Abstract:
Motion of a first component in response to waves is converted to rotary motion of a member of a second component. The two components are magnetically coupled to each other. The relative linear motion of the components causes energy to be transmitted from waves between the two components via the magnetic coupling, and thus no mechanical connection is required for the transmission. This can allow for wave energy conversion without a need for hydraulic or pneumatic systems. Applications for technologies described herein include ocean wave energy converters (OWEC) for generating electricity from wave energy.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/673,209 filed on Apr. 19, 2005, which is incorporated herein by reference. 
     
    
     ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT 
       [0002]    At least some research related to this application was funded by Oregon Sea Grant, contract numbers R/Ec-11-PD and r/Ec-13-PD. The U.S. Government may have some rights in this invention. 
     
    
     FIELD 
       [0003]    This application relates to generating electricity from ocean wave energy. 
       BACKGROUND 
       [0004]    Ocean waves are a potential source of energy for generating electricity. Commonly proposed energy extraction techniques are often based on hydraulic or pneumatic intermediaries that can require high maintenance costs and are often prone to failure. Under operating conditions such as heavy seas, the intermediaries can be damaged by excessive force of the waves. 
       SUMMARY 
       [0005]    Linear motion in response to waves can be converted to rotary motion by moving a first component that is magnetically coupled to a second component. The relative linear motion of the components causes energy to be transmitted from waves between the two components via the magnetic coupling, and thus no mechanical connection is required for the transmission. This can allow for wave energy conversion without a need for hydraulic or pneumatic systems. Applications for technologies described herein include ocean wave energy converters (OWEC) for generating electricity from wave energy. Additionally, the technologies can be used generally in situations where a conversion between linear and rotary motion is desired. 
         [0006]    In one embodiment, a system for converting wave motion to rotary motion (the system being at least partially immersed in a liquid) includes a first component, the first component having an overall buoyancy relative to the liquid, a second component slidably coupled to the first component, and at least one screw rotatably supported by the second component. The first component is configured to slide relative to the second component in response to a force from waves that is exerted on the first component. The first component is magnetically coupled to the screw, and a sliding of the first component relative to the second component in at least one direction causes a rotation of the screw. The sliding of the first component relative to the second component can be relative linear motion. 
         [0007]    In a further embodiment, the first component can include a ferrous metal, and the second component can also include a magnet and a ball screw nut, where the ball screw nut is generally coaxial with the screw, and where the ferrous metal is configured to transfer the force to the ball screw nut through the magnet, which can be a ring magnet. The second component can also include a generator, and the screw can be configured to transfer rotary motion of the screw to the generator. The magnet can be one of a plurality of magnets, and where at least two magnets of the plurality of magnets are separated by a metal pole piece. In one embodiment, the ferrous metal of the first component is generally cylindrical in shape and has one or more salient features. 
         [0008]    An additional embodiment comprises two or more second components which can be configured to transfer energy from the screws of the second components to a generator. 
         [0009]    In another embodiment, the first component also includes a magnet, wherein the second component further also includes a ferrous metal mechanically coupled to a ball screw nut, and wherein the magnet is configured to transfer the force to the ball screw nut through the ferrous metal. Instead of a ball screw, a roller screw can be used. 
         [0010]    In another embodiment, the first component includes a float, and the second component includes a spar. Desirably the spar in one form is approximately neutrally buoyant relative to the liquid. The float can include an opening, such as a central opening, into which the spar is inserted, and the system can also include a mooring system for anchoring the first and second component at an offshore area. The second component can also include a generator coupled to the screw and adapted to generate electricity in response to rotation of the screw, with electrical conductors configured to transmit electricity to a location that is remote from the first and second components. The second component can comprise a hollow interior, and the screw can be entirely contained in the hollow interior to eliminate the requirement of working seals to prevent liquid from entering the interior of the second component. 
         [0011]    In a further embodiment, the first component includes one or more magnets, and the screw includes one or more materials exhibiting generally high electrical resistance and generally low magnetic resistance, such as a silicon iron alloy. The first component can include at least two pole shoes adjacent to the one or more magnets, wherein the pole shoes comprise a main piece and a thread, and wherein the thread extends along at least part of the main piece. The main piece can have a length, and the thread can extend in a generally non-parallel manner along at least part of the length. The at least two pole shoes can include a first pole shoe with a top side and a bottom side, wherein the one or more magnets comprise a first magnet having a north pole and a south pole and a second magnet having a north pole and a south pole, and wherein the north pole of the first magnet is adjacent to the top side of the first pole shoe and the north pole of the second magnet is adjacent to the bottom side of the first pole shoe. The pole shoes can be made of one or more materials exhibiting generally high electrical resistance and generally low magnetic resistance. The screw has a longitudinal axis, and the pole shoes generally extend around the longitudinal axis. 
         [0012]    Another embodiment is an ocean wave energy conversion system including a float and a spar. The spar desirably includes a tube and a screw inside the tube, wherein the float and the spar are configured to undergo relative linear motion as a result of a force applied to the float, and wherein the relative linear motion causes the kinetic energy to be transferred from the float to the screw substantially without a mechanical connection between the float and the spar. The float is magnetically coupled to the spar and can be configured to become magnetically decoupled from the spar when a threshold force is applied to the float. A generator can be mechanically coupled to the screw. 
         [0013]    In another embodiment, a system for converting wave motion to electricity (where the system is at least partially immersed in a liquid) includes a float, the float having an overall buoyancy relative to the liquid; a spar, the spar having an approximately neutral buoyancy relative to the liquid; a screw rotatably supported by the spar component; and a generator. The float is configured to undergo linear movement relative to the spar in response to a force from waves that is exerted on the float. The float is magnetically coupled to the screw, and the movement of the float relative to the spar causes a rotation of the screw, and the screw is configured to transfer rotary motion of the screw to the generator. The system can also include a clutch that is mechanically coupled to the screw and the generator. 
         [0014]    In this application, indefinite articles such as “a” or “an” and the phrase “at least one” encompass both singular and plural instances of objects. For example, when describing a group of multiple objects, “an object” includes one or more than one of the multiple objects. 
         [0015]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter. The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  shows a side view of one embodiment of an ocean wave energy converter system. 
           [0017]      FIG. 1B  shows a top view of the ocean wave energy converter system of  FIG. 1A . 
           [0018]      FIG. 2A  shows a side cross-section view of one embodiment of an ocean wave energy converter system. 
           [0019]      FIG. 2B  depicts a side cross-section view of a magnet piston assembly. 
           [0020]      FIG. 2C  depicts a side cross-section view of an alternate embodiment of the system of  FIG. 2A . 
           [0021]      FIG. 3  depicts side cross-section views of example magnet configurations for a magnet piston assembly. 
           [0022]      FIG. 4  depicts a plot of example finite element analysis results for some configurations of  FIG. 3 . 
           [0023]      FIG. 5  depicts a plot of example generator test results of generator rotation speed as a function of thrust. 
           [0024]      FIG. 6  depicts a plot of example generator test results of generator current as a function of thrust. 
           [0025]      FIG. 7  depicts a plot of example generator test results of generator efficiency as a functions of generator power output. 
           [0026]      FIG. 8  depicts an example equivalent circuit of a permanent magnet synchronous generator. 
           [0027]      FIG. 9  shows a graph of simulated voltage generation for the system of  FIG. 2A . 
           [0028]      FIG. 10  shows a sample oscilloscope waveforms showing a no-load voltage generation for the system of  FIG. 2A . 
           [0029]      FIGS. 11A-11C  show sample oscilloscope waveforms from test results of the system of  FIG. 2A  for output voltage, output current, and output power, respectively. 
           [0030]      FIGS. 12A-12C  show sample oscilloscope waveforms from irregular wave test results of the system of  FIG. 2A  for output voltage, output current, and output power, respectively. 
           [0031]      FIG. 13A  shows a cross-section side view of one embodiment of an ocean wave energy converter system. 
           [0032]      FIG. 13B  shows a close-up cross section side view of a magnet assembly and center screw. 
           [0033]      FIG. 13C  shows a close-up cross section side view of a magnet assembly. 
           [0034]      FIG. 13D  shows a top cross-section view of the embodiment of  FIG. 13A . 
           [0035]      FIG. 14A  shows a side view of one embodiment of an ocean wave energy converter system featuring multiple spars. 
           [0036]      FIG. 14B  shows a top view of the system of  FIG. 14A . 
           [0037]      FIG. 14C  shows a bottom view of the system of  FIG. 14A . 
       
    
    
     DETAILED DESCRIPTION 
     System Overview 
       [0038]      FIG. 1A  shows a side view of one embodiment of a buoy generator system (i.e., an OWEC system)  100 . The buoy generator system  100  comprises an elongated spar  110  and a float  120 . Spar  110  can have a cross section that is round, square, or a number of other shapes, is desirably at least partially hollow, and is preferably constructed of a material that can withstand ocean conditions for a relatively long period of time, such as PVC or composite material. Float  120  is coupled to spar  110  for movement relative to the spar. Desirably the float  120  encircles spar  110  at least in part, but preferably entirely, and can be comprised of any number of buoyant materials as are well known in the art. System  100  can further comprise a ballast weight  130  and a tether  140 .  FIG. 1A  shows tether  140  as being connected to ballast weight  130 , but it can also be connected to other parts of system  100 , e.g., to spar  110 . The remote end of tether  140  is connected to a mooring system  142 , which can be any system or arrangement that allows the system  100  to maintain a relatively constant geographic position. For example, the mooring system could comprise a weight such as an anchor or pilings. An electric cable  144  carries electricity from the system  100  to another location, e.g., a shore-based electric facility  146 . The top end of spar  110  can be sealed by, for example, a cap  150  to protect its contents from the elements. Spar  110  is preferably configured such that it (with its contents) is approximately neutrally buoyant. System  100  can also comprise a wave deflector or wave motion resistor, such as a wave plate  145 , which can be attached or coupled to spar  110 , usually at a right angle to spar  110 . However, wave plate  145  can also be attached at other angles. Wave plate  145  can provide a dampening force to improve a desirable relative linear motion of float  120  and spar  110 .  FIG. 1B  shows a top view of system  100 . 
         [0039]    Generally, generator system  100  can be moored offshore in an area where waves are common. As waves propagate past system  100 , the waves move float  120  generally upwardly and downwardly relative to and along spar  110 . System  100  converts at least some of the relative motion provided by the waves to rotary motion, which is used to turn an electric generator. As will be shown in example embodiments below, system  100  can accomplish this conversion with float  120  and with a power take-off (PTO) system (not shown) inside spar  110 . Preferably, there is a magnetic coupling, but no mechanical coupling, between float  120  and the PTO system inside spar  110  that requires a breach of the wall of spar  110 . (In this application, the term “coupled” encompasses both the direct interconnection of elements and also their indirect connection through or by one or more components.) 
         [0040]    It should be noted that although the motion of float  120  to spar  110  can be described and is often described in the application as “relative linear motion,” other types of motion can also be used. For example, if spar  110  is curved, float  120  can slide along spar  110  in an arcuate motion. In some embodiments, float  120  cans spin relative to spar  110 , but these spins can be dampened by the inertia of float  120 , which can be designed to be larger than that of spar  110 . 
         [0041]    One potential advantage of relying on a magnetic connection (rather than a mechanical connection) is increased durability in severe conditions (e.g., rough seas) of the systems described above. For example, float  120  can be configured to “slip” when a force exceeding a selected threshold is applied to it. When the rough see conditions subside, it can slide back into place on spar  110  and resume normal operation. Cap  150  and plate  145  prevent total separation of float  120  and spar  110  in this example. 
       System Using a Contact-Less Force Transmission System 
       [0042]      FIG. 2A  shows a side cross-section view of system  200  (taken along the lines  2 A- 2 A indicated in  FIG. 1B ), which is one embodiment of system  100  of  FIG. 1 . In this particular embodiment, float  220  comprises air or other buoyant material  223  formed around a concentric cylinder  225  of a ferrous metal such as steel. Cylinder  225  can be the same height as buoyant material  223 , or it can be taller or shorter. Spar  210  forms a cavity  215  which contains at least one ball screw  260 , which can be coaxial with spar  210 . Ball screw  260  can be held in place by cap  250  and desirably is rotatably coupled thereto by a bearing (not shown), but desirably not exposed to the exterior of the cap. In one embodiment, cap  250  is large enough to prevent float  220  from sliding off of spar  210  in, for example, rough seas. A magnet piston assembly  270  is mounted on ball screw  260 . System  200  can also comprise a wave plate  245 . 
         [0043]      FIG. 2B  depicts a side cross-section view of magnet piston assembly  270  in more detail. Magnet piston assembly  270  comprises one or more permanent magnets  272 . Multiple magnets  272  can be interspersed with pole pieces  274 , and both are preferably concentric with ball screw  260 . It is also preferable, but not required, that magnets  272  and pole pieces  274  be generally ring-shaped and completely encircle ball screw  260 . As defined herein, a magnet  272  that is described as “ring-shaped” or as a “ring magnet” can comprise two or more magnets configured to approximate the magnetic performance of a one-piece ring magnet.  FIG. 2B  depicts gaps  284  between pole pieces  274  and harness  282 . These gaps can be of varying sizes or non-existent. 
         [0044]    Generally speaking, magnets  272 , cylinder  225  and ball screw  260  together comprise a ferromagnetic reluctance device, sometimes herein called a contact-less force transmission system (CFTS). Magnets  272  squeeze magnetic flux radially through a central pole piece into cylinder  225 . As float  220  (and cylinder  225 ) moves up and down, a reluctance force develops and is transmitted from cylinder  225  to magnets  272  through the magnetic field that develops between these components. By means not shown in  FIG. 2B , magnets  272  and pole pieces  274  are mechanically connected (e.g., by welding, fasteners or other connections) to a harness  282  and one or two ball screw nuts  280 . Nuts  280  are concentric with ball screw  260 . As float  220  moves up and down, magnet piston assembly  270  is pulled up and down, pushing or pulling ball screw nuts  280  along ball screw  260 , causing ball screw  260  to rotate. Linear motion is thus converted to rotary motion. It should be noticed that that rotary motion can be converted to linear motion by generally reversing this process, e.g., by rotating screw  260  to cause relative linear motion of cylinder  225 . 
         [0045]    Returning to  FIG. 2A , the rotary motion of ball screw  260  turns a coupling  290  and a clutch  291 . Clutch  291  can be a one-way clutch or a two-way clutch. Direct, clutchless coupling is a less-desirable approach. Plate  293  can be added to cavity  215  to protect coupling  290  and clutch  291  from impact with, for example, ball screw nut  280 . Other alternative stop mechanisms can be used. Clutch  291  turns a shaft  294  on electric generator  292 . Accordingly, coupling  290  and clutch  291  comprise one form of an exemplary power take-off (PTO) system. Although this particular embodiment depicts coupling  290 , clutch  291  and generator  292  as being at the bottom end of spar  210 , they can also be arranged at the top of spar  210 . Additionally, in the embodiment depicted in  FIG. 2A , generator  292  is small enough to fit inside spar  210 . This can allow for a greater range of travel of float  220  along the length of spar  210 . In other embodiments, generator  292  can be positioned outside of spar  210 . In such an embodiment, generator  292  can have a diameter greater than that of spar  210 . 
         [0046]    In another embodiment, magnets  272  and metal plates  274  are not inside spar  210 , but are integrated into float  220  in place of cylinder  225 . Cylinder  225  is positioned in spar  210  and mechanically connected to harness  282  and ball nuts  280 , approximately where magnets  272  and metal plates  274  are in the embodiment described above. 
         [0047]      FIG. 2C  depicts another embodiment of system  200 . In this particular embodiment, ball screw  260  and ball screw nuts  280  are replaced with a screw shaft  261  and a roller screw nut  281 , respectively. As roller screws are well known in the art, the inner workings of roller screw nut  281  are omitted from  FIG. 2C . As float  220  moves up and down, magnet piston assembly  270  is pulled up and down, pushing or pulling roller screw nut  281  along screw shaft  261 , causing screw shaft  261  to rotate. In one embodiment, a roller screw nut  281  is on each end of harness  282 . 
         [0048]    Similar to system  100  of  FIG. 1 , system  200  can contain a ballast weight  230  and can be kept in place using a tether  240 . In one embodiment, sea water can be used as ballast, which can allow for tuning of the ballast weight according to output power and sea state. 
         [0049]    As mentioned above, in some embodiments float  220  can be configured to “slip” when a force exceeding a selected threshold is applied to it. In one embodiment, a control system (not shown) can cause generator  292  to rotate ball screw  260 , causing magnet piston assembly  270  to move and “reengage” cylinder  225 . 
         [0050]    Although some embodiments described in this application (e.g., system  200 ) feature the CFTS as part of an ocean wave energy converter, the CFTS is also more generally applicable for other applications where there is a need to translate generally linear motion to generally rotary motion, or vice versa. 
       Configurations of the Contact-Less Force Transmission System Components 
       [0051]      FIG. 3  shows side cross-section views of four exemplary configurations (a)-(d) for magnets  272 , pole pieces  274  and cylinder  225  of system  200 . Those of skill in the art will recognize other possible configurations. Each configuration depicted in  FIG. 3 , is shown relative to a line of axial symmetry  310  that is generally coaxial to spar  210  and ball screw  260 . 
         [0052]    Of the four designs shown, design (a) has a non-salient cylinder  320 , while the other three designs have cylinders  330 ,  340 ,  350  with salients  332 ,  333 , which are raised features protruding from the cylinders. In designs (a)-(c), the middle pole piece  275  is approximately twice as thick as the other pole pieces  274 . An arrangement such as this can be used to create a symmetrical system of equal flux linkage to all phases in order to produce balanced two- or three-phase voltages. Design (d) features pole pieces  274  and middle pole piece  275  that are of approximately equal axial length. Salient  332  on cylinder  330  of design (b) is approximately twice as long (axially) as the other two salients in that design. In designs (c) and (d), salients  333  in each design are of approximately equal size. 
         [0053]    In one group of tests conducted on these designs, it was shown that cylinders  330 ,  340 ,  350  with salients were generally better than the non-salient cylinder  320  at transmitting thrust to the magnets  272 . This group of tests also showed that the thrust transmission of designs (b) and (c) were not significantly different. 
         [0054]    In one embodiment, four ring-type, NdFeB magnets with the following dimensions were used: external diameter, 100 mm; internal diameter 50 mm; axial thickness, 25 mm. The magnets were stacked axially with soft-iron ring-shaped pole pieces 10 mm thick between them. 
         [0055]    Finite element analysis (FEA) was conducted on designs (a)-(d). The dimensions of components modeled in the FEA are shown in Table 1 and Table 2. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Dimensions of magnet and ball screw 
               
               
                 components modeled in FEA. 
               
             
          
           
               
                   
                 NdFeB Magnets 
               
             
          
           
               
                 Design 
                 Diameter of 
                 External 
                 Internal 
                 Axial 
               
               
                 Configuration 
                 ball screw 160 
                 Diameter 
                 Diameter 
                 Thickness 
               
               
                   
               
               
                 Design (a) 
                 ⅜″ 
                  55 mm 
                 25 mm 
                 20 mm 
               
               
                 Design (a), (b), 
                 ¾″ 
                 100 mm 
                 50 mm 
                 25 mm 
               
               
                 (c), (d) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Dimensions of pole pieces and cylinder 
               
               
                 components modeled in FEA. 
               
             
          
           
               
                   
                   
                 Radial 
                 Axial 
                 Axial 
               
               
                   
                   
                 thickness 
                 thickness 
                 thickness 
               
               
                   
                 Diameter 
                 of 
                 of pole 
                 of middle 
               
               
                 Design 
                 of ball 
                 cylinder 
                 piece 
                 pole 
               
               
                 Configuration 
                 screw 160 
                 225 
                 274 
                 piece 275 
               
               
                   
               
               
                 Design (a) 
                 ⅜″ 
                  5 mm 
                  5 mm 
                 10 mm 
               
               
                 Design (a) 
                 ¾″ 
                 20 mm 
                 10 mm 
                 20 mm 
               
               
                 Design (b), (c) 
                 ¾″ 
                 10 mm 
                 10 mm 
                 20 mm 
               
               
                 Design (d) 
                 ¾″ 
                 10 mm 
                 10 mm 
                 10 mm 
               
               
                   
               
             
          
         
       
     
         [0056]    The results of computed force capability as functions of displacement between piston assembly  270  and cylinder  225  are given in  FIG. 4 . (Results for design (c) are not shown, but its performance was very similar to that of design (b).) As shown in the FEA results of  FIG. 4 , the peak thrust of the design (d) is higher than that of design (b). The peak thrust is obtained at a displacement approximately equal to one magnetic pole dimension. However, the thrust characteristics of design (b) are wider than that of design (d), with high thrusts distributed over a wider range of axial displacement. 
         [0057]    The difference in the characteristics of designs (b) and (d) can be attributed to saturation of the central pole (located approximately at middle pole piece  275 ) in design (d) compared to design (b) and the effects of flux leakage. In design (d), the effects of saturation of the central pole make the thrust lower compared to design (b) at higher displacements. On the other hand, the relatively large middle pole piece  275  and consequently larger dimensions in design (b) allow for increased leakage which generally reduces the flux density and thrust. Depending on the required application, either curve can be chosen either to increase the peak thrust (design (d)) or to allow adequate vertical travel (design (b)). The peak thrust values of all four configurations, obtained by FEA, are compared in Table 3. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Peak thrust of design configurations shown in FIG. 3. 
               
             
          
           
               
                   
                 Design 
                 Peak 
               
               
                   
                 Configuration 
                 Thrust, N 
               
               
                   
                   
               
               
                   
                 Design (a) 
                 343 
               
               
                   
                 Design (b) 
                 763 
               
               
                   
                 Design (c) 
                 769 
               
               
                   
                 Design (d) 
                 900 
               
               
                   
                   
               
             
          
         
       
     
         [0058]    The results of Table 3 were compared with experimental test results to determine the peak output thrust for two different prototypes, implemented with different ball screw sizes as shown in Table 4. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Comparison of peak axial thrust from FEA and experimental test data. 
               
               
                 Peak Axial Force (N) 
               
             
          
           
               
                   
                   
                 Design 
                 FEA Model 
                   
               
               
                   
                 Prototype 
                 Configuration 
                 Prediction 
                 Test 
               
               
                   
                   
               
               
                   
                 ⅜″-diameter 
                 Design (d) 
                 122 
                 117.6 
               
               
                   
                 ball screw 260 
               
               
                   
                 ¾″-diameter 
                 Design (a) 
                 900 
                 894.3 
               
               
                   
                 ball screw 260 
               
               
                   
                   
               
             
          
         
       
     
       Experimental Results of the Contact-Less Force Transmission System with Generators 
       [0059]    Testing of one embodiment of the CFTS in system  200  was carried out by applying a known thrust to cylinder  225  and measuring the electrical output of generator  292 . Two permanent magnet generators, generator # 1  and generator # 2 , were used in testing. Parameters for generator # 1  and generator # 2  appear in Table 5 and Table 6, respectively. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Parameters for generator #1. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Manufacturer 
                 AMETEK 
               
               
                   
                 Type 
                 Brushless DC 
               
               
                   
                 Rated Voltage 
                 270 V 
               
               
                   
                 Phase 
                 3 
               
               
                   
                 RPM 
                 12000 
               
               
                   
                 Rs, Xs 
                 0.43 Ω, 0.19 Ω 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Parameters for generator #2. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Manufacturer 
                 MAVILOR MOTORS 
               
               
                   
                 Type 
                 BS073A00010T.00 
               
               
                   
                 Phase 
                   3 
               
             
          
           
               
                   
                 BEMF 
                 241 
                 V 
               
               
                   
                 Peak Stall Torque 
                 13.6 
                 Nm 
               
               
                   
                 Continuous Stall Torque 
                 2 
                 Nm 
               
               
                   
                 KT 
                 0.71 
                 Nm/A 
               
             
          
           
               
                   
                 Max RPM 
                 5600 
               
               
                   
                 Insulation Class 
                 F 
               
               
                   
                 Resolver 
                 2T8 
               
               
                   
                   
               
             
          
         
       
     
         [0060]    In a laboratory setting without water, a known thrust was obtained by attaching weights to cylinder  225  and releasing it to accelerate under gravity. The speed measurement was obtained from an oscilloscope capture of the output waveform of generator  292  by measuring its frequency and using the equation for the speed of a synchronous generator 
         [0000]    
       
         
           
             
               
                 
                   
                     n 
                     s 
                   
                   = 
                   
                     
                       120 
                        
                       f 
                     
                     p 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where p is the number of poles and f is the frequency. From the calculated speed, the axial velocity was obtained from the formula 
         [0000]    
       
         
           
             
               
                 
                   Ω 
                   = 
                   
                     
                       
                          
                         z 
                       
                       
                          
                         t 
                       
                     
                     · 
                     
                       
                         
                           2 
                            
                           π 
                         
                         l 
                       
                        
                       
                         [ 
                         
                           rad 
                           / 
                           s 
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    using the lead, l, of ball screw  260 , where Ω is the mechanical speed of rotation of the shaft and dz/dt is the axial velocity. Input power to this system was the product of the applied thrust and linear velocity. Output power was measured directly as the electrical power was dissipated in resistances that were connected across the generator  292 . 
         [0061]      FIGS. 5-7  show test results for system  200  using generator # 1 .  FIG. 5  shows the shaft speed of the generator under loads of 5, 10, 15 and 20 ohms and during no-load operation. Under no-load operation, the higher speeds can result in higher losses and consequently a non-linear speed-thrust characteristic. Under load, the generator speed is much lower and is more linear with thrust. The current increases fairly linearly with the applied thrust as shown in  FIG. 6 . As seen in  FIG. 7 , the overall system efficiency is greater than 50% for the 10-ohm load but falls as the electrical load is reduced. Similar curves were obtained using generator # 2 , except that its high impedance resulted in significant voltage drops and lower power output. 
       Computer Simulation of the Buoy Generator System 
       [0062]    The buoy generator system  200  of  FIG. 2  was simulated in computer software. In this simulation, the equation of motion of the OWEC, in a single degree of freedom (SDOF) heave mode is given by 
         [0000]        m   v   {umlaut over (z)}+bż+cz=F   0  cos(ω t+ σ)  (3) 
         [0000]    where m v =(m+α) is the total virtual mass of the system  200  including an added mass a; b is the damping of the buoy, comprising the hydrodynamic damping of the waves (b I ) and the damping provided by generator  292  (b G ); c is the spring (buoyancy) constant; F=F 0  cos(ωt+σ) is the exciting force from the waves; z=z 0  cos(ωt) is the heave displacement. The added mass a, hydrodynamic damping b I , and the spring constant c are given for a cylindrical buoy by M. E. McCormick,  Ocean Engineering Wave Mechanics , Wiley, 1973. 
         [0063]    The damping constant of generator  292  can be determined from the following considerations. The relationship between the torque on the shaft T screw  and the axial force F screw  for the ball screw  260  is given by, 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     screw 
                   
                   = 
                   
                     
                       
                         lF 
                         scew 
                       
                       
                         2 
                          
                         
                           πη 
                           f 
                         
                       
                     
                      
                     
                       ( 
                       
                         forward 
                          
                         
                             
                         
                          
                         driving 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     4 
                      
                     a 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     T 
                     screw 
                   
                   = 
                   
                     
                       
                         lF 
                         scew 
                       
                       
                         2 
                          
                         π 
                       
                     
                      
                     
                       
                         η 
                         b 
                       
                        
                       
                         ( 
                         
                           back 
                            
                           
                               
                           
                            
                           driving 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     4 
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where l=screw lead [m/rev], and where η f , η b  are the forward and back drive efficiencies, respectively, of ball screw  260 . Generator  292  basically acts like a brake, opposing the rotation with a torque on the shaft that can be expressed as 
         [0000]        T   screw   =K   T   Ω+T   0   (5) 
         [0000]    where T 0  is the loss torque [Nm], K T  is the braking coefficient of the generator [Nms/rad], and Q is the angular velocity of the shaft. In an embodiment that uses a permanent magnet synchronous generator (PMSG), the introduction of the constant K T  effectively assumes a linear magnetic circuit with no saturation of the rotor and stator iron. With the relatively large effective air gaps (of the magnets themselves) that are common in PMSGs, this assumption does not usually lead to significant errors. 
         [0064]    The total force transmitted to the PTO during an upstroke is then given by 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                     screw 
                   
                   = 
                   
                     
                       
                         2 
                          
                         π 
                       
                       l 
                     
                      
                     
                       ( 
                       
                         
                           
                             K 
                             T 
                           
                            
                           Ω 
                         
                         + 
                         
                           T 
                           0 
                         
                         + 
                         
                           
                             I 
                             
                               m 
                                
                               
                                   
                               
                                
                               G 
                             
                           
                            
                           α 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where I mG  is the moment of inertia of the generator and shaft system, and where for the roller screw 
         [0000]    
       
         
           
             
               Ω 
               = 
               
                 
                   z 
                   . 
                 
                  
                 
                   
                     2 
                      
                     π 
                   
                   l 
                 
               
             
             , 
           
         
       
     
         [0000]    where ż is linear velocity of ball nut  280  or, similarly, velocity of float  220 . Also, 
         [0000]    
       
         
           
             α 
             = 
             
               
                 
                    
                   Ω 
                 
                 
                    
                   t 
                 
               
               = 
               
                 
                   z 
                   ¨ 
                 
                  
                 
                   
                     2 
                      
                     π 
                   
                   l 
                 
               
             
           
         
       
     
         [0000]    is the angular acceleration of the shaft of generator  292 . The generator damping coefficient is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     b 
                     G 
                   
                   = 
                   
                     
                       
                         K 
                         T 
                       
                        
                       
                         ( 
                         
                           
                             2 
                              
                             π 
                           
                           l 
                         
                         ) 
                       
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In an embodiment where generator  292  is decoupled, during the down stroke there is no axial force from the PTO on float  220 . Generator  292  “free wheels,” i.e., it is decelerated by the electrical load connected to it, its own inertia and that of shaft  294  through the unidirectional clutch  291 . In that case, F screw =0 or, 
         [0000]        I   mG   α+T   screw =0  (8) 
         [0065]      FIG. 8  depicts an equivalent circuit of the PMSG. The voltage across a phase of the generator windings can be expressed as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       v 
                       j 
                     
                     = 
                     
                       
                         
                           - 
                           
                             r 
                             j 
                           
                         
                          
                         
                           i 
                           j 
                         
                       
                       + 
                       
                         
                           L 
                           j 
                         
                          
                         
                           
                              
                             
                               i 
                               j 
                             
                           
                           
                              
                             t 
                           
                         
                       
                       + 
                       
                         
                            
                           
                             λ 
                             jf 
                           
                         
                         
                            
                           t 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where r j  is the phase resistance, i j  is the current of j-th phase, λ jf  is the flux linkage in phase j due to the permanent magnet, and L j  is phase inductance. 
         [0066]    The peak value of the induced emf of the PMSG is dependent on speed and can be expressed as 
         [0000]    
       
         
           
             
               E 
               j 
             
             = 
             
               
                 
                    
                   
                     λ 
                     jf 
                   
                 
                 
                    
                   t 
                 
               
               = 
               
                 
                   K 
                   f 
                 
                 · 
                 
                   Ω 
                   . 
                 
               
             
           
         
       
     
         [0000]    The currents can be obtained by rearrangement and integration of Equation 9, noting that v 1 =i 1 R load . 
         [0067]      FIG. 9  shows a graphs of simulated results for the no-load voltage of generator  292  during operation in waves with a unidirectional clutch action on shaft  294  under wave conditions where the wave period T=2.5 s and the significant wave height H s =0.15 m. During free-wheeling, the voltage produced is zero as clutch  291  disengages generator  292  from the rotation and generator  292  is decelerated. Also, unlike operation under the reciprocating action, with a clutch the voltage time area is generally less symmetrical. 
       Wave Flume Testing of the Buoy Generator System 
       [0068]    System  200  (with a ¾″-diameter ball screw  260 ) was tested in a wave flume. The wave flume that was used is 7 feet deep, 30 feet wide, 110 feet long and tapers to a typical beach. There are two sets of hydraulically driven wave makers that are activated in sequence to create irregular waves of approximately 4 feet in height and with approximately four-second dominant periods. System  200  was tested in irregular waves. This particular embodiment was made up of system  200 , with the addition of a rigid shaft between spar  210  and a mooring plate. The shaft was also equipped with a swivel joint that allowed motion in six degrees of freedom. However, the threaded studs of the swivel joint were adjustable to provide a stiff rigid member. For this embodiment, spar  210  is about 1.68 m (5.5 feet) long, and float  220  has an outer diameter of about 0.6 m and is about 0.6 m long. 
         [0069]      FIG. 10  is an oscilloscope capture showing the no-load voltage output of generator  292  during the up-stroke and down-stroke portions of the wave cycle. Because clutch  291  was uni-directional in this tested embodiment, generator  292  free-wheels on the down stroke and no voltage is generated.  FIGS. 11A-11C  show example oscilloscope captures of system  200  operating into a 75-ohm load.  FIGS. 11A-11C  show waveforms for voltage, current, and power outputs, respectively. The peak output power under load was about 69 W. The generator used in the tested embodiment (generator # 1 ) has a high synchronous reactance and a high voltage drop. A generator model of relatively lower impedance can improve output power. 
         [0070]    Wave flume test results for generator outputs under various load conditions are summarized in Table 7. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Wave flume test results. 
               
             
          
           
               
                   
                 Load 
                 Voltage 
                 Current 
                 Power 
               
               
                   
                 Resistance 
                 (Vp) 
                 (Ip) 
                 (Wp) 
               
               
                   
                 ohm 
                 V 
                 A 
                 W 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 16 
                 0.5 
                 6 
               
               
                   
                 30 
                 35 
                 0.7 
                 18.4 
               
               
                   
                 50 
                 52 
                 0.6 
                 23.4 
               
               
                   
                 75 
                 65 
                 0.6 
                 29.3 
               
               
                   
                   
               
             
          
         
       
     
         [0071]      FIGS. 12A-12C  show waveforms (for voltage, current, and power, respectively) caused by irregular motion of spar  210  due to irregular wave excitation. In another embodiment, these effects are reduced using a dynamic control system. 
       System Using a Permanent Magnet Helical Screw Drive 
       [0072]      FIG. 13A  shows a cross-section side view of another embodiment of system  100 . System  1300  comprises a float  1320  which is approximately coaxial with a tube-like spar  1310 . Float  1320  comprises air or other buoyant material  1323  and a magnet assembly  1370 , which is described in more detail below. Float  1320  preferably encircles spar  1310  a full 360 degrees, but it can also encircle spar  1310  less than 360 degrees. Similar to other embodiments described above, spar  1310  can be comprised of a material that can withstand ocean conditions for a relatively long period of time, such as PVC or composite material. System  1300  can further comprise a cap  1350 , a generator  1392  with a shaft  1394 , a clutch  1391  (uni- or bi-directional), a coupling  1390 , a protective plate  1393 , a ballast weight  1330 , and a wave plate  1345 . System  1300  can be secured to an anchor or mooring system by a tether  1340 . Spar  1310  contains at least one center screw  1360 , which is preferably approximately coaxial with spar  1310 . Center screw  1360  is comprised of one or more materials that exhibit high electrical resistance and low magnetic reluctance, such as an alloy comprising about 1-4% silicon steel. As is known in the art, what constitutes “high electrical resistance and low magnetic reluctance” varies from application to application. 
         [0073]      FIG. 13B  shows center screw  1360  and surrounding magnet assembly  1370  in more detail. Center screw  1360  comprises threads such as thread  1376 , which desirably run at least part of the length of center screw  1360 . The threads can have a flat face (i.e., outer surface) and a vertical wall angle, although other face designs and wall angles can also be used. Characteristics of threads  1376  (e.g., pitch, spacing) can be chosen based on a particular application. A choice of thread pitch can be weighed against thrust and speed requirements of system  1300 . 
         [0074]      FIG. 13C  shows magnet assembly  1370  in more detail, without spar  1310  and center screw  1360 . Magnet assembly  1370  comprises two or more pole shoes  1372 , which are arranged generally concentrically with spar  1310 . Pole shoes  1372  can comprise a generally circular or generally semi-circular main piece  1373  and can have a thread  1378  extending along part or all of the inside of main piece  1373 . Pole shoes  1372  and threads  1378  can extend  360  around the inside of float  1320 , or they can extend less than 360 degrees around. In one embodiment, a pole shoe  1372  can be comprised of two or more pole shoe pieces of smaller angular size. The pole shoe pieces can be placed adjacent to each other in an axial plane or, if their size permits, they can be placed non-adjacent in an axial plane. For example, a pole shoe which extends 360 degrees can be comprised of two 180-degree shoes. Pole shoes  1372  are comprised of one or more materials that exhibit high electrical resistance and low magnetic reluctance, such as a silicon iron alloy. Characteristics of threads  1378  (e.g., pitch, spacing) can be chosen based on a particular application. Pitch of threads  1376  can be selected to amplify or reduce the angular speed of a turning center screw  1360 . A choice of thread pitch can be weighed against thrust and speed requirements of system  1300 . Preferably, between two pole shoes  1372  is a ring magnet  1374 . One or more pairs of ring magnets  1374  can be used to create complementary flux densities. In one embodiment, several ring magnets  1374  are stacked axially adjacent to each other with their poles in the same orientation. If desired, threads  1378 , ring magnets  1374  and pole shoes  1372  can be coated with an insulator, preferably a non-conductive, non-corrosive, high-strength, non-magnetic insulation (not shown). 
         [0075]      FIG. 13D  depicts a top cross-sectional view taken along the line  13 D- 13 D indicated in  FIG. 13B . This embodiment shows ring magnet  1374  and the threads  1378  from two 180-degree pole shoes  1372 . (In this view, ring magnet  1374  hides most of the pole shoes  1372  except for threads  1378 .) 
         [0076]    Returning to  FIG. 13A , when relative linear motion occurs between float  1320  and spar  1310  (e.g., when a wave exerts a force on float  1320 ), magnet assembly  1370  moves in a linear direction relative to center screw  1360 . This causes a differential in magnetic flux between center screw  1360  and pole shoes  1372 . This differential flux can result in transaxial forces which pull on screw  1360 , causing it to rotate back into alignment with pole shoes  1372 . This can create relative rotary motion between center screw  1360  and magnet assembly  1370 . As a result, center screw  1360  turns clutch  1391  and shaft  1394  on generator  1392 , creating an electric current. 
         [0077]    Generally, center screw  1360  and magnet assembly  1370  can operate bi-directionally. For example, rotary motion can be converted to linear motion by applying a torque to center screw  1360  or magnet assembly  1370  (or to both). This rotary motion can cause a differential flux (similar to that described above) resulting in a linear motion. 
         [0078]    Although the magnet assembly  1370  and center screw  1360  are described above with respect to an ocean wave energy converter, this combination can be used more generally for applications involving a conversion between linear motion and rotary motion. For example, many applications currently using ball screw assemblies can be redesigned using a magnet assembly  1370  and center screw  1360 . This approach can allow for: less acoustic noise (particularly for operations at relatively high speeds); less wear and maintenance; recovery from overloads with little or no maintenance; amplification of speed or torque (depending upon a “gear ratio”); and improvements in energy transfer efficiency, as losses can generally be limited to radial bearing friction and magnetic hysteresis losses. 
       System Featuring Multiple Spars 
       [0079]      FIG. 14A  depicts an ocean wave energy converter system  1400 , which comprises a float  1420  and two or more spars  1410 . The particular embodiment shown features three spars  1410  surrounded by float  1420 . Spars  1410  are reinforced from above by support structure  1412 , but in other embodiments a support structure on the underside of system  1400  can be added. In another embodiment no support structure is present. Spars  1410  and float  1420  together comprise systems similar to those described previously in this application, e.g., system  200  using the CFTS with either a ball screw or a roller screw, or system  1300  using permanent magnets and the helical center screw. Similar to other embodiments described above, ballast weights  1430  and wave plates  1445  can be attached to spars  1410 , and the spars can be held in place using tethers  1440 . The top ends of the spars  1410  can have caps as in other embodiments, although they are not shown in  FIG. 14A . 
         [0080]    In one embodiment, individual spars  1410  contain a generator (not shown), similar to the systems described above. In another embodiment, spars  1410  transfer rotary energy through a gear system  1452  (or other energy transmission system) to turn a generator  1450 . Harnessing the rotary energy from two or more spars can allow for improved scalability of a multiple-spar system and can also allow for higher generator speeds. 
         [0081]      FIG. 14B  provides a top view of system  1400 , showing float  1420 , spars  1410  and support structure  1412 .  FIG. 14C  is a bottom view of system  1400 , showing generator  1450  and gear system  1452 , as well as float  1420 , ballast weights  1430  and wave plates  1445 . 
         [0082]    In view of the many possible embodiments to which the principles of the disclosed invention can be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.