INTEGRATED MOTOR GENERATOR FLYWHEEL WITH ROTATING PERMANENT MAGNET

Provided is a flywheel system, including: an armature coil set; and a rotor assembly having: a first rotor member; a second rotor member; a permanent magnet disposed between the first rotor member and the second rotor member; and a magnetic circuit formed by the first rotor member, the second rotor member, and the permanent magnet, wherein the magnetic circuit spans a gap between the first rotor member and the second rotor member into which at least part of the armature coil set is disposed.

BACKGROUND

The present invention relates generally to flywheel energy storage and, more specifically, to an integrated motor generator flywheel with a rotating permanent magnet.

2. Description of the Related Art

Often flywheels are used to store energy in the form of rotational kinetic energy. In many cases, when power is available, that power is used to accelerate the rotation of a flywheel and later, when power is not available, the resulting stored energy is drawn upon to supply power. Generally, a flywheel's stored kinetic energy is proportional to its mass, the square of its radius, and the square of its rotational speed (RPM). Thus, relatively large, fast flywheels can have relatively high energy density (i.e., energy per unit mass).

In some cases, energy is added to, and withdrawn from, the flywheel via time and space varying electromagnetic fields, for example, with an electric motor and generator integrally formed with the flywheel. In some flywheels, magnetic fields are established with a field coil disposed within the flywheel (e.g., as shown in FIG. 2 of U.S. Pat. No. 6,323,573, titled “High-Efficiency Inductor-Alternator,” filed Mar. 23, 2000, the entire content of which is hereby incorporated by reference in its entirety for all purposes, as the present techniques may be used in conjunction with the surrounding structure). These field coils, in some cases, include a generally toroidal coil of wire through which a current flows to establish a magnetic circuit with which other components interact to add energy to or remove energy from the flywheel. Such field coils are often not rotating and are surrounded by several hundred pounds of rotating mass, for example, forged ferromagnetic material of the flywheel rotors.

In operation, such field coils present certain disadvantages. For example, driver circuitry and mechanical supports add to the complexity of the flywheel energy storage system. And in some cases, such mechanical supports experience relatively high loads as the relatively heavy field coil is supported by cantilevered members extending from outside the flywheel inward. Further, such field coils often generate additional heat that can be difficult to remove from the flywheel due, in part, to the surrounding rotating mass of the flywheel.

SUMMARY

Some aspects include a flywheel system, including: an armature coil set; and a rotor assembly having: a first rotor member; a second rotor member; a permanent magnet disposed between the first rotor member and the second rotor member; and a magnetic circuit formed by the first rotor member, the second rotor member, and the permanent magnet, wherein the magnetic circuit spans a gap between the first rotor member and the second rotor member into which at least part of the coil set is disposed.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

FIGS. 1 and 2are views of a flywheel assembly that is expected to mitigate some of the above-described issues with traditional systems. In some cases, the illustrative flywheel assemblies may be used in conjunction with the inductor alternators of U.S. Pat. No. 6,118,202, titled “High-Efficiency Inductor Alternator,” filed May 11, 1998, and U.S. Pat. No. 6,323,573, titled “High-Efficiency Inductor-Alternator,” filed Mar. 23, 2000, the entire contents of both of which are hereby incorporated by reference in their entirety for all purposes.

As shown inFIG. 1, flywheel assembly100is generally rotationally symmetric about axis of rotation160and may include an upper rotor130, a lower rotor140, a permanent magnet220(shown inFIG. 2), and a three-phase armature coil set150therebetween. In some cases, flywheel assembly100may include an integrated rotation shaft165.FIG. 2is a cross-sectional perspective view200of flywheel assembly100. (The terms “upper” and “lower” are used to distinguish the two components, and should not be read as imposing any particular limitation with respect to gravity.)

Upper and lower rotors130and140may have a circular cylindrical shape. In some embodiments, upper and lower rotors130and140may be made from a relatively heavy magnetically conductive material (e.g., iron, nickel, cobalt, manganese, or other magnetically conductive material). In some cases upper and lower rotors130and140may weight more than 200 pounds. That said, rotors of less or more weight may be used in embodiments consistent with the present techniques.

In some embodiments, upper rotor130and lower rotor140may include teeth135configured to be interdigitated when the flywheel is assembled. In some embodiments, the arc length of the rotor teeth may be substantially equal to (e.g., within 20% of) the arc length of the spaces between each pair of rotor teeth. Matching arc lengths is expected to reduce bucking voltages within the armature coils. In some cases, the teeth may be formed by having teeth on a single rotor (e.g., upper rotor130), with the other rotor (in these cases lower rotor140) being a relatively smooth disk. In these cases, the rotor teeth on upper rotor130may, for example, be longer than the teeth on both lower rotor140and upper rotor130when both rotors have teeth. In some cases, teeth135of rotors130and140may be formed by having protrusions on both rotors130and140. In some cases, the teeth and the upper or lower rotor are formed from one piece. For example, the upper (or lower rotor) and teeth may be cast in a mold, or the protrusions defining the teeth may be formed by machining material from the upper (or lower) rotor. In other cases teeth135may be connected to the upper or lower rotor (e.g., welded, or via a connector that conducts magnetic flux).

The permanent magnet220may be disposed between the upper and lower rotors130and140. In some cases, permanent magnet220may generally have a circular cylindrical shape with magnetic poles extending in opposite directions towards the adjacent upper or lower respective rotors, e.g., the south pole of the magnet may be directed toward lower rotor140and the north pole towards upper rotor130or vice versa. Or other magnet shapes may be used in other embodiments consistent with the present techniques, e.g., octagonal cylinders, hollow cylinders, square cylinders, structures with non-planar bases, etc. Permanent magnet220may be radially attached to the upper and lower rotors for example, bolted thereto. In some cases, permanent magnet220may be bonded to the upper or lower rotors130and140via glue or other chemical, mechanical or a non-mechanical, non-chemical connector. Upper and lower rotors130and140, and magnet220may define a toroidal volume224and an inter-rotor gap230in which armature coil set150may be disposed. The size of the toroidal volume224and inter-rotor gap230is defined by the size of magnet220and teeth135. In some embodiments, teeth135may include permanent magnets.

In some cases, the upper rotor, the permanent magnet, and the lower rotor may form a single component with zero degrees of relative movement between these components. However, other alternatives are contemplated. For example, in some cases, upper rotor130rotates and lower rotor140is stationary (or vice versa) with magnet220rotating, or upper rotor130rotates and lower rotor140is stationary (or vice versa) with magnet220being stationary.

A variety of different types of permanent magnets may be used. For example, permanent magnet220may be a neodymium iron boron magnet. In this case, permanent magnet220may have a relatively high coercivity (i.e., resistance to being demagnetized, e.g., in the range of −0.70 to −0.50 percent per degrees Celsius in the range of 20-150 degrees Celsius), and may store large amounts of magnetic energy because of the high saturation magnetization of neodymium iron boron magnets. In some embodiments, a neodymium iron boron magnet may be alloyed with other rare earth metals (e.g., terbium or dysprosium) to form permanent magnet220in order to preserve magnetic properties of permanent magnet220at high temperatures. Using a neodymium iron boron magnet may be advantageous because of its relative strength, intense field and for its relatively lower cost (because of its intense filed a small magnet may be used).

In some embodiments, other types of rare-earth magnets may be used. For example, in some embodiments, (e.g., where temperature resistance is more important) permanent magnet220may be a samarium-cobalt magnet for its relatively high temperature resistance and higher coercivity (generally samarium-cobalt magnet may be heated to a temperature between approximately 700° C. and 800° C. before the magnet loses its magnetism). However, other alternatives to rare earth element magnets also consistent with the present techniques. For example, magnetic metallic elements, composites (e.g., ferrite, or alnico), single molecule magnets, single chain magnets, Nano-structured magnets, rare-earth-free permanent magnets, or other types of permanent magnets.

Armature coil set150may include multiple armature coils. In some embodiments, armature coils of armature coil set150may generally have a rectangular (or square) shape (other shapes may also be considered). In some embodiments, the end portions of the armature coils are bent such that each coil includes an outer end portion, an inner end portion, and a left and right leg. In some embodiments, the end portions are substantially parallel to one plane, while the legs are substantially parallel to another plane that forms an angle with the end portions plane. In some cases, the end portions plane and the legs plane are slightly offset. In other cases, the end portions plane and the legs plane form an angle that is approximatively less than 90 degrees. When flywheel100is assembled, the coils of armature coil set150may be layered back to back in two layers (an upper layer and a lower layer) in the inter-rotor gap230between the upper and lower rotor (130and140) such that an inner end portion of each coil is configured to be disposed in the toroid volume224defined by the upper and lower rotors (130and140) and permanent magnet220. In some cases, an outer end portion is configured to be disposed outside of an outer rim of flywheel100. In some embodiments, the legs of the coils in the upper layer and the legs of the coils in the lower layer are substantially parallel to the same plane. In some implementations, coils in the upper layer face upper rotor130and have legs that are bent toward lower rotor140, while coils in the lower layer face lower rotor140and have legs that are bent toward upper rotor130.

In some embodiments, armature coil set150may be a three phase armature coil set. In these cases (and other cases where armature coil sets are poly-phase), the armature coils may be displaced circumferentially about the centerline, some of which are in parallel in a given phase. For example, armature coil set150may include twenty-four armature coils (formed in two layers of twelve coils each, eight coils per phase, and with three electrical phases). In some cases, the twenty-four armature coils may be circumferentially spaced (and nested) every fifteen mechanical degrees. Or other amounts of coils may be used in some embodiments. For example, other three-phase armatures with more or less armature coils (the armature coils being divisible by three to maintain proper phase alignment), or other single or poly-phase armatures may be used consistent with the present techniques.

In some embodiments, coils of armature coil set150may include solid pieces of an electrically conductive, low permeability material (e.g., copper). In some cases, the coils may include turns of wire. Coils of armature coil set150may be wound with enameled copper wire, termed magnet wire, or winding material having a low resistance (to reduce the power consumed by the field coil, and to reduce the waste heat produced by ohmic heating). In some cases, aluminum windings may be used for their relatively low cost. In some embodiments, the turns of wires may consist of a plurality of electrical conductors that are electrically insulated from each other and are electrically connected together in parallel. For example, in some cases, a litz wire constructed of individual film-insulated wires bunched or braided together in a uniform pattern of twists and length of lay may be used. In these cases, a coil formed of litz wire has at least one set of conductors that are parallel to each other coupled together in series with at least one other set of parallel conductors. This configuration may reduce skin effect power losses of solid conductors, or the tendency of high frequency current to be concentrated at the conductor surface. Generally, litz wires have individual strands each positioned in a uniform pattern moving from the center to the outside and back within a given length of the wire. In addition to the reduction of skin effect losses, litz wire and other multi-strand bundles of small gauge wire may produce lower eddy current losses than a single strand of larger wire.

In operation, the upper rotor130, the permanent magnet220, and the lower rotor140may rotate together about the axis of symmetry160, while the armature coil set150may remain generally static, experiencing time varying magnetic flux as the teeth of the rotors rotate past, varying the gap through the magnetic circuit spans. Flywheel100may store rotational kinetic energy. In some embodiments, the amount of energy stored in flywheel100is proportional to the square of the flywheel's rotational speed. In some cases, the flywheel may have revolution rate of a thousand revolution per minute (RPM) or greater. Energy may be transferred to flywheel100by the application of a torque to it by driving a time-varying current through the coils, thereby increasing its rotational speed, and its stored energy. Conversely, flywheel100may release stored energy by inducing a time-varying current in the coils and driving a load with the resulting electrical power. In some embodiments, flywheel100may behave as a unitary rotor formed from a single piece of material. In some cases, flywheel100includes an integral shaft148configured to facilitate rotation of flywheel100about axis160. The use of an integral shaft for rotation of flywheel100may provide an advantage of not limiting the tip speed of the rotor. The resulting time varying magnetic flux passing through the armature coils may drive a current that may be used as a power source in times in which power is absent, and when power is present, appropriately timed and directed current driven through the armature coils may be used to add energy to the rotors.

As shown in the flux diagram ofFIG. 3(and more clearly in the magnetic circuit436ofFIG. 4), the permanent magnet220may establish a magnetic circuit with magnetic flux330passing through the armature coils (FIG. 3shows a sectional perspective view300showing the magnetic flux path300through a flywheel assembly100having a single armature coil150), without the need for a relatively heavy, relatively complicated, and relatively thermally undesirable field coil being positioned between the upper rotor and the lower rotor. This is expected to reduce the cost of the rotor assembly, facilitate use of the rotor assembly in more thermally demanding applications, and improve reliability of the rotor assembly by removing complexity and, in particular, relatively high stress support structures for the field coil. That said, embodiments are consistent with use of a field coil. For instance, the present techniques may be used to reduce the size and load from a field coil by supplementing the field coil's magnetic flux with that of the permanent magnets. In some cases, if magnet220demagnetizes (e.g., in case the rotor heats excessively) a slip ring may transfer energy to the rotor assembly. For example, some embodiments may use a pancake slip ring having conductors arranged on a flat disc as concentric rings centered on the rotor assembly rotating shaft148.

In some embodiments, flywheel assembly100may be used for purposes other than storing power, e.g., controlling the orientation of a mechanical system attached to the flywheel (e.g., transferring the angular momentum of the flywheel to the mechanical system when energy is transferred to or from the flywheel and causing the attaching system to rotate into some desired position). For example, flywheel100may be used to control satellite orientation. In these cases, two counter-rotating flywheels100may be used to orient a satellite's instruments without the use of thruster rockets.

In some embodiments, flywheel100may provide continuous (e.g., over some duration of time, like more than ten seconds, or more than 30 seconds) energy in systems where the energy source is not continuous (e.g., after a failure of grid power). In such cases, the flywheel stores energy when a time varying current is driven by the grid, and flywheel releases the stored energy when the movement of the flywheel induces a current connected to a load.

In some embodiments, flywheel100may supply intermittent pulses of energy at transfer rates that exceed the abilities of its energy source, or when such pulses would disrupt the energy supply (e.g., public electric network). This may be achieved by accumulating stored energy in the flywheel over a period of time, at a rate that is compatible with the energy source, and then releasing that energy at a much higher rate over a relatively short time when it is needed (e.g., flywheel100may be used in riveting machines to store energy from the motor and release it during the riveting operation).

FIG. 4is a cross-section showing another example of a flywheel system400having a rotor assembly412like that described above with reference toFIGS. 1 through 3and an adjustable flux excitation ring414positioned concentrically around the rotor assembly412. The rotor assembly412and the excitation ring414may be generally rotationally symmetric about axis of rotation160. (For simplicity, only one half of the flywheel system400is shown in a half-rotor sectional view.) As noted above, the rotor assembly412may include an upper rotor130and a lower rotor140both in contact with a permanent magnet220.

As described above an upper rotor130, lower rotor140and magnet220may define a toroidal volume424and an inter-rotor gap430in which upper and lower armature coils426and428may be disposed. The rotor assembly412may be mounted to bearings (e.g., magnetic, non-contact bearings), such that the rotor assembly412may rotate while the flux excitation ring414and armature coils426and428remain generally static. For example, the outer portion of the integral shaft may be mounted to the bearings.

The flux excitation ring414may be disposed adjacent and concentrically around outer rim of the rotor assembly412. The flux excitation ring414may include a ring core432made of a magnetically conductive material and a field coil434operative to establish a magnetic flux when a current is driven through the coil434. For example, flux excitation ring414may establish a homopolar magnetic flux within rotor assembly112when energized (e.g., by a DC current).

In some cases, field coil434may be a coil of wire through which a current flows. Coils of field coil434may be wound with enameled copper wire, termed magnet wire, or winding material having a low resistance (to reduce the power consumed by the field coil, and to reduce the waste heat produced by ohmic heating). In some cases, aluminum windings may be used for their relatively low cost. In other cases, silver may be used for its lower resistivity.

In operation, magnetic field lines or magnetic circuit438pass in a continuous loop from the excitation ring414through the rotor assembly412and back through the excitation ring414again. Permanent magnet220may establish a magnetic flux circuit436through the upper and lower rotors (130and140) and armature coils426and428. Flux excitation ring414may establish a magnetic circuit438that also passes through the upper and lower rotors130and140and the armature coils426and428. The flux density of circuit438may be adjusted by adjusting current through coil434to adjust for changes in rotational speed of rotor assembly412.

In some embodiments, when drawing power from the flywheel, as the rotor assembly412slows down, the current through the coil434may be ramped up to increase the magnetic flux from the circuit438and to thereby reduce the amount of decrease in power produced by the flywheel412that would otherwise occur as the rotational speed of the rotor assembly412drops.

In some cases, the current through the coil434may be adjusted in accordance with the rotational speed of the rotor assembly412, such that the power produced by the flywheel assembly400remains generally constant, e.g., in accordance with a feedback control loop implemented with a lookup table of output currents and sensed input rotor speeds.

In some embodiments, flywheel assembly100may be used with an uninterruptible power supply (UPS), where the UPS is powered by the flywheel energy. When utility power fails, the stored energy in flywheel assembly100may be converted to a high frequency (alternative current) AC output voltage from armature coils150. A converter may convert high frequency AC power (e.g., from 300 to about 2,000 Hz or higher) into 50 or 60 Hz power that can be routed to a load. In this case, the UPS provides secondary power for intermittent losses of utility power without chemical batteries, as are traditionally used. Additionally, the UPS may provide secondary power in the event of a total loss of utility power for enough time so that either an orderly shutdown of critical equipment may occur, or until a backup standby generator may be brought on-line. Alternatively, the UPS can be used as a DC energy storage system, in which case it would be connected to the DC buss of a conventional UPS (not shown). Generally, uninterruptible power supply (UPS) devices are ready for immediate use at the instant that the power fails. They generally store small amount of energy which makes them suitable for a few seconds or minutes of use.

In some embodiments, the flywheel assembly100may be used in diesel rotary uninterruptible power supply devices (DRUPS) which combine the functionality of a flywheel-powered UPS and a diesel generator. In theses cases, an electrical generator with a mass functions as motor to store kinetic energy in flywheel assembly100. In combination with a reactor, the electrical generator may also work as an active filter (e.g., frequency variations, harmonics, etc.) If the power fails, energy stored in the flywheel is released to drive the electrical generator, and the diesel engine takes over from the flywheel to drive the electrical generator to provide electricity. The flywheel may support the diesel generator in order to keep a stable output frequency. Typically a DRUPS will have enough fuel to power the load for days or even weeks in the event of failure of the mains electricity supply. Use of a DRUPS having flywheel100may be advantageous compared to battery-powered UPS combined with a diesel-generator because of a higher overall system energy efficiency, smaller footprint, use of fewer components, longer technical lifetime, and lower chemical waste.

The reader should appreciate that the present application describes several inventions. Rather than separating those inventions into multiple isolated patent applications, applicants have grouped these inventions into a single document because their related subject matter lends itself to economies in the application process. But the distinct advantages and aspects of such inventions should not be conflated. In some cases, embodiments address all of the deficiencies noted herein, but it should be understood that the inventions are independently useful, and some embodiments address only a subset of such problems or offer other, unmentioned benefits that will be apparent to those of skill in the art reviewing the present disclosure. Due to costs constraints, some inventions disclosed herein may not be presently claimed and may be claimed in later filings, such as continuation applications or by amending the present claims. Similarly, due to space constraints, neither the Abstract nor the Summary of the Invention sections of the present document should be taken as containing a comprehensive listing of all such inventions or all aspects of such inventions.

It should be understood that the description and the drawings are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description and the drawings are to be construed as illustrative only and are for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed or omitted, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. Headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description.

1. A flywheel system comprising: an armature coil set; and a rotor assembly comprising: a first rotor member; a second rotor member; a permanent magnet disposed between the first rotor member and the second rotor member; and a magnetic circuit formed by the first rotor member, the second rotor member, and the permanent magnet, wherein the magnetic circuit spans a gap between the first rotor member and the second rotor member into which at least part of the armature coil set is disposed.
2. The flywheel system of embodiment 1, comprising: a flux excitation ring disposed circumferentially around the first rotor assembly and the second rotor assembly, the flux excitation ring having a coil and circuitry operative to adjust current through the coil based on a speed of rotation of the rotor assembly.
3. The flywheel system of embodiment 2, wherein magnetic field lines from the excitation ring pass in a continuous loop from the excitation ring through the first rotor member, the second rotor member and the armature coil set and back through the excitation ring.
4. The flywheel system of claim1, wherein the armature coil set is a three-phase armature coil set.
5. The flywheel system of any of embodiments 1-4, comprising: an integrated rotation shaft configured to facilitate rotation of the flywheel system about an axis; and magnetic bearings positioned to confine movement of the rotor assembly other than rotation about the axis.
6. The flywheel system of any of embodiments 1-5, wherein the armature coil set mounted in fixed relation relative to a housing in which the rotor assembly is disposed, and wherein the armature coil set is configured to rotate relative to the armature coil set.
7. The flywheel system of any of embodiments 1-6, wherein the first rotor member or the second rotor member includes multiple protrusions extending therefrom toward the other rotor member in angular spaced relation, and wherein the armature coil set is disposed between the protrusions and the other rotor member.
8. The flywheel system of embodiment 6, wherein both the first rotor member and the second rotor member include a plurality of interdigitated teeth extending toward the opposing rotor member.
9. The flywheel system of any of embodiments 1-8, wherein the magnetic circuit from the permanent magnet passes in a loop through the first rotor member, the second rotor member, and the armature coil set.
10. The flywheel system of any of embodiments 1-9, wherein the permanent magnet is a rare-earth magnet.
11. The flywheel system of any of embodiments 1-10, comprising: a load electrically coupleable to the armature coil set; and an internal-combustion engine generator electrically coupleable to the load.
12. The flywheel system of any of embodiments 1-11, comprising: a feedback control loop configured to adjust current through the armature coil set based on a rotation velocity of the rotor assembly.
13. A method comprising: rotating a flywheel, the flywheel comprising a an armature coil set, a first rotor member, a second rotor member, and a permanent magnet disposed between the first rotor member and the second rotor member; and conducting magnetic flux through a magnetic circuit comprising the first rotor member, the second rotor member, and the permanent magnet, wherein the magnetic circuit spans a gap between the first rotor member and the second rotor member into which at least part of the armature coil set is disposed.
14. The method of embodiment 13, comprising: augmenting magnetic flux in a portion of the magnetic circuit with an excitation ring, the excitation ring being disposed circumferentially around the first rotor member and the second rotor member, wherein at least part of the magnetic circuit is not augmented.
15. The method of any of embodiments 13-14, comprising: adjusting current through the excitation ring in response to a measured or inferred amount of electrical power generated by the flywheel.
16. The method of any of embodiments 13-15, comprising: outputting electrical power from the armature coil set; and converting a frequency of the electrical power.
17. The method of any of embodiments 13-16, comprising: applying a force orthogonal to an axis of rotation of the flywheel with a magnetic rotational bearing; and applying a force parallel to the axis of rotation of the flywheel with a magnetic thrust bearing.
18. The method of any of embodiments 13-17, varying an intensity of the magnetic flux over time in a given portion of the armature coil set
19. The method of any of embodiments 13-18, varying a gap between the first rotor member and the second rotor member into which at least part of the portion of the armature coil set is disposed.
20. The method of any of embodiments 13-19, comprising: storing electrical energy by driving rotation of the flywheel assembly with grid electrical power; drawing electrical energy from the flywheel assembly by inducing a current through armature coil set with rotation of the flywheel assembly; and powering a load with the drawn electrical energy.