U.S. patent application number 11/844877 was filed with the patent office on 2008-07-17 for rigging system for supporting and pointing solar concentrator arrays.
This patent application is currently assigned to CoolEarth Solar. Invention is credited to Jacques Jean Belanger, Eric Bryant Cummings, Kirsten Kaye Pace.
Application Number | 20080168981 11/844877 |
Document ID | / |
Family ID | 39616833 |
Filed Date | 2008-07-17 |
United States Patent
Application |
20080168981 |
Kind Code |
A1 |
Cummings; Eric Bryant ; et
al. |
July 17, 2008 |
RIGGING SYSTEM FOR SUPPORTING AND POINTING SOLAR CONCENTRATOR
ARRAYS
Abstract
Embodiments in accordance with the present invention relate to
the design of inexpensive mounting and pointing apparatuses for
linear arrays of solar energy collectors and converters. Particular
embodiments in accordance with the present invention disclose a
rigging system comprising at least one, and preferably a plurality
of, tensile cables onto which a plurality of solar modules are
fastened. Such an arrangement provides a way of suspending solar
modules over land, vegetation, bodies of water, and other
geographic features without substantial perturbation of the
underlying terrain. Certain embodiments comprise additional tensile
cables fastened to the solar modules, such that differential axial
motion of the cables produces a rotational motion component of the
individual solar modules of the array. This rotational motion
component effects an orientation control along one rotational
axis.
Inventors: |
Cummings; Eric Bryant;
(Livermore, CA) ; Pace; Kirsten Kaye; (Livermore,
CA) ; Belanger; Jacques Jean; (Livermore,
CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
CoolEarth Solar
Livermore
CA
|
Family ID: |
39616833 |
Appl. No.: |
11/844877 |
Filed: |
August 24, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60840156 |
Aug 25, 2006 |
|
|
|
60840110 |
Aug 25, 2006 |
|
|
|
Current U.S.
Class: |
126/600 ; 29/428;
405/259.1; 52/146; 52/690 |
Current CPC
Class: |
F24S 25/50 20180501;
Y02E 10/47 20130101; H01L 31/0547 20141201; Y02E 10/52 20130101;
F24S 30/40 20180501; F24S 2030/133 20180501; H02S 20/10 20141201;
H02S 20/30 20141201; Y10T 29/49826 20150115 |
Class at
Publication: |
126/600 ; 29/428;
52/146; 52/690; 405/259.1 |
International
Class: |
F24J 2/00 20060101
F24J002/00; B21D 39/00 20060101 B21D039/00; E04H 12/20 20060101
E04H012/20; E04C 3/02 20060101 E04C003/02; E21D 21/00 20060101
E21D021/00 |
Claims
1. A method of fastening solar modules to at least one cable under
tension, at least one of the cables connected to a damping
element.
2. A method according to claim 1 providing for axial motion of the
arrays of solar modules by applying a common axial translation to
cables connected to the modules
3. A method according to claim 1 providing for motion of the arrays
of solar modules normal to the cable axis by applying a common
translation of the cables normal to the cable axes.
4. A method according to claim 1 providing for rotation of the
solar modules by applying a relative axial motion between at least
one cable and at least two other cables.
5. A method according to claim 1 providing for rotation of the
solar modules by applying a relative motion to cables normal to the
axial direction.
6. A method according to claim 5 in which said actuation is
accomplished via a tensioned cable.
7. A method according to claim 1 in which at least one cable
comprises a cable having one or more of the functions selected from
mechanical connection, electrical connection, fluid connection,
heat exchanger, optical connection, networking connection.
8. A method according to claim 1 wherein flutter of the tensioned
cable is suppressed by the use of partially filled rigid liquid
conduits.
9. A method according to claim 1 wherein flutter of the tensioned
cable is suppressed by the use of partially filled or substantially
full flexible liquid conduits.
10. An assembly comprising a solar concentrator supported by a
tensile truss.
11. An assembly according to claim 10 wherein the tensile truss
comprises at least two tensioned cables connected by a transverse
element.
12. An assembly according to claim 11 wherein the tensile truss is
configured to exhibit a first order restoring force in resistance
to displacement normal to an axis of the truss along the tensioned
cables
13. An assembly according to claim 11 wherein the tensile truss
further comprises a third tensioned cable connected to the
transverse element between the first tensioned cable and second
tensioned cable.
14. An assembly according to claim 13 wherein the tensile truss is
configured to exhibit a first order restoring force at a connection
between the transverse element and third tensioned cable in
resistance to displacement in any direction along a plane defined
by the transverse element and the third tensioned cable.
15. An assembly according to 14 further comprising a second tensile
truss according to 11 oriented to in a different plane than the
first tensile truss.
16. A ground anchor comprising: a tube having a first end
configured to contact the ground, and an open end opposite to the
first end; a tapered collet having a flared portion disposed within
the tube and a narrow, threaded end protruding from the tube; and a
nut configured to engage the threaded end and rotatable to clamp a
cable disposed within the collet.
17. A method of transferring tensile forces from a truss structure
to the ground, the method comprising: in a first stage, drawing
together a plurality of tensile cables to a group at a pivot; and
in a second stage transferring tensile forces in the cables from
the pivot to the ground.
18. The method of claim 17 further comprising a third stage wherein
tensile forces in the cables are transferred to the pivot after
first drawing a plurality of cables together to a second group.
19. The method of claim 17 wherein the group is created forming a
cable bundle, mechanically mating the cables to a common rigid
part, or mechanically mating the cables to a secondary cable.
20. The method of claim 17 wherein the tensile forces are
transferred from the pivot to the ground by bringing one or more
cables or tensile elements to ground anchors or footings, or by
compressive elements or bending forces.
21. A method of rotating a truss, the method comprising: providing
a truss having a first end and a second end and a contact point,
the truss configured to rotate about a pivot point; providing a
driving mechanism; connecting a first end of a first cable to a
first end of the truss, and connecting a second end of the first
cable to the driving mechanism; connecting a first end of a second
cable to a second end of the truss, and connecting a second end of
the second cable to the driving mechanism; and causing the driving
mechanism to pull on the first cable at a first rate and pull on
the second cable at a second rate, such that the truss rotates and
the contact point engages the first cable or the second cable,
thereby imparting additional rotational moment to pivoting of the
truss.
22. The method of claim 21 wherein the driving mechanism comprises
a rotating drum.
23. The method of claim 22 wherein the first rate is imparted by a
first radius of the drum in contact with the first cable, and the
second rate is imparted by a second radius of the drum in contact
with the second cable.
24. The method of claim 21 wherein the driving mechanism comprises:
a first rotating drum in contact with the first cable and
configured to rotate at a first speed; and a second rotating drum
in contact with the second cable and configured to rotate at a
second speed.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The instant nonprovisional patent application claims
priority to U.S. Provisional Patent Application No. 60/840,110,
filed Aug. 25, 2006 and incorporated by reference in its entirety
herein for all purposes. The instant nonprovisional patent
application is also related to the following provisional patent
applications, the disclosures of which are incorporated by
reference in their entireties herein: U.S. Provisional Patent
Application No. 60/839,841 filed Aug. 23, 2006; U.S. Provisional
Patent Application No. 60/839,855 filed Aug. 23, 2006; and U.S.
Provisional Patent Application No. 60/840,156 filed Aug. 25,
2006.
BACKGROUND OF THE INVENTION
[0002] Solar radiation is the most abundant energy source on earth.
However, attempts to harness solar power on large scales have so
far failed to be economically competitive with most fossil-fuel
energy sources.
[0003] One reason for the lack of adoption of solar energy sources
on a large scale is that fossil-fuel energy sources have the
advantage of economic externalities, such as low-cost or cost-free
pollution and emission. Political solutions have long been sought
to right these imbalances.
[0004] Another reason for the lack of adoption of solar energy
sources on a large scale is that the solar flux is not intense
enough for direct conversion at one solar flux to be cost
effective. Solar energy concentrator technology has sought to
address this issue.
[0005] Specifically, solar radiation is one of the most easy energy
forms to manipulate and concentrate. It can be refracted,
diffracted, or reflected, to many thousands of times the initial
flux, utilizing only modest materials.
[0006] With so many possible approaches, there have been a
multitude of previous attempts to implement low cost solar energy
concentrators. So far, however, solar concentrator systems cost too
much to compete unsubsidized with fossil fuels, in part because of
excessive material and installation costs in the mechanical
supports and solar tracking apparatus for the collectors. While
many solar collectors utilize support trusses, their architectures
lead to excessive material usage, and complicated and time
consuming assembly and installation, rendering them unsuitable for
large-scale solar farming.
[0007] In addition, conventional concentrator support systems
require extensive grounds preparation, often making the land
unsuitable for other uses and destroying natural habitats.
[0008] Accordingly, there is a need in the art for designs that
efficiently and effectively support and align solar concentrators
without an excessive installation burden.
BRIEF SUMMARY OF THE INVENTION
[0009] Embodiments in accordance with the present invention relate
to the design of inexpensive mounting and pointing apparatuses for
linear arrays of solar energy collectors and converters. Particular
embodiments in accordance with the present invention disclose a
rigging system comprising at least one, and preferably a plurality
of, tensile cables onto which a plurality of solar modules are
fastened. Such an arrangement provides a way of suspending solar
modules over land, vegetation, bodies of water, and other
geographic features without substantial perturbation of the
underlying terrain. Certain embodiments comprise additional tensile
cables fastened to the solar modules, such that differential axial
motion of the cables produces a rotational motion component of the
individual solar modules of the array. This rotational motion
component effects an orientation control along one rotational
axis.
[0010] One embodiment in accordance with the present invention
further comprises a plurality of supports that provide for motion
of at least one cable normal to its axis. This produces a
rotational motion component of the individual solar modules of the
array to effect an orientation control along a second rotational
axis.
[0011] Certain embodiments provide for the common translation of a
plurality of cables connected to the modules. This allows the array
of modules to be translated normal to the axis. Another embodiment
provides for the common axial translation of cables such that the
modules can be translated in an axial direction.
[0012] Particular embodiments of the present invention may utilize
a rotary actuator in concert with at least one winch drum having a
variable cross section designed to feed and draw control cables
attached to structures of the present invention, at appropriate and
generally unmatched rates to effect a controlled motion of the
structures.
[0013] Particular embodiments of the present invention may
additionally comprise vibration dampers to reduce motion under wind
loading, etc. Particular embodiments can employ one or more
cable-based actuation systems to adjust orientation along
rotational axes.
[0014] Particular embodiments may use ground mounted rigid posts to
anchor the system and transmit compressive and other loads to the
ground. Particular embodiments may further employ a system of guy
cables and ground anchors to distribute loads to the ground and
reduce bending forces in ground mounting posts. Particular
embodiments of these ground anchors provide for adjustment of the
relative guy cable position by means of a collet whose clamping
force increases with guy-cable load. Particular embodiments of
these ground anchors further provide for tightening of these
collets via mechanical preloads provided by threaded elements.
Particular embodiments of the ground anchors provide mounting and
indexing features to support the use of a removable hydraulic cable
tensioner tool which works in concert with the means of mechanical
preloading to allow convenient and reliable cable-tension
adjustment under high tensile loads. Particular embodiments of
ground anchors further contain mechanical features to allow
installation in the ground via a rotating tool that may further
provide an axial compression or motion that may be coordinated with
the rotation to drive the ground anchor into the ground. Particular
embodiments of this tool and anchor may further provide for
drilling a pilot hole for the anchor prior to or simultaneous with
the driving operation.
[0015] An embodiment of a method in accordance with the present
invention comprises fastening solar modules to at least one cable
under tension, at least one of the cables connected to a damping
element.
[0016] An embodiment of an assembly in accordance with the present
invention comprises a solar concentrator supported by a tensile
truss.
[0017] An embodiment of a ground anchor according to the present
invention comprises a tube having a first end configured to contact
the ground, and an open end opposite to the first end. A tapered
collet having a flared portion is disposed within the tube and a
narrow, threaded end protruding from the tube. A nut is configured
to engage the threaded end and rotatable to clamp a cable disposed
within the collet.
[0018] An embodiment of a method in accordance with the present
invention of transferring tensile forces from a truss structure to
the ground, comprises, in a first stage, drawing together a
plurality of tensile cables to a group at a pivot, and in a second
stage transferring tensile forces in the cables from the pivot to
the ground.
[0019] An embodiment of a method in accordance with the present
invention of rotating a truss, comprises, providing a truss having
a first end and a second end and a contact point, the truss
configured to rotate about a pivot point, and providing a driving
mechanism. A first end of a first cable is connected to a first end
of the truss, and connecting a second end of the first cable to the
driving mechanism. A first end of a second cable is connected to a
second end of the truss, and connecting a second end of the second
cable to the driving mechanism. The driving mechanism is caused to
pull on the first cable at a first rate and pull on the second
cable at a second rate, such that the truss rotates and the contact
point engages the first cable or the second cable, thereby
imparting additional rotational moment to pivoting of the
truss.
[0020] These and other embodiments of the present invention, as
well as its features and some potential advantages are described in
more detail in conjunction with the text below and attached
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 shows a drawing of a high-concentration-factor
two-angle tracking solar collector system that utilizes rigging,
damping, actuation, and ground tackle according to one embodiment
of the present invention. FIG. 1A shows details of an internal
segment of the solar collector system in FIG. 1.
[0022] FIG. 2 shows a perspective view of a one-angle tracking
solar collector system that employs rigging, damping actuation, and
ground tackle according to an embodiment of the present invention.
FIG. 2A shows an enlarged view of the system of FIG. 2.
[0023] FIG. 3 shows a simplified perspective view of a two-angle
tracking solar collector system that illustrates rigging and
actuation according to an embodiment of the present invention.
[0024] FIG. 4A shows a simplified schematic diagram illustrating
one embodiment in accordance with the present invention. The
embodiment of FIG. 4a shows a three-cable rigging assembly of solar
modules according to the present invention, wherein the assembly is
controlled only at the termini.
[0025] FIG. 4B shows a simplified schematic view of a rigging
assembly that is controlled at the termini and an internal point.
Additional internal control points could be used to maintain
control authority over large distances.
[0026] FIG. 5A shows a simplified schematic view of an embodiment
in accordance with the present invention wherein common axial
motion of the control cables translates modules axially. Such an
embodiment would allow an operator to mitigate shading between rows
of modules, for example.
[0027] FIG. 5B shows a simplified schematic view of an embodiment
wherein common motion of the control cables normal to their axis
translates modules accordingly. Such an embodiment would allow an
operator to lower the modules for servicing, for example.
[0028] FIG. 5C shows a simplified schematic view of another
embodiment in accordance with the present invention involving a
three-cable rigging assembly of solar modules. In accordance with
the embodiment of FIG. 5C, cable 3 moves axially with respect to
cables 1 and 2 to rotate solar modules simultaneously.
[0029] FIG. 5D shows a simplified schematic view of another
embodiment in accordance with the present invention involving a
four-cable rigging assembly of solar modules. In accordance with
the embodiment of FIG. 5D, cables 3 and 4 move axially with respect
to cables 1 and 2 to rotate solar modules simultaneously. The use
of one redundant cable prevents wind forces from producing large
torques on the solar modules.
[0030] FIG. 5E shows a simplified schematic view of another
embodiment in accordance with the present invention. In accordance
with the embodiment of FIG. 5E, relative motion of the control
cables normal to their axes produces rotational motion of the
modules about the cable axis.
[0031] FIG. 6 shows a drawing of a truss element actuated via a
circular pulley.
[0032] FIG. 7 shows a drawing of a truss element actuated via a
material efficient drive scheme according to a component of an
embodiment of the present invention.
[0033] FIGS. 8A-B shows views of a single drum roller according to
a component of embodiments of the present invention that can
actuate the truss element shown in FIG. 7.
[0034] FIG. 9 shows a double counter-rotating drum mechanism
according to a component of embodiments of the present invention
that can actuate the truss element shown in FIG. 7.
[0035] FIG. 10 shows a double co-rotating drum mechanism according
to a component of embodiments of the present invention that can
actuate the truss element shown in FIG. 7.
[0036] FIG. 11 shows a tensile truss utilized in the embodiment of
the present invention in FIGS. 1 and 1A.
[0037] FIG. 12 shows a tensile truss that conveys a tensile load to
other apparatus utilized in the embodiment of the present invention
in FIGS. 1 and 1A.
[0038] FIGS. 13A-B shows two alternate three-dimensional tensile
truss components of embodiments of the present invention.
[0039] FIG. 14 shows a compressive truss component of embodiments
of the present invention for supporting tensile structures.
[0040] FIG. 15 shows a compressive truss component that provides
for reduced solar collector shading and actuation using a material
efficient scheme according to a component of embodiments of the
present invention.
[0041] FIGS. 16A-D show views of a compressive truss having
increased out-of-plane stiffness utilized in the embodiment of the
present invention shown in FIGS. 1 and 1A.
[0042] FIG. 17 shows a detail of the out-of-plane stiffener.
[0043] FIGS. 18A-B show an embodiment of an end termination of an
array in accordance with the present invention in which tensile
cables are brought together in one stage and their tensile load
transferred to a pivot axis in another stage and then to ground
tackle in a third phase.
[0044] FIGS. 19A-C show embodiments of an end termination of an
array in accordance with the present invention in which the outer
sets of tensile cables are brought together in on stage, then the
entire set of cables or the tensile loads of the cables are brought
together to a pivot axis in another stage and then to ground tackle
in a third phase.
[0045] FIGS. 20A-D show embodiments of posts and ground tackle for
interior supports within the array.
[0046] FIGS. 21-21A show perspective and detailed views,
respectively, of a ground anchor component of the embodiments of
the present invention shown in FIGS. 1, 1A, and 2.
[0047] FIG. 22 shows a detail of cable-coupler plate that is a
component of the embodiments of the present invention shown in
FIGS. 1 and 1A.
[0048] FIGS. 23A-B show details of cable mounts employed by the
embodiment of the invention in FIG. 1.
[0049] FIG. 24 shows a plan view of a 1 MW farm comprising
sequential and side-by-side arrays of rigged solar modules.
[0050] FIG. 25A shows a sketch of a tensile truss according to an
embodiment of the present invention constructed from a metal
sheet.
[0051] FIG. 25B shows a sketch of part of a chain of tensile truss
units constructed from a long metal sheet, roll, or continuous
rolling operation.
DETAILED DESCRIPTION OF THE INVENTION
[0052] Embodiments in accordance with the present invention relate
to the design of inexpensive and minimum-material mounting and
pointing apparatus for linear arrays of solar energy collectors and
converters. Particular embodiments in accordance with the present
invention disclose the design of a rigging system comprising at
least one tensile cable onto which a plurality of solar modules are
fastened, thus providing a means of suspending solar modules over
land, vegetation, bodies of water, and other geographic features
without preparation of perturbation of the underlying terrain. One
such embodiment 100 is shown in FIG. 1. FIG. 1A shows details for a
typical interior segment of the array in FIG. 1 for clarity.
[0053] Elements 102 in FIGS. 1 and 1A are two-dimensional solar
concentrators that require accurate pointing and tracking of the
sun along two rotational axes. In this embodiment, the diameter of
these concentrators is 2.5 m and the distance between vertical
posts is 20 m. The side-by-side arrangement of these concentrators
provides cost advantages in the ability to share equipment,
minimize the length of support conduits and cabling, etc.
[0054] One rotational axis, 104, coincides with the long axis of
the rigging system. Each concentrator individually pivots about a
second rotational axis 106 along a diameter that is perpendicular
to the first axis.
[0055] Elements 108 are trusses comprised of elements under tension
to provide stiffness in the plane normal to the second rotational
axis of the concentrators. Elements 110 are tensile trusses that
provide stiffness along the secondary axis and further provide a
tension force along the diameter of the concentrators that assists
with maintaining the relative position of elements within the
concentrators and facilitates servicing of the concentrators.
Elements 112 are pivoting compressive trusses that support the
tensile trusses at positions in the interior of the array. Elements
114 are pivoting compressive trusses that support the tensile
trusses at terminal ends of the array.
[0056] Element 116 is a multiple-element cable that includes a
thermal management system that damps vibrations. Embodiments of
such multi-element cables are described at length in U.S.
Provisional Patent Application No. 60/839,855, filed Aug. 23, 2006
and incorporated by reference herein for all purposes. Elements 118
are liquid-air heat exchangers that provide additional vibration
damping.
[0057] Compression forces are required to maintain tensile forces
in the cable. The posts 120 and compressive trusses 112 and 114
support a portion of the compressive load, but the primary
compressive backbone of the tensile truss is the ground on which
the system is mounted via the ground tackle 122. Architectures
according to embodiments of the present invention utilize the
ground as the primary compressive structural element and thereby
externalizes a significant amount of the structural material
cost.
[0058] Elements 124 are features of the compressive trusses that
facilitate actuation along the primary axis of the system. Elements
126 are cables that are drawn and fed from a specialized drum winch
128 to rotate the compressive trusses along the primary axis,
thereby rotating the entire tensile truss system and concentrators.
A similar actuation scheme is used on the secondary axes of the
concentrators. The resulting relative motion of tensile cables
pivot each concentrator.
[0059] Solar energy modules provide electricity, heat, or other
conversion product approximately in proportion to the collection
area of the module. To obtain high total collection area, there are
several advantages to arraying multiple distinct solar modules
rather than a single large solar module, e.g., for ease of
distribution, installation, and service, to provide for more
practical solar tracking, etc. One minimum material implementation
of a solar collector is an inflated concentrator or "balloon" as
shown in FIGS. 1 and 1A. As used herein, the term "balloon" refers
to this specific type of concentrator but is intended to refer more
generally to any type of solar energy collector, e.g., one-sun
solar cells and modules, rigid concentrator mirrors, Fresnel
lenses, etc.
[0060] The conventional approach to mounting solar modules is to
use an extended truss made of beams and extrusions that support
compressive loads and consequently require a significant amount of
material to resist bucking and distortion under wind loading, or
completely separate mounts, each individually tied to a solid
surface, e.g., ground via concrete pads. Such conventional mounts
may offer disadvantages in that they: [0061] may require excessive
material to support tensile loads and prevent deflection; [0062]
may require redundant mounting and pointing apparatus; and/or;
[0063] may require excessive preparation of or access to the
terrain below the modules, impacting the use of land below the
modules and potentially increasing the environmental impact of
solar installations.
[0064] Moreover rigid structures must be designed not to buckle or
fail in bending or shear under the heaviest design loads, which are
generally far more severe than loads at the maximum operating
loads. For example, a solar installation may be designed to survive
a wind storm of 125 mph, but may be designed to operate efficiently
only at a wind speed of 25 mph or lower. In this case, wind
generated forces on the structure at the survival condition are 25
times larger than those encountered in the most severe operating
conditions.
[0065] The use of flexible cables allows one to exploit the large
difference between operating and surviving regimes. For example,
cables can be pretensioned such that they always remain in tension
under operating conditions but may become slack (under compression)
under survival conditions. Because cables generally do not fail in
bending or buckling, stress considerations require cables only to
be designed to withstand maximum tensile loads under survival
conditions.
[0066] Because pointing stability may be important in many
applications, stiffness considerations typically drive the size of
truss elements according to the present invention. Under tension,
the axial stiffness and mechanical behavior of cables is not
substantially different from that of a rigid cylindrical extrusion
having the same material per unit axial length. For a given span
and material use, elongational stiffness is typically far greater
than bending or torsional stiffness. Embodiments according to the
present invention are architected to rely substantially on axial
stiffness, potentially driving a far lower material use per unit
collector area than conventional collector mounts.
[0067] Possible disadvantages of minimum materials structures is
their tendency to exhibit vibrational resonances. The deflection of
such structures can be much larger for dynamic forcing than static
forcing, particularly dynamic forcing having a spectral component
near or at a vibrational resonant frequency.
[0068] Accordingly, approaches to mitigating dynamic deflections
can include one or more various combinations of [0069] increasing
the cross-sectional area of structural elements to make them
stiffer; [0070] adjusting cable tension to drive resonances away
from dominant excitation frequencies; [0071] adding at least one
node to at least one truss at periodic (repeating), aperiodic
(non-repeating), or quasi-aperiodic (occasionally repeating)
intervals; [0072] adding at least one mass to at least one truss at
periodic (repeating), aperiodic (non-repeating), or quasi-aperiodic
(occasionally repeating) intervals, including unequal masses;
[0073] adding at least one spring to at least one truss at periodic
(repeating), aperiodic (non-repeating), or quasi-aperiodic
(occasionally repeating) intervals, including unequal springs;
[0074] constructing at least one truss at least in part from
materials that intrinsically damp vibration; [0075] constructing at
least one truss at least in part from assemblies of materials or
patterned materials that exhibit enhanced vibration damping over
that of the component material or materials; [0076] adding at least
one vibration damper not directly associated with the truss
assembly, e.g., liquid-filled bladders, straight and serpentine
arrangements of tubes with or without gas pockets, multiple-element
flexible cables, dash-pots, skids, elements that rub under
vibration, and the like;
[0077] Certain embodiments incorporating these approaches to reduce
dynamic deflections employ judicious material selection or
operation (e.g., choosing a vibration damping cable material or an
appropriate cable tension); provide secondary functions (e.g.,
using liquid-filled or gas/liquid channels that also facilitate
thermal management). Other embodiments include features that can be
installed conveniently in the field in response to observed
vibration problems (e.g., liquid-filled or liquid-gas filled
dampers that serve no other purpose, masses, or dashpots between
truss elements or between cables and fixed objects).
[0078] Embodiments of the present invention accordingly employ a
plurality of solid, i.e. able to resist static stresses,
high-aspect-ratio tensile members. Alternative embodiments in
accordance with the present invention may employ a plurality of
non-solid members.
[0079] As used herein, the term "cable" may comprise at least one
wire, extrusion, wire rope, natural or synthetic rope, weaves,
fiber-reinforced composites, fiber-reinforced ropes, cable
assemblies, and the like. In a particular embodiment, a flexible
metal tape or strip that is able to buckle under compression
without damage may be used. As used herein, the term "cable" may
also refer to any structural member that is not required to support
substantial bending loads or axial compression in normal operation,
regardless of whether the actual member itself is able to support
bending or compression. Thus, embodiments of the present invention
provide for the use of one or more conventional compressive truss
elements (such as angle extrusions, I-extrusions, C-extrusions,
rods, tubes, or rectangular bars) in place of wire ropes or the
like.
[0080] As used herein, the term "fastened" means at least partly
constrained from relative translation in at least one direction or
partly constrained from relative rotation in at least one direction
over at least a finite range of motion. A further element of this
invention is a method of fastening the balloon to the cables. The
fasteners must attach the balloon firmly to the cable, and allow
for smooth rotation during tracking. These fasteners may comprise
of a swaged ball, sleeve, double sleeve, eye, cable tie, clamp, and
the like. A preferred embodiment of the fastener is a loop of
flexible material such as rope or wire that tightens around the
cable and attaches to the balloon. Because the balloon is held by a
single strand or cable, it is free to rotate around the axis of the
strand or cable, while staying firmly attached. This replacement of
bearings and sliding joints with cables that undergo elastic
torsion is an element of other embodiments of this invention,
including replacing a thrust bearing at the end termination of the
arrays, etc.
[0081] One simple embodiment in accordance with the present
invention comprises a single tensioned cable onto which multiple
solar modules are fastened. However, such an arrangement may
provide little rotational stability for the modules. Accordingly,
another embodiment in accordance with the present invention
comprises a plurality of tensioned cables spaced so that modules
are fastened at two separate points, and are thus constrained in
their rotation about the axis bisecting the cables than for a
single cable. In accordance with a specific embodiment, the cables
may be oriented substantially parallel to each other.
[0082] FIGS. 2-2A show perspective and enlarged views,
respectively, of internal segments of one-dimensional tracking
solar collector 200 according to an embodiment of the present
invention that employs two such tensile cables. Elements 202 are
tensile cables; elements 204 are compressive trusses; elements 206
are compressive posts; and elements 208 are solar collectors, e.g.,
solar panels. Elements 210 are optional vibration damper to reduce
cyclical stresses.
[0083] Elements 204 are designed to support actuation using drawn
and fed cables 212 in concert with a specialized drum 214. However,
a wide range of alternative actuation schemes are possible,
including but not limited to gear motors, ratcheting motors, motor
driven pulleys or chains, rotary hydraulic or pneumatic actuators,
linear electrical, hydraulic, or pneumatic actuators, etc.
Alternatively, a fixed or manual adjustment is possible.
[0084] The expense and complexity of a more sophisticated cable
truss like those in FIGS. 1 and 1A may not be justified for
collectors having a cosine dependence of power on pointing angle
error. However, considerations such as vibrational resonances and
cyclical loading under large-scale vibrations may justify the use
of stiffer designs. Such a non-tracking or one-dimensionally
tracking collector array may be mounted substantially in an
East-West orientation.
[0085] FIG. 3 shows a drawing of internal segments of a
two-dimensional tracking solar collector 300 according to an
embodiment of the present invention. Like in FIGS. 1-2A, the entire
array pivots about one axis 302. In addition, each collector pivots
about individual secondary axes 304 similarly to the embodiment in
FIGS. 1-1A.
[0086] In the embodiment of FIG. 3, this rotation results from
motion 306 of a control cable 308 with respect to another control
cable 310 that doubles in this embodiment as a structural support
cable. Pivoting tether points 312 convert the relative cable motion
to rotary motion of the solar collector.
[0087] In this embodiment 300, the truss comprises a side of the
collector, a compressive element 314 and a tensile element 316.
Such a two-dimensional collector array may be mounted substantially
in a North-South orientation, such that self-shading effects near
dawn and dusk can be reduced by spacing parallel rows of these
arrays further apart in the East-West direction, making less
efficient use of land area, but more efficient use of the solar
collector area.
[0088] In this embodiment 300, a damper 320 is connected to the
truss through an element 318. Damper 320, may be a hollow member
partially filled with liquid or a solid, functions to limit
vibration or flutter of the truss in response to external forces
such as wind.
[0089] Another aspect in accordance with embodiments of the present
invention is the mechanism to provide common axial motion of cables
fastened to the modules to effect axial motion of the modules as
shown in the embodiment of FIG. 5A. Such motion is useful for
example to minimize shadowing effects between adjacent rows of
solar modules disposed on different cable systems at different
times of day and different days of the year. This common axial
displacement can be fixed at install time, manually adjusted, or
actuated.
[0090] A further element of embodiments in accordance with this
invention is a mechanism that provides for the common translation
of at least the cables fastened to the concentrator in a direction
having a component normal to the axes of the cables. This allows
the modules to have a similar translation, as shown in the
embodiment of FIG. 5B. This motion for example could be used to
raise and lower an assembly of modules, for example for ease of
installation or service, to mitigate wind loading, to minimize dust
deposition, or to reduce shadowing effects.
[0091] A further element of embodiments in accordance with this
invention is a mechanism that provides for the tension in the
cables to be raised or lowered. For example, such a mechanism could
slacken cables to facilitate lowering the array e.g., for
maintenance, to avoid wind stresses, etc. This mechanism can be
accomplished by any combination of the following: winch, block,
cleat, pulley system, power winch, chock, clamp, and the like.
[0092] Another element in accordance with an embodiment of the
present invention comprises a third control cable that is fastened
to the module such that the relative axial motion of one or more
cables produces a rotational motion component of the module. This
third control cable thus provides for angular positioning of the
module in one direction as shown in the embodiment of FIG. 5C.
[0093] In general, the number of cables in the tensile truss will
be larger than the number of control cables. The number of control
cables cited in these embodiments refers to structures having the
kinematic effect of a control cable, i.e., the kinematic control
cable could be an assembly of cables and compressive elements that
could logically be replaced by a single cable to perform a
kinematic operation.
[0094] Another embodiment comprises arranging the fastening points
on the solar collector module such that the axis of rotation is
substantially normal to the axis of the cables. Another embodiment
in accordance with this invention, as shown in FIG. 3, comprises a
fourth cable attached to the module. The fourth cable is used in
conjunction with the third cable, such that the differential axial
motion of the third and fourth cable with respect to the first and
second cable produces a rotational motion of the module as shown in
FIG. 5D. The fourth cable is kinematically redundant, but can
reduce the stress or moment associated with rotation or wind loads
on the solar collector module.
[0095] Another embodiment of the present invention comprises an
arrangement of six cables disposed such that relative motion of
three substantially parallel pairs of cables effect substantially
the same rotation to two concentrators arranged side-by-side. Such
a configuration is employed by the embodiment shown in FIGS. 1 and
1A. Such an arrangement uses a rigid connection between the
concentrators to obviate one pair of control cables.
[0096] Another embodiment of the present invention comprises an
arrangement of seven cables having the same structure as the
six-cable system, but providing an extra control cable at the
center. Such a configuration can reduce torsional loads or stresses
in the connection between the concentrators.
[0097] Another embodiment of the present invention comprises an
arrangement of eight cables to service two side-by-side
concentrators. In such an arrangement, the connection between
concentrators, if any, does not necessarily bear any
pointing-related loads.
[0098] A further element in accordance with an embodiment of the
present invention comprises a mechanism for rapidly rotating the
solar modules, e.g., in response to a serious fault such as
critical overheating. A rotation mechanism in accordance with an
embodiment of the present invention may comprise a manual or
automatic release mechanism. Such a release mechanism could be
automatically or manually restored to an operating position.
[0099] A further element in accordance with one embodiment of the
present invention comprises a mechanism that provides for motion
having a component normal to the cable axis of at least one cable
relative to other cables such that the solar modules experience a
motion having a rotational component.
[0100] A further element in accordance with one embodiment of the
present invention comprises a mechanism that provides for motion of
all cables fastened to the solar modules in the trajectory of a
substantially rigid-body rotational motion about some axis. Such
cable motion produces a rotational motion of the solar modules
having a component in the axial direction of the cable, as shown in
FIG. 5E and the embodiments in FIGS. 1-3. It is preferred that the
axis of rotation coincide with a point on the mechanism that is
substantially free of moments from the cable tension and can rotate
about a pivot that is substantially under tension.
[0101] Actuation
[0102] Because the cables only support tensile loads substantially,
a coordinated motion of such mechanisms could occur at least near
the two ends of the cables, as shown in FIG. 4A and possibly also
at points in between, as shown in FIG. 4B and the embodiments in
FIGS. 1-3. This coordinated motion can be accommodated by the use
of a wide variety of mechanisms or schemes, including rigid
mechanical linkages, slaved servo motors, rotary electrical,
pneumatic, or hydraulic actuators or motors or linear electrical,
pneumatic, or hydraulic actuators, etc. The motion of the actuators
could be coupled to the truss through one or more gears, pulleys,
chains, timing belts, etc.
[0103] Any of the above arrangements could support embodiments in
accordance with the present invention. However, a preferred
implementation may employ one or more additional cables in tension
whose axial motion produces a common actuation of all
mechanisms.
[0104] Such a cable or cables could turn a pulley, preferably
designed to index to features on the cable in the manner of a
timing belt, which drives a rotational element such as a gear, gear
train, a wheel or wheel segment or the like to actuate coordinated
motion. The actuation cable may contain cable splices or shunt
connections, e.g., via swages or clamps to facilitate actuation of
a mechanism or mechanisms that lied between the cable terminal.
Alternatively the actuation cable itself can be used directly or
via a pulley or other force-direction-changing element to actuate
mechanisms by applying tensile forces.
[0105] In certain embodiments, the tensile forces may be resisted
by a combination of spring forces, or tensile forces from a
complementary cable. In accordance with one embodiment, the
complementary cable is an extension of the same cable moving in
substantially the opposite direction, e.g., through the action of a
180 degree, direction changing pulley. Another preferred embodiment
employs the complementary axial motion of two splices on
complementary sides of the same cable.
[0106] Such actuation cables may not be sufficiently stiff to
actuate over long runs. In some embodiments according to the
present invention, actuation mechanisms can be placed at shorter
intervals than an entire array to increase stiffness and provide
for better wind-load handling.
[0107] FIG. 6 shows an illustration of a pulley system 600 for
actuating the rotation of a support truss 602 relative to a smaller
pulley 612 from the frame of reference of the truss. In this frame,
the smaller pulley moves in an arc 614 about the pivot point of the
truss. Cable 608 and 610 are simultaneously drawn and fed to
produce this motion. Possible advantages of using a circular pulley
or arc segment B for this rotary actuation may the same rates of
feeding, and drawing 608 and 610 are identical, and that the
rotation rate at a given feed rate is constant. A possible
disadvantage of using a circular-arc pulley is the need for
significant material simply to maintain the circular shape of the
pulley.
[0108] In the illustrated material efficient pulley 604, the region
between spokes is subject to bending under belt tension. Such
bending can reduce the stiffness of the actuation and reduce
tracking accuracy under wind, gravity, and inertial loading. An
amount of material may be needed to make a sufficiently stiff
pulley to maintain rigidity. Moreover the rotation forces can only
be transmitted to the truss frame at points such as 606 where the
pulley is near the truss elements. If these connection points lie
in the interior of a truss element, the strength and stiffness of
the actuation further relies on the bending strength and stiffness
of the truss elements.
[0109] Other embodiments in accordance with the present invention
alternatively use a minimum material cabling scheme that
substantially eliminates bending. This alternative approach may
utilize existing elements of compressive truss assemblies.
[0110] FIG. 7 shows an embodiment 700 of such an actuation method,
taking the frame of reference of the truss. The compressive truss
702 is rotated about its pivot point 720 by drawing and feeding
cables 708 and 710 using mechanism 712. Driving mechanism 712 can
alternatively be mounted on the truss to be actuated and the cables
708 and 710 pull on points and over pivots that lie in another
frame of reference, e.g., that of the ground or of another truss
element. The cables 708 and 710, which can be portions of the same
cable or two distinct cables, pull directly on the truss at points
716.
[0111] Over some angular range, 708 and 710 lay across a contact
point 718 that is held in place by the truss members 704. This
contact point prevents the lever arm between the cables 708 and 710
and the pivot from varying too greatly with the rotation of the
truss. The contact point 718 and the two points 716 can be viewed
as a triangular approximation of a circular arc. The use of
additional pivot points can provide a better arc approximation, if
warranted. As the truss rotates around its pivot, the driving
mechanism 712 moves relative to the truss along an arc 714.
[0112] A possible advantage of this arrangement is material
efficiency for a given actuator stiffness: the actuator stiffness
is primarily derived from the axial stiffness of truss elements.
Moreover truss elements that support a pivot can confer other
advantages.
[0113] For example, in the embodiment of a truss in FIG. 7, the
pivot support elements 704 redistribute compressive stresses in the
truss in such a way that less material can be used in other
elements. The only truss part required specifically for actuation
is the end of the member forming the contact point 718, which is
compact and simple compared to the extra wheel in FIG. 6 (604).
[0114] The approach shown in FIG. 7 could offer a disadvantage in
the need to draw and feed cables at different rates and at rates
that vary with the position of the truss. However, this could be
overcome by operating a coordinated pair of motors driving, e.g.,
conventional drum winches to feed and draw cables separately.
[0115] Alternatively, a mechanical ratching device could be
produced that exploits the finite elasticity of the control cables
and structure to allow a single motor feed and draw cables
alternately while not detensioning the cables. This arrangement has
the advantage of being able to regulate the cable tension by
adjusting the relative amounts of drawing and releasing from their
kinematically matched ideal. However, it has the disadvantage of
mechanical complexity, the need for ratchets and pawls to be
operated frequently under load and the need for the complex
mechanisms to withstand loads at full rated wind speed, unless a
separate braking mechanism is employed.
[0116] An embodiment of the actuation motor employs a specially
designed drum that, by construction, feeds and draws cables when
rotated at substantially the kinematically matched rates required
of the truss as shown in FIGS. 8A-B. The drum can further be
designed such that the rotation rate of the truss is constantly
proportional to the rotation rate of the drum regardless of
angle.
[0117] The design of such drums is straight-forward, but may
require iteration. A first step is to analyze the trajectory of the
cable mounting and pivot points, truss pivot points, and the
position of the drum or drums. In a first iteration, the drum can
be approximated as having a fixed cable diameter. The required feed
and drawing rates for all truss angles could be calculated.
[0118] The instantaneous drum radius should be approximately equal
to the respective feed or drawing rate required. The drum radius
vs. angle curve can then be iteratively refined taking into account
the actual cable angles, out-of-plane motion, and cable
stretch.
[0119] The cable stretch calculation may take into account static
loads on the actuator cables, e.g., from the system weight as well
as cable pretension. A highly refined drum design may result in
less tension variation in actuator cables and improved absolute
angular accuracy, etc. Because of the finite rigidity of all the
elements of the actuator system, however, an approximate drum
design may suffice. It is possible to accommodate drum inaccuracy
by the addition of a compliant support, however this lowers the
stiffness of the actuator. Not all arrangements of drum, pivots,
and attachment points can be accommodated by a drum e.g., since
cables will not follow a local indentation in the drum radius but
will skip from one tangent surface, over the indentation, to the
next tangent surface. Such ill-conditioned drums are typically
encountered with ill-conditioned geometries, e.g., sharp pivot
points.
[0120] To avoid such trouble, cable pivots may be tailored curves
or simple arcs designed such that cables engage and disengage with
the pivots at a point that where the surface is substantially
tangent to the disengaged cable path. If cable pivots are made too
large, they may reduce stiffness by producing off-element-axis
loads that contribute to element bending.
[0121] Because the truss and its angular rotation range is
symmetrical, the drum in FIGS. 8A-B can be decomposed into a pair
of identical drums placed back to back. In some embodiments
according to the present invention, these drums can be separated
and mounted such that they counter rotate at the same rate, as
shown in FIG. 9 or co-rotate as shown in FIG. 10. Both co-rotating
and counter-rotating arrangements provide for convenient driving
from a single rotary actuator (e.g., motor, crank, etc.)
[0122] In some embodiments of the present invention, the relatively
large diameter of the drum relative to common motor shaft diameters
can be exploited to provide a significant effective gear ratio,
e.g., by the use of a small spur, worm, or helical gear on the
motor shaft and a circumferential spur or helical gear mounted,
cast, or machined onto one or more drums. Such an arrangement can
obviate or mitigate any additional gearing requirements on the
rotary actuator that turns the drums. Multiple drums could also be
actuated by drawing a cable, pulley, or timing belt, as described
earlier to reduce the number of actuator drives. In the co-rotating
arrangement in FIG. 10, both cables lie in substantially the same
plane perpendicular to their axes of rotation. It is also possible
to coordinate an axial motion of one or drums with rotation such
that the out-of-plane motion of the cables is substantially
suppressed.
[0123] In some embodiments of the present invention, a braking
device or mechanical latch can be deployed to lock the position of
the array, for example, in preparation for a severe windstorm. Such
a latch may be deployed in positions throughout the angular range
of travel or at one or more specific "home" positions.
[0124] Oscillation Damping and Suppression
[0125] If all cables have a similar catenary shape under loading of
the modules and the cables are parallel to each other, no module
rotation arises from the lowest order modes of oscillation of
excited cables (e.g., by wind forcing). This feature is useful for
solar modules that employ solar concentration at high concentration
factors, since unintended rotation produces angular misalignments
that seriously reduce the module efficiency.
[0126] However, higher-order oscillation modes can produce angular
misalignments. An element of embodiments in accordance with this
invention is to deploy oscillation damping elements on the cables
to reduce high-order oscillations, including damping elements that
cross from one cable to another and elements that propagate along
one cable. Such damping elements could include rigid or flexible
conduits partly filled with liquid, (e.g., water), gels, pastes,
emulsions, thixotropic materials, colloidal suspensions, fibers, or
granular materials (e.g., dirt, sand, sawdust, and the like), or
flexible conduits that are substantially filled with liquid and
possibly suspended particles, such that sloshing of the liquid
under forcing from the cable oscillation damps the oscillation
energy, as shown in the embodiments in FIGS. 1-2.
[0127] A particular embodiment in accordance with the present
invention could employ such conduits further as heat exchange
components for the solar modules, using a coolant as the liquid.
Alternatively, aerodynamic flutter dampers could be employed
similar to those used on power cables in windy areas.
[0128] Low-order relative oscillations of the cables toward and
away from each other can produce compressive and tensile stresses
on the solar modules that could affect their function or lifespan.
One or more linkages that support compression and/or tension
fastened between cables substantially normal to the axis of the
cables can reduce such damage or degradation. These could be
inserted where needed. Relative axial motion of cables produced by
flutter could be suppressed by an arrangement of such linkages
inclined at an angle with respect to the normal of the cable axis.
Such linkages can kinematically support any relative cable motion
that is desired to point the solar modules.
[0129] Tensile Trusses
[0130] The simplest tensile truss is a tensioned cable. Such a
truss generally has considerably larger stiffness in the axial
direction than directions perpendicular to this axis. A tensioned
cable resists axial deflection with a force that is initially
linear with the ratio of the displacement to the cable length,
whereas the resistive force to perpendicular deflections is
initially of third order in the ratio of the displacement to the
cable length and therefore is much less stiff to forces
perpendicular to the cable.
[0131] Some applications, such as unconcentrated or
low-concentration-factor solar collectors may tolerate large
perpendicular deflections. Other applications require greater
stiffness and the use of a more complicated tensile truss
structure.
[0132] The stiffness of cable systems according to the present
invention can be increased by the use of tensile truss structures
such as that shown in FIG. 11. This tensile truss 1100 is part of
the embodiment shown in FIGS. 1-1A.
[0133] The primary cables 1102 are tensioned sufficiently that the
transverse elements (here vertical cables) 1106 are substantially
under tension in normal operation. A third cable 1104 can provide
stiffness along its axis. An axial deflection of a transverse
element is resisted by axial stiffness of cable 1106 and of the
cables 1102. That is, vertical motion of an element requires a
first-order axial displacement of cable 1102. Without such a
tensile truss, the resistance of a cable to deflection
perpendicular to its axis builds up far more slowly, initially as
the cube of the displacement. However, this stiffness advantage can
be lost depending on how cables 1102 are supported.
[0134] Some embodiments of the present invention utilize
compressive trusses to constrain cables 1102 substantially against
motion perpendicular to their axes. Such embodiments rely on
substantially spatially uniform wind loading within the interior of
a series of tensile trusses to suppress differential axial
displacements by symmetry.
[0135] Other embodiments utilize additional compressive trusswork
at various positions, e.g., periodic, aperiodic, or
quasi-aperiodic, within a series of tensile trusses to stiffen the
truss against differential axial displacement of cables 1106. Other
embodiments utilize modest bending stresses and beam flexure to
resist or mitigate these unwanted displacement. In situations where
differential axial motion of cables 1102 is produced by wind
loading, these displacements are likely to be cyclical or
vibrational. In some embodiments of this invention, such cyclical
displacements are mitigated by the use of dampers, as previously
discussed.
[0136] Alternative tensile truss architectures according to
embodiments of the present invention include designs such that
additional tensile cables and stretch from points on compressive
trusses or some or all of the vertical cables 1106 are eliminated
by stretching cables directly to the proximity of mounted
apparatus.
[0137] FIG. 12 depicts another element of the embodiment shown in
FIGS. 1-1A. In particular, FIG. 12 shows a split tensile truss 1200
in which the elements 1206 may comprise apparatus related to the
concentrator.
[0138] Minimum material and robust concentrator designs may benefit
from the application of tensile forces. For example, in the
embodiment shown in FIGS. 1-1A, the tension in element 130 is used
to reduce material in the mount of a solar receiver module and
provide a convenient method for mounting and dismounting
concentrators with access only to the outer edge of the truss e.g.
location 132. Like the truss in FIG. 11, this tensile truss also
stiffens the system.
[0139] FIGS. 13A-B show two embodiments of tensile trusses
according to the present invention that comprise a
three-dimensional frame. Such frames are useful for providing
stiffness or tensile forces in multiple directions.
[0140] The frame 1300 being comprised of a folded tensile truss
1302 is a simple way to produce tensile forces 1304. However,
displacement between a center of load 1308 and the primary
tensioning cables can produce a moment that rotationally deflects
the array, as indicated by 1306. This folded-truss frame may thus
be less useful for high-precision-pointing applications.
[0141] In contrast, the more complicated frame 1350, a part of the
embodiment shown in FIGS. 1-1A, substantially avoids such moments
and their resulting rotational displacements.
[0142] Compressive Trusses
[0143] The architecture of embodiments according to the present
invention utilizes the ground as a primary compressive element, but
it is generally impossible to eliminate additional compressive
elements in any non-trivial design. As used herein, a compressive
truss is an assembly of at least one structural element such that
at least one structural element in the assembly experiences
compressive loads during normal operation.
[0144] A design goal of compressive elements according to the
present invention is to minimize the overall structural material
costs for a given mounting rigidity. The simplest compressive
"truss" is a single element that constrains the motion of at least
one cable.
[0145] For example fixed-angle collectors according to an
embodiment of this invention could be mounted on a tensile truss
strung between at least one post and terminated by at least one
ground-mounted cable. In this embodiment, the compressive truss is
simply the post.
[0146] Many embodiments of the present invention utilize at least
one post as a compressive element, generally but not universally in
concert with additional ground tackle such as cables and ground
anchors. Such posts can employ their resistance to bending to
stiffen the rigging system in addition to or alternative to their
compressive stiffness. If bending resistance is required, including
to resist loads that directly produce bending or to prevent
buckling, preferred embodiments of such posts are those having a
large product of elastic modulus and second moment of inertia in
relation to their unit mass cost.
[0147] Generally, other favorable factors include good vibration
damping ability. Favorable materials are wooden beams and poles,
e.g., utility poles, "railroad ties," pressure-treated, creosoted,
or otherwise environmentally protected lumber, redwood, pine,
spruce, etc., straight and tapered steel and aluminum extrusions,
and the like as is well known in the art. Other favorable materials
include concrete-, sand-, dirt-, foam-, or gravel-, and
water-filled hollow materials, etc.
[0148] In embodiments that require one- or two-angle tracking of
the sun, at least one additional compressive truss element is
needed. This element can be as simple as a single element that
supports more than one tensile element supporting collectors and is
actuated by rotating the element through moments applied to the
element. Alternatively, a single compressive element could be
actuated by one or more elements. These elements could be
compressive, e.g., a thrust rod from a linear actuator, or tensile,
e.g., a cable connecting with an actuator.
[0149] Additional elements to the compressive truss may be added
for a variety of purposes. Examples of such purposes include but
are not limited to, increasing static or dynamic stiffness per
weight or cost, avoiding or minimizing shading of collectors,
supporting actuation, and supporting ancillary hardware such as
interconnections and heat exchangers, etc.
[0150] FIG. 14 shows an example of a tensile truss designed to
support tandem collectors according to an embodiment of the present
invention. This truss 1400 is designed to support rotation about
the pivot point 1402. The compressive arms 1404 typically are
designed to avoid buckling under stress loads. The circular tube
geometry shown has a large specific second moment of inertia per
unit material, but alternative cross sections, e.g., I-beams,
C-channels, square or rectangular extrusions, substantially solid
wood, may be more favorable given application-specific loading and
vibration damping requirements, etc.
[0151] Element 1406 supports a tensile truss as in FIG. 12.
Elements 1408 and 1410 support a tensile truss as in FIG. 11. The
combination of these trusses has the geometry of FIG. 13B (1310 and
1312). Element 1412 is a feature to provide for passages of cables,
e.g., FIG. 11 (1104).
[0152] By design, this truss can be constructed from a minimal
number of different components or differently designed parts.
However, it contains no means for actuation and requires
different-thickness cables for the tensile truss in 1410 and 1408
to equate displacement under load since the cable connected to 1410
may be loaded with more force (for example twice the force) than
the cable connected to 1408, due for example to the forces of wind
and/or gravity.
[0153] FIG. 15 shows another embodiment of a truss design. The
truss 1500 pivots about 1502. Elements 1504 support a tensile truss
cable a 1506. Their tension preloads element 1508 with a tensile
force that offsets the compression developed by constraining the
other tensile truss at points 1510.
[0154] A difference between this truss design and others, is the
incorporation of the element 1518 used with cables 1520 and pivot
1522 to support rotary actuation as described previously. A
pretension in 1520 redistributes loads such that the elements 1514
are always under tension in operation. For this reason, in this
design, 1514 can take the form of a cable or a low-profile rigid
member such as a rod or slender bar, which is fortuitous because it
is in a position to shade concentrators at certain times of the day
and year.
[0155] One issue with both of the substantially planar designs
shown in FIGS. 14 and 15, is their reliance on shear, torsion,
and/or bending to resist out-of-plane twisting. Such twisting could
result from unbalanced or dynamic wind forces.
[0156] FIGS. 16A-D show different views of an alternate design
taken from the embodiment shown in FIGS. 1-1A, that employs an
out-of-plane truss structure to stiffen the compressive truss
against out-of-plane deflection. In addition to the element in FIG.
15, the design 1600 in FIGS. 16A-D contains tensile elements 1602
and compressive elements 1604, which act in concert to produce a
linear axial displacement of one or more elements to out-of-plane
deflections (except for rigid-body rotations of the entire
assembly). The benefit of such out-of-plane stiffness depends on
the nature of wind loading (uniform vs. non-uniform) and the
ability of the structure to dampen vibrations. In some embodiments
of the present invention, such out-of-plane stiffening may be
deployed only where needed, at periodic, a-periodic, or
quasi-periodic intervals.
[0157] The cost in installation time and complexity of such
out-of-plane stiffeners may be an important consideration. FIG. 17
shows details of the interior compressive truss design 1700 of the
embodiment in FIGS. 1-1A. Element 1702 is a screw adjustment that
can extend or shorten compressive element 1704. Extending element
1702 flexes and/or rotates element 1706 such that the cables or
cable 1708 that cross at the intersection of 1704 and 1706 can be
tensioned. Such an adjustment feature is favorable because it can
eliminate costly hardware like turnbuckles, etc. Moreover,
provision for such elements can inexpensively be built in to a
system and only actually installed if the conditions warrant.
[0158] End Terminations
[0159] The ends of rows of cables may require special treatment,
owing to a need to transfer tensile forces from the tensile trusses
to the firmament. One treatment could be to anchor one or more
cables to one or more rigid posts fixed in the firmament, with or
without reinforcements to relieve bending stresses and limit
deflection of the posts. These reinforcements could be cables or
compressive truss elements as known in the art.
[0160] A more complex termination scheme may be needed if the
system tracks the sun through one or two angles. For example, one
or more cables could be anchored to a rigid bar which can pivot on
a rigid structure that is fixed in the firmament. In such
arrangements, elements of the termination are operated in bending,
and therefore may require excessive amounts of materials,
particularly for structures that are more than 5 meters across as
the embodiment in FIG. 1. In accordance with the architectural
methodology in accordance with the present invention, certain
embodiments of end terminations rely substantially on axial forces
on elements and make maximal use of tensile elements to reduce
material cost.
[0161] A tensile collector support structure according to certain
embodiments of the present invention typically contains more than
one cable spaced apart. In order to allow such cables to pivot
about an axis, these cables or the tensile forces they bear, may be
combined to a relatively small region disposed about a pivoting
axis, and to transfer the loads from a relatively small region
disposed about this pivoting axis to the ground.
[0162] In some embodiments, the transfer of tensile stresses to the
ground involves two stages. The first stage is a simultaneous
drawing together of cables or the tensile forces to a pivot, e.g.,
by forming a cable bundle, mechanically mating cables to a common
rigid part or secondary cable, etc. The second stage is the
transfer of those forces from the pivot to the firmament, e.g., by
bringing one or more cables or tensile elements to ground anchors
or footings or by (less materially efficiently) by compressive
elements or bending forces.
[0163] The ability of cables to twist substantially without fatigue
provides an opportunity to obviate a separate pivot means, e.g.,
shaft in a bearing or bushing. A cable segment that is one hundred
to preferably 1000 times its strand diameter or more in length can
repeatedly twist 180 degrees with minimal degradation. Such cable
segments can comprise an inexpensive pivot according to the present
invention.
[0164] In many embodiments of the present invention, it is
desirable to distribute the load to multiple ground anchors or
footings. In some embodiments, it is cost effective to form a
bundle from tensile support cables, use the bundle itself as a
pivot or pass the bundle through a separate pivot structure, then
split more than one of the cables from the bundle to go to separate
ground locations.
[0165] In some embodiments, such as that in FIG. 1, the tensile
structural cables are displaced asymmetrically about the pivot
point. This displacement is often necessary to provide for a full
angular pivot range and to provide for side reinforcement of
interior posts, etc. In such cases, a component of the force in the
tensile structural cables is transmitted to the elements, pivot,
and post of the adjacent compressive truss when the cables are
brought in line with the axis. This additional loading may require
significant changes to the design of the compressive truss and
ground tackle near the termination. Alternatively, or in
conjunction, stages can be added to the transfer of the tensile
forces from the truss structure to the ground.
[0166] FIGS. 18A and B show views of a termination having three
such transfer stages. (FIG. 18A is an end view in which only one of
the posts is visible) In the first stage, the tensile cables 1802
are brought together in a group to a cable plate assembly 1804 that
is at an axis that mitigates the unusual loading profile on the
last compressive truss (indicated by the dashed lines). A
compressive element 1806 takes a substantial amount of the
unbalanced load from transferring the tensile forces to the pivot
axis in the second stages and transfers these loads to the
firmament. The remainder of the tensile loads are transferred to
the firmament in the third stage.
[0167] In order to retain the rigidity of the tensile truss system,
the angular position of the cable plate may be rigidly controlled,
e.g., via a minimum material actuation means 1808, or a wide range
of alternative techniques as previously disclosed. Moreover, it may
be advantageous to add a truss 1810 to stiffen the termination
system.
[0168] The termination embodiment in FIGS. 18A-B produces a large
internal compressive force on the last compression truss. FIGS.
19A-B show an alternative embodiment of a three-stage termination
design in which the cables are grouped or their forces brought to a
compact location at the location of the pivot 1908 after a
plurality of cables, e.g., the outer set of cables 1904, are
combined to a second group, e.g., are bundled, or their carried
forces otherwise brought to a compact location 1906. The reaction
force needed to support this redirection of forces is provided by
the truss system 1910, which may further serve as a component in a
minimum materials actuation system.
[0169] FIG. 19C shows an alternative embodiment that comprises a
tetrahedral truss mounted with one edge along the pivot axis. This
tetrahedral truss utilizes at least two compressive elements 1918.
The other four elements, e.g, 1920, 1922, and 1924 may be purely
tensile. In the embodiment shown, this tetrahedral truss derives
sufficient torsional stiffness with respect to the last interior
compression truss 1902 from the arrangement of cables, compressive
elements, posts, and ground tackle to obviate a separate rotation
actuator. In other embodiments this tetrahedral truss may be
actuated to further stiffen the system.
[0170] In the design in FIG. 19, this actuation system could employ
the same apparatus and drum used by interior actuators. One aspect
of this particular design, however, is that the arms 1912 of the
truss 1910 are long and may be heavily loaded, possibly requiring
excessive material to avoid bucking. In accordance with an
alternative embodiment of the present invention, the truss can be
designed such that cables combine more closely to the pivot point
to reduce forces and compressive element length.
[0171] A wide range of alternative staging architectures according
to the present invention may be designed by one skilled in the art.
Possible architectures include those that minimize material, reuse
components used elsewhere, minimize footprint, etc. Some
embodiments of a termination in accordance with the present
invention provide for substantial open area between ground
elements, e.g., 1914 and 1916, such that maintenance vehicles or
traffic can pass through this region at least part of the day.
Particular embodiments in accordance with the present invention are
envisioned to have access and service roads routed among and
through terminations.
[0172] The mechanism that tensions the ends of the cables should be
able to support the cable tension along with any lateral forces or
increased tensile forces produced by the wind. One embodiment of
such a mechanism is a truss which uses the cables as tensile
elements and a minimum number of compressive elements to perform
its function, since that arrangement should provide for minimizing
the number of cable attachments and the truss material, and
therefore minimizing cost.
[0173] An embodiment of a linkage between the terminal mechanisms
and a mounting firmament is a pole placed or recessed in the
firmament and guyed such that the pole substantially bears a
compressive stress and the guys bear a tensile stress that is
communicated to the mounting firmament via mechanical connectors,
stakes, concrete pads or the like. As used herein the term
"firmament" means a load supporting structure such as a roof-top,
wall, paved surface, ground, bedrock, lake bed, ocean floor,
etc.
[0174] Embodiments in accordance with the present invention may
comprise additional tensile cables fastened to the solar modules.
In accordance with such embodiments, differential axial motion of
cables produces a rotational motion component of the individual
solar modules of the array to effect an orientation control along
one rotational axis.
[0175] An embodiment in accordance with the present invention may
further comprise a plurality of supports. Such supports provide for
motion of at least one cable normal to its axis to produce a
rotational motion component of the individual solar modules of the
array to effect an orientation control along a second rotational
axis.
[0176] An embodiment in accordance with the present invention may
further provide for the common translation of cables connected to
the modules, such that the array of modules can be translated
normal to the axis. Other embodiments in accordance with the
present invention provide for the common axial translation of
cables such that the modules can be translated in an axial
direction.
[0177] FIGS. 20A through D show several embodiments of mounting
systems used to transfer loads to the firmament. These examples
show the use of a post 2002 mounted in the ground whose surface is
indicated by 2004.
[0178] Element 2006 is a ground anchor, footing, or other mounting
element. Element 2008 is a cable arrangement, gusset plate, truss,
or other optional reinforcement that relieves the post of bending
stresses while providing room for cables crossing from the
collectors and collector supports.
[0179] FIG. 20B shows a detail of the top region of the mount shown
in FIG. 20A. In this embodiment, the reinforcement 2010 is arranged
to provide a pivot for a compressive truss that is clear of cables.
For example, the cable 2010 could pass through a hole in a hollow
shaft 2012 or be mounted to 2012. In this embodiment, 2012 endures
cantilever loads and bending from tension in 2010. Because of the
relatively compact geometry, the element 2012 can be designed to
handle such loads without excessive bending, stress, or cost.
[0180] FIG. 20C shows an alternate mounting system in which mounts
are guyed on one side only, leaving the other side clear of cables.
The dots 2013 indicate the cooperation of other structural elements
not shown. In such an embodiment, it is preferable to alternate or
vary the side that is guyed.
[0181] FIG. 20D shows an alternate arrangement in which a single
anchor is shared between two posts to prevent axial motion. This
arrangement may have advantages in material efficiency and
convenience of installation, however, the ground tackle must be
designed to resist side loads.
[0182] The mechanism that tensions the ends of the cables should be
able to support the cable tension along with any lateral forces or
increased tensile forces produced by the wind. One embodiment of
such a mechanism is a truss which uses the cables as tensile
elements and a minimum number of compressive elements to perform
its function, since that arrangement should provide for minimizing
the number of cable attachments and the truss material, and
therefore minimizing cost.
[0183] An embodiment of a linkage between the terminal mechanisms
and a mounting firmament is a pole placed or recessed in the
firmament and guyed such that the pole substantially bears a
compressive stress and the guys bear a tensile stress that is
communicated to the mounting firmament via mechanical connectors,
stakes, concrete pads or the like. As used herein the term
"firmament" means a load supporting structure such as a roof-top,
wall, paved surface, ground, bedrock, lake bed, ocean floor,
etc.
[0184] Embodiments in accordance with the present invention may
comprise additional tensile cables fastened to the solar modules.
In accordance with such embodiments, differential axial motion of
cables produces a rotational motion component of the individual
solar modules of the array to effect an orientation control along
one rotational axis.
[0185] An embodiment in accordance with the present invention may
further comprise a plurality of supports. Such supports provide for
motion of at least one cable normal to its axis to produce a
rotational motion component of the individual solar modules of the
array to effect an orientation control along a second rotational
axis.
[0186] An embodiment in accordance with the present invention may
further provide for the common translation of cables connected to
the modules, such that the array of modules can be translated
normal to the axis. Other embodiments in accordance with the
present invention provide for the common axial translation of
cables such that the modules can be translated in an axial
direction.
[0187] Ground Tackle
[0188] An element of embodiments according to the present invention
is to externalize the cost of the primary compressive backbone of a
solar collector system. Interfaces of the system to the ground is
of interest for such installations.
[0189] Ground anchor solutions are well known in the art. A
particular embodiment of a ground anchor 2100 for use in accordance
with embodiments of the present invention is shown in FIGS. 21-21A.
In FIG. 21A a high-torsional-strength structure, e.g., a square or
circular tube or pipe is connected to a broad helical feature 2104,
as is known in the art of ground anchors.
[0190] The end of the tube 2102 may optionally contain a feature
2106, e.g., a chamfer that sharpens the edge, serrations, which
assist with cutting and displacing soil, clay, or rocks, etc.
Toward the top of the body 2102, is a feature 2108 that provides
for engaging with a rotary tool to drive the ground anchor. Such a
rotary tool may further provide an axial force or preferably
displacement coordinated with rotary motion such that the helix
2104 drives smoothly into the firmament.
[0191] The length of 2102 and the dimensions of 2104 should be
sufficient to hold required loads for a given firmament. The size
and material thickness of 2102 should be coordinated with its
length and the dimensions of 2104 such that the torsional or
compressive strength is at least often not exceeded while driving
the anchor into the firmament.
[0192] Element 2110 is a collet that holds at least one cable in
its bore 2112. A taper of the collet 2114 and anchor cap 2116
ensures that when the collet is loaded, e.g., through the action of
a nut 2118 on a thread, the assembly firmly clamps a cable or
cables passing through 2112. This taper further ensures that
tension in the cable acts to increase the clamping load on the
cable.
[0193] The tapers can work several ways. In one embodiment, the
tapers of the collet and cap are substantially matched. In the
embodiment shown, the tapers of the collet and cap are different
such that compressive forces on the cable are concentrated further
down the collet. Such a mismatch in taper may provide for enhanced
"binding" of the collet and the cap and lower long-term reliance on
continued cable tension or the pretension from 2118.
[0194] The feature 2120 on the anchor is designed to mate with a
hydraulic cable tensioner. Such a tensioner can clamp or threat on
such a feature, clamp to the cable held by the anchor and make
adjustments of the cable length to achieve a desired tension or
displacement.
[0195] Connected to the feature 2120, the hydraulic tensioner
apparatus can further "unbind" or unclamp the collet 2110 e.g., by
turning a nut 2118, then pressing 2118 toward the cap 2108 or by
otherwise releasing a pretensioner 2118, including cases in which
the pretension is manually released, e.g., by manually turning a
nut. Such a device could further provide for automatic or manual
pretensioning following the adjustment, e.g., by a reversal of the
steps to release the cable from the clamp.
[0196] In several positions of the embodiment shown in FIGS. 1-1A,
one or more tensioned cables are preferably coupled to one or more
other cables. For example, FIG. 22 shows a cable plate 2202 that
provides linkage between eight highly tensioned cables 2204 that
comprise the main tensile structure of the truss and 2206, a cable
that communicates the combined tensile forces to the end
terminations of the apparatus. An alternate embodiment of this
plate couples these cables with multiple cables to the end
terminations.
[0197] One method of combining cables known in the art is the use
of various crimps and clamps. The advantage of using a cable plate
as in FIG. 22 is that individual cables can be more easily
tensioned.
[0198] As with the ground anchor in FIGS. 21-21A, the cable plate
employs collets, tapered holes, and pretensioning nuts to clamp
cables. Also like the ground anchor in FIGS. 21-21A, the clamping
structure contains a mounting feature for a hydraulic tensioner
apparatus.
[0199] An aspect of certain embodiments of the present invention is
the ability to flex, bend, or roll at least some of the tensile
structure when not under tension. This can provide for
prefabrication of dimensionally accurate assemblies, convenient
distribution, and convenient and accurate assembly in the field.
For example, complete sections or multiple-section runs of these
cable assemblies could be manufactured using automated apparatus
and distributed on spools, rollers, or other convenient
distribution aides. Accordingly, any element connected to the
cables in such embodiments may be sufficiently flexible or compact
and strong to avoid damage when assemblies are flexed.
[0200] FIGS. 23A and B show designs of cable connections that
illustrate features of such cable connectors. The design in FIG.
23A provides for attaching two substantially orthogonal cables 2306
and 2308 via sandwiching them between two compact rigid stamped
sheet metal plates 2302 and 2304. This connector can be used to
facilitate pivoting of concentrators about their secondary
axis.
[0201] In other embodiments, cables could be held in a single or
multiple piece assembly made via an array of techniques known in
the art, e.g., injection-molding, casting, including zinc alloy
casting, extrusion, etc. In other embodiments, such articles could
be attached to cables via fasteners, e.g., 2314, via clamps,
swages, crimps, adhesives, solders, brazes, welds, etc.
[0202] Welds and high temperature operations may unfavorably affect
the strength of cables. Alternatively, lower temperature mating
techniques, such as molten zinc or soldering could be utilized,
including casting parts directly on cables.
[0203] The design in FIG. 23 provides clearance from cables in the
region 2310 indicated by the dashed line, allowing apparatus
mounted to the location 2312 to be pivoted over a wider angle than
otherwise possible. This design keeps vertical loads on the axis of
the cable 2306 and therefore provides for enhanced vertical
stiffness. Such accommodation is similarly possible for cable
2308.
[0204] In some embodiments, such connectors are required at the
intersection of three cables. The use of one or more compact rigid
pieces at cable intersections can compensate for the finite size of
cables or other apparatus to allow loads to be placed substantially
along the axes of one or more cables. Such crossing connectors may
further provide for cables to pivot during operation or to move
with constraints such that a cable assembly can be rolled without
damage, but is rigid when cables rotate into their operating
position.
[0205] FIG. 23B shows an embodiment of a cable-mounted pivot
according to the present invention. The cable 2316 is clamped in
this instance via forces from a mechanical fastener 2322, but in
other embodiments this connector could be crimped, swaged, or
otherwise mated with the cable as previously discussed. Because
loads from 2318 are off the axis of the cable, the element that
mates with 2318 should be designed to prevent the action of these
generated couples from bending or twisting cable 2316 where
rigidity is important.
[0206] Connections between cables can be made a number of ways. One
favorable technique is to pass a cable through the center of
another cable by separating its strands. The cables can then be
fastened via the use of one or more crimps or swages alone or in
combination with a full or partial sleeve. Cables could
alternatively be secured by soldering, zinc or zinc alloy casting,
or zinc or molten zinc gluing, etc.
[0207] FIG. 24 shows a plan view of a solar farm according to an
embodiment of the present invention. Collector arrays are most
efficiently placed in elongated rows. If these arrays track the sun
in two dimensions, then these rows may be most favorably placed in
a substantially North-South orientation, as disclosed previously.
This orientation allows collectors to be packed as tightly as
possible along the primary axis while avoiding self-shading, which
provides for the minimum material usage. If multiple rows of
concentrators are used, the spacing of these rows should be wide
enough that self shading at the beginning or end of the day is
acceptable.
[0208] The distance between adjacent rows should at least be wide
enough to provide for convenient servicing. The spacing between
rows in some embodiments may be influenced by other factors, e.g.,
the space needed to support parking underneath the array, the space
needed for planting and harvesting, etc.
[0209] A slight Western slope to the land is generally desirable
because it biases the operation window toward the end of the day,
when energy demand is strong. If tracking along one or more axis is
manual, it may be favorable to orient arrays East-West so one axis
need be adjusted only for seasonal variations. Such an orientation
limits the productivity of the system so this tradeoff is not
anticipated to be justified frequently.
[0210] In some embodiments of the present invention, it is
convenient and effective to fabricate one or more elements of the
tensile truss structure from sheets or strips of material, e.g.,
sheet metal, fiberglass, extruded plastic, composites, fabrics,
weaves, and the like. FIG. 25A shows a tensile truss 2502 similar
in operation to that in FIG. 11, but constructed from sheets rather
than ropes, possibly from a single sheet or possibly a plurality of
sheets fastened, welded, sewn, bonded, or otherwise connected
together. The truss supports concentrators at points 2504.
[0211] Like in FIG. 11, the truss is pulled as indicated by the
arrows and the resulting tensile forces are redistributed by a
series of openings 2506 and links 2508 such that tensile forces
resist motion in the plane of the truss. In this embodiment, the
links 2508 fit the description of "cables" as used herein.
[0212] As with the truss in FIG. 11, a sufficient tension should be
applied such that the local forces on links never becomes
compressive in normal operation. This method of making a truss may
be less tolerant of compressive forces in severe loadings than a
cable truss, since it is possible for links to buckle tightly
enough to deform plastically. An advantage of the cutouts 2506 is a
reduction in wind loading, material usage, and mass.
[0213] The thickness of the sheet or sheets and dimensions of the
openings and links should be designed in concert to provide for a
desired wind loading, load bearing, stiffness, and out-of-plane
flexibility. Out of plane flexibility can obviate damage from
buckling and can also allow such trusses to be manufactured and
delivered on rolls.
[0214] FIG. 25B shows a portion of a chain of such trusses 2510
assembled from a long sheet, strip, or roll of material.
Alternatively, the body 2510 could be constructed in a continuous
process. Such a process may include hot or cold rolling of a sheet
of metal including steel, aluminum, stainless steel, etc., possibly
providing spatial patterning to avoid excessive cutting waste;
stamping, punching, plasma-cutting, laser-cutting, water-jet
cutting, etc. of one or more openings; recycling or remelting of
cutout material; heat-treating; annealing; surface treatment, e.g.,
hardening, anti-corrosion, priming, pickling, painting, scale
removal, metal coating such as galvanization; embossing or stamping
of stiffening or strengthening features; the incorporation of
mounting elements and features; spooling into rolls; cutting at
determined lengths; and the like.
[0215] The truss may further provide heat dissipation surface area
in a distributed coolant-air heat exchanger. The truss may also
provide damping directly by judicious design of laminated,
corrugated, and energy-absorbing sheet materials or provide for
mounting of other dampers.
[0216] Having thus described exemplary embodiments of the present
invention, it should be noted by those skilled in the art that the
within disclosures are exemplary only and that various other
alternatives, adaptations, and modifications may be made within the
scope of the present invention. Accordingly, the present invention
is not limited to the specific embodiments as illustrated herein,
but is only limited by the following claims.
* * * * *