and known generically as panemones . Panemones are multi-bladed machines
often designed with some type of articulating mechanism to fold or feather the blades that
are moving upwind, while deploying the working blades moving downwind. Panemone
devices are re-invented at frequent intervals. Their relatively poor eficiency, low blade
speed (a tip-speed ratio necessarily much less than unity), and large surface area have led
panemones into a continual dead end. While simple to manufacture in small sizes, they
have not proven to be cost-effective in other than primitive installations, owing to the need
for large amounts of material and the problem of withstanding high wind loads.
The Savonius rotor (Fig. 3-16) is another innovative vertical-axis device and one which
has somewhat better performance. With its “S-shaped” cross section, the Savonius rotor,
(named for its Finnish inventor) is really more of a semi-lift or low lift-to-drag device.
Since the blades are simple to form from sheet metal, the Savonius inds an occasional
niche in developing countries for tasks such as water pumping. For the same reasons as
the panemone, the Savonius turbine has not been successful for general use.
By the 1950s a number of unconventional wind turbine conigurations had been
explored in some detail. One of the more unusual of these was the Enield-Andreau HAWT
described earlier (Fig. 3-4).
HAWTs with Multiple Rotors
Honnef, in Germany, was an advocate of multiple rotors in the same plane on a single
tower as a means of achieving high power levels with rotors of intermediate size [Honnef
1932, Hütter 1974]. In the early 1940s he tested systems with two counter-rotating rotors
in the 8-m and 10-m diameter range, but his large-scale designs never got further than
artist's drawings. Nevertheless, the idea of multiple rotors continues to attract followers.
Studies in the early 1970s concluded that it was more cost-effective to use multiple turbines
or larger turbines than to pay for the complex structure needed to support and yaw a
multiple-rotor system. One company briely went into limited production of a six-rotor,
single-tower HAWT in 1987.
Multiple rotors in the same plane increase net swept area and should not be confused
with multiple rotors on the same axis. In some cases the latter are counter-rotating.
Advocates of such systems usually misunderstand the physics behind wind energy conver-
sion. The effects of induced velocity limit the theoretical maximum power coeficient of
two rotors on the same axis to little more than that of a single rotor. Counter-rotation does
decrease the rotational energy lost in the wake, but this beneit is trivial compared to the
costs of the second rotor and associated gearing. These systems have rarely been success-
ful, much less cost-effective. The Noah HAWT, privately developed in northern Germany
in the mid-1970s, is the most recent example known.
Translating Units
Translating wind power systems have been conceived as a means of achieving power
levels far beyond that of an individual wind turbine. These concepts generally involve a
large circular or oval track upon which a “train” of vehicles moves, each vehicle supporting
a sail or vertical wing of some type. The wheels of the vehicles turn generators, and power
is conducted away through a third rail, in the reverse of a conventional electric railway