NZ618498B2 - A wave energy extraction device and method - Google Patents
A wave energy extraction device and method Download PDFInfo
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- NZ618498B2 NZ618498B2 NZ618498A NZ61849812A NZ618498B2 NZ 618498 B2 NZ618498 B2 NZ 618498B2 NZ 618498 A NZ618498 A NZ 618498A NZ 61849812 A NZ61849812 A NZ 61849812A NZ 618498 B2 NZ618498 B2 NZ 618498B2
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- wave energy
- paddle
- wave
- extraction device
- energy extraction
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Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
- F03B—MACHINES OR ENGINES FOR LIQUIDS
- F03B13/00—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates
- F03B13/12—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy
- F03B13/14—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy
- F03B13/16—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem"
- F03B13/18—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore
- F03B13/1805—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore and the wom is hinged to the rem
- F03B13/181—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore and the wom is hinged to the rem for limited rotation
- F03B13/182—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore and the wom is hinged to the rem for limited rotation with a to-and-fro movement
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
- F03B—MACHINES OR ENGINES FOR LIQUIDS
- F03B13/00—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates
- F03B13/12—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy
- F03B13/14—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy
- F03B13/16—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem"
- F03B13/18—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore
- F03B13/1845—Adaptations of machines or engines for special use; Combinations of machines or engines with driving or driven apparatus; Power stations or aggregates characterised by using wave or tide energy using wave energy using the relative movement between a wave-operated member, i.e. a "wom" and another member, i.e. a reaction member or "rem" where the other member, i.e. rem is fixed, at least at one point, with respect to the sea bed or shore and the wom slides relative to the rem
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
- F03B—MACHINES OR ENGINES FOR LIQUIDS
- F03B17/00—Other machines or engines
- F03B17/06—Other machines or engines using liquid flow with predominantly kinetic energy conversion, e.g. of swinging-flap type, "run-of-river", "ultra-low head"
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
- F03B—MACHINES OR ENGINES FOR LIQUIDS
- F03B17/00—Other machines or engines
- F03B17/06—Other machines or engines using liquid flow with predominantly kinetic energy conversion, e.g. of swinging-flap type, "run-of-river", "ultra-low head"
- F03B17/062—Other machines or engines using liquid flow with predominantly kinetic energy conversion, e.g. of swinging-flap type, "run-of-river", "ultra-low head" with rotation axis substantially at right angle to flow direction
- F03B17/063—Other machines or engines using liquid flow with predominantly kinetic energy conversion, e.g. of swinging-flap type, "run-of-river", "ultra-low head" with rotation axis substantially at right angle to flow direction the flow engaging parts having no movement relative to the rotor during its rotation
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/20—Hydro energy
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/30—Energy from the sea, e.g. using wave energy or salinity gradient
Abstract
wave energy extraction device (Fig. 2b) for converting wave energy into mechanical motion is curved in the horizontal plane via a connection (16). A concave side (11) facing the incoming waves increases energy capture and a convex side (12) on the lee side to reduce energy dissipation. The concave surface is substantially parabolic or semi-elliptical and the convex side is optionally provided with a substantially Gaussian profile (4) to improve the hydrodynamics and additional strength. To simplify connection to flexible coupling (18) to a base platform (19) the paddle curvature tapers to a straight edge (1) at the mounting points and the base device can be located either above or below the water surface. One embodiment is also curved about a vertical as well as the horizontal plane to create a spoon- or cup-shaped paddle. surface is substantially parabolic or semi-elliptical and the convex side is optionally provided with a substantially Gaussian profile (4) to improve the hydrodynamics and additional strength. To simplify connection to flexible coupling (18) to a base platform (19) the paddle curvature tapers to a straight edge (1) at the mounting points and the base device can be located either above or below the water surface. One embodiment is also curved about a vertical as well as the horizontal plane to create a spoon- or cup-shaped paddle.
Description
A wave energy extraction device and method
Technical Field
This invention relates to a wave energy extraction device or
paddle for converting energy within water waves, particularly
sea waves, into useful mechanical motion. The invention also
extends to a wave energy conversion apparatus and to a method of
extracting energy from water waves.
The world' s oceans and other water bodies contain a vast amount
of energy that has accumulated from passing weather systems and
propagates as waves along the surface. This energy is frequently
recognised as a 'green' power resource that could meet the
world's energy needs many times over.
There are a variety of methods used to extract energy from the
water waves. The main systems include:
1 . Point absorbers that use the relative displacement between
floating units at (or near) the surface and a fixed point to
generate mechanical energy.
2 . Attenuators that use a group of floating units that follow
the surface and generate mechanical energy through their
relative motion.
3 . Wave paddles that cut vertically through the water waves and
generate motion by interrupting the particle orbits to create a
pressure gradient across the paddle that drives it forward and
backward. Energy from this motion is captured by a hydraulic
piston or ram to pump water, generate electricity, or generate
or store energy in any other suitable manner.
Background Ar
Focussing on the third type of system mentioned above, different
paddles designs have been proposed. For example, US-A-
2008/0018113 and GB-A-2 ,333,130 describe downward hanging, flat,
symmetric paddles that intersect the wave motion near the
surface. These devices gradually absorb energy and flatten the
waves as they pass beneath.
WO-A-2004/097212 and EP-A-2 ,292,924 are designed for shallow
water with a symmetric paddle hinged about the bottom. WO-A-
2004/097212 describes a substantially flat paddle with a slight
vertical curvature. While the paddle in EP-A-2, 292, 924 is
constructed from a sequence of horizontal tubular sections that
give both faces a contoured vertical profile. The paddle
additionally includes a pair of 'end effectors' that protrude
evenly from each vertical side.
Though the constructions of the paddles within these devices all
differ, they are mostly designed around the presumption that
wave conditions driving a paddle in the direction of the energy
movements ("forward") are equivalent, though applied to the
opposite paddle face, to the forces that drive it against the
energy movement ("backward"). Therefore each of these designs
would operate equally well if installed in reverse. However, the
flow of ocean energy typically has a strong directional bias:
e.g. in the northern North Sea 95% of the incoming energy
arrives within an arc of +/- 30° of the mean direction.
Each of these paddles also have a substantially flat face on the
lee side of the paddle. Therefore, when these paddles move they
dissipate a significant amount of energy through the creation of
secondary waves that propagate from the paddle. In an ideal
system there would be no significant waves on the lee side of
the paddle. One way to achieve this is to remove the water on
the lee side of the paddle and create an air gap within which
the paddle can move freely. However, this is both complicated
and expensive to build and would require regular maintenance of
the high-pressure waterproof seals.
What is needed is a straightforward paddle design that takes
advantage of the directional bias in waves, in particular ocean
waves, to increase the wave energy captured, while
simultaneously reducing waves generated on the lee side to
maximise its overall energy absorbing properties.
It is an object of the invention to provide the public with at
least a useful alternative.
Summary of Invention
In accordance with the present invention, there is provided a
wave energy extraction device for use with a wave energy
conversion apparatus to extract and convert energy from water
waves, the device having a height and a width and being
arrangeable on the wave energy conversion apparatus so as to be
at least partially submerged in the water and comprising of a
first surface arranged to oppose a mean water wave direction,
the first surface being concave about a vertical axis of
symmetry of the device and a second surface disposed opposite
the first surface and being convex about the vertical axis of
symmetry of the device so as to be able to extract energy from
the crest and the trough of the wave. In embodiments of the
invention, there therefore exists an asymmetry between the first
and second surfaces of the device (or paddle). This asymmetry
both increases energy collection as the paddle moves in response
to incident waves and reduces energy dissipation caused by the
creation of new waves on the second (or lee) side of the device.
The result is overall increased power extraction compared to a
symmetrically shaped or flat device. The asymmetry further
enables the device to take advantage of directional biases in
water waves.
In an embodiment, the paddle device is arranged to be submerged
in the water over a majority proportion of its height, for
example to approximately 80% or over of its height. This
maximises the energy collection for a given height of paddle.
However, the paddle is preferably not entirely submerged and
waves preferably should not be able to escape over the top of
the paddle ("overtopping") since this would result in energy
being lost. In some embodiments, the centre of the paddle may be
higher than the edges, particularly when the paddle is wide, to
prevent overtopping by the incident waves as they are focussed
towards the centre, while minimising the use of material at the
edges. Preferably, when viewed from a front elevation, the
paddle is substantially rectangular in outline.
In an embodiment, the convex second surface has a cross-
sectional profile that additionally extends to a generally
pointed tip at the vertical axis of symmetry of the device. The
cross-sectional profile of the second surface may be formed from
any suitably smooth curve that comes to a relatively sharp
point, such as a curve based on a Gaussian function. This
provides the device with a hydro-dynamically efficient profile,
improving laminar flow around the second surface, which reduces
the dynamic drag pressure on the lee surface and reduces energy
dissipation .
In an embodiment, the horizontal extremities of the concave
first surface comprise tips arranged to substantially oppose the
mean wave direction during use. The tips may comprise generally
aerofoil shaped tips, the aerofoil shaped tips having a leading
edge arranged to oppose the mean wave direction during use. The
tips result in a clean separation of the flow into the concave
face of the paddle and around it.
In one embodiment, the first surface extends continuously across
the wave energy extraction device between horizontal extremities
thereof. In this way, the entire width of the first surface is
movable in response to incident waves to generate power.
Alternatively, the device comprises two or more components
including: a wave energy absorber component having a first width
and adapted to move in response to incoming water waves for
extracting power; and a wave energy concentrator component for
concentrating water waves from across a second width greater
than the first width down to the first width and guiding the
concentrated wave energy towards the wave energy absorber
component. In this embodiment, the amount of wave energy that
can be captured by the paddle for each wave crest is increased
by channelling the wave on either side of the paddle towards the
paddle surface using a wave concentrator that is preferably
stationary relative to the incoming waves. The channelled
portion of the wave would otherwise have moved past the paddle
and some of the potential energy lost. Advantageously, this
means that a relatively small paddle, having consequently
smaller drag and simplified construction, is able to capture the
same energy as a much larger moving paddle.
In an embodiment, the device includes an attachment point for
attachment to the wave energy conversion apparatus. The
attachment point may comprise a hinge point for hinged
attachment to the wave energy conversion device. The attachment
point may be located at one of the upper and lower end of the
device so as to pivotally hinge the device to the wave energy
conversion apparatus. The device is then able to hang vertically
from the wave energy conversion apparatus if the attachment
point is at the upper end, or stand generally upright in the
water if the attachment point is at the lower end.
The device may have a cross-sectional profile that varies over
the device height. The cross-sectional profile may taper towards
a flat profile adjacent the attachment point for ease of
attaching the device to the wave energy conversion apparatus.
Additionally, having a generally flat profile in an attachment
point adjacent the sea bed is easier to seal to restrict flow of
water beneath the paddle and preventing energy from being lost.
In another arrangement, where the paddle is adapted for
rotational motion about an attachment point, the curvature of
the paddle body changes at least partly along the height of the
paddle, the body being more curved at a first point thereon
which is a further distance away from an axis of rotation of the
paddle than at a second point thereon.
In one embodiment, the attachment point is operable to adjust
the relative spacing between the wave energy extraction device
and the wave energy conversion apparatus. A flexible joint that
permits the device to move up and down in response to changes in
sea level (due to tides, for example) leads to improved
extraction of wave energy by optimising the position of the
device relative to sea level. A flexible joint would also assist
in moving the paddle relative to the energy conversion apparatus
for ease of maintenance and installation.
The device may include one or more generally horizontally
disposed shelves for limiting fluid flow vertically along the
first surface of the device. These shelves increase power
absorption, particularly when they are located at or near the
top and/or bottom of the device to prevent water overtopping or
flowing underneath it, respectively, and provide additional
structural strength. The depth of each shelf may vary from
approximately 1/10 of the device depth to full depth of the
device. The shelf may be flat, but is preferably contoured in
profile when attached to either the top or bottom of the device
to provide a smooth transition towards the second surface. This
decreases drag. Additionally, the concave first surface of the
device is preferably also curved to connect smoothly with the
shelf. A smooth profile reduces stress concentrations as waves
impact the surface of the device.
In a preferred embodiment, a first internal volume is formed
between the convex second surface and the generally pointed tip.
The generally pointed tip may be formed by a third surface that
is separate to and fixedly arranged on the second convex surface
to provide the first internal volume. The pointed tip
advantageously reinforces the strength and rigidity of a central
portion of the wave energy device where the wave energy is
concentrated by the concave first surface. The first and second
surfaces may also be separate components fixed together around
their peripheries to form a second internal volume between them.
The internal volumes may be air voids configured to provide
buoyancy or storage space inside the device. In this design,
some form of access to the internal void is included and the
void may be provided with one or more hooks, clips or other
equipment attachment means for housing cables and any other
equipment. The internal volumes may also contain power
conversion apparatus or desalination equipment such as osmotic
membranes and filters.
Each of the first concave surface and the second convex surface
may be substantially parabolic or semi-elliptic in form and the
form of the first concave surface and form of the second convex
surface may be different to each other. In some embodiments, the
parabolic or semi-elliptic form is approximated using a
plurality of straight sections for the ease of manufacture
and/or attachment to the wave power conversion apparatus.
In some embodiments, either or both of the first surface and the
second surface are also concave about a horizontal axis to form
a cup-shaped paddle.
The concavity of the first surface preferably has a depth that
is between 1/16 and 1/4 of length of an expected dominant
wavelength of the water waves.
The device may include one or more hydraulic rams arranged on
the second surface for operable connection to the wave energy
conversion apparatus. The wave energy conversion apparatus may
comprise a base platform operably attached to the bed of a body
of water in use of the apparatus. In an embodiment the base
platform is anchored to the bed of the body of water.
Alternatively, the base platform may be fixedly disposed on a
pontoon that is anchored to the bed of the body of water.
Preferably the base platform can rotate about a vertical axis,
to redirect the device depending upon changing incident wave
direction and energy, thereby providing control to the amount of
energy extracted by the device. For example, energy extraction
could be maximised by turning the device directly towards
incident waves or the device can be turned at 90° to the
incident waves during repair or maintenance of the device.
The paddle device may be operably connected to the wave energy
conversion apparatus by a power conversion means for converting
movement of the device to useful forms of power, usually
electrical power. However, the apparatus could also be used for
example to pump pressurised water or to generate hydrogen. In an
embodiment, the power conversion means comprises a hydraulic
accumulator operably coupled to the hydraulic ram for pumping of
a working fluid to a hydraulic accumulator, a hydraulic motor
arranged to be driven by the hydraulic accumulator and an
alternator arranged to be driven by the hydraulic motor.
In one embodiment the wave energy conversion apparatus further
comprises a wave condition sensor and a controller in operable
communication with the wave condition sensor and the hydraulic
ram, the sensor to relay a sensed condition of incident waves to
the controller, the controller to control movement of the
hydraulic ram according to the sensed wave condition. In this
manner, the apparatus can be operated efficiently according to
the strength of the sensed wave conditions and in extreme
circumstances the paddle can be withdrawn away from the water
surface and the most active part of the waves to prevent damage
to the apparatus.
Another aspect of the invention provides a method of extracting
useful energy from water waves comprising: at least partially
submerging a wave energy conversion apparatus in a body of
water; angling a first surface of the wave energy conversion
apparatus to oppose an approximate mean water wave direction,
the first surface being concave about a vertical axis; and
extracting energy from movement of the wave energy conversion
apparatus caused by both the crest and the trough of incoming
waves. Advantageously, the concave surface facing into the wave
direction guides and channels wave energy towards the centre of
the apparatus, such that this method extracts more useful energy
from each crest and trough of incoming waves.
Preferably, the wave energy conversion apparatus is provided
with a second surface angled to be on the lee side of the
apparatus pointing in the approximate mean water wave direction
and shaped to minimize water resistance in order to decrease the
amount of useful energy lost to drag.
B2012/000408
In another aspect, there is provided a device for absorbing
water wave energy comprising a paddle having a backward-facing
surface on a first side of the paddle and a forward-facing
surface on a second side of the paddle opposite the first side,
the backward-facing surface having a high resistance to water
motion and the forward-facing surface having a low resistance to
water motion, the paddle adapted to be submerged in water such
that the backward-facing surface is angled predominantly towards
incoming waves and is moved backwards and forwards by incoming
waves to absorb their energy with the majority of the paddle
remaining underwater over the range of motion of the paddle. In
a similar manner to other aspects, the high resistance backward-
facing surface increases the amount of energy that is absorbed
from each incoming wave while the low resistance forward-facing
surface decreases the energy lost as the paddle moves through
the water.
In one embodiment, the paddle is hinged about either of an upper
or a lower end of the paddle and adapted to rotate backwards and
forwards about the hinge in response to the movement of incoming
waves. The hinge is a simple but effective manner of ensuring
that the paddle moves smoothly backwards and forwards with
incoming waves crests and troughs, absorbing the maximum amount
of energy.
Another aspect of the invention provides a water wave energy
absorbing paddle having a first surface, the first surface
having a curvature which varies over a height of the paddle from
a flat portion at a first end of the paddle to a concave portion
at an opposite end of the paddle, the flat portion for
connection to a hinge permitting rotation of the paddle, the
concave portion for focusing incoming waves causing rotation of
the paddle about the hinge to absorb the energy of the incoming
waves. This aspect of the invention provides a balance between
having a curved paddle, which increases the energy absorbed from
incoming waves, and a flat paddle, which is easier to connect to
a hinge to constrain the paddle to rotating backwards and
forwards with the waves.
In one embodiment, the paddle has a second surface, opposite the
first surface, the second surface having a curvature which
varies over the height of the paddle from a flat portion at the
first end of the paddle to a convex portion at the opposite end
of the paddle, the convex portion to reduce the resistance of
the second surface to motion through water and preferably having
a Gaussian profile. Similar to other embodiments, the Gaussian
profile minimises the energy lost to drag as the paddle moves
through the water.
In another aspect there is provided a method of extracting
energy from water waves comprising: at least partially
submerging a water wave paddle in a body of water, channelling
incoming waves from each side of the paddle towards the paddle,
thereby focusing the energy of the waves onto a front surface of
the paddle, and converting movement of the paddle caused by the
incoming waves into usable energy. This aspect of the invention
increases the amount of wave energy that can be captured by the
paddle for each wave crest by channelling the wave on either
side of the paddle towards the paddle surface. The channelled
portion of the wave would otherwise have moved past the paddle,
losing energy. Advantageously, this means that a relatively
small paddle, having conseguently smaller drag and simplified
construction, is able to capture the same energy as a much
larger one.
Another aspect of the invention provides a water wave energy
extraction system, comprising: a wave energy absorber having a
first width and being at least partially submerged in water and
adapted to move in response to incoming water waves, the
movement of the wave energy absorber being convertible into
useful energy; and a wave energy concentrator for concentrating
water waves from across a second width greater than the first
width down to the first width and guiding the concentrated waves
towards the wave energy absorber. Similar to the previous
aspect, by concentrating wave energy from a width greater than
the width of the wave energy absorber itself the energy absorbed
from each wave crest is increased without increasing the size of
the wave energy absorber itself.
Unless the context clearly requires otherwise, throughout the
description and claims the terms “comprise”, “comprising” and
the like are to be construed in an inclusive sense, as opposed
to an exclusive or exhaustive sense. That is, in the sense of
“including, but not limited to”.
Brief Description of Drawings
Further features and advantages of the present invention will
now be described with reference to the accompanying drawings in
which:
Figure 1a is a schematic plan view of a first wave paddle
embodying the present invention;
Figure 1b is a 3D side elevation view of the wave paddle of
Figure 1a connected to a base device;
Figure 2a is a schematic plan view of a second wave paddle
embodying the present invention;
Figure 2b is a 3D side elevation view of the wave paddle of
Figure 2a connected to a base device;
Figure 2c is a side view of the system of Figure 2b;
Figure 2d is a side view of an alternative arrangement of the
system of Figure 2b, demonstrating an internally provided energy
conversion apparatus;
Figure 3a shows a schematic plan view of a third wave paddle
embodying the present invention;
Figure 3b is a 3D side elevation view of the wave paddle of
Figure 3a connected to a base device;
Figure 3c is a side view of the system of Figure 3b;
Figure 4a is a schematic plan view of a fourth wave paddle
embodying the present invention;
Figure 4b is a 3D side elevation view of the wave paddle of
Figure 4a;
Figure 5a is a schematic plan view of a fifth wave paddle
embodying the present invention;
Figure 5b is a 3D side elevation view of the wave paddle of
Figure 5a connected to a base device;
Figure a is a schematic plan view of a sixth wave paddle
embodying the present invention;
Figure b is a 3D side elevation view of the wave paddle of
Figure 6a connected to a base device;
Figure 7 is a schematic plan view of a seventh wave paddle
embodyin the present invention;
Figure 7b is a 3D side elevation view of the wave paddle of
Figure 7a connected to a base device;
Figure 8a shows a schematic plan view of a wave paddle and wave
concentrator embodying the present invention;
Figure 8b is a 3D side elevation view of the system of Figure 8a
with the wave paddle connected to a base device;
Figure 8c is a side view of the system of Figure 8b;
Figure 9 is a schematic plan view of the leading edge of the tip
of a wave paddle embodying the present invention;
Figures 10a to 10p are schematic plan views illustrating a range
of possible cross-sections for wave paddles embodying the
present invention;
Figures 11a to 11f are schematic plan views illustrating a range
of possible cross-sections for wave paddles and wave
concentrators embodying the present invention;
Figure 12 is a diagrammatic view of a wave powered energy-
generating system;
Figure 13a is a diagrammatic view of a wave powered water
desalination system;
Figure 13b is a 3D side elevation view of eighth wave paddle
embodying the present invention with vertical structural tubes
that also house osmotic membranes for a water desalination
system;
Figure 14a is a 3D side elevation view of an array of paddles
and base devices in situ;
Figure 14b is a 3D side elevation view of a paddle embodying the
present invention mounted to a floating base device tethered to
the ocean floor.
Figure 15 illustrates how the dynamic pressures vary with depth
beneath the crest and trough of a wave;
Figure 16 illustrates how the dynamic pressure varies either
side of a wave paddle beneath a wave crest;
Figures 17a and 17b illustrate the relative flow velocities
around a paddle, and the dynamic pressure on the forward- and
backward- paddle faces; and
Figures 18a and 18b present experimental results for an
asymmetric curved paddle and symmetric flat paddle,
respectively.
Description of Embodiments
Embodiments of the present invention relate firstly to the
design and shape of paddles 24 to extract energy from water
waves, particularly ocean/sea waves but also waves in
bays/harbours, lakes, lochs, estuaries, reservoirs or other
suitable natural or man-made bodies of water. The paddles take
advantage of directional biases in wave motion by being
asymmetric in a plane perpendicular to the mean direction of
wave energy travel. This asymmetry increases energy collection
and reduces energy dissipation compared to known, substantially
flat, paddle designs. Paddles embodying the invention deliver a
smooth power stroke that reduces fatigue from sudden impact
loads .
The described paddles 24 are intended for use with a wave-power
conversion system or "base device" 15 which together extracts
energy from the movement of the paddle and converts it into a
useful form, usually electricity. It may also be used directly
to desalinate sea-water, for example.
Figures la and lb are schematic plan and 3D side elevation
views, respectively, of a first wave paddle embodying the
present invention. The paddle has a concave surface 11 that
points towards the incoming waves 40 (the "backward face") and
an opposing convex surface 12 that points in the direction of
the wave travel (the "forward face") . The curvature of the
paddle reduces along its height, tapering to a flat edge 1 at
the bottom of the paddle for easy connection to a base device 15
below the surface of the water, with the top curved edge of the
paddle emerging above the surface of the water. At its maximum
curvature, the shape of the paddle is preferably a parabola or
semi-ellipse in a horizontal plane with the side edges or tips 2
of the paddle pointing substantially into the incoming wave
direction.
In use, the paddle is connected to a base device 15 via a hinge
13 allowing the paddle to rotate or rock backwards and forwards
in response to incident wave crests and troughs. The base device
comprises one or more energy converting means such as
hydraulic rams 16 (only one shown) to convert motion of the
paddle into useful energy. The ram 16 has a flexible coupling 18
that connects to a support 19 on the convex forward face 12 of
the paddle. The support 19 preferably extends the whole width of
the paddle and is preferably located at one third of the
paddle's height from the bottom. This configuration gives the
paddle vertical flexibility, reducing the chance of fracture,
particularly near the surface when sudden impact loads are most
common due to breaking waves or impacts with other floating
bodies.
The concave backward face 11 preferably has a drag coefficient
(C ) greater than 2.0. As fluid enters the concave face 11
between the paddle tips 2 its inability to go either sideways or
down leads to high stagnation pressures on the paddle face and
an increase in the water surface elevation ("run-up"). The
concave face 11 also channels the incoming wave crests towards
the centre of the paddle, which further magnifies the crest
elevation and the dynamic pressures against the paddle.
Additionally, the concave surface of the backward face 11
ensures a smooth transition of power from the wave to the
paddle, with the wave crests meeting first the tips 2 and then
gradually coming into contact with more and more of the backward
face 11 of the paddle.
During the formation of a wave trough, when the wave fluid
particles move backward away from the paddle's concave backward
face 11, the concave shape now acts to magnify the depth of the
trough creating a lower water surface than would otherwise occur
without the paddle. Consequently, the magnitude of the dynamic
pressure (D ) is again increased during this phase of the wave
motion, though now of opposite sign.
In general, regardless of paddle shape, the volume of fluid
displaced as a paddle moves is directly related to its swept
area. The convex forward face 12 of the paddle embodying the
present invention displaces water over a larger circumference
than a flat paddle with a consequent reduction in the fluid
velocities normal to the paddle surface, leading to smaller
waves that reduce the dynamic pressures (D ) on the forward face,
and a substantial reduction in energy dissipation.
A further advantage of the convex forward surface 12 arises from
the fact that the incident waves move faster than the paddle. As
the waves overtake the paddle on the outer side of the tips 2,
the curved forward surface 12 improves laminar flow around the
paddle. This mobilises the water ahead of the paddle so that its
motion is more closely aligned with the motion of the paddle.
This greatly reduces the dynamic pressure (D ) against the
forward face 12, which increases the pressure differential
between the forward 12 and backward 11 faces leading to
increased power absorption. Furthermore, the overtaking waves
merge smoothly together on the front side of the paddle creating
less turbulence (or eddies) that would otherwise cause energy
losses.
One drawback of the convex forward face 12 is that skin friction
and drag due to flow parallel to the surface is increased,
though this can be expected to be relatively small for the
velocities concerned. Nevertheless the forward face 12 is
provided with a smooth finish to minimise drag.
To maximize energy capture, the top edge of the paddle should be
slightly higher than the maximum expected height of wave crests
(the "freeboard") taking into account changes in the water
height, changes in angle of the paddle over its range of motion,
and the run-up due to the concave backward face 11. In deep
water, the paddle preferably extends to a depth approximately
half the dominant wavelength to capture the majority of the
wave's energy, and the base device includes a horizontal shelf
to restrict downward flow. In shallow water, the paddle
extends the full water depth and the base device 15 includes a
seal 14 against the seabed preventing water flowing beneath the
paddle. Consequently, the height of the paddle depends upon the
water depth, wave conditions and the position of the base
device .
As an example in shallow water of 10m depth and assuming that
the base device stands l from the seabed, that the significant
wave height is Hs=l . m , and that the peak period, T =5.9s, then
the paddle should be approximately 11.5m high. This allows the
paddle to tilt through a range of +/-22° and retain at least lm
freeboard when the paddle is tilted most and sitting lower in
the water. Without this freeboard the waves can wash over the
paddle reducing the energy it can extract. The freeboard should
not be larger than necessary since this would increase the
weight of the paddle and, in extreme seas, a limited freeboard
allows excess energy to escape and constrains the loads on the
paddle. In very extreme seas the paddles should be withdrawn
from the water surface, either by lifting it out of the water or
lowering it against the seabed. In some environments it may be
preferable for the paddle to operate less efficiently by
remaining fully submerged, in particular where there is: excess
wave energy for the power requirements; surface traffic (e.g.
shipping lanes or local leisure craft) ; or environmental
concerns (e.g. visual impact).
Preferably the base unit 15 can be raised or lowered in response
to changes in water depth caused by the tide and storm surge so
that substantially the same area of the paddle is exposed to the
incident waves .
The width of the paddle, being the dimension parallel to wave
fronts when the paddle is in use, is determined by factors
including the power required and the average expected wavelength
of incident waves at the point of deployment: as the width of
the paddle increases relative to the average incident
wavelength, the relative motion between the paddle and the
surrounding fluid starts to approximate a flat surface removing
many of the advantages associated with the present invention.
Other limitations to the paddle width include its weight and the
strength of the materials from which it is constructed.
In deep water the paddle can be wider than it is high to
maximize power absorption. However, in shallow water, where the
base device can be mounted directly on the sea bed, the paddle
will typically be higher than it is wide so that it extends the
full depth of water. However the relative proportions of height
to width, will depend, for example, on anticipated wave
conditions and power requirements.
It is also not essential that the height of the paddle be
constant across its width. As the curved backward face 11
focuses waves towards the centre of the paddle, the wave height
will increase. To reduce the chance of overtopping at the
centre, the centre of the paddle may therefore be higher than
the edges.
The depth of the concavity in the backward face 11 will
typically be within a range equal to the width of the paddle to
about a quarter of the width of the paddle to ensure that a
suitable curvature can be obtained. The preferred depth of the
concavity at the water surface relates to the expected wave
conditions and should ideally lie between 1/16 and 1/2 of the
dominant wavelength. A greater depth of the concavity in the
backward face 11 captures more of the incoming wave energy, by
limiting the egress of excess water in a wave crest/trough
around the paddle tips 2, such that a depth of less than 1/16 of
the wavelength would capture an insignificant proportion of
incident wave energy. If other constraints (such as weight or
variable seas) require the depth to be less than about 1/16 of
the incident wavelength, the tips 2 can be extended in the
forward direction, parallel to the direction of wave travel, to
limit the effects of egress. On the other hand, a depth greater
than about 1/2 of a wavelength would bridge sequential wave
crests and troughs, leading to conflicting flow directions at
different points over the paddle surface.
The tips 2 on the edges of the paddle, facing the incoming
waves, are positioned to be substantially parallel to wave
particle motions, i.e. substantially perpendicular to the wave
crests. As illustrated in Figure 8 , each tip is provided with a
blunt shape with a cross-section similar to an aerofoil but with
neutral lift. This ensures a clean separation of the flow into
and around the paddle. The precise angle and width of the tips
will depend on the expected variance in the dominant wave
direction. In general, the width of the tip should be largest
near the water surface. Also a wider tip should be used when
there is greater variance in the dominant wave direction in
order to reduce vortices, maintain laminar flow, and limit
structural vibrations and fatigue. Where there is significant
variance in wave direction, typically of more than +/- 30°, the
entire paddle should be rotatable toward the incoming waves to
maximize power extraction.
The paddle may be constructed in a number of different ways. One
simple method of construction is to roll a flat sheet of
suitable metal (e.g. aluminium) into the require curvature, or
the paddle can be moulded from a suitable plastic or composite
material (e.g. glass reinforced plastic). Another way of
constructing the paddle is to form it from a plurality of hollow
tubes welded or otherwise fixed together in the desired shape.
Each hollow tube would have an internal volume, which would
improve buoyancy of the paddle if sealed and can also be used
for storage of equipment (see discussion below in connection
with Figures 2 and 13b) .
Figures 2a, 2b and 2c are a schematic plan, 3D and side
elevation views, respectively, of a second wave paddle embodying
the present invention. This second embodiment is identical to
the first embodiment except for the addition of a forward-facing
tip 4 and a shelf 6 on the backward face 11.
Specifically, the convex forward face 12 is provided with a
relatively sharp tip 4 which points in the direction of wave
travel and tapers towards the flat edge 1 at the bottom of the
paddle. The forward-pointing tip 4 further improves laminar flow
around the paddle, which further enhances the water mobilisation
effect described in connection with Figure 1 above.
Preferably, the shape of the forward pointing tip 4 is similar
to the drag zone 41 (see Figure 17a) that would otherwise be
created. For example, a suitable shape is a Gaussian function
with a variance of between 0.2 and 1 and scaled to merge
smoothly with the convex forward surface 12 of the paddle. Other
shapes having a hydrodynamic profile (generally, a sharp forward
edge and a wide base) may be used instead of a Gaussian
function. The horizontal distance from the front of the forward-
pointing tip 4 to the centre of the backward concave face 11 is
typically around 1/2 of width of the paddle to create a suitably
smooth overall profile.
Starting from a paddle similar to that shown in Figure 1 , formed
from a curved sheet of metal for example, the forward-pointing
tip 4 may be formed as a separate sheet and then attached to the
main body of the paddle 24 creating internal volume 5 . The
additional material defining the internal volume 5 increases the
paddle second moment of area, in each of the principal
horizontal dimensions, giving the centre of the paddle
additional structural strength and rigidity.
Another option is to form or mould the paddle, including the
forward pointing tip 4 as a single piece, with or without an
internal volume 5 . Alternatively, a plurality of tubular members
can be fixed together.
The internal volume 5 is preferably sealed and water-tight, but
may also be open to water. The internal volume 5 can house
additional structural members to provide even more structural
strength and, if sealed, provides buoyancy that gives the paddle
a generally vertical bias within the water. The internal volume
may also house energy conversion or water desalination
equipment. If the internal volume 5 is sealed this will protect
equipment stored inside it from the corrosive effects of sea-
water .
Also in this second embodiment, a horizontally disposed shelf 6
is optionally located at the top of the backward face 11 of the
device. The shelf 6 can be formed by curving a top portion of
the backward face 11 or by fixing a shelf portion to the
backward face 11. In either arrangement, the shelf should
transition or merge smoothly with the backward face 11, as best
illustrated in Figures 2c and 2d to reduce structural stress
concentrations. The shelf helps to prevent water flowing over
the top of the backward face 11 in use ("overtopping"), thereby
increasing power absorption, and provides additional structural
strength.
The depth of the shelf 6 maybe equal to the depth of the
concavity in the backward face 11, particularly if rough seas
are expected with a significant chance of overtopping or if the
top of the device is to be located at or below the surface of
the water. Alternatively, the shelf 6 may extend only 1/10 of
the depth of the concavity, for example, in calmer water or
where there is a large freeboard. Conversely, where there is a
substantial risk of extreme waves (also called freak waves) it
may be desirable to permit overtopping by selecting the
dimensions of the shelf 6 to provide sufficient additional
strength without completely constraining the water within the
concavity .
Although not shown in the figures, the forward face 12 and
forward-pointing tip 4 preferably also merge with the curvature
of the shelf 6 to provide a smooth transition towards the second
surface, reducing drag.
In use, bending moments on the paddle are largest where the
support 19 connects to the paddle and this part of the paddle is
preferably the strongest. To increase structural strength,
tapering of the curvature of the paddle is preferably gradual
from the top of the paddle to the connection point, with a more
rapid taper down to the straight edge 1 below the connection
point, as illustrated in Figure 2c.
Figure 2d is a side elevation view of an alternative arrangement
of this second embodiment. In this arrangement, at least part of
an energy conversion apparatus (only the hydraulic ram 16 and
flexible hydraulic pipes 17 are shown) is mounted within the
internal volume 5 . In this configuration the connection 31
between the hydraulic ram 16 and the base device 15, is located
close to the hinge 13. This arrangement advantageously protects
the equipment from the harsh external salt-water environment,
reducing the need for protective casings and/or coatings,
thereby reducing costs. Furthermore, the integrated design
permits installation or removal of the power conversion
apparatus together with the paddle 24. One disadvantage is that
a flexible high-pressure seal is required at the point where the
piston from the hydraulic ram 16 exits the internal volume 5 .
Figures 3a, 3b and 3c show schematic plan, 3D and side elevation
views, respectively, of a third wave paddle embodying the
present invention. This third embodiment is identical to the
second embodiment but has been adapted to be top-mounted to a
base device 15 from which the paddle hangs vertically downwards.
The curved backward 11 and forward 12 faces, and the forward-
pointing tip 4 , taper to a flat top edge 22 for easy connection
to the base device 15.
With a top-mounted paddle, the magnitude of the paddle's linear
horizontal speed as it rotates around the top edge 22 increases
with depth. However, particle motions within incoming waves
reduce with depth. Preferably, therefore, the paddle should not
extend below the point where the speed of the bottom tip of the
paddle exceeds the particle velocities within the incoming waves
at that depth.
A bottom plate 6 is preferably connected across the bottom of
the concave backward face 11 blocking fluid flow down this face.
The bottom plate 6 is preferably curved upward towards the
forward-pointing tip 4 to further reduce drag resistance as the
paddle moves forward through the water.
Figures 4a and 4b are schematic plan and 3D side elevation
views, respectively, of a fourth wave paddle embodying the
present invention. This fourth embodiment retains the concave
backward surface 11 and convex forward surface 12 of other
embodiments but is curved both horizontally and vertically to
create a cupped or spoon shape 9 . This shape may be formed as a
hollow ellipsoid or as a surface of revolution created by
rotating a parabola or other suitable shape about its central
axis of symmetry. The paddle is connected by a connecting rod 8
to a base device, which is preferably either directly above or
below the paddle, but may be at any other orientation. The
connecting rod 8 preferably has an elliptical cross-section with
the sharper ends of the ellipse pointing in the mean wave
direction to minimise drag.
Figures 5a and 5b are schematic plan and 3D side elevation
views, respectively, of a fifth wave paddle embodying the
present invention. This fifth embodiment is identical to the
first embodiment except that the curved backward 11 and forward
surfaces 12 of the paddle do not taper to a flat edge, but
instead have a constant curvature along the height of the
paddle. Such a paddle is easier to construct, is suited for use
with a base device 21 that relies on a lateral rather than
rotational movement of the paddle, and is preferable for use in
shallow water waves where the flow velocities of waves decays
slowly with depth.
As illustrated in Figure 5b, for connection to a base device 21
relying on lateral movement, a support 19 is connected to the
centre of the convex forward face 12 of the paddle for
connection to a ram 16 of the base device 21. A plate or shelf 6
is preferably connected across the bottom of the concave
backward face 11 blocking fluid flow down this face and under
the device. The bottom plate 6 is preferably curved upward
towards the forward face 12 to create a smooth join with the
forward face 12 to reduce drag resistance as the paddle moves
forward through the water. The shelf 6 preferably extends
backwards for the full depth of the concavity in the backward
face 11 to minimise the egress of water.
If the device is intended to be fully submerged or with a small
freeboard then preferably a further shelf 6 is connected across
the top of the concave backward face 11, blocking flow up this
face and over the device. Again, the shelf 6 is preferably
curved to a smooth transition with the forward face 12 to reduce
drag resistance. Also, the shelf 6 preferably extends backwards
for the full depth of the concavity in the backward face 11 to
minimise the egress of water, but may be less than the full
depth if significant overtopping is not anticipated or if there
is a need to limit the maximum energy extracted from
particularly large waves.
Further horizontal shelves or plates 6 of any suitable depth may
also be provided along the height of the backward face 11 to
provide additional structural strength.
Figures 6a and 6b are schematic plan and 3D side elevation
views, respectively, of a sixth wave paddle embodying the
present invention. This sixth embodiment is identical to the
second embodiment except that the curved surfaces 11,12 of the
paddle and the forward pointing tip 4 do not taper to a flat
edge 1 and instead have a constant curvature along the height of
the paddle. As with the fifth embodiment, such a paddle is
easier to construct. The paddle may be used with a base device
relying on lateral motion, similar to the fifth embodiment
described above. Alternatively, as illustrated in Figure 6b, the
paddle can be adapted to pivot relative to the base device 15 by
providing a structural member 20 supported within the internal
volume 5 and extending from the paddle to make a pivoting
connection 22 with the base device 15. The ram 16 of the base
device 15, for converting motion of the paddle into usable
energy, may be connected to the structural member to avoid
placing stress on the paddle itself.
As illustrated in Figure 6b, the paddle may be located below the
base device, but can also be positioned above the base device,
or at any other orientation. Preferably, a bottom plate 6 is
connected across the bottom of the concave backward face 11, as
with the fifth embodiment above.
In one variation, the structural member 20 is moveably or
flexibly connected to the paddle 24. This allows the vertical
position of the paddle to be adjusted relative to the base
device 15 to keep its position constant relative to the water
level. Since sea level, for example, changes with tides,
changing the position of the paddle ensures optimal power output
with changing water depths. This vertical adjustment is
preferably achieved using the buoyancy that results from the
internal volume 5 of the device, which allows it to adjust
automatically .
Figures 7a and 7b are schematic plan and 3D side elevation
views, respectively, of a seventh wave paddle embodying the
present invention. This seventh embodiment is identical to the
sixth embodiment except that the paddle tips 2 are enlarged to
incorporate structural members 26 for connection to the base
device 15. A moveable or flexible connection exists between the
structural members 26 and the paddle tips 2 , which allows the
vertical position of the paddle to be adjusted relative to base
device 15 similar to the sixth embodiment above. Also shown is a
rod or bar 28 that connects the paddle tips 2 to improve the
structural rigidity of the device and prevent excessive lateral
deformation of the device.
As with the third embodiment, preferably a bottom plate 6 is
connected across the bottom of the concave backward face 11
blocking fluid flow down this face. The bottom plate 6 is
preferably curved upward towards the forward-pointing tip 4 to
further reduce drag resistance.
Figures 8a, 8b and 8c show schematic plan, 3D and side elevation
views, respectively, of an eighth wave paddle 2 4 embodying the
present invention. The paddle design shown in these Figures is
identical to that of Figure 2 , but may instead be of any
suitable design, including a totally flat design. Placed either
side of the paddle on the backward-face side are two wave
deflector or wave concentrator panels 2 3 . In one embodiment, the
panels do not move in response to incoming waves but instead
channel incident wave energy towards the movable paddle 24,
magnifying both the height of the crests and the depth of the
troughs received by the movable paddle. These panels 2 3 are
useful where the maximum width of the paddle 2 4 has been reached
due to size, weight or other limits, but where the system is
capable of extracting more useful energy from each wave.
In another embodiment, the panels 2 3 are also movable in
response to incoming waves and each panel 2 3 and the paddle 2 4
is provided with an energy extraction device such as a hydraulic
ram. The panels 2 3 and paddle 2 4 may all be mounted on a single
base device or may be mounted on separate base devices for
flexibility in the relative position of the panels 2 3 and paddle
2 4 depending upon wave conditions. Having several independently
movable components means that the different surfaces can react
out of phase with each other to, in particular, receive
directional wave energy. The independent multipart design also
increases flexibility in manufacture and deployment, through the
use of smaller, individual but combinable components.
Preferably, similar to the paddle embodiments described above,
the tips on the edges of the panels 2 3 facing the incoming waves
have a blunt, aerofoil-like shape as illustrated in Figure 9 .
The embodiments described above have either constant curvature
along the height of the paddle or the curvature tapers gradually
to a straight edge along the height of the paddle. However, the
present invention is not limited to these particular
arrangements. Other embodiments of the invention may have a
constant curvature along parts of the height of the paddle and
portions where the curvature changes along the height of the
paddle. Changes in curvature are also not limited to transitions
from a curve to a straight edge. Different parts of the paddle
may have different general curvatures depending upon anticipated
wave conditions and the base device design with smooth
transitions between curvatures along the height of the paddle.
Figures 10a to lOp illustrate a range of different paddle 24
curvatures or cross-sections that may be used in embodiments of
the present invention. Figures 10a to lOd show parabolic or
semi-elliptical paddles similar to the first embodiment
described above. The paddle illustrated in Figure 10c includes
backward facing straight extensions 27 to the tips 2 which
extend in a direction generally parallel to the incident waves
to limit egress of water around the edges of the paddle.
Figure lOe shows a forward-pointing tip 4 design similar to the
second embodiment described above but formed from a single-piece
panel such that the backward face 11 has the same profile as the
forward face 12. Figure lOf shows an alternative to lOe with a
blunted forward tip 4 for increased structural strength at the
tip.
Figures lOg to lOi show paddles formed from two, four and eight
flat sections, respectively, to approximate a smoothly curved
surface. Figure 10 shows a paddle with a substantially flat
central portion curving at the ends to provide tips 2 pointing
towards the incoming waves. This flat profile may be necessary
for connection to some base devices.
Figures 10k to 10p show a range of compound paddle designs
formed by using different cross-sections on the backward and
forward faces of the paddle. Each compound paddle has an
internal volume 5 that provides structural rigidity and
buoyancy. Figure 10k shows a paddle with the forward and
backward faces each formed from two flat sections similar to
Figure 10g. Figure 10l shows a paddle having a smoothly curved
concave backward face similar to Figure 10a and a forward face
formed from two flat sections. Figure 10m shows a paddle with a
smoothly curved concave backward face and a forward-pointing tip
4 similar to Figure 10e while the forward-pointing tip 4 on the
paddle shown in Figure 10n is blunted similar to the paddle
shown in Figure 10f. Figure 10o shows a paddle with profiles of
slightly different curvature on the backward and forward faces.
Figure 10p shows a paddle with a flat central portion to the
backward face, similar to Figure 10j, with a forward-pointing
tip 4.
For each of the paddles shown in Figures 10a to 10p, the drag
coefficient (C ) of the backward face 11 is greater than 2.
Depending upon the materials used, the drag coefficients (C ) for
the forward faces 12 lie between 0.3 and 2.0, the flatter design
of Figure 10j having the largest drag on the forward face 12 and
the paddles with forward-pointing tips shown in Figures 10k-10n
have amongst the lowest drag on the forward face 12. Symmetric,
substantially flat paddles such as those known from the prior
art have typical drag coefficient greater than 2 for both faces.
Figures 11a to 11f illustrate a range of paddle 24 and panel 23
combinations, similar to the embodiments described above in
connection Figure 8. In the arrangements shown in each of these
Figures, the panels 23 may be fixed in place and simply serve to
guide and concentrate wave energy onto a movable paddle 24, or
each panel 23 may be independently movable as described above.
Figure 11a shows two smoothly curving panels 23 having a
curvature that meets smoothly with an elliptically curved paddle
24 similar to the paddle illustrated in Figure 10b. Figure lib
shows a combination of four panels/paddles which have an overall
shape similar to the paddle of Figure lOg, but where any or all
of the four panels/paddles may be independently movable to
extract wave energy.
Figures 11c to lie show panel 23 and paddle 24 combinations,
where the panels and paddle are each made up of a number of flat
sections. Figure lie, in particular, has a totally flat forward
surface. Nevertheless, many of the advantages of the present
invention are still realised due to the overall asymmetry in the
horizontal plane perpendicular to the incident wave direction.
The side panels 23 therefore focus wave energy smoothly onto the
paddle 24.
Figure llf shows the combination of two wave concentrating
panels 23 and a Gaussian-profiled paddle 24 similar to Figure 8 .
Figure 12 shows a possible design of a wave powered hydraulic
system designed to generate electricity. Other designs are well
known in the art. The movement of the wave powered paddle
compresses fluid in either end of a double acting hydraulic ram
16 and pumps it through one of two non-return valves 52 into a
hydraulic accumulator 55. A hydraulic motor 58 pumps water to
drive an alternator 59 to generate electricity. The volume of
the accumulator 55 is preferably large relative to the volume of
the ram 16, with a large plan area so the resulting pressure
head available for the hydraulic motor 58 remains relatively
constant. To reduce the height of the accumulator, while
maintaining the pressure head, the void 56 in the head of the
accumulator may be pressurized with gas.
The flow of fluid to the motor 58 is adjusted through control
valve 57. Exhaust fluid passes to a reservoir 60 from which it
is available to be sucked back into either end of the hydraulic
ram 16 through a second set of non-return valves. In order to
smooth any pressure fluctuations in the pipes from the wave
paddle assembly to the accumulator, which may be of considerable
length, a pressure chamber 54 is provided.
Where the base device has more than one ram 16, the overall
resistance of the paddle movement can be altered by either
engaging or disengaging additional rams. A convenient method to
achieve this is to open a bypass valve 64 that permits fluid to
flow from one side of the double acting hydraulic ram to the
other. Preferably, an array of multiple paddles will share the
same core system with only the elements within box 63 being
repeated for each paddle. Some of the elements in box 63 may be
located in an internal volume 5 of a paddle.
The system is controlled through a central computer unit 62. An
electronic sensing system 61 monitors incoming waves and through
the operation of a valve 51 the movement of the hydraulic ram
16, and therefore the paddle, can be tuned to suit wave
conditions. The volume of fluid pumped is measured by a flow
meter 53.
Figure 13a shows a possible design of a wave powered
desalination system to convert sea-water to fresh-water. The
elements in box 70 in this system are similar in operation to
the elements in box 63 in Figure 12, except that the working
fluid is sea-water drawn in through filter 71. The sea-water is
pressurised by the ram 16 and delivered via a flow meter 53 and
control valve 57 to reverse osmosis equipment 73, which creates
fresh water for collection in reservoir 72 and a concentrated
brine solution for collection in reservoir 75. Control valves 74
and 57 are controlled through a central computer unit 62 to
maintain the appropriate pressure across the reverse osmosis
equipment 73.
Some of the elements of Figure 13a may be located in an internal
volume 5 of a paddle. For example, Figure 13b illustrates a
possible paddle design 24 in which reverse osmosis tubes are
incorporated within vertical structural tubes 76 located between
flat panels defining the forward 12 and backward 11 faces. The
remainder of the equipment illustrated in Figure 13a can be
located in the front pointed tip 5 of the device, with a low
pressure pipe delivering desalinated water ashore.
Figure 14a illustrates a group of three paddle units arranged to
form a wave farm array. Each unit is positioned such that
incoming waves strike them at different times in order to smooth
the power extracted over the entire array. Preferably a large
number of paddle units are used in parallel to smooth the
delivery of water to the accumulator 55 or desalination
equipment 73. The paddles may also be aligned so they
collectively act as a breakwater reducing the size and intensity
of the waves that approach the shore.
Figure 14b illustrates one method of locating a paddle in deep
water where it would be difficult or impossible to fix a base
device 15 directly to the seabed. The paddle is attached to a
base device 15 that is itself mounted on a submerged pontoon 30.
The pontoon is anchored to the seabed using cables 29 which may
be shortened or lengthened to position the paddle at the correct
height depending upon wave conditions or to withdraw it from the
surface of the water entirely in bad weather.
Figures 15 to 17 illustrate pressure variations and flow
velocities in the region of paddles embodying the present
invention. These figures are provided to assist in understandi
the mathematical and theoretical basis behind the present
invention .
The pressure field associated with wave is derived from the
unsteady Bernoulli equation for an deal fluid:
where f is velocity potential, p is density, g is gravitational
acceleration and z is the vertical position above Still Water
Level (SWL) .
Figure 15 illustrates the total pressures on a vertical paddle
beneath a non-breaking wave, which are predominantly hydrostatic
(i.e. P=~pgz) , with an unsteady contribution near the surface
from dt in Eq. 1 . The presence of a wave at the surface only
has a local impact on the pressures below, so that at a depth of
approximately half the dominant wavelength the pressures become
purely hydrostatic with respect to only the SWL. The difference
between the actual pressure beneath the wave and the hydrostatic
pressures relative to SWL, is known as the dynamic pressure (Di) ,
which alternates from a positive pressure under a wave crest to
a negative pressure beneath a wave trough, as illustrated in
Figures 17a and 17b.
Figure 16 illustrates how the dynamic pressure might vary on the
forward and backward sides of a wave paddle beneath a wave
crest. As the paddle moves more slowly than the waves, the
concave curvature of the backward face 11, constrains the fluid
motion and amplifies the pressures D along this face: both
positively within a crest and negatively within a trough.
On the forward face 12 the dynamic pressure is derived from a
combination of processes, which include:
1) An inertia force due to the relative acceleration of the
paddle against the water, which causes a change in the inertia
of the surrounding fluid. Therefore this pressure will
typically be largest at either extent of the paddle motion
when the paddle 24 has the greatest acceleration. As the
motion of the water on the backward face 11 drives the paddle,
the region of water affected by this paddle acceleration can
be idealised as a body of water 41 in front of the paddle.
This is typically Gaussian in shape and resists changes in the
paddle velocity. In the literature this is usually accounted
for by adding mass to the paddle, and is therefore referred to
as “added-mass”.
2) Once the paddle is moving there is a corresponding drag force,
which is largely responsible for creating a wave that
propagates at a perpendicular angle from the paddle’s forward
surface 12 and dissipates energy. Energy within dissipated
waves is proportional to the square of the wave velocity, so
it is beneficial to minimise these velocities. The convex
forward face 12 and the pointed tip 4, both help to reduce
these perpendicular wave velocities.
3) A constructive pressure (D ) due to incident waves 42 that
pass around the tips 2 of the paddle on the outer side, and
travel around the forward surface 12 of the paddle. During a
passing crest (Figure 17a), the wave 42 mobilises the fluid in
front of the paddle, creating a negative dynamic pressure (D )
along the front face 12 of the paddle. During a passing trough
(Figure 17b), the wave 42 now travels in the opposite
direction, creating additional positive dynamic pressure along
the front face 12.
This third process works to draw the paddle in the direction of
travel of the wave, thus increasing the energy available to the
paddle. During initial testing of embodiments of the present
invention, this was an unexpected effect of the smooth curvature
on the forward face 12 of the paddle that was anticipated to
only reduce the drag force mentioned in process 2 above. The
energy captured by the device was therefore significantly
greater than anticipated. In effect, the device captures energy
over a wave front that is wider than the actual paddle width.
Balancing the main forces that act on the paddle:
where F represents the force between the paddle and the piston
that extracts power, m is the mass of the paddle, and ¾dded
the added mass.
During the paddle's forward movement and D act together, and
are constrained by D2 (see Figure 17a) , while the component on
the right hand side of Eq. 2 represents stored, extracted and
lost energy, respectively. During the paddle's backward movement
D is now predominantly negative due to the trough that forms on
the paddles backward face 11, D turns positive, and D become
negative .
The power extracted from the paddle can be approximated by:
Power — (Eq. 3 )
where T is the dominant wave period, D is the horizontal travel
of the paddle and F is the mean force between the paddle and
piston. As G is fixed by the given wave conditions the only way
to change the power extracted from the waves is through a change
to the product F - .
There are two limiting conditions for this equation when no
power is extracted:
1 ) If the paddle is locked so it acts as a rigid vertical wall
then D =0 and F will be maximized. However, with no movement and
assuming other losses are small, then almost all of the energy
is reflected so it travels back against the incoming waves.
2 ) If the paddle moves freely with the waves, so F =0, then D
is maximized. If the paddle is sufficiently light to allow it to
move at the same velocity as the fluid on the incoming wave
side, then almost all of the wave energy that reaches the paddle
is absorbed and immediately dissipated on the opposite side
through the creation of new secondary waves.
Laboratory test have been conducted for a range of curved test
paddle designs and a flat symmetric test paddle that forms a
base for comparison. The laboratory facility is 15m long, 2.5m
wide, with lm of water depth. The tank contained computer
control wave generators at one side and a beach to absorb the
waves at the other. Each test paddle had width and height of
exactly lm, such that the projected area towards the incident
waves was the same. All the paddles were mounted in a generally
vertical orientation above a fixed frame, 20cm above the tank
floor, which allowed the paddles to rotate forward and backward
inline with the incident wave direction. This rotation was
constrained by a pneumatic piston connected through a force
transducer to the top of the test paddle, with valves on the
piston configured to provide suitable resistance, while also
pumping air with each stroke (similar to the configuration shown
in box 63 of Figure 12).
Figures 18a and 18b presents some sample results for a curved
(Test #123) and flat (Test #139) paddle, respectively, for near
identical incident waves. The curved paddle was of form similar
to that shown in Figure 2a, with a depth of concavity of
approximately 0.3m. The incident wave height for each case was
approximately 16cm with a period of 1.3s, which equated to
incident wave energy of about 31W/m.
Comparing the Force and Displacement curves for each paddle, it
is evident that the curved paddle delivered a much greater force
and consequently a large displacement. From these two curves Eq.
3 is used to compute the power generated and mean power output
over 2s window (shown with a dashed line). These results
demonstrate the curve paddle delivers a mean power output of
about 51W/m, while the flat paddle delivered only about 23W/m. A
further unexpected result is that the curved paddle delivers
approximately equal power during both a passing wave crest and
wave trough, whereas a flat paddle deliver ~30% less power from
a trough.
A range of other wave frequencies and heights were also tested,
with the curved design consistently delivering between 40% and
150% more power. With optimisation of the curved paddle design
based on wave conditions, even greater relative improvements can
be expected.
Claims (31)
1. A wave energy extraction device to extract energy from water waves through forwards and backwards motion of the device for 5 use with a wave energy conversion apparatus to convert that energy to a useful form, the device having a height and a width and being arrangeable on the wave energy conversion apparatus so as to be at least partially submerged in the water and comprising of a first surface arranged to oppose a mean water 10 wave direction, the first surface being concave about a vertical axis of the device and a second surface disposed opposite the first surface and being convex about the vertical axis of the device so as to be able to extract power from the crest and the trough of the wave.
2. A wave energy extraction device as claimed in claim 1 in which a cross-sectional profile of the second surface additionally extends to a generally pointed tip at the vertical axis of symmetry of the device.
3. A wave energy extraction device as claimed in any one of the preceding claims in which a cross-sectional profile of the second surface is formed with a generally Gaussian profile. 25
4. A wave energy extraction device as claimed in any one of the preceding claims in which the cross-sectional profile of the second surface has a low resistance to motion through the water.
5. A wave energy extraction device as claimed in any one of the 30 preceding claims in which horizontal extremities of the first concave surface are formed as tips arranged to oppose the mean wave direction during use.
6. A wave energy extraction device as claimed in claim 5 in which the tips are generally aerofoil shaped, the aerofoil shaped tips having a leading edge arranged to oppose the mean wave direction during use.
7. A wave energy extraction device as claimed in any one of the preceding claims in which the first surface extends continuously across the wave energy extraction device between horizontal extremities thereof.
8. A wave energy extraction device as claimed in any one of claims 1 to 6, comprising: a wave energy absorber component having a first width and adapted to move in response to incoming water waves for 15 extracting power; and a wave energy concentrator component for concentrating water waves from across a second width greater than the first width down to the first width and guiding the concentrated wave energy towards the wave energy absorber component.
9. A wave energy extraction device as claimed in any one of the preceding claims, which comprises a paddle.
10. A wave energy extraction device as claimed in any one of the 25 preceding claims, which is arranged to move in response to waves to extract power therefrom.
11. A wave energy extraction device as claimed in any one of the preceding claims, further comprising an attachment point for 30 attachment to the wave energy conversion apparatus.
12. A wave energy extraction device as claimed in claim 11 in which the device has an upper end and a lower end during use thereof, the attachment point being located at one of the upper and lower end of the device.
13. A wave energy extraction device as claimed in claim 11 or 5 12, having a cross-sectional profile that varies over the device height.
14. A wave energy extraction device as claimed in claim 13, in which the cross-sectional profile tapers toward a flat profile 10 adjacent the attachment point.
15. A wave energy extraction device as claimed in any one of claims 11 to 14 wherein the attachment point comprises a hinge point for hinged attachment to the wave energy conversion 15 apparatus.
16. A wave energy extraction device as claimed in any one of claims 11 to 15 in which the attachment point is operable to adjust the relative spacing between the wave energy extraction 20 device and the wave energy conversion apparatus.
17. A wave energy extraction device as claimed in any one of the preceding claims, in which the lower end of the device includes one or more generally horizontally disposed shelf for limiting 25 fluid flow vertically along the device during use thereof.
18. A wave energy extraction device as claimed in claim 17 in which the generally horizontally disposed shelf is contoured in profile.
19. A wave energy device as claimed in claim 2 in which the generally pointed tip is formed by a third surface that is fixedly attached to the convex second surface to provide a first internal volume between the second convex surface and the third surface.
20. A wave energy extraction device as claimed in claim 19 which 5 is formed of separate components and in which the generally pointed tip is a component separate to a component forming the convex second surface and reinforces the strength and rigidity of a central portion of the wave energy device. 10
21. A wave energy extraction device as claimed in claim 20 wherein the concave first surface is adapted to focus wave energy toward the reinforced central portion of the wave energy extraction device. 15
22. A wave energy extraction device as claimed in any one of the preceding claims in which the first surface and the second surface are separate components fixed together to create a second internal volume. 20
23. A wave energy extraction device as claimed in any one of claims 19 to 22 in which the at least one of the first and second internal volumes is sealed against the ingress of water to provide buoyancy to the device. 25
24. A wave energy extraction device as claimed in any one of claims 19 to 23 in which at least one of the first and second internal volumes is configured to provide storage space inside the device. 30
25. A wave energy extraction device as claimed in any one of claims 19 to 24 wherein at least one of the first and second internal volumes is configured to receive one or more sea-water desalination units.
26. A wave energy extraction device as claimed in any one of the preceding claims, in which each of the first concave surface and the second convex surface are substantially parabolic or semi- elliptic in form.
27. A wave energy extraction device as claimed in claim 26 in which the form of the first concave surface and form of the second convex surface are different to each other. 10
28. A wave energy extraction device as claimed in claim 26 or claim 27 in which the parabolic or semi-elliptic form is approximated using a plurality of straight sections.
29. A wave energy extraction device as claimed in any one of the 15 preceding claims in which the first surface is concave also about a horizontal axis.
30. A wave energy extraction device as claimed in any one of the preceding claims in which the second surface is convex also 20 about a horizontal axis.
31. A wave energy extraction device as claimed in any one of the preceding claims in which the concavity of the first surface has a depth that is between
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB1107377.2A GB2490515B (en) | 2011-05-04 | 2011-05-04 | A wave energy extraction device |
GB1107377.2 | 2011-05-04 | ||
PCT/GB2012/000408 WO2012150437A2 (en) | 2011-05-04 | 2012-05-03 | A wave energy extraction device and method |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ618498A NZ618498A (en) | 2015-06-26 |
NZ618498B2 true NZ618498B2 (en) | 2015-09-29 |
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ID=
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