U.S. patent application number 17/429087 was filed with the patent office on 2022-04-28 for shallow draft, wide-base floating wind turbine without nacelle.
The applicant listed for this patent is Northeastern University. Invention is credited to Andrew T. Myers, Jeremy J. Papadopoulos.
Application Number | 20220128033 17/429087 |
Document ID | / |
Family ID | |
Filed Date | 2022-04-28 |
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United States Patent
Application |
20220128033 |
Kind Code |
A1 |
Myers; Andrew T. ; et
al. |
April 28, 2022 |
SHALLOW DRAFT, WIDE-BASE FLOATING WIND TURBINE WITHOUT NACELLE
Abstract
Disclosed are wind turbines suitable for floating application.
The wind turbines include multiple floats and multiple towers
connected to the floats, a turbine rotor, including a hub and a
plurality of blades, structurally supported by the plurality of
towers, the turbine rotor coupled to an electrical generator; and
have a very shallow draft even for rated capacities of at least 1
MW. The wind turbines can have a single mooring line for yawing
eliminating the need for a nacelle, and can allow for deck-level
belt driven electrical generators without the need for gear
boxes.
Inventors: |
Myers; Andrew T.; (Milton,
MA) ; Papadopoulos; Jeremy J.; (Chestnut Hill,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Northeastern University |
Boston |
MA |
US |
|
|
Appl. No.: |
17/429087 |
Filed: |
February 18, 2020 |
PCT Filed: |
February 18, 2020 |
PCT NO: |
PCT/US20/18647 |
371 Date: |
August 6, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62896903 |
Sep 6, 2019 |
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62806085 |
Feb 15, 2019 |
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International
Class: |
F03D 13/25 20060101
F03D013/25; F03D 15/00 20060101 F03D015/00; B63B 35/44 20060101
B63B035/44; F03D 9/25 20060101 F03D009/25 |
Claims
1. A wind turbine, (i) comprising a) a plurality of floats and one
or more connectors interconnecting the plurality of floats; b) a
plurality of towers connected to the plurality of floats; and c) a
turbine rotor, including a hub and a plurality of blades,
structurally supported by the plurality of towers, the turbine
rotor coupled to an electrical generator; and (ii) (a) having a
rated capacity of at least about 1 MW and a draft of less than
about 1 meter per 1 MW of rated capacity, or (b) having a mass of
at least 30,000 kg and a draft of less than about one sixth of the
length of the blades.
2. The wind turbine of claim 1, having a draft of less than 10
meters.
3. The wind turbine of any one of the preceding claims, wherein the
plurality of floats are spaced apart defining an area A between the
floats, the blades have a length l, and l/ {square root over (A)}
is less than 5.
4. The wind turbine of any one of the preceding claims, wherein
connections between the plurality of floats include one or more
rigid connectors selected from beams and trusses.
5. The wind turbine of any one of the preceding claims, wherein
connections between the plurality of floats include taut
cables.
6. The wind turbine of any one of the preceding claims, wherein at
least two floats of the plurality of floats are rigidly
connected.
7. The wind turbine of any one of the preceding claims, wherein at
least three floats of the plurality of floats are rigidly
connected.
8. The wind turbine of any one of the preceding claims, wherein at
least four floats of the plurality of floats are rigidly
connected.
9. The wind turbine of any one of the preceding claims, wherein the
plurality of floats is 2 to 10 floats.
10. The wind turbine of any one of the preceding claims, wherein
the plurality of floats includes floats that are approximately
biconical.
11. The wind turbine of claim 10, wherein the floats which are
approximately biconical include an apex-up top cone with truncated
top and an apex-down bottom cone connected below the apex-up top
cone.
12. The wind turbine of claim 11, wherein the apex-down bottom cone
is truncated to provide a central hole.
13. The wind turbine of any one of the preceding claims, having a
mass of at least 30,000 kg and a draft of less than about one ninth
of the length of the blades.
14. The wind turbine of any one of claims 1 to 9, wherein the
plurality of floats are round surface piercing floats, and the wind
turbine has a draft of less than twice the diameter of the round
surface piercing floats.
15. The wind turbine of any one of the preceding claims, wherein
the floats are adapted to provide a maximum buoyancy of about 120%
to about 300% the weight-required buoyancy for the wind
turbine.
16. The wind turbine of any one of the preceding claims, wherein
the turbine rotor is positioned such that the blades rotate between
at least two towers of the plurality of towers.
17. The wind turbine of any one of the preceding claims, wherein
each tower of the plurality of towers is connected on top of a
float.
18. The wind turbine of any one of the preceding claims,
characterized by a heave frequency larger than about 0.2 Hz, when
it is not operating.
19. The wind turbine of any one of the preceding claims, wherein
the wind turbine has a mass of less than 1,000,000 kg.
20. The wind turbine of any one of the preceding claims, not having
a nacelle.
21. The wind turbine of any one of the preceding claims, wherein
the electrical generator is positioned between the plurality of
towers.
22. The wind turbine of any one of the preceding claims, wherein
the electrical generator, when the wind turbine is floating on
water, is positioned closer to the water than the turbine
rotor.
23. The wind turbine of any one of the preceding claims, wherein
the turbine rotor is coupled to the electrical generator with a
coupling comprising a sheave holding a belt, the sheave connected
to the turbine rotor to rotate with rotation of the turbine rotor
and the belt wrapping a smaller drum on a shaft which drives the
electrical generator.
24. The wind turbine of claim 23, wherein the sheave has a diameter
which is about 25% to about 35% the length of the blades.
25. The wind turbine of any one of the preceding claims, wherein
the sheave has a diameter which is about 10% to about 20% of the
diameter of the turbine rotor.
26. The wind turbine of any one of the preceding claims, further
comprising a gear box coupled to a shaft of the electrical
generator.
27. The wind turbine of any one of the preceding claims, wherein
the wind turbine, when floating, is adapted to allow the plurality
of towers to yaw as a single unitized structure to orient the
turbine rotor against the wind.
28. The wind turbine of any one of the preceding claims, further
comprising a mooring cable attached to a fixed underwater mooring
point, which, when under tension, points from the underwater
mooring point to the hub or to a point in space within a distance
from the hub which is less than about 20% of the length of the
blades.
29. The wind turbine of claim 28, wherein the mooring cable, when
under tension, has a slope of about 0.7:1 to about 3:1.
30. The wind turbine of claim 28 or 29, wherein the wind turbine
further comprises an axle to which the hub is rotably connected,
and the mooring cable is attached to a fixed underwater mooring
point and windward attached to the axle.
31. The wind turbine of claim 28 or 29, wherein the mooring cable
is supported windward of the base by a standoff structure.
32. The wind turbine of any one of claims 28, 29, and 31, wherein
the wind turbine comprises an axle to which the hub is rotably
connected, and the mooring cable is not attached to the axle.
33. The wind turbine of any one of claims 28 to 32, wherein yawing
of the turbine rotor is the result of movement of the entire wind
turbine.
34. The wind turbine of any one of the preceding claims, wherein
the plurality of towers is a plurality of lattice towers.
35. The wind turbine of any one of the preceding claims, wherein
the plurality of floats includes four floats configured in a square
arrangement with a distance of about 36 meters to about 72 meters,
the plurality of towers includes four lattice towers, each of the
flour lattice towers attached to the top of one of the four floats,
the four lattice towers sloping upward to structurally support the
turbine rotor, each pair of diagonally opposite floats being
rigidly connected.
36. A wind turbine comprising a) four floats configured in a
rectangular arrangement with a perimeter of about 144 meters to
about 288 meters, each pair of diagonally opposite floats being
rigidly interconnected; b) a turbine rotor, including a hub and a
plurality of blades, each blade having a length between about 70
and 130 meters; c) four lattice towers, each of the four floats
structurally connected on top to one of the four lattice towers,
the four lattice towers sloping upwards to structurally support the
turbine rotor positioned approximately above the centroid of the
rectangular arrangement; d) a sheave connected to the turbine rotor
to rotate with rotation of the turbine rotor, the sheave holding a
belt coupled to a shaft of an electrical generator, the sheave
having a diameter of about 15 to 40 meters; the electrical
generator mounted closer to the floats than the turbine rotor.
37. The wind turbine of claim 36, having a rated capacity of at
least 10 MW and a draft of less than about 5 meter.
38. A wind turbine, (i) comprising a) a plurality of floats and one
or more connectors interconnecting the plurality of floats; b) a
plurality of towers connected to the plurality of floats; c) a
turbine rotor, including a hub and a plurality of blades,
structurally supported by the plurality of towers, the turbine
rotor coupled to an electrical generator; and d) a mooring cable
attached to an underwater mooring point, which, when under tension,
points from the underwater mooring point to the hub or to a point
in space within a distance from the hub which is less than about
15% of the diameter of the turbine rotor, and with a slope of about
0.75:1 to about 3:1.
39. The wind turbine of claim 38, wherein the mooring cable is
positioned within the wind turbine at a position below 40% of the
height of the hub.
40. The wind turbine of claim 38 or 39, wherein the mooring cable
is attached to an underwater mooring point and attached to the
non-rotating axle.
41. The wind turbine of claim 38 or 39, wherein the mooring cable
is held windward of the base by a standoff structure.
42. The wind turbine of any one of claims 38, 39, and 41, wherein
the wind turbine comprises an axle to which the hub is rotably
connected, and the mooring cable is not attached to the axle.
43. The wind turbine of any one of claims 38 to 42, wherein yawing
of the turbine rotor is the result of movement of the entire wind
turbine.
44. The wind turbine of any one of claims 38 to 43, having a rated
power of at least about 1 MW and a draft of less than about 1 meter
per 1 MW of rated capacity.
45. The wind turbine of any one of claims 38 to 44, having a mass
of at least 30,000 kg and a draft of less than about one sixth of
the length of the blades.
46. The wind turbine of any one of claims 38 to 44, characterized
by a natural frequency in heave exceeding 0.2 Hz when mooring lines
have been removed.
47. The wind turbine of any one of the preceding claims, wherein
electric power generated by the wind turbine is used to produce
ammonia, hydrogen, liquid fuels, metals, or distilled water.
48. The wind turbine of any one of the preceding claims, comprising
a single mooring rope of controlled slope to prevent wind induced
pitch while permitting rising with the tide, and yawing to follow
the wind.
49. The wind turbine of any one of the preceding claims, comprising
a single mooring rope, wherein the mooring rope is a neutrally
buoyant rope to prevent catenary sag and compliance.
50. The wind turbine of any one of the preceding claims, having no
ballast.
51. The wind turbine of any one of the preceding claims, comprising
one or more pressure wheels on a belt carried by a sheave to permit
minimum belt tension.
52. The wind turbine of any one of the preceding claims, further
comprising a water source controlled to water-flood the sheave,
when driven, to prevent damage in the event of overload.
53. The wind turbine of any one of the preceding claims, wherein
the turbine rotor has a rotor axle extending through the hub which
is structurally supported by the towers at both ends of the rotor
axle.
54. The wind turbine of any one of the preceding claims, wherein at
least one tower is downwind and one tower is upwind.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of priority to U.S.
provisional patent application Ser. No. 62/806,085, filed Feb. 15,
2019, and U.S. provisional patent application Ser. No. 62/896,903,
filed Sep. 6, 2019. The entire teachings of the above applications
are incorporated herein by reference.
BACKGROUND
[0002] Offshore wind resources offer high average power density and
leading wind-power countries (Denmark, England, Germany) are
developing cost-effective ways to exploit them. Recently, the U.S.
Bureau of Ocean Energy management auctioned three lease areas 20
miles off the coast Martha's Vineyard for an astounding total price
of $405 M or $250 K/km.sup.2, a price comparable to that of
lucrative oil and gas leases. The three auction winners were
Equinor, Shell and Copenhagen Infrastructure Partners and the high
prices indicate the importance of this emerging market in the
United States. The development of these areas is expected to use
fixed bottom rather than floating structures because of the higher
technology readiness of fixed bottom technology. Offshore wind
turbines made with existing technology are expensive with capital
expenditures for installation around $4 M USD/MW of power capacity
and operational expenses around $160 K USD/MW/year. In Europe, the
fixed bottom offshore wind energy industry is fairly mature with
.about.15 GW of installed capacity. In the United States, the fixed
bottom offshore wind energy industry is in its infancy, but
expected to grow very quickly. But by simply transplanting a
heavy-tower land turbine and placing it on a massive underwater
base, costs are accepted that make the produced electricity (about
$0.10/kWh) unaffordable without subsidy (down to $0.07/kWh). And
even then this cost is not truly competitive compared to fossil
plants under $0.04/kWh). Globally, the floating offshore wind
energy is in its infancy, consisting of only a handful of
demonstration projects, none of which are in the United States.
These demonstrations also have essentially transplanted
conventional (slender tower with heavy nacelle) onshore turbines,
onto massive floating bases that exceed the bottom-fixed underwater
bases in cost. So their electricity is even more expensive.
[0003] In support of ocean wind turbines, U.S. states have
committed to generate a large defined power output from ocean wind
by 2030. About 2200 large (e.g. 10 MW, 120 m tower) offshore
turbines will be installed in ten years, a rate of 220/year. But to
seriously address climate change concerns, all U.S. fossil fuel
power plants need to be replaced as they retire. This will take
120,000 turbines. At the currently-planned installation rate this
will take an unacceptable 500 years. A lower-cost turbine producing
cost-saving electricity, and manufacturable in large volumes with
existing infrastructure, could profitably be installed at a rate of
4,000 per year, so fossil fuel electricity could theoretically be
supplanted by 2050.
[0004] Therefore, wind turbines, particularly for offshore
application, are needed that are much less expensive to build,
install, and operate on a per MW rated capacity basis compared to
current offshore wind turbines.
SUMMARY
[0005] Floating wind turbines are disclosed which are several times
less expensive on a per MW rated capacity basis compared to current
technology. They are far easier to manufacture by current
businesses with little investment, they are usable in deeper waters
off California and Maine, they can be installed quickly with
conventional small tugboats rather than slowly with expensive
European jack-up ships, and they can be inexpensively repaired
according to a swap-out and tow-to-port strategy, where a
replacement unit is installed in less than a day, and the damaged
unit is repaired at port.
[0006] One embodiment is a wind turbine, (i) comprising a) a
plurality of floats and one or more connectors interconnecting the
plurality of floats; b) a plurality of converging towers connected
to the plurality of floats; and c) a turbine rotor, including a hub
and a plurality of blades, structurally supported by the plurality
of towers, the turbine rotor coupled to an electrical generator;
and (ii) (a) having a rated capacity of at least about 1 MW and a
draft of less than about 1 meter per 1 MW of rated capacity, or (b)
having a mass of at least 30,000 kg and a draft of less than about
one ninth of the length of the blades.
[0007] Another embodiment is a wind turbine comprising a) four
floats configured in a rectangular arrangement with a perimeter of
about 160 meters to about 240 meters, the floats being rigidly
interconnected; b) a turbine rotor, including a hub and a plurality
of blades, each blade having a length between about 70 and 130
meters; c) four lattice towers, each of the four floats
structurally connected on top to one of the four lattice towers,
the four lattice towers sloping upwards to structurally support the
turbine rotor positioned approximately above the centroid of the
rectangular arrangement; d) a sheave connected to the turbine rotor
to rotate with rotation of the turbine rotor, the sheave holding a
belt coupled to a shaft of an electrical generator, the sheave
having a diameter of about 10 to 40 meters; the electrical
generator mounted closer to the floats than the turbine rotor.
[0008] A further embodiment is a wind turbine, (i) comprising a) a
plurality of floats and one or more connectors interconnecting the
plurality of floats; b) a plurality of towers connected to the
plurality of floats; c) a turbine rotor, including a hub and a
plurality of blades, structurally supported by the plurality of
towers, the turbine rotor coupled to an electrical generator; and
d) a mooring cable attached to a fixed underwater mooring point,
which, when under tension due to wind thrust, points from the
underwater mooring point to the hub or to a point in space within a
distance from the hub which is less than about 15% of the diameter
of the turbine rotor, and with a slope of about 1:3 to about 3:1.
In aspects of this embodiment, the slope is about 1:1.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The foregoing will be apparent from the following more
particular description of example embodiments of the invention, as
illustrated in the accompanying drawings. The drawings are not
necessarily to scale, emphasis instead being placed upon
illustrating embodiments of the present invention.
[0010] FIG. 1 provides an illustration of an embodiment of a 10 MW
rated capacity wind turbine suitable for offshore application; the
anchor system is not shown.
[0011] FIG. 2 provides an illustration of an embodiment of a 10 MW
rated capacity wind turbine suitable for offshore application, when
floating on water.
[0012] FIG. 3 provides a schematic view of an embodiment of a wind
turbine, when floating on water, and illustrating the use of a
mooring line to facilitate wind-following yawing of the wind
turbine; in deep (e.g., >300 m) water as illustrated here, three
mooring anchors position an underwater mooring buoy, in less deep
water a single anchor suited to omnidirectional pull is
appropriate.
[0013] FIG. 4 illustrates force vectors for the wind turbine in
FIG. 3, specifically, of the mooring force, thrust, buoyancy, and
gravity. It shows that a mooring line directed at a common center
of force results in buoyant support with a fixed line of action,
hence no turbine pitch; and that the towers are then loaded purely
vertically.
[0014] FIG. 5 shows an embodiment of the connection between lattice
towers and a rotor axle supported at both ends, of a wind turbine
of the present disclosure.
[0015] FIG. 6 illustrates an embodiment of a hub where three blade
mounts join to a bore tube that will revolve about an axle fixed to
the towers.
[0016] FIG. 7 provides a schematic cross-sectional view of a
turbine rotor and how it can be structurally supported by the
lattice towers. It includes a fixed tubular axle, hub roller and
thrust bearings, blade pitch bearings with elastic matching, a
rapid blade-pitch system driven by rotor rotation, a welded tubular
hub, and a large belt sheave if belt drive is used.
[0017] FIG. 8 illustrates steps of an embodiment of a method for
crane-less construction of a wind turbine of the present
disclosure, including the steps of hoisting the growing structure
from the ground (position A) to a height (position B) sufficient to
move and structurally connect (e.g., weld) a further tower segment
(in or from position C) to the elevated tower segment (in hoisted
position B).
[0018] FIG. 9A provides a schematic side-view of an embodiment of a
short lifting tower suitable for hoisting of the growing turbine
according to the method illustrated in FIG. 8.
[0019] FIG. 9B further illustrates the embodiment of a lifting
tower shown in FIG. 9A.
[0020] FIG. 9C illustrates the lifting tower of FIG. 9A and FIG. 9B
with a leg (e.g., lattice tower) of the partially assembled wind
turbine structure jacked to the height at which a further leg
segment can be structurally attached at the bottom of the elevated
leg segment.
[0021] FIG. 10 illustrates a float including a rudder-shaped
open-bottom tube which allows for both steering (by rotation to a
desired direction) and vertical force control (by valving of air
above an enclosed water column).
DETAILED DESCRIPTION
[0022] The various concepts introduced above and discussed in
greater detail below may be implemented in any of numerous ways, as
the described concepts are not limited to any particular manner of
implementation. Examples of specific implementations and
applications are provided primarily for illustrative purposes.
[0023] Floating wind turbines are disclosed which are several times
less expensive on a per MW basis compared to current bottom-fixed
or floating support technology. Advantages of the disclosed wind
turbines include that they can be several times (e.g., four to ten
times) lighter than current technology and they can have a much
shallower draft than current technology, enabling quayside
construction and launch in shallow ports requiring little or no
dock-strengthening, and no heavy cranes nor special installation
ships, and saving greatly on capital expenditures. The thin metals
associated with light weight provide a great advantage in
manufacturing, because they can be bent and joined with existing
common fabrication equipment. So unlike existing offshore turbines
that cannot be built in the U.S. until billions of dollars of
manufacturing and transport upgrades are made, the disclosed
invention can be manufactured without delay, leading to many U.S.
jobs. Additionally, unlike any existing or planned offshore
turbines, the disclosed invention is suited to rapid installation
by small-boat towing and a quick connection to the mooring line and
electric cable. This alters the maintenance equation, because when
a turbine is either damaged or due for refitting, a replacement can
first be installed (with only 1 day of downtime) and the damaged
unit can be returned to port for repair at leisure, with more
resources and lower daily cost. Further advantages include that the
wind turbines can have fewer wearable components than existing
technology, and some of the wearable components that remain can be
located near sea level, rather than up in the hub, where they are
much more easily maintained, saving on operation and maintenance
costs. For example, the gearing and generator need not be sited
near or at the rotor axis. Further advantages include that a
nacelle is not needed or does not need to slew, the tower does not
need to be slender (hence heavy), and compared to the tremendous
ballast of current floating technology, little or none is
needed.
[0024] In brief, once a turbine can orient to the wind by swinging
at anchor, many expensive features may be avoided. The nacelle does
not need to slew, the tower does not need to be slender, and the
gearing/generator need not be sited at the rotor axis. These
factors allow the weight to be reduced and lowered to deck level,
leading to reduced flotation requirements.
[0025] Compared with existing offshore turbines, the wind turbines
of the present disclosure can also use a cheaper generator, and, in
embodiments, dispense with a nacelle yawing system and/or a
gearbox.
[0026] One embodiment is a wind turbine, (i) comprising a) a
plurality of floats and one or more connectors interconnecting the
plurality of floats; b) a plurality of towers connected to the
plurality of floats; and c) a turbine rotor, including a hub and a
plurality of blades, structurally supported by the plurality of
towers, the turbine rotor coupled to an electrical generator; and
(ii) (a) having a rated capacity of at least about 1 MW and a draft
of less than about 1 meter per 1 MW of rated capacity, and/or (b)
having a mass of at least 30,000 kg and a draft of less than about
one sixths of the length of the blades.
[0027] A further embodiment is a wind turbine, comprising a) a
plurality of floats and one or more connectors interconnecting the
plurality of floats; b) a plurality of towers connected to the
plurality of floats; and c) a turbine rotor, including a hub and a
plurality of blades, structurally supported by the plurality of
towers, the turbine rotor coupled to an electrical generator.
[0028] A further embodiment is a wind turbine, comprising an axle,
a hub with blades connected to the hub, the hub rotatablely
connected to the axle, a plurality of towers converging to and
structurally supporting the axle, and the hub coupled to an
electrical generator.
[0029] As used herein, the hub "coupled" to an electrical
generator, refers to a coupling that allows transfer of rotational
energy of the hub to rotational energy of a shaft that drives the
electrical generator. In embodiments, this coupling is achieved by
use of a mechanical drive system comprising a sheave fixedly
connected to the hub, a belt, a sheave of a first diameter, a drum
of a second diameter connected to a shaft which drives the
electrical generator, the first diameter being larger than the
second diameter (e.g., 15:1 to 40:1).
[0030] A further embodiment is a wind turbine, wherein, when
floating, the wind turbine has a part below water, and the below
water part has a weight, not including weight of ropes and anchors,
that is less than 50% of total weight of the wind turbine.
[0031] In some embodiments, the wind turbine, when floating, has a
part below water, and the below water part has a weight, not
including weight of ropes and anchors, that is less than 50% of
total weight of the wind turbine.
[0032] In some embodiments, the blades of the wind turbine form
more than 20% of system weight, not including weight of ropes and
anchors.
[0033] In some embodiments, the turbine has a weight in metric tons
(MT) that is less than 35*(rated capacity of the wind turbine in
MW){circumflex over ( )}1.5.
[0034] In some embodiments, the wind turbine has a rated capacity
of at least about 500 kW, at least about 1 MW, at least about 2 MW,
at least 5 MW, or at least about 10 MW. In other embodiments, the
wind turbine has a rated capacity of about 500 kW to about 20 MW.
In aspects of these embodiments, the wind turbine has a draft of
less than about 1 meter per 1 MW of rated capacity, less than about
2 meter per 1 MW of rated capacity, or less than about 50 cm per 1
MW rated capacity.
[0035] As used herein, "rated capacity" refers to the intended
full-load sustained output of a wind turbine.
[0036] In some embodiments, the wind turbine has a draft of less
than about 1 meter per 1 MW of rated capacity, less than about 2
meter per 1 MW of rated capacity, or less than about 50 cm per 1 MW
rated capacity.
[0037] In some embodiments, the wind turbine further comprises an
axle and the hub is rotatably connected to the axle. During
operation of the wind turbine (i.e, typically, when the wind speed
is at least the cut-in speed) the hub, with the blades attached to
it, rotates around the axle, the axle being structurally supported
by the plurality of towers.
[0038] The wind turbines described herein have a shallow draft
compared to conventional floating wind turbines. As used herein,
"draft" refers to the lowest point of a wind turbine, when floating
offshore and without substantial wind (i.e., less than 5 mph),
below the water level, except for non-buoyant material such as, for
example, power cables, mooring line(s), or tubes with opening to
allow water to fill the tube to a certain fill level. Thus, for
illustration purposes, if a wind turbine structure has a solid
metal rod extending vertically 50 meters below water level and
thereby also below all of the floats, this does not mean that the
draft of that wind turbine is 50 meters. Typically, the draft of a
wind turbine is the depth above which 90% of the submerged buoyant
volume is found.
[0039] In some embodiments, the wind turbine has a mass of at least
about 30,000 kg, of at least about 100,000 kg, of at least about
250,000 kg, of at least about 500,000 kg, or at least about
1,000,000 kg; and a draft of less than about one sixth, one
seventh, one eighth, or one ninth of the length of the blades. In
an aspect of this embodiment, the draft is less than about one
ninth of the length of the blades.
[0040] In some embodiments, the wind turbine has a draft of less
than about one sixth, one eighth, one ninth, or one tenth of the
length of the blades.
[0041] In some embodiments, the wind turbine has a draft of less
than 15 meters, of less than 10 meters, of less than 8 meters, of
less than 6 meters, of less than 5 meters, of less than 3 meters,
or less than 2 meters.
[0042] In some embodiments, the wind turbine comprises a plurality
of floats which are spaced apart defining an area A between the
floats, the blades have a length L, and L/ A is less than about 3.
The area A is measured in the plane at waterline, the vertices of
the area A being provided by the centroids of the floats in that
plane.
[0043] In some embodiments, the wind turbine comprises a plurality
of floats which are spaced apart defining an area A between the
floats, the blades have a length L, and L/ A is between about 1 and
about 3.
[0044] In some embodiments, the wind turbine comprises a plurality
of floats which are spaced apart defining an area A between the
floats, the blades have a length L, and L/ A is between about 1 and
about 2.5.
[0045] In some embodiments, the wind turbine has connections
between the plurality of floats which include one or more rigid
connectors selected from beams and trusses.
[0046] In some embodiments, the connections between the plurality
of floats include tensioned cables.
[0047] In some embodiments, at least two floats of the plurality of
floats are rigidly connected. In an aspect of this embodiment,
floats in positions approximately opposite to the centroid of the
arrangement formed by the floats are connected with rigid
connectors.
[0048] Rigid connectors can be, but are not limited to beams and
trusses. A variety or materials in a variety of shapes can be used.
Typically, for sea water applications of the turbine, materials of
the wind turbine exposed to sea water will be chosen to be
corrosion resistant or must be painted. Such materials can be
commercially obtained.
[0049] In some embodiments, at least three floats of the plurality
of floats are rigidly connected.
[0050] In some embodiments, at least four floats of the plurality
of floats are rigidly connected.
[0051] While the number of floats of the wind turbine is not
limited, a large number (e.g., over 10) floats is less efficient.
In some embodiments, the wind turbine of any one of the preceding
claims, wherein the plurality of floats is 2 to 10 floats.
[0052] As referred herein, a "float" provides at least 5% of the
required buoyancy for the entire wind turbine. If it provides less,
it is not considered a "float". The shape of the floats is not
limited to a particular shape; however, more typical floats have a
shape that includes cylindrical or conical shapes, for example,
bifrustum (e.g., biconical), one-sided truncated bifrustum (e.g.,
one-sided truncated biconical), or two-sided truncated bifrustum
(e.g., truncated biconical). Additionally, typically, the float has
an approximately equiaxed geometry in which the width (at the
waterline) is not much less than the total depth of penetration or
draft, nor much greater (a flat barge is structurally
inefficient).
[0053] In some embodiments, the plurality of floats includes floats
that are approximately biconical.
[0054] In some embodiments, the floats which are approximately
biconical include an apex-up top cone with truncated top and an
apex-down bottom cone connected below the apex-up top cone.
[0055] In some embodiments, the apex-down bottom cone is truncated
to reduce the draft. The truncation may be solidly capped, or it
can be left open to partially equalize internal pressure (where the
air may be trapped by a membrane or bladder), or it can be left
open for the purpose of mounting a rudder or a water trapping
tube.
[0056] In some embodiments, the plurality of floats are cylindrical
surface piercing floats, and float has a draft of less than half
its diameter.
[0057] In some embodiments, the plurality of floats are adapted to
provide a maximum buoyancy of about 120% to about 300% the
weight-required buoyancy for the wind turbine
[0058] In some embodiments, the turbine rotor is positioned such
that the blades rotate between at least two towers of the plurality
of towers: at least one upwind tower, and at least one downwind
tower, which support the fixed axle from both ends in a way that
reinforces the tower structure. In an aspect of this embodiment,
the wind turbine does not have a nacelle.
[0059] In some embodiments, each tower of the plurality of towers
is connected on top of a float.
[0060] In some embodiments, the wind turbine is characterized by a
heave frequency larger than about 0.2 Hz, when it is not
operating.
[0061] In some embodiments, the wind turbine has a mass of more
than 1,000,000 kg.
[0062] In some embodiments, the wind turbine does not have a
nacelle.
[0063] In some embodiments, the wind turbine is floating.
[0064] In some embodiments, the electrical generator is mounted on
a structural support connected to the one or more connectors.
[0065] In some embodiments, the electrical generator, when the wind
turbine is floating on water, is positioned closer to the water
level than the turbine rotor.
[0066] In some embodiments, the turbine rotor is coupled to the
electrical generator with a coupling comprising a sheave holding a
belt, the sheave connected to the turbine rotor to rotate with
rotation of the turbine rotor and the belt connected to a shaft
which drives the electrical generator.
[0067] In some embodiments, the sheave has a diameter which is
about 10% to about 30% the length of the blades.
[0068] In some embodiments, the sheave has a diameter which is
about 5% to about 40% of the diameter of the turbine rotor.
[0069] In some embodiments, the wind turbine further comprises a
gear box coupled to the electrical generator.
[0070] In some embodiments, the wind turbine, when floating, is
moored so as to allow the plurality of floats and towers to yaw as
one to orient the turbine rotor against the wind.
[0071] In some embodiments, the wind turbine, when floating, is
adapted to allow the plurality of towers to yaw to orient the
turbine rotor against the wind.
[0072] In some embodiments, the wind turbine comprises a mooring
cable whose lower end is attached to an underwater mooring point,
which, when under tension, points from the underwater mooring point
to the hub or to a point in space within a distance from the hub
which is less than about 20% of the length of the blades.
[0073] In some embodiments, the mooring cable, when under tension,
has a slope of about 0.5:1 to about 3:1. In aspects of this
embodiments, the mooring cable has a slope of about 1.5:1.
[0074] In some embodiments, the mooring cable is attached to an
underwater mooring point and attached to the rotor axle.
[0075] In some embodiments, the mooring cable is held windward of
the tower or floats by a standoff structure.
[0076] In some embodiments, the mooring cable is not attached to
the rotor axle.
[0077] In some embodiments, yawing of the turbine rotor is the
result of movement of the entire wind turbine.
[0078] In some embodiments, the towers are lattice towers.
[0079] In some embodiments, the wind turbine includes a plurality
of floats, wherein the plurality of floats includes four floats
configured in a square arrangement with a distance of about 30
meters to about 60 meters, the plurality of towers includes four
lattice towers, each of the flour lattice towers attached to the
top of one of the four floats, the four lattice towers sloping
upward to structurally support the fixed rotor axle, each pair of
diagonally opposite floats being rigidly connected.
[0080] In some embodiments, the wind turbine includes a plurality
of floats, wherein the plurality of floats includes four floats
with a distance between any two of the four floats of about 36
meters to about 72 meters, the plurality of towers includes four
lattice towers, each of the flour lattice towers attached to the
top of one of the four floats, the four lattice towers sloping
upward to structurally support the fixed rotor axle, each pair of
diagonally opposite floats being rigidly connected.
[0081] In some embodiments, the wind turbine comprises a) four
rigidly interconnected floats configured in a rectangular
arrangement with a perimeter of about 144 meters to about 288
meters; b) a turbine rotor, including a hub and a plurality of
blades, each blade having a length between about 70 and 130 meters;
c) four lattice towers, each of the four floats structurally
connected on top to one of the four lattice towers, the four
lattice towers sloping upwards to structurally support the turbine
rotor positioned approximately above the centroid of the
rectangular arrangement; d) a sheave connected to the turbine rotor
to rotate with rotation of the turbine rotor, the sheave holding a
belt coupled to a shaft of an electrical generator, the sheave
having a diameter of about 10 to 40 meters; the electrical
generator mounted closer to the floats than the turbine rotor.
[0082] In some embodiments, the wind turbine has a rated capacity
of at least 10 MW and a draft of less than about 5 meter.
[0083] In some embodiments, the wind turbine, (i) comprises a) a
plurality of floats and one or more connectors interconnecting the
plurality of floats; b) a plurality of towers connected to the
plurality of floats; c) a turbine rotor, including a hub and a
plurality of blades, structurally supported by the plurality of
towers, the turbine rotor coupled to an electrical generator; and
d) a mooring cable attached to an underwater mooring point, which,
when under tension, points from the underwater mooring point to the
hub or to a point in space within a distance from the hub which is
less than about 15% of the diameter of the turbine rotor, and with
a slope of about 0.5:1 to about 3:1.
[0084] In some embodiments, the wind turbine includes a mooring
cable, and the mooring cable is attached at the rotor axle, a tower
near the rotor axle, or a standoff structure above or below the
water surface.
[0085] In embodiments, the wind turbine includes a mooring cable
attached to a standoff structure. In aspects of this embodiment,
the standoff structure extends beyond the area defined between the
floats.
[0086] In some embodiments, a position along the upper length of
the mooring cable is connected to the wind turbine at a position
below 40% of the height of the hub. As used herein, "mooring cable
is connected" refers to a connection adapted for the wind forces
expected during operation of the wind turbine.
[0087] In some embodiments, the mooring cable is attached to an
underwater mooring point and attached to the hub.
[0088] In some embodiments, the mooring cable is held windward of
the base by a standoff structure.
[0089] In some embodiments, the mooring cable is (i) not attached
to the hub, and/or (ii) attached to a structure of the wind turbine
within 10 m of water level.
[0090] In some embodiments, the yawing of the turbine rotor is the
result of movement of the entire wind turbine.
[0091] In some embodiments, the wind turbine has a rated capacity
of at least about 1 MW and a draft of less than about 1 meter per 1
MW of rated capacity.
[0092] In some embodiments, the wind turbine is characterized by a
natural frequency in heave exceeding 0.2 Hz when mooring lines have
been removed.
[0093] In some embodiments, electric power generated by the wind
turbine is used to produce ammonia, hydrogen, liquid fuels, reduced
metals, or distilled water at a nearby floating plant.
[0094] In some embodiments, the wind turbine comprises a single
mooring cable of controlled slope to prevent wind induced pitch
while permitting rising with the tide, and yawing to follow the
wind.
[0095] In some embodiments, the wind turbine comprises a first
mooring cable and a second mooring cable, both mooring cables
attached underwater along their bottom lengths at a single
underwater mooring point, the first mooring cable attached along
its top length to a first standoff structure at a first connection
point, the second mooring cable attached along its top length to a
second standoff structure at a second connection point, the rotor
being positioned vertically above a rotor point at water level, and
the rotor point being between a first line, defined by the first
underwater mooring point and the first connection point, and a
second line, defined by the second underwater mooring point and the
second connection point. In aspects of this embodiment, the first
standoff structure and the second standoff structure are the same
structure. In a further aspect, the first and/or second mooring
cable can be controlled in their lengths, and controlling the
lengths changes the wind turbine yaw.
[0096] In some embodiments, the wind turbine comprises a single
mooring rope, wherein the mooring rope is a neutrally buoyant
synthetic rope to prevent catenary sag and compliance.
[0097] Another mooring option is to use a buoy fixed in position at
the surface, and a rotational connection of the turbine standoff to
that fixed point, wherein the connection has a sloped or slanted
freedom to move up and down with wind turbine rise due to, for
example, waves. For example, a slanted tube, or a rod with a
wheeled carriage, such that the rod or tube normal points at or
near the rotor center, provides the proper mooring force
direction.
[0098] In some embodiments, the wind turbine has substantially no
ballast.
[0099] In some embodiments, the wind turbine further comprises one
or more pressure wheels on a belt carried by a sheave to permit
reducing belt tension without causing slip.
[0100] In some embodiments, the wind turbine further comprising a
water source controlled to allow for water flooding of driven
sheave during overload to prevent slip damage.
[0101] In some embodiments, the turbine rotor has a rotor axle
extending through the hub which is structurally supported by the
towers at both ends of the rotor axle.
[0102] In some embodiments, at least one tower is downwind and one
tower is upwind.
[0103] An further embodiment is a shallow draft, wide-base floating
wind turbine without nacelle, secured by a properly sloped
anti-pitch mooring line, and transmitting power to base with a
step-up belt drive.
[0104] A further embodiment is a floating wind turbine supported on
at least three, approximately equi-axed floats with rigid
interconnection, with draft in no-wind conditions less than 1.0
times float diameter or equivalent diameter (D-float or
sqrt(A-float)), where A-float is measured at the waterline.
[0105] A further embodiment is a floating wind turbine with at
least one tower leg upwind of the rotor, at least one tower leg
downwind of the rotor, wherein each tower leg is supported on a
distinct equiaxed float, where the leg-support floats have no-wind
draft less then 1.0 sqrt(A-float). In an aspect of this embodiment,
the wind turbine does not have a nacelle.
[0106] In some embodiments, one or more equal-altitude-angle
mooring lines from a single fixed mooring point have a mean
direction which aims near the rotor center. In an aspect of these
embodiments, the one or more mooring lines are connected to the
turbine structure within 1/3 of hub height from water surface.
[0107] In some embodiments, mooring of the wind turbine is to a
heavy SPM surface buoy via a sloped low-friction interface which
constrains mooring force direction to aim at the rotor.
[0108] Well known quick connect technology for the rope and
electrical cable can be used to permit floating turbine swap out in
a few hours, for convenient and safe in-port maintenance without
interruption of power production.
[0109] In some embodiments, the wind turbine has a draft in meters
which is less than three times the square root of the rated
capacity in MW.
[0110] In some embodiments, the wind turbine has a draft which is
less than 2.5 times the square root of the rated capacity.
[0111] In some embodiments, the wind turbine has a draft which is
less than 2.0 times the square root of the rated capacity.
[0112] A further embodiment is a floating turbine with draft less
than 2*sqrt(rated capacity) with a standoff upwind ballast system
comprising struts to a floating base and one or more tensile
element to the top of the tower, supporting a water-containing
vessel (e.g., such as a fabric bladder or vertical thinwall capped
steel tube or cone).
[0113] A further embodiment is a wind turbine with rated power
exceeding 0.1 MW with natural frequency in heave (Hertz) that is
greater than 0.3*(1/MW){circumflex over ( )}0.25.
[0114] In some embodiments, the wind turbine has a rated power
exceeding 0.1 MW and a natural frequency in heave (Hertz) that is
greater than 0.3*(1/MW){circumflex over ( )}0.25.
[0115] A further embodiment is a floating wind turbine, comprising
a rotor but no nacelle, at least one supporting tower upwind of the
rotor and at least one supporting tower downwind of the rotor.
[0116] A further embodiment is a wind turbine, comprising a
plurality of low-draft floats rigidly connected to form a floating
island, wherein the floating island has a width that carries a
spread-leg tower supporting a turbine rotor. In an aspect of this
embodiment, the wind turbine has a single mooring rope from a
single mooring point, not pointing toward the rotor center.
[0117] A further embodiment is a wind turbine having a light
multi-float shallow draft platform, with a spread base tower, and a
rope connection point upwind of the turbine. In an aspect of this
embodiment, the wind turbine further comprises a tank connected to
a standoff with compressive strength, with a tension member (e.g.,
rope) from the standoff to the top of the tower. In aspects of this
embodiment, the tank can be filled or emptied by pumps, or by
removing or injecting air above an open-bottom water column,
possibly in connection with wave pressures raising and lowering the
water level in the tank.
[0118] If a single mooring rope or multiple mooring ropes attach to
the underside center of the floating island, wind yawing (automatic
aiming) of the turbine will typically not occur. And ballast may be
needed. But the island could be rotated, for example, with the use
of propeller thrusters.
[0119] If dual mooring ropes from a single mooring point attach to
two points on the island, the island will always `face` the mooring
point. Adjustable ballast could still be needed. But if the waves
affect the turbine position, it may not accurately face the wind.
Then the island orientation could be altered to precisely face the
wind by adjusting the lengths of the two cables.
[0120] Multi-line mooring may also prevent island rotation. That
is, multiple lines to multiple attachment points. There could be
three somewhat slack ropes from three anchors, and the wind would
pull one or two tight. (Or there could be multiple heavy catenary
chains from points around the island.) If the ropes do not point
toward the rotor, then ballast may or may not be needed. In order
to adjust yaw to maintain turbine alignment with the wind, either
the mooring rope attachment points could be on carriages that are
motor-moved around the island, in effect rotating the island
relative to the anchors. Or the towers could sit on wheels (like a
land embodiment) so the tower can be motor-rotated on a circular
rail on the island. Any ballast would also have to rotate with the
tower.
[0121] Exemplary embodiments and aspects of the disclosed wind
turbines further are (1) a mooring system, (2) a mechanical drive
system from rotor to generator, (3) a light weight tubular hub and
axle, (4) needle bearings for rotor turning, (5) needle bearings
for blade pitch, (6) a rapid blade-pitch mechanism without electric
or hydraulic actuator, (7) lattice towers that diverge to a wide
base, (8) craneless erection on unimproved soil, followed by
air-cushion launch into shallow water, (9) multifunctional turbine
floats, and (10) control of wave-driven resonant motion in the
water, and are described in the following. Suggested dimensions
relate to a 10 MW rotor as devised for the DTU (Danish Technical
University) reference turbine.
[0122] These embodiments and aspects of the wind turbine are can
also be advantageous on land. For example, the above (3), (4), (5),
and (6). (2), (7) and (8) can be employed if a land turbine has the
space to mount four legs to a circular rail. Crane-less erection on
land (as described below) can also be advantageous. Such an
embodiment would have no foundation, a cheaper and easily
transported and erected tower, the resulting cheaper hub and axle,
and a cheaper and overload tolerant belt drive.
Mooring System, Including Cable-Free
[0123] The use of an angled mooring force aimed at the rotor center
can provide a number of advantages. FIG. 1, FIG. 2, FIG. 3 and FIG.
4 each illustrate a submerged mooring line held by a mooring
standoff with a collinear tension brace connected to the fixed axle
of the turbine rotor. The angled mooring line, aimed at the rotor
center, can be connected at or near the rotor center, or connected
somewhat away from the rotor center, for example between 30 m below
and 30 m above the waterline. The intended effect can be achieved,
for example, with a two-part line, one connected to the structure
of the wind turbine including the floats, connectors between the
floats and any mooring standoff, which provides a platform for the
towers, turbine rotor and generator) to moor the turbine, and a
collinear brace from the line connection point to a non-rotating
point near the rotor center. While the slope can vary, a slope of
about 1.5 is economically beneficial and provides a good tradeoff
between distance from anchor point, versus downpull on floating
turbine requiring additional buoyancy. Whereas a slope of about 0.5
reduces fore-aft surge motion from wave lift, but requires either a
much longer standoff, or leaves the cable moving about freely in
air.
[0124] With a single mooring line, the path of the line defines the
direction and line of action of the mooring force. Accordingly, the
mooring line typically aims at or in the vicinity of the rotor
center. In case of a SPM buoy (a single point mooring), this
typically exerts only horizontal force, not aimed at the rotor
center. To rectify this defect, the connection point of the turbine
can be attached to a carriage on the SPM buoy, that slides up and
down a sloped track. The line perpendicular to the track is the
mooring force. So the track could have a slope of -0.67, then the
mooring force could have a slope of -1/(-0.67)=1.5, pointing at the
rotor center.
[0125] FIG. 3 provides a schematic view of a wind turbine 305 of an
embodiment described herein (e.g., the floating wind turbine of
FIG. 1 or FIG. 2) floating on water 310. In this schematic
cross-sectional view, only two floats 312 (here, of approximately
bifrustum shape (e.g., here, truncated top cone connected to bottom
cone)). The wind turbine 305 includes a mooring standoff 315
providing a suitable standoff distance (i.e., distance from a
turbine structure attachment point (e.g., slip ring 320), where a
mooring line or cable 325 is held or connected (here, held by
confinement to the slip ring 320 opening) to the mooring standoff
315, to near the fixed axle 330 of the wind turbine 305,
approximately above the center of gravity (cg) of the wind turbine
335. 327 is a collinear brace transmitting mooring force to (or
near) the rotor axle. 327 can be a continuation of mooring cable
325 (i.e., one single cable extending from 340 to 330) The slope of
the mooring line 325 (here, 1.5:1 (illustrated with rise 3 divided
by run 2)) from a point underwater 340 (e.g., a buoy or a seafloor
point if the water is <300 m deep) where the mooring line is
held or attached, to the attachment point (e.g. slip ring 320) is
determined by the length of the mooring line 325 from point 340 to
point 320. For a given depth of point 340 in the water, the length
of the mooring cable determines a yaw radius (e.g., here
illustrated as R.sub.yaw=167 m). When the mooring line 325 extends
all the way to the hub 330 (not illustrated here, where a rotary
junction with slip ring conveys mooring force from cable 325 to
cable 327), typically, the length of the mooring line and the
standoff distance is chosen such that the slope of the mooring line
is about the same below and above the attachment point 320. FIG. 3
further illustrates that the underwater buoy 340 can be held in
place by three or more (here, three) lines or cables 345
attached/anchored to the sea floor 350. When power generated by the
wind turbine needs to be transmitted, a power cable 355 for this
transmission can be placed and attached along the mooring standoff
315 and along the mooring line 325 from about the location of the
mooring holding point 320 to the buoy 340 to then reach the
seafloor 350. A tension limiter 360 can optionally be used, to
prevent damage from a moving turbine being brought up short by a
suddenly-taut mooring cable.
[0126] FIG. 4 illustrates force vectors for the wind turbine in
FIG. 3, specifically of the mooring force F.sub.mooring, thrust
F.sub.thrust, buoyancy F.sub.buoyancy, and gravity F.sub.gravity.
These force vectors will typically need to be considered for any
floating wind turbine. If Fb and Fg are vertically aligned, then Fm
must point at the intersection of Fb+Fg and Ft. This will occur
without any pitch if Fm already points at the rotor center in the
case of small Ft.
[0127] The best way to control mooring line force angle is to
employ neutrally buoyant lines, which pass straight from an anchor
point to the turbine attachment point. (A steel line or chain would
require floats to remain straight at small tensions.) If there is a
water current impinging on the line, freely rotatable light plastic
shrouds (streamline covers) can be used that will align to the
current, cutting drag by a factor of 10 or so to minimize current
force tending to displace and curve the cable.
[0128] The sloped mooring force need not be produced by a sloped
rope extending downwards and away from the hull. If a Single Point
Mooring (SPM) structure has been provided at the water surface, its
interaction with the turbine can be via a non-vertical attachment
guide, so the force on the turbine has the desired altitude slope.
For example, a roller on the float, interacting with a slanted rod
on the turbine structure, or vice versa, will produce a mooring
force aimed in the vicinity of the rotor center. Various other
geometric or linkage arrangements can also be used to control the
mooring force direction at the attachment point.
[0129] If single or double angled mooring lines are used, which is
an easy way to control the mooring force direction, their length
must be considered. If very short (for example, 10 m) then heave of
the turbine will change the line angle a lot. Connected to the
rotor hub this would not matter, but connected to the hull it will
mean some wind-induced pitch of the turbine. The short-line case,
for example, in the case of shallow water, is a good application
for the above-mentioned slanted rod or equivalent.
[0130] Normally the force vector variation due to turbine heave is
minimized by making the line longer than, for example, 50 m. But if
it ends up very long (in deep water), at a shift in wind direction
the turbine has to travel a large distance around the watch circle,
so realignment is slow. Additionally, the distances between wind
turbines would be large to ensure that wind turbines do not hit
each other during realignment. Accordingly, when water is deep,
mooring the turbine to a subsurface buoy, tethered in a fixed
location, for example, by three converging anchor lines, as shown
in FIG. 3, is preferred. This reduces the radius of the circle
traveled by the turbine (i.e., the yawing radius), however it adds
the cost of additional anchors and the buoy; and it impedes fishing
over a greater area. Thus, typically, the single-line lengths
(attaching to a hull or mooring-standoff) will be between 30 m and
300 m, with a three-line buoy only if necessary in deeper
water.
[0131] In embodiments where the wind turbine includes a mooring
line, needed additional features are "mooring line overload fuse",
and "mooring line twist prevention" (particularly in relation to
the electric power cable) (See FIG. 3 which illustrates a power
cable 355 positioned along part of the path of the mooring
line).
Overload Fuse
[0132] The mooring overload fuse is related to the potential impact
damage of a straight line, if the turbine surges toward, then away
from, the anchor or mooring point (due to waves, in light wind).
The line will slacken, then after the turbine reverses direction
the line will suddenly become straight and stiff, resulting in a
high-force impact. This could damage the line, damage the hull, or
unseat the anchor. It is one of the reasons for using expensive
curved catenary chains. For a horizontal wind thrust of, for
example, 200 MT the sloped mooring line will experience a steady
tension (360 MT) which is proportional. Anchors and lines can be
sized for double or triple that, in case they degrade over time.
High impact force can be reduced or prevented by capping the
tension with an overload fuse set to roughly 120% of expected
steady tension (440 MT).
[0133] A reversible force limiter (e.g., tension limiter 360 in
FIG. 3) can be, for example, a collapsed piston-in-cylinder,
installed in line with the mooring cable, and held compressed by
the water pressure at its installation depth. For example, at 100 m
depth a 2.4 m-diameter piston in a tube will move toward the open
end only if pulled with a cable force of about 440 MT. The cable
force must balance hydrostatic pressure on one end of the piston,
minus water vapor-pressure at the other end. Equivalently, a bundle
of four 1.2 m-diameter tubes can be employed. The needed
displacement to absorb likely turbine kinetic energy is around 1 m,
with an additional 2 m useful for guiding a long piston. This tube
can be steel or concrete, with only the wall thickness to prevent
buckling from external pressure. The piston must be long enough or
include roller guides to prevent tipping/jamming. The chamber can
be sealed by a rolling diaphragm or an O-ring seal or lip seal.
Fluid leakage can be tolerated if a pump or a spring return of the
piston can clear accumulated fluid ingress from time to time. Note
that the force-limiting function can be enhanced if the device can
also dissipate stored energy. As an example, the displaced-piston
stored energy can be dissipated by admitting water into the empty
space. Slowly pumping that water out would reset the system.
[0134] When installed in deeper water, such an overload fuse can be
made of a smaller diameter, e.g., 400 m depth would allow just 1.2
m diameter.
[0135] Another way to achieve overload fusing is to employ
re-usable axially loaded buckling rods. Important characteristics
include axial near-rigidity until the buckling load, then axial
shortening at virtually constant force, until bending failure.
Unlike a force limiter based on a linear spring, there is no need
to use any of the strain-energy capacity to react the working
tension. The amount of needed material (typically less than 500 kg)
is found by equating energy to be stored, with maximum bending
energy of a bar. (Note that fiberglass is a preferred choice
because it does not corrode, and stores more energy per volume than
any metal.) The dimensions are selected so it buckles at the
desired protective load. One practical way to employ one or more
such bars is to connect the bar in line with the mooring cable,
then flip it end for end so the cable tensions place it in
compression. For stability, the cables must pass through guide eyes
at the bar ends.
[0136] Buckling rods as suggested here can be used in diverse ways
to achieve desired force characteristics. For example, making a
square of cables, a buckling rod can be installed as a transverse
diagonal, and this assembly will show post-buckling stiffening as
the axial diagonal is stretched. Also, placing a cable or axial
spring in parallel with such a buckling bar can limit its
compression, or provide a desired constant stiffness.
Preventing Cable Twist
[0137] Since typically the generator is meant to deliver electrical
power to a power cable on the seafloor, twisting damage is a
concern. If the turbine swings around its mooring to follow the
changing direction of the wind, there is a possibility that it can
wind clockwise more than counterclockwise (though this might take
days or weeks), and eventually damage the electric cable. There are
several possible ways to reduce or eliminate the risk of cable
damage. For example, one method includes measuring the wind speed
and determining or predicting a time when the wind completely
switches direction (i.e., to a wind direction which will cause the
wind turbine to move to a position opposite to the prior
direction), which typically provides a time window of very small
wind velocity. During that time window, applying a small bias will
cause the turbine to rotate in either desired sense (cw or ccw).
That bias can be provided, for example, by pre-rotating (using
individual pitch control of the rotor blades, or pushing with a few
small outboard motors) as the wind dies off. Alternatively, or
additionally, the method of power production using a wind turbine
described herein, includes a temporary break from power production,
and during the break, parking and feathering the turbine rotor for
minimum thrust, and using one or more water propellers to move the
wind turbine along the watch circle in a determined direction
(e.g., determined by a method that includes keeping track of the
movement of the wind turbine around the watch circle over time) to
untwist the entire floating structure one or two rotations. Another
option is to mount a rotary joint or slip ring where the mooring
cable joins the hull (e.g. at a mooring standoff), preferably above
the water level for easy access and to prevent water ingress. To
keep the mooring cable and festooned wire twist free, a heavy duty
thrust bearing can support the mooring force, and a slow, high
torque alignment motor can be used to rotate it properly relative
to the hull to keep it twist free. The same housing can contain
electrical brushes or mercury slip rings suited to the current. It
can be oil-filled to inhibit corrosion or electric arcing.
Wind Turbines without Mooring and/or Electric Cables
[0138] The original offshore turbines were mounted on foundations
sitting on, or buried into, the (shallow) seafloor. Floating wind
turbines have occasionally been deployed in deeper waters,
exchanging costly tower and foundation capabilities for even more
expensive moored floating platforms. In addition, all current
offshore wind turbines are used to generate electricity for onshore
use, employing expensive undersea electric cables and grid
connections to deliver the power to land.
[0139] Alternatively, floating wind turbines can be used that (a)
dispense with the anchors and mooring cables and/or (b) dispense
with the electric connection. While options (a) and (b) can be used
independently, use of both options together is advantageous.
[0140] A floating turbine can hold a fixed location despite the
wind pressure acting on the rotor, by providing a large underwater
`station-keeping` thruster, for example, a large water propeller.
Such a device must resist the very large rotor wind force of
approximately 150 MT, which requires it to have a large diameter as
explained below. It must also react the lesser (but potentially
off-axis) wave forces: this calls for some directionality such as a
thrust-steering vane or directional propeller. Accordingly, in some
embodiments, the wind turbines described herein have one or more
thrusters.
[0141] In embodiments, the wind turbine disclosed herein are a
station-keeping floating wind turbine with one or more water
thrusters having a diameter exceeding 15% of the turbine rotor
diameter.
[0142] With the turbine at one fixed position (not moving) the
rotor force and water-propeller force have the same magnitude F.
The power taken from the wind is proportional to F*Vwind and the
power delivered to the water is proportional to F*Vwater, where
Vwater is the fluid motion created by the propeller. Thus the
fraction of power lost is (Vwater/Vwind).
[0143] That velocity ratio can be estimated from the force
balance:
(.rho.wind/2)*Awind*(Vwind).sup.2.about.(.rho.water/2)*Awater*(Vwater).s-
up.2 where .rho.=density, A=rotor area
[0144] This implies that
(Vwater/Vwind).about.sqrt[(.rho.wind*Awind)/(.rho.water*Awater)]
[0145] For each rotor A is proportional to R.sup.2, while
sqrt(.rho.wind/.rho.water)=0.035
[0146] Therefore the fraction of generated power devoted to station
keeping is 0.035*(Rwind/Rwater).
[0147] If Rwater=Rwind/6 (e.g., Rwater=90 m/6=15 m), this means
0.035*6=0.21 or 21% losses. A larger water propeller can reduce the
losses to about 15% or even about 10%. This might be
cost-ineffective for a conventional shallow depth offshore wind
farm, but when water depth is much greater (which would require
expensive mooring) it can be cost-effective.
[0148] Station keeping (which also implies self-propulsion) can be
particularly advantageous if grid-connected electricity is no
longer produced. If the wind power can provide other valuable
goods, it seems possible that the turbine could be allowed to work
in international waters, with minimum permitting or planning,
moving periodically to locations with optimum wind, and also
avoiding storms.
[0149] Various known power-requiring processes can be considered,
like purifying salt water, or splitting water to capture hydrogen.
But the most attractive high density products would be liquid or
solid fuels. For example, electrolytic hydrogen and liquefied
atmospheric carbon dioxide can be processed into SYNGAS. With that
precursor a liquid crude-oil substitute (e-crude, useful for
plastics or fuel production) can be synthesized, apparently about
25,000 bbl of green fuel per year from 10 MW electric power. The
big advantage of a liquid product is that a towable bladder can be
filled over the course of a month, and several bladders can be
towed to shore every few months. In embodiments, the wind turbines
described herein have no mooring but a propeller for station
keeping, and the wind turbines include or are moored adjacent to
devices to convert generated electricity or shaft work into
chemical energy (e.g., in the form of synthesized compounds such as
liquid or solid fuels, hydrogen, or ammonia) or compressed gas
stored at depth to await later expansion.
[0150] Ammonia is another valuable product (both a fuel and a
fertilizer) that might be synthesized, from electrolytic hydrogen
and atmospheric nitrogen. It can be liquefied at reasonable
pressures for easy transport
[0151] In embodiments where the floating wind turbine is adapted
for station keeping, the water propeller can be powered
mechanically (for example by a belt from the generator axis to the
propeller axis). However, the mooring force is then no longer
directed at the rotor axis as is needed to prevent pitch from wind
thrust. Tipping the propeller to provide that sloped force would be
highly inefficient (its thrust would have to be much bigger than
the horizontal wind force). To balance the overturning couple
formed by wind thrust and water thrust, the simplest step is to
provide some upwind ballast. For example, the tip of the standoffs
where the mooring line is typically attached is approximately 80 m
in front of the 120 m high rotor axis, and would balance the
overturning wind moment with a weight of 225 MT, i.e. about (6
m){circumflex over ( )}3 of water. A water-holding steel or
concrete tube of 7 m diameter and 6 m height above the water plus 3
m below provides that weight, or even more if the wind pushes extra
hard. It can either be used full at all times (in which case, at
low wind thrust the turbine is modestly pitched), or it can be
adjusted in weight to match conditions, either by a pump or by
valving air in or out during wave motion.
[0152] A floating turbine can hold itself in place, and resist
pitching from wind force, at a cost of less than 20% of produced
power. By adopting this design choice, the expense of anchoring
(and possibly also leasing/permitting) can be avoided.
The Mechanical Drive System
[0153] A conventional wind turbine rotor rotates around a slender
tower to align with the wind, with its gear transmission and
generator in an enclosed nacelle. A wind turbine with no nacelle as
described herein can yaw the entire tower and its floats to orient
the rotor (the tower can be broad for efficient connection to the
floats), and the generator can be placed on a structural support,
typically, protected from water. In these cases, a speed-increasing
mechanical power transmission can power the generator, without a
complex gearbox which in conventional designs is an expensive and
unreliable component housed by the nacelle in proximity to the
rotor axis.
[0154] In embodiments, the wind turbines described herein include a
mechanical transmission which comprises a large diameter sheave
connected to the turbine rotor, a belt carried by the sheave, and a
small diameter sheave (or belt drum) mechanically coupled to the
shaft of the electrical generator. A mechanical transmission can be
lightened and its stages reduced if the input gear or sheave can be
large-diameter. A sheave or gear can be used that is far larger
than any conventional nacelle, for example, between 15 m and 40 m
in diameter (see, e.g., FIGS. 1, 2, and 5).
[0155] FIG. 5 shows the top part 500 of a wind turbine (e.g., of a
wind turbine of FIG. 1 or FIG. 2). Only the top parts of the four
lattice towers 505 are illustrated, with two towers supporting each
end of the fixed tubular axle. A belt sheave 510 (here, e.g., 30 m
in diameter) is braced and driven by a plurality of wire spokes 515
from a rotating hub 520. In addition to the rotating hub 520 and
the belt sheave 510, the turbine rotor includes three blades 525
(each only shown where it connects to the hub 520), three blade
ring gears 530 (one per blade) for individual blade pitch, integral
with segmented blade slewing rings, blade pitch input bevel gear
535 and blade pitch reversing gears 540 which drive contra-rotating
power shafts to pitch each blade, blade pitch air clutches 550
which connect either power shaft to its pitch drive pinion, and a
large bevel gear 555 fixed to the non-rotating rotor axle 560 which
extends through the bore of the hub 520. The sheave 510 carries a
hard stainless steel drive band 565 (i.e., an example of a belt;
weather shroud not shown). Reversing gears 540 are on top of blade
pitch input bevel gears 535.
[0156] For wind turbines with high rated capacity and accordingly
large turbine rotor diameter (e.g., diameters substantially larger
than 30 meters, e.g., 80 m to 250 m) it is uneconomical to install
a gear, sheave, or magnets/coils near the blade tips.
[0157] A simple belt (or drive) to manufacture is a 301 stainless
fully hard steel band, welded then ground down along a shallow bias
cut to make a continuous belt. In line with the previous paragraph,
a rather light large sheave (e.g., about 30 m in diameter) would
drive the belt, which would turn a smaller diameter (e.g., 1 m)
drum on the generator with substantial speedup (e.g., 30:1).
Properly aligned hard steel belts are known to be extremely
efficient, and the cost of this system are projected to be very
low. It has to be sized so that the combination of bending around
the small drum, and tension difference between slack and tight
sides, do not cause fatigue. This can be determined using a Goodman
rule, applied to full hard 301 stainless of about 1 mm thickness
and 2 m width.
[0158] In case the generator locks up and the belt slips on that
drum, it will heat up and reduce its hard temper. An instant of
slip should not be so damaging as long as the belt keeps moving to
distribute the heat input; and severe heating can be prevented by
immediate water flooding. Water is a coolant, but more importantly
as a lubricating film it will let the belt slip with almost no heat
generation. In embodiments, the wind turbine includes a source of
water which can be controlled to bring the belt in contact with the
water for cooling and lubricating purposes.
[0159] One of the concerns is transmitting torque by friction,
without needing high belt tensions. This is possible if friction
coefficient is adequate, but there is always a risk that months of
use could burnish the surfaces and reduce the friction coefficient.
A solution is pinch rollers although special coatings (rubber,
ceramic) can also be used on the belt or drum. If fully inflated
truck tires are used to press the belt to the drum, non-slipping
drive is possible even with a low coefficient of friction. Despite
their rolling resistance, truck tires are preferable to a hard
pinch roller because the applied force is spread out resulting in
lower belt pressure.
[0160] In case the metal belt displays an unexpected vibration or
durability problem, there are other reasonable options for gaining
most of its advantages:
[0161] In embodiments, the wind turbine includes a rope drive. The
steel and synthetic ropes used in deep elevators can also be
suitable for driving a deck-level generator. If a turbine rotor
included a ring at the blade tips a single high-speed rope can be
sufficient. But a 3-blade rotor would end up a lot more expensive
if it required a sufficiently strong circumferential ring. Using
instead a 15 m radius sheave, multiple ropes (with attendant cost
and practical disadvantages) would be needed. This is because
twisted or braided ropes have poor fatigue behavior, and therefore
have to be used at very low axial and bending stress.
[0162] In embodiments, the wind turbine includes a mine-conveyor
belt as part of a mechanical drive system. A mine-conveyor belt is
a useful component where many thin wire ropes or strong fabrics are
unitized by rubber in a protected wide sheet. It would be a more
practical way to handle a dozen or more ropes.
[0163] In embodiments, the wind turbine includes a bevel gear shaft
drive as part of the mechanical drive system. It is advantageous to
avoid the cost, fragility, inefficiency and maintenance needs of
precision gearing, which could alternatively be used despite the
high cost. Some of the advantages can be achieved by using a very
large input gear, much bigger in radius than the conventional
nacelle. The efficiency is highest when the number of gear meshes
is least, so a preferred approach is a very large input bevel gear,
driving a small bevel gear on a long vertical tube, braced against
whirl. This is the layout of a manual eggbeater or hand-crank
drill. Possibly the step-up ratio would be supplemented by small
deck-level 3:1 gear drive or industrial belt drive at the
generator. For best manufacturability (and cost) the large gear can
be designed for construction in small pieces, similar to proposed
segmental blade pitch bearings. High hardness commodity steel plate
can be roughed out by waterjet, then precision machined by CNC into
identical sub-parts to be pinned, bolted, and possibly also bonded
or soldered, into a precise assembly when the turbine is
erected.
Light Weight Hub and Axle
[0164] A conventional wind turbine axle is solid, and the rotor is
cantilevered from one end. A conventional blade-mounting hub is a
massive casting, which seemingly has not been designed to work in
membrane stress but rather is thickened to tolerate local bending
stress.
[0165] It can be desirable to fabricate the hub so blade-root
bending moments are taken primarily by membrane stresses. Then
large-diameter steel tubes with small (e.g., sub-inch) wall
thickness, can be manufactured near the assembly dock, and could
reduce weight by a factor of 3 or more.
[0166] When a blade transmits a bending moment to its support, this
is reacted by membrane stresses if the blade is attached to a
same-diameter tube, which is incorporated into the hub. The
principle of joining thinwall structures is that any place where
tangents are not aligned should have a third-direction surface (a
diaphragm) all along that joint. And joint fillets should be
generous. FIG. 6 illustrates an example of a hub 600 where three
blade mounts 610 join to bore tube 620. Internal diaphragms are
placed on the symmetry planes between blade mounts.
[0167] FIG. 7 provides a schematic cross-sectional view of the
turbine rotor 700 and how it can be structurally supported by the
lattice towers 705. The central component at the tower top is a
tubular non-rotating axle 706 (i.e., a fixed tubular axle unitizing
the towers) that connects all towers and bears the loads of the
heavy rotor. It is shown here with a point 702 at the upwind end,
for connection to a mooring rope or tower brace 703 from the
mooring attachment point. Two towers converge on an upwind clamping
saddle 750 and two converge on a downwind clamping saddle 750 to
support the axle. A tubular steel hub 735 with blade connection
stubs 755 rotates on the axle, supported at both ends of the bore
tube by needle rollers 715 (or hydrostatic fluid film bearings).
Wind thrust forces are carried by downwind thrust rollers 716. The
hub carries multiple components. Three composite blades 745 are
mounted on slewing rings 740 (on blade stubs 755) constructed from
segments of hardened steel plate. These have double rows of needle
bearings and integral gear teeth, in order to rapidly adjust blade
pitch on the fly for optimum aerodynamic efficiency, reduction in
gust forces, and control of the rotor center of pressure in order
to enhance yaw control of the floating turbine. A large sheave 710
supported by wire spokes in tension carries a 1 mm thick 2 m wide
stainless steel drive belt, for low cost power transmission with
rpm increase to a low level generator. The spokes are angled to
provide torsional and axial bracing. Each blade is equipped with a
gear system (725 and 731) to rapidly control blade pitch. The idea
is that rotor kinetic energy is rapidly clutched to any blade to
develop and then remove several degrees of blade pitch, while the
blade is traversing one quadrant of the rotor circle. Blade
pitching is effected through interaction with a single fixed bevel
gear 730 mounted on the fixed axle. While this gear never turns,
observed from the rotating hub it appears to be turning, so it can
be used to power mechanisms on the hub. The fixed bevel gear
interacts with an input bevel gear 731 for blade pitch for each
blade. As long as the rotor turns, the three input bevel gears also
rotate on their shafts. Above the input bevel gear for any blade is
a pair of reversing spur gears. Not visible in this figure, the
second spur gear and its vertical shaft is behind the first. As the
input bevel gear rotates in one direction, the gear and shaft above
it rotates in the same direction, and its meshing partner and
second shaft rotate in the opposite direction. At the top of these
two shafts for each blade are air clutches 720 with external gears.
When either air clutch is energized with air pressure, its gear is
caused to turn, pitching the blade about its long axis. When
energized, one air clutch causes the blade to pitch in the positive
sense during rotor turning; the other air clutch will cause the
blade to pitch in the negative sense. If both clutches are
undesirably energized simultaneously, they will both slip at
maximum torque until the rotor comes to rest. Braking of a large
rotor is best accomplished by control of blade pitch to produce
decelerating aerodynamic torque. Energizing both clutches at once
is useful as a holding brake only. 751 is optional sheltered
equipment that takes advantage of a sealed dry space.
[0168] Blades are conventionally connected by T bolts to the blade
pitch bearings (discussed below).
[0169] The hub bore tube (See, e.g., FIG. 6 and FIG. 7) can also be
a thin-wall large-diameter steel tube. Such a tube is close to an
optimal cross section when moments might be applied in any
direction, as long as thickness-to-radius is sufficient to prevent
local buckling.
[0170] The axle (See FIG. 7) can likewise be a large thin-wall tube
fitting through the hub bore tube. It is supported (built in) to
the towers at both ends, and otherwise is as short as possible.
Then bending stresses will be small in comparison with shear
stresses.
Bearings for Rotor Turning
[0171] Bearing contact stresses under load (which define the degree
of surface damage) can be computed from force per projected area.
Stresses are minimized by a large projected area, and by using
rollers rather than balls.
[0172] The main rotor bearing loads include the vertical (from
weight, belt and heave acceleration); the axial (rotor thrust from
wind and horizontal acceleration); and net moment from tilting
(pitch or yaw) acceleration and precession velocity. The moments
are expected to produce the main radial loads on the rotor
bearings.
[0173] One of the cheapest and thinnest (with least impact on shaft
diameter) bearings are needle rollers. Hardened precision shafting
e.g. 1'' diameter is widely available as a commodity, and can be
cut into lengths to construct full complement needle bearings
(e.g., 715). Axial loads can also be carried by needle rollers
(e.g., 716) (possibly guided by a cage to preserve orientation)
forming a thrust bearing. No special raceways will be needed as
long as the contact stress is proportioned to the strength of the
axle and hub bore tube materials. See FIG. 7.
[0174] An alternative way to support the turning rotor is a fluid
film bearing maintained by a pump at high pressures (typically 1000
psi or greater). Such well-known `hydrostatic` bearings must be
designed to be stable against pressure gradients. This is commonly
achieved by allowing multiple support areas to provide different
pressures, e.g. through pressure reducing orifices leading to
reduced pressure when flow is large. Then reduction of clearance at
any point will lead to increased pressure, and vice versa, keeping
the rotor centered and aligned. On the possibility that the
hydrostatic pump ever fails, the moving surfaces can be provided
with metal bushings for short time rotational service
Bearings for Blade Pitch
[0175] The blade pitch bearings (e.g., 740) transmit blade bending
moments by thrust forces and require 5 m diameter raceways. A
segmental design is preferred using multi-layer pieces can be
shaped precisely by conventional CNC milling, then assembled
ruggedly and precisely with bolts and pins. The material of choice
is quenched and tempered abrasion-resistant commodity plate,
probably the AR500 grade. This can be Blanchard-ground for
smoothness, rough cut by waterjet or plasma cutting, then precise
dimensions (including integral gear teeth) can be produced with
carbide tooling. The idea is to form the needed large rings by
multiple overlapping layers of arcuate segments that are 1.5 m-2 m
long. Once precision-cut, these segments can be aligned and
compressed with shoulder bolts, then some holes can be taper-reamed
and taper-pinned for a load-bearing fit. If additional unitizing is
needed, soldering or adhesive bonding can be adopted.
[0176] The races (e.g., of 740) will cooperate with well-known
blade T bolts, and will provide integral gear teeth for control of
blade pitch. Axial loading will be transmitted by a flange on one
part (e.g., the blade), supported by needle rollers above and below
in a double flange (U shape) connected to the hub. The much lower
radial loads will be carried by a single row of rollers. Preferably
the design incorporates elastic matching, such that the
load-induced slope of the flange on the blade matches the
load-induced slope of the flange on the hub. See FIG. 7.
[0177] Such a design is meant to be assembled onsite into a
precision ring assembly that is too large for convenient trucking.
By avoiding the manufacture of full rings, equipment such as large
ring rollers, heat treating furnaces and grinders are not needed.
This saves cost and lead time for a customer, and reduces the
investment required of the producer.
Rapid Blade-Pitch Mechanism without Electric or Hydraulic
Actuator
[0178] Rapid pitch control of individual blades means that blade
pitch can be tuned to the different wind velocity at different part
of the large turbine disk. This optimizes power production and
reduces unwanted gust loads. In addition, such control can be used
to create intentional yawing torques, in order to correct undesired
wave-induced yawing of the floating turbine.
[0179] The limiting factor in blade motion is the required power of
the actuator. A high-power controllable actuator capable of
rotating the blade several degrees in less than a second is costly.
Conventional electric or hydraulic actuators can be dispensed with
by utilizing the kinetic energy of the rotor. In the frame of the
rotor the fixed axle appears to be turning at 10 rpm. This
`unstoppable` rotating shaft will be used for a power source that
can be clutched independently to any or all blades, to pitch any
blade in either direction at any moment. Since pitch rate of a
blade cannot be changed instantaneously, the clutching process
involves slip and energy dissipation. For example, blade spin
inertia about its long axis can be estimated as a mass of 40,000 kg
at a radius of 2.5 m. At a pitch rate of 12 deg/s its
circumferential velocity is 0.5 m/s. In a clutching move to create
velocity match, the work done is mv.sup.2, half going to kinetic
energy and half to slip. The acceleration time is mv/F, and power
required is Fv. The unknown is the slipping force F to be applied.
Selecting a force F=100,000 N, the time to reach maximum pitch rate
is 0.2 s, during which time the rotor turns 12 degrees. The power
used in frictional work and in overcoming inertia during that time
is 50,000 W, about 0.5% of wind power during 3% of rotor turning
time. To decelerate the pitch rate is expected to be energy neutral
(just eliminating kinetic energy that was already created). With
doubling the time (to deal with blade de-pitching) and tripling the
power (to take into account 3 blades), we are considering a loss of
0.5% of wind power during 18% of the time, which seems
negligible.
[0180] Gearing and clutching can be achieved as follows. FIG. 5 and
FIG. 7 illustrate conventional spur gears (driven by a bevel gear
on the stationary rotor axle, with added meshes to achieve velocity
reversal) and industrial pneumatic clutches. But other approaches
are possible, such as the use of capstan cables without gear teeth:
the blade can be wrapped in two directions to be able to pitch it
with tension only. The two ends would each be dragging lightly
around fixed axle-drums, until engaged with pneumatic or other
actuation to grip. If designed like wrap spring clutches, many
conventional gripping and releasing strategies (involving a movable
dog on the hub) become available.
[0181] When the rotor is at rest (either becalmed or parked), there
is still a need to pitch any blade. Therefore, a backup
battery-powered system is typically provided. This could be a 5 kW
motor with 1:200 gearing and electric clutch, able to slowly rotate
the blade when engaged.
Lattice Towers
[0182] The wind turbines disclosed herein are adapted to allow
either the entire wind turbine or at least the towers together with
the turbine rotor to yaw against the wind. Thus, there is no need
to provide a slender (and structurally inefficient) tower, as is
the case in conventional designs. A set of support columns (also
referred to herein as "towers") on a spread base (i.e. spaced to
rest on the support floats) is very efficient, and if carefully
designed the support columns will be subjected to compression only.
The towers structurally support the turbine rotor.
[0183] A support column with enough area to resist compressive
yield will, if long enough, buckle. To prevent buckling a
sufficiently large radius of gyration is needed. If this is
provided in tubular form, at great enough length the wall has to be
so thin that local buckling appears. A more efficient structure in
that case is a lattice tower, similar to a tall radio mast. (A
lattice tower also has less aerodynamic drag area, and its
fabrication can be performed by conventional manual welding.)
[0184] The lattice towers can be fabricated in boltable 40-ft
lengths, in a fixture that permits easy rotation about the column
axis for weld access.
[0185] Traditional concerns for lattice towers include corrosion
and bird nesting. Cutting the lacing members so the diagonals are
vertical and horizontal sheds water more easily and discourages
perching. Additional protective material can be used as
necessary.
Craneless Erection on Unimproved Soil, Followed by Air-Cushion
Launch into Shallow Water
[0186] In embodiments, the floating wind turbine is meant to have a
shallow draft, for example, a draft of around 4 m, in contrast to
most floating turbine concepts with spar buoyancy exceeding 20 m
draft. A shallow draft allows for boat-ramp launching into shallow
water, after construction at the water's edge. It can also be
transferred from a dock surface above the water surface, onto
grounded box-floats, that can then be air-filled to rise a few
inches and float the turbine away. Once in slightly deeper water
the boxes can be lowered by bleeding air, until they can be
removed.
[0187] In order to travel freely on flat land and smoothly enter
very shallow water, the erected turbine can be provided with four
air cushion transporters each able to support 25% of the turbine
weight, e.g., each able to support 300,000 lb. Assuming a pressure
of 6 psi, these would be about 20 ft.times.20 ft (or at 3 psi,
close to 30 ft.times.30 ft), and they would float under load in
water at a depth of 14 ft (like the shallow berth of the New
Bedford Marine Commerce Terminal) or, with 3 psi, at 7 ft.
[0188] The air cushion transporters can each be formed as two
half-cushions, each shaped to transport a half turbine float.
Half-floats would be moved over to the wind turbine legs with
attached air cushions, then joined to the special bottoms of the
legs (also referred to herein as "towers") without requiring the
structure to be raised. Finally, the turbine with floats can be
transported over soil or pavement into the water. By flooding and
removing the air-cushion segments, the turbine can be supported by
its floats and the air cushions can be returned to land.
[0189] An advantage of the wind turbines which include
multi-segment (e.g., 10-segment) lattice towers is that they can be
constructed using a cost-efficient craneless erection method. A
lattice segment may be 40 ft long weighing about 2 tons, or any
other convenient length.
[0190] The method comprises (i) providing a turbine axle (e.g.,
706) carrying surrounding rotatable hub (e.g., 735) with belt
sheave if used, (ii) structurally coupling the topmost lattice
tower segments to the axle ends with bolted tube saddles (e.g.,
750) (see, e.g., FIG. 7 for such a coupling), (iii) hoisting the
first lattice tower segments simultaneously by their feet to a
height that allows a plurality of second lattice tower segments to
be structurally connected (e.g., bolted or welded) to the first
plurality of lattice tower segments, (iv) structurally connecting
(e.g., welding) the plurality of second lattice tower segments to
the plurality of first lattice tower segments; (v) lowering the
second lattice tower segments to the ground and moving the hoists
to grip the bottom ends of the second lattice tower segments, and
repeating steps (iii)-(v) until the wind turbine has a plurality of
lattice towers of the desired height. Temporary braces between the
growing towers can be used until the final bottom inter-float and
inter-tower braces are installed. The hoisting can be achieved
using multiple appropriately dimensioned inexpensive hoist towers.
For example, if the lattice tower segments are to be of a certain
length, the lifting tower will typically allow to lift the so-far
assembled lattice tower to a height that is sufficient to attach
one further lattice tower segment. Blades can be attached once the
length of the assembled towers and accordingly the height of the
so-far assembled wind turbine structure is of a sufficient height
to allow it.
[0191] FIG. 8 illustrates the steps of jacking the bottom-most
tower segment 805 from the ground 810 (position A) to a height
(position B) sufficient to move and structurally connect (e.g.,
weld) a further tower segment 815 (in or from position C) to the
elevated tower segment 805 (in position B). This method is utilized
simultaneously for each of the plurality of legs (e.g. lattice
towers) of the wind turbine.
[0192] FIG. 9A provides a schematic side-view of an embodiment of a
lifting tower 900 suitable for jacking of tower segments. The
lifting tower includes a winch 910, upper and lower pulleys 915,
and a work platform 920, and a structural frame 925 that supports
lifting the desired weights and straddling of the tower segments.
Pulleys can be used to amplify winch force, for example, a 10 ton
winch force can be amplified to 100 tons. FIG. 9B further
illustrates the embodiment of a lifting tower 900 shown in FIG. 9A.
This view further shows optional retractable wheels for moving of
the lifting tower as well as upper pulleys 915. FIG. 9C illustrates
the lifting tower of FIG. 9A and FIG. 9B with a leg (e.g., lattice
tower) of the partially assembled wind turbine structure jacked to
the height at which a further leg segment can be structurally
attached at the bottom of the elevated leg segment.
[0193] The crane-less erection scheme (FIG. 9A-9C) can be based on
40-ft leg-truss segments, and four 40-ft hoists able to
simultaneously lift the bottom of each leg of a partially assembled
structure, and also winch a truss segment under that structure to
join to what already exists. The structure will need temporary
cable braces until it is complete, and the lifting hoists may also
need anchoring or bracing for each lift to make sure they are
stable.
[0194] In the very first stage, the axle with hub is connected to
the first four truss segments. This might be accomplished by the
same hoisting process, or possibly would be performed at ground
level after which the beginning assembly is pulled upright. At that
point when the hub is still close to the ground, two blades can be
installed sequentially in `rabbit ear` position (namely 10:00 and
2:00). A blade on the ground needs to be cable- (or rocker-)
supported near its CM so its hub end can easily be tipped and
slightly displaced to meet the hub pitch bearing, so bolting can
draw the two components together.
[0195] Once one blade is mounted, a temporary kingpin on the hub,
and an anchored cable or a dead weight will be able to rotate the
first blade well above horizontal, so the second blade can be
installed in a manner similar to the first. With two blades
installed and braced to prevent rotation away from `rabbit ears`
position, the tower can be lengthened by extending its four legs.
Once it is nearly finished, the third blade can be hoisted to the
axle in a vertical orientation, for bolting to the bottom (6:00)
pitch bearing.
[0196] The assembly process is to lift the entire structure at four
points (the four current feet) a height of 40 ft. When the turbine
is nearly complete this requires a lift force of about 150 tons for
each leg. Then the next two-ton leg segment is hoisted and joined,
after which the structure is lowered to rest on its new feet.
Lastly the hoists are rolled to the new foot positions and the
process is repeated.
[0197] The hoists can be designed so a short vertical support post
can be installed as a very final step, resting on a compact
reinforced slab. Then with the hoists withdrawn, float halves on
air cushion transporters can be connected to each other around the
post (which may finally be removed for reuse, allowing the turbine
to rest on the floats supported by air cushions).
Multifunctional Turbine Floats
[0198] Typically, wind turbines with higher power capacity will
require, among other things, larger blades, larger rotor diameter,
larger hub height, and larger towers, which leads to larger weight
which the floats have to support. The wind turbines described
herein have much shallower draft compared to known floating wind
turbines. Still, the floats must provide the desired water
displacement at the desired draft, with a significant reserve
buoyancy (e.g., double the minimum) to deal with higher wind thrust
possibly combined with a large pitch angle caused by wave-face
slope. Further, the floats can must provide sufficient `stiffness`
to stabilize the turbine in pitch and roll. As is well known,
buoyant stabilization of a collection of shallow-draft floats is
based primarily on the waterline area of each float, times the
weight density of the water, times the square of the distance of
each float from the central rotation (i.e. tipping) axis, summed
over all floats. (The sum of areas times squared distances is
approximately the second moment of waterline area, which to be
precise should include the second moments of each individual area
around its own center). Multiplied by the weight density of water,
it effectively forms a torsional spring constant to resist tipping
or capsizing.
[0199] In embodiments, the floats are dimensioned and spaced
relative to each other such that the wind turbine attains buoyant
natural frequencies that are high in comparison to typical wave
frequencies. This can reduce wave-excited motion.
[0200] In order to minimize material usage for the floats, a low
stress design to carry leg force via membrane (not bending)
stresses can be used. Cylindrical shapes achieve low
circumferential stress for radial pressure, but suffer high bending
stress if there is a flat pressure-bearing bottom or top. A conical
shape (or a pyramidal approximation) of 10 mm-16 mm thickness will
experience low stress from fluid pressures, and a bottom point will
help in reducing slamming force in case a float ever leaves water.
Another option for saving material is to leave the float bottom
open, with air trapped in a bladder. In general tank material is
minimized for an equi-axed submerged shape (height in proportion to
width). If the bottom is open, then a height of half the width can
be better.
[0201] Radian heave frequency of a flexibly supported object is
sqrt(k/M), where k is the stiffness (restoring force per unit
displacement) and M is the mass, here of one fourth the entire
turbine. For a body in fluid of mass density rhoF the effective
vertical stiffness is (rhoF)(g)(A) where A is the area where the
body is intersected by the fluid surface. If the waterline float
radius is R, A=pi*R{circumflex over ( )}2, the heave frequency is
approximately R*sqrt(rhoF g pi/M) showing proportionality to R if
we seek a high frequency with fixed M. It is also well known that
heave frequency may be approximated as sqrt(g/draft) where draft is
an approximation to how far a vessel's weight pushes it into the
water. Therefore, high natural frequency is associated with small
draft. Natural frequency in heave is also influenced by `added
mass`--water that moves vertically along with the float. That mass
is large for a wide flat float, and is small for a sharply conical
float. Our designs attempt to minimize added mass, to keep natural
frequency high. Note too that the stiffness of a taut mooring rope
also contributes to the natural frequency.
[0202] In some embodiments, the one or more, or all of the
plurality of floats have an apex-up top cone (e.g., with about a
radius of 6 m and 45 degrees slope) truncated to conveniently
transfer leg load to the sloped cone surface. The top cone rests on
top of an apex-down bottom cone with similar shape, which can be
truncated to reduce draft. If we leave a central hole the
communication of hydrostatic pressure may reduce the stress, and
there will be room to mount hardware such as a rudder.
[0203] Since the float will have dynamic interactions with the
water, the following features for steering, to reduce or minimize
drag, for buoyancy control, and/or direction control, can be
desirable for one or more, or all of the plurality of floats of the
wind turbine.
[0204] In some embodiments, a float has a rotatable rudder for
steering relative to the moving fluid of a wave surface. This can
help mitigate the yaw-disturbing effects of waves and off-axis
wind. The rudder can generate horizontal steering forces from wave
orbital velocity. A rudder on each float can either be steered with
an actuator, or it can generate force by using trim tabs to orient
relative to the flow.
[0205] FIG. 10 illustrates a float 1000 including a wing-shaped
tube 1005 which is hollow and has an opening in the bottom part of
the tube (not shown). The wing-shaped tube 1005 can act as a
steering rudder in case it is needed to steer in big waves coming
from an angle. The rudder might be turned one way as the turbine
ascends the wave face, and be turned another way (responsive to
turbine wandering) as it descends the back face. The wing-shaped
tube 1005 can further act as a vertical-force control system. The
length of the wing-shaped tube 1005 increases depth of water
penetration, but not with buoyant materials, accordingly, the
length from the tube opening to the water level within the tube is
not part of the "draft". The water in this tube could rise and fall
as the float moves up and down with a wave. The top of the tube
represents fast-acting valves 1010, that can control airflow. This
can leave the tube with an excess of water, so it becomes a large
added mass to prevent lifting up, or an excess of air providing
reserve buoyancy. Control of the valve allows resisting both the
rise and fall of interior water, so it can become a very effective
damper of any oscillations relative to the water surface. The float
1000 further includes a truncated biconical hull 1015 (shown in
cross-section) as well as a cylindrical interior wall 1020 (shown
in cross-section) which together define a hollow space 1025 with
air which provides the desired buoyancy to support the low-weight
turbine with a margin of safety.
[0206] In some embodiments, a float can be shaped with a prow and
stern in order to reduce or minimize drag forces of wave orbital
velocities. In aspects of these embodiments, the float can be
shaped with a prow and stern, that may be re-orientable to match
predominant wave direction.
[0207] In some embodiments, a float allows for buoyancy control.
For example, by temporarily trapping air or water the float can
gain mass, and gain or lose buoyancy. The needed actuation is to
rapidly control a large airport, for example, with a flap valve or
a rolling ribbon valve. Both of these have small actuation forces
and can be opened or closed in a fraction of a second. Some
potential uses include: (A) If a float is rising out of the water,
trapping a large weight of water will increase the restoring force
from further rise. (And if penetrating more deeply than normal, air
can be trapped, to increase the resistance to further penetration.)
(B) If a float is moving up and down relative to the water surface,
delaying water entry and egress by controlling air motion will
provide significant damping. To enhance these effects, once in deep
water the turbine may deploy a vertical tube at each float, with
air-valving at its above-water top. This tube could also perform
the rudder function, and it can be located on the float centerline
rather than outside the float perimeter. Such deployable tubes are
not part of the draft and do not impede launch in shallow water,
nor lower the natural frequency as long as the valves are open.
[0208] In some embodiments, a float may be equipped with powered
steerable thrusters such as propellors. The cost can be justified
by the ability to untwist the mooring cable, to maintain a desired
heading with heavy waves but light wind, or to make small
corrections to small disturbances before they become too large.
[0209] At least in level water, a float possesses a property of
`added mass for vertical acceleration`, in which some water is
accelerated along with the float, increasing its effective mass and
reducing the natural frequency. For a vertical cylindrical float,
the added mass is roughly that of a hemisphere of water, capping
the float bottom. For shallow wide floats, this added mass can
exceed the wind turbine mass. To reduce added mass (note that
in-phase wave excitation may not involve added mass, since water is
already moving with the wave), N smaller separated vertical
cylinders with the same total cross section can be used. Then the
added mass is proportional to 1/sqrt(N), easily reducing its
influence. Added mass reduction is useful when attempting to keep
the natural frequency above wave frequencies. This goal is quite
opposite of normal practice, where natural frequency is pushed
below wave frequency, by use of reduced waterline area and a
stabilization approach relying on large-draft ballast.
[0210] To the extent that the waves cause turbine pitching, the
rotor will experience a wind velocity varying from (V+v) to (V-v).
Assuming blade pitch is properly adjusted to suit, the time average
power delivered to the rotor (proportional to the cube of relative
velocity) is augmented by the factor (1+3v.sup.2/V.sup.2). When
waves are significant this can add a few percent to the captured
power below rated wind speed. In other words, the turbine in waves
may be viewed as a combined wave and wind energy harvester.
Control of Resonant Motion in the Water
[0211] As described above, the floats can be adapted to be able to
trap water. To control roll resonance the effective mass of the
floats can be increased by trapping water, thereby lowering the
natural frequency in roll.
[0212] To reduce the risk of damaging resonance (either rigid-body
turbine motion, or elastic deformation of a component) arises from
periodic forcing, a "tuned mass damper" or TMD can be used. This
splits a resonant response peak into two peaks, one on either side
of a defined frequency. It would be typically used where forcing
frequency is invariable. But since the floating wind turbine may be
susceptible to a range of disturbing frequencies, it can be
valuable to retune the TMD to cancel any observed growing
vibration. Classically this can be done by shifting a spring
support point. It should also be possible to add or subtract water
mass. But a particularly convenient approach is to use a pneumatic
spring (e.g. Firestone Air Spring). Especially if two air springs
are loaded in opposition to squeeze a vibrating TMD mass, then the
stiffness they provide is proportional to air pressure, which can
be quickly changed by an order of magnitude without moving
significant mass.
[0213] In further embodiments, the wind turbine is not ballasted,
so stability is derived by its waterline area. This can lead to
very shallow draft (e.g. 2 m), permitting assembly at a shallow
port.
[0214] In further embodiments, the wind turbine can yaw at its
mooring to face the wind, driven either by the wind or other ways
as describe herein.
[0215] In further embodiments, the mooring connection consists of a
single cable, or other device making the restraint force act in a
defined direction. That defined slope (e.g., 1.5:1) affects many
aspects of the design, including the anchor force, the flotation
needed, the hull length, and required tower strength
[0216] In further embodiments, a mooring cable is attach to the
seafloor.
[0217] In further embodiments, a mooring cable is attached to a
point defined by three anchor cables converging on a sub-surface
buoy.
[0218] In further embodiments, the restraint force from mooring is
directed to intersect the wind force and system weight at a single
point. This means that changes in wind exert no pitching moments
about this point and the barge/turbine assembly does not pitch
under this loading.
[0219] In further embodiments, the floats have a vertically
prismatic hull. If a turbine with no wind is in balance (center of
mass over center of buoyancy) and if the buoyant material above
that float is prismatic, and symmetric relative to the vertical
line through the intersection of mooring force and wind thrust,
then the turbine remains level at any value of wind thrust.
[0220] In further embodiments, the mooring cable (or a structural
member collinear with the mooring cable aimed near the rotor axle)
is anchored to the rotor axle to make the tower loads purely
vertical. Using water-level standoffs for the mooring cable
attachment point adds to pitch stability.
[0221] In further embodiments, the wind turbine does not have a
yawing system at the rotor axis.
[0222] In further embodiments, the wind turbine does not have a
single tower design.
[0223] In further embodiments, a dual or quadruple lattice tower
supports the rotor axis at both ends, eliminating overhung loads,
and permitting use of cheap hydrostatic rotor bearings.
[0224] In further embodiments, the generator is sited on deck. This
allows for better stability and easier maintenance.
[0225] In further embodiments, a mechanical drive is used to couple
rotor rotation to the generator. A belt drive or rope drive costs
far less than a gearbox, and is overload tolerant. With this system
there would be no maintenance needs at the rotor shaft. A full hard
stainless belt offers advantages compared to other materials and
configurations.
[0226] In further embodiments, three neutrally buoyant anchor ropes
converge at a submerged small buoy. From the buoy to a point on the
floating wind turbine, a single mooring line rises at a defined
angle to intersect the wind thrust line above the system center of
gravity, to control the angle of the mooring force reacting wind
thrust. (That angle is slightly steeper than the slope of the plane
defined by any two anchor lines.) For example, a 1.5:1 slope of the
single line. It may be attached near the rotor axle for structural
efficiency, and its position may be controlled relative to the
floating hull with rigid standoffs.
[0227] In further embodiments, the wind turbine does not have a
nacelle. In these embodiments, a nacelle is not needed because
there is no yawing bearing or drive, no gearbox, and no generator
at the rotor hub. The two-sided support of the axle reduces
structural weight and makes large hydrostatic bearings or needle
roller bearings economical.
[0228] In further embodiments, a rotation-transmitting mechanical
drive can be used to transmit rotor torque to a generator at the
tower base. This can include but is not limited to a belt-type or
rotary shaft drive. Previous approaches used a rope driven at the
rotor periphery (for modern large rotors this leads to excessive
sheave size and impractical rope speed) or a small-sheave belt
drive (this leads to excessive belt tensions, meanwhile retaining
an expensive gearbox to achieve high rpm of the generator). We find
that a metal belt 1.5 m wide and 1 mm thick driven by a 30 m
diameter sheave can transmit rotor power to the barge deck, at an
rpm that allows use of a small and inexpensive high speed
generator. This approach is inexpensive, offers a useful fatigue
lifetime, and virtually eliminates tower-top maintenance. To work
in all load cases without high tension in the slack side of the
belt (which stresses the tower), pinch rollers may be useful.
[0229] In further embodiments, the wind turbine has a single
mooring cable. This so-called "single line mooring" allows for
weathervane yawing of the entire turbine structure with controlled
mooring force direction (to permit rise and fall with the tide,
while preventing platform pitch due to wind load).
[0230] In further embodiments, the mooring cable is a synthetic
mooring rope attached near the rotor axis to reduce tower strength
requirements and increase stability.
[0231] In further embodiments, the towers include upwind and
downwind towers. This reduces needed axle weight because of support
from both ends, but the downwind towers must be sloped to prevent
strike of a deflected blade.
[0232] In further embodiments, the turbine rotor is supported on a
fixed axle with no nacelle. In an aspect of these embodiments, the
towers can be lattice construction, framed to the rotor axle.
[0233] In further embodiments, the angled towers mate to a wide
base of floating barges, to enhance pitch stability.
[0234] In further embodiments, power is transmitted mechanically
down to a low-level generator. The generator can be a small
high-speed generator at deck level, with no expensive gearbox.
[0235] In further embodiments, the belt drive is a band of
stainless steel or multiple wire ropes encased in rubber (like a
mine conveyor belt).
[0236] In further embodiments, the wind turbine further comprises
traction control pressure wheels at the small generator sheave.
Further Definitions
[0237] As used herein, the singular forms "a," "an" and "the"
include plural referents unless the content clearly dictates
otherwise. For example, reference to "a cell" includes a
combination of two or more cells, and the like.
[0238] As used herein, "about" will be understood by persons of
ordinary skill in the art and will vary to some extent depending
upon the context in which it is used. If there are uses of the term
which are not clear to persons of ordinary skill in the art, given
the context in which it is used, "about" will mean up to plus or
minus 10% of the particular term.
EXAMPLES
[0239] The present examples are non-limiting implementations of the
present technology.
Example 1: 10 MW Wind Turbine
[0240] A 10 MW floating wind turbine 100 is illustrated in FIG. 1.
Four floats 105 (e.g., floating tanks, are here shown cylindrical
with implied internal bracing, but can be of a variety of shapes
with or without bracing) support the system weight with very
shallow draft (for example, about 2 m). The horizontal spread of
these floats (here, at the corners of a 40 m square) prevents
tipping. They are connected together with taut cables 115 and
horizontal trusses 110, which also support the small high-speed
generator 120. In operation the wind comes from the belt side, i.e.
from the left in this image. The axle 130 and hub 132 defining the
rotor axis (there is no nacelle which conventionally covers a
gearbox, a rotating rotor shaft, a generator, and a yawing system)
are about 120 m above the water where it structurally joins four
towers 135, two upwind and two downwind. Under a strong wind, the
blades (which can be commercially available blades; the specific
shape of the blades is not illustrated and can vary) 140 will bend
downwind, away from the drive belt, but not far enough to strike
the downwind legs. The rotor carries three about 90 m long blades
140 following the DTU reference design, and an about 30 m diameter
sheave 145, which drives a belt 150 (e.g., a ribbon of full-hard
stainless steel, 1.5 m wide and 1 mm thick). The belt 150 wraps
around an about 1 m diameter small drum (not shown) at the
generator 120. The mooring cable 155 (e.g., a synthetic mooring
rope) emerges from an underwater mooring point (not shown). The
mooring cable 155 can pass all the way to the axle 130 at the top
of the tower (in this case 157 is also the mooring cable). Here,
the mooring cable 155 connects to an attachment point 156 on a
standoff structure 160. 157 is a different cable, collinear (or
typically at least substantially collinear) with the mooring cable
155. Alternatively, 157 can be a tensile brace like a metal tube.
The upper end of 157 connects the axle 130, to eliminate horizontal
tower loads. At the water level it is held windward of the base, by
two neutrally buoyant standoff tubes (i.e., a standoff structure)
160. When the wind is blowing, the entire turbine floats downwind
of the mooring point and automatically faces the wind. The axle 130
does not rotate and the hub 132 (with blades 140 and sheave 145)
rotates around the axle on which it sits, between the towers.
Example 2: 10 MW Wind Turbine
[0241] A 10 MW floating wind turbine 200 is illustrated in FIG. 2.
Four floats 205 (e.g., floating tanks, here of approximately
bifrustum shape (e.g., truncated cone on top connected at its base
perimeter to the base perimeter of a cone with apex facing
downward); here, only the part of the float above water line is
illustrated) support the wind turbine weight with very shallow
draft (for example, about 4 m). The horizontal spread of these
floats (here, with corners of a 60 m square) prevents tipping. They
are connected together with cables 215 and rigid connections (e.g.,
horizontal trusses below the water surface) 210, which help support
the small high-speed generator 220 on a cable-supported platform
222 elevated relative to the waterline. The wind may come from the
left to strike the sheave and drive belt before the rotor. The hub
and axle 230 defining the rotor axis (there is no nacelle) (see
FIGS. 5 and 7 for further illustration) is about 120 m above the
water where it structurally joins four towers 235, two upwind and
two downwind. Under a strong wind, the blades (which can be
commercially available blades; the specific shape of the blades is
not illustrated and can vary) 240 will bend downwind, away from the
drive belt, but not far enough to strike the downwind legs. The
rotor carries three about 90 m long blades 240 and an about 30 m
diameter sheave 245, which drives a belt 250 (e.g., a ribbon of
full-hard stainless steel, 2 m wide and 1 mm thick). The belt 250
wraps around an about 1 m diameter small drum 221 at the generator
220. The mooring cable 255 (e.g., a synthetic mooring rope) is
attached to an underwater mooring point (not shown), and rises at a
slope (e.g., 1.5:1 or 3:1 slope) to connect to an attachment point
256 at water level. At the water level it is held windward of the
base, by two neutrally buoyant standoff tubes (i.e., a standoff
structure) 260. From the attachment point 256, a rope (likely but
not necessarily different) or other tensile element 257 rises at
the same or substantially the same slope and connects to the
non-rotating axle 230, to reduce tower loads.
[0242] The four lattice towers 235 are each square with 100 inch
sides. The corner members are 7.times.7.times. 3/16, of A570 grade
50, each with a cross section area of 5.25 in {circumflex over (
)}2. The single-lacing diagonal members butt-welded at 30 degrees
from the column axis are 4.times.4.times.0.083 (=14 gauge). The
towers can be manually welded in boltable 40-ft lengths, in a
fixture that permits easy rotation about the column axis for good
welding access.
RELATED PATENT LITERATURE
[0243] German Patent Application Publication No. 102012009145A1,
entitled "Wind turbine with horizontal rotor shaft and with
rotatable tower", filed May 8, 2012.
[0244] German Patent Application Publication No. 202016001490U1,
entitled "Tower construction for a wind turbine", filed Mar. 8,
2016.
[0245] PCT Application No. PCT/EP2012/054552, entitled "An offshore
floating wind turbine for electric power generation", filed Mar.
15, 2012, and published as WO 2013/135291A1 (incorporated by
reference).
[0246] U.S. Pat. No. 8,729,723, entitled "Removable offshore wind
turbines with pre-installed mooring system", issued May 20, 2014
(incorporated by reference).
[0247] PCT Application No. PCT/EP2014/052224, entitled "Wind
turbine", filed Feb. 5, 2014, and published as WO 2014/122165A1
(incorporated by reference).
[0248] U.S. Pat. No. 8,801,363, entitled "Wind turbine with pulley
transfer box apparatus", issued Aug. 12, 2014 (incorporated by
reference).
[0249] PCT Application No. PCT/EP2014/052224, entitled "Floating
wind turbine structure", filed Apr. 17, 2014, and published as WO
2014/170027A1 (incorporated by reference).
[0250] U.S. Pat. No. 9,976,540, entitled "Floating wind turbine
structure", issued May 22, 2018 (incorporated by reference).
[0251] Chinese Patent Application Publication No. CN 105569928A,
entitled "Single point mooring type deep sea floating type draught
fan, filed Dec. 23, 2015.
[0252] U.S. Pat. No. 8,178,993, entitled "Floating wind turbine
with turbine anchor", issued May 15, 2012 (incorporated by
reference).
[0253] The teachings of the documents cited herein are hereby
incorporated by reference.
EQUIVALENTS
[0254] The present technology (including present methods) is not
limited to the particular embodiments described in this
application, which are intended as individual illustrations of
aspects of the present technology. Many modifications and
variations of the present technology can be made without departing
from its spirit and scope, as will be apparent to those skilled in
the art. Functionally equivalent methods and apparatuses within the
scope of the present technology, in addition to those enumerated
herein, will be apparent to those skilled in the art from the
foregoing descriptions. Such modifications and variations are
intended to fall within the scope of the appended claims. The
present technology is to be limited only by the terms of the
appended claims, along with the full scope of equivalents to which
such claims are entitled. It is to be understood that this present
technology is not limited to particular methods, compounds,
compositions, disease pathologies, or devices, which can, of
course, vary. It is also to be understood that the terminology used
herein is for the purpose of describing particular embodiments
only, and is not intended to be limiting.
* * * * *