Advanced Wind Turbine Program Next Generation Turbine ... - NREL

posed in conjunction with the direct-drive generator concept, GE’s concept evolved to consider it
as a possible replacement for the bedplate main frame regardless of the drive train arrangement.
(a) Large Bearing Support (b) Tubular Nacelle
Figure 4. Direct-Drive Support System
The motivation for using welded steel sheet is that the latter could be cut, rolled, and welded
quite inexpensively, either in a factory setting or even in a field assembly facility. Minimal finish
machining would be required, and the entire structure should save substantial money. Significant
main frame weight reductions were predicted.
Extensive structural analysis of the tubular nacelle concept was conducted. This analysis was
based around replacing the bedplate for the 1.5 MW POC turbine with the welded steel tubular
nacelle. Internal structure was added in order to add rigidity to the cylindrical skin structure and
to distribute the loads to the skin and the rest of the support structure in a reasonable fashion. An
optimization of the tubular nacelle design was conducted in an attempt to minimize the amount
of material used while maintaining structural integrity. Welded steel does not have particularly
high fatigue strength, only 4.5 ksi, or 31 MPa. Figure 5 below illustrates the stress distribution
for the final “optimized” design. Even this design, exhibiting areas of excessive fatigue (gray
areas in Figure 5), weighs over twice what the existing bedplate weighs. As a result, this configuration
is expected to cost more than the GE 1.5 MW bedplate and fiberglass nacelle. Further optimization
of the design might be expected to shave more cost,, but not enough to provide a significant
benefit. As shown in Figure 5, the distribution of material does not match the distribution
of stress. A great amount of material above the centerline and behind the tower is very
lightly stressed. This design was an inefficient structure for this application.
17
`