TECHN ICAL FI ELD OF TH E I NVENTION AND BACKGROUN D ART

The present invention relates to a generator for generating electric energy from kinetic energy of moving sea water. "Sea water" is to be interpreted as water present outdoors in any type of constellation , such as for example in oceans, lakes, rivers and even dams. The movement from which kinetic energy is converted to electric energy in such a generator may be caused by waves on the surface of said water or water currents of different types within such sea water constellations. However, the present invention is particularly directed to generating electric energy from power of waves of sea water, which is the reason for hereinafter mainly describing the invention for that application without restricting it thereto. Such wave power generators could be con- nected to a buoy that is either on the surface of the water or some meters below the surface, and energy is extracted from the sea waves when this buoy moves.

Characterizing for wave power is that it is delivered with very low speeds and large forces. The speeds of movements caused by waves where generators of this type are arranged may often be below 1 .0 m/s and even below 0.5 m/s in wave climates with smaller waves, such as for example in the seas surrounding Sweden . These low speeds makes it challenging to efficiently convert energy from sea waves to electric energy.

The most common generator type used for wave power is a longitudinal flux permanent magnet synchronous generator. This generator perform poorly at low speeds by providing a weak damping force and having large losses at these low speeds. The weak damping force at the low speeds limits the possi bilities to control the buoy movement by controlling the current in the generator, since it is desirable to have large forces at low speeds to get a proper buoy control . These generators are also large and expensive, so that a large part of the cost for a wave power plant having such generators is the cost of the generators.

The present invention is for that sake directed to another type of such generators, namely transverse magnetic flux generators, i .e. generators in which the magnetic flux caused to vary by a move- ment of moveable parts of the generator is flowing transversally to the direction of such movement.

SUM MARY OF THE I NVENTI ON The object of the present invention is to provide a generator for generating electric energy from kinetic energy of moving sea water being improved in at least some aspect with respect to such generators already known . This object is according to the invention obtained by providing a generator according to the preamble of appended claim 1 with the features of the characterizing part of that claim.

By having the total length I of the electric conductor forming the winding by being wound around a core section < Nb4V ^r 4, in which A is the cross section area of said section of the core enclosed by the winding , N is the number of turns of the electric conductor around this core section and b is 1 .7, 1 .5, 1 .4 , 1 .3 or 1 .2, the winding may be made considerably shorter than in known such generators for delivering an alternating voltage aimed at, which means that the resistance of said winding will be lower for a determined electric conductor used , so that even a cheaper conductor, such as of Al instead of Cu , with slightly lower conductivity may be used and still a considerably higher electric current may be generated in the generator. This results in a high damping force already at low speeds, and that the efficiency of the generation at these speeds may be increased considerably. The higher damping force in the form of higher force density does also mean that the generator may be made smaller for the same power rating saving costs. Furthermore, the high forces at low speeds makes this generator a very useful tool for buoy control .

According to an embodiment of the invention said magnet pile, second piles and core are designed to simultaneously d uring op- eration of the generator gather a magnetic flux from at least three consecutive magnets in said magnet pile as seen in the direction of extension of this pile in said core section around which said electric conductor is wound . This results in a short winding with a low resistance for a certain amount of magnetic flux resulting in high damping forces at low speeds and by that a high power rating at these speeds.

According to another embodiment of the invention the cross section of the magnetically permeable material of said core section enclosed by said winding has rectangular, square or circular shape. Such a shape of the cross section enclosing the magnetic flux, especially the circular shape, makes it possible to have a short electric conductor for enclosing a certain magnetic flux while generating a certain voltage, so that the damping force pro- vided by the generator will then be high and by that also the power generated by the generator even at low speeds.

According to another embodiment of the invention said magnet pile has three consecutive sections in the direction of extension of the pile each having a plurality of permanent magnets separated by a said first member, adjacent magnet pile sections are separated by a member of magnetically non-permeable material , that the generator comprises three said cores of magnetically permeable material , one for each magnet pile section , configured to form a closed loop from one second pile to the other for allowing magnetic flux to pass from a said magnet pile section to one second pile and back through the core and the other second pile, each said core has a section around which an electric conductor is wound , and said members of magnetically non-permeable ma- terial separating the magnetic pile sections are dimensioned so as to displace an alternating voltage induced by said mutual movement of the magnet pile and said second piles in each said winding by 120 electrical degrees with respect to the alternating voltage induced in the other two windings so as to generate a three-phase alternating voltage. This generator will efficiently generate a three-phase alternating voltage to be fed to consumers through possible interconnection of active rectifiers with current control , power converters and transformers. According to another embodiment of the invention said magnet pile is immovable with respect to said core, and said second piles are configured to be moved by being influenced by a movement of sea water. By having the permanent magnets in the non-moving part all magnets may be used all the time irrespectively of the mutual position of this magnet pile with respect to said second piles then moving , so that the number of magnets of the generator may be reduced with respect to having the magnets in the moving part. This results in a considerable reduction of costs for the generator, since that permanent magnets used in generators of this type are expensive.

According to another embodiment of the invention the generator comprises two said first magnet piles having a fixed mutual position and three said second piles having a fixed mutual position , each first magnet pile has a said second pile on two different sides thereof, each magnet of one said magnetic pile has an opposite direction of magnetization than the magnet of the other magnet pile arranged directly laterally thereto with respect to the direction of extension of the magnetic piles, and said core is form- ing a closed loop from one second pile to another second pile for allowing magnetic flux to pass from one magnet pile to a first second pile to the other magnet pile to a second second pile and back to the core and a third second pile. Such a series connection of the two magnet piles obtained in this way results in a lower magnetic flux for a certain damping force obtainable with respect to a parallel connection of the magnet piles, so that said core may be given a smaller cross section and still able to take the magnetic flux. Magnetically permeable material , normally iron , used for the core may by that be saved at the cost of more turns and more material of the electric conductor of the winding . It is pointed out that the generator may have an arbitrary amount of said magnet piles and second piles arranged as in this embodiment of the invention for optimizing the relationship between cost and performance of the generator.

According to another embodiment of the invention said piles have a rectilinear extension . This is a suitable extension of the piles for connecting the moving part to a said buoy by a linear direct coupling thereto, whereas in another embodiment of the invention at least one first magnet pile and said second piles extend along a circle and the movability of the first and second piles with respect to each other is provided by having one of them rotatably arranged so as to be rotated by movement of sea water acting thereupon , which then requires a simple conversion from linear to rotating motion for example by using a winch , when connecting the part rotatably arranged to a said buoy.

According to another embodiment of the invention directed to the embodiment having piles with a rectilinear extension the genera- tor has two sets of at least one first magnet pile and two second piles, and the two sets have a first said core section around which an electric conductor is wound in common and separate core sections magnetically connecting the respective set to said first core section . This means that the magnetic flux from both sets will be concentrated in one and the same core section provided with said winding resulting in a high voltage induced per turn of the winding and by that a possibility to have a short winding with a low resistance and still obtain a high voltage and by that a high current and a strong damping force. According to another embodiment of the invention constituting a further development of the embodiment last mentioned the piles of one set are arranged to extend in parallel with the piles of the other set, and said core has a H-like shape as seen in the direction of extension of said piles with the web of said H formed by a said first core section provided with said winding separating the piles of the two sets from each other and the legs of the H have the two ends connecting to one set of piles each . The H-shape makes the generator with core and piles symmetric and the winding may be applied around one core section being in common to two parallel magnetic circuits.

According to another embodiment of the invention said first magnet pile has a plurality of permanent magnets arranged side by side in a row in the direction perpendicular to the extension of the pile and to the direction this pile is facing a said second pile with a said first member separating each such row of permanent magnets, an elongated rigid element extends along the extension of the magnet pile on each side thereof corresponding to opposite ends of said rows of permanent magnets, stabilizing rods pene- trate through said first members transverse to the extension of the magnet pile from one said elongated element to the other for connecting these elements to each other while bearing under pretension against opposite sides of the magnet pile. This construction of the magnet pile makes it possible to obtain a sufficient stiffness even to withstand magnetic forces from the magnets tending to bend the structure even if the structure of the magnet pile is long and thin .

According to another embodiment of the invention said stabilizing rods are provided with a bolt joint to be tightened for pressing said elongated elements against the sides of said magnet pile, and according to another embodiment of the invention each said magnet pile has at least one beam of an electrically insulating material extending in the direction of extension of the pile while dividing said row of permanent magnets so as to stabilize the magnet pile. The network of stabilizing rods and beam or beams will then form a grate that will be very stiff without disturbing the electromagnetic performance of the generator. This is ensured by having the elongated elements and/or stabilizing rods made of or coated by an electrically insulating material according to another embodiment of the invention .

The invention also relates to a plant for generating electric power from kinetic energy of moving sea water provided with at least one generator according to the invention . The advantages and advantageous features of such a plant appear clearly from the above discussion of the different embodiments of a generator according to the invention .

Further advantages and advantageous features of the invention appear from the description following below.

BRI EF DESCRI PTION OF DRAWI NGS

With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a simplified perspective view illustrating vital parts of a generator according to a first embodiment of the invention ,

Fig 2 is a simplified view of the generator shown in Fig 1 from above, Figs 3 and 4 are simplified views illustrating how the generator shown in Fig 1 works,

Figs 5 and 6 are graphs of force density versus speed and power density versus speed for a generator according to the present invention in comparison to a known conventional generator with a longitudinal magnetic flux, Fig 7 is a simplified side elevation of a magnet pile of the generator shown in Fig 1 , is a view corresponding to that in Fig 7 of the mag net pile from above,

Fig 9 is a simplified view corresponding to Fig 3 and 4 of a part of a generator according to a second embodiment of the invention , and

Fig 10 is a simplified perspective view of a part of a generator according to a third embodiment of the invention .

DETAI LED DESCRI PTION OF PREFERRED EMBODI MENTS OF THE I NVENTION

A generator according to a first embodiment of the invention is schematically and very simplifiedly illustrated in a perspective view in Fig 1 . It is shown how the generator 1 is connected to a buoy 2 located at the surface of a sea 3 for being moved by sea waves and by that also move a movable part of the generator for generating electric energy from the kinetic energy of the buoy 2. The general construction of the generator shown in Fig 1 will be explained while at the same time also making reference to Figs 2-4. The generator has two first piles 4, 5 of a plurality of permanent magnets 6 with magnetization in the direction in extension of the pile. Adjacent magnets are separated by a first member 7 of magnetically permeable material and have opposite magneti- zation direction . "Magnetically permeable material" is in this disclosure defined to be a material which has a relative permeability of more than 50 in any direction at a magnetic flux density of 0.5 T. The two magnet piles 4, 5 have a fixed mutual position by being interconnected in a suitable way not shown , and each magnet of one magnet pile has an opposite direction of magnetization than the magnet of the other magnet pile arranged directly laterally thereto with respect to the direction of extension of the magnet piles, which appears from Figs 3 and 4. The generator also has three second piles 8-10 of second members 1 1 of magnetically permeable material , in which each first magnet pile has a second pile 8-10 on each side thereof. Adjacent second members 1 1 of each second pile have a mutual distance corresponding to twice the distance of two consecutive first mem- bers 7 of the magnet piles 4, 5, and the three second piles have a fixed mutual position by being connected to each other as schematically illustrated by a connecting member 12 in Fig 1 . Each second member 1 1 of one second pile is as seen in the pile extension direction arranged at equal distance to two consecutive second members of the second pile located next to this second pile.

Each magnet pile 4, 5 has in this embodiment three consecutive section 13- 15 in the direction of extension of the pile each having a plurality of permanent magnets separated by a said first member 7, and adjacent magnet pile sections are separated by a member 16 of magnetically non-permeable material . The generator has three cores 1 7-19 of magnetically permeable material each forming a closed loop from one second pile 8 to another 10 for allowing magnetic flux to pass from the magnet piles 4, 5 through the second piles 8-10 and back through the core. Each core has a section 20 around which an electric conductor 21 is wound by for example in the order of 200 turns.

The magnet piles 4, 5 and the second piles 8-10 are movably arranged with respect to each other by having the magnet piles arranged immovable with respect to the cores and the second piles configured to be moved by being influenced by a movement of sea water as simplifiedly illustrated in Fig 1 by a connection of a line 22 to the connecting member 12. Such a mutual movement of the second piles and the magnet piles along the extension of these piles will generate a time varying magnetic flux in the cores transversally to the direction of this mutual movement and an alternating voltage in the windings 23. The members 16 of magnetically non-permeable material separating magnet pile sections are dimensioned to displace an alternating voltage induced by said mutual movement of the magnet piles and the second piles in each said winding by 120 electrical degrees with respect to the alternating voltage induced in the other two windings so as to generate a three-phase alternating voltage.

The generator according to this embodiment of the invention has two sets 24, 25 of two magnet piles and three second piles, and the two sets have a first core section 20 around which an electric conductor 21 is wound in common and separate core sections 26- 29 magnetically connecting the respective set to said first core section . The two sets 24, 25 of piles have a rectilinear extension and extend in parallel with each other. Each core has a H-like shape as seen in the direction of extension of the piles as seen in Fig 2 with a web of the H formed by a first core section 20 separating the piles of the two sets from each other and the legs of the H have each two ends connecting to one set of piles each .

How the magnetic flux through the cores is brought to be varying with time is schematically illustrated through Figs 3 and 4. It is shown how the magnetic flux from the permanent magnets will find a way through the first members of these piles and the second members of the second piles and further in said closed loop through the core in one direction when the second piles have a position shown in Fig 3 with respect to the magnet piles 4, 5 and how the magnetic flux direction has changed to be the opposite when the second piles have been moved from the position shown in Fig 3 to the position shown in Fig 4. The magnet piles may be called "flux concentrators" by having the magnets directed towards each other with a said first member 7 therebetween . This means that the magnetic flux density out from the first members 7 is normally higher than the magnetic flux density through the magnets, since the first members have normally a smaller surface than the permanent magnets. The total length I of the electric conductor 21 of each winding 23 is much shorter than in conventional generators of this type per unit induced voltage and will by that have a much lower winding resistance. More exactly, this total length I is < Nb4V ^r 4, in which A is the cross section area of said first core section 20 enclosed by the winding , N is the number of turns of the electric conductor around this core section and b is here with a first core section with the cross section of a circular shape as low as 0.9 (I = 0.88 ^■ 4N ^4 for a circular cross section and I = 4N ^4 for a square cross section for the case only of one winding layer). This lower re- sistance results in a higher current possi ble for a certain voltage induced in the windings, so that a higher damping force may be obtained and by that more electric power may be generated at a given speed of movement of said buoy 2 and by that the second piles.

The graph of Fig 5 illustrates how the force per m ² active area (the active area is the air gap area on both sides of each magnet pile and here calculated to be height x width x 2 for each magnet pile. The total active area of the generator is the sum of the active area of all magnet piles) obtainable by a generator according to the invention (solid line) is depending upon said speed of the movement compared to a conventional typical longitudinal flux permanent magnet synchronous generator (dashed line). It appears that a generator according to the invention has a maximum damping force being high (60 kN/m ² ) already at a speed of less than 0.1 m/s, whereas this force is much lower for the conventional machine, especially for the low speeds normal for waters like the ones surrounding Sweden . For such speeds the damping force per square meter active area (the shear stress) of the generator according to the invention may be as much as 10 times higher, which then also results in an electric power per square meter active area generated by the generator according to the invention being that much higher (as illustrated in Fig 6), since the power is said damping force multiplied with the speed . Even for a speed of 0.7 m/s that power per square meter will be five to ten times higher than for the conventional generator.

It appears from Fig 1 that each magnet pile 4, 5 has a plurality of permanent magnets arranged side by side in a row in the direction perpendicular to the extension of the pile and to the direction this pile is facing the second piles next thereto with a first member 7 separating each such row of permanent magnets. This way of arranging permanent magnets and first members in a magnet pile will result in magnetic forces trying to bend the structure, and how the structure of the magnetic piles are constructed for being sta- bile will now be described while making reference to Figs 7 and 8. An elongated rigid element 30, 31 of electrically insulating material or coated by such a material extends along the extension of the magnet pile on each side thereof corresponding to opposite ends of said rows of permanent magnets for example of neodym- ium and separated by laminated iron . These two rigid elements are interconnected by stabilizing rods 32 also of electrically insulating material or coated by such a material penetrating through the first members transverse to the extension of the magnet pile. The stabilizing rods are provided with a bolt joint 33 to be tight- ened for pressing the elongated elements 30, 31 against the sides of the magnet pile. Each magnet pile has also beams 34 of electrically insulating material or coated by such a material extending in the direction of extension of the pile while dividing the rows of permanent magnets and stabilizing the magnet pile. The problem to get the long-thin structure sufficiently stiff to withstand magnetic forces is obtained by this network of stabilizing rods and beams forming a grate that will be very stiff without disturbing the electromagnetic performance of the generator, since the material of this grate will be electrically insulated with respect to the rest of the piles and not form any electrically conducting loops therein . The second piles are held together in a similar way by elongated rigid elements 35, 36 of electrically insulating material on each side thereon . Fig 9 is a view corresponding to Figs 3 and 4 of a part of a generator according to a second embodiment of the invention differing from the one shown in Fig 1 mainly by having only one magnet pile 40 and two second piles 41 , 42. The function of this generator will be the same as for the generator shown in Fig 1 . An ad- vantage of having more than one magnet pile connected in series as in the first embodiment is that the same magnetic flux will be obtained in the core, so that this has not to have a greater cross section requiring more material to be used , but the magnetic force and by that the damping force of the generator will still be higher.

A part of a generator according to a third embodiment of the invention is schematically illustrated in Fig 10, and this has a first magnet pile 50 and second piles 51 , 52 extending along a circle. The movability of the magnet pile and the second piles with re- spect to each other is provided by having the magnet pile rotata- bly arranged so as to be rotated by movement of sea water acting thereupon by a suitable connection thereof to a buoy or the like through a transmission which may convert a linear motion to a rotating motion , so that the magnet pile is the rotor and the rest the stator of the generator. Otherwise, the function of this generator will be similar to that of the generator shown in Fig 1 and described above with a time varying magnetic flux directed transversal to the extension of the magnetic pile, which is here according to a circle arc. Thus, pile is in this disclosure defined as an arrangement of pieces directly or indirectly superimposed and having an arbitrary extension .

The different members of magnetically permeable material in a generator according to the invention , such as said first and second members and said cores are formed by laminate structures of a plurality of thin laminate pieces, typically of a thickness of 0.1 - 0.5 mm , separated by a thin insulating layer so as to keep eddy current losses at low levels.

The invention is of course not in any way restricted to the embod- iments described above, but many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the scope of invention as defined in the appended claims. Although it is shown that the two second piles arranged next to a magnet pile extend along opposite sides thereof, this is not necessary, but they do only have to extend along two different sides of the magnet pile to form a double-sided generator to which the present invention is directed .

"a mutual distance corresponding to twice the distance of two consecutive first members" is in this disclosure to be interpreted to cover mutual distances deviating slightly from twice said distances, such as by for example a couple of mm as long as the function will be the same with respect to the magnetic flux through consecutive such first members. The distance may for instance deli berately be made 1 mm larger than "twice", which means that the performance is lowered slightly since the maximum magnetic flux will not hit all first members simultaneously, but vibrations of components of the generator may by that be lowered . The same reasoning is also applicable to the use of "equal" as used in this disclosure with respect to the arrangement of said second members in the second piles.

The magnet piles and the second piles may be movably arranged with respect to each other in other ways than described above. The buoy may be secured to the stator (core), which is typically the case if the generator is integrated in the buoy, and the second piles will then instead be secured to the sea bottom.