Structural Dynamic Considerations in Wind Turbine Design - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 586

Wind Turbine Design Philosophies
There are certain design philosophies that transcend many of the other determinants of
turbine configuration or “type”, such as HAWT vs. VAWT, large vs. small, upwind vs.
downwind, etc. Modern wind turbine configurations follow a system architecture that
embodies one or more of these design philosophies, an architecture which seeks to combine
efficient and durable static and dynamic structures with high-performance machinery and
controls. Engineers are often more comfortable with requirements than philosophies, but
nevertheless, the end product of their work is always a record of their design philosophy.
The emphasis in this chapter is on system design philosophy and, in particular, on
system architectures based on compliance with the forces of nature, rather than resistance
to these forces. For the moment, we shall depart from the more-objective approach
followed elsewhere in this topic and advocate the compliant architecture philosophy of wind
turbine design. The opinions expressed are based on design and operating experience with
several large-scale prototype HAWTs which have demonstrated the benefits of compliant
architecture, as well as on observations of commercial wind turbines that do or do not
embody this philosophy. Contradictory opinions can easily be found if one consults other
experienced wind turbine designers. One only has to look at the wide variety of installed
turbine configurations illustrated in Appendix E to see that design tradeoffs, to date, have
usually been resolved in favor of resistance rather than compliance.
Two factors may explain the current emphasis on resistive system architectures. First,
the great majority of installed wind turbines are medium-to-small in scale, and any weight
and cost penalties associated with this type of architecture are manageable. Second, the
compliant architecture is viewed as less reliable or less durable because of a lack of long-
term, successful field experience with turbine configurations based upon it. However, as
the world-wide trend toward larger and more powerful commercial turbines continues and
as positive experience with compliant subsystems accumulates, we can expect more of a
balance between compliant and resistive architectures in future wind turbines.
Stiff vs . Soft Design Philosophy
A prime issue in establishing a design philosophy is that of stiff vs. soft design. All
structural dynamic systems can be simulated by a series of interconnected masses, springs ,
and dampers. In a stiff system concept, transient air loadings go into the structural springs
as physical strains and (sooner or later) induce fatigue failure. In the soft system concept,
transient air loadings are reacted primarily by subsystem masses with little strain energy
involvement. Hence, there is a low proclivity for fatigue failure in a dynamically soft
system.
Representative of wind turbines with stiff structural systems are the early 200-
kW NASA/DOE Mod-OA and the 2.0-MW General Electric Mod-1 HAWTs (Figs. 3-
29 and -31) and typical 3-bladed Danish-type turbines currently in service worldwide
( e.g. , Fig. 4-22). Softer structural systems are embodied in the Boeing 2.5-MW Mod-2 and
3.2-MW Mod-5B HAWTs (Figs. 3-39 and 2-1), and, to an even greater extent, in the
Hamilton Standard/KKRV WTS-3 and WTS-4 (3.0 and 4.0 MW, respectively; Figs. 3-34 and
4-24) and, most recently, in the 2.0-MW WEST Gamma 60 shown in Figure 10-1.
In addition to the load response aspects of selecting a philosophy of system design, the
choice of concept will also affect both the achievable load control and the ease or difficulty
of an adequate simulation of the system.