Patent Publication Number: US-11641140-B2

Title: Electromechanical battery

Description:
RELATED APPLICATIONS 
     This application is a National Stage application under 35 U.S.C. § 371 of International Patent Application No. PCT/US2019/040186, filed Jul. 1, 2019, which claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/694,853 filed Jul. 6, 2018, both applications are incorporated, fully herein by reference in their entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a device and method for the storage of electrical energy in the form of kinetic energy—the mechanical energy of motion. 
     INTRODUCTION 
     The general trend away from the use of fossil fuels for the generation of electrical energy has taken many paths. Significant renewable energy sources like solar, wind and tidal power provide energy intermittently, temporally decoupled from energy demand. One approach to mitigating this has been the development of batteries to facilitate the storage of intermittent energy sources. 
     Electrochemical electrical energy storage has limitations: Of prime concern is the life cycle and cost of such batteries. Then, of course, there is the issue of toxic materials used in manufacture. Enter the electromechanical battery, commonly termed an energy-storage flywheel, FES, or simply, a flywheel. 
     Electromechanical batteries are comprised of substantially inert standard construction materials: permanent magnets, common metals, polymer composites and the like. In one recent design iteration, electromechanical batteries comprise a columnar Halbach array of magnets, a stator centered in the Halbach magnetic field, a flywheel coupled to the Halbach array where the flywheel is put in motion to store energy. To minimize as many sources of frictional resistance as possible, the high-speed rotating parts are placed in an evacuated chamber. An example of a flywheel incorporating a Halbach magnet array may be seen in Merritt, et al., Halbach Array Motor/Generators— A Novel Generalized Electric Machine. 
     In this configuration, electromechanical batteries can be superior to current electrochemical batteries. For example, power densities of 5-10 kW/kg, several times that of most internal combustion engines and 100 times that of current best electrochemical batteries, can be achieved. And the efficiency of energy recovery can exceed that of typical electrochemical batteries. 
     Perhaps most interestingly, the life cycle for electromechanical batteries is virtually limitless, with estimates of more than a decade of constant charging/discharging cycles being regularly touted. This enables electromechanical batteries to also be utilized in storage applications that can have very many fast charge/discharge cycles, such as phase and load-matching applications, power leveling, and power conditioning, in addition to providing the aforementioned storage mitigation for intermittent power sources. 
     Electromechanical batteries are not, however without problems. 
     One fundamental problem with levitated non-contacting electromechanical batteries is described by Earnshaw&#39;s Theorem, which demonstrates that there is no possible static configuration of magnets that by themselves can stably levitate an object against gravity in three dimensions. Many in the prior art have attempted to address this limitation using the dynamic motion of the flywheel to circumvent the limitation of Earnshaw&#39;s Theorem in an effort to ensure that the system is passively stable—that is, stable without an intervening active control. 
     Instead, this invention leverages low-cost modern computation to actively levitate and stabilize the flywheel system using very little power. This novel active stabilizing system substantially avoids the parasitic power drain of current active stabilizing systems, minimizes the sets of permanent magnets required, and in the process, eliminates the need for expensive, finely balanced flywheels. 
     Even with modern mass-manufacturing techniques, fabricated flywheel rotors typically exhibit imbalances that must be corrected by a post-manufacturing balancing process. Elimination of this process can speed manufacture and reduce costs. 
     Further, under the stresses generated by high-speed rotation, the real materials comprising the flywheel can strain in a non-ideal manner. As real materials age, such strains can cause the inertial center and axis of the spinning rotor to migrate, and thus create potentially huge imbalance forces. Even small imbalances can induce cyclic fatigue in rotor materials, limiting lifetimes, reducing energy storage capacity, or causing catastrophic failure. 
     These imbalance forces can be mitigated if the flywheel is allowed to rotate about its inertial axis (as opposed to a fixed geometric axis) even though the inertial axis may vary in position due to changes in stress over time. 
     Typical magnetic bearings, while contactless, do not inherently eliminate geometric axis imbalance forces, and can be complicated, expensive, and power-hungry. Magnetic suspension may also be a source of radial instability. A flywheel employing passive magnetic bearings, and disclosure of associated rotor instabilities may be seen in U.S. Pat. No. 7,876,010, which is fully incorporated herein by reference, including drawings. 
     In summary, balance-related phenomena in flywheels ultimately impose expensive precision manufacturing, derating of energy storage capacity, and service life reduction due to material aging effects that must be corrected through maintenance or repair. These serious limitations all derive from the prior art&#39;s spinning flywheels about geometrically defined axes rather than inertial rotational axes. 
     The art has long employed various combinations of Z-axis magnetic levitation configurations, using combinations of active and passive magnets such as an attractive pair above a rotor and a repulsive pair below. None has achieved the simplicity and utility provided by the novel magnetic suspension system of the instant invention. 
     A further limitation of the flywheel art relates to the need to operate high speed flywheel rotors in a vacuum. Energy storage flywheels operate at very high speeds. The periphery of most such flywheels has a velocity several times the speed of sound in air. When a rotor is operated at high speed in air, energy is lost to aerodynamic friction and shock wave phenomena. The art has long known that energy storage flywheels must operate in a vacuum to avoid such losses. 
     Maintenance of a vacuum for long periods of time without active means is difficult. Materials of construction such as composite matrix materials can outgas to a degree that rapidly degrades a vacuum environment. It is common in the art to attach active vacuum pumps to mitigate such outgassing. These pumps add cost, scheduled maintenance, and unscheduled downtime to the energy storage flywheel system. Sudden vacuum pump failure can cause catastrophic flywheel failure. In general, they reduce the service lifetime of energy storage flywheels. 
     Additional vacuum degradation may arise from penetrations through the vacuum barrier, such penetrations being needed for electrical power and signal conductors and heat rejection components. Heat rejection presents a particularly difficult vacuum sealing problem due to thermomechanical changes in seal dimensions under varying heat loads. These thermally induced dimensional changes break metal vacuum seals in extended service. Elastomer seals are commonly used, but these allow diffusion of gases into the vacuum environment, necessitating active vacuum maintenance measures such as pumps. 
     Rotation of magnets such as the rotor&#39;s Halbach array commonly induces electrical currents in conductive components that may be immersed in said rotating fields. This can destabilize a rotor, degrade control system effectiveness, and reduce energy storage efficiency due to energy dissipation as eddy current heating. The instant invention can mitigate this limitation through magnetic shielding using magnetically permeable components. 
     The foregoing illustrates, without limitation, several serious limitations in the art of energy storage flywheels that are overcome by this invention. 
     SUMMARY 
     A flywheel according to the instant invention comprises the following: 
     at least one rotor assembly that spins substantially about its inertial axis as determined by its mass distribution, thereby obviating a need for precision rotor centration or balance, said critical utility enabled by: 
     a novel double-lift attractive magnetic levitation system having static radial stability while not imposing rotation about a fixed geometric axis, and a novel control system that employs heretofore unutilized magnetic features of unipolar cylindrical single pole-pair Halbach magnetic arrays to provide rotor translation and/or tilt, as well as eliminating sub-synchronous instabilities through an efficient electronic damping functionality, and further comprises: 
     optionally, a novel means of thermal transfer whereby heat generated within the flywheel stator assembly is transported to the external environment without penetration of the flywheel&#39;s vacuum enclosure, and further comprises; 
     optionally, magnetically permeable shields that isolate magnetic fields and prevent detrimental interactions of such fields that would otherwise degrade operation of the flywheel, and further comprises; 
     sensors of rotor position in three mutually orthogonal axes X, Y, and Z, and further comprises; 
     computational means that accept sensor data, perform control computations, and energize control effectors, and further comprises; 
     at least one stator with electromagnetic coils disposed such that, in combination with a unipolar cylindrical Halbach magnet array, effects reversible transformation of electrical energy into rotational kinetic energy as well as providing for the disposition of rotor position and stability control effectors, and further comprises; 
     electronic means of interfacing with the external environment, and further comprises; 
     an enclosure and mechanical framework that together provide mechanical support, a low-pressure environment, and means for mounting the flywheel assembly to fixed support in the environment external to said enclosure. 
     These elements of the invention are discussed in more detail below. 
     Double-lift magnetic levitation system:  FIG.  1 D  schematically depicts without limitation, one potential configuration of an attractive levitator magnet assembly, which is comprised of inner ring magnet  217 , outer ring magnet  216 , electromagnetic control coil  218 , and magnetically permeable component  215 , the assembly being positioned perpendicular to the local gravity vector. Arrows indicate opposing magnetic polarity of ring magnets  216  and  217 . The assembly attracts magnetically permeable component  219 , which is mounted on the rotor assembly (not shown). 
     In this invention, one such assembly is employed at the upper portion of the rotor, while another is employed at the lower portion of the rotor, substantially about the rotor&#39;s inertial center. Importantly, both assemblies lift up on their respective magnetic components fixed to the rotor (not shown). Vertical rotor position is sensed and control coil  218  in at least one assembly is energized to modulate the total lifting force provided by the permanent magnets, to match the rotor&#39;s weight, whereupon the rotor is levitated. 
     During operation, magnetic flux generated by ring magnets  216  and  217  is conducted through magnetically permeable enclosure  215  and returns in free space or in part through magnetic component  219 . When energized, electromagnetic control coil  218  modulates flux in the magnetic circuit by changing magnetic saturation of enclosure  215  at the region between ring magnets  216  and  217 . This controls the degree of magnetic attraction exerted by the assembly on magnetic rotor component  219 . Magnet dimensions, materials and strength are generally selected such that magnets  216  and  217  contribute approximately equal magnetic energy to the magnetic circuit (up magnetic flux equaling down magnetic flux). It will be apparent to those skilled in the art that a suitable magnetic circuit may be achieved with other than two magnets, including, without limitation, only one magnet. 
     During operation, electromagnetic control coil  218  is energized (in response to Z-axis rotor position sensors and suitable computation, such as a PID control loop) to maintain the rotor in levitation by modulating the assembly&#39;s attractive force. 
     A further aspect of this double lift levitation configuration is a small stabilizing centration force the levitator magnets impart to the rotor. 
     A novelty of this invention is use of fringing fields to control rotor stability. Fringing fields, as depicted in  FIG.  1 B , have been regarded in the art as nuisances or useless epiphenomena. The instant invention employs these fields to effect rotor positioning by means of interaction with electromagnetic coils positioned within the fringing fields. The uniform unipolar magnetic field within the central region of the Halbach array cannot be employed to effect rotor displacement perpendicular to the axis of rotation. The array&#39;s fringing fields exhibit components that may be acted upon by controllable electromagnets to stabilize the rotor. 
     Electromagnetic control coils are disposed as in  FIG.  2   , elements  12  and  14 . So disposed, these coils, when energized with an electric current, exert a force on the rotor assembly through their interactions with respective fringing fields  18  and  19 . Said force may be positive or negative according to the energizing current polarity. Due to the complexity of the fringing fields, the detailed interactions between said fields and control coils may be derived, without limitation, by empirical measurements, by modeling, or by a combination thereof. 
     Radial perturbations in rotor position are sensed. Forces generated by the control coils can interact with the rotating fringing fields to damp measured instabilities in radial position. 
     Overall, control of rotor position comprises, without limitation:
         a. Measure the rotor&#39;s average geometric location at two measurement positions, one above and the other below the rotor&#39;s approximate center of mass, over an integral number of rotations. Due to the weak centration force, these measurements and computations approximate the inertial centers of rotation at the measurement positions.   b. Map the movement of upper and lower inertial centers and calculate their velocities.   c. Calculate control coil currents appropriate to damp these velocities (a variable electronic damper).   d. So long as the velocities of the upper and lower inertial centers are small, the rotor will remain in stable rotation primarily about its inertial axis.       

     Heat generation imposes other limitations on flywheels constructed according to the prior art. Joule heating loss (I 2 R, or resistive heating loss) in the stator windings is the primary contributor to heat generation under fast energy charge or discharge conditions. The flywheel vacuum environment can complicate heat transport. 
     In the instant invention, the stator assembly comprises an enclosure within which are disposed rotor control coils and motor/generator coils. A stator enclosure can provide vacuum isolation and mechanical support for components carried within the stator. The stator enclosure may also contain a suitable dielectric fluid (gas or liquid) that contacts heat-generating stator components to restore convective heat transport to the external environment. The stator assembly is thermally connected to at least one portion of at least one inner surface of the external flywheel enclosure. Using a phase-change material as the dielectric fluid may greatly enhance convective heat transport. 
    
    
     
       DESCRIPTION 
       Brief Description of the Figures 
         FIG.  1 A : Uniform Magnetic Field in the Core of a Single Pole-Pair Halbach Array 
         FIG.  1 B : Schematic of “Fringing Fields” at Ends of Columnar Single Pole-Pair Halbach Array 
         FIG.  1 C : Conventional Vertical-Axis Flywheel Suspension, shows a schematic sectional view of a conventional flywheel rotor, its supporting bearings and its geometric and inertial rotational axes. The depicted flywheel and bearings are theoretically perfect and ideal with respect to geometry and mass distribution. Rotor  201  rotates on an axle  203  that is supported radially by bearings  202  and is supported against gravity by thrust bearing assembly  204 . The figure depicts a condition of perfect balance and two conditions of imperfect balance. In the ideal case, geometric rotation axis  205  matches the inertial rotation axis, which are colinear and perfectly centered and coaxial with axle  203  and its connected rotor  201 , these all being parallel to the Z-axis of  200 . In the first depicted imbalance case, a mass distribution inhomogeneity (greater mass on the left rotor half of the sectional view) has caused inertial rotational axis  206  to no longer be colinear with geometric rotational axis  205  (displacement exaggerated for clarity). Both inertial and geometric axes remain parallel because the mass distribution inhomogeneity is purely radial and is symmetric about a horizontal plane bisecting rotor  201  normal to the rotor (with respect to the Z-axis) at exactly its Z-axis midpoint. The second imbalance case depicts rotor  201  having an asymmetric mass distribution such that at the instant depicted in the Figure, the rotor has more mass on its left side and on its upper half, leading to the displaced and tilted inertial axis  207  (displacement again exaggerated for clarity). Geometric axis  205  and inertial axis  207  are neither parallel, nor even necessarily intersecting within the rotor  201 . 
         FIG.  1 D : Example of an Attractive Levitator Assembly, This Figure schematically depicts a sectional view of one example of a levitator magnet assembly comprised of inner ring magnet  217 , outer ring magnet  216 , electromagnetic control coil  218 , and ferromagnetic housing  215 , the assembly being positioned parallel to the XY plane of coordinate system  200  and positioned relative to rotor inertial axis  206 . Arrows indicate magnetic polarity of ring magnets  216  and  217 . A magnetic rotor component that is controllably attracted by said levitator magnet assembly so as to levitate the attracted rotor (not shown) is depicted. The assembly of enclosure, magnets, and control coil is fixed to a stationary frame (not shown) and attracts magnetic component  219 , which is mounted on the rotor assembly (not shown). 
         FIG.  2   : Example of Fringe Field Interaction 
         FIG.  3   : Example of Optical Occlusion Radial Position Measurement 
         FIG.  4   : Example of a Double-Lift Configuration 
         FIG.  5   : Example of a Double-Lift Configuration with Control Coils to Modulate Lift 
         FIG.  6   : Example of Optical Occlusion Vertical Position Measurement 
         FIG.  7   : Flywheel Mass Contiguous to the Halbach Array 
         FIG.  8   : Example of Magnetic Subsystem Shielding 
     
    
    
     DISCUSSION 
     It is understood that, with regard to this description and the appended claims, reference to any aspect of this invention made in the singular includes the plural and vice versa unless it is expressly stated or unambiguously clear from the context that such is not intended. For instance, a reference to a “sensor” in the singular includes two or more sensors. 
     As used herein, any term of approximation such as, without limitation, near, about, approximately, substantially, essentially and the like, mean that the word or phrase modified by the term of approximation need not be exactly that which is written but may vary from that written description to some extent. The extent to which the description may vary will depend on how great a change can be instituted and have one of ordinary skill in the art recognize the modified version as still having the properties, characteristics and capabilities of the word or phrase unmodified by the term of approximation. In general, but with the preceding in mind, a numerical value herein that is modified by a word of approximation may vary from the stated value by ±15%, unless expressly stated otherwise. 
     As used herein, the use of “preferred,” “preferably,” “more preferred,” “presently preferred” and the like refers to preferences as they existed at the time of filing of this patent application. 
     As used herein, an “electromechanical battery,” sometimes referred to in the relevant literature as a “motor-generator,” also sometimes referred to as a “flywheel”, refers to a device that stores energy in a rapidly rotating rotor assembly and releases that energy as electrical energy as the rotor slows down. Previous manifestations of such batteries are revealed in U.S. Pat. Nos. 5,705,902, 6,566,775, 6,858,962, 7,679, 247, each of which is incorporated by reference, including drawings, as if fully set forth herein. These and other publications describe in detail the use of a Halbach array that is coupled to a flywheel that is made to rapidly spin and thereby store energy for eventual conversion to electrical as the flywheel spins down. As such, it is not considered necessary to recite herein all of the basic elements of a Halbach array electromechanical battery as such can be readily gleaned from the cited references as well as numerous other patent references and technical articles. The import of the present invention lies in its use of novel technology to stabilize the electromechanical battery&#39;s rotating assembly using an extremely low expenditure of energy and stabilizing the rotating assembly substantially about its inertial rotational axis, maximizing energy storage and conversion efficiencies, single charge lifetime and overall operational lifetime of the battery. 
     As used herein, a “rotor assembly” or “rotor” refers to a construct comprising at least the following elements:
         a. columnar single pole-pair Halbach array  5 ,  FIG.  1 B , disposed vertically along the z-axis of a standard xyz coordinate system. For the purpose of this disclosure such disposition of the Halbach array is referred to as a “vertical columnar array”. The Halbach array has a through-hole down the entire center of the array.   b. annular flywheel  8 ,  FIG.  2   , coupled to the Halbach array. A flywheel herein has the generally accepted definition, that is, a rotating disk, cylinder of wheel wherein mechanical energy is stored in the rotating mass. For the purposes of this invention, the mass constituting a flywheel may be coupled directly to the Halbach array or the main mass of the flywheel may be displaced outward from the axis of rotation of the flywheel by one or more spacers in order to increase energy stored in the flywheel. A ‘spacer’ refers to a relatively light weight, compared to the mass of the flywheel, radial member that connects the mass of the flywheel to the Halbach array. A spacer may be one of a plurality of spokes, rods or bars that connect the Halbach array to the flywheel mass. In the alternative, a spacer may comprise a solid ring of relatively light weight material where the inner radius of the ring that allows that surface of the ring to be contiguous with and coupled to the outer edge of the Halbach array and the outer radius of the ring to be contiguous with and coupled to the flywheel mass;   c. tubular stator  10   FIG.  2   , is disposed in the through-hole of the Halbach array  5 . Stator  10  does not contact the inner wall of the Halbach array; rather, the two ends of the stator are coupled to the top and bottom surfaces of a framework or housing that surrounds the battery as such is described in the cited extant literature. Stator comprises windings  11 ,  FIG.  2   , which may comprise one phase, two phase, three phase or higher windings as such are commonly known and referred to in the art. Presently preferred is a three-phase winding.       

     As used herein, a “Halbach array” refers to a specialized arrangement of permanent magnets as known and understood in the art wherein the magnets augment the magnetic field inside the through-hole of the arrangement while essentially cancelling the magnetic field on the exterior. A top view of a columnar Halbach array is shown in  FIG.  1 A . In brief, the arrows on the magnets depict the direction of the magnetic field, the arrow head designating north and the tail of the arrow designating south for each participating magnet. For the purposes of this invention, the array is set up to bias the magnetic field into the core through-hole of the array and to essentially eliminate any field outside the columnar array. The magnetic field in the through-hole, indicated by the dashed lines in  FIGS.  1 A and  1 B , is completely directional as indicated in the figures. Of particular note is the fact that the magnetic field within the through-hole of the array is unidirectional and uniform. The uniformity can be fine-tuned by increasing the number of permanent magnets used to create the columnar array and virtually any number of magnets may be used and such columnar Halbach arrays are within the scope of this invention but 12 permanent magnets are presently preferred. While the magnetic field in the through-hole of the columnar Halbach array is uniform, the magnetic field at each end of the array is nonuniform, the magnetic field there exhibiting both radial and longitudinal (or “vertical” when the Halbach array is vertically situated as is the case in the present invention) components,  FIG.  1 B . These end portions of the Halbach array magnetic field are hereafter referred to as “fringe fields”. 
     Key to this aspect of the invention are two fixed coils, one being located at or near one end of the Halbach column and the other being located at or near the other end of the Halbach column. The coils are disposed so as to be capable of interacting with the two fringe fields of the Halbach array. Thus, the coils may be located anywhere in the through-hole so long as the aforesaid interaction is possible. It is presently preferred, however, that the two coils be disposed inside and at either end of stator  10  such that first coil  12 , shown in  FIG.  2   , is disposed adjacent to top edge  15  of Halbach array  5  and second coil  14  is disposed adjacent to bottom edge  7  of Halbach array  5  ( FIG.  2   ). As can be seen, the coils are at least partially within top fringe field  18  and bottom fringe field  19  of Halbach array  5 . 
     In addition to the rotor and stator elements  4 ,  10 , respectively, an electromechanical battery of this invention includes a sensor, the function of which is to detect changes in the radial position of the rotor assembly when the rotor assembly is levitated. The sensor would, of course, be capable of detecting the radial position of the rotor assembly when it is not levitated, but such is of little consequence to the instant invention. In addition to detecting the position of the rotor assembly when levitated, the sensor is capable of detecting changes in the position of the rotor assembly when it is levitated and rotating. In fact, it is this last function of the sensor that is, at present, of most importance to this aspect of the invention. In electromechanical batteries, the flywheel is often spinning extremely rapidly. Thus, inherent resonance frequencies of the various parts of the battery when spinning and the high speed at which the flywheel is rotating can result in negative effects on the battery such as, without limitation, fatigue and construction material failure. This aspect of the present invention minimizes and potentially eliminates such adverse events. 
     Once the sensor has detected changes in the radial position of the rotor assembly, it transmits that information to a controller with which it is in communication and the controller, in turn, directs a current through the coils that are at least partially within the fringe magnetic fields thereby interacting with those fields and generating reaction forces on the rotor assembly. 
     While virtually any manner of sensor can be used for the above purpose and any and all such sensors are within the scope of this invention, it is presently preferred that the sensor comprises a light source and change in light intensity detector. This is shown in  FIG.  3   , which is a top down view of Halbach array  5 . Light source  40  is directed toward vertical edge  45  of Halbach array  5  in such a manner that a portion of the light is impeded by Halbach array  5  and a portion of the light passes by vertical edge  45  and impinges on detector  55 . If Halbach array  5  is spinning out of its preferred axial alignment, the amount of light that detector  55  sees will vary. The change in the amount of light seen by detector  55  is relayed to a controller (not shown), which then performs its function as set forth above.  FIG.  3    shows a light source being directed at the body of Halbach array  5  but the sensor/controller pair will work equally well if any edge of the flywheel or other portions of the rotor assembly are used and such are obviously within the scope of this invention. Any light source may be used but it is presently preferred that the light source be an LED, which is inexpensive, has a long operational lifetime and generates minimal heat. 
     A further aspect of this invention relates to fine-tuning the levitation of the rotor assembly. An added benefit of the novel mechanism for doing so is a passive positive effect on centration of the rotor assembly on its longitudinal (vertical) axis of rotation. This aspect of the invention uses at least two assemblies of permanent magnets. While it will be evident from the discussion that follows how the technique discussed would apply when using more than two assemblies of magnets, this description will be directed solely to the presently preferred embodiment which is the use of just two assemblies of magnets. While the following discussion refers to an embodiment in which one magnet of each assembly of levitation magnets is fixed to the rotor assembly, it discloses and applies equally to the levitation magnet assembly of  FIG.  1 D , which depicts one of the at least one assembly of levitation magnet assemblies configured such that no magnets are fixed to the rotor. 
     In  FIG.  4   , the polarity of magnets  70 ,  71 ,  72 , and  73  are as shown using the conventional arrow depiction of polarity. As can be seen, magnetic pair  70  and  71  and magnetic pair  72  and  73  are both in attractive mode. The pairs of magnets are both orthogonal to the longitudinal axis, that is, the approximate inertial axis of rotation  77  of rotor assembly  50 . With attention to achieving torque balance around the CG of the rotor&#39;s radial axis, these could be located anywhere between the ends of rotor assembly  50  but it is presently preferred that they be located equidistantly from the center of gravity (CG)  79  of rotor assembly  50 , one pair being above the CG and one pair being below the CG. The magnetic force generated by the two pairs of permanent magnets in attractive mode is selected so as to be sufficient to levitate rotor assembly  50  with its spoke/flywheel. 
     The use of magnets to offset the gravimetric weight of rotor assemblies/flywheels has long been known in the art. Commonly used configurations for vertical-axis flywheels, however, differ from that of this invention in that, in the art, either repulsive arrays are preferred, or else an attractive array is used near the top of the rotor, while a secondary array at the bottom is either absent entirely, or configured to interact in repulsion. The problem with this is that, when in repulsive mode, magnets are radially divergent; that is, they push each other away and to one side or another. With regard to an electromechanical battery, this inherent tendency to diverge necessitates additional methods to achieve and maintain rotor assembly stability around its axis of rotation. Generally, rotor assembly stability imposes tight constraints on rotor mass balance to limit imbalance forces. This, then, requires more expensive rotor manufacturing procedures. Even then, it is well-known that rotor mass distribution often changes during operation. In the short term, rotor mass distribution may change as the rotor expands nonuniformly during spin-up. Over the long term, rotor mass distribution may change as rotor materials creep due to prolonged exposure to radial and circumferential stresses at high rotation speeds. In addition, rotor mass distribution may change suddenly as the result of limited material failure under rotational stress. In contrast to prior art, the instant invention can overcome these limitations. 
     With reference to  FIG.  4   , magnet  72  and magnet  73  are set to be in attractive mode, as are magnets  70  and  71 . This configuration minimizes divergent forces, at the perceived cost of having to be actively modulated to achieve vertical stability. Flipping the direction of the poles between the upper pair and the lower pair, while not necessary for function of the invention, can make the net magnetic fields introduced by the levitation magnetic sets effectively balance each other out when considered from more remote distances. 
     By employing two levitators both of which act in attractive mode, each levitator exerts a passive radial centering force, referred to herein as “centration” on the rotor and thus requires no compensation for radial divergence effects inherent in repulsive levitators. The weak radial restoring force permits large radial clearances between the rotor assembly and adjacent stationary structure, which allows rotation approximating that which would result from a rotor spinning purely according to its inertial mass distribution without radial constraint. This enhanced clearance in turn accommodates changes in the rotor assembly/flywheel mass distribution due to spin-up/spin-down nonuniform strains, material aging and local microstructural failures. In addition, the two pairs of levitation magnets in attractive mode present essentially identical characteristics with respect to development of hardware production and assembly and can simplify control software design. These and other advantages of the “double-lift” design with both pairs of levitating magnets being in attractive mode will become clear to those skilled in the art based on the discussion herein and knowledge of the shortcomings of alternative suspension systems. 
     It is also an aspect of this invention that magnet supporting elements  77  are L-shaped with the leg of the L facing downward toward and aligned with a projection upward from magnet supporting elements  78 . While magnet supporting elements  77  and  78  may be constructed of any material of sufficient strength to perform their support function, it is presently preferred that magnet supporting elements  77  and  78  be constructed of a high magnetic permeability material. An example, without limitation, of such a material would be a ferromagnetic material such as steel. This is equally applicable to elements  215  and  219  of  FIG.  1 D . If steel elements are used, the “ring” formed by the magnets themselves and the magnet supporting elements including the L-shaped portion of element  77  and the magnet supporting elements including the L-shaped portion of element  77  and the upward facing portion of element  78  create a pathway for continuous circulation of magnetic flux, which serves to strengthen the magnetic field in the vicinity of the magnets and thereby enhance the attractive force between the magnets to further facilitate levitation. 
     To deliver the above advantages, the double-lift design of the instant invention requires active control of the vertical position of the rotor assembly/flywheel of this invention. This is accomplished by the inclusion of control coils  90  and  91 , shown in  FIG.  5   , and by control coil  218  in  FIG.  1 D . In  FIG.  5   , the control coils are situated adjacent to stationary magnets  71  and  73 . A controller (not shown) is in communication with a sensor, which detects changes in the vertical position of the levitated rotor assembly and relays that position data to the controller, which then directs a current through control coils  90  and  91  to alter the magnetic field intensity between magnets  70  and  71  and magnets  72  and  73  to affect the vertical position of rotor assembly  50 . Many control algorithms are suitable, for example, a PID controller. 
     The sensor for detecting changes in vertical position of rotor assembly  50  is similar to the sensor for detecting radial divergence of rotor assembly  50 . Again, any type of sensor that will achieve the result described below can be used and is within the scope of this invention. It is presently preferred, however, that the sensor comprises a light source and change in light intensity detector. This is shown in  FIG.  6   . Light source  100  is directed toward the edge of top surface  15  or rotor assembly  50  in such a manner that a portion of the light is impeded by rotor assembly  50  and a portion of the light passes by top surface  15  and impinges on detector  105 , which is located on the other side of rotor assembly  50  from light source  100 . If rotor assembly  50  is moving upward or downward from its current location, the amount of light that detector  105  sees will vary. Changes in the amount of light detected by detector  105  are relayed to a controller, which then performs its function as set forth above. While  FIG.  6    shows the light source being directed across the top surface of rotor assembly  50 , it is equally permissible and is within the scope of this invention for the light source to be trained across the bottom surface of levitated rotor assembly  50 . As before, any light source may be used but it is presently preferred that the light source be an LED. 
     It should be noted that, while the above double-lift mechanism for levitating a rotor assembly/flywheel is described as an addition to the earlier described device comprising means for controlling radial displacement of the rotor assembly, it is possible and is an aspect of this invention that the double-lift mechanism may be used without radial displacement control, the result being a device with active vertical placement control only. 
     The flywheel of an electromechanical battery of this invention may be appended to the rotor assembly in at least two distinct ways. The flywheel mass may be contiguous with and directly connected to the Halbach array. This is shown in  FIG.  7    where, as stated, flywheel  8  is contiguous with and joined directly to Halbach array  5 . In the alternative, the mass of flywheel  8  may comprise a separate annular ring some distance radially from rotor assembly  50  and Halbach array  5 . This is shown in  FIG.  2    where flywheel  8  is separated from the Halbach core and is attached to the rotor assembly  50  using spacing elements  110 . The spacing elements may simply be spokes such as those found on any manner of wheel connected to a hub, such as an automobile or bicycle tire. The spacing element, on the other hand, may be a solid construct in which an inner radius of the spacing element is completely contiguous with the surface of a rotor assembly and an outer radius of the spacing element is completely contiguous with the inner surface of the flywheel mass. Of course, other spacing elements for connecting the rotor assembly to the flywheel will be evident to those skilled in the art based on the disclosures herein and all such alternative spacing elements are within the scope of this invention. 
     Another aspect of this invention is a device for attenuating detrimental magnetic field interactions within an electromechanical battery. Electromechanical batteries such as that of this invention use permanent and electromagnets to generate powerful magnetic fields that may interfere with one another. For example, the magnetic fields used for the motor/generator of a flywheel-based electromechanical battery may interfere with the magnetic fields generated for systems such as those described above for vertical positioning. To attenuate these interferences, an element made of magnetically permeable material may be interposed between a magnetic field and those locations from which it is desired that a magnetic field be excluded or, at least, its intensity at the location be diminished. The permeable element provides a preferential path for magnetic flux thereby diminishing residual magnetic force that might detrimentally affect another element of the overall electromechanical battery system. Any type of highly magnetically permeable material may be used but it is presently preferred that the material be ferromagnetic and have a magnetic permeability of at least 2000. This would, it is anticipated, enhance the stable operation of all elements of the system. Such an element is shown in  FIG.  8   . 
       FIG.  8    shows rotor assembly  50  with double-lift permanent magnet pairs  70  and  71  and  72  and  73 . Placed between the top surface  150  of Halbach array  5  and double lift magnet pair  72  and  73  is annular element  160 . Likewise, annular element  161  is placed between bottom surface  151  of Halbach array  5  and double lift magnet pair  70  and  71 . The elements are annular so as to allow stator  10  to protrude from both ends of rotor assembly  50  and to be attached to a top and a bottom surface of a housing (not shown) containing the entire electromechanical battery. As shown in  FIG.  8   , annular elements  160  and  161  isolate the magnetic fields created by permanent magnets  70 ,  71 ,  72 , and  73  from fringe fields  18  and  19  created by the Halbach array (not shown but see  FIG.  2   ). This then mitigates interferences between the magnetic fields and any attendant negative effect on the stability of rotor assembly  50 . 
     PRIOR ART CITED 
     
         
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