Abstract:
Apparatuses, systems and methods are described for a flywheel system incorporating a rotor made from a high-strength material in an open-core flywheel architecture with a high-temperature superconductive (HTS) bearing technology to achieve the desired high energy density in the flywheel energy storage devices, to obtain superior results and performance, and that eliminates the material growth-matching problem and obviates radial growth and bending mode issues that otherwise occur at various high frequencies and speeds.

Description:
TECHNOLOGICAL FIELD 
     The present disclosure relates to flywheel energy storage devices and, more particularly, to hubless, or open-core flywheel storage devices having improved stability and performance. 
     BACKGROUND 
     Flywheel energy storage devices and systems are known for storing energy and releasing stored energy on demand. Known flywheel assemblies have a traditional rotor design sometimes made with carbon fiber composites. Such rotors have a shaft on which the motor/generator (M/G) and bearing permanent magnets (PMs) are mounted. The shaft is conventionally connected to the rim via a hub. The shaft-and-hub flywheel design is limited in terms of its achievable upper-end velocity. Matching useable materials for components in the flywheel assembly has been problematic since the radial growth of the components varies as the rotor velocity increases. The hub must mechanically couple the shaft to the rim without introducing bending modes into the rotor structure through the range of operating frequencies in the operating speed range of the flywheel. However, the shaft often exhibits negligible radial growth while the rim exhibits significant radial growth. This imbalance in component growth during flywheel operation restricts flywheel performance and can lead to flywheel system failure. 
     BRIEF SUMMARY 
     The present disclosure is directed to a flywheel and flywheel architecture that eliminates the material growth-matching problem and obviates radial growth and bending mode issues that otherwise occur at various frequencies and speeds. More specifically, disclosed herein are flywheel assemblies having an “open-core” (hubless) architecture as opposed to a shaft-and hub architecture. 
     The present disclosure is directed to a novel open-core flywheel energy storage system that will obtain high energy, high power-density and efficiency, while having a significantly reduced size profile. The flywheel storage systems of the present disclosure comprise high-temperature superconducting (HTS) bearings and rotors comprising high-strength materials. Preferred high-strength materials include but are not limited to carbon fiber-containing materials, glass fiber-containing materials, metal-containing materials, etc. and combinations thereof. 
     The desired properties inherent in the fabricated rotors of the present disclosure result in significantly improved flywheel performance in terms of significantly increased speed, increased power storage/generation and increased system durability. 
     Still further, disclosures are directed to a flywheel assembly for storing and releasing energy comprising a hollow substantially cylindrical rotor assembly having a rotor having an inner and outer surface. The rotor comprises a material preferably having a preferred tensile strength of from about 2 GPa to about 20 GPa. A stator assembly is positioned in close proximity with the rotor assembly with at least one flexible rotor magnet affixed to the inner surface of the rotor and at least one stator magnet affixed to the stator. The flexible rotor magnet preferably comprises FeBNd powder. The stator magnets have an attractive force value at rest and are dimensioned to a predetermined width to substantially maintain the attractive force value with the rotor when the rotor is operating at circumferential velocities of from about 300 m/s to about 3000 m/s. The rotor magnets and stator magnets are positioned relative to one another to facilitate levitation of the rotor during operation. The flywheel architecture is preferably an open-core architecture, wherein the rotor preferably achieves a velocity at its outer radius of from about 300 m/s to about 3000 m/s during operation. 
     Still further, variations are directed to a method for storing energy for subsequent release upon demand comprising the steps of providing a hollow substantially cylindrical rotor assembly comprising a rotor having an inner and outer surface. The rotor comprises a carbon-fiber-containing, glass-fiber-containing or metal-containing material (or a combination thereof) with the material having a tensile strength of from about 2 GPa to about 20 GPa. A stator assembly is provided and positioned in close proximity with the rotor assembly, preferably in an open-core architecture. At least one flexible rotor magnet is affixed to the inner surface of the rotor and the stator and rotor are positioned relative to one another to facilitate levitation of the rotor during operation. Preferably, the rotor achieves a circumferential velocity at its outer radius of from about 300 m/s to about 3000 m/s during operation. The carbon-fiber-containing, glass-fiber-containing or metal-containing material preferably comprises a matrix of materials selected from the group consisting of graphite, E-glass, S-glass, silica, aluminum, titanium, steel and combinations thereof. One particularly preferred material is a carbon nanotube-containing material, and is preferably a single-walled carbon nanotube-containing material. 
     In preferred variations, the flywheel assemblies have particular usefulness as a sustainable power source for use in stationary applications and mobile applications such as, for example, manned and unmanned vehicles, including aircraft, spacecraft and terrestrial and surface and subsurface water-borne vehicles, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       Having thus described variations of the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1( a )  is a cross-sectional view of a prior art shaft-and-hub flywheel assembly; 
         FIG. 1( b )  is a cross-sectional view of an open-core flywheel assembly; 
         FIG. 2  is a partially exploded view of a high-temperature superconducting bearing; 
         FIG. 3  is a grid showing directional magnetization of a low-order Halbach array; 
         FIG. 4  is a graph showing radial magnetic field over complete pole pitch circumferential length; 
         FIG. 5  is a graph comparing the rotational rates and voltages of the open-core and shaft-and-hub flywheels; 
         FIG. 6  is a close-up cross-sectional view of an alternate flywheel assembly shown in  FIG. 1( b ) ; 
         FIG. 7  is a close-up cross-sectional view of the flywheel assembly shown in  FIG. 1( b ) ; 
         FIGS. 8( a )-8( e ) and 9( a ) and 9( b )  are close-up cross-sectional views of various bearing configurations and magnetizations. 
         FIG. 10  is a cut-away view of a variation of the present disclosure showing an inner surface of a rotor showing PMs with direction of magnetization; and 
         FIG. 11  is a cut-away view of a variation of the present disclosure showing an inner surface of a rotor showing PMs and a copper conductor; 
     
    
    
     DETAILED DESCRIPTION 
     According to the present disclosure, there are several key technologies that are incorporated into the open-core flywheel architecture to achieve the desired high energy density in the flywheel energy storage devices to obtain superior results and performance. Such advances include incorporating rotors made from high-strength materials, and incorporating a rotor in an open-core (hubless) flywheel architecture with a high-temperature superconductive (HTS) bearing technology. 
     Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material. These cylindrical carbon molecules have unusual properties that are valuable for nanotechnology, electronics, optics and other fields of material science and technology. Because of their thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Individual nanotubes naturally align themselves into “ropes” held together by van der Waals forces, more specifically, pi-stacking. 
     CNTs are among the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus. This strength results from the covalent sp 2  bonds formed between the individual carbon atoms. MWCNTs were tested to have tensile strength of about 63 gigapascals (GPa). For illustration, this translates into the ability to endure tension of a weight equivalent to 6422 kg on a cable with a cross-section of 1 mm 2 . Individual CNT shells have a strength of up to about 100 GPa. Since CNTs have a low density for a solid of from about 1.3 to about 1.4 g/cm 3 , their specific strength of up to about 48,000 kN·m·kg −1  is the best of known materials, compared to, for example, high-strength carbon steel having a specific strength of about 154 kN·m·kg −1 . 
     Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes leads to significant reductions in the effective strength of multi-walled carbon nanotubes and carbon nanotube bundles down to only a few GPa&#39;s. However, applying high-energy electron irradiation, which crosslinks inner shells and tubes, effectively increases the strength of these materials to about 60 GPa for multi-walled carbon nanotubes and about 17 GPa for double-walled carbon nanotube bundles. 
     Standard single-walled carbon nanotubes (SWCNTs) can withstand a pressure up to about 24 GPa without deformation. They then undergo a transformation to superhard phase nanotubes. Maximum pressures measured using current experimental techniques are about 55 GPa. However, these new superhard phase nanotubes collapse at an even higher, albeit unknown, pressure. 
     Multi-walled carbon nanotubes (MWCNTs) are multiple concentric nanotubes precisely nested within one another. These CNTs exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. 
     According to the present disclosure, CNTs are used directly in the manufacture of the composite rotors. MWCNT yarns having a density of about 0.2 gm/cm 3  are believed to yield a conservative minimal material strength of at least about 45 GPa, for twist-free composite structures. 
     The preferred CNTs for use in the fabrication of the novel rotors of the present disclosure preferably have a wall thickness of about 0.075 nm and an effective wall thickness of about 0.34 nm with a physical wall strength of from about 150 to about 260 GPa. This provides a preferred material having volume fractions of up to about 65% of 30 nm diameter MWCNTs with metallic, glassy and/or polymeric matrices. Inducing defects into the MWCNTs is believed to improve inter-wall strength to improve mechanical load transfer between the MWCNT strands to inner strand “walls” by a factor of about 2. 
     The preferred CNTs used in the present disclosure are specifically formulated by controlling the degree of orientation and volume fraction in a matrix to afford the finished composite material and product desired physical properties (such as, for example, higher rotor tensile strengths) than are presently known. 
     In addition, ceramic-type magnets have been used in flywheel assemblies have not been practical at higher rotational speeds (circumferential velocities) due to their inherent characteristics including, but not limited to, their brittleness, for example. Therefore, as rotational flywheel speeds increase, various magnet types are needed. Known ceramic magnets are generally limited to circumferential velocities of less than about 300 m/s. The present disclosure contemplates incorporating flexible magnets having desirable properties, including their ability to expand as the rotor material itself expands in operation at very high speeds. Preferred flexible magnets comprise FeBNd powder. 
     Some variations of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all variations of the disclosure are shown. Indeed, the present disclosure may be embodied in many different forms and should not be construed as limited to the variations set forth herein. Instead, these illustrative variations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. For example, unless otherwise indicated, referencing something as being a first, second or the like should not be construed to imply a particular order. Also, something may be described as being “above” something else and, unless otherwise indicated, may instead be “below”, and vice versa. Similarly, something described as being to the left of something else may instead be to the right, and vice versa. Like reference numerals refer to like elements throughout. 
       FIG. 1( a )  shows a cross-sectional view of a traditional shaft-and-hub flywheel assembly  10  that displays limited performance at, for example, various frequencies and higher speeds. A fiber-composite rim rotor  12  is attached to hub  14  that, in turn, is attached to shaft  16 . Sintered permanent magnets (PMs)  15  and  18  exert attractive and repulsive forces on a lift PM  20  and a high temperature superconductor  22  that are attached to shaft  16 . PM  20  is shown attached to support  17 . A stator coil  24  from the motor/generator (M/G) is shown suspended between the M/G PM  26  and support  17 . 
       FIG. 1( b )  shows a cross-sectional view of a flywheel architecture  30  made according to the present disclosure. In this “hubless” open-core flywheel architecture (with dot-dashed line indicating a centerline), elastic permanent magnets (PMs)  34 ,  36  and  38  are shown affixed to a fiber-composite rim rotor  32 . Lift bearing stator PM  48  and stator coil  42  from the motor/generator (M/G) are attached to support structure  43 . High temperature superconductor (HTS)  45  is positioned proximate to support  46 . PMs  48  and  34  comprise the lift bearing, and elements  45  and  38  comprise the stability bearing. 
     The open-core architecture of the present disclosure presents a novel design that enables the fiber-composite rim and the HTS bearing to achieve maximum performance without the design limitations of component radial growth disparities inherent in the shaft-and-hub flywheel design. It is understood that the entire open-core flywheel  30  in its shown vertical orientation, is contained within a vacuum chamber (not shown). In a preferred vertical orientation, the ring-shaped rotor  32  is preferably suspended by a passively stable magnetic bearing comprising a lift bearing PM  48  and  34  at one end or “top” and a HTS stability bearing  45  and  38  at a second end, or “bottom”. Preferably, a brushless PM motor/generator  36  and  42  delivers power in and out of the rotor. As shown in  FIG. 1( b ) , the rotor PMs  34 ,  36  and  38  are positioned along the inner surface  33  of the rotor  32 . According to the present disclosure, these PMs must be sufficiently flexible to accommodate the radial growth or, “dimensional expansion”, of the flywheel without breaking or otherwise compromising structural integrity or performance. As such, the PMs desirably have a relative low Young&#39;s modulus in the range of from about 0.01 MPa to about 2 MPa. An example materials for these magnets include those comprising FeBNd powder dispersed in rubber. The coldhead of a small cryocooler (not shown) thermally conducts to the HTS stability bearing  45  to maintain a desired temperature of from about 30 K to about 90 K, and preferably about 60 K. A small turbo-molecular or getter-sublimation pump (not shown) maintains the vacuum inside the chamber. 
     The use of the HTS bearing is important to the present disclosure and allows the flywheel rotor to rotate at high velocity and take advantage of the benefits of the open-core architecture. The HTS bearing remains passively stable so long as the temperature of the HTS components remains below 80 K. The heat capacity of the HTS combined with low heat leak into the HTS results in the ability to maintain a sufficiently low temperature to keep stability and operate the bearing. 
     In earlier known HTS bearings, the HTS elements were bathed in liquid nitrogen. Advanced HTS bearings do not require a liquid cryogen.  FIG. 2  shows schematically the HTS part of the system  60  according to the present disclosure, including the cryogenic cooling. Cryocooler  64  comprises coldhead  66 . Coldhead  66  connects to cables  68  that may be flexible, and that preferably act as thermal conductors at cryogenic temperature. The cables preferably comprise copper, copper alloys, aluminum, aluminum alloys, and combinations thereof, etc. Cables  68  connect to a preferably flat, thermally conducting plate  70  by means of a conducting lug  72 . HTS element  62  rests on top of thermally conducting plate  70 . Thermally conducting plate  70  preferably rests on, and is supported by non-thermally conducting plate  74 . Lugs  72  preferably penetrate non-thermally conducting plate  74  through openings in plate  74  in one or more places, and preferably do not contact plate  74 . Plate  74  is mechanically connected by a non-thermally conducting support  76  that connects to ground support  78 . The cryogenic portion of the system may be covered in one or more sheets of film (not shown) having a low emissivity to reduce heat input to the system by means of radiation. 
     This configuration is similar to the stator component of a superconducting stability bearing used in a 5-kWh, 3-kWh flywheel assembly as reported in Materials Science and Engineering B 151 (2008) 195-198 M Strasik, J. R. Hull, P. E. Johnson, J. Mittleider, K. E. McCrary, C. R. McIver, A. C. Day,  Performance of a Conduction - cooled High - temperature Superconducting Bearing . As indicated by the experimental bearing loss values, the presence of the copper thermal bus under the HTS elements did not significantly increase the bearing loss. The gap is the distance between the bottom of the flywheel rotor magnet and the top of the HTS crystals. A gap of from about 2 mm to about 4 mm is preferred for the HTS bearing. The rotational loss in an HTS bearing is proportional to (ΔB) 3 /Jc, where ΔB is the inhomogeneity of the magnetic field of the PM component measured in the rotational direction, and Jc is the critical current density in the HTS. 
     According to further variations of the present disclosure, for the HTS bearing to operate optimally, the stator part of the bearing preferably must be kept at cryogenic temperatures of lower than about 80 K, and more preferably from about 30 K to about 80 K. This is accomplished by establishing a flexible mechanical connection with high thermal conductance between the support base for the bulk HTS and the coldhead of a cryocooler. One preferred cryocooler contemplated for inclusion in the preferred flywheel systems of the present disclosure is the Sunpower Cryotel™ (SunPower Inc., Athens, Ohio). The preferred cryocooler is a linear, free-piston, integral Stirling-cycle machine that uses air bearings and has no friction-based failure modes, and has the ability to provide up to about 15 W of cooling at about 77 K. In addition, the preferred cryocooler has the ability to throttle input power when less cooling is required, and should provide cooling to the HTS bearing for flywheel sizes up to about 100 kWh. 
     According to the present disclosure, the M/G works as a conventional radial-gap brushless design in that in motor mode currents pass through the stator coils in a timed manner to interact with the magnetic field of the rotor PMs to produce torque. In generator mode, the magnetic flux of rotating PMs sweeps through the stator coils and produces voltage according to Faraday&#39;s law. At low speeds, a Hall-effect sensor measures the magnetic field from the M/G PMs to control the timing of the stator currents. At high speeds, the back electromagnetic field on the coils provides the input for this control. In a conventional radial-gap M/G, the stator coil is typically located radially outward from the PMs. However, according to preferred variations of the present disclosure, in the preferred open-core design, the locations are reversed, with the stator coils located radially inward of the PMs, as shown in  FIG. 1( b ) . 
     According to the present disclosure, the PMs of the open-core M/G are magnetized in a low-order Halbach array, as shown in  FIG. 3 . The magnetization of a low-order Halbach array over a single pole length of 8 degrees is shown in the circumferential (x) direction. The value “z” represent the vertical and “y” the radial directions. The thinness of the bonded magnet shell dictates that the circumferential pole length cannot be too great without severely limiting the available flux and distorting the desired sinusoidal waveform in the stator core. The pole length is preferably greater than about 10 times the gap between the rotor PM inner radius and the stator coil outer radius. Experimental calculations for an example flywheel, shown in  FIG. 4  indicate that a 90-pole machine, with a gap of about 5 mm between PMs and stator, provides sufficient flux and waveform.  FIG. 4  shows a radial magnetic field over complete pole pitch μ at 5 mm radially inward from the PMs shown in  FIG. 3 . The preferred maximum electrical frequency for such a M/G is about 30 kHz. A stator comprising Litz wire windings without a ferromagnetic core is sufficient to provide the required power output without creating a substantial eddy current or other parasitic loss. 
     The high speed of the rotor and the large number of poles create a high power density. Further, for the relatively low power requirements of the flywheels made according to certain variations of the present disclosure, the radial thickness of the stator windings is relatively small, such as, for example, from about 1 mm to about 10 mm. 
     One significant advantage of the disclosed open-core flywheel architecture of the present disclosure is that rotor growth with speed significantly widens the speed range over which the power electronics can efficiently extract energy from the flywheel. According to variations of the open-core architecture, the rotor&#39;s dimension grows radially as the flywheel speed increases. In addition, as the PMs of the M/G move farther away from the stator coils, the magnetic flux through the coil diminishes. This results in a voltage that is relatively constant over the upper speed range of the flywheel. An example calculation for the outer flywheel of the design is shown in  FIG. 5 . The rotor radius increases by about 4.2 mm in increasing speed to about 48,500 rpm. Standard power electronics can typically remove energy from the flywheel when the generator voltage is between about 0.6 to about 1.0 of the maximum design value. This limits the available energy from a shaft-and-hub flywheel to 64% of the maximum kinetic energy. As seen in  FIG. 5 , in the open-core design of variations of the present disclosure, 60% of the maximum voltage is available for speeds greater than about 15,000 rpm, and over 90% of the maximum kinetic energy is available for the load. In the example shown in  FIG. 5 , the maximum voltage occurs at about 40,000 rpm, and decreases slightly at speed in excess of about 40,000 rpm. 
       FIG. 6  shows an open-core flywheel  100  concentric about a centerline  102 . The flywheel comprises rotor  110  and stator  120 . Rotor  110  preferably comprises a fiber-composite rim  112 , an upper stability bearing permanent magnet (PM)  114 , a lower stability permanent magnet PM  116 , and a motor/generator permanent magnet (PM) array  118 . The stator  120  comprises an upper stability bearing HTS array  124 , a lower stability bearing HTS array  126 , a stator coil assembly  128 , and mechanical supports  134 ,  136  and  138 . Mechanical support  134  supports the upper stability bearing HTS  124 . Mechanical support  136  supports lower stability bearing HTS array  126 . Mechanical support  138  supports stator coil assembly  128 . The mechanical supports  134 ,  136  and  138  are fixedly attached to a vacuum chamber (not shown) that surrounds flywheel assembly  100 . It is understood that while supports  134  and  138  are shown immediately adjacent to one another, such supports may be spaced a desired distance from one another. Flywheel rotor  110  is magnetically levitated via the magnetic bearing components, including the upper stability bearing (comprising rotor PM  114  and stator HTS  124 ), and the lower stability bearing (comprising rotor PM  116  and lower stator HTS array  126 ). Rotational acceleration of rotor  110  about centerline  102  is achieved by the electromagnetic interaction between the rotor PM  118  and the stator coil  128 . Mechanical support  136  thermally insulates the HTS array  126  from the ground. There is also typically a thermally conducting structure (not shown) located between the HTS array  126  and thermally insulating structure  136  that connects HTS array  126  to a cold source, e.g. a cryocooler, etc. as shown in  FIG. 2 . Similarly, mechanical support  134  thermally insulates the HTS  124  from the ground, and there is typically a thermally conducting structure (not shown) positioned between HTS  124  and support  134  that connects HTS  124  to a cold source. 
       FIG. 7  shows a further variation where the open-core flywheel  150  is concentric about a centerline  152 . The flywheel comprises rotor  160  and stator  170 . Rotor  160  comprises a fiber-composite rim  162 , lift bearing PM  164 , stability PM  166 , and a motor/generator PM array  168 . The stator  170  comprises a lift bearing PM  174 , a HTS assembly  176 , a stator coil assembly  178 , and mechanical supports  184 ,  186  and  188 . Mechanical support  184  supports stator lift bearing PM  174 . Mechanical support  186  supports HTS array  176 . Mechanical support  188  supports stator coil assembly  178 . The mechanical supports  184 ,  186  and  188  are fixedly attached to a vacuum chamber (not shown) that preferably surrounds flywheel assembly  150 . Flywheel rotor  160  is magnetically levitated via the magnetic bearing components, including the lift bearing (comprising rotor PM  164  and stator PM  174 ), and the stability bearing (comprising rotor PM  166  and stator HTS  176 . Rotational acceleration of rotor  160  about centerline  152  is achieved by the electromagnetic interaction between the rotor PM  168  and the stator coil  178 . Mechanical support  186  thermally insulates the HTS array  176  from the ground. There is also typically a thermally conducting structure (not shown) positioned between the HTS bearing assembly  176  and thermally insulating structure  186  that connects HTS  176  to a cold source, such as, for example, a cryocooler, etc., as shown in  FIG. 2 . 
     A number of configurations are contemplated by regarding the lift bearing in the novel open-core flywheel assemblies.  FIG. 8  is directed to one variation showing an upper portion of the flywheel assembly. PM  204  is attached to the upper part of composite rim  202 . Stator PM  206  is located vertically above PM  204 , and is attached to mechanical support  208 . The black arrows in  FIG. 8( a )  designate the preferred direction of magnetization. In this example, there is an attractive force upward on PM  204  that helps lift the rotor  202  against the force of gravity. The stator PM  206  is sufficiently wide such that the attractive force is nearly uniform as the rotor composite rim  202  grows outward radially. 
     An alternative lift bearing is shown in  FIG. 8( b ) , showing a second stator PM  207  located below and radially inward from rotor PM  204 . The magnetic force in this case is repelling, and the location of the stator PM  207  below the rotor PM  204  preferably provides an additional upward force on the rotor  202 . It is understood that additional magnets may be added to increase the force as indicated in  FIG. 8( c ) . In this instance, there is an additional attractive force with resulting additional upward force on rotor  202 , between the interactions of stator PM  207  and rotor PM  210 . There is also an additional repulsive force, with resulting upward force on the rotor, between the interactions of rotor PM  210  and stator PM  212 . 
     As shown in  FIG. 8( d )  additional variations presently disclosed contemplate magnetizations that are not vertical, e.g. radial magnetization, etc.  FIG. 8( e )  shows additional PM  207  attached to mechanical support  209 . 
     Further, the present disclosure contemplates orienting the stability bearing into different arrangements.  FIG. 9( a )  shows the magnetization of PM  116  as it would exist in  FIG. 6 . In  FIG. 9( b ) , an alternative shows HTS  306  located radially inward from the stability bearing PM  304 . In this orientation, PM  304  is magnetized in the radial direction. While  FIG. 9( b )  shows the magnetization direction as radially inward, it is understood that such magnetization could be directed radially outward. 
     In addition,  FIG. 6  shows a further alternative where the lift bearing is replaced by a second stability bearing. The motor/generator PM  118  shown in  FIG. 6 , and discussed relative to  FIGS. 3-5 , shows magnetizations that are radial, circumferential, or a combination of the two.  FIG. 9  shows a further contemplated variation directed to an arrangement where the motor/generator PM has vertical magnetizations that alternate direction about the circumference. 
     In  FIG. 10 , the rotor  460  of an open-core flywheel assembly preferably comprises a fiber composite rim  462 , an upper PM  464 , a lower PM  466  and a PM ring  468 . It is understood that rotor  460  is substantially cylindrical, and that PMs  464 ,  466  and  468  are understood to preferably extend about the entire circumference of the inner surface of rotor rim  462 . Centrally positioned PM  468  is shown magnetized according to the arrows, with the direction of magnetization alternating in the vertical upward or downward direction. 
       FIG. 11  shows an alternate variation for a rotor of the present disclosure. Rotor  510  of an open-core flywheel assembly preferably comprises a fiber composite rim  512 , an upper PM  514 , a lower PM  516  and a ladder-shaped copper conductor  518 . It is understood that rotor rim  512  is substantially cylindrical, and that PMs  514 ,  516  and  518  are understood to preferably extend about the entire circumference of the inner surface of rotor rim  512 . In this variation, the motor/generator function is preferably performed with an induction motor topology. 
     According to the present, disclosure, incorporating into an open-core flywheel architecture, rotor materials having significantly improved strength/density ratios, including preferred MWCNTs will increase flywheel rotor energy densities from presently known values of about 264 Wh/kg to at least about 473 Wh/kg, and a commensurate increase of and fiber tensile strength of from about 11 to about 63 GPa (an increase in efficiency and strength of at least about 80% from known devices). Indeed, when wall thickness of the MWCNTs is normalized to about 0.075 nm, theoretical wall strengths of at least about 300 GPa are achievable. It is further understood that single-walled CNTs (SWCNTs) are also contemplated by the present disclosure and may be incorporated into the rotor components of the inventive flywheel assemblies presented herein, since SWCNTs may provide adequate or even superior mass efficient reinforcement. A typical SWCNT has a diameter of about 1.35 nm. Using this diameter with a 1 atom interatomic spacing Vfs of only 39% are achievable. A diameter of 3 nm would yield Vfs of 60%. It is understood that the optimal CNTs for use in connection with variations of the present disclosure balance CNT diameter, achievable Vf, and efficiency of the CNT reinforcement. 
     According to preferred variations of the present disclosure, most of the flywheel rotor comprises a filament-wound fiber composite that is magnetically levitated by a HTS bearing. The HTS bearing comprises a PM rotor and HTS stator. Because of the superconducting properties of the HTS stator, the levitation is passive, requiring no significant feedback or active controls. The HTS stator preferably comprises an array of individual HTS crystals of Y—Ba—Cu—O, or other materials where Y is replaced by other rare earth elements such as, for example, Gd, Nd, Eu, etc., that are cooled by thermal conduction to the coldhead of a cryocooler to a temperature of from about 70 K to about 80 K. Preferably no cryogenic fluids (for example, liquid nitrogen, etc.) are required for bearing operation. The brushless M/G comprises a PM rotor and a stator preferably comprising copper windings in a ferromagnetic yoke. M/G stator cooling is accomplished by thermal conduction to the vacuum chamber walls. No parasitic energy is required for this function. An energy-absorbing containment liner is placed between the rotating flywheel and the outer vacuum shell. It is preferred to keep the weight of both the vacuum chamber, and the stationary components inside the vacuum chamber to a minimum to meet the energy density requirements of the flywheel array. Other major components of the preferred system include a lift bearing, a touchdown bearing, and power electronics. 
     While the preferred variations and alternatives have been illustrated and described, it will be appreciated that various changes and substitutions can be made therein without departing from the spirit and scope of the disclosure. Accordingly, the scope of the disclosure should only be limited by the accompanying claims and equivalents thereof.