Wave Energy Watercraft

Abstract

A watercraft comprising a plurality of linearly ordered, connected segments that are configured to enable the connected segments to articulate relative to each other in at least one degree of freedom. The watercraft harvests mechanical energy from ambient waves by means of the articulation of the segments, and converts the mechanical energy into electrical energy that powers a propulsion mechanism.

Description

BACKGROUND

There is an increasing need for unmanned, autonomous sensor platforms for use in the maritime environment that support monitoring operations for very long durations without human intervention. These platforms need to communicate, navigate and maneuver to position sensors at desirable locations, water depths, and times. Long duration missions require the vehicle to carry or obtain the energy necessary for locomotion, sensor power, and communications.

SUMMARY

An apparatus, comprising a plurality of linearly ordered segments, the plurality having two extremity segments, and wherein at least two segments are connected; a connecting assembly joining each of the two connected segments of the plurality of linearly ordered segments, the connecting assembly configured to enable the two connected segments to articulate relative to each other in at least one degree of freedom; a wave energy harvesting mechanism operatively connected to the connecting assembly that extracts mechanical energy from ambient liquid waves and converts the mechanical energy into electrical energy; a propulsion mechanism attached to an extremity segment, operatively connected to and powered by the wave energy harvesting mechanism.

DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts an embodiment of the invention in 7 linear segments.

FIG. 2 depicts an embodiment of the invention wherein wave energy harvesting is by means of hydraulic pumps.

FIG. 3 depicts a wave energy harvesting mechanism employing a gear train.

FIG. 4 depicts an embodiment of the invention wherein wave energy harvesting is by means of linear Lorentz style generator.

FIG. 5 depicts a wave energy harvesting mechanism employing a circular Lorentz style generator.

FIG. 6 depicts a close up view of the coil rings of a circular Lorentz style generator.

FIG. 7 depicts a wave energy harvesting mechanism employing piezoelectric elements and flex-tension amplifiers.

FIG. 8 depicts the propulsion mechanism of an embodiment of the invention.

FIG. 9 depicts the attenuation of water velocity with depth for a wave.

DETAILED DESCRIPTION

The embodiment described in the above summary solves the problem of providing a means for long duration, watercraft, sensor platforms by extracting energy from ambient water waves, converting the wave energy into electricity, and then using that energy for locomotion, sensor power, and communications. Excess energy is stored onboard the vehicle for use when ambient wave energy is insufficient for operations. The embodiments described in detail below are comprised of multiple segments flexibly connected together. Electric energy is produced by differential motion of adjacent segments resulting from wave-induced motion of the water. Relative motion between adjacent segments can be converted to electricity by a number of means; electromagnetic means (magnets and inductors), pump fluids to spin electric generators, or strain piezoelectric elements to produce electricity. Power generation may be augmented with photovoltaic elements attached to the upward-facing surfaces of the vessel.

As shown in FIG. 1, embodiments of the invention may be composed of one or more segments 110 connected with one or more degrees of freedom. The segments are able to move relative to one another in concert with the wave motion of their environment. In this way, certain portions of two segments will alternatively move closer together and further apart in accordance with the save motion. The energy of this relative motion caused by waves can be captured by wave energy harvesting mechanisms 120. The segments can sit at varying levels with respect to waterline 130, including completely submerged. Individual segments may have length to width ratios (“L/W”) near unity or they may be elongated. The cross-sectional shape could be of any geometric design such as circular, square, triangular, oval or any other symmetrical or asymmetrical shape. For instance a segment could be spherical in low length to width ratios or cylindrical in shape for large L/W ratios. It could be cubic in low L/W ratio or rectangular prism in large L/W ratios. Embodiments of the invention may be composed of a single line array of connected segments or multiple arrays of segments to form two or three dimensional shapes of any desired configuration.

Techniques for conversion of wave energy to electric power are known in the art. For example, the Pelamis system manufactured by Pelamis Wave Power Limited provides off shore power generation by means of a line array of multiple floating segments is anchored is a wave field. See for example “Pelamis Technology” and “Pelamis Wave Power Brochure” both published by Pelamis Wave Power Limited and both of which are incorporated by reference into this specification.

Wave action will typically align the array to be at a large angle to the wave crests. As waves move under the Pelamis system, segments are tilted differentially to conform to the wave shape. The segments are movably connected so that segments adjacent to the connection can be at different angles when projected into a vertical plane. The connection point may also allow the segments to move differentially in the horizontal plane. Consequently, the space between adjacent segments opens at some points and closes at others as a wave passes across the segments. When the wave crest is positioned at the connection point between segments, the gap between segments is opened at the top of the connection point and closed at the bottom. As the trough of the wave moves past the connection point, the gap at the top of the connection points is closed and the bottom is opened.

As shown in FIG. 2 and in the Pelamis system, a wave energy harvesting mechanism can be implemented by means of hydraulic pumps. The cyclic opening and closing of space between segments actuates hydraulic pumps 210, which are integrated with a connecting assembly 240, joining segments together. One end of the connecting assembly is attached to a first segment and another end of the connecting assembly is integrated with the hydraulic pump piston and attached to a second segment. With appropriate valves, piping and hydraulic accumulators 220, this wave energy harvesting mechanism spins a rotary hydraulic motor which turns an electrical generator 230. See for example, U.S. patent application publication number US20130239562 A1, which is incorporated by reference into this specification. The electrical generator is operatively connected to propulsion mechanism 270 attached to an extremity segment of the watercraft.

A wave energy harvesting mechanism system employing hydraulic pumps is capable of producing 750 kW of power with 70% efficiency of its power-take-off system. The wave energy flux of water in waves is estimated by the equation

$P=\frac{\rho g^2}{64\pi }H^2T,$

where H is the wave height (trough to crest), T is the wave period, ρ is the density of water and g is the acceleration due to gravity. For a modest sea offshore characterized by 2 meter high waves with a wavelength of 80 meters, a period of 7 seconds and velocity of 11.1 m/s, the potential power is about 13.4 kW/m of wave crest.

One means of transforming the mechanical forces of wave motion acting on multiple hull sections is to convert the flexing moment of the hull sections into rotary motion by means of a gear train as illustrated in FIG. 3. Adjacent hull segment are connected at a flexible joint 310 allowing at least one degree of freedom. In this embodiment, the hulls are allowed to flex in the vertical plane. Other arrangements of the flexible connection could allow for flexure in the horizontal plane and other dimensions as well. For simplicity and clarity, a single degree of freedom in the vertical plane will be used to describe the implementation.

A segment of a circular ring gear 320 affixed to Hull Segment D is concentric with the pivot point of the hull segments C and D. The ring gear is engaged with a smaller gear 330 mounted on Hull Segment C which is coaxially connected to a larger diameter gear which drives one or more additional gears to increase the angular velocity. The overall gear ratio of each gear and combination of gears in the gear-train 340 can be selected to accommodate an average flexure (for and average sea height) between hull segments and the angular velocity best suited for a particular rotary electric generator 350. Amplification of angular velocity will typically be on the order of hundreds to thousands.

Because the direction of flexing changes periodically with the passage of each wave crest and trough, the flexural amplitude changes with sea state. The design of the gear-box and vehicle characteristics can be tuned for maximum efficiency for a particular sea state or change dynamically to accommodate a range of changes in sea state. Efficiency may be optimized by converting bidirectional flexural input to unidirectional output. Conversion of the flexural motion could also be converted to output constant velocity rotational output. Electrical power generated will be conditioned to achieve the power type and voltages needed by propulsion hardware, navigation, communication and power storage equipment.

Provided that the generator can also be used as a motor, it and the gear train can be used in combination with position sensors mounted on the hull segment to position the hull segments in desirable configurations. Examples of desirable configurations might be those advantageous for maneuvering or to synchronize movement of the multiple joints simultaneously to mimic the swimming motion of a water snake. It may also be desirable to drive a small diameter rotary electric generator or a larger diameter circular motor similar to those illustrated in an alternative embodiments described below.

For illustration purposes, it is useful to provide an example of one method for estimating the power generated by an embodiment of the invention. Returning to FIG. 1, three power modules may be connected on either side by rigid hull sections 1 meter in diameter and about 7 m long. Each power module is free to bend to conform to the wave surface as it passes by.

Each 7 m segment has a hull 5 m long and thus a displacement of about of 3925 kg if neutrally buoyant. Assuming 20% positive buoyancy, this embodiment produces a displacement of 3142 kg. Distance from center of power module to center of gravity for the adjacent rigid section is 3.5 m.

Torque (T)=force*lever arm. Therefore, T=3142 kg*9.8 m/s2*3.5 m=107,770 Nm. Power is torque multiplied by the angular velocity of action.

In the sea state illustrated, each hull segment sweeps through 2.5 degrees up and down (or 5.0 total) every 7.1 seconds as the crest and trough moves under each power module. Therefore, P=(T/7.1 s)*5.0 deg*6.28 rad/360 deg=9,400 J/7.1 s.

Each power module has a rigid hull section either side of it so power is doubled for each power module. Therefore, P=18,800 J/7 s=2,686 watt for each power module.

All three power modules produce 8,058 W or 10.8 hp if all of the power at the power module was converted to electricity. Assuming an efficiency of power conversion of 30%, 3.25 hp would be available for system operation (e.g., propulsion, on-board system use and power storage).

Speed to length ratio (“S/L”) for a displacement hull is estimated using the empirical relationship S/L=(1200/((Disp/hp))̂(−3). Total displacement is estimated to be 5(3,142 kg) or 34,562 pounds. Therefore, S/L is estimated to be 0.48.

For a displacement hull with a waterline length of 90 ft (as described in FIG. 5) operating at the surface, the expected speed of the vessel in the described sea-state is 4.6 kt. Because of the large length to beam ratio, the speed may be greater. When the vessel is submerged, the resistance of cutting the air-water interface and wave-drag are reduced and the speed may be greater yet.

The angle of attack with respect to the vessel and the wave crest also affects the conversion factor of wave energy into electrical power. The scenario described above assumes the vessel was operating perpendicular to the waves. The direction of travel affects the power conversion too. Propelling the vessel in the direction of the waves reduces the observed period of the wave and so reduces the power extracted. Heading the vessel parallel to the wave crest will reduce the power converted.

The operating efficiency of a multi-segmented wave energy extraction system can be optimized for the dominant sea state of the operating area. Segments too long in short wavelength seas will not be as efficient as shorter segments. Bending angle between adjacent segments will be maximized when segment lengths are one quarter of the wavelength. Use of many short segments increases the number of connection and power generation components that comprise the same length of array and also increases the labor and complexity of construction, operation and maintenance.

Another means of transforming the mechanical forces associated with differential relative movement of adjacent segments is to employ linear electric motors such as the Lorentz type of actuator. Common implementations are induction motors and synchronous motors. In a Lorentz type actuator, current and magnetic field strength are related by the equation:

({right arrow over (F)}=q{right arrow over (v)}×{right arrow over (B)})

FIG. 4 illustrates one such arrangement using magnet linear generators such as that produced by ASLM, Inc. Magnets 410 and coils 420 positioned appropriately on adjacent segments transform the linear motion of the gap opening and closing into electricity. This electricity may be transmitted along connecting wires or some similar transmission structure 440 to propulsion mechanism 270. The electric energy may be collected and converted at converter 430. The converter may also be integrated with a batter to store excess energy for future use.

In FIG. 4 only a single degree of freedom for the Lorentz type actuators is show. However, other arrangements can be configured to allow two or more degrees of freedom using mechanism similar to a universal joint.

Lorentz style generators and actuators can be formed into circular configurations as illustrated in FIG. 5, and detailed view FIG. 6. It may be desirable to drive a small diameter rotary electric generator or a larger diameter circular motor 510. The configuration of the circular Lorentz motor/generator can be configured to suit the particular engineering application. For instance, the magnet 520 and coil 530 rings may be oriented with their long axis perpendicular to the axis of the ring as illustrated in FIG. 5 or they may be oriented coaxially. Either of the rings (magnet or coil) may be fixed to one of the hull segments and the other rotated with respect to the fixed ring. An angular velocity amplifying gear train similar that described above drives the rotating ring to improve generator efficiency. Similar to the small diameter rotating motor/generator, the large diameter circular Lorentz type motor generator and gear train working in conjunction with position sensors can be used to set the angle of flexure between adjacent hull segments. In most embodiments it will be desirable to coat the magnets and coils in water-proof materials. In some embodiments the Lorentz type motors take advantage of the cooling benefits of partial submersion relieve excess heat from the generator.

The opening and closing of the gap between segments can be used to generate power in other ways. For instance as show in FIG. 7, piezoelectric elements positioned in the gap and rigidly connected to the end-caps of adjoining segments would experience a compression and tension as wave crests or troughs pass across the segment connection point. The strain in the piezoelectric elements caused by the compression and tension of produces an electrical current which is harvested and stored. The strain acting on piezoelectric elements can be magnified by a mechanical device known as a flex-tensional amplifiers 710. Examples of suitable flex-tensional amplifiers are those such as that depicted in U.S. Pat. Nos. 8,154,173 and 6,629,922, both of which are incorporated by reference into this specification. These types of flex-tensional amplifiers may be scaled up such that they are suitable for generating the energy needs of embodiments of the present invention.

Several flex tensional amplifiers and associated piezoelectric elements can be connected to each end cap of adjoining segments. While providing the mechanical connection necessary between segments, the flex-tensional amplifiers would work in concert to generate electricity as the segments flex due to wave induced differential movement of the segments. A radial pattern is as shown in 720 but other arrangements may be implemented that are suitable for the geometry of the embodiment.

The electric power generated by the wave energy harvesting mechanisms employed in embodiments of the invention is stored in a battery for onboard use. This energy is used to power the electric propulsion mechanism, but can also be used to power a navigation system, data collection and storage, and transmission systems.

In addition to energy extracted from wave motion, additional power can be extracted from solar energy while the vessel is operating on the surface. Energy can be extracted from photovoltaic collectors affixed to the upward facing surfaces of the vessel. These can be conformal with the vessel hull (e.g. a curved surface) or mounted on hull structures that present a larger projected area facing upward such as a single horizontal surface or multiple surfaces more favorably oriented to capture solar energy. In sea state conditions that do not support wave energy extraction, solar energy can be collected for propulsion, storage and on-board system consumption.

At least one propulsion mechanism 270 located at end of array in direction of travel (bow). Pulling vessel through water rather than pushing it. Multi-element, flexible line array is hard to push through water. A forward mounted propulsion mechanism in underwater vehicles is described in U.S. Pat. Nos. 6,725,797, 6,701,862, and 6,701,862, each of which is hereby incorporated by reference into this specification.

At relatively slow speeds, control surfaces for horizontal or vertical maneuvering of a vessel have much reduced effect. As show in FIG. 8, the propulsion mechanism may implement steerable propeller thrust 810 coupled with a steerable shroud 820 that further directs the flow of water to impart maneuvering forces on the vessel. In one embodiment, the shroud is a circular cross section coaxially positioned in front of a cylindrical vessel with a gap between the propeller shroud and the vessel. The shroud and the vessel share a similar diameter, and the shroud is open to the sea at the distal end. A propeller is positioned inside the shroud which provides a duct for improved propeller efficiency. The shroud is movably attached to the vessel in such a way that it can be rotated about orthogonal axes; notionally a horizontal and a vertical axis.

When the orientation of the axis of the shroud is coincident with that of the vessel, the propeller thrust is symmetrical around the perimeter of the shroud and so no asymmetric maneuvering force (e.g., up, down, left, right) is imparted on the vessel. When the shroud and duct are rotated about one or more of the axes, the direction of thrust of the propeller is changed which imparts a maneuvering force on the vessel. When turned, the shroud opens the gap between the shroud and vessel at a location in the direction desired to maneuver while simultaneously closing the gap on the opposite side. In this way the thrust of the propeller is preferentially emphasized.

A similar propulsion mechanism can be added at the other end of the vessel to provide increased thrust for speed, increased maneuvering forces and additional maneuvering options when running the systems in the same or opposite direction of thrust.

The steerable cowling is capable of producing maneuvering forces to the left, right, upward and downward when it is enabled to rotate on two orthogonal axes. Maneuvering forces in combination with a variable ballast system and autonomous control provide the means for embodiments of the invention to change depth in a controlled fashion. There are several benefits of being able to operate submerged. Propulsion efficiency is increased when the cowling and propeller are fully submerged. A submerged vessel has the added benefit of increased efficiency because the vessel avoids the deleterious effects of piercing the air-water interface and potentially adverse wind conditions. When submerged, a watercraft avoids exposure to potentially adverse wind conditions.

Wave motion propagates downward in the water column so that a wave powered vessel can extract energy from the environment though at dramatically reduced rates. In deep water that is deeper than the wavelength of the waves, wave induced velocity of water diminishes exponentially with depth below the surface. The relative magnitude of the vertical component of wave induced water velocity at a depth z (Vz) is estimated by the equation V_z≈e^ω2z/g

Angular velocity is defined by ω=(2πg/L)̂(½) where g is the gravitational acceleration and L is the wavelength of the wave. The chart at FIG. 9 illustrates the rapid attenuation of water velocity with depth for a wave with L=50 m. Displacement of the particle is proportional to velocity so the relative attenuation of wave induced water velocity also predicts the relative displacement of the displacement. It is evident that wave “height” at a depth about 10% of the wavelength is reduced to about 50% of the value at the surface. Because wave energy flux is proportional to the square of the wave height as previously discussed, the amount of energy that could be extracted from wave energy at a depth 10% of wavelength is about 25% of that at the surface. Energy can be extracted from wave induced motion of water at depth below the surface but the amount of energy available drops of exponentially with depth.

With the large volume of space enclosed by the hull segments of certain embodiments of the present invention and the desire to partially submerge the vehicle so that water resistance is decreased, as previously mentioned it would be advantageous to carry electric energy storage devices such as batteries. Batteries enable embodiments of the invention to store energy harvested in excess of that needed for propulsion. The stored energy is then available to provide propulsion when harvested wave energy was inadequate for desired transit speeds. On-board storage of energy also enables the embodiments of the invention to maintain operation in the absence of wave energy to harvest. These circumstances may occur when wave heights are very low or when the watercraft is submerged in a water column below the effects of wave energy on the surface. Submerging embodiments of the invention is desirable to avoid navigation obstacles or reach particular locations necessary to make observations or measurement. Embodiments of the invention may also submerge to a particular depth and even rest on the bottom to avoid heavy weather or make observations and measurements of phenomena better observed at depth. Replenishment of stored power would be accomplished returning to the surface or close to the surface where wave energy could be harvested again.

Claims

1. An apparatus, comprising a plurality of linearly ordered segments, the plurality having two extremity segments, and wherein at least two segments are connected;a connecting assembly joining each of the two connected segments of the plurality of linearly ordered segments, the connecting assembly configured to enable the two connected segments to articulate relative to each other in at least one degree of freedom;a wave energy harvesting mechanism operatively connected to the connecting assembly that extracts mechanical energy from ambient liquid waves and converts the mechanical energy into electrical energy;a propulsion mechanism attached to an extremity segment, operatively connected to and powered by the wave energy harvesting mechanism.

2. An apparatus, comprising a plurality of linearly ordered segments, the plurality having two extremity segments, and wherein at least two segments are connected;a connecting assembly joining each of the two connected segments of the plurality of linearly ordered segments, the connecting assembly configured to enable the two connected segments to articulate relative to each other in at least one degree of freedom;a wave energy harvesting mechanism operatively connected to the connecting assembly that extracts mechanical energy from ambient liquid waves and converts the mechanical energy into electric energy, the connecting assembly having two ends, each end attached to a separate segment, andthe wave energy harvesting mechanism, further comprising a hydraulic pump having two ends and containing hydraulic fluid, a first end attached to a first end of the connecting assembly and a second end attached to a second end of the connecting assembly, and wherein each end of the hydraulic pump can move relative to the other such that the hydraulic fluid is compressed or expanded; anda propulsion mechanism attached to an extremity segment, operatively connected to and powered by the wave energy harvesting mechanism.

3. An apparatus, comprising a plurality of linearly ordered segments, the plurality having two extremity segments, and wherein at least two segments are connected;a connecting assembly joining each of the two connected segments of the plurality of linearly ordered segments, the connecting assembly configured to enable the two connected segments to articulate relative to each other in at least one degree of freedom;a wave energy harvesting mechanism operatively connected to the connecting assembly that extracts mechanical energy from ambient liquid waves and converts the mechanical energy into electrical energy; the connecting assembly having two ends, each end attached to a separate segment, andthe wave energy harvesting mechanism, further comprising a piezoelectric element operatively connected to a flex-tensional amplifier, the combination rigidly connected between each end of the connecting assembly; anda propulsion mechanism attached to an extremity segment, operatively connected to and powered by the wave energy harvesting mechanism.