Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 38

The Development of Small Wind Turbine Generators
The advent and development of the airplane in the irst decades of the twentieth century
gave rise to intense analysis and design studies of the propeller that could immediately be
applied to the wind turbine, as we may now properly call it. Professor Albert Betz evolved
in clear form the magic value of 16/27 as the ideal maximum utilization factor (or the Betz
limit , as it is sometimes called) pertaining to the lux of energy available from the wind
[1926]. He was also interested in several other detailed aspects of aerodynamic
performance, but the times were not auspicious for any considerable undertakings because
of the worldwide depression of the 1930s. One notable feature, however, was the
introduction in several countries of a new type of fast-running wind turbine having two or
three blades with an aerodynamic proile more suitable for generating electricity.
In the United States, the design of Marcellus and Joseph Jacobs could be called the
archetypal battery-charging wind generator ; it was immediately popular for a quarter of a
century [Jacobs 1973]. Figure 1-20 shows the Jacobs Wind Electric power plant. Its
development started in 1925, commercial installation began in 1931, and production ended
in 1957. Jacobs' wind generators had a reputation for high performance, minimal
maintenance, and excellent structural integrity. A 32-V DC model delivered up to 2,500
W, and a 110-V DC unit produced up to 3,000 W. An interesting feature of the
development period was the selection of a three-bladed rotor. A serious vibrational
condition was observed in a two-bladed model during changes in wind direction. As it
yawed, the rotor experienced a series of jerks produced by changes in gyroscopic inertia
about the tower axis each time the blades passed from a vertical to a horizontal position.
Three blades, however, provided a constant inertia and solved the vibration problem. (This
dificulty with two-bladed rotors appears to have been overcome by the use of a teetered
hub in these later years.) Note that nearly all Danish wind turbines are still three-bladed,
along with most other European types, and these turbines appear to be among the leaders
in on-line availability in wind power stations worldwide.
A successful two-bladed, propeller-type turbine for providing direct current was the
Wincharger , originally available in sizes from 200 to 1,200 W. The 200-W model had a
rotor only 1.8 m in diameter, was much less expensive than the Jacobs machine, and was
sold in very large numbers for powering radios and perhaps a few lights on farms and
ranches.
An innovative type of wind turbine rotor, the Savonius rotor, was named after its
inventor, Finnish engineer S. J. Savonius, and irst tested in 1925 [Savonius 1931; Bach
1931]. The inventor's interest had been aroused by the Flettner rotor ship with its large,
rotating cylindrical “sails.” Wind passing over these cylinders created lift by the Magnus
effect , which propelled the ship forward. He was intrigued by the possibility of substituting
wind power for the external motor power used to rotate these cylinders on the Flettner ship.
His experiments resulted in a rotor with an S-shaped cross section which, in its simplest
form, could be constructed by cutting a circular cylinder in half longitudinally and rejoining
opposite edges along an axle. Tests did indeed show that his S-rotor could spin a Flettner
cylinder with enough power to propel a small boat at speeds up to 15 knots, sailing at all
angles to the wind.
According to the inventor, the Savonius rotor achieved some popularity in Finland, but
it has not prospered commercially as a means for driving an electrical generator. It did
have a vogue in the 1960s and '70s, largely because its design and construction were very
simple. It could be mode from an oil drum and a piece of pipe, which indeed it was by
enthusiastic amateurs in many places around the world. Its other advantages were high
starting torque and the ability to accept wind from any direction; its drawbacks were low