Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 111

FIGURE 6.12 Blade templates for Carter 25 wind turbine, fabricated by AEI. There are three different air-
foils, and thus there are different shapes along with different chords and twists at the ten stations.
with the proper twist (Figure 6.12). Then the blade templates were used to construct a plug, from
which two molds, top and bottom, were constructed. After fabrication of the blade skins, they were
attached to a Carter 25 spar and hub and tested in the field in a side-by-side comparison with a pro-
duction unit [29, 30]. Data were collected at low, medium, and high wind speeds for clean, medium,
and heavy surface roughness conditions. The roughness conditions were simulated with the applica-
tion of grit on 2.5 cm wide tape on the upper (0.02 chord) and the lower (0.05 chord) leading edge.
Results of the tests showed little power difference at low wind speed, the reduced power from the
outer part of the blade could not be tested since the teetering hub reduced high flap loads, and the
new airfoils were much less sensitive to surface roughness for medium and high wind speeds.
Essentially the same amount of power can be obtained from one blade rotating fast or more
blades rotating slower, or from the same number of blades with different chord lengths. From the
performance prediction programs, as solidity increases for a given rotor area, the tip speed ratio
that gives the maximum power coefficient becomes smaller. For a given size rotor operating at fixed
rpm, different size generators (rated power) can be placed on the unit by increasing the rated wind
speed. In the past a number of wind turbines were built with the same diameter, 10 m; however, they
had the following rated powers: 8, 12, 15, 25, 40, and 90 kW. Today, most wind turbines have rated
powers at wind speeds from 10 to 13 m/s.
The design engineers of wind turbines have a number of parameters to select just for the rotor:
airfoil, planform, solidity, number of blades, radius, tip speed ratio (variable or fixed), etc. The most
efficient blade from an aerodynamic basis is generally more difficult to construct from a practical
and manufacturing standpoint. Early blades were made from wood, the same as propellers, and a
commonly used airfoil was the NACA 4400 series, because the bottom side of the airfoil was flat.
Other airfoils with better lift to drag were used, such as the NACA 23000 series and the LS1 airfoil.
These airfoils had camber, curved on the bottom side, which made them somewhat more difficult
to construct. An aerodynamic efficient blade will have the largest twist and chord at the root, which
then decreases toward the tip; however, because of other considerations, in general, the inner part
of the blade is only designed for some efficiency and starting torque, because the outer third of the
blade generates most of the power. Therefore, that part of the blade must be aerodynamically effi-
cient. Finally, the design of the tip of the blade is important for noise considerations and to reduce
tip losses if possible. The outer portion of the General Electric blade is now swept back and the
Skystream has sweep blades, which means the outer portion is curved like a scimitar (sword).