U.S. patent number 3,918,839 [Application Number 05/508,016] was granted by the patent office on 1975-11-11 for wind turbine.
This patent grant is currently assigned to The United States of America as represented by the United States Energy. Invention is credited to Bennie F. Blackwell, Louis V. Feltz, Randall C. Maydew.
United States Patent |
3,918,839 |
Blackwell , et al. |
November 11, 1975 |
Wind turbine
Abstract
A wind turbine rotatable about a shaft may include a drive rotor
with one or more elongated blades each having a central outwardly
curved portion of airfoil shape which produces rotary motion when
the blade rotates in wind at a blade tip velocity to wind velocity
ratio greater than about three or four, additional wind rotor means
disposed at both ends of the curved portions of the elongated blade
for rotatably accelerating the drive rotor to the desired velocity
ratio, and means coupled to said rotors for utilizing the rotation
thereof.
Inventors: |
Blackwell; Bennie F.
(Albuquerque, NM), Feltz; Louis V. (Albuquerque, NM),
Maydew; Randall C. (Albuquerque, NM) |
Assignee: |
The United States of America as
represented by the United States Energy (Washington,
DC)
|
Family
ID: |
24021039 |
Appl.
No.: |
05/508,016 |
Filed: |
September 20, 1974 |
Current U.S.
Class: |
416/175;
416/197R; 416/203; 416/240; 416/197A; 416/227A; 416/227R |
Current CPC
Class: |
F03D
3/065 (20130101); F03D 3/061 (20130101); F05B
2240/212 (20130101); F05B 2240/213 (20130101); Y02E
10/74 (20130101) |
Current International
Class: |
F03D
3/00 (20060101); F03D 3/06 (20060101); F03D
003/02 () |
Field of
Search: |
;416/132,175,197,203,227,240,110,111,119 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
961,999 |
|
May 1950 |
|
FR |
|
1,021,619 |
|
Feb 1953 |
|
FR |
|
39,680 |
|
Mar 1957 |
|
PO |
|
Primary Examiner: Powell, Jr.; Everette A.
Attorney, Agent or Firm: Carlson; Dean E. King; Dudley W.
Constant; Richard E.
Claims
What is claimed is:
1. A wind turbine comprising a rotatable shaft; a drive rotor
having an elongated blade with a central curved portion of airfoil
shape transverse to said curvature, and means for supporting said
blade on said shaft with said airfoil shape directed along the path
of movement of said blade for exerting significant driving force on
said shaft when said curved blade portion attains a linear velocity
to wind velocity ratio greater than about three; starter rotor
means disposed on said shaft having vanes out of registry with the
curved portion of said drive rotor for rotatably accelerating said
shaft to said velocity ratio; and means coupled to said shaft for
utilizing the rotation of said shaft.
2. The turbine of claim 1 wherein said drive rotor includes a
plurality of said blades, each having a central outwardly curved
portion of airfoil shape.
3. The turbine of claim 2 wherein said outwardly curved portions of
said blades are arcuately shaped approximating a portion of a
troposkien shape.
4. The turbine of claim 1 wherein said blade supporting means
includes substantially straight blade segments connecting and
supporting between them said curved portion of airfoil shape.
5. The turbine of claim 4 including means for separating the ends
of said curved portion of said blade from said blade segments.
6. The turbine of claim 4 including tip plate spoilers at each end
of said curved portion of said blade.
7. The turbine of claim 6 wherein said tip plate spoilers are
positioned generally perpendicular to said shaft.
8. The turbine of claim 4 including weight members positioned at
each end of said curved portion of said blade.
9. The turbine of claim 4 wherein said curved portion of said blade
includes a high strength, elongated strap disposed at the center of
said curved portion, a foam core disposed about said strap in said
airfoil shape, and an outer substantially impervious skin
thereover.
10. The turbine of claim 9 wherein said strap is bent into an
arcuate shape, said foam comprises inner and outer airfoil shape
segments adhered to each other and to said strap on both sides of
said strap, and the outer surface of said foam segments is coated
with said impervious skin.
11. The turbine of claim 1 wherein said velocity ratio is from
about 5 to 7.
12. The turbine of claim 1 wherein said shaft is generally vertical
and said starter rotor means include a first self-starting rotor
disposed above said curved portion and a second self-starting rotor
disposed below said curved portion; each of said self-starting
rotors including a plurality of hollow-shaped vanes facing in
opposite directions with respect to each other, and means for
supporting said vanes on said shaft partially overlapping each
other in a generally S-shaped fashion for directing wind caught by
the hollow portion of one vane into the hollow portion of at least
another vane in each rotor.
13. The turbine of claim 12 wherein the outer radii of said
self-starting rotors are less than the outer radius of said drive
rotor.
14. The turbine of claim 13 wherein the ratio of said radii is from
about 5 to 6 to 1 and aid in rotation of said turbine at and above
said velocity ratio.
15. The turbine of claim 12 wherein said vanes of said
self-starting rotors are interdigitated with respect to each
other.
16. The turbine of claim 1 including a plurality of said drive
rotors supported one above the other on said shaft, each succeding
drive rotor having a diameter greater than the next adjacent lower
rotor.
Description
BACKGROUND OF INVENTION
Wind was one of the first natural energy sources to be harnessed by
man with the use of various windmill driven apparatus. The use of
windmills, however, declined drastically after the development of
the steam engine, internal combustion engine, and other fossil
fueled energy conversion machines. Recently, with the increasing
cost of fossil and other presently widely used energy sources,
interest is again being directed to the use of wind as a
competitive source of energy.
For example, it has been estimated that greater than about
10.sup.12 kilowatt hours of electricity could be produced from
practical wind power sites in the Unites States alone, the energy
available being proportioned to the air density and wind speed, the
latter affecting energy by the third power. Since the amount of
energy available in wind may be significant when compared to the
energy needs of the world, such wind driven power sources may
become of increasing importance, especially if the location at
which the energy is required is remote or where alternate energy
sources require high cost fuel to produce power.
Various wind driven machines or turbines have been proposed or
utilized, such as the well known horizontal axis windmills. These
windmills have used various designs and arrangements of rotors
which have achieved rotor tip velocity to wind velocity ratios of
as great as 6 to 1. However, because of the inherent limitations of
such horizontal axis windmills which require the rotor to be
aligned in a particular direction with respect to the wind
direction (which of course is not constant) these windmills often
include complex windmill rotation drive mechanisms to maintain the
proper windmill rotor attitude or direction with respect to the
wind direction. These mechanisms, besides being complex, generally
must be attached to the windmill adjacent to the axis of the rotor
and are thus supported well above ground level, at least as high as
the radius of the rotor. This also adds to the complexity, cost and
weight of the supporting towers and other previously referred to
mechanisms of the entire windmill system.
Vertical axis wind turbines have been proposed and tested to
overcome some of these shortcomings. Most vertical axis wind
turbines, however, have very low rotor tip velocity to wind
velocity ratios and are thus very inefficient or require an
additional power source to accelerate the rotor to a velocity at
which the rotor can produce positive torque. In addition, some
prior vertical axis wind turbines have utilized rather complex and
expensive rotor blade designs or have been of relatively low
strength for practical applications. Even though vertical axis wind
turbines are often capable of operating from a wind coming from any
direction and with power generating equipment and tower structure
which may be of relatively simple construction, vertical axis wind
turbines have not been developed or widely used.
SUMMARY OF THE INVENTION
In view of the above, it is an object of this invention to provide
a relatively simple and low cost wind turbine arrangement.
It is a further object of this invention to provide a vertical axis
wind turbine which is self-starting and which is capable of
providing a relatively high blade tip velocity to wind velocity
ratio.
It is a still further object of this invention to provide a
vertical axis wind turbine having a novel rotor blade
configuration.
It is a further object of this invention to provide a high
efficiency vertical axis wind turbine system.
Various other objects and advantages will appear from the following
description of the invention, and the most novel features will be
particularly pointed out hereinafter in connection with the
appended claims. It will be understood that various changes in the
details, materials and arrangements of the parts, which are herein
described and illustrated in order to explain the nature of the
invention, may be made by those skilled in the art within the
principles and scope of this invention.
This invention relates to a wind turbine having a drive rotor
including an outwardly curved, airfoil shaped blade extending
between end portions of a rotatable shaft; and additional rotor
means disposed at both end portions of the shaft and out of
registry with torque producing portions of the drive rotor to bring
the entire rotor assembly up to a speed at which the drive rotor
may maintain a rotary driving force to the shaft and which
thereafter continues to contribute a driving force thereto at the
higher speeds.
DESCRIPTION OF DRAWINGS
The invention is illustrated in the accompanying drawing
wherein;
FIG. 1 is a somewhat simplified perspective view of the wind
turbine assembly of this invention showing the relative positions
of the rotor elements;
FIG. 2 shows diagrammatically the preferred shape of the blades in
the main drive rotor of the wind turbine;
FIG. 3 shows diagrammatically a comparison of the blade shape of
this invention with other possible blade curvatures;
FIG. 4 is a cross-sectional view of the airfoil portion of the
blade shown in FIG. 2;
FIG. 5 is a graph of efficiency versus velocity ratios for the
respective rotor portions of the present wind turbine;
FIG. 6a and FIG. 6b illustrate by cross section various shapes that
the straight segments may have for the blades shown in FIGS. 1 and
2;
FIG. 7 illustrates diagrammatically in a top section view the
positions of the vanes of the starter rotor utilized in the wind
turbine assembly of FIG. 1;
FIG. 8 is a perspective view of another starter rotor arrangement
which may be used with the turbine of FIG. 1;
FIG. 9 shows diagrammatically a modification to the drive rotor
blade and blade shape;
FIGS. 10a and 10b illustrate further modifications of the drive
rotor blade to effectively increase the aspect ratio of the rotor
blade;
FIG. 11 illustrates a mofified version of the wind turbine which
utilizes a vertical stacking of drive rotors; and
FIG. 12 is a simplified diagrammatic view of an arrangement of the
drive rotor blades in which the blade segments may be folded to
reduce the wind profile of the turbine.
DETAILED DESCRIPTION
The wind turbine of this invention includes a wind driven main
power or drive rotor 10 and a pair of wind driven starter rotors 14
and 16 coupled to a rotatable shaft 12, as indicated in FIG. 1. It
is preferred that the wind tubine be supported in a vertical
position as shown, so that any wind, regardless of direction, will
always cause rotation of the wind turbine rotors without adjustment
of the turbine axis. Each of the rotors 10, 14 and 16 are fixed to
the shaft 12 so as to rotate together about fixed platform or tower
18 with the shaft 12 maintained in the desired vertical position.
Shaft 12 may be rotatably mounted on platform 18 by appropriate
rotary bearings, and the like and may be stabilized by appropriate
guys or other supports 19 from upper portions of the shaft, if such
is desirable, depending on the size of the wind turbine and the
wind velocities in which it is to be operated. In addition, shaft
12, and consequently rotors 10, 14 and 16, may be coupled directly
or via an appropriate drive system, such as represented by gears 20
and 22, to a suitable utilization means 24 which may convert or
otherwise utilize the energy produced by the rotation of shaft 12.
Utilization means 24 may be an appropriate apparatus or mechanism
which may convert the rotary motion of the wind turbine into
electricity or some other form of energy, for example an alternator
or generator, or which may provide some other operation or
function, for example pumping of a fluid from a well or operating
another apparatus or mechanism.
The main drive or power rotor 10 may include one or more generally
vertically disposed elongated blades, such as the three blades 26a,
26b and 26c shown, which are fastened or coupled to shaft 12 at
their extremities through an appropriate collar or other support.
The blade or blades may be positioned around shaft 12 so as to
balance each other or may be provided with appropriate
counterweights, or the like to provide this balance. Each blade, as
indicated by blade 26a, may include a central outwardly curved,
arcuate portion 28 connected through straight segment 30 to an
upper portion of shaft 12 and through another straight segment 32
to a lower portion of shaft 12. More or less blades than the three
shown may be utilized in rotor 10 but with some lowering of
efficiency and/or inordinate increase in cost, the efficiency of
drive rotor 10 being a function of the ratio of blade area to the
blade swept area. The shaft 12 may be a single solid or hollow rod,
concentric rods rotatable with respect to each other, or a lattice
or truss-like structure, depending on its proposed strength and
size and the apparatus used to support the same.
It has been found that if a perfectly flexible cable of uniform
density and cross section is attached at its ends to two points on
a vertical axis and is then spun at a constant angular velocity
about the vertical axis, the cable will assume the curvature
indicated by the dotted line 34 shown in FIG., 2 referred to
hereinafter as a troposkien shape, regardless of the angular
velocity. When the cable assumes this shape and is rotated about
the vertical axis, the stresses produced in the cable are
essentially tensile stresses. It has further been found that for
purposes of this invention, the troposkien shape can be
approximated by a circular arc 34a, at the outermost portion of the
troposkien shape, and a pair of straight segments 34b and 34c
coupled between the ends of the circular arc 34a and the rotation
axis. With this approximation, the cable is still subjected to
essentially tensile stresses with only negligible bending stresses.
This approximation is utilized as the desired shape for the power
rotor 10 blades illustrated in FIG. 1.
FIG. 3 illustrates the differences between a troposkien-shaped
curve 34 and that of a circular arc 36 and a catenary-shaped curve
38. The catenary-shaped curve 38 approximates the shape assumed by
a perfectly flexible cable of uniform density and cross section
hanging freely from two fixed points. A rotated blade having either
of the shapes 36 or 38 will produce greater bending stresses than
the shape 34 or its approximation. As described above, the
troposkien-shaped curve 34 minimizes the bending stresses produced
in the vertical blade when subjected to rotary motion, while the
approximation of a troposkien shape, as illustrated by the circular
arc segment 34a and the straight segment sections 34b and 34c in
FIG. 2 and the corresponding curved portion 28 and straight
segments 30 and 32 of the blade 26a in FIG. 1, provide minimized
bending stresses while insuring a blade configuration which may be
manufactured with a relatively simple shape at relatively low cost.
The blade configuration shown may be selected to provide a close
approximation of the troposkien shape to minimize bending stresses
by minimizing the maximum separation distance between curve 34 and
the approximation segments 34a, 34b and 34c, or by otherwise
adjusting the approximation shape. In addition, since the rotors 14
and 16 are located in a position where they may normally interfere
with an air stream or wind directed against the blades of rotor 10
at the upper and lower extremities thereof, the straight segments
30 and 32 of the rotor 10 blades can be formed as structural
members with little or no aerodynamic lift or torque producing
effects. Further, since the torque or rotary force produced by the
blades of rotor 10 increases as the blade distance from the
rotation axis increases, the use of the curved portion 28 as the
principal or only drive section makes more effective use of wind
energy as other portions of the blade, namely the straight
segments, inherently produce lower torque levels from equal wind
energy.
The curved portion 28 of the blades 26a, 26b and 26c are provided
with an airfoil shape or cross section transverse to the blade
curvature and facing the direction of rotation of rotor 10 so as to
provide a lift force when the rotor 10 turns in a wind. A typical
cross section is shown in FIG. 4 which is selected to provide an
optimum Lift-to-Drag ratio, thus increasing power producing
performance.
Because of the nature of rotor 10 and the circular movement of the
blades, each blade curved airfoil section 28 will experience both
positive and negative angles of attack during a revolution so that
there is no apparent advantage in using a nonsymmetrical airfoil.
In addition, the lift for airfoils increases with increasing angle
of attack up to the point where the flow separates from the
airfoil, which condition may cause a stall and is generally to be
avoided, the maximum lift being higher for increasing aspect ratios
(the ratio of airfoil length to airfoil chord length). However,
with the rotor 10, the wind felt on curved portion 28 is not simply
the absolute wind speed or velocity but rather the absolute wind
velocity minus vectorially the absolute blade velocity. Also, in a
rotating airfoil, the angle of attack is the angle between the
relative wind speed (that is the apparent wind direction) and the
chord line of the airfoil blade, the angle of attack being
dependent on wind velocity, rotational blade velocity and the blade
position with respect to the turbine. For a given blade position,
the angle of attack decreases with increasing blade velocity to
wind velocity ratio. Therefore, for a sufficiently high ratio, the
airfoil may never stall during a revolution while at low ratios the
airfoil may be stalled over an appreciable portion of the blade
revolution. At high ratios, the angle of attack decreases
consequently decreasing the chord-wise component of lift. There is
thus maximum rotor efficiency at some tip velocity (linear velocity
of blade or vane at its maximum diameter) to wind velocity ratio as
indicated by curve 40 in FIG. 5, as determined by analytical
studies and wind-tunnel testing. It has been found that the most
efficient velocity ratios for the rotor 10 of this invention to
produce maximum power is from about 5 to 7, typically with a
maximum at about 6.
A symmetical airfoil shape which has a large lift-to-drag ratio may
be the NACA 0012 airfoil (National Advisory Committee for
Aeronautics). Such an airfoil or similar airfoil may be formed, as
shown in FIG. 4, with a high-strength backbone or tensile stress
element 42 surrounded by a rigid foam core 44. The stress element
42 may be a steel, aluminum or fiber composite leaf or strap which
is roll or otherwise formed in the desired arcuate curvature shown
in FIG. 2 by curve 34a so as to act as the supporting element for
the curved portion 28 and as the strength member to withstand the
tensile forces produced in the blade from the rotation of rotor 10.
The rigid foam core 44 may be formed from light-weight polyurethane
or the like foam bodies, as described below. Suitable fasteners or
attachments, such as hinges or pins (not shown), may be affixed to
the ends of element 42 at this time for convenience in connecting
the curved portion 28 of the blade to the straight segments 30 and
32. The rigid core 44 may be shaped in the desired airfoil
configuration and suitably adhered to the stress element 42, such
as by forming the core 44 by machining or the like two separate
rigid foam body halves from suitable foam blades into the desired
complementary shapes or sections 44a and 44b and then attaching the
sections on either side of the curved stress element 42. The outer
surface of the core 44 may then be appropriately coated, such as
with a fiberglass resin skin 46 in either mat, cloth or sprayed
form, to provide a smooth and erosion resistant surface around the
core 44 which will protect the same from impacts by objects carried
by the wind and from rain, hail, or the like. The skin 46 may be
smoothed and polished and further coated to minimize friction and
other aerodynamic losses and to provide the desired final shaping
and balancing of the airfoil.
The straight segments 30 and 32 of the blades 26a, 26b and 26c may
be formed of any convenient shape which provides minimal wind
resistance and which has sufficient tensile strength to support the
curved portion 28 under maximum stress conditions and are attached
in appropriate manner to the fasteners connected to curved portion
28. For example, the straight segments may be formed with an
airfoil shape to aid in providing a drive force or to minimize drag
resistance to rotor 10 by bending a sheet into an airfoil shape and
welding the trailing edges of the sheet, as shown by the straight
segment cross section 50a in FIG. 6a. However, since the straight
segments may contribute very little drive force due to their
position with respect to the rotors 14 and 16 and with respect to
shaft 12, economy may dictate the use of a simple circular hollow
or solid rod or other shape as indicated by the cross section 50b
in FIG. 6b. The straight segments are generally made of rigid
materials to support the blades when the turbine is at rest and may
include suitable supports (not shown) from shaft 12 to aid in this
support. There may also be applications where it would be desirable
to form the segments 30 and 32 out of a flexible material, such as
a steel cable, which would assume the troposkien shape upon
rotation of the turbine. In these arrangements, some other support
of the airfoil portions may have to be provided, as needed, when
the turbine is at rest.
As illustrated by the curve 40 in FIG. 5, rotor 10 must be driven
to a blade tip speed to wind velocity ratio of about 3 before the
rotor 10 blades begin to exert or provide a significant driving
force sufficient to offset drag, inertia, and other losses and to
accelerate the turbine to peak operating levels. In order to
achieve this velocity, starter rotors 14 and 16 are appropriately
supported at upper and lower portions of rotor 10 coupled to the
common shaft 12 and out of registry with curved portions 28 of the
drive rotor 10. A particularly effective starter rotor is
illustrated in FIG. 7 in which a pair of arcuate or simicircular
shaped rectangular vanes 52 and 54 are supported on shaft 12 with
hollowed portions facing in opposite directions with a portion of
each vane overlapping the shaft 12 and the other vane in a
generally S-shape fashion. With the vanes so positioned, wind
directed against the hollow portion or chamber on the inside of one
of the vanes, such as the portion 56 of vane 52, will apply a
driving force against vane 52 in the direction of the arrow 58 and
will be directed through the channel 60 between vane 52 and shaft
12 against the hollow portion of vane 54, again producing a driving
force in the direction of arrow 58. Such a rotor exhibits an
efficiency to rotational velocity ratio characteristic as indicated
by the curve 62 in FIG. 5 showing that the peak performance of the
rotor shown in FIG. 7 occurs at a ratio of approximately one. The
ratio of the diameter of rotor 10 to rotors 14 and 16 should thus
be sized to be from about 5 to 6 to 1, so that both the starting
and drive rotors are operating at their peak performance at about
the same rotational velocities. It has also been found that the
starter rotors 14 and 16 may be provided with a height which is
approximately the same as their diameter to minimize blocking of
the most effective portion, that is the curved portion 28 of rotor
10 as indicated in FIG. 1, or they may extend from said curved
portion 28 to beyond the ends of the drive rotor 10 blades. The
vanes 52 and 54 of the starter rotors may be made in the form shown
or with varible thickness in an airfoil shape to provide increased
efficiency. For purpose of economy, and since the additional
aerodynamic performance may not be significantly greater to warrant
the additional fabrication costs, the vanes 52 and 54 are
preferably formed from sheet metal with the vane chamber or hollow
portion forming a segment of an arc having constant radius. The
vanes of the upper starter rotor 14 should be positioned, as shown
in FIG. 2, so as to be out of phase with the vanesof the lower
starter rotor 16, that is, interdigitated or perpendicular one with
respect to the other, so that the wind turbine is self-starting
from wind coming from any direction and so as to smooth out the
starting torque produced by the starting rotors. Other types of
starter rotors, such as certain drag-type rotors may be utilized
but with lower over all efficiencies and drive power, such as the
type shown in FIG. 8 utilizing three buckets 62a, 62b and 62c
appropriately connected to shaft 12.
The respective rotors 10, 14 and 16 connected to the common shaft
12 may be rotated in a wind to a velocity of from 3 to 4 times that
of the wind by the proper proportioning of the size and radius of
the starter rotors and the power rotor, as described above. The
starter rotors will self-start without any external application of
power (other than wind) and will automatically regulate the correct
airfoil starting velocity as a function of any wind velocity within
the operating range and limitations of the turbine. The starter
rotor may continue to produce driving power even at the operating
velocity of the power rotor without degrading the latter operation.
With the blade design described above, the forces produced in the
blade are substantially tensile in nature and readily absorbed by
the system. The utilization means 24 may then be operated to
provide whatever power, energy or operation desired from the
rotation of the wind turbine in a highly efficient, simple and low
cost system.
If it is desired to provide increased driving torque but with
somewhat higher tensile stresses, the blades of rotor 10 may be
modified by positioning appropriate mass or weight members at the
junction between the straight segments and the curved portion of
the blade, such as shown by weight members 64 and 66 in FIG. 9.
These masses will tend to straighten out and change the arc of the
curved portion of the blades of the previous troposkien description
into a new arc shape or curved portion 28a which increases the
swept area of the rotor 10 blades. In other words, the airfoil
portion of the blades are more vertical and thus provide a greater
average radius from the rotor shaft to the drive portion of the
power blade and a greater area of blade sweep. Since the blade
curved portion is still in the form of an ace, the stresses within
the curved portion will still be tensile but may require a higher
strength joint or junction between the curve portion 28a and the
straight segments of the blade.
The blades of rotor 10 may be further modified by installing tip
plates of larger dimension than the blade cross section at the
junction between the curved portion 28 and the straight segments 30
and 32 of the power blades of rotor 10. These tip plates are most
effective when the angles of attack are high to increase the
effective aspect ratio (ratio of blade length to blade chord
length) of the blade airfoil by preventing the higher pressure air
inside the airfoil from "spilling" around the end of the airfoil
into the low pressure side. The tip plates may be installed
perpendicular to the blade as shown in FIG. 10a by tip 68a or
perpendicular to the vertical axis or shaft 12 of the turbine as
indicated by tip 68b in FIG. 10b. In the latter configuration, the
tip plate 68b would minimize interference with the air flow over
the blade itself and would not have to rotate against the air
stream at the rotational velocity or rotor 10.
Since the fabrication cost of a wind turbine of the type described
above may increase substantially as the size of the wind turbine is
increased and since wind velocities often increase with distance
above ground level, it may be desirable to stack wind turbines one
above the other on a common shaft 72, as indicated in FIG. 11 by
turbines 70a and 70b. Because of this increase in wind velocity
with height, it may also be desirable that the upper wind turbine
70b be provided with a diameter greater than lower turbines to
provide a more efficient utilization of the wind energy. The
turbines 70a and 70b (and additional stacked turbines) and their
common shaft 72 may be appropriately supported at the ground and
with suitable guy and collar arrangements 74a and 74b at
intermediate and upper positions of the turbines. The turbines can
thus be positioned so as to occupy a limited area of ground without
any wind interference between turbines. It will be understood that
these turbines may be provided with one or more similar starter
rotors as described above.
In order to protect the wind turbines of this invention from
excessive winds, the turbines may be provided with demountable or
foldable junctions or fasteners at the connection between the
curved portions and straight segments of the blades and between the
blades and shaft 12 so that the blades may be folded or collapsed
to a much smaller diameter which will have significantly lower wind
resistance and which may be suitably covered, if desired. For
example, if the blades of rotor 10 are provided, as shown in FIG.
12 with a hinge-like connector between each of the upper straight
blade segments 30' and 30" and curved portions 28' and 28" and
between the lower straight segments 32' and 32" and the vertical
shaft 12' , the lower straight segments are demountable from the
curved portion, the lower straight segments may be detached from
the curved portion of the blade and folded against the shaft while
the upper straight segment and curved portion are pivoted against
the shaft and appropriately fastened or strapped thereto. As can be
seen, the wind profile of the turbine is thus drastically
reduced.
* * * * *