Environmental Engineering Reference
In-Depth Information
proceedings [8-10]. The U.S. Department of Energy (DOE) discontinued funding for this program
after a few years.
Winglets or tips (dynamic inducer) on the ends of the blades [11], which reduce the drag due
to the tip vortex, were tested by Aerovironment and the University of Delft, the Netherlands. The
results were inconclusive due to the variability of the wind speeds. In some cases, energy production
could be improved, but the cost of the winglets could be offset by increasing the radius of the blades.
Where the wind speed variability is not a major factor, winglets can reduce drag and increase lift,
like on some airplanes.
A simple sail wing consisting of a pipe spar, and a trailing cable was designed and built by
Sweeney [12]. The advantages are light weight and ease of repair. The patent rights were pur-
chased by Grumman, who built a couple of prototypes but never put the unit into production. WECS
Tech installed a number of sail wing units on a wind farm in Texas and others on wind farms in
California. The operating history was very poor, as high winds destroyed the sails and most units
were destroyed within a short time. The same sail wing design was used on a prototype project by
the Instituto de Investigación Electricas in Mexico.
The idea of a confined vortex, a tornado, was invented by T. J. Yen. DOE funded theoretical and
model studies of this concept. Another concept was to use unconfined vortices produced along the
edges of a delta wing and then place two rotors at those locations. Again, DOE funded model stud-
ies. Existing structures could be modified or new buildings would incorporate features to increase
the wind speed, which then would be captured by a WECS. Since wind speed increases with height,
if rotors could be placed in low-altitude jets by use of tethered balloons or airfoils, a large amount
of energy could be obtained from small-size rotors.
Other ideas are lift translators with horizontal or vertical axis, which is similar to the idea of
railroad cars with wings, except cables hold the sails or airfoils and the wind turbine resembles a
moving clothes line. Both concepts need wind from a predominant direction, as large units cannot
be oriented. A number of foundations were constructed, and a few lift translators were built during
the early 1980s in California; however, they were never really operational.
An idea for reducing weight was to use cables for tension, as proven in suspension bridges, to
support long cage-containing blades. An oscillating vane or airfoil could extract energy from the
wind, but the intercept area is fairly small for the amount of material.
There have numerous designs and a number of wind turbines have been built with different com-
binations and unusual blade shapes. A few examples are Darrieus or giromill wind turbines with
Savonius rotors on the inner shaft for start-up torque, wind turbines with double rotors (some rotors
close together, some farther apart), multiple rotors on a single shaft (either vertical or horizontal),
double-bladed giromills, and blades with nontraditional shapes (curved like a helix) on horizontal
or vertical axes. A wind system with three stacked Darrieus units (4 kW each) was built at a news-
paper office in Florida. Other units have enclosures to increase the wind speed or are designed to be
incorporated into tall buildings.
The Noah wind turbine had two rotors ( Figure 5.20 ) , which were close to one another, each
with five blades, and the wind rotors were counterrotating, with one connected to the stator
and the other to the rotor of a generator, so a gearbox was not needed. The wind turbine had
a unique overspeed control, which consisted of a counterweight that tilted the rotor assembly
to the horizontal position, which then had to be reset manually. Another system has multiple
rotors on a coaxial shaft [13], where the line of the rotors is kept at an angle to the wind to
improve influx of the wind to the downwind rotors. Units with two to seven rotors have been
built (two and three blades) with rated power from 2 kW (diameter, 2.4 m; two rotors, 3.7 m
apart) to 4 kW. One unit even has 13 two-bladed rotors, each with a diameter of 0.5 m, and
rated at 400 W.
Lagerway built a unit with two conventional wind turbines (25 kW each) mounted on a horizon-
tal cross-beam at the top of the tower. Then another was built that resembled a tree, as it had two
more levels, for a total of six wind turbines.
Search WWH ::




Custom Search