Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED WAVE ENERGY CONVERTER (WEC) DEVICE AND SYSTEM
BACKGROUND OF THE INVENTION
This invention relates to the conversion of energy present in surface
waves on large bodies of water to useful energy.
In related United States Patent No. 6,772,592, filed February 4,
2003, there is disclosed a surface wave energy conversion system comprising
two
separate, but interacting components, each for capturing energy from surface
waves.
A first of the system components comprises a float on the water
surface which bobs up and down in response to passing waves. Such bobbing
motion tends to be in phase with the passing waves, i.e., the float rises in
response to a passing cresting wave.
The second component of the system comprises a submerged
member dependent from the float and including a compressible fluid responsive
to
water pressure variations. In response to an overpassing cresting surface wave
and an increase in water pressure, the compressible fluid is compressed
resulting
in a decreased volume and corresponding decreased buoyancy of the second
component. Thus, the second component tends to sink relative to the float in
out-
of-phase relation with the passing waves.
Of significance is that the two components tend to move in opposite
directions in response to the same passing wave. Thus, by interconnecting an
energy transducer, e.g., a linear electrical generator, between the two
components, energy generation is obtained.
SUMMARY OF THE INVENTION
A wave energy converter comprises two floats, a first of which is
configured to rise and fall generally in phase with passing surface waves on a
body of water, and the second of which is configured to rise and fall
generally out
of phase with passing waves.
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In one embodiment, the float comprises an annular member having
a central opening, and the second float comprises an elongated spar disposed
within the central opening for vertical out-of-phase movements relative to the
first
float; the wall of the central opening serving as a bearing for the moving
spar.
An energy converter, e.g., a linear electrical generator, is connected
between the two floats for converting relative movements therebetween into
useful
energy. Significantly, because energy is obtained from relative movements
between the floats, neither needs to be anchored to the floor of the body of
water.
According to one aspect of the present invention, there is provided
apparatus for capturing energy from surface waves on a body of water
comprising
first and second floats, each having, when the apparatus is deployed in a body
of
water, an intercept with the water surface, a power take-off element connected
between said floats for converting relative movements therebetween into useful
energy, and wherein said first float generally extends horizontally along the
water
surface and is configured to rise and fall in in-phase relation with passing
surface
waves, and said second float extends generally vertically above and below the
first float and is configured to rise and fall in out-of-phase relation with
said
passing waves, and wherein said first and second floats have configuration
values
g/Z which are greater and less than w2, respectively, where: g = acceleration
due
to gravity; Z = the effective depths of the floats; and c,J = the angular
frequency of
the passing waves; and where: Z (effective depth) = VD/As, where: VD is the
volume of the water displaced by the float including hydrodynamic added mass;
and AS is the waterplane area of the float.
According to another aspect of the present invention, there is
provided an apparatus for capturing energy from waves on a body of water
comprising an elongated spar having positive buoyancy and having top and
bottom ends, said bottom end being anchored to the floor of the water body by
a
gimbal joint allowing tilting of the spar away from a vertical axis, and said
top end
extending to and beyond the water body surface, a circular float disposed
around
the spar upper end for vertical movements relative to the spar in response to
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passing surface waves on said water body, and a power take-off device
interconnected between said float and said spar for converting relative
movements
between said float and said spar into useful energy.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings are schematic and not to scale.
FIGURE 1 is a vertical section of a system according to the
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invention deployed in a body of water;
FIGURE 2 is a horizontal section taken along line 2-2 of Fig.
1;
FIGURE 3 is a view in perspective showing a variation of a
mooring arrangement shown in Fig. 1;
FIGURE 4 is a view similar to Fig. 1 but showing a variation
of the mechanical configuration of the system;
FIGURE 5 is an enlarged view in section showing electrical
elements of a linear electrical generator in one of the floats;
FIGURES 6 and 7 show a series of graphs illustrating the phase
relationships among the system components shown in Fig. 1 and
surface waves driving the system;
FIGURE 8 shows a single float system;
FIGURES 9-14 show modified spar floats useable according to
the present invention;
FIGURES 15A-C illustrate, partially by comparison with a spar
similar to the one shown in Fig. 1, further variations of spar
floats useable according to the present invention; and
FIGURE 16 is a view in elevation of an inflatable system,
shown in fully inflated configuration; while
FIGURE 17 shows the same system shown in Fig 16 in deflated
condition.
DETAILED DESCRIPTION OF THE INVENTION
One embodiment according to the present invention is
illustrated in Figs. 1 and 2. Therein, two floats 100 and 200 are
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shown; the float 100 being generally "flat", in the sense of having
a relatively large horizontal surface area with a relatively small
submerged depth; and the float 200 having a relatively small
horizontal surface and a relatively large submerged depth. By
"horizontal surface" is meant that plane of a float lying in the
plane of the mean level surface of the water. Hereinafter, such
;.horizontal surfaces are referred to as "waterplane areas".
.The float 100 has an annular shape, including a rim 102
enclosing a central opening 104. The float 200 is elongated and
extends through the central opening of the float 100.
The physical characteristics of the two floats are selected
such that they move generally out of phase with one another in
response to passing waves.
Fig. 1 also shows, schematically, a mooring arrangement for
the dual float system. Thus, separate buoys 600 are provided
fixedly anchored in place. The buoys 600 are loosely connected, by
flexible cables, to the float 100 which is thus free to bob up and
down while being moored in place. Fig. 3 shows an alternative
arrangement with a loose fitting collar 201 slidably disposed on
the float 200. The collar 201 is anchored by one or more cables.
It can be shown that whether a float heaves in-phase or out-
of-phase with a passing surface wave is dependent on whether the
float displaces a small or large volume of water relative to the
float's waterplane area. In the case of in-phase motion, the float
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displaces a relatively small volume of water for a given waterplane
area, and may be described as a low-inertia float. Conversely, for
the case of out-of-phase motion, the float displaces a relatively
large volume of water for a given waterplane area, and may be
described as a high-inertia float. It can be shown that the
properties of a float relative to a surface wave of angular
frequency w are such that the float displays in-phase or out-of-
phase behavior depending on the relative values of g/Z which are
greater or less, respectively, than w2, where:
w is the angular frequency of the passing surface waves;
g is the acceleration due to gravity; and
Z is the "effective depth" of the float where:
Z = VD/AS (1)
where:
VD L.s the volume of water displaced by the float including
hydrodynamic added mass effects; and
AS is the waterplane area of the float.
Thus, for the float 100, moving in phase with the passing
waves:
g/Z > w2 (2)
or
Z < g/w2 > VD/AS (3)
The expression g/w2 is known as the "resonance depth", i.e., a
body with an effective depth (Z) equal to the resonance depth will
have a natural period of oscillation equal to the frequency of the
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surface waves.
Floats, such as the float 100 shown in Fig. 1, having
effective depths less than the resonance depth tend to bob up and
down in phase with the passing waves.
Conversely, floats having effective depths, Z, greater than
the resonance depth tend to bob up and down out of phase with the
waves.
For floats having an effective depth close to the resonance
depth, the phase relationships between the floats and the waves can
be variable, depending upon various damping effects such as viscous
damping. Accordingly, for definite in and out of phase movements
of the floats relative to the waves and to one another, the
effective depths, Z, of the floats are designed to be either
greater or less than the resonance depth. As noted, the effective
depth, Z, is equal to the displacement of a float divided by its
waterplane area As. For a given volume, related to the desired
power generation of the system, the principal design variable is
the area As. From Equation (3), with a given VD, a float will move
in phase with the surface waves provided AS is sufficiently large.
Conversely, a float will tend to move out of phase with the
surface waves provided AS is sufficiently small.
In Fig. 1, the float 100 has a large AS relative to the volume
of water displaced (VD) by the float; whereas the float 200 has a
small AS relative to its VD.
The buoyancy or "heave" force on a vertically oriented
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cylindrical float is a function of the water pressure at the bottom
of the float multiplied by the float bottom area. When a cresting
wave passes a relatively shallow float, the momentarily increased
depth of the float gives rise to an increased water pressure at the
float bottom and hence an increased force. Due to the low inertia
of the float, the float tends to respond immediately to the force,
and thus tends to move in phase with the passing wave.
The same forcing mechanism applies with an elongated spar
except that the increased inertia of the spar causes the spar to
tend to be out of phase with the passing wave. (It is known, for a
sinusoidally forced high-mass system with negligible position-
dependent restoring forces, that the motion of the system tends to
be out of phase with the forcing on said system.)
A further factor influencing the movement of a float is that
the amount of water pressure increase at the bottom of the float in
response to a passing wave crest decreases with increasing depth of
the float. Because vertical movements of the floats in response to
passing waves are in response to water pressure variations at the
bottom of the floats, reductions in such water pressure variations
reduce the forces applied to the floats. This reduction of water
pressure variation or heave force with depth is known, and for an
upright floating spar, the manner in which the heave force on the
spar varies with depth is given by the equation:
as = cosh[K(d-D)]/cosh(Kd) (4)
where:
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K is the wavenumber, defined as 27t/?, where 2 is the distance
between wave crests (i.e. the wavelength)
d is the depth of the body of water; and
D is the draft or submerged length of the spar relative to the
mean water level.
The factor 6 is based on pressure due to surface waves propagating
in the absence of any impediment (i.e. the float) and hence is
closely related to the known "Froude-Krylov" force. Specifically,
6 is the ratio of the Froude-Krylov force for a given floating body
to the Froude-Krylov force integrated along the underside of the
waterplane area of that body.
In deep water, d >>D, the reduction of water pressure variation
defined in Equation 4 may be expressed as:
o = exp (-KD) . (5)
This a factor affects both the shallow and elongated floats, and
preferably the 6 of each float is as large as possible.
Accordingly, in some embodiments of the invention, each has as
short an effective depth as possible within the constraint of
Equation 3. Another practical constraint is that the shallow float
should have an effective depth not less than the typical wave
amplitude to assure hydrodynamic interaction between the shallow
float and the wave.
In one embodiment, the shallow float, which tends to move in phase
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with the wave elevation, has an effective depth Z that is
substantially less than the resonance depth. The spar, which tends
to move out of phase with the wave elevation, has an effective
depth Z that is not much larger than the resonance depth.
In connection with typical power take-off devices which function
most efficiently at higher speeds, it is beneficial to make the
effective depth Z of the elongated spar as close to the resonance
depth as possible so as to increase its oscillation amplitude.
This increase in oscillation amplitude leads to more efficient
conversion of energy by the power take-off device.
By way of example of a system according to this invention, and
in reliance upon Equation (2) for the float 100 (and the inverse of
Equation (2) for the spar, i.e., g/Z < w 2), assume that the system
is intended for use where the surface waves have a dominant wave
period of T=8 seconds (2=100m, k=.063, w = 2n/T, so W2 = 0.62 sec-
2). Consequently, g/Z for the float 100 must be greater than 0.62
sec-2, and g/Z for the spar must be less than 0.62 sec-2. Taking
9.81 m/s2 as the acceleration due to gravity, the float 100 must
have an effective depth Z less than 15.9 in, and the spar 200 must
have an effective depth greater than 15.9 m.
Further by way of example, assuming a circular float 100, for
ease of mooring (as explained hereafter), and dominant surface
waves of 100 meters wavelength, the float outer diameter is 2
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meters and has a central opening of 1.2 meters. The waterplane
area of the float 100 is thus approximately 2 square meters. For
stability, the float 100 is ballasted to have 1/2 of its height
below water. Thus, with a total height of 3.0 meters, the float
submergence depth is 1.5m.
The force tending to lift the float is a function of the area
of the float which, in the above example, is approximately 2 square
meters.
The spar float 200, in this example, is a cylinder having an
outer diameter of 1.15 meters, hence a waterplane area of
approximately 1.0 square meters. The height of the float is 20
meters and the float is ballasted to have a submerged depth of 17
meters. The effective depth, Z, of the float is thus approximately
17 meters.
The a factor for the shallow float (assuming deployment in deep
water) is exp(-kZ)=exp(-0.063*1.5)=0.91. The a factor for the
elongated spar is exp(-kZ)=exp(-0.063*17)=0.35.
Due to the higher a factor for the shallow float, which corresponds
to increased wave forcing pressure, the waterplane area of the
shallow float is, in one embodiment, larger than the waterplane
area of the elongated spar. In one embodiment, the ratio of the
waterplane area of the shallow float to that of the elongated spar
is not too large, or the mass of the elongated spar will be
inadequate for it to react against the power take-off device
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disposed between the two components.
How the two floats are configured to obtain the desired
relationships is a matter of choice to the designer. A feature of
the present invention, however, is the particular relationship
between the two floats 100 and 200. Thus, by disposing the float
200 within the central opening 104 of the float 100, the movements
of the two floats relative to one another are constrained, with the
float 100 serving as a bearing for the float 200. To further
control the relative movements between the two floats, a collar 106
can be added to the float 100 as shown in Fig. 1. Also, for
biasing the spar 200 to remain in upright position, the lower
portion of the float is preferably weighted, i.e., by a weight 202
shown in Fig. 1.
In Fig. 4, the bearing function of the floats is reversed.
The elongated float 200A encompasses the flat float 100A and
provides a bearing surface for the float 100A.
Energy is converted by virtue of relative movements between
the two floats and a suitable energy converter, e.g., a hydraulic
pump 110 shown in Fig. 1, connected between the two floats. While
relative vertical movements are required, uncontrolled angular
rotation of the two floats relative to one another is preferably
restricted to avoid the need for complicated interconnections to
and between the floats. To this end, the cross-sections of the
inter-fitting float parts are preferably non-circular. For
example, as shown in Fig. 2, the shape of the central opening 104
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through the float 100C and the corresponding cross-sectional shape
of the float 200C are rectilinear.
In Fig. 2, the float is shown of circular outer shape. This
provides the advantage that no particular orientation of the float
is required with respect to the passing waves. A limitation on the
diameter of the float, however, is that it be relatively small in
comparison with the wavelength of the passing waves, e.g., not more
than 10% of such wavelength. This is to avoid "cancellation"
effects, i.e., when the float is simultaneously exposed to both
lifting and falling forces. For example, if the float diameter
were equal to a surface wave wavelength, the net heave force on the
float would be zero.
One means for increasing the size of the float 100 while
avoiding cancellation effects is to enlarge the float in a
direction perpendicular to the direction of advance of the waves.
This requires, however, that the proper orientation of the float be
maintained relative to the wave direction.
The two floats 100 and 200 acquire kinetic energy as they bob
up and down in response to the passing waves. One means for
extracting energy from the moving floats is to interconnect each
float to a separate energy converter, e.g., a hydraulic pump,
connected between a respective float and a stationary ground point,
e.g., the ocean bed. An advantage of the dual float system of the
present invention, however, is that each float can serve as a
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ground point for the other, with neither float rigidly
interconnected to the ocean bed. This is particularly advantageous
in deep water situations.
Thus, as shown in Fig. 1, for example, an energy converter,
e.g., a hydraulic pump 110, can be interconnected between the two
floats with the relative vertical motions of the two floats being
used to pump the pump 110 for pressurizing a hydraulic fluid
therein. The fact that the two floats are constrained to move in
preselected paths relative to one another greatly simplifies the
mounting and interconnecting of an energy converter on and between
the two floats.
In one embodiment, electrically conductive members comprising
elements of an electrical generator are provided on the surfaces of
the two floats which slide past one another, i.e., conductive
elements 112 (Fig. 5) are provided on the inner surface 114 of the
rim 102 of the float 100, and conductive elements 212 (Fig. 11) are
provided on the outer surface 214 of that length of the float 200
which slides within the float central opening 104. The relatively
movable conductive members can be configured to comprise a linear
electrical generator.
As described, the two floats tend to move in opposite vertical
directions in response to passing surface waves. This is
illustrated in Fig. 6 where, in graph A, vertical movements of the
two floats are plotted against time.
In graph B, vertical movements of passing surface waves are
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plotted against the same time scale as used in Graph A. In Graph A,
the vertical movements of the float 100 are shown by the curve 120
and those of the float 200 by the curve 220. Curve 120 for the
float 200 is in phase with the surface waves, while curve 220 for
the float 200 is 1800 out of phase with the waves. Curve 150 plots
the relative movements or separation between the two floats 100 and
200.
The movements illustrated by curves 120 and 220 for the two
floats 100 and 200 are those for freely moving floats. In actual
use, the two floats 100 and 200 are interconnected by an energy
converter, and the effect of such interconnection, and energy
removal from the floats, is shown in Graph C in Fig. 7. Because of
the interconnection between the two floats, through the energy
converter, the two floats are no longer 1800 out of phase with one
another.
As noted, the float 100 serves as a bearing for the spar float
200 in the embodiment shown in Fig. 1, and vice versa in the Fig. 4
embodiment. The illustrated mechanical interaction between the two
floats, for maintaining them in desired physical relationships even
in a heaving water surface, is so advantageous that such mechanical
relationship is retained in a system illustrated in Fig. 8. As
shown, only one float, e.g., the float 100, is free for vertical;
movements, while the other float, the spar 200, is fixedly anchored
to the ocean floor by means of a known type of gimbal joint 700
allowing tilting of the spar but no vertical movements. Thus, only
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the float 100 moves in response to the passing waves for capturing
energy from the waves
While the spar 200 shown in Fig. 8 is vertically stationary,
it is an effective means for mooring the float in place while
allowing free vertical movements of the float. Additionally, it is
generally known that protection of a floating object against storm
damage can be obtained by submerging the object. By constraining
the vertical movements of the float along the anchored spar 200,
protective flooding of ballast tanks in the float 100 can cause it
to sink in a controlled manner downwardly along the spar and in
fixed location. Upon blowing of the ballast tanks, the float 100
rises to its previous position.
Other features and structural variations of the invention are
shown in Figs. 9-15.
In Fig. 9, a spar 200B is shown having a heavy weight 220 at
the bottom end 222 and a plurality of air-filled cells 224 at the
top end 226. The arrangement illustrated is effective for
maintaining the spar in vertical orientation.
In Fig. 10, a spar 2000 is shown with an indented region 240
for receipt, as previously mentioned, of a series of conductive
elements 212 (Fig. 11) forming, in connection with conductive
elements on the inside surface 115 (Fig. 5) of the annular float
100, a known type of linear generator.
In Figs. 12 and 12A, a spar 200D comprises a plurality of
telescoping concentric pipes 250 for greater ease of storage and
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transportation. When in use, the adjoining sections are locked
together.
In Fig. 13, a spar 200E comprises a plurality of hollow
annular members 254 vertically stacked in fixed angular relation
along a central column 256.
In Fig. 14, a mass - spring system 270 is disposed within a
spar 200F. The system includes a weight 272 mounted between two
springs 274 and a selectively movable mechanism 276 for allowing or
preventing vertical movements of the weight. The effect of this
internal degree of freedom of the spar is to increase the lowest
natural oscillation frequency of the spar, providing a means for
the designer of the WEC apparatus to tune the device for greater
energy conversion efficiency. For example, for an embodiment of
the present invention intended for deployment in a region where
dominant waves have a range of wave periods, it may be advantageous
to design an elongated spar to a length which leads to optimal
energy conversion for the longer wavelengths. In the presence of
long period waves, the mass-spring system 270 is locked against
movement, and thus the system is tuned. In the presence of shorter
period waves, the mass-spring system is allowed to oscillate,
causing the spar to resonate at a frequency closer to that of the
shorter period waves, leading to improved energy capture.
In Figs 15A, 15B, and 15C, three possible configurations of
spars are shown. Fig 15A shows a spar 200 similar to the spar 200
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shown in Fig. 1 and of a certain waterplane area and a certain
effective depth Z. The spar 200 comprises a single cylinder of
uniform diameter. In Figure 15B a spar 2000 is shown in a dual-
cylindrical configuration, i.e. the spar 2000 is comprised of an
upper cylinder 280 which has a diameter greater than the diameter
of a lower cylinder 281. The spar 2000 shown in Fig. 15B is
configured such that its waterplane area is equal to that of the
spar 200 shown in Fig. 15A. The lower cylinder 218 of the spar
200G is configured such that the total volume of water displaced by
the spar 2000 is equal to the volume of water displaced by the spar
200 shown in Figure 15A. Because the spars 200 and 200G have
equivalent waterplane areas and displace equivalent volumes of
water, they have substantially equivalent effective depths. The
advantage to the embodiment of the spar 200G is that its a factor
is greater than the u factor for spar 200. The increase in a
factor comes about because the lower surface of the upper cylinder
280, in comparison with the lower surface of the spar 200,
interacts with a portion of pressure field closer to the surface of
the water, hence experiences larger variations in pressure with
passing waves. This leads to larger forces for improved power
conversion efficiency.
In the spar 200H shown in Fig. 15C, the lower cylinder 281
(Fig. 15B) of the spar 200G is replaced with a dense cable or chain
282, the length of which substantially exceeds the distance from
the bottom of the spar 200H to the floor 283 of the body of water.
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The cable 282 can have multiple floats 285 attached along its
length, the purpose of which is to assure that the volume of the
cable 282 plus the volume of the floats 285 equals the volume of
the lower cylinder 281 of the spar 200G.
The advantage to the Fig. 15C embodiment is that the lower end
of the chain or cable can be fixed to an anchor 284 on the floor
283, thus providing a means for mooring the spar. In one
embodiment, a lower length 286 of the cable 282 rests on the floor
283, which cable length varies as the spar heaves with passing
waves. Preferably, the density of the cable lower length 286 is
significantly less than that of the remainder of the cable such
that variations in the hanging length of the cable with vertical
movements of the spar do not substantially change the buoyancy
characteristics of the spar 200H.
Fig. 16 shows a float 100 -- spar 200 system similar to that
shown in Fig. 1 except for the materials used. Thus, both the
float and the spar are made from impervious, stretchable
materials, and the structural shapes shown in Fig. 16 are
obtained by filling structures shown in Fig.17 with water and
pressurized air. The float and spar are closed, hollow members
formed from commercially available materials used, for example,
in inflatable rafts, e.g., PVC coated rubber tubing. In the
empty condition shown in Fig. 17, the spar is folded along
horizontal pleats, accordion style, and pouring water into the
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spar causes it to expand. The desired final weight and buoyancy
of the spar is tuned by the quantity of air pumped into the spar.
A weight is fixedly contained in the spar lower end.
The float shown in Fig. 17 is likewise caused to expand into the
size shown in fig. 16 by adding water and pressurized air.
When deployed, the system functions as does the system shown in
Fig. 1.
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