Evolution of Modern Wind Turbines Part A: 1940 to 1994 - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 142

structural dynamic behavior of a HAWT becomes increasingly more dificult to analyze and
to accommodate as the number of blades is reduced from three to two to one.
Research on one-bladed rotors has been conducted in Germany (since the 1930s), Italy,
and in the U.S. on the Mod-0 experimental HAWT. Structural dynamics questions
associated with asymmetric loading are a worrisome issue for the one-bladed concept. A
one-bladed, 24-m diameter 370-kW Monopteros wind turbine erected near Bremerhaven,
Germany, was tested with suficient success that three 50-m, 640-kW commercial units
were erected at nearby Wihelmshaven. An Italian-German team produced several sizes of
one-bladed Riva Calzoni wind turbines, including a 350-kW unit 33 m in diameter.
During the early stages of design evolution, large-scale wind turbines around the world,
both government and privately funded, tended more to use two blades, rather than one or
three. The beneits of higher tip-speed ratios were expected to be more important to larger
turbines with their intrinsically lower shaft speeds, higher step-up ratio requirements, very
large torques, and higher ratios of rotor cost to total installed cost. Moreover, large-scale
system manufacturers generally have proportionately more engineering and analytical facili-
ties and budgets available to solve structural dynamics problems. Exceptions to this trend,
retaining the three-bladed coniguration, included the two 630-kW Nibe turbines in Denmark
and larger commercial units (up to 500 kW) built with Danish utility and European Economic
Community (EEC) funds. For the converse set of reasons, smaller commercial turbines have
generally used three blades.
Rotor Blade Materials
Early electricity-producing wind turbines used wood blades, as did many of their water-
pumping predecessors. Early experiments into larger systems (such as the Mod-0 HAWT)
used aluminum rib/spar/skin construction typical of airplane wings. This design, however,
is labor intensive and expensive, and the many rivets and bolts are subject to fatigue failure.
Other early machines used everything from riveted and bolted steel (the Smith-Putnam
turbine) to laid-up iberglass , cured in molds (the Hütter turbines). Fiberglass, manufac-
tured in this manner, remains one of the dominant materials for blades of all sizes.
The Hamilton Standard-KKRV WTS-3 and -4 wind turbines successfully utilized
ilament-wound iberglass blades each 24 m long. Other large turbines, such as the Mod-2
and the 3.2-MW Mod-5B have utilized welded steel rotors , and one would expect that to
remain the case for rotor diameters larger than about 60 m, particularly if the blades contain
outboard mechanisms for tip pitch control. However, an increasing market may allow
further investment in automated ilament-winding machinery, which could cause the trend
to shift toward iberglass and away from welded steel. Composite materials more exotic
than conventional iberglass have been used, but rarely. The German Growian and Aeolus
II HAWTs, for example, had some carbon iber composite in their blades, but in most cases
the economics of wind turbines preclude the large-scale use of such expensive materials.
In a blade technology program at the NASA Lewis Research Center, numerous blade
designs and materials were examined and tested [Linscott et al . 1984]. The chronology of
this extensive program, with ield installations of the various materials developed, is
illustrated in Figure 3-27. Some techniques failed, but several iberglass variants were
among those that were successful. Both ilament- and tape-wound iberglass were shown
to be amenable to fabrication over mandrels on automated machinery [ e.g. Weingart 1981].
Reducing hand labor should lead to lower costs and improved quality control, always an
issue in highly-stressed composites.