Patent Publication Number: US-11661997-B2

Title: Flywheel energy storage device

Description:
PRIORITY CLAIM 
     This application claims the priority filing benefit as a continuation of U.S. patent application Ser. No. 15/929,939 filed May 29, 2020 for “Catenary Flywheel Kinetic Battery” of James B. Clegern, which claims the priority filing benefit of U.S. Provisional Patent Application No. 62/857,088 filed Jun. 4, 2019 for “Catenary Flywheel Kinetic Battery” of James B. Clegern, each hereby incorporated by reference in its entirety as though fully set forth herein. 
    
    
     BACKGROUND 
     Flywheels have been well established for over 2000 years as energy storage devices in the form of spinning kinetic energy. In larger form factors (typically 10 Kilowatt hours or higher), they can provide the core of an Energy Storage System (ESS) for use with renewable and traditional electrical grids ranging from multiple-module utility systems to single unit residential/commercial systems. Vacuum enclosed flywheels coupled with ultra-low friction magnetic bearings are particularly well suited for ESS applications that require frequent charge/discharge cycles per day since they essentially have no parts that wear-out. Examples of these high cycle ESS applications include supporting the intermittent nature of renewable energy, powering electric vehicle charging stations, and stabilizing grid frequency regulation. Generally, however, the current generation of flywheels with the potential for use in these applications rotate at relatively lower speeds and are fabricated with large to massive amounts of structural materials such as steel or composite fiber-resin laminates. These high mass flywheels are often cost prohibitive for the energy they are able to store, both in initial cost and installation and limit applications to where volume or mass are not constrained. 
     When examining the energy storage capacity of a flywheel, they are largely limited by their structural design and selection of structural materials. The structural design dictates how the flywheel attaches to the rotating shaft to transfer torque and where the structural mass is placed to maximize the rotating moment of inertia. The structural material properties (i.e., strength, density, elasticity, fatigue resistance, etc.) dictate the flywheel operational speed and lifecycle endurance based on the structural design. For example, with each evolution of structural materials, from the stone age, bronze age, steel age and now composite age, the improved structural material properties of the flywheel have allowed for faster rotational speeds of a given structural design. 
     The flywheel kinetic energy storage (Ek) capability is governed by the standard equation Ek=½Iω{circumflex over ( )}2 where the structural design mass placement in the moment of inertia (I) and the flywheel rotational speed (ω) are the two energy storage drivers. Of the two energy storage drivers, increasing the structural mass and moment of inertia (I) for a given structural material has been the standard path forward for greater energy storage. This involves adding thickness or height to a traditional disk, cylinder-rim, or ring-rim type structural flywheel designs once a maximum speed has been reached for a given structural material. Utilizing composite materials for a traditional mass focused flywheel was demonstrated commercially with a 1,250 kilogram unit storing 25 kilowatt-hours of energy. Utilizing steel materials for a traditional mass focused flywheel was demonstrated commercially with a 2,270 kilogram unit storing 50 kilowatt-hours of energy. Even the utilization of concrete materials for a traditional mass focused flywheel was demonstrated commercially with a 3,000 kilogram unit storing 10 kilowatt-hours of energy. 
     Increasing the flywheel energy storage by adding mass becomes a challenge on three fronts. First, the flywheel energy storage capacity increases linearly with the increase in flywheel mass, resulting in practical operational limits for manufacturing, delivering and emplacing massive flywheels of high energy storage capacity. Second, as the flywheel mass increases each of the supporting flywheel subsystems (i.e. bearings, support structure, and vacuum enclosure) also must also be “upsized” to handle the added mass, adding to overall system mass and complexity. Third, the overall mass increase and subsystem complexity to attain increased energy storage becomes a significant cost driver, pricing many flywheel energy storage systems well outside a reasonable Return On Investment time frame. 
     The second energy storage driver is to focus on increasing the flywheel speed since it improves a flywheel energy storage capability by a power of two. Two flywheel development areas allow an increase in speed: structural design and stronger structural materials. Utilizing lighter and stronger composite materials in place of heavier metals and fiberglass has been the mainstay of traditional disk, cylinder-rim, or ring-rim type traditional structural mass flywheel designs. New structural flywheel designs which explored increased energy storage capacity were examined in the 1970&#39;s, employing the flexibility of the new manufacturing technique of continuous filament winding of resin-impregnated composite filament material. These new structural designs had promise, but only augmented the traditional ring-rim mass flywheel designs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an example flywheel energy storage device. 
         FIG.  2    is a side view of an example elliptical ovoid flywheel. 
         FIG.  3    is a cross-sectional view of the example elliptical ovoid flywheel, including bearing subsystems and transmission subsystem. 
         FIG.  4    is a detailed cross-sectional view of the right bearing subsystem and the transmission subsystem. 
         FIG.  5    is a detailed cross-sectional view of the left bearing subsystem. 
         FIGS.  6 - 7    are cross-sectional views of other example elliptical ovoid flywheels. 
     
    
    
     DETAILED DESCRIPTION 
     A high speed, compact flywheel system is disclosed, that is suitable for storing excess energy from renewable and traditional utility power facilities and dispensing energy when energy is required. An example high speed, elliptical ovoid flywheel includes a composite shell formed of helically wound ribbons of resin-impregnated composite filament material combined with internal composite structures providing compressive support. In an example, the filament wound composite shell is coupled to the rotating shaft by paired structural flange plates concentrically disposed on either side of the shaft where the plurality of the composite shell filament ribbons are attached for torque transfer. The majority of the flywheel shell interior is hollow to maximize the flywheel inertial moment mass at the maximum allowable shell radius. 
     The flywheel ovoid shape and elliptical curvature are formed to support the centripetal forces varying with the radius of each fiber of the helical overwrap. This permits two plane structural support, increasing the allowable tip speed significantly compared with a traditional disk or cylindrical flywheel of equal diameter and same structural materials. The all-composite nature of the elliptical flywheel structure increases operational safety and allows maximum and consistent utilization of the composite material strength in its optimal embodiments. 
     In an example, the elliptical ovoid flywheel includes a plurality of sets of helical wraps, each set of helical wraps having a uniform width. Each set of helical wraps has a layer extending around the outer surface of the shell and the hub on both sides of the axis of rotation. In another example, each set of helical wraps has a near continuous, single ended filamentary material wound in a unidirectional relationship and encased in a resin matrix. In another example, each set of helical wraps is uniformly stressed over the width of each wrap during rotation. In another example, each set of helical wraps has a radially oriented, layered composite shell. In another example, the helical wraps are disposed in an overlapping relationship with one another on portions of the hub at locations adjacent to the axis of rotation. 
     In an example, each layer of the radially oriented helical wrap is angled initially about 5 degrees and stepped to about 35 degrees to shift wrap layer mass build-up away from the axis of rotation to increase flywheel moment of inertia. 
     In an example, the radially oriented helical wrap is configured to transfer tensile tip stress to the thickened structure near the axis of rotation under compressive stress, thereby maximizing the effective strength of the filament wound composite laminate. 
     The elliptical ovoid flywheel may have integrated internal structures attached to and providing compression support for the external ovoid shell. In an example, the internal structures are formed from metal or a near continuous single ended filamentary material wound in a unidirectional relationship in a plurality of layers and encased in a resin matrix. In another example, the internal structures are formed from metal, composite laminate wound in a unidirectional relationship in a plurality of layers and that are bonded to the inner layer of the external ovoid shell. The internal structures may have a minimum inner-to-outer ratio of about 0.1 when compared with the outer diameter of the ovoid shell. 
     In an example, the inner and outermost peripheral surface of the ovoid shell is in the configuration of a rotational ellipse. In an example, the ovoid shell helical wrap layers are secondarily used to control the inertial mass of the flywheel. 
     In an example, adding or subtracting layers configures the flywheel energy storage capacity to be increased or decreased by the plurality of wrap layers. 
     The elliptical ovoid flywheel may also have dual boss plates attached to the innermost and outermost ovoid shell layers to provide the attachment of the ovoid shell to the shaft for co-rotation and torque transfer. By keeping the boss plate radius under 15% of the elliptical ovoid radius, the structural strength of the boss plates are never exceeded, eliminating the traditional flywheel structural weak point. 
     There are several energy storage advantages to the systems and methods described herein. Those include, but are not limited to, providing a flywheel that more efficiently uses its structural mass; providing means to increase the rotational speed of the flywheel without structural failure; providing a flexible flywheel structure that can adapt to its load profile; providing an economical flywheel structure that can be tailored to a customer&#39;s energy storage needs; and combining the advantages of a smaller, high energy density flywheel with smaller, compact subsystems to yield a low cost energy storage system. Still further advantages will become apparent from a study of the following description and accompanying drawings. 
     Before continuing, it is noted that as used herein, the terms “includes” and “including” mean, but is not limited to, “includes” or “including” and “includes at least” or “including at least.” The term “based on” means “based on” and “based at least in part on.” 
       FIG.  1    is a perspective view of an example flywheel energy storage device  10 . An example of the system  10  includes a sealed flywheel assembly  12  on a mounting structure  14 . A vacuum port  16  is provided through the sealed flywheel assembly  12 . The flywheel assembly  12  may be operatively associated with a motor or generator  18  mounted to platform  20 . For example, the flywheel assembly  12  may be operatively associated with the generator  18  via transmission link  22 . In an example, the generator  18  may include cooling lines  24  operatively associated with a cooling subsystem  26 . 
     In an example, the generator  18  may be operatively associated with a power control subsystem  28  having power connection(s)  30  and control and monitoring connection(s)  32 . For example, the power control subsystem  28  may be operatively associated with the generator  18  via engagement controls  34 . 
     Control of the subsystem(s) and/or transmission may be based at least in part on operating conditions in the flywheel assembly  12 . For example, control may be based at least in part on one or more sensor(s) via connections  36 . 
     In an example, the flywheel assembly  12  houses a high speed, three-dimensional (3D) elliptical ovoid flywheel  50  mounted on a shaft  52  associated with transmission  22 .  FIG.  2    shows a side view of an example elliptical ovoid flywheel  50 . The elliptical ovoid flywheel  50  offers a significant inertial mass and operating speed improvement over a two-dimensional disk, ring or cylinder shape, even one made of the same structural material and having the same or similar diameter. Table 1 provides a comparison of example flywheel capabilities for the same geometric and material constraints. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Flywheel Capabilities 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Design Factors 
                 2D Solid Steel 
                 2D Steel Hub, 
                 All Composite 
               
               
                 1-meter radius 
                 Cylinder 
                 Composite Hollow 
                 Structure 3D 
               
               
                 50 kWh  
                 Flywheel 
                 Cylinder Flywheel 
                 Elliptical Ovoid 
               
               
                 Operational 
                   
                   
                 Flywheel 
               
               
                 Energy 
                   
                   
                   
               
               
                 1.5 Safety Factor 
                   
                   
                   
               
               
                 Materials  
                 ASTM 
                 ASTM A514 
                 Carbon 
               
               
                 Strength 
                 A514 Steel 
                 Steel + Carbon 
                 fiber/laminate 
               
               
                   
                 7800 mPa Yield 
                 fiber/lam inate, 
                 3000 mPa 
               
               
                   
                   
                 3000 mPa 
                   
               
               
                 Rotor Mass (kg) 
                 5,500 
                 1,630 
                 250 
               
               
                 Inertial Mass 
                 689 
                 175 
                 24.1 
               
               
                 (kg-m 2 ) 
                   
                   
                   
               
               
                 Operational  
                 7,000 
                 14,000 
                 36,000 
               
               
                 Speed (RPM) 
                   
                   
                   
               
               
                 Rotor Length (m) 
                 0.9 
                 1.06 
                 0.45 
               
               
                 Energy Density 
                 4.5 
                 15.1 
                 205 
               
               
                 (w-h/kg) 
                   
                   
                   
               
               
                 Energy Volume 
                 35.4 
                 30.1 
                 490 
               
               
                 (w-h-liter) 
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     In an example, the elliptical ovoid flywheel  50  includes layers of helically wound resin-impregnated composite filament material forming the shape of an ovoid shell. This configuration provides the means to shift a significant inertial mass to the maximum flywheel diameter. 
     Adjusting the number of helically wound layers of the elliptical ovoid flywheel  50  allows tailoring of the flywheel inertial mass. Each layer of helically wound composite filament material is in nearly pure tension, maximizing the structural material performance, while the curvature of the ovoid composite shell minimizes stress concentrations detrimental to structural integrity. The 3D nature of the ovoid composite shell also allows the composite filament structural material at the flywheel maximum diameter to share stress in two planes, similar to membrane stress within a spherical pressure vessel, allowing the flywheel to exceed the classic hoop stress limits for a hoop wound disk, ring or cylinder by a significant portion. 
     During operation, the vacuum sealed flywheel  50  transfers torque and energy through the sealed vacuum enclosure  12  through a two part magnetic transmission attached to the flywheel shaft  52  and attached to the external motor/generator  18 , geared to match the flywheel rotational speed with the operational speed of the external motor/generator  18 . 
     Although not shown, an enclosure may also be provided to cover the motor/generator and connections (e.g., with the flywheel vacuum enclosure  12 ) and external magnetic transmission. In an example, one or more sensor(s)  36  provide position, temperature, vibration, flywheel revolutions per minute (rpm) and pressure readings which are relayed to the control display through wiring embedded in the vacuum enclosure  12 . 
     The external motor/generator  18  is supported by a translating platform  14 , control  28  to allow coupling and uncoupling of the external magnetic transmission from the vacuum enclosure. This allows the vacuum sealed elliptical ovoid flywheel  50  to disengage the external motor/generator  18 , removing a prominent source of standby power loss. A small stand-by chemical battery, which is typically charged by the motor/generator  18  when in operation, or other external power maintains the required power to the sensor package and active axial thrust bearing when the motor/generator  18  is disengaged. With the disengaged motor/generator  18 , the standby until 50% power remaining capability of the elliptical ovoid flywheel ESS can exceed 250 hours. Utilizing external plug-in power or a solar array to recharge the stand-by chemical battery, the standby until 50% power remaining capability of the elliptical ovoid Flywheel ESS can exceed 350 hours. 
       FIG.  3    is a cross-sectional view of the example elliptical ovoid flywheel  50 , including bearing subsystems  54   a ,  54   b  and transmission subsystem  56 .  FIG.  4    is a detailed cross-sectional view of the right bearing subsystem  54   a  (from detail area  4  in  FIG.  3   ) and the transmission subsystem  56 .  FIG.  5    is a detailed cross-sectional view of the left bearing subsystem  54   b  (from detail area  5  in  FIG.  3   ). 
     The example elliptical ovoid flywheel  50  may comprise an ovoid composite shell  58  with radial composite fiber overwrap on an integrated composite compressive support structure  60 . The example elliptical ovoid flywheel  50  may be mounted on the shaft  52  via boss plate attachments  62  on internal boss plate supports  64  and outer balancing boss plates  66  and outer shell guide plates  68  and inner boss plate  70 . The elliptical ovoid flywheel  50  is supported on each end of shaft  52  via an axial touchdown bearings and electromagnet axial stabilizer  72  and radial touchdown bearings  74 . Rotating PM bearings  76 , stationary PM bearings  78 , axial electrodynamic bearings  80  are also visible and carry the majority of the load and system stabilization. Internal PM transmission  82 , moderator plate  84 , back bearing housing plate  86 , and bearing housing  88  are also visible. 
     The elliptical ovoid flywheel  50  may be constructed on an internal compressive support  90 . In an example, an internal compressive support  90  is a circumferentially wound hollow cylinder of resin-impregnated composite filament material of radius r 1  and a thickness that provides compressive support when the flywheel is in operation. The internal compressive support is bonded in place to the inner elliptical ovoid wrap of the outer ovoid shell. 
     In an example, the composite fiber material for the elliptical ovoid flywheel structure is selected to have minimal elongation while under stress (e.g., like carbon/graphite fibers) as to not compromise the structural integrity of the flywheel against the vacuum enclosure. Certain composite materials (e.g., like reinforced plastics, fiberglass, ultra-high density polyethylene, basalt and aramid fibers) that continuously elongate or “creep” under load, may be unsuitable for this flywheel application due to its high continuous load requirements while in active operation. 
     In an example, the outer shell of the elliptical ovoid flywheel  50  is comprised of a plurality of helically wrapped resin-impregnated composite filament laminate layers comprising thickness t 0  with each layer completely covering the circumference of the flywheel radius r 0 . The inner laminate layer follows an inner elliptical ovoid form that is removed after wrap completion, creating an internal void. The inner laminate layer follows a low angle helical winding path, bypassing the flywheel shaft and shaft support, and is bonded to the outer face of the inner boss flange plates. Each additional laminate layer increasingly increments its helical winding path angle to a maximum angle equal to the inner support cylinder outer radius r 1 . 
     After the desired number of laminate layers has been added to provide the stress support and inertial mass, the outer laminate layer finishes with the optimum helical angled elliptical path. In each of the laminate layers between, the elliptical path transitions to a vertical path through an elliptical section across the thickness of the internal hollow cylinder support to allow the compressive stresses to be concentrated at the region of greatest internal structural support. 
     The elliptical ovoid flywheel  50  is supported and stabilized along its spin axis and shaft  52  by an active electromagnet thrust bearing, shaft attached radial magnetic bearings and enclosure fixed radial magnetic bearings, axial electrodynamic bearings, and back-up touchdown bearings. A portion of the outer boss flange plate on either side of the flywheel provides balancing mass for fine tuning the inertial axis along the axis of rotation. 
     Elliptical ovoid flywheel construction is visible in  FIG.  3   , showing the internal compressive support of radius r 1  and thickness t 1 , external boss flange plates, internal boss flange plates, shaft support and internal shaft supports. Also included in this view is a helical wrapping guide plate bonded to the ends of the internal compressive support. 
       FIG.  3    also illustrates the full elliptical shape of radius r 0  outer helical wrap layer set with a sole compressive support of width W that doubles as the shaft compressive support.  FIG.  3    also illustrates a partial elliptical shape of radius r 0  outer helical wrap layer set with multiple internal compressive supports of width W. The side view illustrates locations of the internal compressive supports for example systems relative to the flywheel diameter. 
     An example elliptical ovoid flywheel outer shell, showing a stepped composite filament-resin wrapping with the starting low angles out to the optimal helical angled elliptical path that intersects the outer radius of the hollow inner compressive support cylinder. The plurality of helically wrapped resin-impregnated composite filament laminate wraps comprises each layer of the ovoid composite shell. 
     Finite Element Analysis results of the example elliptical ovoid flywheel  50  with example inner and outer elliptical helical wrap laminate layers with the internal hollow cylinder compressive support. The predicted Safety Factor (SF) is included to allow interpretation of the flywheel stress analysis at the 36,000 rpm operating speed. 
     Before continuing, it should be noted that the examples described above are provided for purposes of illustration, and are not intended to be limiting. Other devices and/or device configurations may be utilized to carry out the operations described herein. 
     In an example, the components and connections depicted in the figures may be implemented for the following operations. The operations shown and described herein are provided to illustrate example implementations. It is noted that the operations are not limited to the ordering shown. Still other operations may also be implemented. 
     In an example operation, the elliptical ovoid flywheel  50  is rotated around a horizontal or vertical axis on shaft  52  within a vacuum enclosure  12  supported by magnetic bearings ( FIGS.  3 - 5   ). Magnetic bearings provide support in the radial and axial directions. The speed of the elliptical ovoid flywheel  50  can be increased or decreased, and hence its energy storage level can be increased or decreased, by transferring torque through the vacuum enclosure  12  (e.g., by means of a magnetic transmission attached to the flywheel shaft  52  and external motor/generator  18 ). The magnetic transmission acts as an intermediary between the elliptical ovoid flywheel  50  and the external motor/generator  18 , allowing the rotational energy to transfer via magnetic coupling through the wall of the vacuum enclosure  12 . Power input and output is provided by the external motor/generator  18 , which pulls power from a user source to speed up the elliptical ovoid flywheel  50  to convert electrical power into rotational kinetic energy, and then pushes power to a user as it slows down the elliptical ovoid flywheel  50 , converting the rotational kinetic energy back into electrical power. 
     When the elliptical ovoid flywheel is in operation, various design aspects increase the available energy storage. In an example, the inner and outer elliptical arches of the ovoid shell slightly extend (e.g., about 1-2% compared to a stationary state) due to centripetal forces pulling the structure radially. This slightly extends the flywheel outer radius, increasing the inertial moment while the flywheel is in operation, allowing 3-5% additional energy storage capability. This flexibility allows the 3D structural design to adapt to its load profile while contributing a positive gain to the flywheel energy storage capability. 
     In an example, the elliptical ovoid flywheel  50  may also utilize carbon/graphite fiber composite filaments that have an approximate 1-2% total elongation under load. With these filaments helically wrapped in a radial orientation, the expected elongation slightly extends the flywheel outer radius while under load. This increases the flywheel inertial moment while the flywheel is in operation, allowing about 3-5% additional energy storage capability. This flexibility allows the 3D structural design to adapt to its load profile while contributing a positive gain to the flywheel energy storage capability. 
     In an example, the inner structural composite cylinder also allows flexing both radially (from the ovoid shell compressive stress) and from the radial dilation of the hoop oriented composite filaments. The resulting slight increase in the composite cylinder radius flexes into the upper void space, and does not impart added stress to the outer ovoid shell as would a traditional ring-rim overwrap. 
     In an example, by shifting the helical wrap angle upward for each subsequent wrap layer of the ovoid shell, the filaments (that would otherwise concentrate around the shaft) are shifted to greater radii, increasing the effective flywheel moment of inertia, while still spreading the tensile stress of the filament layers through the increased shell cross sectional thickness radiating outward from the shaft. 
     In an example, the overlapping layers of the helical overwrap placed in the elliptical arch create the 3D nature of the ovoid composite shell. This allows the structural composite material in the flywheel ovoid shell to share stress in two planes, similar to membrane stress within a spherical pressure vessel. This allows the elliptical ovoid flywheel to exceed the classic hoop stress limits for a hoop wound disk, ring or cylinder of the same radius by over about 40%, allowing a significant increase in the elliptical ovoid flywheels operational rotational speed and thereby an increase in its energy storage level. As the elliptical ovoid flywheel width-to-diameter ratio approaches 1.0, and the flywheel takes on a spherical shape (instead of an ovoid), the two plane stress improvement approaches 100% over the classical hoop stress. The added compressive stress from increasing the width-to-diameter ratio must be handled by adding internal flywheel structure. 
     When these five aspects are combined into the elliptical ovoid flywheel structure, the energy storage capability increases up to five times higher when compared with a traditional flywheel of the same structural material, mass and diameter. 
     EXAMPLE 
     An example elliptical ovoid flywheel as shown in  FIG.  1    was designed and evaluated by Finite Element Analysis (FEA). The example flywheel was designed with the ovoid shell and internal support wound with medium strength 3000 mPa carbon composite fiber laminate with an epoxy resin impregnate. The boss flanges were simulated utilizing stainless steel AISI A514. The simulated flywheel was modeled in an enclosed vacuum environment with near frictionless operation with magnetic bearings, as illustrated in  FIG.  3   . The maximum diameter of the simulated flywheel was about 1.0 meters with a width of about 0.45 meters on the attached shaft. 
     After analysis, the stored energy in the simulated flywheel was determined to be about 50.9 kilowatt-hours, with a flywheel inertia of about 24.1 kilogram-meters squared, and an operational speed of about 36,000 revolutions per minute. 
     System stresses maintained a safety factor of about 1.4 to 2.2. Not only was the simulated flywheel operation well within an expected safety factor for commercial applications, but it also achieved a gravimetric energy density of about 205 watt-hours per kilogram based on the total mass of the flywheel, and about 490 watt-hours per liter volumetric energy density based on the volume of the flywheel. This is compared to approximately 20 watt-hours per kilogram and 45 watt-hours per liter for traditional 2D cylindrical flywheels. Fatigue analysis indicates the elliptical ovoid flywheel will provide over 200,000 charge/discharge cycles during its lifecycle. 
     Utilizing higher strength carbon composite fiber laminate with a strength of 4100 mPa, in the same simulated flywheel geometry, the operational speed exceeded 49,000 rpm for the same safety factor, and achieved an energy storage capacity of about 97 kilowatt-hours. 
       FIGS.  6 - 7    are cross-sectional views of other example elliptical ovoid flywheels. The elliptical ovoid flywheel  150  in  FIG.  6    is more conical than the elliptical ovoid flywheel  50 . The elliptical ovoid flywheel  150  is the same width (width W 1  is about the same as width W of flywheel  50 ), and has an internal support structure  153  near shaft  152 . The elliptical ovoid flywheel  250  in  FIG.  7    is wider (width W 2 ) with a center internal support structure  253  and another support structure  254  near shaft  252 . 
     The systems and methods disclosed herein provide an elliptical ovoid flywheel capable of greater stored energy per unit mass than previously known rotational energy storage devices. It will be appreciated that the elliptical ovoid flywheel described herein is particularly adaptable to supplying energy storage with a small operational footprint and mass so as to be utilized at deployed utility and renewable energy locations, residential and commercial locations, and potentially in transportation, e.g., powering electric vehicle charging stations. 
     It is noted that the examples shown and described are provided for purposes of illustration and are not intended to be limiting. Still other examples are also contemplated, including but not limited to a vertically oriented elliptical ovoid flywheel.