Note: Descriptions are shown in the official language in which they were submitted.
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'i CA 02347398 2001-02-26
Wave Energy Converters Utilizing Pressure Differences
Background of the Invention:
This invention relates to the conversion of energy from naturally
occurring sources of mechanical energy, and particularly to the conversion
of the mechanical energy present in ocean surface waves to useful energy,
particularly electrical energy.
In many known systems for capturing surface wave energy, a float is
used for being vertically oscillated in response to passing waves. The float
is rigidly coupled to an energy converter which is driven in response to
vertical movements of the float. In one system, described in U.S. patents
4,773,221 and 4,277,690 (the subject matter of which is incorporated
herein by reference), an open-ended hollow tube is rigidly suspended
beneath a float, the tube being completely submerged and in vertical
orientation.
The tube vertically oscillates in the water in correspondence with
movements of the float and, in the absence of anything within the tube, the
tube moves freely relative to the column of water within the open-ended
tube. In one embodiment, a movable piston is disposed within the tube for
blocking relative movements between the water column and the tube. As f
the tube and float oscillate within the water, the mass of water within the
tube tends to block corresponding movements of the piston, hence the
piston moves relative to the tube. Actual movements of the piston do
1
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CA 02347398 2001-02-26
occur, however, and provided the entire system is oscillating at its natural
resonant frequency, relatively large amplitude oscillations of the piston can
occur. The moving piston drives an energy converter fixedly mounted,
e.g., within the float, for converting the piston movements to useful energy.
While these float driven tube systems work, efficient operation
requires that the natural resonant frequency of the system closely matches
the frequency of the ocean waves driving the system. While this can be
generally accomplished at a specific site and specific time, particularly if
means for adjusting the resonant frequency of the system in response to
changing surface wave frequencies are provided, a problem is that, at any
instant, multiple random surtace waves are present whereby much of the
wave energy present can not be efficiently transferred to the oscillating
system. Also, the means for adjusting the resonant frequency of the
device generally involves changing the water mass within the device.
Since this mass is quite large, it is not readily changed.
A feature of the present invention is that a relatively high efficiency of
operation is obtained which is relatively insensitive to random variations of
wave frequencies and amplitudes.
Summary of the Invention:
In a first embodiment of the invention, an open-ended, hollow tube is
disposed in vertical, submerged and fixed location relative to the mean
water level. Specifically, the tube is not in "floating" (moveable)
relationship with the passing waves. The length of the tube and the depth
2
~~d d6~' ~~O IO ST
9
CA 02347398 2001-02-26
of the top end of the tube beneath the mean water level are selected, as
described hereinafter, depending upon the frequency and amplitude of the
most prevalent anticipated surface waves, as well as the water depth.
While maximum efficiency of operation is attained when the anticipated
waves are present, the fall-off of efficiency of operation is relatively small
with variations of wave conditions.
During operation, pressure variations, at the top, open end of the
tube (caused by passing waves) in comparison with a relatively fixed
pressure at the open, bottom end of the tube (unaffected by passing
waves) cause vertical flows of water through the tube which are used for
driving an energy converter, preferably by means of a movable piston
within the tube.
In a second embodiment of the invention, a hollow tube having a
closed top end and a bottom open end is disposed in vertical, submerged
but relatively movable relation with the mean water level. In a preferred
embodiment, the tube is secured for vertical cyclical movements relative to
a float fixedly submerged beneath the water surface and disposed within
the tube. The dimensions of the tube and its at-rest location relative to the
water surface are in accordance with the tube of the first embodiment
except, in the second embodiment, the piston movable within the tube of
the first embodiment comprises the closed top end of the tube of the
second embodiment. During operation, pressure variations against the top
end of the tube cause vertical oscillations of the tube relative to the fixed
float, and such oscillations are used for driving an energy converter fixed
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CA 02347398 2001-02-26
within the float and coupled to the movable tube. In a variation of the
second embodiment, the transducer is not mounted in a float but fixedly
secured to the ocean bottom. The movable tube, which need not be
hollow, is secured to the transducer
In all embodiments, movements of the tube relative to the adjoining
water are caused not by wave-induced displacements of a float on the
water surface, but in response to pressure variations caused by passing
waves.
Description of the Drawings:
The drawings are schematic and not necessarily to scale.
FIGURE 1 is a sketch for identifying various relevant dimensional
parameters of a system according to the present invention deployed in a
body of water;
FIGURES 2, 2A and 3-6 are side sectional views showing different
embodiments of power converting systems in accordance with a first
embodiment of the present invention deployed in bodies of water, e.g., an
ocean;
FIGURE 7 is a side elevational view of an energy converter in
accordance with a second embodiment of the invention;
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CA 02347398 2001-02-26
FIGURE 8 is an end view of the converter looking in the direction of
the arrows 8-8 in Fig. 8;
FIGURE 9 is an isometric view of the converter shown in Figs. 7 and
8; and
FIGURE 10 is a side elevational view of a modified version of the
embodiment shown in Figs. 7-9.
Description of Preferred Embodiments of the Invention:
An apparatus according to a first embodiment of the present
invention is shown in Figure 1. Shown schematically is an open-ended
tube 10 disposed (as herewith described) in fixed, vertical orientation below
the mean water level of a body of water, e.g., an ocean having wind driven
surface waves. Figure 1 also identifies parameters important in the
practice of the invention, i.e., wave height, wave length, water depth, depth
of the top of the tube below the water surface, length of the tube, and the
diameter of the tube. The optimum depth for the lower end of the tube is
dependent on the wavelength (~,) of the longest waves to be utilized in an
efficient manner. The principle of operation is that the changes in water
energy level, which can be expressed as changes in pressure, due to the
passage of wave peaks and troughs, is highest near the surface, and these
pressure changes decay exponentially with depth below the surtace. Thus,
the top of a long tube experiences relatively large pressure variations while
s
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CA 02347398 2001-02-26
the bottom of the tube experiences an almost steady pressure that is equal
to the pressure due to the weight of water above it at the mean water level.
The energy levels at different water depths under a wave field can be
calculated with Equations 1 and 2. The equations are for deep water
waves and are modified somewhat by the depth in more shallow water
(depths less than ~,/2). The water energy levels due to waves of a given
size are a function of wave length and water depth. There is little practical
value in extending the tube bottom any deeper than 1/2 the wavelength of
the longest waves to be optimally used because the energy level is already
greatly reduced from its near surface value.
Ed = ESexp(-2rcdl~,) (1 )
where: ES is the energy due to a wave at the water surface, and
Ed is the energy due to a wave at a depth equal to d, and
~. is the wave length of the waves being considered
The wave length of deep water waves may be calculated by the formula:
~, = gT212~ (2 )
where: g is the gravitational constant, 9.8 meters per second per
second, and
T is the period of the waves in seconds
As an example using equation 2, for waves with a period of 7 seconds, the
wave length is
~,7 = 76.43 meters
As a second example, for waves with a period of 5 seconds, the wave
length is
6
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_~.w~..w..~._~.n..~....~~..~.. .~,.. ~._~ __.__..~.~ _ _.._.......~...~..~~
,._..~. ...~.....~ .- ..,..... ......._..._.. .. ..
CA 02347398 2001-02-26
~,5 = 38.99 meters
For waves with a period of 7 seconds and a wave length of 76.43 meters,
and waves with a period of 5 seconds and a wave length of 38.99 meters,
the energy at different depths can be calculated as percentage of the
energy at the surface, using Equation 1. This is shown in Table 1.
Table 1.
eneray water death (ml depth as a % of ~,~ E ~ as % of Es depth as a % of ~.5
Ed5 as % of Es
0.1 0.1 99.2 0.3 98.4
0.5 0.6 96.0 1.3 92.3
1.0 1.3 92.1 2.6 85.1
19.1 25 20.8 49 4.6
38.21 50 4.3 98 0.2
76.43 100 0.2 196 0
Table 1 shows that when waves with a period of 7 seconds are present,
and the tube 10 has its top end at depth of 0.5 meters below the surface,
and its bottom end at a depth of 38.21 meters below the surface, the top
will experience pressure changes 91.7% (96 - 4.3) larger than the bottom.
These conditions will cause water to flow down the inside of the tube when
a wave peak is over the top end, and water to flow up the inside of the tube
when a wave trough is at the top of the tube. This pressurized water flow
provides the opportunity to extract mechanical power from the wave
energy. Extending the tube from 38.21 meters to 76.43 meters in length
only increases the pressure differential by 4.1 % (4.3 - 0.2).
Further study of the Table 1 shows that when waves with a 5 second
period are present, a tube with its bottom at 38.21 meters below the
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CA 02347398 2001-02-26
surface has an even lower pressure variation at the bottom, 0.2%, than
when 7 second period waves are present. Thus wave energy from shorter
wave length and shorter period waves can be collected efficiently. When
waves of longer period are present, the energy, or pressure variations, at
the tube bottom gradually increase. Thus, the efficiency of energy
collection will gradually decrease. However, the range of efficient
operation is much larger than in the previously described known devices
that are tuned for specific wave periods for resonant and efficient
operation. These devices can suffer a significant loss of efficiency when
the wave period changes even a few seconds.
Table 1 also shows that as the wave period decreases, the
importance of the top of the tube being near the surface increases. For
example, with a water depth of the top of the tube being 0.5 meters under
the surface in 7 second period waves, the energy has decreased at the
tube top to 96% of its maximum, while in 5 second waves the energy has
decreased to 92.3% of its maximum.
Regular waves are waves that have a consistent period. A sine wave
is an example of a regular wave. Regular waves at a constant period
would allow the tuning of a resonant wave energy capture device to the
specific wave period, even though the wave period may change with the
negative impact mentioned above. In practice, ocean and sea waves are
both random and irregular and simultaneously contain waves with different
periods. An example of this is a case when ocean swells with a 10 second
period are present along with wind waves with a 5 second period. The
inventive apparatus has the ability to capture energy efficiently from
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CA 02347398 2001-02-26
irregular waves as well as from regular waves. This is because the
apparatus is not optimized for a specific period but is driven dependent
upon the instantaneous quantity of water above or below the mean water
level (but subject to "cancellation effects" discussed hereinafter).
The theoretical amount of energy that can be captured at a site with
a given water depth and wave characteristics can be determined as
follows. Bernoulli's Equation for fluids in unsteady irrotational flow is:
~.~+P~+gY,+V~2 - ~2+P?+gY2+V2
st p 2 st p
Where (1 ) is a point in the fluid and (2) is another point in the fluid, and
where:
s~lst is the differential of the velocity potential in meters squared per
second squared (m21s2), at a point, and
gy is the gravitational constant times the depth at a point in mZ/s2, and
Plp is the pressure at a point divided by the fluid density in m2/s2, (to
achieve these dimensions it should be remembered that mass can be
expressed as force divided by acceleration), and
V2/2 is the velocity squared of the fluid at that point m2/s2.
For example, if point 1 is considered to be near the water surface as is the
top of the tube, and point 2 is deeper as is the bottom of the tube, the
Bernoulli Equation can be the basis for analysing the different forms of
energy available at each point as time passes.
Principle of Operation - Energy Capture: The simple long tube
described above provides a situation where there are two different
pressure levels appearing simultaneously at each end of the tube. Two
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CA 02347398 2001-02-26
preferred methods of capturing energy from the available energy in the
tube are as follows:
1. A piston 12, shown in Figure 2, placed in the tube is forcefully driven
up and down by water in the tube moving up and down due to the
varying pressure differentials above and below the piston. This
forceful movement is converted to mechanical power by attaching a
device to the piston that resists its movement. One example is the rod
of a hydraulic cylinder. The motion of the cylinder rod pumps a
pressurized fluid (hydraulic fluid) through a hydraulic motor which then
rotates. The mechanical power produced by the hydraulic motor is
converted to electrical power by a generator attached to the motor. In
Figure 2, the water driven piston 12 and its shaft are shown to move
up and down while guided by the Piston Shaft Support and Shaft
Bearings 16. To reduce mechanical drag on the system the piston
preferably does not touch the sides of the tube. A clearance between
the piston rim and tube of 3 to 6 millimeters will permit some water to
leak past the piston. This represents a loss of power but is a small
percentage of the area for a piston that is larger than 1 meter in
diameter. A hydraulic cylinder rod 18 (from a hydraulic cylinder 20) is
attached to the top of the piston shaft 12. A hydraulic cylinder support
22 fixedly attaches the cylinder 20 to the tube 10. Hydraulic hoses 24
carry the hydraulic fluid back and forth to a watertight compartment
that contains an hydraulic motor and electric generator. A double-
ended cylinder (rod extends from both ends) is preferred because the
cylinder performance is the same in both stroke directions. The piston
is made buoyant enough to cause the piston - piston shaft - hydraulic
to
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CA 02347398 2001-02-26
cylinder rod assembly to be neutrally buoyant, and therefore move up
or down equally with the same applied forces. A preferred
arrangement of the components is shown in Figure 2A. In this
arrangement, the piston 12 slides up and down on the hydraulic
cylinder itself. Both ends of the hydraulic cylinder rod 18 are fixedly
attached to the tube 10 by the hydraulic cylinder rod supports 22. A
watertight compartment 26 is part of the piston assembly and contains
the hydraulic motor and electric generator. This compartment is
buoyant enough to cause the entire piston assembly to be neutrally
buoyant.
Figure 2 also shows an arrangement for mooring the power
converting system. This is later described.
Figure 3 shows an arrangement where the area of the piston 12 is
larger than the area of the tubel 0 at its top and bottom ends. This is to
illustrate that the piston area can be either larger or smaller than the
tube end areas. In a given situation, the arrangement in Figure 3 will
produce a higher force and a shorter stroke than if the piston and tube
ends have the same area. This is because the tube length and depth
determines the pressure differential on the piston, and the tube end
areas determine the volume of water flow. Thus, the same pressure on
a larger piston area produces more force, but more water volume is
required to move the larger piston. Figure 3 illustrates that the piston
size can be varied to match desired piston forces and strokes.
However there are losses of energy incurred whenever the moving
11
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CA 02347398 2001-02-26
water is caused to change direction as it does when the piston area is
different than the tube end areas. Thus, the most energy efficient
configuration is when the piston and tube ends have the same area.
A second power take-off approach (not illustrated) is to attach a rod
to the piston that moves vertically with the piston. Instead of this piston
rod being attached to a hydraulic cylinder, it is attached to a positive
drive belt (the belt and sprockets having teeth that are positively
engaged), that is around two vertically arranged sprockets. As the
piston is driven up and down by the wave energy it drives one side of
the belt up and down causing the sprockets to rotate. One of the
shafts of a driven sprocket is coupled to a generator to produce electric
power.
A third power take-off approach (not illustrated) is to directly drive a
linear generating device, such as a linear electric motor, with the piston
movement. Due to the sub-surface marine environment, the hydraulic
approach is preferred.
2. (n the system shown in Figure 4, a turbine 40 is disposed within a tube
for being driven to rotate by the water flow in the tube moving up
and down due to the varying pressure differential at the top and bottom
of the tube. This rotation produces mechanical power by, for example
coupling the shaft of an electric generator 42 to the turbine shaft 44.
The tube preferably has, as shown, large diameter ends, and a small
diameter turbine section in order to increase the water flow velocity
through the turbine.
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CA 02347398 2001-02-26
The moving piston approach (1 ) is preferred because in general, the
inventive systems are more readily designed for providing powerful strokes
of limited length rather than providing rapid water flow. Each approach is
described in more detail below:
1. Moving piston approach: As a piston such as shown as 12 in Figs. 2,
2A, and 3 moves against resistance it produces a force (Newtons). The
piston moves a certain distance in a given time (meters per second). The
product of this force times velocity is Newton-meters per second (Nm/s)
which converts directly to watts of power. One Nm/s is equal to one watt.
POWerWatts = FOrCeNewtons x VelOCltymete~s/second (3)
A longer stroke in a given time at a lower force can produce the same
amount of power as a shorter stroke in the same time at a higher force, or
vice versa. (n practical applications of the piston approach, there is a limit
on the length of piston stroke allowed. This is because practical devices
such as a hydraulic cylinder have a certain amount of stroke, and
exceeding that physical limit damages the cylinder. Also, in a given
location, the waves are normally in a known range of sizes during the year.
Thus, it would be economically impractical to provide equipment that could
stroke farther than would be caused by the normally present waves.
Prevention of damage by larger than normal waves, such as storm waves
(not illustrated), is by pressure relief doors in the tube 10 above and below
the pistons 12 range of motion. If a wave produces a pressure differential
(and resulting piston force) across the piston that is more than a
13
G I 'id dEE ~ ~O I O S T
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..,w.v,..M .T...,.,Wr.,. ~......m. _......._
CA 02347398 2001-02-26
preselected valve, the doors are pushed open. This allows water to
bypass the piston, reducing its force and preventing damage to the device.
A piston system will normally have provision for a certain physical
stroke range such as 1 meter, but could be longer or shorter. However,
the force can be increased or decreased by simply making the unit and its
piston larger or smaller. This is an important factor in the design of a
practical system, and is based on the fact that fluid pressure does not
depend on the size of the area it is acting upon. For example, assume that
waves are expected to be present that provide an average pressure
differential of 2,000 Pascals (Pa) between the top and the bottom of the
tube, as described above. A Pascal is a pressure of one Newton per
square meter. Also, assume that the waves have a period of 5 seconds,
that is, a wave will move from a peak to a trough in 2.5 seconds. If the
piston stroke is limited to 1 meter, and it is moving its full stroke, it will
have
an average velocity of 1/2.5 = 0.4 m/s. If 1000 watts, or 1000 Nm/s, is
desired from the system, then the average force must be {1000
Nmls)l(0.4m/s) = 2,500 N. Since the pressure differential is 2,000 Pa or
2,000 N/m2, the piston area must be (2,500 N)/( 2,000 N/m2) = 1.25 m2.
This corresponds to a piston diameter of 1.26 meters (D).
The mass of water in the tube and piston area moves along with the
piston. It must be accelerated in one direction, decelerated to a stop,
accelerated in the other direction, decelerated to a stop, and so on.
Therefore, some of the force produced by the pressure differential on the
piston must be used to accelerate the water mass. This can be calculated
as force equals mass times acceleration, or FWa~er = mwatera. Some of this
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CA 02347398 2001-02-26
force is recoverable from the decelerating water. However, having to
accelerate and decelerate a large water mass causes the optimum tube 10
length to be shorter than'/Z the waves length. This is because a longer
tube captures a higher pressure differential than a shorter tube, but also
contains more water. The optimum tube 10 length can be calculated for a
specific wave length, wave height, and water depth using Bernoulli's
Equation as previously discussed.
By way of concise summary, characteristics of the moving piston
approach are:
1. A tube long enough to create a significant varying pressure differential
between its top and bottom ends when placed in waves with a range of
wavelengths.
2. A piston within the tube that causes the varying pressure from the top
of the tube to occur at the top surface of the piston, and the relatively
constant pressure from the bottom of the tube to occur on the bottom
surface of the piston.
3. Because of 1. and 2. the piston is driven up and down with force and
velocity.
4. A means, such as a hydraulic cylinder and motor, to converk the
reciprocating mechanical power of 3. into rotary mechanical power.
5. An electric generator to convert the rotary mechanical power of 4. into
electrical power.
6T 'I~d d~E ~tr0 ZO ST
CA 02347398 2001-02-26
6. The piston diameter can be larger or smaller than the tube diameter,
producing either a relatively high force low velocity motion or a
relatively low force high velocity motion.
7. The sizes of the system tube and piston components affect the amount
of water mass enclosed within the system which affects the amount of
acceleration of the piston and water that can be achieved from a given
wave environment.
Additional requirements, discussed further hereinafter, are:
8. The system can utilize a fixed mooring to the sea bottom, or a mooring
that provides a floating unit to balance the piston forces with a properly
sized buoyant section.
9. The system can utilize a mooring that combines a fixed buoyant
mooring and additional buoyancy to compensate for tidal variations by
moving the tube top up and down with the tide.
Various mooring arrangements are described hereinafter.
2. Description of turbine approach: The principles of operation of units
that use a water turbine instead of a moving piston for power extraction are
very similar. The key difference is the need for a high water flow velocity.
As shown in Equation 3, force and velocity make equal contributions to
power output.
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CA 02347398 2001-02-26
PowerWatts = FOrCeNewtons X Velocltymeterslsecond
The moving piston approach must limit the stroke and hence the velocity
for practical reasons. Thus, the force is emphasized by providing a large
piston area. When using a turbine to extract power from flowing fluids a
high velocity is desirable to overcome initial static friction to insure that
the
turbine starts rotating, and to provide efficient operation. The power
available for capture from a cross sectional area of fluid flow is given by:
Powerwacts = 0.5 x A x p x V3 (4)
Where A is the cross sectional area of flow in m2, p is the density of the
fluid (1000 kg/m3 for water), and V is the velocity in meters per second.
A high average velocity is desirable to optimize power output. The
maximum thrust, or force, on a turbine in fluid flow is given by:
ThrUStNewcons = (3~8) x A x p x V2 (5)
Once the velocity profile has been determined for a certain tube
configuration and wave profile, the expected power output can be
calculated from Equation 4. Also, the necessary buoyancy volume to
balance the thrust force and maintain stationary position for the tube can
be calculated from Equation 5.
Mooring of the above-described embodiments is now described.
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CA 02347398 2001-02-26
In the example shown in Figure 2, a mooring attachment to the sea
bottom is shown. The mooring attachment acts as a mechanical datum to
resist the upward and downward forces of the piston and keeps the tube
fixed in place. The mooring attachment must be strong enough to
withstand the downward forces produced by the unit, and heavy enough to
resist the upward forces produced by the unit. It must be strongly attached
to the ocean bottom to resist the forces produced by storm waves.
In other sites the water may be too deep for practical bottom
mounting. Figure 5 shows an arrangement for mooring the inventive
systems in virtually any depth of water because the length of its mooring
chain 17 is variable. The tube 10 is held in vertical position by buoyancy
tanks 50 attached to its outer perimeter. These buoyancy tanks are
sufficiently buoyant to float the unit to the surface were it not held by its
mooring chain or cable. The tanks are buoyant enough to support the
weight of the unit plus at least the maximum downward force exerted by
the piston against the tube. This prevents the tube from moving lower
during normal operation of the power producing tube. The mooring chain
must be at least strong enough to resist the net upward force of the
buoyancy tanks, plus the maximum upward force produced by the piston
against the tube. The anchor also must weigh at least as much as the net
upward force of the buoyancy tanks plus the maximum upward force
produced by the piston against the tube to prevent lifting of the anchor.
The fixed depth mooring arrangements shown in Figures 2 and 5 will
allow tidal changes in water depth to affect power capturing performance.
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CA 02347398 2001-02-26
In normal tides, for example 1 meter, the effect is small. A preferred
mooring plan is to moor the unit at its planned depth below the surface at
the midpoint of the tidal change. Then some times it will be deeper below
the surtace (high tide), and some times it will be closer to the surface (low
tide) than planned. Table 1 indicates that the energy level at the fop of a
tube that is 1 meter below the mean surface of waves with a 7 second
period is 4% less than if it were 0.5 meters below the mean surface. Thus
a unit that was moored 1 meter below the surface at mid-tide in a 1 meter
tidal environment would range from plus or minus 0.5 meters from the
planned depth during a day. A unit moored so shallow that wave troughs
expose the top of the unit suffers little or no loss in power output.
Therefore, such a unit fixedly moored as discussed above will produce
approximately at its average planned level in a normal tidal environment.
In areas with high tides, the unit is preferably mounted lower in the water to
prevent excessive exposure during wave troughs. This will reduce the
average power that the unit can capture as can be estimated from Table 7.
To meet a certain power goal, a slightly larger unit is required than if the
site had smaller tidal changes. The simplicity of a fixed mooring
arrangement generally outweighs the power loss in sites with range of
depths and tides that are not extreme.
A second mooring approach, shown in Figure 6, combines a fixed
bottom mounting and a floating tube top. The fixed bottom mounting
provides the simplicity discussed above, and the float provides tidal
compensation. In this case, the top portion of the tube 15 is flexible and
can be extended upward by the buoyancy of a small float 62 when the tide
19
dSE~~0 TO ST
CA 02347398 2001-02-26
is high and raises the mean water level. When the tide is low, the float 62
follows the water level downward compressing and shortening the flexible
top tube section 15. The float maintains the top of the tube at a relatively
fixed depth below the water surface, e.g., 1 meter. The apparent change
in the water height above the tube is approximately the same whether the
water is rising and falling above a fixed open tube top, or is rising and
falling above the tube extension. In this arrangement, the large forces
produced by the piston working in its pressure driven mode are countered
by the fixed mooring buoyancy tanks 50, while the added buoyancy tanks
62 only raise and lower the top of the tube.
A second embodiment of the invention, illustrated in Figures 7
through 9 is now described.
Figures 7-9 show a hollow tube 110 having a closed top end 112 and
an open bottom end 114. As previously described in the first embodiment,
the tube 110 is in vertical, submerged orientation but, unlike the tube 10 in
the first embodiment, which is preferably fixed in place, the tube 110 of the
second embodiment is vertically movable relative to a fixed support. Such
support can be a rigid structure mounted on the water bed, but, especially
in deep water, is preferably a float 116 fixedly moored to the water bed 118
by an anchor 120 and a chain or cable 128.
Most conveniently, the tube 110 encloses the float 116 and, because
the tube is vertically elongated, the float 116 is similarly elongated.
'Id dSE ~ ~0 T 0 S Z qa~
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CA 02347398 2001-02-26
The float 116 has a large buoyancy, and corresponds to a fixed
structure rigidly mounted on the water bed but with the exception that some
horizontal displacement of the float can occur in response to horizontal
water movements. Such horizontal displacements of the float will generally
occur at a slow rate and, essentially, the function of the float is to provide
a
definite position of the tube relative to the water bed. In situations where
large changes in the water level occur, means, generally known, are used
for adjusting the distance between the float and the water bed for
maintaining a fixed distance between the float and the water surface.
However, and as explained in the description of the first embodiment,
power generation is relatively insensitive to moderate water level changes
and, typically, the float is positioned for optimum performance at the
average water level and not thereafter changed in position with water level
changes.
The tube 110 is secured to the float 116 by means of a hydraulic
pump 122 of known type comprising a rigid casing 124 with a piston rod
126 (for pumping fluids within the pump) extending entirely through and
outwardly from both ends of the casing 124. Herein, the pump casing 124
i is rigidly secured to the movable tube 110 by a spoke-like bracket 121 (so
as not to impede water movement within the to
be 110). The upper end of
the pump casing 124 is rigidly secured to the closed top end of the tube
110 but with one end 126b of the piston rod 126 extendin thr
g ough the
tube end. (Optionally, a navigation aide 127 is attached to the rod end
i
126b and extends above the surface of the water.) The other end 126a of
the piston rod 126 is rigidly secured to the float 116. The tube is neutrally
i
21
g~-td d9E~b0 TO ST
CA 02347398 2001-02-26
buoyant and includes a hollow buoyancy chamber 125. Being neutrally
buoyant, the tube 110 vertically oscillates in response to tube top-to-bottom
pressure variations caused, as previously described, by passing waves.
Vertical oscillations of the tube 110 relative to the fixed float 116 thus
cause relative movements between the pump casing 124 and the pump
piston rod 126, the result being the generation of alternating hydraulic
pressures within the pump which can be used for pressure circulating a
fluid through hoses 111 a for driving a hydraulic motor-electrical generator
111.
Factors influencing the design of the overall system are similar to
those described in the description of the first embodiment. Therein, a
piston within a stationary tube moves in response to passing waves.
Herein, the closed (upper) end 112 of the tube 110 functions as a piston
movable relative to a fixed support.
During operation, the tube upper end 112 remains submerged for all
passing waves within a range of wave sizes with which the system is
designed to operate. For avoiding excessive forces due to extra large
waves, pressure relief valves are used, e.g., in the form of spring biased
doors 130 shown in Fig. 10 at the top end 112 of the tube 110. Herein,
four doors 130 are shown. If the pressure differential between the water
above the tube and the water inside the tube exceeds a preselected level,
two of the doors open downwardly to equalize the pressure within and
outside the tube 110. The other two doors 130 open upwardly to relieve
internal excess pressures due to excessively deep wave troughs passing
22
9yld d9E~b0 TO ST qa~
CA 02347398 2001-02-26
over (or beneath) the tube upper end 112. The spring bias for the doors
130 can be obtained from weights or buoyant compartments on the doors.
An advantage of the cable anchored arrangement shown in the
figures is that the unit is free to move horizontally due to wave action. This
reduces the horizontal forces imposed on the mooring and reduces the
mass of the required mooring. Large horizontal movements tend to lower
the tube upper end 112 relative to the water surface. This lowering tends
to reduce the output power from the unit otherwise obtainable when the
tube upper end 112 is optimally spaced beneath the water surface
(previously described). However, as previously noted, changes in power
production with increased spacings of the tube end from the water surface
are rather gradual, and useful power production continues even with large
horizontal leanings of the unit.
Relative horizontal movements between the float 116 and the tube
110 are preferably avoided for avoiding damage of the mechanical
coupling therebetween. For such movements to occur, in response to
lateral movements of the tube 110, water must move within the tube 110
from side to side of the float 116. Such water movements, and attendant
relative lateral movements of the float 116 relative to the tube 110, are
essentially prevented by the use of vertically elongated, radially extending
fins 117 shown in Figs. 8 and 9.
The above-described arrangement of a float 116 within a tube 110
provides a self-contained unit which can be readily assembled on-shore
23
G~ 'id dLE ~ b0 T O S T
CA 02347398 2001-02-26
and transported for simple placement at an ocean site. )n such
arrangement, the float 116 serves as a fixed support on which a transducer
is fixedly mounted; the transducer, in turn, being connected to and driven
by the movable tube 110.
In an alternate arrangement, shown in Fig. 10, a transducer 222
(e.g., a hydraulic tube or the like) is fixedly mounted on the floor of the
ocean (preferably by a mechanical coupling, e.g., a ball-socket joint 232,
allowing pivoting of the transducer 222), with the movable piston rod 226 of
the transducer rigidly connected to the bottom end of a neutrally buoyant
tube 210 identical to the tube 110 shown in Figs. 7-9 but not including an
internal float. The tube 210 is connected to the piston rod 226 (again,
preferably by a pivoting coupling) by an anchoring link 228 which can be an
anchor chain or, preferably, a solid rod having a high modulus of elasticity,
i.e., low straining with applied stress.
In the very first patent (USP 4,404,490) issued to the owner of the
present invention, reference is made to "cancellation effects", i.e., the
energy robbing effect when the dimensions of the wave energy collector
are a significant fraction of the wave length of the surface waves (the
subject matter of such patent being incorporated herein by reference).
Herein, for example, top to bottom pressure variations across the lengths
of the various herein disclosed tubes occur in response to passing waves.
If, for example, the diameter of the tubes were equal to the wave lengths of
the passing waves, the pressure increases caused by the wave crests
overlying the tube top ends would be cancelled by the simultaneous
24
s~ -~d dGE ~ b0 T O S i
CA 02347398 2001-02-26
presence of the overpassing wave troughs. Thus, no vertical oscillations of
the tubes would occur. Ocean waves, however, tend to be quite large and,
for practical reasons, the diameter of the tubes are so small in comparison
with the wave lengths that cancellation effects can be ignored - provided
that the tube diameters are not in excess of a relatively small proportion of
the wave length, e.g., 10%.
Any such cancellation effect occurs in directions parallel to the
directions of movements of the waves. No cancellations occur in directions
normal to the wave directions, hence quite large area tubes can be used of
rectangular cross-section provided the axis of greater length (in excess of
10% of the wave length) is maintained perpendicular to the wave direction.
With, in the embodiment shown in Fig. 10, the transducer 222
disposed below and outside the tube 210, a hollow space within the tube
210 for containing the transducer 222 and a float 116 (as in Fig. 7) is not
required, and the tube 210 need not be hollow and need not have an open
bottom end. The only requirements for the tube 220, in accordance with
the present invention, are that it is similar to the tube 110 in that it has
the
same outside dimensions (for use with the same wave environment) and
has a closed top end serving as a piston responding to surface wave
pressure variations. The tube 220, similarly as the tube 110, must be
neutrally buoyant, for vertical oscillations in responses to top to bottom
pressure variations caused by overpassing waves, but the tube can be
hollow or solid to any extent as may be desired.
g~ ~Id dLE ~ ir0 Z 0 S T Gad
