This application relates to generating electricity from ocean wave energy.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.)
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.
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.
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
Returning to
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.
Similar to system 100 of
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.
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.
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.
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.
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.
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.
The results of computed force capability as functions of displacement between piston assembly 270 and cylinder 225 are given in
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.
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.
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.
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
where p is the number of poles and f is the frequency. From the calculated speed, the axial velocity was obtained from the formula
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.
The buoy generator system 200 of
m
v
{umlaut over (z)}+bż+cz=F
0 cos(ωt+σ) (3)
where mv=(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 (bI) and the damping provided by generator 292 (bG); c is the spring (buoyancy) constant; F=F0 cos(ωt+σ) is the exciting force from the waves; z=z0 cos(ωt) is the heave displacement. The added mass a, hydrodynamic damping bI, and the spring constant c are given for a cylindrical buoy by M. E. McCormick, Ocean Engineering Wave Mechanics, Wiley, 1973.
The damping constant of generator 292 can be determined from the following considerations. The relationship between the torque on the shaft Tscrew and the axial force Fscrew for the ball screw 260 is given by,
where l=screw lead [m/rev], and where ηf, ηb 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
T
screw
=K
T
Ω+T
0 (5)
where T0 is the loss torque [Nm], KT 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 KT 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.
The total force transmitted to the PTO during an upstroke is then given by
where ImG is the moment of inertia of the generator and shaft system, and where for the roller screw
where ż is linear velocity of ball nut 280 or, similarly, velocity of float 220. Also,
is the angular acceleration of the shaft of generator 292. The generator damping coefficient is given by
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, Fscrew=0 or,
I
mG
α+T
screw=0 (8)
where rj is the phase resistance, ij is the current of j-th phase, λjf is the flux linkage in phase j due to the permanent magnet, and Lj is phase inductance.
The peak value of the induced emf of the PMSG is dependent on speed and can be expressed as
The currents can be obtained by rearrangement and integration of Equation 9, noting that v1=i1Rload.
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.
Wave flume test results for generator outputs under various load conditions are summarized in Table 7.
Returning to
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.
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.
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.
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.
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.
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.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US2006/014848 | 4/19/2006 | WO | 00 | 7/7/2008 |
Number | Date | Country | |
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60673209 | Apr 2005 | US |