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

Figure 2. Concept layout of Single Stage Gearbox, Medium Speed Generator System
The initial concept planned for the unit to be integrated into a tubular nacelle, as described in
Section 2.1.3.3. Therefore, some of the projected support structure cost savings resulted from
that integration. Nevertheless, data shows that the stand-alone medium-speed generator should
weigh and cost no more than a conventional generator, while it can be expected that simply integrating
the single-stage gearbox with the medium-speed generator would result in a significant
weight and cost savings for the gearbox, translating into a cost savings of nearly 2.5% of the installed
turbine cost.
No estimate was made of the improvement in gearbox efficiency resulting from elimination of
the second and third stages of the conventional gearbox, although it can be expected that anywhere
from 1%–2% loss is associated with each stage. Therefore, it would not be unreasonable
to assume that the loss of efficiency associated with the medium-speed generator is offset by a
gain in efficiency on the gearing. The impact on the energy capture could be expected to be
minimal.
Therefore, this concept could potentially result in some small reduction in COE. However, a
relatively high amount of risk is believed to be associated with the concept, given that no medium-speed
wound rotor induction generators are known to exist, and it would represent a substantially
new technology.
2.1.2.2 Medium-Speed Squirrel Cage and Synchronous Generators
GE did not expend substantial effort in studying medium-speed squirrel cage induction, wound
rotor synchronous, or permanent magnet synchronous generators. Based upon the results reported
in Section 2.1.2.1, it is possible to design a medium-speed wound rotor induction generator
similar in weight and cost to a conventional-speed machine. The NGT design team con­
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