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A
Simple Self Feathering
Wind Turbine for Light Winds
Project Summary
A new wind turbine design has
moved through conceptual and analysis stages and is ready for Component
Development under DOE Technical Area 2. The expected advantage of this
turbine over earlier designs is better control of worst-case wind forces
under extreme turbulent conditions. With better control and lower maximum
forces on the blades, bearings, and tower of a wind installation, it should
be possible to use a larger turbine in relation to other components, thus
sweeping more wind area, gathering more energy from low winds, and reaching
full-power capacity at a lower windspeed.
This increased energy-gathering
capacity is expected at little or no extra cost, compared to the same
type installation fitted with a smaller turbine of earlier design. The
turbine is slightly more complex than the extremely simple “one-piece”
three-blade designs common in small turbines, since each blade freely
rotates about a pitch-change axis directed along the length of the blade.
The design is simpler than other turbines with blade pitch control, however,
since the blade pitch angle is controlled passively by wind and inertia
forces. There are no linkages or servo controls in the rotor hub, and
each blade acts like a single moving part. While both two- and three-blade
design variations are possible, it is expected that this turbine can operate
smoothly and quietly in an upwind, two-blade configuration, thus offsetting
some or all of the cost of added blade complexity. The blades are expected
to exhibit an inherent tendency to orient smoothly and slowly upwind,
without developing the gyroscopic vibrations that trouble most upwind
two-bladed turbines using a tail.
Quiet operation with low peak
wind forces is expected because the turbine blades feather quickly in
response to gusts and turbulent eddies, streamlining themselves to the
local wind affecting each blade. The blades avoid aerodynamic stall and
the noisy, turbulent buffeting and vibration associated with stalled operation.
While operation in turbine blades that govern power by aerodynamic stall
is highly sensitive to blade shape and surface condition, operation of
the blades of the proposed turbine is insensitive to surface condition.
There is a sharp transition from maximum efficiency in low winds to maximum
governing power in high winds, with no peaking to excess power. Thus,
requirement for over-design of the generator is reduced. Since generators
and alternators lose efficiency when operating well below maximum capacity,
a reduction of generator over-capacity for handling transient power peaks
is expected to improve average energy-gathering performance while reducing
costs. The transition to governing can be optimized through familiar techniques
of electronic control of power inversion into a utility line.
Passive aerodynamic control
of turbine blade operation is accomplished as follows. The untapered blades
are warped from root to tip, so that the airfoil at the blade root provides
the high lift normally achieved by tapering blades to be wider at the
root. The blade tip is warped for the lower maximum lift normally achieved
with a narrower airfoil at the tip. The extra blade chord width at the
tip functions as an aerodynamic trim tab, causing the tip region to passively
seek a constant lift angle in relation to the wind moving past the blade.
Each blade changes pitch angle freely (within bounds) and independently
by its rotation about the pitch-change axis running along the center of
each blade. When one turbine blade encounters different aerodynamic conditions
from the other one or two blades, it responds differently and independently,
seeking a constant lift angle relative to its local wind. Since the velocity
of the blade is primarily tangential, resulting from turbine rotation,
the aerodynamic force magnitude developed by the blade is dependent primarily
on rotor rotation speed and almost independent of instantaneous windspeed.
The effect of increasing wind
is to rotate the blade force vector in the direction of rotor torque,
so that high winds tend to speed up the rotor. To limit this speed-up
to a safe governing rotation speed, a weight on each blade, at the end
of a flat spring steel band, moves centrifugally outward from the blade
surface with increasing rotation speed. Centrifugal force on this weight
produces a blade-feathering torsion force, offsetting the pitch-controlling
torsion from the trim tab region of the blade. The blade progressively
feathers. Thus, the blades spill progressively more wind as wind and rotor
speeds. Very high winds pass through unimpeded in direct contrast to a
stalled rotor. The blades in this design “weathervane” rapidly
in changing and turbulent winds, dynamically seeking an angle that lets
strong wind pass harmlessly through the rotor.
The Davis Collaborative proposes
to design, fabricate, instrument, and test this turbine mounted above
the bed of a truck equipped with turbulence and gust inducing baffles.
This proposal also includes building a full size unit for installation
and testing at DOE or Altamont Pass. The blades, slightly modified from
constant cross-section by warping the thin trailing edges, are compatible
with inexpensive composite fabrication, e.g., pultrusion, making a system
appropriate for domestic manufacture, international sale, and fossil fuel
saving. Three investigators specialize in: 1) design and analysis, 2)
instrumentation and data collection, 3) fabrication, testing, and reporting.
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