Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 73

5 Wind Turbines
Wind turbines are classified according to the interaction of the blades with the wind (aerodynamics),
orientation of the rotor axis with respect to the ground, and innovative or unusual types of machines.
The aerodynamic interaction of the blades with the wind is by drag or lift, or a combination of
the two.
5.1 DRAG DEVICE
In a drag device, the wind pushes against blade or sail ( Figure 5.1 ), and drag devices are inher-
ently limited in efficiency since the speed of the device or blades cannot be greater than the
wind speed. For a drag wind turbine, the wind pushes on the blades, forcing the rotor to turn on
its axis.
Examples of drag devices are cup anemometers, vanes, and paddles, which are shielded from the
wind or change parallel to the wind on half the rotor cycle ( Figure 5.2 ). Clams shells, which open
on the downwind side and close on the upwind side, are another example of a drag device. There are
no commercial drag wind turbines for producing electricity, since they are inefficient and require a
lot of material for blades. However, drag devices are popular with inventors and homebuilders, as
they are easy to construct ( Figure 5.3 ) . Invariably, the inventors become irate when they are told that
the inefficient aerodynamics and large amount of material for blades for drag devices limits their
commercialization.
5.2 LIFT DEVICE
Most lift devices use airfoils for blades similar to propellers or airplane wings; however, other con-
cepts have been used. Using lift, the blades can move faster than the wind and are more efficient
in terms of aerodynamics and amount of material needed for the blades. The tip speed ratio is the
speed of the tip of the blade divided by the wind speed. At the point of maximum efficiency for a
rotor, the tip speed ratio is around 7 for a lift device and 0.3 for a drag device. For a lift device the
ratio of amount of power per material area is around 75, again emphasizing why wind turbines using
lift are used to produce electricity. The optimum tip speed ratio also depends on the solidity of the
rotor. Solidity is the ratio of blade area to rotor swept area.
So one blade rotating very fast can essentially extract as much energy from the wind as many
blades rotating slowly ( Figure 5.4 ). A wind turbine with one blade would save on material; how-
ever, a counter weight is needed for balance. Most modern wind turbines have two or three blades
because of other considerations, and almost all large wind turbines in the commercial market have
three blades. The MBB Monopteros and the FLAIR designs were single-bladed wind turbines built
in Germany, and a one-bladed (5 kW) unit was built by Riva Calzoni, Italy. The Monopteros had
full-span pitch control and the rotor was upwind. MBB and Riva Calzoni collaborated on a 20 kW
one-bladed unit, and then Riva Calzoni built a 330 kW unit. Chalk [1] invented a rotor with a large
number of blades based on the design of a bicycle wheel. There have been some modern wind tur-
bines with four to six blades.
A Savonius rotor ( Figure 5.5 ) is not strictly a drag device, but it has the same characteristic of
large blade area to intercept area. This means more material and problems with the force of the
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