Abstract:
The underwater electrical generator includes a stator core and counter-rotating turbine blades with permanent magnets in the hubs and windings in the core housing. When tethered by the nose of the housing, the water current rotates the turbine blades generating electrical current which is supplied for commercial use via the tether to a land-based utility. Ballast tanks are provided to maintain the generator at a predetermined depth. Small stator windings and permanent magnets are provided in the rotating hubs and core housing respectively to charge batteries in the hubs for altering the pitch of the blades such that the blades can be feathered for maintenance outages.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a generator responsive to water current for generating electricity and particularly relates to an underwater electrical current generator having turbine blades rotatable about a center body. 
     With increasing demand for environmentally friendly electrical power generation, many alternatives to carbon produced electrical power have been proposed. For example, in U.S. Pat. No. 3,209,156 there is disclosed an underwater generator for supplying limited electrical power to underwater measuring and sensing units. The supplied electricity provides power for underwater communications and management of oceanographic instrumentation. The generator in the afore-mentioned patent includes a central housing mounting a plurality of circumferentially spaced blades which rotate a hub journaled on the housing such that an arrangement of permanent magnets and stator windings generate electricity for powering the instrumentation. The disclosed generator, however, turns at relatively low speed compared to conventional turbines and the resulting power production potential is quite low. Commercial generation of electricity for land use is not particularly feasible employing the construction of that patent. Accordingly there is a need to develop an underwater current generator for producing electricity for commercial land uses. 
     In a preferred embodiment of the present invention, there is provided an electrical generator for disposition in a water current comprising: a central body carrying a plurality of stator coils within the body and defining a generally longitudinal axis; a pair of annular hubs rotatable about and carried by the central body, each of the hubs mounts a plurality of turbine blades and a plurality of magnets for generating magnetic flux, the hubs being located about the stator coils enabling generation of electricity by the interaction of the magnetic flux and the coils in response to rotation of the blades and hubs about the central body; the blades being raked back in the direction of the water current and having free ends. 
     In another embodiment of the present invention, there is provided an electrical generator for disposition in a water current comprising a central body carrying a plurality of stator coils within the body and defining a generally longitudinal axis; a pair of annular hubs rotatable about and carried by the central body, each of the hubs mounting a plurality of turbine blades and a plurality of magnets for generating magnetic flux, the hubs being located about the stator coils enabling generation of electricity by the interaction of the magnetic flux and the coils in response to rotation of the blades and hubs about the central body; at least portions of the blades being rotatable about axes non-parallel with the longitudinal axis to enable rotational movement of said blade portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a water current electrical generator constructed in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a cross sectional view thereof; 
         FIG. 3  is a fragmentary enlarged cross sectional view thereof illustrating a method and apparatus for tethering the blades during battery recharging operations; 
         FIG. 4  is an enlarged fragmentary cross sectional view illustrating a water bearing; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing figures, particularly to  FIG. 1 , there is illustrated an underwater generator, generally designated  10 , for producing electricity and including a stator core housing  12  and a rotor including at least a pair of counter rotating impeller or turbine blades  14  and  16  mounted on counter-rotating turbine hubs  18  and  20 . As illustrated, the impeller blades  14  and  16  mounted on the hubs are circumferentially spaced one from the other and are open at their radially outer ends i.e. the tips are not connected to one another. The generator  10  is constructed for tethering to an underwater anchor  22 . The tether  24  interconnects the anchor  22  and a coupling  26  on a nose of the stator housing  12 . Additionally, as illustrated in  FIG. 1 , electricity generated by the generator  10  is preferably transmitted by electrical lines carried by tether  24  to the anchor and then via electrical lines  28  to a land base utility site for commercial distribution. 
     As illustrated in  FIG. 2 , each of the hubs  18  and  20  carries a plurality of circumferentially spaced magnets  32  in annular axially spaced arrays thereof. In radial opposition to the permanent magnets  32  are a plurality of stator coils  34  wound about iron cores and carried by the core housing  12 . It will be appreciated that as the hubs  18  and  20  rotate relative to the fixed stator core, the magnetic flux in cooperation with the stator windings produces electricity. Because electric power generation is a function of the water current velocity cubed, it is very beneficial to locate the device in high current areas and to enhance the velocity of the water current passing over and through the blades. To this end, by having a large housing located in the center of the device, water which would normally pass through this area, must pass around the central housing. This increases the velocity of the water passing through the blade area, enhancing the power production. For example, for a one half megawatt turbine, the core housing diameter may be approximately 14 feet with an overall diameter of 34 feet. With those dimensions and the center of the generator plugged by the core housing  12 , the velocity through the rotor blades is raised substantially e.g., about 17% resulting in an increase in theoretical power availability of about 60%. 
     Referring to  FIGS. 2 and 4 , the hubs  18  and  20  each terminate in tapered aft-bearing surfaces. For example, bearing surface  40  for hub  20  is tapered or inclined in an aft and radial outward direction. The cooperating bearing surface  42  on the stator core housing  12  is similarly tapered in an aft, radial outward direction. The surfaces  40  and  42  are preferably formed of a polymeric material. As a result of the taper of the surfaces, the water current acting on the blades, seats and centers the hubs  18  and  20  on axis against the tapered surfaces  42  of the core housing  12 . Additionally, a hydrodynamic film is provided between the bearing surfaces  40  and  42 . This is accomplished by a pump  46  located within the core housing  12 . Pump  46  draws in sea water and pumps the sea water between the bearing surfaces  40  and  42  via conduits  48 . The hydrodynamic film reduces friction and wear and is especially beneficial during start-up since once the turbine blades are up to speed, the blades are centered magnetically. This additionally would assist in offsetting axial thrusts at speed. Further, the filtration of the forced lubricating water by the installation of magnets and filters in the suction line to the pump would help keep debris from penetrating the gap between the stator and rotor and maintain magnetic materials away from the magnetic areas caused by the permanent magnets in the rotor and the coils in the stator. 
     As best illustrated in  FIG. 2 , the blades  14  and  16  are raked back in the direction of the current flow. Additionally, as illustrated in  FIG. 1 , the blades  14  and  16  are open at their outer ends. This assists in maintaining the blades free of debris enabling the blades to slough off debris from the rotor. 
     As previously noted, the rotors are counter-rotated. Thus the net torque on the core housing can be balanced out to zero whereby the housing  12  is prevented from spinning and is maintained in a substantially non-rotatable orientation. While the blades preferably counter-rotate independently of one another, the blades may be geared to one another so that they turn at the same rotational speed. This can be accomplished with mechanical gearing or by placing thrust wheels between the two rotors. Also, with the two stators are tied together electrically, the two rotors will be forced electrically to turn at the same rotational speeds which may eliminate any need for mechanical synchronization. Further, the forward rotor is anticipated to be more efficient in the water current than the aft rotor. The aft rotor may therefore have a larger diameter than the forward rotor in order to substantially balance the torques. 
     Ballast compartments are provided in the core housing  12 . Particularly, a forward ballast compartment  50  and an aft ballast  52  are provided. A pump  54 , illustrated in  FIG. 2 , is provided for pumping sea water to and from the fore and aft ballast compartments  50  and  52 , respectively. The pressure side of the pump  54  communicates through a valve  56  with either the forward ballast compartment  50 , the aft ballast compartment  52 , or both compartments. The ballast compartments may be ballasted to maintain the generator at an adjustable and optimal depth below the water surface. The device can therefore be raised or lowered to optimize power production by locating the device in the highest current. Additionally, should the current be too strong at a given depth, alternate depths can be achieved by adjusting the ballast. The ballast compartments as illustrated are located in the lower portion of the core housing leaving the upper portion of the core housing free for installation of necessary electrical equipment. Consequently, the ballast acts as a pendulum and facilitates preventing the generator central body from rotating. Since there is only one point connection i.e., the tether  24 , the generator is free to pivot in any direction around the anchor point to remain oriented into the current without causing excessive strain on one side or the other. Also, by having both a fore and aft ballast tank, the attitude of the device in the current can be adjusted. 
     The rotation of the blades can be stopped when maintenance on the generator is necessary notwithstanding the continuous flow of the underwater current. Thus, it may be necessary to stop the rotation of the blades and the generation of electricity to service the generator. Stopping the generation of electricity may be accomplished by disconnecting the load. However, this does not stop the rotation of the blades and may increase their rotational velocity in the absence of the load. Moreover, the generator may continue to create voltage and that voltage and parasitic currents could be hazardous to maintenance personnel, as well as to the equipment due to overheating and insulation breakdown. 
     To stop the blade rotation, and to stop the rotation of the blades which would otherwise increase the drag of the generator and hence increase the load on the tether, the rotor blades are feathered. As illustrated, the blades are twisted along their lengths and there is no typical position of the blades, even when feathered, which would absolutely prevent rotation. The turbine blades  14  and  16  are therefore preferably formed in two parts i.e. an inner blade portion  58  and an outer blade portion  60 . A shaft  62  ( FIG. 3 ) interconnects the outer and inner blade portions. Consequently, by rotating the shaft  62  relative to the inner blade portion  58 , the outer blade portion  60  may be rotationally aligned into a feathered position, i.e., a position with twists in the opposite direction than the twist of the inner portion  58  of the blade. The opposite twists of the inner and outer blade portions thus tends to feather the overall blade. 
     The underwater environment of the electrical generator poses the problem of how to feather the blades in that environment. Referring to  FIG. 3 , there is provided, preferably in each of the hubs  18  and  20 , a field winding  70  in opposition to a plurality of permanent magnets  72  on the stator housing  12 . Both the hub and the stator housing serve as a stator and rotor respectively. The stator windings  70  may be very small and only generate sufficient power to maintain a storage charge on batteries located in the hub e.g., battery  74 . The batteries  74  in turn are coupled to an electrical actuator  76  mechanically coupled to the shaft  62  to rotate the shaft and hence the outer portion  60  of the blade. Mechanical interlocks between the inner and outer blades may be provided once the outer portions  60  of the blades are rotated to the desired operational and feathered positions. The stator winding  70  and permanent magnets  72  do not impose a drag on the rotation of the hubs except when charging the battery  74 . When fully charged, there is no load imposed on the hub. Electrical signals to effect feathering of the blades i.e. actuation of the actuator  76  can be accomplished by signals between the core housing  12  and the rotors. The outer portions of the rotors also can be adjusted relative to one another to balance the load carried by the generator. Consequently when the generator is taken out of service e.g., for maintenance, the outer portions of the blades are rotated to a position where there is little or no resulting torque created by the water current on the blades. Additionally, when feathered, the resulting drag on the generator body is dramatically less than if the blades are merely stopped e.g., by brakes. Once the blades are stopped, the ballast in the core housing  12  is adjusted to float the generator to the surface for maintenance. In this manner, the electrical and mechanical hazards due to rotor rotation can be minimized. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.