Abstract:
A buoyant energy generating housing apparatus submersed in fluid currents. The disclosed embodiments comprises rotary turbines that harvest the kinetic energy in the currents, and buoys that house equipment and provide buoyancy to support the system. Movements and rotations are restrained by multiple cables or tendons that are anchored on the seabed, in combination with the internal active ballast system in the buoys. Applications in currents with direction change are possible with the use of two-buoy embodiments, further assisted by the optional use of weathervanes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable 
       BACKGROUND 
       [0003]    Renewable energy plays a critical role in the world-wide efforts to reduce fossil fuel use and curtail pollutions and carbon-dioxide emissions. Presently, wind and solar dominate the development and deployment of the renewable energy. Another source of the renewables is the kinetic energy in the moving waters on our planet. Although water typically moves slower than wind, water is more than 800 times heavier than air, and therefore has high energy density when it moves. Traditional methods of harvesting hydraulic energy often involve the construction of dams in/on rivers in order to increase flow velocity, which alters the natural flow of rivers and impose major disruptions to the environmental and ecological system in nature. 
         [0004]    A major hydrokinetic energy source that is largely untapped is the moving water in the ocean. Great amounts of kinetic energy exists in the form of waves, tides, and ocean currents. A wave is the movement of water on the sea surface due to wind. The tide is the movement of water due to the gravity interactions of the earth, the moon and the sun. Ocean currents are the long distance movement of ocean water due to wind, temperature gradient, rotation of the earth, sanity gradients, etc. The present invention is a device used for the power extraction from ocean currents, tidal streams, and other fluid currents. 
         [0005]    In the past there have been many different devices proposed in prior art to harvest the kinetic energy in water currents involving hydraulic turbines with rotating blades. Different types of turbines have been developed and are commercially available. Generally, submerged hydraulic turbines can either have a horizontal axis or a vertical axis. In many forms, the present invention uses the turbine with a horizontal axis. In some forms, the invention is a buoyant device that includes the hydraulic turbine, and a new method to tether the device in the middle of the moving water current, so that electricity can be continuously and effectively generated. 
         [0006]    In several embodiments, the present invention has several distinctions and advantages, which may include: 1) in several embodiments, the present invention has a buoyant device tethered by mooring cables/tendons that are preferably in high tension; 2) in several embodiments, the present invention is totally submerged in the water and stays below the ship traffic; 3) in several embodiments, the present invention has the rotary turbine with its axis parallel to water flow direction and its blades sweeping in the plane normal to the water flow direction; 4) in several embodiments, the present invention uses single rotary turbine, but can easily be modified to house multiple-rotor turbines; 5) in several embodiments, the present invention uses three or more, parallel, non-coplanar, equal-length tethering cables/tendons; 6) in several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments; 7) in several embodiments, the present invention uses CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments; 8) in several embodiments, the present invention has yaw rotational capability by using two buoys, one rotating and the other non-rotating; and 9) in several embodiments, the present invention utilizes a weathervane to increase sensitivity to currents&#39; direction change. 
         [0007]    US 2009/0140524 to Keijha discloses a device with a single mooring line and single rotor turbine, but without discussion on the control of levelness and the roll and pitch motions. The operating position, as well as positions in different water flow conditions, may be very different from what is shown in the drawing. Keijha does not disclose the use of multiple, parallel, non-coplanar, equal-length tethering cable; use of differential tensions in multiple, spaced cables to resist rotational moments; or use of CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments. 
         [0008]    U.S. Pat. No. 7,737,570 to Costin discloses a device with a single mooring line and single rotor turbine. Costin depends on large and spread buoyant bodies and internal water tanks to provide stability for the device. The device of Costin is not completely submerged and there is a need to pump water in and out of the device. Costin does not disclose total submergence in the water; use of multiple, parallel, non-coplanar, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; use of yaw rotational capability by using two buoys, one rotating and the other non-rotating; or utilization of an optional weathervane. 
         [0009]    US 2010/0327583 to Hunt discloses a device tethered at one single point. Therefore, the levelness and stationary position of the device is maintained through the adjustment of the CO location and the hydrodynamic wings in response to the flow speed change. In US 2010/0326343, Hunt discloses a mooring method with minimum two cables connected to a single point on the device. The two-cable system as shown in Hunt cannot handle large flow direction change. Hunt does not disclose use of multiple, parallel, non-coplanar, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; use of yaw rotational capability by using two buoys, one rotating and the other non-rotating; and use of optional weathervane. 
         [0010]    U.S. Pat. No. 7,291,936 to Robson is similar to Hunt—US2010/0327583, except it uses twin counter-rotating rotor, single mooring cable. The ballast system is used to introduce pitch and adjust angle of attack to control the depth of the device. Robson does not disclose use of a single rotary turbine, not dual rotors with opposite spin direction; use of multiple, parallel, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; use of a yaw rotational capability by using two buoys, one rotating and the other non-rotating; and an optional weathervane. 
         [0011]    U.S. Pat. No. 8,766,466 to Dehlsen discloses two or more counter-rotating rotor assemblies. Dehlsen is a multi-rotor device with nonparallel mooring lines. Dehlsen also discloses that the vertical mooring line will not be engaged all the time. Dehlsen discloses that the wing depressor is used to provide a downward force in fast current. Dehlsen does not disclose use of a single rotary turbine; use of multiple, parallel, non-coplanar, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; use of a yaw rotational capability by using two buoys, one rotating and the other non-rotating; optional use of a weathervane. 
         [0012]    U.S. Pat. No. 7,541,688 to Mackie discloses a semi-submerged device, with a minimum of one surface piercing strut always staying above the water surface. It is therefore a fundamentally different device from the present invention, which is completely submerged. The extensive use of flaps, hydrofoil wings, and lateral thrusters as disclosed in Mackie would interrupt the flow and introduce large drag, reduce the efficiency of the device. Mackie does not disclose the use of multiple, parallel, non-coplanar, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; use of a CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments; use of yaw rotational capability by using two buoys, one rotating and the other non-rotating; or use of an optional weathervane. 
         [0013]    US 2010/02309711 to Mackie discloses the use of mooring cables with different angles to create the so-called “geofixed” buoy at the prescribed depth, essentially a virtual seabed above the natural seabed to provide an anchor place for the tethered turbine. The geofixed buoy and the turbine are two parts separated by long cable or tendon. The proposed mooring method of Mackie may be strong in restraining the linear movements of the buoy, but not as strong in restraining rotations. In several embodiments, the present invention has a rotational part and a non-rotational part closely attached through a shaft to form one device. The non-rotational part is not fixed in space. Rather, it moves as the speed and direction of the current changes, and is strong in resisting pitch and roll rotations. In several embodiments of the present invention the mooring cables maintain parallel, and the device translates without rotation during movement with the water flow. Mackie does not disclose use of multiple, parallel, non-coplanar, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; rotational capability by using two buoys, one rotating and the other non-rotating; or use of an optional weathervane. 
         [0014]    U.S. Pat. No. 7,492,054 to Catlin discloses a floating device used at the water surface of river/tidal stream, deployed in plurality and interconnected by cables. Catlin is mainly floating, unlike several embodiments of the present invention. Catlin does not address the issues facing a submerged device with single turbine. Catlin does not disclose: use of multiple, parallel, non-coplanar, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; use of CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments; us of yaw rotational capability by using two buoys, one rotating and the other non-rotating; or optional use of a weathervane. 
         [0015]    U.S. Pat. No. 8,272,831 to Johnston discloses a device with two counter-rotating rotors. It is totally submerged in the water and stay below the ship traffic. Johnston does not disclose: use of a single rotary turbine; uses of multiple, parallel, non-coplanar, equal-length tethering cables; use of differential tensions in multiple, spaced cables to resist rotational moments; use of CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments; yaw rotational capability by using two buoys, one rotating and the other non-rotating; or optional use of weathervane. 
         [0016]    U.S. Pat. No. 4,306,157 to Wracsaricht discloses devices primarily using solid stiff supports, but two of its embodiments do use tethering cables, which are not parallel. The turbine is suspended from the floats above, and the only possible way to provide a stiff support from the floats is to use high tensions in the suspension cables. However, it is difficult to do so without causing slack in other cables. There is no discussion on pitch and roll motion control, an important issue for all floating devices. The device of Wracsaricht is unable to rotate to align with the changing flow direction. Key differences from the present invention include: In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0017]    U.S. Pat. No. 6,109,863 to Milliken discloses a submersible buoyant apparatus using dual rotors with vertical axes. The vanes (blades) of the rotors consist of many sub vanes that can open and close in designated directions, and thus to drive the rotor to rotate as the current passes through the device. The global stability of the device, not the focus of the invention, is questionable as the device is fully submerged and using only one mooring cable. Key differences from the present invention include: In several embodiments, the present invention uses the rotary turbine that has its axis parallel to water flow direction and its blades sweeping in the plane normal to the water flow direction. In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses CG eccentricity method from self-weight and/or water weight an internal active ballast system to counter balance rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0018]    U.S. Pat. No. 6,531,788 to Robson discloses the use of a twin counter-rotating rotor, single mooring cable. In Robson, a ballast system is used to introduce pitch and adjust angle of attack to control the depth of the device. The Robson device does not disclose: In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0019]    U.S. Pat. No. 6,856,036 to Belinsky discloses a semi-submersible platform with a Darrieus type turbines (vertical axis) suspended under the platform to harvest the kinetic energy of the flowing current. The device of Belinski lacks: In several embodiments, the present invention is totally submerged in the water and stays below the ship traffic. In several embodiments, the present invention the rotary turbine has its axis parallel to water flow direction and its blades sweeping in the plane normal to the water flow direction. In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0020]    U.S. Pat. No. 7,682,126 to Parker discloses an airplane-like device with dual rotors and single mooring cable. Parker depends on ballast water for pitch and roll control. The device of Parker lacks: In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0021]    U.S. Pat. No. 7,902,687 to Sauer has the main focus of the invention on the parts and assembly of the modular device and generator. It is a different kind of turbine with many foil shaped blades on a horizontal axis perpendicular to the water flow. The device of Sauer is not able to rotate with the change flow direction, although flow reversal is claimed to be allowed. Sauer lacks: In several embodiments, the present invention the rotary turbine has its axis parallel to water flow direction and its blades sweeping in the plane normal to the water flow direction. In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0022]    U.S. Pat. No. 8,344,535 to Pitre discloses a platform-like device on water surface extracts power from the moving fluids on which it floats. Pitre uses helicoids flights (screw-like threads) blades on dual turbines. Pitre lacks the following from the present invention: In several embodiments, the present invention is totally submerged in the water and stays below the ship traffic. In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0023]    US 2007/0231072 to Jennings discloses a submersible planar platform, consisting of a number of impellors, to be secured in the water column by mooring cables. Jennings uses taut cables from the floor and to the water surface. Jennings lacks the following: In several embodiments, the present invention is totally submerged in the water and stays below the ship traffic. In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
         [0024]    US 2008/0018115 to Orlov discloses a semi-submersible platform for power harvesting from the ocean current partially above the sea surface and with dual turbines. The device of Orlov has no rotational capability. Orlov lacks the following: In several embodiments, the present invention is totally submerged in the water and stay below the ship traffic. In several embodiments, the present invention uses single rotary turbine, not dual rotors with opposite spin direction. In several embodiments, the present invention uses multiple, parallel, non-coplanar, equal-length tethering cables. In several embodiments, the present invention uses differential tensions in multiple, spaced cables to resist rotational moments. In several embodiments, the present invention uses CG eccentricity method from self-weight and/or water weight in internal active ballast system to counter balance rotational moments. In several embodiments, the present invention uses yaw rotational capability by using two buoys, one rotating and the other non-rotating. In several embodiments, the present invention utilizes a weathervane. 
       SUMMARY 
       [0025]    In several embodiments of the present invention, the present invention operates under several parameters. 
         [0026]    In several embodiments, the present invention is a buoyant power generation housing device that harvests power and generates electricity from water current. In some embodiments, the device is completely submersed under the water surface during operation. It consists of a single rotary turbine and a submerged, substantially hollow buoy. In some embodiments of the present invention, the rotary turbine has a plurality of rotational blades installed on a central shaft that is aligned to the water flow direction. The buoy houses the electricity generator, gear box, controller, and all other mechanical and electric parts for electricity generation and transmission as known in the art. 
         [0027]    In some embodiments, during operation the entire device has net positive buoyancy, and is anchored to the seabed with three or more tethering cables or tendons. The tethering cables are substantially equal-length, parallel but non-coplanar, and of substantially equal resistance capacity. The tethering cables are mechanically attached to the secure anchors on the seabed. In some embodiments, when the water flows the device moves to and stay at a new position where new equilibrium is achieved. The movement is substantially translation with minimum rotation due to the mooring method disclosed. The differential tension forces in the spaced tethering cables provide the buoy with substantial rotational resistance. 
         [0028]    In some embodiments of the present invention, the rotation blades located external to the buoy are in mechanical communication with the electricity generator in the buoy through the horizontal shaft. Electricity is generated in the normal manner in the generator when the shaft rotates driven by toque from the blades. 
         [0029]    In some embodiments of the present invention, the submerged buoy can have different shapes and arrangements, all based on the same principles disclosed in this document. In some embodiments of the present invention, the submerged buoy consists of longitudinal and transverse cylindrical-shaped tubes, interconnected in a symmetric manner about the horizontal axis aligned to the water flow direction. In some embodiments of the present invention, the submerged buoy consists of substantially saucer shape. 
         [0030]    In several embodiments of the present invention, the buoy contains water tanks or chambers and an internal active ballast system. The active ballast system moves water from lower water tanks to high water tanks after the buoy losses levelness. The resulting change of the CG location of the buoy and its content contributes to the counter balance of the applied moments on the buoy from the water flow and power harvesting. 
         [0031]    In several embodiments of the present invention, the combined use of the differential tensions in the spaced tethering cables and the CG eccentricity of the buoy and its ballast water results in a substantially leveled, stationary, and stable housing buoy in the flowing water, enabling the continuous energy extraction and electricity generation from the water current. 
         [0032]    In several embodiments of the present invention, the present invention comprises a fluid submersed energy generating housing apparatus comprising a first buoy with a substantially hollow interior, and a second buoy with a substantially hollow interior attached to the first buoy with the use of a vertical shaft. The second buoy is located on top of the first buoy, and is able to move in yaw rotation relative to the first buoy that does not rotate when it moves. 
         [0033]    In several embodiments, the first buoy is tethered to the seabed anchors through three or more tethering cables or tendons. The tethering cables are substantially equal-length, parallel but non-coplanar, and of substantially equal resistance capacity. 
         [0034]    In several embodiments, the rotary turbines with rotational blades are in communication with the second buoy through the horizontal shaft that is always aligned to current flow direction and extended into the electricity generator. The second buoy houses the electricity generator, gear box, controller, and all other mechanical and electric parts for electricity generation and transmission as known in the art. 
         [0035]    In several embodiments, when the water flow change direction the second buoy can rotate accordingly to align its longitudinal direction to the water flow direction. The turbine blades are therefore able to rotate in the plane substantially normal to the water flow direction. 
         [0036]    In several embodiments of the present invention, the first buoy contains water tanks or chambers and an internal active ballast system. The active ballast system moves water from lower water tanks to high water tanks when the first buoy losses levelness. The resulting change of the CG location of the first buoy and its content contributes to the counter balance the applied moments on the buoy from the water flow and power harvesting. 
         [0037]    In several embodiments of the present invention, the combined use of the differential tensions in the spaced tethering cables and the CG eccentricity of the first buoy and its ballast water results in a substantially stationary, stable, and level housing buoy in the flowing water, enabling the continuous energy extraction and electricity generation of the device from the water currents. 
         [0038]    In several embodiments, internal active ballast system can also be introduced inside the second buoy to help control the pitch motion of the second buoy. In some embodiments, the eccentric self-weight distribution in the transverse direction of the second buoy is used to control the roll motion caused by the rotary blades. 
         [0039]    In several embodiments of the invention, in the two-buoy embodiments, a fluid weathervane is further attached distal to the second buoy and adjacent to the rotation blades. The use of weathervane increases the device&#39;s sensitivity to water flow&#39;s direction change and enable applications in flows with direction reversal such as in tidal streams. The use of weathervane also reduces vibration in the direction of yaw rotation during operation. 
         [0040]    In several embodiments, the present invention is a fluid submersed energy generating housing apparatus comprising; a buoy with a substantially hollow interior; a rotary turbine comprising a body, rotation blades in communication with said body, and an axis in a substantially parallel direction to a fluid flow (wherein said body is housed within said buoy and said rotation blades in communication with said body); a mooring system comprising substantially equal length tethering cables wherein said tethering cables are substantially parallel and of substantially equal resistance capacity (said buoy being mechanically attached to said tethering cables); anchors mechanically attached with said tethering cables; and an active ballast system with chambers and pumps located in the hollow interior of said buoy (said active ballast system with chambers and pumps further comprising a water weight movement system in internal active ballast system to counter balance rotational moments of said buoy). In several embodiment of the invention said buoy with a substantially hollow interior is substantially saucer shaped. In several embodiments of the invention, said buoy with a substantially hollow interior comprises an assembly of substantially cylindrical shaped hollow sections. In several embodiments of the invention, said substantially equal length tethering cables comprise as least three in number. In several embodiments of said invention, said equal length tethering cables are arranged in a non-coplanar fashion. In several embodiments of the invention, said rotary turbine comprising blades and a body further comprises a multi armed body, with multiple turbines. 
         [0041]    In several embodiments, the present invention, is a fluid submersed energy generating housing apparatus comprising; a first buoy with a substantially hollow interior, a second buoy with a substantially hollow interior attached in rotational and mechanical communication with the first buoy; a rotary turbine comprising body, rotation blades in communication with said body, and an axis in substantially parallel direction to a fluid flow (said body being housed within said second buoy and said rotation blades in communication with said body); a mooring system comprising substantially equal length tethering cables wherein said tethering cables are substantially parallel and of substantially equal resistance capacity (said first buoy being mechanically attached to said tethering cables); anchors mechanically attached with said tethering cables; and an active ballast system with chambers and pumps located in the hollow interior of said first and second buoys (said active ballast system with chambers and pumps further comprising a water weight movement system in internal active ballast system to counter balance rotational moments of said first buoy and second buoy). In several embodiments of the present invention, said first buoy with a substantially hollow interior is substantially saucer shaped. In several embodiments of the present invention, said second buoy with a substantially hollow interior is substantially cylindrical shaped. In several embodiments of the present invention, said second buoy with a substantially hollow interior is further comprised of a fluid weathervane attached to the exterior of said second buoy. In several embodiments of the present invention, said substantially equal length tethering cables comprise as least three in number. In several embodiments of the present invention, a fluid weathervane attached distal to said second buoy and adjacent to said rotation blades. In several embodiments of the present invention said equal length tethering cables are arranged in a non-coplanar fashion. 
         [0042]    In several embodiments of the present invention, the present invention is a fluid submersed energy generating housing apparatus comprising; a first buoy with a substantially hollow interior; a second buoy with a substantially hollow interior attached in rotational and mechanical communication with the first buoy; a rotary turbine assembly comprising a body with arms, a plurality of rotation blades in communication with said rotary turbine assembly (said body with arms being partially housed within said second buoy and said plurality of rotation blades in communication with said rotary turbine assembly, in further communication with said body located external to said second buoy); a mooring system comprising substantially equal length tethering cables wherein said tethering cables are substantially parallel and of substantially equal resistance capacity (said first buoy being mechanically attached to said tethering cables; anchors mechanically attached with said tethering cables); an active ballast system with chambers and pumps located in the hollow interior of said first and second buoys (said active ballast system with chambers and pumps further comprising a water weight movement system in internal active ballast system to counter balance rotational moments of said first buoy and second buoy). In some embodiments, said first buoy with a substantially hollow interior is substantially saucer shaped. In some embodiments, said second buoy with a substantially hollow interior is substantially cylindrical shaped. In some embodiments, said second buoy with a substantially hollow interior is further comprised of a fluid weathervane attached to the exterior of said second buoy. In some embodiments, said substantially equal length tethering cables comprise as least three in number. In some embodiments, a fluid weathervane attached distal to said second buoy and adjacent to said rotation blades. In some embodiments, said equal length tethering cables are arranged in a non-coplanar fashion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions to be taken in conjunction with the accompanying drawings describing specific embodiments of the disclosure, wherein: 
           [0044]      FIG. 1  illustrates one embodiment of the present invention in one quarter side view. 
           [0045]      FIG. 2  illustrates one embodiment of the present invention in side view. 
           [0046]      FIG. 3A  illustrates one embodiment of the present invention from upstream without blades. 
           [0047]      FIG. 3B  illustrates one embodiment of the present invention from upstream without blades. 
           [0048]      FIG. 4  illustrates one embodiment of the present invention in side view. 
           [0049]      FIG. 5  illustrates one embodiment of the present invention in one quarter side view. 
           [0050]      FIG. 6  illustrates one embodiment of the present invention in one quarter side view. 
           [0051]      FIG. 7  illustrates one embodiment of the present invention in top view of partial cross section of the buoy. 
           [0052]      FIG. 8  illustrates one embodiment of the present invention from upstream view of the buoy with blades. 
           [0053]      FIG. 9  illustrates one embodiment of the present invention in side view. 
           [0054]      FIG. 10  illustrates one embodiment of the present invention in one quarter side view. 
           [0055]      FIG. 11  illustrates one embodiment of the present invention in side view. 
           [0056]      FIG. 12  illustrates one embodiment of the present invention in top view partial cross section of the buoy. 
           [0057]      FIG. 13  illustrates one embodiment of the present invention from upstream. 
           [0058]      FIG. 14  illustrates one embodiment of the present invention in one quarter side view. 
           [0059]      FIG. 15  illustrates one embodiment of the present invention in one quarter side view. 
           [0060]      FIG. 16  illustrates one embodiment of the present invention in one quarter side view. 
       
    
    
     DETAILED DESCRIPTION 
       [0061]    In the following description, certain details are set forth such as specific quantities, sizes, etc. . . . so as to provide a thorough understanding of the present embodiments disclosed herein. However, it will be evident to those of ordinary skill in the art that the present disclosure may be practiced without such specific details. In many cases, details concerning such considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present disclosure and are within the skills of persons of ordinary skill in the relevant art. 
         [0062]    Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing particular embodiments of the disclosure and are not intended to be limiting thereto. Drawings are not necessarily to scale. 
         [0063]    One, or more, illustrative embodiments incorporating the invention disclosed herein are presented below. Applicants have created a revolutionary and novel buoyant device for power extraction from fluid current using tethering cables with high tension and method of use for the same. 
         [0064]    While most of the terms used herein will be recognizable to those of ordinary skill in the art, it should be understood, however, that when not explicitly defined, terms should be interpreted as adopting a meaning presently accepted by those of ordinary skill in the art. In cases where the construction of a term would render it meaningless or essentially meaningless, the definition should be taken from Webster&#39;s Dictionary, 11th Edition, 2008. The following terms are defined for use in this application:
       Buoy: an enclosure, sealed during operation, with substantially hollow interior and net positive buoyancy. Buoy can be of large size to provide large buoyancy and large internal housing space for equipment, internal ballast system, etc.   CG Eccentricity: Refers to the scenario where the center of gravity (CG) of the buoy and its contents is not located on the vertical line that passes the center of buoyancy (CB). The offset or eccentricity is sometimes purposely introduced to counter balance the rotational moment the buoy is subject to.   Moment: A pair of forces with equal magnitude and opposite directions form a couple. The moment of a coupe is the product of the magnitude of the forces and their spacing. Moment causes an object to rotate in the same plane in which the opposite force couple occupies. Two parallel forces in the same direction but with different magnitudes contain a moment that is equal to the product of the spacing of the forces and their deviation to the average magnitude.   Seabed Anchor: Device with fixed location on the seabed, allowing connection to the tethering cables or tendons. It has adequate capacity to resist the pull-up forces from the tethering cable. It can comprise piles driven into the seabed soil/rock, or simply heavy blocks with large enough weight to overcome the buoyancy and maximum pull-up force.   Weathervane: A weathervane comprises a rod with one end rigidly connected to the yaw rotatable buoy and the other end free in the horizontal plane in the water. The free end is installed with a blade in the vertical plane so that any lateral pressure from water caused by the current direction change will force the weathervane to rotate. Rather than to point the current direction, the weathervane in this document is used to apply a rotational moment on the buoy to help it aligned to the current direction.       
 
         [0070]    Although several preferred embodiments of the present invention have been described in detail herein, the invention is not limited hereto. It will be appreciated by those having ordinary skill in the art that various modifications can be made without materially departing from the novel and advantageous teachings of the invention. Accordingly, the embodiments disclosed herein are by way of example. It is to be understood that the scope of the invention is not to be limited thereby. 
         [0071]      FIG. 1  illustrates one embodiment of the invention. In some embodiments, the invention is a buoyant power generation device  10  that harvests power and generates electricity from water current. As shown in this embodiment, the device  10  can be completely under the water surface  11  during operation. In some embodiments, the device  10  consists of a rotary turbine  20  and a stationary submerged buoy  30 . In some embodiments the rotary turbine  20  has a plurality of rotational blades  21  installed on a central shaft  22  that can be aligned to the water flow direction  12 . In several embodiments of the present invention, turbine  20  may be a turbine assemblies  120  and constructed in such a manner as to have multiple struts  127 . See  FIG. 16 . 
         [0072]    Buoy  30  may house the electricity generator, gear box, controller, and all other mechanical and electric parts for electricity generation and transmission as known in the art. These parts and equipment for underwater application are commercially available from vendors. The present invention enables larger scale underwater electricity generation and thus may stimulate the commercial manufacturers to develop larger capacity equipment to add to their product list. In several embodiments, the flow of water forces the blades  21  to turn, which drives the shaft  22  that extends into the generator located inside buoy  30  to spin. Electricity is generated as the shaft spins, in the normal manner used in other power generation occasions such as in the common wind turbines widely installed. In several embodiments, one aim of this invention is to disclose an effective method of building a stationary and stiff device or platform  10  placed under the water surface of the deep water region to house the electricity generating equipment. 
         [0073]    In some embodiments, the stationary buoy  30  can have different shapes and arrangements. In several embodiments, two horizontal axes are introduced for buoy  30 . The longitudinal axis  31  is preferably aligned to the direction of the flow of the water current  12 . The transverse axis  32  is preferably perpendicular to the water flow direction  12 . An additional vertical axis  33  can be introduced, which can be substantially perpendicular to the horizontal plane formed by axes  31  and  32 . 
         [0074]    As shown in  FIG. 1 , the submerged buoy  30  may consist of a longitudinal tube  34 , and two transverse tubes,  135  on left side and  235  on the right side. Axis  31  may also be the longitudinal axis of tube  34 , which therefore is aligned to the flow direction  12  during operation. In some embodiments, shaft  22  is extended into tube  34 . Tubes  34  and  135 ,  235  are to be kept as much in the level position as possible during operation. In several embodiments, device  10  consists of hallow enclosed tubes  34 ,  135 ,  235  that house equipment and also provide buoyancy. The device  10  should possess sufficient strength to resist the large water pressure but preferably its self-weight should be kept as small as possible. Steel, aluminum, and other alloy and composite materials are the possible candidates for the tube plates and shells. 
         [0075]    In some embodiments, during operation, the device  10  has net positive buoyancy, and is anchored to the seabed  13  with three or more tethering cables or tendons. In the three-cable configuration, as shown in  FIG. 1 , one cable  36  is connected to the longitudinal tube  34  at the upstream, and the other two cables  137  and  237  are connected to the ends of transverse tubes  135  and  235 , respectively. In some embodiments, high pretensions in tethering cables  36 ,  137  and  237  are purposely introduced through the use of the large net buoyancy of device  10 . In some embodiments, the tethering cables or tendons are anchored at the seabed  13  through the use of seabed anchors  14  or  15 , which are devices with fixed locations on the seabed, and are able to remain fixed when subject to forces. The anchors provided in this application have the adequate capacity to resist the pull-up forces from the tethering cables. The anchors can be piles  14  driven to the seabed soil/rock, or simply heavy blocks  15  with sufficient weight to overcome the buoyancy and maximum pull-up force. 
         [0076]    In some embodiments of the present invention, water flows in direction  12 . Buoy  30  is located at the upstream side while the rotary turbine  20  is located at the downstream side. The flowing water current  12  drives the blades  21  and shaft  22  to rotate about the axis  31  while buoy  30  remains stationary. In this embodiment, power is generated and transmitted out using underwater power lines  1036 , which can be (loosely) attached to the tethering cables near the device before connecting to the main transmission line. 
         [0077]    In several embodiments, turbine  20  used in this invention has a horizontal central shaft  22  aligned to the direction of the water flow  12 . In several embodiments, a single rotary turbine is used, which allows the use of long blades with a single-point support at the central shaft location. The support from buoy  30  to turbine  20  is sufficiently stiff and strong to resist the forces and moments on the turbine in all directions. The single turbine configuration is most efficient in energy collecting due to its large sweeping area, but poses the greatest rotary torque loads on the device. Most discussions in this document will be focused on embodiments with single turbine installation. However, the same principles can be easily applied to the multi-turbine installations that are subject to smaller applied torques. 
         [0078]    In several embodiments of the present invention, buoy  30  is stationary during operation. In some embodiments, buoy  30  contains ballast tanks that can be filled with water to make the whole system buoyancy negative during installation, as known in the art. However during operation, in some embodiments, the internal water content is reduced and buoy  30  has positive buoyancy. The blades  21  and shaft  22  may be designed and fabricated to be near buoyancy neutral, therefore the discussions on the buoyancy and the gravity in this document will be focused on the buoys only. 
         [0079]    As shown in  FIG. 2 , in several embodiments, the center of buoyancy CB and the center of gravity CO of the buoy  30  is devised to be near the center of the triangle formed by the three end connection points of the tethering cables  36 ,  137 , and  237 . High pretensions can be purposely introduced in the tethering cables through the use of large buoyancy of buoy  30 . The three cables have the equal length, and the locations of their anchors at the seabed mirror the connection points on buoy  30 . The cables are therefore parallel. 
         [0080]    In several embodiments, water flow not only causes the turbine  20  to spin about the longitudinal axis  31 , it also drags the device  10  to move to a new position in the water flow direction  12 . The horizontal drag force is resisted by the horizontal component of the tension forces in the tethering cables. 
         [0081]    As shown in  FIG. 2 , in several embodiments, the tethering cables  36 ,  137  and  237  have equal length and are designed to stay in the vertical direction in still water. In moving water the device is pushed by the water flow  12  and will move horizontally as demonstrated in  FIG. 2 . During the movement, the weight and buoyancy of the buoy  30  do not change, and the high tensions in the mooring cables  36 ,  137 , and  237  remain to be high. In some embodiments of the present invention, the cables will remain parallel to each other because of their equal length. The longitudinal tube  34  and shaft  22  will remain to be level during this movement in a steady and uniform flow condition. The tethering cables develop the same angle of tilt, a, in the new position in the moving water. The total tension force T of all tethering cables is aligned to the tilted cable direction. 
         [0082]    In some embodiments of the present invention, the equilibrium in the vertical direction requires that the vertical component of T, denoted by F, i.e., F=T cos α, be equal to the difference between buoyancy B and the weigh G as shown earlier (F=B−G). The total horizontal component, H, of the total cable force in the new position is equal to F tan α. Device  10  will cease to move once it reaches to the position at which the total drag force from the water flow, D, is equal to H. In this embodiment, buoy  30  will be stationary at this position as a state of equilibrium is achieved in the horizontal direction, while blades  21  spin continuously about axis  31  to generate the electricity. 
         [0083]    In some embodiments of the present invention shown in  FIGS. 1 and 2 , the connecting points of the tethering cables  36 ,  137 , and  237  are shown to be located at the elevation of shaft  22  in order to avoid the formation of an overturning moment from H and D, which causes pitch motion of tube  34 . More disclosures on pitch motion control are provided later in this document. 
         [0084]    At times, water current speed is usually highest at the depth in proximity to the water surface. It is therefore desired, in some embodiments to install the device in that high speed region for maximum energy harvesting. For example, in deep water ocean application device  10  can be installed at such depth that the tip  121  of blades  21 , when at their highest position, is thirty meters below the sea surface  11 . At this depth the device  10  stays clear of the majority of the navigational activities and avoid major wave forces at the sea surface. This desired location can result in a significant distance from device  10  to the seabed  13  to which the tethering cables are anchored. In some embodiments, cables  36  and  137 , and  237  can be tens, hundreds or thousands of meters long. In some deep water application, large variation of the angle of tile α will cause significant position change for device  10  in both the horizontal and vertical directions. Angle α will be larger in faster moving current in order to provide a greater horizontal restraining force (H) to resist the greater drag (D) associated to the higher current speeds. In some embodiments of the present invention, variation in the depth of the device location in different water speeds should be kept small in order to capture the energy of the high speed current near the sea surface  11  while keeping the device within the safe water depth range under different flow speed conditions. 
         [0085]    As shown in  FIG. 2  a large horizontal restraining force H can be achieved by increasing either angle α or the total pretension F. High pretension through the use of large buoyance of buoy  30  is preferred in this invention. When high pretensions are present in the tethering cables, a small angle of tilt will produce a large enough change to horizontal force component H to counter balance the drag force change. With the high pretension, the tethering cables will need to tilt only a small angle from the vertical position during operation; its depth change in response to current speed change will be small. 
         [0086]    The disclosed high pretension method is hence effective in the depth control and enables device  10  to stay within a small range of depth. In prior arts it was proposed to use hydrofoil wings or to adjust angle of attack for depth control. Those methods are problematic for the low flow speed application, and impose large disruption to the steady water current near the device that will compromise its efficiency and productivity. 
         [0087]    In some embodiments, when the current drives the blades  21  and shaft  22  to spin about axis  31 , it also drives the buoy to spin about the same axis and causes the roll motion of tube  34 . An overturning moment in the transverse vertical plane, M T , is applied to buoy  30  from the rotating blades ( FIGS. 3A and 3B ). While spin of the blades is welcomed, spin of the buoy must be restrained in order for buoy  30  to remain stationary and device  10  to work properly. The moment M T  must be counter balanced. A moment can only be countered by other moments in the same plane. A counter moment can be formed by two opposite force components spaced in the plane of rotation, equal to the product of the magnitude of the (opposite) forces and distance of their spacing. In several embodiments of the invention, two sources of force components are used to form the counter moments. One is the differential tensions in cables  137  and  237 . The other is the opposite gravity and the buoyancy forces, by introducing an offset/eccentricity between the two to form a moment. 
         [0088]    As shown in  FIG. 3A , in some embodiments, the rotation of the buoy  30  about axis  31  is restrained by cables  137  and  237 . Often in some embodiments, to hold a buoyant buoy  30  both cables  137  and  237  are in tension but at different levels when M T  is present, with larger tension in cable  137  than that in cable  237  for the shown direction of M T . The differential tensions, acting along with the spacing between the two cables, forms a counter moment that is in the opposite direction of the M T . 
         [0089]    It is the difference of the vertical components of the cable tensions (F in  FIG. 2 ) that contributes to the counter balance of the M T . The differential vertical forces result partly from the tension difference in the cables, and partly from the different tilt angles of the cables. For brevity the counter moment from the cable forces are generally being referred to as due to the differential cable tension forces throughout this document. It should be understood that whenever tension force change is called out for a tethering cable, it usually means that changes of both the tension force and the tilt angle occur to the cable. 
         [0090]    In some embodiments, using the above differential cable force method alone to counter M T  has limitations. In developing a higher vertical force from cable  137 , tube  135  moves higher than tube  235 , cable  137  has to be in a smaller angle of tilt than cable  237  due to the reason demonstrated in  FIG. 2 , which shows that a smaller angle α is associated with a higher cable top elevation. This cable angle difference of the two sides also causes the buoy to twist slightly about vertical axis  33 , resulting in tube  235  to stay slightly ahead of the tube  135  in the flow direction  12 . This twist about vertical axis  33  is not welcomed as it causes the blades to be in a plane not perpendicular to the water flow  12  and thus reduces the productivity of the device. The twist is small and acceptable if the overturning moment M T  is not large. 
         [0091]    A CO eccentricity method is based on the fact that in some embodiments, since the entire device is submerged in the water, the magnitude and the center of the buoyancy of the device do no change. In some embodiments the device is likely to have a symmetric shape, and therefore the CB is located at the geometric center of the surface shape. The force of the buoyancy is upward and the force of gravity is downward. If the weight of the device is so distributed that the CG is eccentric to the CB, a moment is formed between the gravity and the buoyancy. As shown in some embodiments in  FIG. 3B , tubes  235  and  135  can be fabricated with different self-weights. An eccentricity between CG and CB, e, is purposely introduced. The resulting moment due to the eccentricity is used to counter the overturning moment M T . Assume CB is in the central location, when the product of G and e equals M T , the overturning moment is balanced by this CG eccentricity moment alone. The tension forces in cable  137  and  237  are equal, and no twist about the vertical axis  33  mentioned earlier will occur. 
         [0092]    At times during operation the device is also subject to the overturning moment in the longitudinal plane, M L , which causes the pitch motion as shown in  FIG. 4 . Moment M L  may be developed due to the non-uniform distribution of the water speed in the vertical direction, which results in a total drag force D not located at the center of the turbine. The combined use of the cable differential tension method and the CG Eccentricity method to counter the rotary torque M T  disclosed earlier can also be used to counter the M L . As shown in  FIG. 4 , for a M L  caused by a drag force above axis  32 , cable  36  develops a larger tension than cable  137  and  237 . The force difference between the front cables  137  and  237  and the rear cable  36 , coupled with their spacing, forms a counter moment in the opposite direction of M L . Similarly, if the CG of buoy  30  is not coincident to CB but rather is located in the back side of the CB as shown in  FIG. 4 , another counter moment due to the eccentricity e between the upward B and the downward G is formed to counter M L . 
         [0093]    The eccentricities of CG just discussed are caused by the nonsymmetrical distribution of the material self-weight, and are therefore fixed in the transverse and longitudinal directions. The eccentricities can also be caused by contents inside the buoy and be variable with the use of the internal active ballast system. As demonstrated in  FIG. 5 , water tanks  38 ,  139  and  239  are installed at the ends of tubes  34 ,  135 , and  235  respectively. An internal active ballast system is installed to move water between these water tanks. Levelness sensors are installed in buoy  30  in both the longitudinal and transverse directions. The water tanks  38 ,  139  and  239  are connected by water pipes with pumps and valves inside tubes  34 ,  135  and  235 . If the slope of the buoy exceeds a preset threshold value, the internal pumps will be automatically activated and water will be pumped from the tank(s) at the lower elevation to the tank(s) at the higher elevation. The levelness of the buoy  30  will be restored. 
         [0094]    In several embodiments, before the pumps are activated, the tethering cable connected to the water tank with the highest elevation has the greatest tension. As the pumps are activated and water moves to this highest tank, its elevation is lowered and the CG of the buoy  30  moves toward this side. The tension in the cable connected to this tank is reduced while the tension in the cable connected to the tank with outgoing water increases. This active ballast system can dynamically move the CG of the buoy  30  in both the longitudinal and horizontal directions. Therefore, it can provide the required CG eccentricity to counter both M T  and M L . In some embodiments of the present invention, an active ballast system with water tanks  38 ,  139  and  239  is used in combination with highly tensioned parallel tethering cables to control both the pitch and roll motions and to keep the buoy level in both the longitudinal and transverse directions. 
         [0095]    In some embodiments the water tanks can be built inside the buoy.  FIG. 6  illustrates an embodiment of the invention in which buoy  70  has a circular disk shape with the disk plane positioned horizontally. As shown, four tethering cables that are tied to four points in a symmetric pattern at the outside edge of buoy  70 . Cables  172  and  272  are located in the front of the buoy  70  while cables  371  and  471  are located in the back side of buoy  70   
         [0096]      FIG. 7  illustrated the top view of one embodiment of an internal partition of buoy  70  with four (4) chambers provided to store the ballast water. The front chambers are labeled  172  and  272  to which the connections to tethering cables  171  and  271  are attached. Similarly, the rear chambers are labeled  372  and  472  to which the connections to tethering cables  371  and  471  are attached. The chambers are interconnected by pipes  73  with control valves  74  and pumps  75 . In many embodiments there are still sufficient open spaces inside buoy  70  and in the chambers to provide large buoyancy required to produce high tensions in the tethering cables. 
         [0097]    As shown in  FIG. 8 , if a M T  about axis  31  causes the buoy&#39;s left side to ascend and right side to descend, tensions in cables  171  and  371  on the left side will increase while tensions in cables  271  and  471  on the right side will decrease. As the buoy losses levelness in the transverse direction, the internal ballast system is activated and water in the right, lower chambers  272  and  472  will be pumped to the left, higher chambers  172  and  372 , which gradually restores the levelness of buoy  70  and reduces the tension difference between cables on the left side ( 171  and  371 ) and on the right side ( 271  and  471 ). As a result, chamber  172  and  372  have more ballast water than chambers  272  and  472  as shown in  FIG. 8  after levelness of the buoy is restored. 
         [0098]    As shown in  FIG. 9 , if a M L  about axis  32  causes the buoy&#39;s back side to ascend and front side to descend, tensions in cables  371  and  471  in the back row will increase while tensions in cables  171  and  271  in the front row will decrease. As the buoy loses levelness in the longitudinal direction, water in the front, lower chambers  172  and  272  will be pumped to the rear, higher chambers  372  and  472 . The water movement gradually restore the levelness of buoy  70  and reduces the tension difference between the front cables  171  and  271  and the back cables  371  and  471 . As a result, chamber  372  and  472  have more ballast water than chambers  172  and  272  as shown in  FIG. 9  after levelness of the buoy is restored. Water movement is in the inverse direction if M L  is in the direction opposite to that shown in  FIG. 9 . 
         [0099]    M T  and M L  usually are both present; therefore water movements in the longitudinal and transverse directions occur at the same time. The amount of water in each chamber is generally different. Many of the above disclosed embodiments can be used in water condition where the flow  12  has no or very little direction change. When the direction change occurs, the device  10  will twist about the vertical axis  33 . The tethering cables will no longer be parallel, and the buoy will lost levelness. The blades are not in the plane normal to the flow direction and even in danger of fouling the tethering cables. Therefore, for application in places with significant flow direction change such as in tidal streams, it is desired to minimize the twist moment about the vertical axis by introducing rotation capability of the turbine. Embodiments with this features are disclosed subsequently. 
         [0100]    In one embodiment of the present invention, as disclosed in  FIG. 10 , two buoyant buoys are used, namely, a rotatable cylindrical shaped buoy  50  installed on top of a nonrotating, circular disk shape buoy  70  with the disk plane positioned horizontally. An axial shaft  60  perpendicular to disk plane of buoy  70  is rigidly connected to the center of buoy  70 . Buoy  50  is also connected to shaft  60 , however, the connection allows buoy  50  to rotate about shaft  60 . Both buoys  50  and  70  have positive buoyancy. The blades  21  and shaft  22  are again assumed to be buoyancy neutral for the purpose of the discussion. 
         [0101]    In several embodiments, turbine  20  is connected to buoy  50 , which houses the electricity generator, gear box, controller, and all other mechanical and electric parts for electricity generation as known in the art. The generated electricity can be transmitted out through shaft  60  and buoy  70  before reaching the external transmission lines as known in the art (not shown). As the water flow changes direction, the top buoy  50  along with turbine  20  rotates accordingly about axis  33  so that the longitudinal direction of buoy  50  is always aligned to the water flow direction  12 . The blades  21  of the turbines will always spin in the plane that is normal to the water flow direction  12 . Buoy  70  also moves with the changing current direction  12  but mainly through translation. 
         [0102]    In some embodiments, by allowing buoy  50  to rotate to always maintain alignment to the varying water flow direction  12 , the rotational moment about the vertical axis  33  on buoy  70  is essentially eliminated. Only the moments about longitudinal axis  31  (M T ) and transverse axis  32  (M L ) need to be managed during operation. Axes  31  and  32  are the horizontal axes located in the central disk plane of buoy  70 . Axis  31  and axis  32  are always aligned and perpendicular to the longitudinal direction of buoy  50 , respectively, and thus change direction as buoy  50  rotates with the changing water flow direction  12 . 
         [0103]    As shown in  FIG. 11 , in some embodiments, an overturning moment M L  on buoy  70  about axis  32  is likely to be present all the time. Even when the velocity of the water flow is uniform, the center of drag force on turbine  20  and buoy  50 , denoted by Di in  FIG. 11 , is likely to act at the level close to the center of buoy  50  and thus higher than axis  32  that is in the central plane of buoy  70 . The tethering cables may be connected to points near the center plane of the buoy  70  where axes  31  and  32  are located. Therefore, D 1  will create a M L  in the counter-clockwise direction about axis  32  on buoy  70 . This moment can be partially countered using the buoyancy of buoy  50 . As shown in  FIG. 11  the buoyancy on Buoy  50 , denoted by B 1 , is located at the position that generates a moment in the clockwise direction about axis  32 , which is opposite to the moment generated by Di about axis  32 . 
         [0104]    Additional counter moment can be provided by adjusting the CG location of the buoy  50  using an active ballast system inside buoy  50  with only two chambers at the two ends in buoy  50 , as shown by chambers  151  and  251  in  FIG. 11 . The two chambers are connected by water pipes with valves and pumps. The CG location of buoy  50  can be dynamically adjusted in response to the variation of M L  due to the change of flow velocity and distribution. 
         [0105]    In some embodiments, additional resistance to M L  comes from the combined use of differential tensions in cables  171 ,  271 ,  371  and  471 , and the CG eccentricity resulting from ballast water movement in the active ballast system in buoy  70  following the same principle disclosed earlier.  FIG. 12  shows the top view of the components inside buoy  70  in one embodiment. The internal partition of buoy  70  is similar to the embodiment shown in  FIG. 7  except without the presence of the electrical generator, the gear box, and many other mechanical and electrical parts that are located in buoy  50 . Rather, an anchor base  77  for shaft  60  is present at the center. Notice that the tethering cables are always aligned to water flow  12  in the horizontal plane. Axis  31  is the longitudinal direction of buoy  50  (not shown) and therefore is also always aligned to the water flow direction. Axis  32  also changes direction to remain perpendicular to axis  31  when flow direction changes. Moment M L  causes buoy  70  to rotate about axis  32  and loss levelness in the direction along axis  31 . If as a result the area in the downstream side of axis  32  descends and the area in the upstream side of axis  32  ascends, tensions in the downstream side cables  171  and  271  will decrease and tensions in the upstream side cables  371  and  471  will increase. If the overturning moment is large, the internal ballast system is activated to move the water from chambers in the descending downstream side  172 ,  272  to chambers in the ascending upstream side  372 ,  472 . The tension difference in the cables are reduced and the levelness of buoy  70  is restored. The principle here is same as the one used in the embodiment shown in  FIG. 7 . 
         [0106]    In some embodiments, M T  is overturning moment about the longitudinal axis  31 , caused primarily by the rotation of the blades  21 . Resistance to M T  comes from the combined use of differential tensions in cables  171 ,  271 ,  371  and  471 , and the CG eccentricity resulting from ballast water movement in the active ballast system in buoy  70  following the same principle disclosed earlier. Refer also to  FIG. 12 , where axis  31  and axis  32  are aligned and perpendicular to the water flow direction  12 , respectively. Moment M T  causes buoy  70  to rotate about axis  31  and loss levelness in the direction along axis  32 . If as a result the area in the left side of axis  31  ascends and the area in the right side of axis  32  descends, tensions in the left side cables  171  and  371  will increase and tensions in the right side cables  271  and  471  will decrease. If the overturning moment is large, the internal ballast system is activated to move the water from chambers in the descending right side  272 ,  472  to chambers in the ascending left side  172 ,  372 . The tension difference in the cables are reduced and the levelness of buoy  70  is restored. The principle here is same as the one used in the embodiment shown in  FIG. 7 . 
         [0107]    As shown in  FIG. 13 , additional weight  52  can be added at the ascending side of buoy  50  due to M T  to counter the moment as the direction M T  with respect to buoy  50  does not change. In general, M L  and M T  are usually both present. The movements of ballast water take places in both directions at the same time, and the contents in all chambers are different. 
         [0108]    As shown in  FIG. 14 , in some embodiments, to help the turbine  20  and the top buoy  50  exhibit better response the direction change of the water flow, a weathervane  25  can be installed at the downstream. It is connected to the turbine through the use of the non-rotational axel member  26  that is aligned to the horizontal shaft  22 . The blade of the weathervane  25  is always in the vertical plane. A small side pressure on the blade  25  will cause the turbine  20  and buoy  50  to rotate about shaft  60 , making it sensitive to the change of the flow direction. The weathervane is especially helpful to rotate and align the device to the flow direction in the event of significant flow direction change, such as in tidal streams where the current direction reverses. It also reduces the potential rotational oscillation/vibration about the vertical axis, the irregular yaw movement due to flow turbulence. The weathervane if used needs to be located distant enough from the turbine blades  21  to avoid the turbulence cause by the movement of the blades. 
         [0109]    In some embodiments, axel  26  and shaft  22  are coaxial, and shaft  22  rotates with the blades. One method to install the weathervane can be to use a hollow cylindrical member for shaft  22 , which rotates outside the non-rotational axel  26 . Axel  26  must be rigidly connected to the inside of buoy  50 . The rotational movement of shaft  22  is transmitted to the power generator through the use of gears, bypassing the fixed connection base of axel  26  inside buoy  50 . The weathervane  25  and the connecting member  26  can be made buoyancy neutral. 
         [0110]    Another way to install the weathervane, in an alternative embodiment, can be to leave the connection joint between axel  26  and shaft  22  at downstream side of the blades. The two members have an overlapping segment that forms a joint. Rotation between the two members at the joint is allowed. The lower part of the weathervane blade  25  is made of material with much higher density than water, and the upper part of the weathervane blade  25  is made of light material that has positive buoyancy. The weathervane is therefore always in vertical position, and rotation of shaft  22  relative to the non-rotational axel  26  occurs at the joint during operation. 
         [0111]    As shown in  FIG. 15 , an alternative to weathervane  25  is to add the vertical fins  52  near the downstream end of buoy  50 . This embodiment is less sensitive to the water direction change, but may be a more economical solution if it is able to provide sufficient sensitivity to the flow direction change. 
         [0112]    In several embodiments, the disclosed invention provides an apparatus that creates a large vertical working plane normal to the water flow direction. Although in each embodiment shown in  FIGS. 1 to 15  the blades of a single rotary turbine occupy this vertical plane, other energy harvesting devices with different turbine types and numbers can be installed in the plane in lieu of the blades of the single turbine. For example, by modifying the embodiment shown in  FIG. 14 , a new embodiment is disclosed in  FIG. 16  in which a plurality of horizontal-axis turbine assemblies  120  are connected to buoy  50  by struts or arms  127 . Each turbine spins about its own axis and generate electricity in its own generator  128 . The struts or arms  127  rigidly connect the turbine assemblies  120  to buoy  50  instead of spinning about axis  31 . If opposite spin directions are used in the pairs of the turbines, the rotary torque about axis  31  on buoy  70  is significantly smaller than those in the single turbine embodiments shown in  FIGS. 1 to 15 . Therefore, the principles disclosed early on embodiments with single turbine are directly applicable to the less loaded device with multiple turbines such as the one shown in  FIG. 16 . Other embodiments shown in  FIG. 1 to 15  can be modified similarly to use multiple turbines in lieu of single turbine. 
         [0113]    Although several preferred embodiments of the present invention have been described in detail herein, the invention is not limited hereto. It will be appreciated by those having ordinary skill in the art that various modifications can be made without materially departing from the novel and advantageous teachings of the invention. Accordingly, the embodiments disclosed herein are by way of example. It is to be understood that the scope of the invention is not to be limited thereby.