U.S. patent application number 13/925743 was filed with the patent office on 2014-01-30 for novel electrical generators for use in unmoored buoys and the like platforms with low-frequency and time-varying oscillatory motions.
This patent application is currently assigned to OMNITEK PARTNERS LLC. The applicant listed for this patent is Richard T. Murray, Jahangir S. Rastegar. Invention is credited to Richard T. Murray, Jahangir S. Rastegar.
Application Number | 20140028027 13/925743 |
Document ID | / |
Family ID | 40252505 |
Filed Date | 2014-01-30 |
United States Patent
Application |
20140028027 |
Kind Code |
A1 |
Rastegar; Jahangir S. ; et
al. |
January 30, 2014 |
Novel Electrical Generators For Use In Unmoored Buoys And the Like
Platforms With Low-Frequency and Time-Varying Oscillatory
Motions
Abstract
A buoy for generating electrical energy from an roll/pitch
motion resulting from a passing wave. The buoy including: a body; a
pendulum member rotatably disposed on the body at a first end of
the pendulum member; a pendulum mass disposed at a second end of
the pendulum member; an exciter mass positioned on the pendulum
member between the first and second ends; and one or more
generators for generating electrical energy, the one or more
generators being positioned in the body such that swinging of the
pendulum member due to the roll/pitch motion causes a portion of
the exciter mass to engage the one or more generators to generate
the electrical energy.
Inventors: |
Rastegar; Jahangir S.;
(Stony Brook, NY) ; Murray; Richard T.;
(Patchogue, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rastegar; Jahangir S.
Murray; Richard T. |
Stony Brook
Patchogue |
NY
NY |
US
US |
|
|
Assignee: |
OMNITEK PARTNERS LLC
Ronkonkoma
NY
|
Family ID: |
40252505 |
Appl. No.: |
13/925743 |
Filed: |
June 24, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13418288 |
Mar 12, 2012 |
|
|
|
13925743 |
|
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|
12217655 |
Jul 8, 2008 |
8134281 |
|
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13418288 |
|
|
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|
60958946 |
Jul 10, 2007 |
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Current U.S.
Class: |
290/53 ;
310/339 |
Current CPC
Class: |
Y02E 10/38 20130101;
H02N 2/183 20130101; F03B 13/20 20130101; F05B 2250/40 20130101;
H02N 2/18 20130101; Y02E 10/30 20130101; H01L 41/1136 20130101;
F05B 2220/709 20130101 |
Class at
Publication: |
290/53 ;
310/339 |
International
Class: |
F03B 13/20 20060101
F03B013/20; H02N 2/18 20060101 H02N002/18 |
Claims
1. A buoy for generating electrical energy from an roll/pitch
motion resulting from a passing wave, the buoy comprising: a body;
a pendulum member rotatably disposed on the body at a first end of
the pendulum member; a pendulum mass disposed at a second end of
the pendulum member; an exciter mass positioned on the pendulum
member between the first and second ends; and one or more
generators for generating electrical energy, the one or more
generators being positioned in the body such that swinging of the
pendulum member due to the roll/pitch motion causes a portion of
the exciter mass to engage the one or more generators to generate
the electrical energy.
2. The buoy of claim 1, wherein the exciter mass has tips for
engaging the one or more generators when the pendulum member swings
in first and second directions.
3. The buoy of claim 1, wherein the pendulum mass is releasably
retained against a surface of the body and released by the
roll/pitch motion.
4. The buoy of claim 1, wherein the exciter mass is separate from
the pendulum mass.
5. The buoy of claim 1, wherein the one or more generators are
piezoelectric generators.
6. The buoy of claim 1, wherein the body has a cross-section such
that it is stabile in a rotation direction.
7. The buoy of claim 1, wherein the body has one or more fins so as
to be stabile in a rotation direction.
8. A method for generating electrical energy from an roll/pitch
motion resulting from a passing wave, the method comprising:
rotatably disposing a pendulum member on a body of a buoy at a
first end of the pendulum member; disposing a pendulum mass at a
second end of the pendulum member; positioning an exciter mass on
the pendulum member between the first and second ends; and
generating electrical energy resulting from a swinging of the
pendulum member due to the roll/pitch motion by causing a portion
of the exciter mass to engage at least one generator to generate
the electrical energy.
9. The method of claim 8, further comprising offsetting the
pendulum mass such that the body of the buoy is heeled to one side
when floating in water.
10. The method of claim 9, wherein the wherein the offsetting
comprises releasably retaining the pendulum mass against a surface
of the body.
11. The method of claim 10, wherein the pendulum mass is released
from the surface of the body by the roll/pitch motion to cause the
exciter mass to engage the at least one generator to generate the
electrical energy.
12. The method of claim 11, wherein, after the release of the
pendulum mass from the surface, the pendulum mass is releasably
retained on another surface of the body.
13. The method of claim 12, wherein the pendulum mass is released
from the another surface of the body by the roll/pitch motion to
cause the exciter mass to again engage the at least one generator
to generate the electrical energy.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation application of U.S.
application Ser. No. 13/418,288 filed on Mar. 12, 2012, which is a
Continuation application of U.S. application Ser. No. 12/217,655
filed on Jul. 8, 2008, which claims benefit to U.S. Provisional
Application 60/958,946 filed Jul. 10, 2007, the entire contents of
each of which is incorporated herein by reference. This application
is also related to U.S. application Ser. No. 12/142,739 filed Jun.
19, 2008, the entire contents of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to devices for
harvesting energy from waves and generating electrical energy and,
more particularly, to devices for harvesting energy from waves and
generating electrical energy for use in unmoored buoys and other
similar platforms.
[0004] 2. Prior Art
[0005] In recent years, particularly following the development of
low-power electronics, sensors and wireless communications devices,
electrical energy generators that harvest energy from the
environment have seen renewed attention. The most common means of
generating electrical energy is the use of magnets and coils using
rotary type of generators. Generators that harvest energy from
vibration that use either coils and magnets or active materials
such as piezoelectric materials based devices have also been
developed, particularly for low power consuming electronics. In the
latter area, piezoelectric materials have been used widely to
generate electrical energy from the ambient vibration.
[0006] To efficiently generate electrical energy from mechanical
energy using active materials such as piezoelectric elements or by
using various types of magnets and coils arrangements, the
frequency of the input motion must be relatively high and not time
varying. This means that if the input motion is rotary, then the
rotational velocity must be relatively constant and high,
preferably in the order of several thousands. On the other hand, if
the motion is oscillatory, such as vibratory or rocking or the
like, then the frequency of vibration or rocking must be high,
preferably in the order of a few thousands when using magnet and
coil type of mechanical to electrical energy conversion devices and
even higher frequencies if, for example, piezoelectric based
mechanical to electrical energy conversion devices are
employed.
[0007] However, in many applications, for example in platforms that
rock through relatively small angles such as buoys, ships, trains
or vehicles; the rocking or oscillating frequency is very low and
even in the order of 0.1-0.5 Hz and time varying, thereby making
the operation of all currently available energy harvesting devices,
i.e., mechanical energy to electrical energy conversion devices,
extremely inefficient. Such low frequency and time varying motions
are also encountered by floating platforms in the oceans and seas
if heaving of waves is to be used to generate electrical
energy.
[0008] Similarly, in rotary machinery such as windmills or turbines
used to harvest tidal or ocean waves or other similar flows, the
input rotary speed is relatively low and varies significantly over
time, thereby making the operation of all currently available
electrical energy generators highly inefficient. In fact in most
such turbo-machinery such as windmills, to make the generation
cycle efficient, gearing or other similar mechanisms have to be
used to increase the output speed and in many applications to also
regulate the output speed. Input speed increasing gearing and speed
control mechanisms are, however, costly and significantly increase
the system complexity and cost, particularly those related to
maintenance and service.
SUMMARY OF THE INVENTION
[0009] A need therefore exists for apparatus and methods that can
be used to develop electrical energy generators that could
efficiently generate electrical energy from slow and time varying
rocking (oscillatory) platforms such as buoys and other floating
platforms. It is noted that to achieve high mechanical energy to
electrical energy conversion efficiency, the above method is highly
desirable to lead to generators that operate at high and relatively
constant input motion frequencies.
[0010] In particular, there is a need for energy harvesting
generators that could efficiently generate electrical energy from
the motion of floating platforms such as buoys, particularly
unmoored buoys, that undergo heaving and rocking motions through
relatively small angles, in which the rocking frequency could vary
significantly over time and even from one cycle of motion to the
next, with frequencies that could be as low as 0.1-0.5 Hz or even
lower.
[0011] In particular, there is a need for energy harvesting
generators that could efficiently generate electrical energy from
the motion of platforms that rock through relatively small angles,
such as buoys, ships, trains or trucks, and heaving of unmoored
platforms such as buoys, boats and ships, in which the rocking and
heaving frequency could vary significantly over time and from one
cycle of motion (oscillation) to the next, with frequencies that
could even be in the order of 0.1-0.5 Hz or even lower. It is noted
that to achieve high mechanical energy to electrical energy
conversion efficiency, the energy harvesting generators must
operate at high and relatively constant input frequencies of the
orders of tens to hundreds Hz or even higher.
[0012] Accordingly, a apparatus and methods are provided that could
be used to develop electrical energy generators for harvesting
electrical energy, i.e., convert mechanical energy to electrical
energy, from slow and time varying rocking (oscillatory) and
heaving motions of buoys, ships and the like platforms. With this
method, the generator device does not require devices such as speed
increasing devices and/or speed regulating devices. In addition,
the disclosed method provides the means to develop highly efficient
mechanical energy to electrical energy conversion devices since the
resulting mechanical to electrical energy conversion devices would
generally operate at appropriately high and relatively constant
input motion frequencies.
[0013] In addition, a new class of highly efficient piezoelectric
based energy harvesting electrical energy generators is disclosed
for mounting on platforms that oscillate (undergo rocking or linear
or rotary vibration) at relatively low to moderate frequencies
based on the aforementioned method. The maximum amount of available
mechanical energy during each cycle of platform oscillation
(rocking motion) can be shown to be proportional to the inertia of
the oscillating element; the frequency and amplitude of platform
vibration; and the size of the generator.
[0014] Such generators can be based on piezoelectric elements to
convert mechanical energy to electrical energy. However, it is
appreciated by those familiar with the art that other active
materials or appropriate coil and magnet type of mechanical to
electrical energy conversion devices may be used instead or in
combination with piezoelectric elements.
[0015] The present apparatus and methods are based on two-stage
operating mechanisms. The input (rocking, heaving, or the like) of
the platform (e.g., the buoy) motion drives the first stage
mechanisms. The first stage mechanisms in return intermittently
transfer mechanical energy (excite) a second stage vibrating system
that is tuned to vibrate at a fixed prescribed frequency. Then
following each excitation of the second stage vibrating system, the
mechanical energy transferred to the (second stage) vibrating
system is transformed into electrical energy preferably using
piezoelectric elements (particularly for relatively small
platforms), even though coil and magnet type of electrical energy
generating devices could also be used.
[0016] The electrical energy generators developed based on the
present apparatus and methods are very simple, can efficiently
operate over a very large range of input oscillatory frequencies or
rotary speeds, and require minimal service and maintenance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] These and other features, aspects, and advantages of the
apparatus of the present invention will become better understood
with regard to the following description, appended claims, and
accompanying drawings where:
[0018] FIG. 1 illustrates a schematic view of a mass-spring system
mounted on a vibrating platform that is vibrating in the vertical
direction.
[0019] FIG. 2 illustrates a schematic view of a pendulum system
mounted on a rotating platform.
[0020] FIG. 3 illustrates the heaving and pitch and roll motions of
a buoy in waves and a coordinate system used to indicate these and
other possible motions of a buoy.
[0021] FIG. 4 illustrates the plot of harvested power as function
of forcing period for wave tank experiment.
[0022] FIG. 5 is the plot of Pierson-Moskowitz ocean wave frequency
spectral density for different wind speeds.
[0023] FIG. 6 illustrates the schematic of a two-stage energy
harvesting device for harvesting energy from the slow and variable
amplitude heaving motion of a buoy.
[0024] FIG. 7A illustrates the overall view of the two-stage energy
harvesting device for harvesting energy from the slow and variable
amplitude heaving motion of a buoy shown in FIG. 6.
[0025] FIG. 7B illustrates a detailed portion of FIG. 7A labeled as
7B-7B in FIG. 7A.
[0026] FIG. 8 illustrates the schematic of a two-stage energy
harvesting device for harvesting energy from the slow and variable
amplitude roll/pitching motion of a buoy.
[0027] FIG. 9 illustrates the schematic of one embodiment of the
two-stage energy harvesting device for harvesting energy from the
slow and variable amplitude roll/pitching motion of a buoy.
[0028] FIG. 10 shows a close-up view of the embodiment of FIG. 9
around the second stage vibratory element of the energy harvesting
system.
[0029] FIG. 11 illustrates the schematic of a second embodiment of
the two-stage energy harvesting device for harvesting energy from
the slow and variable amplitude roll/pitching motion of a buoy.
[0030] FIG. 12 shows a close-up view of the embodiment of FIG. 11
around the second stage vibratory element of the energy harvesting
system.
[0031] FIG. 13 illustrates the schematic of a third embodiment of
the two-stage energy harvesting device for harvesting energy from
the slow and variable amplitude roll/pitching motion of a buoy.
[0032] FIG. 14 shows a close-up view of the embodiment of FIG. 3
around the second stage vibratory element of the energy harvesting
system.
[0033] FIG. 15 illustrates the schematic of the four-bar linkage
mechanism used in the embodiment of the present invention shown in
FIGS. 13 and 14.
[0034] FIG. 16 shows the velocity field of ocean waves showing its
depth-dependency.
[0035] FIGS. 17A and 17B illustrate the schematic of two possible
buoy cross-sections for maintaining their vertical orientation in
waves.
[0036] FIG. 18 illustrates the schematic of another method of
maintaining the vertical orientation of buoys in waves.
[0037] FIGS. 19a and 19b illustrate a schematic of the first method
of forcing the vertical axis of the spar buoy to stay nearly normal
to the surface of the wave.
[0038] FIG. 20 illustrates an approximate ocean wave surface
showing the limit of included angle for ocean surface waves being
around 120 degrees.
DETAILED DESCRIPTION OF THE METHOD AND PREFERRED EMBODIMENT
[0039] For a vibrating platform, the first question that has to be
answered is the amount of mechanical energy that is made available
by the platform for harvesting. The answer to this question has to
be known if one is to know if a method of harvesting energy, in the
present case harvesting mechanical energy and converting it to
electrical energy, is an efficient method. To this end, consider
the mass-spring system 10 shown in FIG. 1. The system 10 consists
of a mass element m and a spring element k, indicated by numerals
11 and 12, respectively. The mass-spring system 10 is mounted on a
vibrating platform 13 that is vibrating in the vertical direction
indicated by the vector Y(t) and enumerated as 14. Let the platform
motion Y(t) be a simple harmonic motion with a frequency .omega.
and amplitude A. Now if the spring is kept from deflection, i.e.,
if the mass m is kept a fixed distance from the vibrating platform
13, then during each cycle of platform motion, the mass m is raised
and then lowered a total maximum distance of 2A, i.e., its
potential energy is varied by a maximum amount of 2Amg relative to
the (fixed) ground, where g is the gravitational acceleration.
Here, it is assumed that the inertia of the vibrating platform is
significantly larger than that of the mass-spring system. Thus,
2Amg is the maximum amount of energy that a vibrating platform 13
can transfer to the vibrating mass-spring system 10, assuming that
there are no losses. This is therefore the maximum amount of energy
that becomes available during each cycle of platform vibration for
harvesting and transferring into electrical energy.
[0040] In addition, if the frequency of vibration of the platform
.omega. is indicated in cycles/sec (Hz), the maximum amount of
power that could possibly be harvested becomes (2Amg.omega.). If
the amplitude A is in meters, the mass m is in kg, g is in
meter/second square, and .omega. in Hz, the above power will have
the units of Watts.
[0041] As expected, the maximum amount of mechanical energy that is
available for harvesting is proportional to the amplitude of the
platform vibration and the inertia of the energy harvesting power
source. Thus, given a vibrating platform, the only parameter that
can be varied to increase the available energy for harvesting is
the inertia of the mass-spring system of the energy harvesting
power source, while using a properly designed spring element 12.
However, by increasing the inertia (mass) of the energy harvesting
power source, its size is also generally increased. It can
therefore be concluded that to minimize the size of energy
harvesting power sources for a specified power requirement, the
vibrating mass has to be constructed with high-density materials,
and attempt has to be made to mount most of the components of the
power source system onto the vibrating mass to maximize the inertia
of the mass-spring system without increasing the overall size of
the power source.
[0042] If the oscillatory motion of the platform is rotational,
such as the rocking motion of a buoy or a ship, then the simplest
method of generating potential energy for harvesting is the use of
a pendulum 20, FIG. 2, or some other pendulum-like mechanism. In
FIG. 2, a pendulum 20 of length r and carrying a mass m, indicated
by numerals 21 and 22, respectively, is shown. The pendulum 20 is
attached to the rocking platform 23 at the pin joint 24. If the
amplitude of the platform rotational oscillations is a, a properly
designed pendulum would undergo oscillations of the same amplitude.
During each cycle of its oscillations, the pendulum mass m is
raised twice a distance h (FIG. 2) above its vertical positioning
indicated by dashed lines, thereby giving it a relative potential
energy mgh. The distance h=r(1-cos .alpha.) is proportional to the
length r. Therefore, to increase the amount of mechanical energy
available for harvesting, the energy harvesting generators have to
be constructed as a tall pendulum, which is not considered to be
practical. Alternatively, the device could be made wider to
accommodate a series of parallel pendulums, or a traveling mass,
which is in fact a pendulum with infinite arm length r as described
later in this disclosure.
[0043] When attempting to harvest energy from a free-floating
(unmoored) buoy in ocean waves, there are two distinct motions
available to the designer of the energy harvesting system: [0044]
1. Heaving motion of the buoy as it rises and falls vertically on
the surface of a passing wave. This is illustrated as y in FIG. 3.
[0045] 2. Roll/Pitch motion of the buoy as buoyancy and
hydrodynamic forces cause rocking motion about the x and y axes as
shown in FIG. 3.
[0046] The primary problems in attempting to harvest energy from
any of the above oscillatory motions of an unmoored buoy and
convert it into electricity are the very low and varying frequency
of such motions. In general, the oscillatory motion of the platform
(in this case buoy) is used to excite a vibratory (resonating)
system, and the mechanical energy transferred to the vibratory
system is then used to generate electrical energy using an
appropriate mechanical to electrical energy conversion device such
as a magnet and coil system or piezoelectric elements.
[0047] For other larger (mostly moored) platforms, mechanical
energy of the oscillatory system is transferred to a
mechanical-to-electrical energy conversion system through
intermediate mechanical energy "conditioning" systems such as a
pumping system that drives some type of turbine, which in turn
generates electrical energy. All aforementioned methods have
"mechanical coupling" problem with the intermediate mechanical
devices (e.g., the aforementioned vibratory system) that is to
provide input to the mechanical to electrical energy conversion
system due to the low and varying frequency nature of the ocean
waves.
[0048] Regarding the mechanical to electrical energy conversion
systems, when the input motion is rotational, the generation of
electricity from mechanical motion is traditionally accomplished
through motion of a conductive loop in a magnetic field. Such
magnet and coil systems have also been used to generate electrical
energy from linear or rotary oscillatory motions. For such systems,
the voltage and current developed in the loop is directly
proportional to the rate of change of magnetic flux through the
loop. In a system of fixed geometry and construction, this leaves
only the speed of the loop relative to the magnetic field to
determine the output. Therefore, to efficiently generate any
appreciable power, the system must be actuated with relatively high
input velocity. This requirement is directly conflicting with the
slow yet powerful motion of ocean waves.
[0049] Additionally, the most efficient means of transforming
mechanical energy to electrical energy operate at relatively high
and constant rotary or oscillatory motions. This is also directly
at odds with the random frequency and amplitude characteristics of
ocean waves. In past and current applications, attempts to multiply
and regulate the input speed of electrical generators powered by
low-and-variable-speed sources (by methods such as gearing in
systems such as turbines harvesting energy from tidal waves) have
lead to efficiency and reliability problems in the speed control
mechanisms even in relatively benign operating environments.
[0050] The aforementioned shortcomings of the existing methods and
devices for harvesting energy from sea waves are particularly
limiting to unmoored buoys due to their limited size, volume and
buoyancy, particularly for the case of communications buoys which
are relatively slender (of the order of 3 inches in diameter) and
relatively short (of the order of 3 feet long). To address these
shortcomings of all previous mechanical energy harvesting systems
as applied to buoys, a novel class of two-stage energy harvesters
are provided which convert low and varying frequency oscillatory
and/or rotary motions into high frequency and constant vibratory
oscillations that could then be harvested using mechanical to
electrical energy harvesting devices such as those constructed
using piezoelectric materials or customarily used magnet and coil
systems. These devices are shown to be ideally suited to harvesting
energy from the motions of spar buoys.
[0051] The maximum energy available from the heaving (vertical)
motion of a buoy is equivalent to the work done on the buoy by the
wave, namely the product of the wave height and the force required
to lift the buoy. For a spar buoy of 3.0'' diameter and 20.0''
draft in seawater, the buoy mass, equivalent to the mass of
displaced water, is approximately 2.4 kg in seawater with a density
of 1025 kg/m.sup.3. This, combined with a typical wave height
(crest-to-trough) of 3.75 m, yields 88 Joules of energy per wave
cycle, or 9.8 Watts for seas with a temporal wave period of 9
seconds. This energy however may only be collected by a buoy which
is grounded by being moored or employing a sea anchor. This is
because, for an unmoored buoy, the net force on all components of
the buoy from gravitational and buoyancy forces will be downward at
all times. With no reversing force available to cycle the
generation mechanism, any method of energy harvesting which draws
on the heaving motion of a buoy will require an elastic element
(such as a spring) to cycle the device for continuous operation.
Therefore, without a grounding element, the only possible means of
harvesting any appreciable energy from wave heaving motion is by
exciting a mass-spring element into resonance as is commonly done
in many energy harvesting and vibration damping applications. This
method, however, cannot be used in applications of extremely low
and varying frequency such as ocean waves; even the highest
frequencies of waves are only near 0.4 Hz. Constructing a practical
system with such a low natural frequency is of extraordinary
difficulty. Add to this the typical variation of ocean wave
frequency (from 0.1 Hz to 0.2 Hz) and tuning a system for
resonance-based energy harvesting in this spectrum becomes nearly
impossible.
[0052] Construction of a heaving-based resonating magnetic field
type generator was attempted by Grilli, et al. (Grilli, A. R.,
Merrill, J., Grilli, S. T., Spaulding, M. L., and Cheung, J. T.,
Experimental and Numerical Study of Spar Buoy-Magnet/Spring
Oscillators used as Wave Energy Absorbers, Proc. 17th Offshore and
Polar Engng. Conf (ISOPE07, Lisbon, Portugal, July 2007), 489-496).
The resulting 1:10 scale prototype (a 6 cm diameter, 50 cm draft
spar buoy) generated 200-300 mW when excited by a test machine at
exactly its natural frequency of 0.7 Hz (period of 1.42 s.). This
ideal operational output however cannot be taken as a legitimate
performance claim. Firstly, the natural frequency of the system was
far too high to be excited by actual ocean waves. In fact, had the
device been deployed in relatively high frequency ocean waves of
0.14 Hz (period 7 sec), the power generated would have been
negligible according to the results of the experiment, shown in
FIG. 4. To tune the device for these actual conditions, given an
oscillator mass of 0.205 kg, the required spring constant for the
oscillator would be a mere 0.18 N/m. If manufactured, a spring that
soft would buckle and collapse under its own weight. Additionally,
even if the device could be constructed to achieve this natural
frequency, the normal variance of ocean wave periods would be far
outside the excitation period envelope of the device, approximately
0.2 seconds. According to the Pierson-Moskowitz spectrum shown in
FIG. 5, the normal variance in period of ocean waves is from 6 to
20 seconds (Pierson, W. J., and L. Moscowitz, 1964, A proposed
spectral form for fully developed wind seas based on the similarity
theory of S. A. Kitaigorodskii. J. Geophys. Res., 69,
5181-5190).
[0053] The forces (moment) resulting from roll/pitch motion of a
buoy, unlike heaving motion, do undergo complete reversal, and
therefore a restorative force (spring) is not required to cycle any
generator motion induced by the roll/pitch buoy motion. Without the
necessary inclusion of a spring, the system has a natural frequency
of zero and can be excited by a cyclic force of any frequency, low
or high; constant or varying. The total energy available from the
roll/pitch motion of a spar buoy can be calculated by examining a
buoy in a quiescent pool which has been displaced angularly in roll
or pitch as follows: [0054] The work done to displace the buoy may
be calculated by examining the relationship between the three
principle centers of a floating body, namely the buoy's center of
mass, center of buoyancy, and metacenter. [0055] The maximum work
will be required to displace a buoy which has maximum stability.
This is characterized by a buoy with a low center of gravity and a
high center of buoyancy. [0056] The work required to capsize a
3.0'' diameter, 20.0'' draft spar buoy can be stated as:
[0056] E=mgh sin(.theta.)tan(.theta.) [0057] Where: [0058] m=mass
of the buoy (concentrated at lowest possible point on axis [0059]
g=gravitational acceleration [0060] h=distance from mass center to
buoyancy center [0061] .theta.=amplitude of roll/pitch angular
displacement [0062] Conservative estimates of spar buoy roll/pitch
response indicate a roll/pitch amplitude of approximately 10
degrees (Analysis of Buoy Motions and their Effects; Woods Hole
Oceanographic Institute). This translates to 700 mJ per wave cycle
or 70 mW for a relatively long wave period of 10 seconds.
[0063] In the particular case of relatively small buoys such as
those used for communications purposes, for example the
aforementioned 3.0'' diameter and 20.0'' draft spar buoys,
harvesting energy from the buoy roll/pitch and heaving motions
become very difficult due to the small volume and shape of the
buoy. In such buoys, because of the relatively short transverse
motion available inside a 3.0'' diameter spar buoy and because of
the very low frequency of oscillation, no single-stage electrical
generator can be used to harvest any appreciable portion of the
available mechanical energy from the system.
[0064] The disclosed two-stage systems, however, is shown to
provide the means for such buoys to efficiently harvest energy from
the aforementioned heaving and pitch/roll motions. The basic
principle of the system for energy harvesting from the
aforementioned heaving motion of the buoy is shown in the schematic
of FIG. 6, with an exemplary implementation of such embodiment
shown in FIGS. 7A and 7B.
[0065] The embodiment of FIG. 6 operates as follows. The rigid
element 43 (53 in FIG. 7B) is positioned inside the buoy chamber
housing 41 (51 in FIG. 7A), a portion of the wall and bottom
surface of which is shown in FIG. 6. The rigid element 43 is
allowed to travel in the vertical direction in the direction of the
arrow 49 (59 in FIG. 7A). To the rigid element 43 is attached a
relatively rigid bar 44 (54 in FIGS. 7A and 7B), which can travel
in a guide 45 (55 in FIG. 7A), thereby moving the element 43 up and
down within the buoy housing 41. The bar 44 is in turn attached to
the sea anchor (not shown), indicated as 60 in FIG. 7A, by a
relatively inextensible "rope" (cable or the like) 61.
[0066] The elements 53, 54, 61 and 60 shown in FIGS. 7A and 7B
constitute the main elements of the primary stage. While the buoy
62 is floating, the heaving motion of the waves will result in the
sea anchor 60 to pull the rod 54 down as the water raises the
floating buoy due to a passing wave, and allow the rod 54 to move
back up inside the buoy housing (with the help of a spring that is
compressed as the rod is moved down--not shown in FIGS. 6, 7A and
7B) as the buoy is lowered by the passing wave.
[0067] It is noted that the function of the sea anchor 60 is to
resist upward motion as the buoy 62 is raised by the passing wave,
thereby allowing the rod 54 to be pulled down against the resisting
compression spring. In this scheme, to create the relative motion
between the rod 44 (54 in FIGS. 7A and 7B) and the buoy housing 41
(51 in FIG. 7A) that is necessary for the present heaving-based
energy harvesting device, a device commonly known as a sea anchor
is shown to be deployed from the buoy chassis some time after the
buoy itself is deployed. The concept of a sea anchor is to use the
relatively still water below the waves as a drag-based grounding
system. As the buoy rises and falls on the ocean surface, the sea
anchor will resist being taken up and allow for relative vertical
motion between parts the rod 44 and the buoy housing 41 of the
energy harvesting mechanism.
[0068] After the buoy itself is deployed, the parachute-like sea
anchor 60 may be deployed from the bottom of the buoy 62. The sea
anchor 61 will descend to a depth on the order of several meters to
quiescent waters. The attachment cable 61 will provide the
necessary driving force as the buoy 62 is lifted by the waves and
the sea anchor 60 resists the motion. The cable 61 can be attached
to the primary energy harvesting system through a bellows (not
shown) to isolate the interior of the buoy from the water. It
should be noted that such a "pull-only" sea anchor will require a
restorative force to cycle the mechanism. This feature may be built
into the bellows assembly, or may be applied as a constant-force
spring (not shown) in the primary mechanism inside the buoy
chassis.
[0069] A major advantage of employing a sea anchor in the
application of a small communications buoy is that the sea anchor,
while providing resistance to vertical motion, will also prevent
drifting of the buoy from its initially deployed latitude and
longitude.
[0070] At least one secondary vibratory system 40 (indicated as 50
in FIG. 7B) is attached to the inner wall of the buoy housing 41.
In the schematic of FIG. 6, the secondary vibratory systems 40 are
constructed as beam elements 48, to the surface of each of which a
mechanical to electrical energy conversion devices 42 (preferably
bimorph piezoelectric elements operating in tension and compression
layers as the beam 48 vibrates) are attached. As the element 43 is
moved up and down by the rod 44 due to the heaving motion of the
buoy, a tip of the protruding element 46 reaches the tip 47 of the
beam elements 48 and excite its natural mode of vibration. As a
result, part of the mechanical energy of the waves is transferred
to the secondary vibratory systems 40. The elements 42 are then
used to transform the mechanical energy of the systems 40 to
electrical energy. An appropriate electronics circuitry (not shown)
can then harvest the generated electrical energy from the
piezoelectric elements and direct it for use by certain load or for
storage in appropriate electrical energy storage devices such as
capacitors and/or rechargeable batteries. The above methods and
devices for harvesting the electrical energy and regulating it for
direct use (e.g., lighting or communication devices) or for storage
in capacitors and rechargeable batteries are well known in the
art.
[0071] It is noted that the design presented in the schematic of
FIG. 6 is merely for the sake of illustrating the method of
operation of the embodiment and for harvesting energy from the
heaving motion of the buoy. In practice, however, such two-stage
energy harvesting power sources may be designed in a variety of
different types. For example, contact between the tips 46 and 47
would result in rapid wear and inefficiency in the transfer of the
mechanical energy from the primary stage to the secondary vibratory
elements. To make the operation of such a system significantly more
efficient, opposing pole magnets can be used instead of physically
contacting tips 46 and 47, as was described previously.
[0072] In addition, although the element 43 and tip 46 shown in
FIG. 6 is associated with one set of secondary vibratory systems
40, such secondary vibratory systems 40 can also be used on the
opposite side of the element 43 (as shown in FIG. 7B with secondary
vibratory systems 50 on both sides of the element 53). In fact the
secondary vibratory elements 50 may be distributed in any pattern
inside the buoy housing as long as an appropriately shaped and
sized element 53 is provided that could excite the elements 50 as
it traverses along the length of the buoy housing 51. For example,
the tips 46 can be placed around a circumference of the mass and
each have secondary vibratory systems 40 associated therewith.
Still further, more than one element 43 may be attached to the rod
44, allowing each row of tips 46 to engage the secondary vibratory
systems 40.
[0073] The above two-stage energy harvesting method is readily used
to develop devices to harvest energy from the aforementioned
roll/pitch motion of a buoy. The basic operation of such energy
harvesting devices is best illustrated by the energy harvesting
device 30 shown schematically in FIG. 8. The primary system of the
embodiment 30 consists of a simple housing 32, which is attached
directly to the rocking platform 31 (in this case a buoy). The
roll/pitch oscillation of the buoy 31 is considered to be about an
axis perpendicular to the plane of the page. As the buoy 31
undergoes rotary (roll/pitch) oscillations, the traveling mass 33
begins to slide from the side that has been raised, travels the
length of the housing 32 and ends on its opposite end of the
housing. At least one secondary vibratory element 34 is attached to
the top portion 39 of the housing 32. Each vibratory element
consists of a relatively flexible beam 35, to the tip of which is
preferably attached a mass 37 to allow for optimal tuning of the
natural frequency of the first mode of vibration of the vibratory
elements. The tip of the beam 35 and mass 37 assemblies can be
provided with a pointed tip 38 for engagement with the traveling
mass 33 as described below.
[0074] As the traveling mass 33 passes the secondary vibratory
elements 34, it engages their free tips 38 and causes the beams 35
to bend slightly in the direction of its travel. The traveling mass
33 then passes under an engaged secondary vibratory element 34,
moving to the next secondary vibratory element 34. The potential
energy stored in the released beam element 35 causes it to vibrate.
A mechanical to electrical energy conversion means such as a
piezoelectric element 36 that is attached to the surface of the
beam element can then be used to harvest the available mechanical
energy and convert it to electrical energy for collection by an
appropriate electronics circuitry (not shown) and direct usage or
storage in a storage device such as a capacitor or rechargeable
battery (not shown). The use of piezoelectric elements for the
conversion of mechanical energy to electrical energy and related
electronics circuits for collecting the charges generated by
piezoelectric or other similar elements and storing them in storage
devices such as capacitors and/or rechargeable batteries are well
known in the art.
[0075] All contacting surfaces can be designed to minimize
frictional losses. The spacing of the secondary vibratory elements
and the total deflection of the beams 35 and their bending
stiffness can also be selected to maximize the transfer of
potential energy from the traveling mass 33 to the secondary
vibratory elements and to ensure that the total potential energy
stored in each beam element 35 is harvested by the piezoelectric
elements 36 before the next strike of the traveling mass 33. As can
be seen, during each cycle of oscillation of the rocking platform
31, each secondary vibratory element is struck twice by the
traveling mass 33.
[0076] It is noted that one major source of loss in devices such as
the embodiment of FIG. 8 is the mechanical interface where the
traveling mass 33 (i.e., the exciter element) contacts the
secondary vibratory elements 34, i.e., the tip elements 38. To
eliminate these contact losses, non-contacting magnet elements can
be employed, preferably on both traveling mass 33 surface and on
the tips 38 of the secondary vibratory elements 34. The two magnets
are preferably of opposite poles and as the traveling mass 33
passes under the tip 38 of a secondary vibratory element 34, the
two magnets are attracted to each other and as the traveling mass
33 has moved a far enough distance, the secondary vibratory element
24 is released and begin to vibrate, primarily at the frequency of
its first mode of vibration. Such an arrangement would allow for a
strong interaction between exciters and secondary elements owing to
the close proximity of opposite poles.
[0077] The amount of mechanical energy available can be seen to be
proportional to the width L, of the housing 32 of the energy
harvesting device 30, and the mass of the traveling mass 33. The
basic embodiment shown in FIG. 8, however, requires a relatively
long length L (i.e., buoy width or diameter) to generate a
significant amount of power. In the application of this basic
principle to a small buoy, the traversing mass will also be a
significant portion of the total buoy mass. In addition, attempting
to elevate the traversing mass will result in buoy instability and
the buoy may rotate about its vertical axis to regain stability. In
other words: once the traversing mass is elevated even a small
distance, the buoy will become unstable. There are now two paths
back to stability: 1) releasing the mass and allowing it to
traverse "downhill" on the buoy chassis exciting the secondary
elements, or 2) rotating the buoy about its axis until the mass is
on the "downhill" side of the buoy. Because it is desirable to
delay the release of the mass for as long as possible to gain
maximum potential energy, the buoy should be prevented from
rotating about its vertical axis. This concept is akin to the
necessity of "grounding" a heaving-based buoy generator, and can be
accomplished similarly by using the wave motion to maintain the
angular orientation about the buoy's vertical axis. Methods to
prevent unwanted axial rotation of the buoy include fixed or
deployable fins, buoy chassis cross-section designs, and stacking
of several energy harvesting devices in different orientations
along the length of the buoy. Such embodiments are described
below.
[0078] The basic method of harvesting energy from the rocking
motion of the platform 31 shown in FIG. 8 can be used to develop
numerous different designs for devices to harvest energy from the
roll/pitch motion of buoys, all with the common characteristic of
being designed with two stages, a primary stage that transforms the
low and variable frequency and usually small amplitude roll/pitch
oscillations into potential energy that becomes available to a
secondary stages of vibrating elements with significantly higher
and fixed frequency of vibration appropriate for efficient energy
harvesting utilizing various means such as piezoelectric
elements.
[0079] Regarding the sources of forces/moments that induce
roll/pitch motion, there are four moments acting on a free-floating
spar buoy (Berteaux, H. O., Goldsmith, R. A., and Schott, W. E.,
Heave and Roll Response of Free Floating Bodies of Cylindrical
Shape, Woods Hole, Mass. 02543, February, 1977) to create the total
roll/pitch response: [0080] The righting moment caused by the
displacement of the center of buoyancy. [0081] The damping moment
due to buoy motion in the water. [0082] The friction moment due to
drag forces induced on the buoy by horizontal water particle
velocity. [0083] The inertia moment due to inertia forces induced
on the buoy by horizontal water particle acceleration.
[0084] The amount of resulting roll/pitch motions is dependent on
the point of application of their resultant forces with respect to
the mass center of the buoy. Identification of these moments and
their combined effect on any particular buoy design will allow the
designer to create a buoy optimized for maximum response in almost
any seas.
[0085] A free-sliding or rolling mass shown in FIG. 8 is in general
not practicable in an application such as the present slender
buoys, and a more constrained exciter that could operate
efficiently in small width (diameter) buoy is desired for
transferring energy to the secondary vibrating system. The
following are several embodiments for executing the basic concept
of two-stage energy harvesting from roll/pitch motion shown in FIG.
8, with constrained motion of the primary exciter mass and capable
of operating efficiently in a small width (diameter) buoy.
[0086] One embodiment 70 of the present invention for harvesting
energy from the roll/pitch motion of a slender unmoored buoy is
shown in FIG. 9. The embodiment 70 consists of a sealed buoy
housing 71 with a top cap 72. A pendulum 73 is attached on one end
to the top cap 72 by a hinge 74. An exciter mass 75 is attached to
the pendulum 73, preferably as close as possible to its free end as
shown in FIG. 9 to maximize the amount of mechanical energy that
becomes available for harvesting during roll/pitch motion of the
buoy. At least one relatively rigid element 76 with relatively
sharp tips 77 are attached to the pendulum 73 as shown in the
close-up view shown in FIG. 10.
[0087] The system starts with the mass 75 held against one side of
the buoy interior by a magnet, detent ball, or some similar means
(not shown). The offset pendulum mass will effect the stability of
the system, and the buoy will rest in quiescent waters heeled to
one side. Upon interacting with the first half-cycle of a wave, the
buoy chassis will rotate through some roll/pitch angle until the
pendulum mass is released. Upon release, the pendulum 73 will
swing, and the tip 77 strike the at least one secondary vibratory
element 78, transferring mechanical energy to the secondary
vibratory element 78, and continuing at lower speed to rest at the
opposite side of the buoy on a magnet or similar retention device.
During the second half-cycle of a wave, the process is reversed,
and the pendulum 73 will again strike the secondary vibratory
element 78. Because of the low frequency of the ocean waves, all
vibratory energy from the first strike can be readily be removed
before re-excitation.
[0088] As an example, for the buoy dimensions previously indicated,
a mass 75 of 400 grams can be readily accommodated. With an
estimate of a roll/pitch angle of 10 degrees, the energy available
for harvesting per wave cycle will be approximately 70 mJ, or 8.75
mW for a temporal wave period of 8 s.
[0089] In the embodiment of the present invention shown in FIGS. 9
and 10, only one secondary vibratory element 78 and corresponding
exciter tip 77 are shown. It is appreciated by those familiar with
the art that more than one secondary vibratory element and/or
exciter tips may also be used to maximize the amount of harvested
energy.
[0090] Another embodiment 80 for harvesting energy from the
roll/pitch motion of a slender unmoored buoy is shown in FIG. 11.
The embodiment 80 consists of a sealed buoy housing 81. At least
one eccentric mass based energy harvesting device 82 is positioned
inside the buoy housing. The device 82 consists of an eccentric
mass 83, which functions similar to the pendulum 73 of the
embodiment of FIGS. 9 and 10, with the main difference of rotating
about a vertical axis in a plane perpendicular to the long axis of
the buoy, thereby allowing several such eccentric energy harvesting
devices 82 to be stacked along the length of the buoy to increase
the total harvested power output. Here, the eccentric mass 83
pivots about the center of the buoy housing (chassis) 81. For
compactness, at least one bending secondary vibratory element 84 is
attached to the periphery of the buoy inner housing and are excited
at their tips 85 (towards the center) by the tips 86 of the exciter
element 87, which is fixed to the eccentric mass 83. It is noted
that in the schematic of FIGS. 11 and 12, the exciter element 87 is
shown with two tips 86, but fewer or more tips 86 may also be used
depending on the size of the eccentric mass 83, the size (diameter)
of the buoy, the size of the secondary vibratory elements 84, etc.,
for optimal energy harvesting. The number of primary eccentric
masses 83 and secondary vibratory elements 84 beams may be
optimized for a particular application.
[0091] In this embodiment, the system starts with the eccentric
mass 83 held against one side of the buoy interior by a magnet,
detent ball, or some similar means (not shown). The offset
eccentric mass 83 effects the stability of the system and the buoy
will rest in quiescent waters heeled to one side. Upon interacting
with the first half-cycle of a wave, the buoy chassis will rotate
through some roll/pitch angle until the eccentric mass 83 is
released. Upon release, the eccentric mass 83 will rotate and the
tip 86 strike the tip 85 of the at least one secondary vibratory
element 84, transferring mechanical energy to the secondary
vibratory element 84, and continuing at lower speed to rest at the
opposite side of the buoy on a magnet or similar retention device.
During the second half-cycle of a wave, the process is reversed,
and the eccentric mass 83 will strike the same or another secondary
vibratory element 84. Because of the low frequency of the ocean
waves, all vibratory energy from the first strike can be readily
removed before re-excitation.
[0092] In another embodiment 90 shown in FIG. 13, at least one
four-bar linkage mechanism 91 is positioned inside the buoy housing
92 and is used to function as an inverted pendulum to perform the
basic function of the pendulum 73 of the embodiment of FIGS. 9 and
10. A close-up view of the four-bar linkage mechanism based energy
harvester is shown in FIG. 14. The use of a four-bar linkage
mechanism 91 also has the advantage of being stackable like the
eccentric mass based generators 82 of the embodiment of FIGS. 11
and 12.
[0093] The four-bar linkage mechanism 91 consists of two links 93,
which are attached to the buoy housing (chassis) 92 by the hinges
94 on one end, and to the coupler link 95 by hinges (cannot be
viewed in FIG. 14) on the other ends. The mass of the coupler link
95 also act as the aforementioned mass of the inverted
pendulum.
[0094] The schematic drawing of such a four-bar linkage mechanism
100 is shown in FIG. 15. The mechanism consists of two links 101,
which are attached to the buoy housing 101 via revolute joints
(hinges) 102 on one end and to the coupler link 103 via the
revolute joints 104 on the other end. The coupler link 103 is
provided with a amount of mass, which acts as the mass of its
equivalent inverted pendulum. This rightward rotation of this
equivalent inverted pendulum is limited by the inside wall portion
105 of the buoy housing 101 as shown in FIG. 15. The leftward
rotation of the equivalent inverted pendulum is similarly limited
by the inside wall portion 106 of the buoy housing 101. In the
schematic drawing of FIG. 15, two exciter tips 107 are also shown
to be provided on the coupler link 103, which are provided for
engagement with the secondary vibratory elements shown in FIGS. 13
and 14.
[0095] It is noted that the naturally stable positions of the
four-bar linkage mechanism 91 against the surfaces 105 and 106 of
the interior wall of the buoy housing 101 can be used instead of a
magnet or detent to control the roll or pitch angle required to
release the mass.
[0096] The system starts with the coupler link (95 in FIGS. 14 and
103 in FIG. 15) held against one side of the buoy interior against
either the wall surface 105 or 106, FIG. 15 (by right side wall of
the housing 92 in the schematic of FIG. 14). The offset (coupler
link) mass 95 effects the stability of the system and the buoy will
rest in quiescent waters heeled to one side. Upon interacting with
the first half-cycle of a wave, the buoy housing (chassis) 92, FIG.
14, will rotate through some roll/pitch angle until the (inverted
pendulum) mass 95, FIG. 14, is released. Upon release, the
(inverted pendulum), i.e., links 93, will swing, and the tip 96
(107 in FIG. 15) strikes the at least one secondary vibratory
element 97, transferring mechanical energy to the secondary
vibratory element 97, and continuing at lower speed to rest at the
opposite side of the buoy against the opposite interior wall of the
buoy housing 92 (i.e., the coupler link moves from its position
shown in FIG. 15 and comes to rest on the interior wall surface
indicated as 106). During the second half-cycle of a wave, the
process is reversed, and at least one tip 96 coupler link 95 will
again strike at least one other (oppositely positioned) secondary
vibratory element 97. Because of the low frequency of the ocean
waves, all vibratory energy from the first strike can be readily be
removed before re-excitation.
[0097] In the schematic of FIG. 14 is shown to use secondary
vibratory elements 97 toward the sides of the buoy and two exciter
tips 96 on the coupler link (inverted pendulum mass) 95. This
configuration can require the exciter tips 96 to be rigid only in
one direction so the coupler 95 can traverse the entire buoy
diameter, gaining energy, before striking a secondary vibratory
element 97. This and numerous other variants of secondary vibratory
element arrangements are possible and appropriate depending on the
size of the buoy and other design parameters of the system.
[0098] The most significant advantage of the four-bar linkage
mechanism based embodiment of FIGS. 13-14 over the pendulum based
embodiment of FIG. 9 is that a plurality of four-bar linkage based
generators can be stacked at the lower end of the buoy, thereby
increasing the amount of energy that can be harvested by the
overall buoy system. The size, quantity, and arrangement of the
individual four-bar linkage based energy harvesting modules can be
optimized to match each specific application.
[0099] It is noted that the four-bar linkage mechanisms shown in
FIGS. 13-15 are of parallelogram type and are installed
symmetrically within the buoy housing. This would allow symmetric
rotational travel of the coupler link (inverted pendulum) to each
side of the buoy housing. It is, however, appreciated by those
familiar with the art that the link lengths (including the ground
link) may be selected such that the aforementioned motions are
non-symmetric and biased to one side. By such configurations, the
amount of energy that is transferred to the secondary vibratory
element during one of the motions can be made to be significantly
higher. Such configurations may be warranted if the width
(diameter) of the buoy is very small and the amount of roll/pitch
angle is expected to be relatively small, thereby allowing a
relatively significant amount of mechanical energy to be
transferred to the secondary vibratory elements during each cycle
of the buoy roll/pitch oscillation.
[0100] As mentioned above, the moment generated by the rotation of
the pendulum mass 75 in the embodiment of FIG. 9 (eccentric mass 83
in the embodiment of FIG. 12 and coupler link mass 95 in the
embodiment of FIG. 14) will also tend to bring the buoy to its
stable position during the roll/pitch motion of the buoy. If the
buoy is allowed to be rotated to its stable position at all times,
then the aforementioned energy harvester mass elements (pendulum
mass 75 in the embodiment of FIG. 9, eccentric mass 83 in the
embodiment of FIG. 12 and coupler link mass 95 in the embodiment of
FIG. 14) cannot gain potential energy and thereby harvest
mechanical energy from the roll/pitch motion of the buoy in the
waves. It is also noted that since the period of the waves is
relatively long, the inertia of the buoy cannot be used to resist
the aforementioned stabilizing moments. However, since these
stabilizing moments are relatively small, the countering moments
required to maintain axial rotational orientation of the buoy
during its roll/pitch motion will be equally small and may be
accomplished using, for example, one or more of the following
techniques depending on design constraints for the exterior size
and shape of the buoy housing (chassis).
[0101] Axial orientation may be maintained by using the horizontal
water flow seen in FIG. 16 (Introduction to Ocean Waves &
Tides, Salmon, 2006). In this illustration, the arrow 110 indicates
the direction of the wave travel, and the arrows 111 indicate the
flow velocity profile at the specified locations. By using buoy
chasses with cross-sections such as those for the buoys 115 and 116
shown in FIGS. 17A and 17B, respectively, the buoy's longer
transverse axis will tend to maintain alignment with the horizontal
flow of the waves. This cross section may be constant (A), or can
be varying and discontinuous (B), depending on the constraints of
any particular design. The only requirement is that the overall
profile of the wetted portion of the buoy must counteract the
tendency of the buoy to rotate in response to the aforementioned
instability.
[0102] Axial orientation of a buoy 120, FIG. 18, may also be
maintained through the use of fins 122 applied to the exterior 121
of the buoy 120 (anywhere along the length of the buoy). These fins
may be fixed or deployable depending on the requirements of a
particular application.
[0103] It is also noted that the roll/pitch response of the buoy is
driven by the depth-varying horizontal water movement in the wave.
Given that floating bodies will rotate about their mass centers,
the ideal configuration for a large-response roll/pitch buoy is to
locate the mass center as deeply as possible. For a 3.0'' diameter
spar buoy with a 20.0'' draft, this equates to an ideal
configuration of around 2.4 kg of buoy mass concentrated around
20.0'' below the waterline. In this configuration, the water
particles impinging on the buoy surface will have the greatest
effect on the capsizing moment when acting near the water surface
(farthest from the mass center). Coincidentally, this is where the
horizontal water velocity is fastest, and will have the most
momentum to transfer.
[0104] It is also noted that surface finishes and features are
commonly employed to optimize the forces applied to a body by a
fluid medium. Such methods may also be employed to the following
embodiments. Here since the response of the roll/pitch motion is to
be maximized, a surface texture or roughness can be applied to that
end.
[0105] In another embodiment, the amplitude of the roll/pitching
motion, the vertical axis of the spar buoy 130 is forced to
maintain its orientation nearly normal to the surface of the wave.
In one embodiment, three or more fixed or deployable outriggers 131
are used as shown in FIG. 19a will work to this end and the buoy
130 will experience roll/pitch motion nearly equal to the maximum
angle of the wave surface as shown in FIG. 19b. The limit of
included angle for ocean surface waves is 120 degrees (Bascom, W.,
Waves and beaches: The dynamics of the ocean surface, Garden City,
N.Y.: Doubleday and Company, 1964) as illustrated in FIG. 20. This
translates to a maximum angular buoy displacement of around 30
degrees from the vertical--three times the displacement observed
without forcing surface tangency (Analysis of Buoy Motions and
their Effects; Woods Hole Oceanographic Institute).
[0106] While there has been shown and described what is considered
to be preferred embodiments of the invention, it will, of course,
be understood that various modifications and changes in form or
detail could readily be made without departing from the spirit of
the invention. It is therefore intended that the invention be not
limited to the exact forms described and illustrated, but should be
constructed to cover all modifications that may fall within the
scope of the appended claims.
* * * * *