Patent Publication Number: US-2023163673-A1

Title: Flywheel systems and flywheel bearing modules

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/682,331 filed Feb. 28, 2022, which is a continuation of U.S. patent application Ser. No. 16/758,298 filed Apr. 22, 2020, which is a 35 U.S.C. § 371 filing of International Application No. PCT/DK2018/050265 filed Oct. 22, 2018, which claims the benefit of priority to U.S. Provisional Application No. 62/575,489, filed Oct. 22, 2017, and Danish Patent Application No. PA 2018 00643 filed Sep. 26, 2018, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     A flywheel system is a mechanical device that stores rotational energy in a mass. The amount of energy stored in the rotor is proportional to the square of the rotor&#39;s rotational speed. The rotor may be magnetically coupled with an electromagnetic generator stator to allow the flywheel system to convert between rotational energy of the rotor and electrical energy. The generator stator may decelerate the rotor to produce electrical energy from the rotational energy extracted from the rotor, and the generator stator may receive electrical energy and convert this electrical energy to rotational energy of the rotor resulting in acceleration of the rotor. Flywheel systems may be designed to have large energy storage capacity, and are further capable of both delivering and absorbing energy rapidly. Common uses of a flywheel system include (a) peak-shaving of the power output of another energy source such as a combustion generator stator, (b) energy storage, (c) backup power supply, and (d) rapid energy delivery. 
     Low-loss energy storage in a flywheel system requires that the rotor rotates with very little friction. Therefore, the rotor of a high-performance flywheel system typically is magnetically levitated. 
     SUMMARY 
     In an embodiment, a flywheel system includes a rotor configured to rotate about a rotation axis. The flywheel system further includes a fixture and an active magnetic bearing module for actively stabilizing the rotor relative to the fixture. The active magnetic bearing module includes a plurality of first magnetizable elements mechanically coupled to or integrated in the rotor, and a plurality of electromagnets mechanically coupled to the fixture and configured to magnetically couple with the plurality of first magnetizable elements to actively stabilize the rotor relative to the fixture. Each of the first magnetizable elements is farther than each of the electromagnets from the rotation axis. 
     Optionally, each of the first magnetizable elements is a soft magnetic composite. 
     Optionally, the first magnetizable elements are arranged along a first diameter about the rotation axis, the electromagnets being arranged along a second diameter about the rotation axis, the first diameter being greater than the second diameter. 
     Optionally, in dimensions orthogonal to the rotation axis, the electromagnets and the first magnetizable elements are away from each other by a first radial gap, the first radial gap being in range between 2 and 10 millimeters. 
     Optionally, extent, along the rotation axis, of the first magnetizable elements exceeding extent along the rotation axis, of the electromagnets. 
     Optionally, the rotor forms a first void encircling the rotation axis, the plurality of first magnetizable elements being positioned at a first surface of the first void that encircles the rotation axis and faces the rotation axis, each of the electromagnets being positioned in the first void to magnetically couple with the first magnetizable elements across a portion of the first void. 
     Optionally, permanent magnets are mechanically coupled to or integrated in the rotor and positioned at a second surface of the first void that encircles the rotation axis; anda generator stator is mechanically coupled to the fixture and positioned in the first void to magnetically couple with the permanent magnets, so as to convert between rotational energy of the rotor and electric current in windings of the generator stator. 
     Optionally, the fixture comprises an endplate positioned adjacent a top end or a bottom end of the rotor, the electromagnets and the generator stator being attached to the endplate. 
     Optionally, the generator stator is between the electromagnets and the endplate, the second surface being closer than the first surface to the endplate. 
     Optionally, the electromagnets are between the generator stator and the endplate, the first surface being closer than the second surface to the endplate. 
     Optionally, the endplate forms a base adjacent the bottom-end, the flywheel system further comprises: first load bearing magnets mechanically coupled to or integrated in the rotor at bottom surface of rotor; second load bearing magnets, mechanically coupled to the base, for magnetically coupling with the first load bearing magnets to magnetically levitate the rotor above the base. 
     Optionally, in the first void, there is at least one passive magnetic bearing for stabilizing the rotor relative to the fixture if the active magnetic bearing loses power, each passive magnetic bearing comprising: second permanent magnets mechanically coupled with the rotor; and third permanent magnets mechanically coupled to the fixture and positioned in the first void to magnetically couple with the second permanent magnets, so as to provide backup stabilization of the rotor relative to the fixture. 
     Optionally, the at least one passive magnetic bearing comprises a plurality of passive magnetic bearings located in different respective positions. 
     Optionally,in dimensions orthogonal to the rotation axis, the electromagnets and the first magnetizable elements are apart from each other by a first radial gap, and the generator stator and the permanent magnets are apart from each of by a second radial gap that exceeds the first radial gap. 
     Optionally, each of the first surface and the second surface facing the rotation axis, the first surface and the second surface are respective portions of a common cylindrical surface. 
     Optionally, each of the first surface and the second surface facing the rotation axis, diameter of the first surface are different from diameter of the second surface. 
     Optionally, each of the first surface and the second surface facing the rotation axis, the first void span across the rotation axis. 
     Optionally, the first void is a groove that encircles the rotation axis but does not coincide with the rotation axis, the rotor further forms a central void closer than the groove to the rotation axis, the flywheel system further comprising: permanent magnets are mechanically coupled to or integrated in the rotor and positioned at a second surface of the central void that faces and encircles the rotation axis; and a generator stator is mechanically coupled to the fixture and positioned in the central void to magnetically couple with the permanent magnets, so as to convert between rotational energy of the rotor and electric current in windings of the generator stator. 
     Optionally, the fixture comprises an endplate positioned adjacent a top end or bottom end of the rotor, the electromagnets and the generator stator being attached to the endplate. 
     Optionally, the endplate forms a base positioned adjacent the bottom end of the rotor, the flywheel system further comprising: first load bearing magnets mechanically coupled to or integrated in the rotor at bottom surface of rotor; and second load bearing magnets, mechanically coupled to the base, for magnetically coupling with the first load bearing magnets to magnetically levitate the rotor above the base. 
     Optionally, in one or both of the groove and the central void, there is at least one passive magnetic bearing for stabilizing the rotor relative to the fixture if the active magnetic bearing loses power, each passive magnetic bearing comprising: second permanent magnets mechanically coupled to or integrated in the rotor; and third permanent magnets mechanically coupled to the fixture and positioned in the groove or the central void to magnetically couple with the second permanent magnets, so as to provide backup stabilization of the rotor relative to the fixture. 
     Optionally, the at least one passive magnetic bearing comprises a plurality of passive magnetic bearings located in different respective positions. 
     Optionally, there is a power supply for powering the plurality of electromagnets to adjust position of the rotor relative to the fixture; and at least one sensor for sensing a movement characteristic of the rotor relative to the fixture, the at least one sensor being communicatively coupled with the power supply to enable adjustment of the position of the rotor relative to the fixture in response to the movement characteristic. 
     Optionally, there is at least one passive backup bearing including second permanent magnets for stabilizing the rotor relative to the fixture if the power supply fails to provide power to one or more of the plurality of electromagnets. 
     In an embodiment, a bearing module for a flywheel system includes a plurality of first magnetizable elements arranged along a first diameter and configured to be mechanically coupled to a rotor of the flywheel system, and a plurality of electromagnets configured to be mechanically coupled to a fixture and magnetically couple with the first magnetizable elements to stabilize the rotor relative to the fixture. The electromagnets are bounded by a second diameter that is smaller than the first diameter to enable positioning of the electromagnets inside the first diameter. 
     Optionally, each of the first magnetizable elements is a soft magnetic composite. 
     Optionally, there is at least one sensor for sensing a position characteristic of the rotor relative to the fixture; and a power supply, communicatively coupled with the at least one sensor, for powering the electromagnets to adjust position of the rotor relative to the fixture in response to the position characteristic. 
     Optionally, there is at least one passive magnetic bearing including (a) a plurality of first permanent magnets configured to be mechanically coupled to the rotor and (b) a plurality of second permanent magnets configured to be mechanically coupled to the fixture and magnetically couple with the first permanent magnets to stabilize the rotor relative to the fixture if the power supply fails to provide power to the electromagnets. 
     Optionally, there is a plurality of third permanent magnets arranged along a third diameter and configured to be mechanically coupled to the rotor; and a generator stator for magnetically coupling with the third permanent magnets, to convert between rotational energy of the rotor and electric current in windings of the generator stator. 
     Optionally, the generator stator is bounded by a fourth diameter that is smaller than the third diameter to enable positioning of the generator stator inside the third diameter. 
     Optionally, the electromagnets are mounted on the generator stator. 
     Optionally, there is first load bearing magnets configured to be mechanically coupled to the rotor; and second load bearing magnets configured to be mechanically coupled to the fixture and magnetically couple with the first load bearing magnets to magnetically levitate the rotor above the second load bearing magnets. 
     In an embodiment, a bearing module is integrated with a generator. The integrated bearing module and generator are configured for use in a flywheel system and include a plurality of permanent magnets, a generator stator, and an active magnetic bearing. The plurality of permanent magnets are arranged along a first diameter and configured to be mechanically coupled to a rotor of the flywheel system. The generator stator is configured to be mechanically coupled to a fixture. The generator stator is bounded by a second diameter that is smaller than the first diameter to enable positioning of the generator stator inside the first diameter to magnetically couple with the permanent magnets, so as to convert between rotational energy of the rotor and electric current in windings of the generator stator. The active magnetic bearing includes (a) a plurality of first magnetizable elements arranged along a third diameter and configured to be mechanically coupled to the rotor, and (b) a plurality of electromagnets arranged along a fourth diameter and configured to be mechanically coupled to the fixture and magnetically couple with the first magnetizable elements. The third diameter is greater than the first diameter to enable positioning of the first magnetizable elements at greater distance than the permanent magnets from rotation axis of the rotor. The fourth diameter is greater than the first diameter to enable positioning of the active magnetic bearing at greater distance than the permanent magnets from the rotation axis. 
     Optionally, each of the first magnetizable elements being a soft magnet. 
     Optionally, the fourth diameter is greater than the third diameter to enable positioning of the first magnetizable elements closer than the electromagnets to the rotation axis. 
     Optionally, the fourth diameter is smaller than the third diameter to enable positioning of the first magnetizable elements farther than the electromagnets from the rotation axis. 
     Optionally, there is an endplate that forms at least a portion of the fixture, the generator stator and the electromagnets being mounted on the endplate. 
     Optionally, first load bearing magnets are configured to be mechanically coupled to the rotor; and second load bearing magnets configured to be mechanically coupled to the fixture and magnetically couple with the first load bearing magnets to magnetically levitate the rotor above the second load bearing magnets. 
     Optionally, there is at least one sensor for sensing a position characteristic of the rotor relative to the fixture; and a power supply, communicatively coupled with the at least one sensor, for powering the electromagnets to adjust position of the rotor relative to the fixture in response to the position characteristic. 
     Optionally, at least one passive magnetic bearing includes (a) a plurality of second permanent magnets configured to be mechanically coupled to the rotor and (b) a plurality of third permanent magnets configured to be mechanically coupled to the fixture and magnetically couple with the second permanent magnets to stabilize the rotor relative to the fixture if the power supply fails to provide power to the electromagnets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a flywheel system in an exemplary use scenario, according to an embodiment. 
         FIG.  2    illustrates a flywheel system that includes an active magnetic bearing to actively stabilize the rotor of the flywheel system relative to the foundation of the flywheel system, according to an embodiment. 
         FIG.  3    schematically illustrates an active magnetic bearing module for a flywheel system, according to an embodiment. 
         FIG.  4    illustrates an active magnetic bearing for use in a flywheel system, according to an embodiment. 
         FIG.  5    illustrates a flywheel system having an active magnetic bearing positioned in a void of the rotor, according to an embodiment. 
         FIG.  6    illustrates an alternate flywheel system implementing an active magnetic bearing at a shaft of the flywheel system. 
         FIG.  7    illustrates a flywheel system having an active magnetic bearing that is integrated with a generator of the flywheel system, according to an embodiment. 
         FIG.  8    illustrates a bearing module having both an active magnetic bearing and a passive magnetic backup bearing, according to an embodiment. 
         FIG.  9    illustrates a flywheel system that has an active magnetic bearing integrated with a generator of the flywheel system and further includes one or more passive magnetic backup bearings, according to an embodiment. 
         FIG.  10    illustrates another flywheel system having an active magnetic bearing that is integrated with a generator of the flywheel system, according to an embodiment. 
         FIG.  11    illustrates a flywheel system having an active magnetic bearing that is integrated with a generator of the flywheel system with both the active magnetic bearing and the generator being mounted above a top end of the rotor of the flywheel system, according to an embodiment. 
         FIG.  12    illustrates a flywheel system including a generator and an active magnetic bearing positioned at greater radii than the generator, according to an embodiment. 
         FIG.  13    illustrates another flywheel system including a generator and an active magnetic bearing positioned at greater radii than the generator, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Flywheel systems are being considered for use in offshore or onshore environments not connected to a conventional electrical grid but instead relying on a so-called micro grid. In these environments, flywheel systems may serve as a source of energy and, for example, provide power functionalities such as peak-shaving or frequency control. In addition, flywheel systems may serve to rapidly meet a high, short-term power demand. When implemented in environments that are potentially unstable, such as onboard a drillship, a semisubmersible drilling platform, or another marine vessel, the flywheel system is subject to substantial forces which can affect both the performance and lifetime of the flywheel system. 
     Disclosed herein are active magnetic bearings configured to actively stabilize the rotor of a flywheel system relative to the foundation of the flywheel system. These active magnetic bearings enable implementation of flywheel systems in both offshore environments and unstable onshore environments such as locations subject to earthquakes. Herein, an “active magnetic bearing” refers to a bearing that is adjustable based upon an input signal. An active magnetic bearing may be coupled with one or more sensors in a feedback loop. 
       FIG.  1    illustrates one flywheel system  100  in an exemplary use scenario including a rotor  110  and a fixture  120  that supports rotor  110 . Rotor  110  is configured to rotate about a rotation axis  190 , as indicated by arrow  192  or in the direction opposite arrow  192 . Fixture  120  couples rotor  110  to a foundation  180 , for example the floor of a building or a deck onboard a marine vessel. Fixture  120  is substantially rigidly coupled to foundation  180  and therefore moves with foundation  180  when foundation  180  moves. Foundation  180  may undergo movement in a variety of directions, for example horizontal translation as indicated by arrow  152 , vertical translation as indicated by arrows  154 , and rotation as indicated by  150 , or a combination thereof. Fixture  120  may form part of a housing around rotor  110 , such as a vacuum enclosure. In one example, the weight of rotor  110  is between 10 and 10,000 kilograms. 
       FIG.  2    illustrates one flywheel system  200  that includes an active magnetic bearing to actively stabilize the rotor of the flywheel system relative to the foundation of the flywheel system. Flywheel system  200  is an embodiment of flywheel system  100  that further includes an active magnetic bearing  210  that utilizes magnetic coupling between rotor  110  and fixture  120  to stabilize rotor  110  relative to fixture  120 . Active magnetic bearing  210  includes electromagnets to actively adjust the position of rotor  110  relative to fixture  120 . Active magnetic bearing  210  may serve to maintain minimal or no friction during rotation of rotor  110  about rotation axis  190  and/or prevent damage to flywheel system  200  associated with excessive physical contact between rotor  110  and fixture  120 . In one example, active magnetic bearing  210  ensures that the direction of rotation axis  190  remains sufficiently constant relative to fixture  120  to ensure satisfactory performance of flywheel system  200  and to prevent damage to flywheel system  200 . 
       FIG.  3    is a block diagram that schematically illustrates one active magnetic bearing module  300  for a flywheel system such as flywheel system  200 . Active magnetic bearing module  300  is an embodiment of active magnetic bearing  210 . Active magnetic bearing module  300  includes a plurality of magnetizable elements  310  and a plurality of electromagnets  320  configured to magnetically couple with magnetizable elements  310  as shown schematically by arrows  330 . Each magnetizable element  310  may be a soft magnetic composite, a stack of laminated transformer steel, a stack of non-oriented electrical steel, or a magnetic material with intrinsic coercivity less than 1000 Ampere/meter. When active magnetic bearing module  300  is implemented in flywheel system  200 , magnetizable elements  310  are mechanically coupled to rotor  110  or integrated in rotor  110 , and electromagnets  320  are mechanically coupled to fixture  120 . Electromagnets  320  enable active adjustment of the position of rotor  110  relative to fixture  120 . 
     In an embodiment, active magnetic bearing module  300  further includes one or more sensors  340  and at least one power supply  350 . In operation, sensor(s)  340  senses a property of the position or motion of rotor  110  relative to fixture  120  and communicates this property to power supply  350  which controls the current passing through one or more of electromagnets  320  according to the property. 
     Active magnetic bearing module  300  cooperates with rotor  110  and fixture  120  to form an embodiment of flywheel system  200 . 
       FIG.  4    illustrates one active magnetic bearing  400  for use in a flywheel system. Active magnetic bearing  400  is an embodiment of magnetizable elements  310  and electromagnets  320  and may be implemented in flywheel system  200 . Active magnetic bearing  400  includes (a) a plurality of magnetizable elements  410  arranged along a diameter  412  and (b) a plurality of electromagnets  420  arranged along a diameter  422  that is smaller than diameter  412  such that electromagnets  420  may be positioned within the ring of magnetizable elements  410 . The number of magnetizable elements  410  and electromagnets  420  may be different from that shown in  FIG.  4   , without departing from the scope hereof. Also without departing from the scope hereof, the number of magnetizable elements  410  may be different from the number of electromagnets  420 . 
     In one implementation of active magnetic bearing  400  in flywheel system  200 , each of diameters  412  and  422  is centered about rotation axis  190 , as illustrated. 
       FIG.  5    illustrates one flywheel system  500  having an active magnetic bearing  530  positioned in a void of the rotor. Flywheel system  500  is an embodiment of flywheel system  200 , and active magnetic bearing  530  is an embodiment of active magnetic bearing  400 . Flywheel system  500  includes a rotor  510  and a fixture  520 . Rotor  510  forms a void  512 . Void  512  faces fixture  520  and encircles rotation axis  190 . Void  512  may, but does not need to, span across rotation axis  190 . A tip  514  extends from rotor  510  toward fixture  520 . Without departing from the scope hereof, tip  514  may be omitted from flywheel system  500 . 
     Active magnetic bearing  530  includes a plurality of magnetizable elements  532  mechanically coupled to, or integrated in, rotor  510  at a surface  516  of void  512  facing rotation axis  190 . Active magnetic bearing  530  further includes a plurality of electromagnets  534  mechanically coupled to fixture  520  via a mount  522 . Mount  522  may form a hollow  523  that accommodates tip  514 . In embodiments that do not include tip  514 , mount  522  may be solid across rotation axis  190 . Electromagnets  534  are configured to magnetically couple with magnetizable elements  532  across the portion of void  512  between electromagnets  534  and magnetizable elements  532 . The nominal radial gap  535  between magnetizable elements  532  and electromagnets  534 , when rotor  510  is radially centered about mount  522 , may be in the range between 2 and 10 millimeters. In operation, electromagnets  534  exert forces  538  on magnetizable elements  532  at surface  516  to actively stabilize rotor  510  relative to fixture  520 . 
     The axial extent (along rotation axis  190 ) of magnetizable elements  532  may exceed the axial extent of electromagnets  534 , such that the magnetic coupling between magnetizable elements  532  and electromagnets  534  is the same or similar even in the presence of axial movement of rotor  510  relative to fixture  520 . In one implementation, the axial extent of magnetizable elements  532  exceed the axial extent of electromagnets  534  by 10% in both axial directions. 
     In an embodiment, active magnetic bearing  530  includes one or more sensors  536  that senses a property of the position and/or motion of rotor  510  relative to fixture  520 . Sensor(s)  536  form an embodiment of sensor(s)  340 . Flywheel system  500  may further include power supply  350  as discussed above in reference to  FIG.  3   . 
     Fixture  520  may be positioned below a bottom end of rotor  510  (as shown in  FIG.  5   ) or above a top end of rotor  510 . Herein, the “bottom end” and “top end” of a rotor refer to the bottom end and top end, respectively, of the rotor when the rotation axis is vertical. It is understood that a flywheel system may be oriented with a non-vertical rotation axis, for example prior to installation in an operating environment, or when the operating environment causes the orientation of a nominally vertical rotation axis to deviate from vertical (e.g., during movement and/or oscillation of the foundation supporting a flywheel system designed to operate with a generally vertical orientation axis). Similarly, the terms “above” and “below”, as used herein, are referenced to the rotation axis. Fixture  520  is, for example, an endplate of a housing around rotor  510 . In certain embodiments, fixture  520  is a base of flywheel system  500 . In such embodiments, flywheel system  500  may further include a plurality of permanent magnets  540  mechanically coupled to rotor  510  and a plurality of permanent magnets  542  mechanically coupled to fixture  520 . Permanent magnets  540  and  542  are configured to magnetically couple with each other to bear the load of rotor  510  so as to magnetically levitate rotor  510  above the base formed by fixture  520 . 
     Without departing from the scope hereof, active magnetic bearing  530  may be provided as a standalone bearing to be implemented in a third party flywheel system. Active magnetic bearing  530  may be provided together with one or more of power supply  350 , permanent magnets  540 , and permanent magnets  542 . 
       FIG.  6    illustrates an alternate flywheel system  600  implementing an active magnetic bearing at a shaft of the flywheel system. Flywheel system  600  includes a rotor  610 , a shaft  612  attached to rotor  610  (or integrally formed therewith), and a base  620  positioned below a bottom end of rotor  610 . Flywheel system  600  further includes (a) magnetizable elements  632  attached to shaft  612  and (b) electromagnets  634  extending up from base  620  to magnetically couple with magnetizable elements  632  on shaft  612 . When utilizing electromagnets  634  to stabilize rotor  610  relative to base  620 , electromagnets  634  exert forces  638  inward on shaft  612 . These forces concentrate significant stress on the area  614  where shaft  612  and rotor  610  connect to each other. 
     In contrast, forces  538  in flywheel system  500  are directed outward onto a larger surface of rotor  510  and do not generate the stress caused by forces  638  in flywheel system  600 . The configuration of flywheel system  500  thereby reduces or eliminates any adverse effect of active magnetic bearing  530  on the performance and lifetime of flywheel system  500 . 
       FIG.  7    illustrates one flywheel system  700  having an active magnetic bearing that is integrated with a generator of the flywheel system. Flywheel system  700  is an embodiment of flywheel system  500 . Flywheel system  700  includes a rotor  710  and fixture  520 . Rotor  710  forms a void  712 . A portion of void  712  closer to fixture  520  has diameter  788 , and a portion of void  712  farther from fixture  520  has diameter  786 . Flywheel system  700  includes (a) a plurality of permanent magnets  742  mechanically coupled to, or integrated in, rotor  710  at a surface  718  of void  712  characterized by diameter  788 , and (b) a generator stator  740  mounted to fixture  520 . Generator stator  740  includes a plurality of windings  744  that magnetically couple with permanent magnets  742  to convert between rotational energy of rotor  710  and electrical energy in windings  744 . Generator stator  740  may function in both “generator mode” and “motor mode”. In “generator mode”, generator stator  740  decelerates the rotation of rotor  710  to generate electrical energy, in the form of electrical energy in windings  744 , from rotational energy of rotor  710 . In “motor mode”, generator stator  740  uses electrical energy, supplied from an external source to windings  744 , to accelerate the rotation of rotor  710  and thereby increase the rotational energy of rotor  710 . In one implementation, windings  744  are water cooled, air cooled by forced air, or passively air cooled. Flywheel system  700  further includes active magnetic bearing  530  positioned in void  712 . Flywheel system  700  implements magnetizable elements  532  at a surface  716  of void  712  characterized by diameter  786 . Flywheel system  700  implements electromagnets  534 , and optionally sensor(s)  536  in a mount  722  above generator stator  740 . 
     A pair of tips  714  and  715  extend from rotor  710  toward fixture  520 . Mount  722  and generator stator  740  may form respective hollows  723  and  743  to accommodate tips  714  and  715 . Without departing from the scope hereof, tips  714  and  715  may be omitted from flywheel system  700 . 
     Fixture  520  may be positioned below a bottom end of rotor  710  (as shown in  FIG.  7   ) or above a top end of rotor  710 . Fixture  520  is, for example, an endplate of a housing around rotor  710 . In certain embodiments, fixture  520  is a base of flywheel system  700 . In such embodiments, flywheel system  700  may further include permanent magnets  540  mechanically coupled to rotor  710  and permanent magnets  542  mechanically coupled to fixture  520 , to magnetically levitate rotor  710  above the base formed by fixture  520 . 
     In an embodiment, flywheel system  700  further includes a sensor array  750  positioned in fixture  520  or mechanically coupled to fixture  520 . Sensor array  750  senses motion properties of fixture  520  and may serve to impose limitations on the operation of flywheel system  700  according to such motion properties. For example, the rate of acceleration and/or deceleration of rotor  710  may be limited during time periods when fixture  520  undergoes relatively large movement. 
     The nominal radial gap  735  (when rotor  710  is radially centered above generator stator  740  and mount  722 ) between magnetizable elements  532  and electromagnets  534  may be smaller than the nominal radial gap  745  between permanent magnets  742  and generator stator  740 , so as to provide active stabilization with sufficient accuracy to ensure that permanent magnets  742  do not come into physical contact with any portion of generator stator  740 . In one example, nominal radial gap  745  is at least twice the value of nominal radial gap  735 . Nominal radial gap  735  may be similar to nominal radial gap  535 . 
     Without departing from the scope hereof, diameters  786  and  788  may be identical such that surfaces  716  and  718  are respective portions of a common cylindrical surface. 
     Also without departing from the scope hereof, active magnetic bearing  530  and generator stator  740  may be provided as a standalone integrated bearing module to be implemented in a third party flywheel system. This integrated bearing module may further include one or more of mount  722 , power supply  350 , permanent magnets  540 , and permanent magnets  542 . 
       FIG.  8    illustrates one bearing module  800  having both an active magnetic bearing and a passive magnetic backup bearing. Bearing module  800  is an extension of active magnetic bearing module  300  that further includes permanent magnets  810  mechanically coupled to rotor  110  and permanent magnets  820  mechanically coupled to fixture  520 . Permanent magnets  810  and  820  are configured to magnetically couple with each other (as indicated by magnetic coupling  830 ). In the event that electromagnets  320  should be incapable of sufficiently stabilizing rotor  110  relative to fixture  120 , for example if power supply  350  fails, permanent magnets  810  and  820  form a passive magnetic bearing configured to provide at least some degree of stabilization of rotor  110  relative to fixture  120 . The backup stabilization provided by permanent magnets  810  and  820  may be sufficient to prevent catastrophic damage of a flywheel system implementing bearing module  800  and, for example, safely stabilize rotor  110  during deceleration to a standstill. 
       FIG.  9    illustrates one exemplary flywheel system  900  that has an active magnetic bearing integrated with a generator of the flywheel system and further includes one or more passive magnetic backup bearings. Flywheel system  900  is an embodiment of flywheel system  700  that further includes one or more passive backup bearings  910 . Each bearing  910  includes permanent magnets  810  and  820  respectively coupled to rotor  710  and fixture  520  (directly or indirectly).  FIG.  9    shows several exemplary locations of bearings  910 . In embodiments including multiple bearings  910 , smaller and/or less powerful permanent magnets  810  and  820  may suffice to achieve the same backup magnetic force as in embodiments utilizing a single, more powerful bearing  910 . Without departing from the scope hereof, flywheel system  900  may include more or fewer bearings  910  than shown in  FIG.  9   , and bearing(s)  910  may be located in position(s) different from those shown in  FIG.  9   . Also without departing from the scope hereof, one or more bearings  910  may be implemented in flywheel system  500 . 
     Also without departing from the scope hereof, active magnetic bearing  530 , generator stator  740 , and passive magnetic bearing(s)  910  may be provided as a standalone integrated bearing module to be implemented in a third party flywheel system. This integrated bearing module may further include one or more of mount  722 , power supply  350 , permanent magnets  540 , and permanent magnets  542 . 
       FIG.  10    illustrates another flywheel system  1000  having an active magnetic bearing that is integrated with a generator of the flywheel system. Flywheel system  1000  is similar to flywheel system  700  except that, in flywheel system  700 , generator stator  740  is closer than active magnetic bearing  530  to fixture  520  whereas, in flywheel system  1000 , generator stator  740  is farther than active magnetic bearing  530  from fixture  520 . In flywheel system  1000 , generator stator  740  and permanent magnets  742  are in the portion of void  712  associated with surface  716 , and active magnetic bearing  530  is in the portion of void  712  associated with surface  718 . In flywheel system  1000 , electromagnets  534  are mechanically coupled to fixture  520  via a mount  1022  that is similar to mount  722 . 
     Although not shown in  FIG.  10   , it is understood that flywheel system  1000  may further include one or more passive backup bearings  910  as discussed above in reference to  FIG.  9   . 
       FIG.  11    illustrates one flywheel system  1100  having an active magnetic bearing that is integrated with a generator of the flywheel system with both the active magnetic bearing and the generator being mounted above a top end of the rotor of the flywheel system. Flywheel system  1100  is similar to flywheel system  1000 . However, as compared to flywheel system  1000 , rotor  710  is replaced by a rotor  1110  that is upside down relative to rotor  710  such that void  712  faces upwards. Mount  1022  and generator stator  740  are suspended from a top plate  1130  positioned above a top end of rotor  1110 . In flywheel system  1100 , top plate  1130  and fixture  520  may form respective endplates of a housing that encloses rotor  1110 . It is understood that each of flywheel systems  700  and  900  may be modified in a similar manner with bearings and generator stator being suspended from above. 
       FIG.  12    illustrates one flywheel system  1200  including generator stator  740  and an active magnetic bearing  1230  positioned at greater radii than generator stator  740 . Flywheel system  1200  is an embodiment of flywheel system  500 . Flywheel system  1200  includes fixture  520  and a rotor  1210 . Rotor  1210  forms (a) a groove  1232  encircling rotation axis  190  and having an inner diameter  1286  and (b) a central void  1212  that is similar to void  512  and has a diameter  1282  which is smaller than inner diameter  1286 . A tip  1214  extends from rotor  1210  toward fixture  520  inside void  1212 . Void  1212  accommodates generator stator  740  which may form a hollow for accommodating tip  1214 . Without departing from the scope hereof, tip  1214  may be omitted from flywheel system  1200 . 
     Flywheel system  1200  further includes permanent magnets  742  positioned at a surface  1216  of void  1212 . Windings  744  of generator stator  740  magnetically couple with permanent magnets  742  as discussed above in reference to  FIG.  7   . Groove  1232  accommodates electromagnets  534  mechanically coupled to fixture  520  and configured to magnetically couple with magnetizable elements  532  across a portion of groove  1232 . Groove  1232  may further accommodate sensor(s)  536 . Magnetizable elements  532  are mechanically coupled to, or integrated in, rotor  1210  and arranged along a diameter that is smaller than the diameter associated with electromagnets  534 . In flywheel system  1200 , magnetizable elements  532  and electromagnets  534  cooperate to form an active magnetic bearing magnetic. 
     Flywheel system  1200  may include one or more passive backup bearings  910 .  FIG.  12    shows exemplary locations of such bearings  910 . Alternatively, one or more passive backup bearings  910  may be positioned at least partly in groove  1232 . In an embodiment, flywheel system  1200  includes permanent magnets  540  and  542  configured as discussed above in reference to  FIG.  5   . 
     Without departing from the scope hereof, rotor  1210  may be turned upside down in a manner similar to that discussed for rotor  1110  in reference to  FIG.  11   . 
     Also without departing from the scope hereof, active magnetic bearing  1230  and generator stator  740 , and optionally passive magnetic bearing(s)  910 , may be provided as a standalone integrated bearing module to be implemented in a third party flywheel system. This integrated bearing module may further include one or more of power supply  350 , permanent magnets  540 , and permanent magnets  542 . 
       FIG.  13    illustrates one flywheel system  1300  including generator stator  740 , permanent magnets  742 , and an active magnetic bearing  1330  positioned at greater radii than generator stator  740  and permanent magnets  742 . Flywheel system  1300  is similar to flywheel system  1200  except that active magnetic bearing  1230  is replaced by active magnetic bearing  1330 . Active magnetic bearing  1330  is similar to active magnetic bearing  1230  except that, in active magnetic bearing  1330 , magnetizable elements  532  are disposed farther than electromagnets  534  from rotation axis  190 . 
     Combinations of Features 
     Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. For example, it will be appreciated that aspects of one flywheel system or bearing module described herein may incorporate or swap features of another flywheel system or bearing module described herein. The following examples illustrate possible, non-limiting combinations of embodiments described above. It should be clear that many other changes and modifications may be made to the methods and device herein without departing from the spirit and scope of this invention: 
     (A1) A flywheel system may include a rotor configured to rotate about a rotation axis, a fixture, and a bearing module for at least one of (a) supporting the rotor on the fixture and (b) stabilizing the rotor relative to the fixture. 
     (A2) In the flywheel system denoted as (A1), the bearing module may include an active magnetic bearing for actively stabilizing the rotor relative to the fixture. 
     (A3) In the flywheel system denoted as (A2), the active magnetic bearing may include a plurality of first magnetizable elements mechanically coupled to or integrated in the rotor, and a plurality of electromagnets mechanically coupled to the fixture and configured to magnetically couple with the plurality of first magnetizable elements to actively stabilize the rotor relative to the fixture. 
     (A4) In the flywheel system denoted as (A3), each of the first magnetizable elements may be a soft magnetic composite. 
     (A5) In either of the flywheel systems denoted as (A3) and (A4), each of the first magnetizable elements may be farther than each of the electromagnets from the rotation axis. 
     (A6) In any of the flywheel systems denoted as (A3) through (A5), the first magnetizable elements may be arranged along a first diameter about the rotation axis, and the electromagnets being arranged along a second diameter about the rotation axis, wherein the first diameter is greater than the second diameter. 
     (A7) In any of the flywheel systems denoted as (A3) through (A6), in dimensions orthogonal to the rotation axis, the electromagnets and the first magnetizable elements may be away from each other by a first radial gap, the first radial gap being in range between 2 and 10 millimeters 
     (A8) In any of the flywheel systems denoted as (A3) through (A7), the rotor may form a first void encircling the rotation axis, and the plurality of first magnetizable elements may be positioned at a first surface of the first void that encircles the rotation axis and faces or faces away from the rotation axis. 
     (A9) In the flywheel system denoted as (A8), each of the first magnetizable elements may extend along a portion of the rotation axis. 
     (A10) In either of the flywheel systems denoted as (A8) and (A9), each of the electromagnets may be positioned in the first void to magnetically couple with the first magnetizable elements across a portion of the first void. 
     (A11) The flywheel system denoted as (A10) may further include (i) permanent magnets mechanically coupled to or integrated in the rotor and positioned at a second surface of the first void that encircles the rotation axis, and (ii) a generator stator mechanically coupled to the fixture and positioned in the first void to magnetically couple with the permanent magnets, so as to convert between rotational energy of the rotor and electric current in windings of the generator stator. 
     (A12) In the flywheel system denoted as (A11), the fixture may include an endplate positioned adjacent a top end or bottom end of the rotor, and the electromagnets and the generator stator may be attached to the endplate. 
     (A13) In the flywheel system denoted as (A12), the generator stator may be between the electromagnets and the endplate, and the second surface may be closer than the first surface to the endplate. 
     (A14) In the flywheel system denoted as (A12), the electromagnets may be between the generator stator and the endplate, and the first surface being closer than the second surface to the endplate. 
     (A15) In any of the flywheel systems denoted as (A12) through (A14), the endplate may form a base adjacent the bottom-end, and the flywheel system may further include (I) first load bearing magnets mechanically coupled to or integrated in the rotor at bottom surface of rotor, and (II) second load bearing magnets, mechanically coupled to the base, for magnetically coupling with the first load bearing magnets to magnetically levitate the rotor above the base. 
     (A16) Any of the flywheel systems denoted as (A12) through (A15) may further include, in the first void, at least one passive magnetic bearing for stabilizing the rotor relative to the fixture if the active magnetic bearing loses power. 
     (A17) In the flywheel system denoted as (A16), the at least one passive magnetic bearing may include a plurality of passive magnetic bearings located in different respective positions. 
     (A18) In the flywheel system denoted as (A16), each passive magnetic bearing may include second permanent magnets mechanically coupled to or integrated in the rotor, and third permanent magnets mechanically coupled to the fixture and positioned in the first void to magnetically couple with the second permanent magnets, so as to provide backup stabilization of the rotor relative to the fixture. 
     (A19) In any of the flywheel systems denoted as (A11) through (A12), each of the first surface and the second surface may face the rotation axis. 
     (A20) In the flywheel system denoted as (A19), the first surface and the second surface may be respective portions of a common cylindrical surface. 
     (A21) In the flywheel system denoted as (A19), diameter of the first surface may be different from diameter of the second surface. 
     (A22) In any of the flywheel systems denoted as (A19) through (A21), the first void may span across the rotation axis. 
     (A23) In the flywheel system denoted as (A10), the first void may be a groove that encircles the rotation axis but does not coincide with the rotation axis. 
     (A24) In the flywheel system denoted as (A23), the first surface may face away from the rotation axis, such that the first magnetizable elements are closer than the electromagnets to the rotation axis. 
     (A25) In either of the flywheel systems denoted as (A23) and (A24), the rotor may further form a central void closer than the groove to the rotation axis, and the flywheel system may further include permanent magnets mechanically coupled to or integrated in the rotor and positioned at a second surface of the central void that faces and encircles the rotation axis, and a generator stator mechanically coupled to the fixture and positioned in the central void to magnetically couple with the permanent magnets, so as to convert between rotational energy of the rotor and electric current in windings of the generator stator. 
     (A26) In the flywheel system denoted as (A25), the fixture may include an endplate positioned adjacent a top end or bottom end of the rotor, and the electromagnets and the generator stator may be attached to the endplate. 
     (A27) In the flywheel system denoted as (A26), the endplate may form a base positioned adjacent a bottom end of the rotor, and the flywheel system may further include (I) first load bearing magnets mechanically coupled to or integrated in the rotor at bottom surface of rotor, and (II) second load bearing magnets, mechanically coupled to the base, for magnetically coupling with the first load bearing magnets to magnetically levitate the rotor above the base. 
     (A28) Any of the flywheel systems denoted as (A25) through (A27) may further include, in one or both of the groove and the central void, at least one passive magnetic bearing for stabilizing the rotor relative to the fixture if the active magnetic bearing loses power. 
     (A29) In the flywheel system denoted as (A28), the at least one passive magnetic bearing may include a plurality of passive magnetic bearings located in different respective positions. 
     (A30) In either of the flywheel systems denoted as (A28) and (A29), each passive magnetic bearing may include second permanent magnets mechanically coupled to or integrated in the rotor, and third permanent magnets mechanically coupled to the fixture and positioned in the groove or the central void to magnetically couple with the second permanent magnets, so as to provide backup stabilization of the rotor relative to the fixture. 
     (A31) Any of the flywheel systems denoted as (A3) through (A30) may further include a power supply for powering the plurality of electromagnets to adjust position of the rotor relative to the fixture. 
     (A32) The flywheel system denoted as (A31) may further include at least one sensor for sensing a movement characteristic of the rotor relative to the fixture, wherein the at least one sensor is communicatively coupled with the power supply to enable adjustment of the position of the rotor relative to the fixture in response to the movement characteristic 
     (A33) The flywheel system denoted as (A32) may further include at least one passive backup bearing that includes second permanent magnets for stabilizing the rotor relative to the fixture if the power supply fails to provide power to the one or more electromagnets. 
     (B1) A bearing module for a flywheel system may include a plurality of first magnetizable elements arranged along a first diameter and configured to be mechanically coupled to a rotor of the flywheel system, and a plurality of electromagnets configured to be mechanically coupled to a fixture and magnetically couple with the first magnetizable elements to stabilize the rotor relative to the fixture, wherein the electromagnets are bounded by a second diameter that is smaller than the first diameter to enable positioning of the electromagnets inside the first diameter. 
     (B2) In the bearing module denoted as (B1), each of the first magnetizable elements may be a soft magnet. 
     (B3) Either of the bearing modules denoted as (B1) and (B2) may further include at least one sensor for sensing a position characteristic of the rotor relative to the fixture, and a power supply, communicatively coupled with the at least one sensor, for powering the electromagnets to adjust position of the rotor relative to the fixture in response to the position characteristic. 
     (B4) Any of the bearing modules denoted as (B1) through (B3) may further include at least one passive magnetic bearing including (a) a plurality of first permanent magnets configured to be mechanically coupled to the rotor and (b) a plurality of second permanent magnets configured to be mechanically coupled to the fixture and magnetically couple with the first permanent magnets to stabilize the rotor relative to the fixture if the power supply fails to provide power to the electromagnets. 
     (B5) Any of the bearing modules denoted as (B1) through (B4) may further include a plurality of third permanent magnets arranged along a third diameter and configured to be mechanically coupled to the rotor, and a generator stator for magnetically coupling with the third permanent magnets, to convert between rotational energy of the rotor and electric current in windings of the generator stator. 
     (B6) In the bearing module denoted as (B5), the generator stator may be bounded by a fourth diameter that is smaller than the third diameter to enable positioning of the generator stator inside the third diameter. 
     (B7) In the bearing module denoted as (B6), the electromagnets may be mounted on the generator stator. 
     (B8) Any of the bearing modules denoted as (B1) through (B7) may further include first load bearing magnets configured to be mechanically coupled to the rotor, and second load bearing magnets configured to be mechanically coupled to the fixture and magnetically couple with the first load bearing magnets to magnetically levitate the rotor above the second load bearing magnets. 
     (C1) A bearing module integrated with a generator for use in a flywheel system may include (1) a plurality of permanent magnets arranged along a first diameter and configured to be mechanically coupled to a rotor of the flywheel system, (2) a generator stator configured to be mechanically coupled to a fixture, the generator stator being bounded by a second diameter that is smaller than the first diameter to enable positioning of the generator stator inside the first diameter to magnetically couple with the permanent magnets, so as to convert between rotational energy of the rotor and electric current in windings of the generator stator, and (3) an active magnetic bearing including (a) a plurality of first magnetizable elements arranged along a third diameter and configured to be mechanically coupled to the rotor, wherein the third diameter is greater than the first diameter to enable positioning of the first magnetizable elements at greater distance than the permanent magnets from rotation axis of the rotor, and (b) a plurality of electromagnets arranged along a fourth diameter and configured to be mechanically coupled to the fixture and magnetically couple with the first magnetizable elements, so as to actively stabilize the rotor relative to the fixture, wherein the fourth diameter is greater than the first diameter to enable positioning of the active magnetic bearing at greater distance than the permanent magnets from the rotation axis. 
     (C2) In the bearing module denoted as (C1), each of the first magnetizable elements may be a soft magnet. 
     (C3) In either of the bearing modules denoted as (C1) and (C2), the fourth diameter may be greater than the third diameter to enable positioning of the first magnetizable elements closer than the electromagnets to the rotation axis. 
     (C4) In either of the bearing modules denoted as (C1) and (C2), the fourth diameter may be smaller than the third diameter to enable positioning of the first magnetizable elements farther than the electromagnets from the rotation axis. 
     (C5) Any of the bearing modules denoted as (C1) through (C4) may further include an endplate that forms at least a portion of the fixture, and the generator stator and the electromagnets may be mounted on the endplate. 
     (C6) Any of the bearing modules denoted as (C1) through (C5) may further include first load bearing magnets configured to be mechanically coupled to the rotor, and second load bearing magnets configured to be mechanically coupled to the fixture and magnetically couple with the first load bearing magnets to magnetically levitate the rotor above the second load bearing magnets. 
     (C7) Any of the bearing module denoted as (C1) through (C6) may further include at least one sensor for sensing a position characteristic of the rotor relative to the fixture, and a power supply, communicatively coupled with the at least one sensor, for powering the electromagnets to adjust position of the rotor relative to the fixture in response to the position characteristic. 
     (C8) Any of the bearing modules denoted as (C1) through (C7) may further include at least one passive magnetic bearing including (a) a plurality of second permanent magnets configured to be mechanically coupled to the rotor and (b) a plurality of third permanent magnets configured to be mechanically coupled to the fixture and magnetically couple with the second permanent magnets to stabilize the rotor relative to the fixture if the power supply fails to provide power to the electromagnets. 
     Changes may be made in the above systems and methods without departing from the scope hereof. It should thus be noted that the matter contained in the above description and shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover generic and specific features described herein, as well as all statements of the scope of the present systems and methods, which, as a matter of language, might be said to fall therebetween.