Patent Publication Number: US-2016241106-A1

Title: Flywheel Energy System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/153,216, filed Jun. 3, 2011, which claims the benefit of U.S. Provisional Patent Application No. 61/352,810, filed Jun. 8, 2010. The contents of each of these applications are expressly incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to energy storage systems, and more specifically to energy storage systems capable of storing electrical energy as kinetic energy of a rotating flywheel, for release of the stored kinetic energy as electrical energy when required. 
     DESCRIPTION OF THE PRIOR ART 
     Large-scale energy storage has the potential to solve many challenges related to modernizing electrical power distribution. Some of these challenges include managing intermittent renewable energy generation, electricity load shifting, black-start capabilities, managing electricity price fluctuations, and back-up power supply. 
     Currently, there are several large-scale energy storage technologies that attempt to address the challenges facing the energy storage industry. These technologies include advanced batteries, electrochemical capacitors (EC), pumped hydro, compressed air energy storage, and flywheel technologies. 
     With respect to the advanced batteries technologies, one such technology—the lead acid battery, has been a popular choice for power quality and UPS applications due to the low cost associated with such batteries. However, the effectiveness of lead acid batteries for large-scale applications is limited by the very short life cycle of such batteries, and the variable discharge rate. Li-ion batteries are often seen as an alternative or replacement for lead acid batteries because of their much longer life cycle. Development of the Li-ion battery has been driven to date primarily by the automobile industry, with potential applications for vehicular, residential and commercial use. The effectiveness of Li-ion batteries as suitable energy-storage technology is, however, limited by the high cost associated with the manufacture of such batteries, and by security concerns associated with large-scale implementations of Li-ion batteries. Metal-Air batteries are the most compact and potentially the least expensive battery to manufacture. However, the effectiveness of Metal-Air batteries is limited by the very short life cycle and low efficiencies (e.g., approximately 50%) of such batteries. One particular battery technology that has shown promise as a solution for large-scale implementations is the sodium-sulphur (NaS) battery technology. NaS batteries have high energy density but require high operating temperatures and have a relatively short life span. The above-identified battery technologies typically have an average AC to AC round-trip efficiency of approximately 64%. Moreover, electrochemical battery technology, in general, have a usable life that is degraded by the number of charge/discharge cycles. 
     Electrochemical capacitors (EC) are also used as an energy storage solution. ECs are energy storage devices that have longer life cycles and are more powerful than lead-acid batteries. However, it is not feasible to implement ECs on large-scale projects due to their high cost and low energy density. 
     A potential solution to large-scale implementations of energy storage technology is pumped hydro. Conventional pumped hydro uses two water reservoirs, which are separated vertically and thus have an energy potential associated with the energy of the water travelling from the elevation of higher potential energy to the elevation of lower potential energy by means of gravity. During off-peak hours, electrical power is used to pump water from the lower reservoir to the upper reservoir. As demand for electrical energy increases, the water flow is reversed to generate electricity. Pumped storage is the most widespread energy storage system in use on power networks. The main applications for pumped hydro are energy management and frequency control. The main drawbacks associated with pumped hydro are the unique site requirements and the large upfront capital costs. 
     Another potential energy-storage solution is compressed air energy storage (CAES). CAES uses a combination of compressed air and natural gas. A motor pushes compressed air into an underground cavern at off-peak times. During on-peak times, compressed air is used in combination with gas to power a turbine power plant. A CAES uses roughly 40% as much gas as a natural gas power plant. A CAES has similar wide-scale use limitations as pumped hydro: the site locations and large upfront capital costs. 
     Another proposal for large-scale energy storage implementations is flywheel energy storage systems, which have emerged as an alternative to the above-identified energy storage technologies. Such systems are currently used in two primary commercial applications: uninterruptible power supply (UPS) and power frequency regulation (FR). Both UPS and FR require extremely quick charge and discharge times that are measured in seconds and fractions of seconds. Flywheel technologies have many advantages over other energy storage technologies, including higher reliability, longer service life, extremely low maintenance costs, higher power capability, and environmental friendliness. Flywheel energy storage systems store energy in a rotating flywheel that is supported by a low friction bearing system inside a housing. A connected motor/generator accelerates the flywheel for storing inputted electrical energy, and decelerates the flywheel for retrieving this energy. Power electronics maintain the flow of energy into and out of the system, to mitigate power interruptions, or alternatively, manage peak loads. Traditional flywheel designs limit their use to the above mentioned short duration applications due to high electrical parasitic losses associated with electromagnetic bearing systems. 
     One way to support a flywheel for rotation at high speeds is with rolling element mechanical bearing assemblies such as ball bearing assemblies. The life of such mechanical bearing assemblies is strongly influenced by the loads that such mechanical bearing assemblies must carry. In order to extend the life of flywheel energy storage systems using mechanical bearing assemblies, a magnetic bearing can be used in combination with the mechanical bearings for the purpose of reducing the load on the mechanical bearings. In such, an, example, the rotor portion of the flywheel typically rotates about a vertical axis and the mechanical bearing assemblies provide radial support while the magnetic bearing assembly carries or supports the axial load of the flywheel. Traditionally, flywheel designs have utilized electromagnetic thrust bearings for this purpose. 
     U.S. Pat. No. 6,710,489, issued Mar. 23, 2004, (hereinafter “Gabrys I”) discloses the use of a plurality of magnetic bearing assemblies that are used to support axially the flywheel rotor portion. Such a flywheel energy storage system also has multiple mechanical bearing assemblies which each provide radial support for the flywheel rotor portion, but do not axially restrain the flywheel rotor portion. The design of such a system having mechanical bearing assemblies that are unrestrained axially substantially ensures that the entire axial load of the flywheel or rotor is distributed on the magnetic bearings, thus reducing the wear on the mechanical bearing assemblies. In this manner, such a flywheel rotor portion effectively “floats”. The systems of Gabrys I utilize magnetic bearings to locate the rotor axially, either repulsive bearings for passive (permanent) magnets, or attractive bearings for actively controlled electro magnets. Where attractive bearings are used, a control system is required to adjust the axial location of the flywheel by adjustment of the attractive force. Such systems are relatively complex and absorb significant power while in operation thus limiting their use to short duration applications. 
     U.S. Pat. No. 6,806,605, issued Oct. 19, 2004, (hereinafter “Gabrys II”) also discloses the use of magnetic bearings for supporting rotating objects. More specifically, Gabrys II discloses a permanent magnetic thrust bearing with an electromagnetic radial magnetic bearing having a rotating portion with a circumferential multi-piece construction. This electromagnetic radial magnetic bearing provides radial stiffness, which is desirable because applications wherein a flywheel will, be rotating at high speeds require that the flywheel be rotating true to its rotational axis. Thus, Gabrys II discloses a flywheel energy storage system which uses magnetic forces to produce (i) axial forces that suspend the flywheel, and (ii) radial forces that centre or stabilize the flywheel in an effort to maintain a true axis of rotation. Gabrys II further discloses a flywheel system wherein the flywheel is axially and radially supported by means of repulsive magnetic forces that generate a thrust that purportedly maintains a stable levitation of the flywheel. Repulsive magnetic forces generated from permanent magnets are known to degenerate over time; and accordingly there is the possibility of mechanical failure of the device. 
     A paper entitled Low Cost Energy Storage for a Fuel Cell Powered Transit Bus, authored by C S Hearn describes a flywheel structure in which passive lift magnets are used to reduce the axial loads on mechanical bearings. The mechanical bearings axially locate the rotor of the flywheel. The magnetic path resulting from the structure shown in Hearn is relatively dispersed, which, together with the mechanical bearing arrangement disclosed, provides a relatively inefficient support system. 
     It is therefore an object of the present invention to obviate or mitigate the above disadvantages. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, an energy storage system is provided including a first housing having an end face and a flywheel. The flywheel can have a rotor and a drive shaft defining a substantially vertical axis about which the rotor is mounted for rotation within the first housing. The energy storage system can also include a permanent magnetic bearing assembly positioned between the end face and the rotor and having a permanent magnet mounted on the first housing or the rotor, and the other of the first housing or the rotor having ferromagnetic properties, to attract the rotor towards the end face. The energy storage system can further include a first mechanical bearing assembly acting between the first housing and the rotor to provide radial positioning of the rotor and to limit upward axial movement of the rotor in relation to the end face, the rotor being spaced from the end face by a clearance gap. The energy storage system can include a second mechanical bearing assembly spaced from the first mechanical bearing assembly along the drive shaft and acting between the first housing and the rotor to provide radial positioning of the rotor, the second mechanical bearing assembly permitting relative axial movement between the drive shaft and the first housing. 
     In another aspect, an energy storage system is provided that can include a flywheel housing and a flywheel positioned within the flywheel housing. The flywheel can include a rotor and a drive shaft defining a substantially vertical axis about which the rotor is mounted for rotation within the flywheel housing. The energy storage system can include a motor/generator housing detachably attached to the flywheel housing and a motor/generator positioned within the motor/generator housing. The motor/generator can be detachably attached to the rotor. 
     In a further aspect, an energy storage system is provided that can include a motor/generator housing, and a motor/generator positioned within the motor/generator housing. The energy storage system can include a flywheel housing having a vacuum port and a flywheel positioned within the flywheel housing The flywheel can include a rotor, and a drive shaft defining a substantially vertical axis about which the rotor is mounted for rotation within the flywheel housing. The energy storage system can include a vacuum pump connected to the interior volume of the flywheel housing via the vacuum port. The vacuum pump can be configured to be energized from electricity supplied by the motor/generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which, 
         FIG. 1  is a front perspective view of an energy storage system. 
         FIG. 2  is a cross-sectional view along the line II-II of  FIG. 1 . 
         FIG. 3  is a view similar to that of  FIG. 2 , in a partly disassembled state. 
         FIG. 3 a    is a view similar to  FIG. 3  further disassembled. 
         FIG. 4  is an enlarged view of an upper portion of  FIG. 2 . 
         FIG. 5  is an enlarged view of a lower portion of  FIG. 2 . 
         FIG. 6 a    is bottom plan view of a first alternative embodiment of magnetic thrust bearing assembly. 
         FIG. 6 b    is a cross-sectional view along line  6 B- 6 B of  FIG. 6A . 
         FIG. 6 c    is an enlarged view of the encircled area  6 C of  FIG. 6B . 
         FIG. 7 a    is bottom plan view of a second alternative embodiment of magnetic thrust bearing assembly. 
         FIG. 7 b    is a cross-sectional view along sight line  7 B- 7 B of  FIG. 7   a.    
         FIG. 7 c    is an enlarged view of the encircled area  7 C of  FIG. 7   b.    
         FIG. 8  is a plot of an area of  FIG. 4 , illustrating the circular magnetic flux pattern created by the magnetic thrust bearing assembly. 
         FIG. 9  is a perspective view of an array of energy storage systems contained within a collective container, with the collective container being partially cut away. 
         FIG. 10  is a perspective view of an array of collective containers, each similar to the collective container illustrated in  FIG. 9 . 
         FIG. 11  is a perspective view of an array of above grade domed vaults that each house an energy storage system. 
         FIG. 12  is a cross-sectional view of an array of below-grade vaults that each house an energy storage system. 
         FIG. 13  is an alternative configuration of energy storage system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto. The entire disclosures of all references recited above are incorporated herein by reference. 
       FIG. 1  is a perspective view of an energy storage system  20  that is constructed as a modular system having two major components: a first housing  21  containing a flywheel (not visible in  FIG. 1 ) rotatably mounted therein as will be described more fully below, and a second housing  22  releasably mounted atop the first housing  21 . The second housing  22  contains a motor/generator (not visible in  FIG. 1 ) coupled to the flywheel to either drive the flywheel or be driven by the flywheel, upon operation of the system in a manner that will become more apparent as description unfolds. 
     As best seen in  FIG. 1 , the first housing  21  has a cylindrical outer wall  28  that terminates at its upward extent in a radially outwardly projecting peripheral flange  23 , and is closed at its lower extent by an annular base plate  33 . The base plate  33  preferably projects beyond the cylindrical outer wall  28  a radial distance substantially equal to that of the peripheral flange  23 . The cylindrical outer wall  28  is reinforced at regular intervals around its circumference by a plurality of spaced vertical ribs  29 , which extend between the base plate  33  and the radially outwardly projecting peripheral flange  23 . The first housing  21  is closed adjacent its opposite, upper end by means of an annular top plate  27 , which is releasably affixed to the radially outwardly projecting peripheral flange  23  by a plurality of circumferentially spaced machine screws  31   a . Each machine screw  31  a engages a corresponding plurality of complimentary threaded bores  31   b  (see  FIG. 2 ) formed in the radially outwardly projecting peripheral flange  23 . The housing thus formed is of rigid and robust construction, suitable to contain the flywheel. 
     In the embodiment shown, the second housing  22  is formed with a cylindrical outer wall  2   a  (of smaller diameter than the cylindrical outer wall  28  of the first housing  21 ), which cylindrical outer wall  22   a  terminates at its lower extent in a radially outwardly projecting peripheral flange  64 , The second housing  22  is closed adjacent its upper end by a cylindrical top plate  35  attached to the cylindrical outer wall  22   a  by means of, for example, a plurality of machine screws  37 , arranged around the periphery of the top plate  35  and received in complimentary threaded bores (not shown) formed in the upper edge of the cylindrical outer wall  22   a.    
     It is preferred that the housings  21 ,  22  are formed from non-ferromagnetic materials. Non-ferromagnetic materials are especially preferred for this purpose to minimise the magnetic drag that slows down the flywheel&#39;s rotation and lessens the time the motor/generator is available for energy release during a discharge cycle. Suitable materials may be selected from a group including, but not limited to, stainless steel, aluminum, plastics, fibreglass, concrete, and combinations thereof, which materials may also be reinforced with composite materials, including, but not limited to, carbon fibre, Kevlar™, or the like. 
     As can be seen in  FIGS. 2 and 3 , the first housing  21  contains a flywheel  24  that is supported for rotation within the housing  21  on bearing assemblies  47   a,    47   b.  The flywheel  24  includes a rotor  25  and an upper drive shaft segment  24   a  and lower drive shaft segment  24   c  segment. The rotor  25  and drive shaft segments  24   a,    24   c  are integrally formed from a forged blank. The rotor  25  is cylindrical with its axis aligned with drive shaft segments  24   a,    24   c.  The diameters of the drive shaft segments  24   a,    24   c  may differ due to the different loads applied. The drive shaft segments  24   a , 24   c  together define a substantially vertical axis A about which the rotor  25  is mounted for rotation within the first housing  21  in a manner that will be described in more detail below. Rotor  25  has an upper planar end surface  25   a  and lower planar end surface  25   b  with a peripheral surface  25   c  extending between the upper and lower planar surfaces. A pair of radial grooves  25   d  are formed between the end faces  25   a,    25   b  to facilitate heat transfer during manufacture. While the first housing  21  may be sized and otherwise constructed to accommodate more than one flywheel rotating therein, in the preferred embodiment illustrated, a single flywheel  24  is shown, as this is the simplest to illustrate and describe, and, as will become more apparent as this description proceeds, the preferred arrangement readily supports ordered and regular modular expansion of the subject energy storage system by adding further flywheels, one at a time, with each contained within a respective first housing  21 . 
     It will also be appreciated that while a solid rotor  25  and drive shaft  24   a,    24   b  has been described, a fabricated rotor with separate drive shaft segments may be used. Alternatively, a separate drive shaft extending through the rotor  25  and attached thereto for driving rotation thereof could be used. 
     The rotor  25  is made from a material having ferromagnetic properties, such as, for example, high density steel. In alternate embodiments, other ferromagnetic materials from which the rotor  25  may be manufactured are iron, nickel, cobalt, and the like. The higher the mass of the rotor  25 , the greater the kinetic energy the energy storage system  20  is able to store at the same RPM of the flywheel. In contrast, the higher the mass of the rotor  25 , the greater the potential frictional, losses that can occur through the mechanical bearings used to mount same for rotation, and the greater the need for precision engineering and robustness of the system in order to prevent potentially dangerous accidents through component failure at high RPMs. 
     It will be appreciated that the rotor  25  may be made as a composite structure with part ferromagnetic materials if preferred, and may be shaped other than cylindrical, provided it is balanced for high speed rotation. A cylindrical, steel rotor appears to be the most economical. 
     The preferred embodiment illustrated in  FIGS. 1 to 5  further comprises a magnetic thrust bearing assembly  26  that acts between the housing  21  and flywheel  24  to support a significant portion of the weight of flywheel  24  thus relieving the mechanical bearing assemblies  47  of axial loading. The magnetic thrust bearing assembly  26  has at least one annular permanent magnet  26   a  that is mounted on the first housing  21 , as described more fully below. During operation of the preferred embodiment, the annular permanent magnet  26   a  remains fixed, and does not rotate, thereby providing a very stable support mechanism for the flywheel  24  which lies beneath. The magnetic thrust bearing assembly  26 , and more specifically, the annular permanent magnet  26   a,  is mounted on the first housing  21  in stationary centred relation about the vertical axis A, so as to be juxtaposed with end face  25   a  of the rotor  25 . The annular permanent magnet  26   a  may be constructed as a unitary annulus having a single layer of ferromagnetic metal material, as shown in  FIGS. 2 through 6C , or may vary in its construction, as discussed further below. 
     As the rotor  25  is made from a ferromagnetic material, the positioning of the permanent magnet above the end face  25   a  attracts the rotor  25  axially upwardly towards a lower face  26   d  of the annular permanent magnet  26   a.  The attractive magnetic forces between the annular permanent magnet  26   a  and the rotor  25  at least partially, and ideally, totally, support the weight of the flywheel  24 . 
     As best seen in  FIGS. 2 through 4 , magnetic thrust bearing assembly  26  comprises annular permanent magnet  26   a,  together with an annular backing plate  26   b  and a non-magnetic spacer ring  26   c  composed of a non-ferrous metal material, or a polymer, such as “REANCE F65”—a flexible neodymium iron boron magnet—manufactured by The Electrodyne Company, Batavia, Ohio. The annular backing plate  26   b  is constructed from a ferromagnetic metal, and is mounted to the underside or end face  21   a  of the annular top plate  27  of the first housing  21 , also in stationary centered relation about the vertical axis A. A plurality of machine screws  60  engages corresponding threaded bores formed in the annular backing plate  26   b  to secure the backing plate  26   b  to the top plate  27 . The annular backing plate  26   b  extends radially beyond the outer radial edge of the annular permanent magnet  26   a,  and beyond the outer radial edge of the non-magnetic spacer ring  26   c,  to form a downwardly projecting perimeter skirt portion  61 . The downwardly depending perimeter skirt portion  61  preferably has an outer radius at least equal to the radius of the rotor  25 , with the non-magnetic spacer ring  26   c  interposed between the outer radial edge of the annular permanent magnet  26   a  and the inner radius of the downwardly depending perimeter skirt portion  61 . The annular backing plate  26   b  preferably has a shoulder portion  59  arranged around its outer circumferential edge, which rests in close-fitting nested relation upon a complimentary internal annular ledge  65  formed adjacent to the upper edge of the cylindrical outer wall  28  of the first housing  21 . 
     To enhance the support of the rotor  25 , the magnetic bearing  26  is configured to constrain the flux path through the rotor  25 . The perimeter skirt portion  61  has a lower face  85  that is vertically substantially co-terminus with the lower face  26   d  of the annular permanent magnet  26   a,  thereby to also maintain the same minimum clearance gap  30  between the rotor  25  and the lower face  85  of the perimeter skirt portion  61 . The perimeter skirt portion  61  helps shape the magnetic field and thus contributes to the inherent stability of the rotor  25  while it rotates during operation of the energy storage system. With the arrangement shown, the annular permanent magnet  26   a,  the annular backing plate  26   b,  the non-magnetic spacer ring  26   c,  and the perimeter skirt portion  61  constrain the magnetic flux field to enhance the support capacity of the bearing  26 . 
     The annular permanent magnet  26   a  of  FIGS. 2 through 5  is preferably affixed to the annular backing plate  26   b  by magnetic attraction thereto, and such affixation may be supplemented by the use of low out-gassing adhesive, such as HS-4 Cyanoacrylate Adhesive manufactured by Satellite City, Simi Valley, Calif., or an epoxy. 
     In the embodiment shown in  FIGS. 1-5 , the annular permanent magnet  26   a  is shown as being formed as a unitary, rigid structure of conventional magnetized metal, rare earth metal, or the like. In alternative embodiments, the annular permanent magnet  26   a  may, instead, be formed from one or more sections or layers of magnetic material. This provides, in most cases, for easier and less costly fabrication. For example, the annular permanent magnet  26   a  may be fabricated from a flexible magnetic material, such as rare earth magnetic particles mixed with a polymer binder (such as is used in the construction of conventional fridge magnets). In one such alternative embodiment, shown in  FIGS. 6 a    through  6   c,  a single layer of such flexible permanent magnetized material may be formed from this material in a series of concentric circles  26   e  of widening radius wrapped around the vertical axis A in a radially expanding manner. The magnetic poles of the layer of flexible magnetic material are aligned in the same direction, and preferably run in parallel relation to the vertical axis A, as shown by the arrows in  FIG. 6   c.    
     In a further alternate embodiment (shown in  FIGS. 7 a  through 7 c   ), the annular permanent magnet  26   a  can be built up from a plurality of patches  26   f  of the aforesaid flexible magnetic material laid in a regular patchwork array having one or more layers positioned one above the other. As shown in  FIGS. 7 a    through  7   c,  the patchwork may be of rectangular strips (1.5″×0.125″), and the plurality of layers shown is three layers  78   a,    78   b,  and  78   c.  It will again be noted from  FIG. 7 c    that the magnetic poles of each of the layers  78   a,    78   b,  and  78   c  of flexible magnetic material are aligned in the same direction, preferably running in parallel relation to the vertical axis A. Patches of flexible magnetic material of other shapes and sizes, for example, square patches, may be substituted for the rectangular patches shown in  FIGS. 7A  through  7 C, and the number of layers utilized in a particular installation will vary according to the strength required to support the target percentage of weight of the flywheel  24  to be carried by the magnetic thrust bearing assembly  26  in that particular application. 
     Similar forms of affixation may be used for each layer of permanent magnet material illustrated in the alternate embodiments illustrated in  FIGS. 6 a  through 6 c  and 7 a  through 7 c    as were previously described in relation to the embodiment of  FIGS. 1 through 5 . 
     Although the permanent magnet could be formed on the upper surface of the rotor  25 , the stationary mounting of the magnet  26   a  permits the use of such flexible permanent magnetic material in the construction of a magnetic thrust bearing assembly  26 . Such flexible magnetic material is too soft and fragile to sustain high speed rotation (i.e., above 1,000 RPMs, and more typically above 10,000 RPM) for prolonged periods of time, particularly where the flexible magnetic material is circumferentially wrapped or laid in a layered array. By reason of the high centrifugal forces exerted thereon during high speed rotation the material would be subject to radial distortion, and possible rupture or de-lamination. 
     As illustrated in  FIGS. 2 through 4 , an electrical rotary machine that may function as a motor or generator, referred to as a motor/generator  72  is releasably coupled to the upper drive shaft segment  24   a  by means of a coupling shaft  34 . The shaft  34  has an annular collar  34   a  that projects downwardly from the motor/generator  72  in order to provide for an axially slidable engagement with the upper drive shaft segment  24   a.  The collar  34   a  of coupling shaft  34  is releasably coupled to the upper drive shaft segment  24   a  by means of a bolt  36 . A key  34   b  and mating keyway engage one another to operatively connect the coupling shaft  34  with the upper drive shaft segment  24   a  of the drive shaft for transfer of torque from the motor/generator  72  to the flywheel  24  (and vice versa). Alternatively, mating splines (not shown) may be used on the coupling shaft  34  and the upper drive shaft segment  24   a,  respectively, in place of the key and keyway illustrated. 
     The upper mechanical bearing assembly  47   a  is mounted within a top portion of the first housing  21 , about the upper drive shaft segment  24   a.  The upper mechanical bearing assembly  47   a  provides axial positioning of the rotor  25  in order to limit at least upward axial movement of the rotor  25  in relation to the lower face  26   d  of the annular permanent magnet  26   a . More particularly, the upper mechanical bearing assembly  47   a  limits the upward axial movement of the rotor  25  so as to define a minimum clearance gap  30  between the lower face  26   d  of the annular permanent magnet and the end face  25   a  of rotor  25 . The upper mechanical bearing assembly  47   a  may also be preferably configured to limit downward axial movement of the rotor  25  in relation to the lower face  26   d  of the annular permanent magnet. In this regard, the upper mechanical bearing assembly  47   a  is preferably a thrust bearing. This configuration allows the upper mechanical bearing assembly  47   a  to further define a maximum clearance gap  30  between the lower face of the annular permanent magnet and the rotor  25 , which maximum gap  30  is equal to the minimum clearance gap  30  in the preferred embodiment illustrated. Restraining movement of the upper mechanical bearing assembly  47   a  in both axial directions assures that the gap  30  maintained between the lower face  26   d  of the annular permanent magnet and the rotor  25  is within operative tolerances, thereby assuring reliable lift by the annular permanent magnet  26   a  of the rotor  25 . 
     As best seen in  FIG. 4 , the upper drive shaft segment  24   a  has a precision ground bearing support that terminates at a shoulder  48 . The upper mechanical bearing assembly  47   a  is preferably comprised of two rolling element bearing sets  42  contained within a removable bearing cartridge  42   a  to facilitate the quick and easy replacement of worn or damaged bearing assemblies. The rolling element bearing sets  42 , 42  are both preferably ceramic angular contact ball bearing sets, and most preferably very high speed, super precision, hybrid ceramic bearing sets, meaning, the balls are comprised of ceramic material which run in precision ground steel races. 
     The cartridge  42   a  includes a bearing support housing  43 , a bearing axial fixing ring  44  and machine screws  45  and  46 . The support housing  43  has a radial flange  43   a  and a bearing recess  43   b.  The bearing sets  42  are located in the recess  43   b  and retained by the ring  44 . The outer races of the rolling element bearing sets  42  are restrained axially between lower surface  44   a  of bearing axial fixing ring  44  and end face  49  the bearing recess  43   b  and the ring  44  secured by machine screws  45 . The bearing support flange  43  is retained axially via machine screws  46  to the upper surface  51  of the annular backing plate  26   b,  which in turn is fixed to the annular top plate  27  of the first housing  21  as previously described. 
     The lower surface  34   c  of collar  34   a  of coupling shaft  34  bears against the inner races  42   b  of the rolling element bearing sets  42  and is secured by a bolt  36  that is received in the drive shaft  24   a.  The bolt  36  acts through the shaft  34  to apply a preload to the rolling element bearing sets  42  by adjustably compressing the inner races between the lower surface  34   c  of the coupling shaft  34  and bearing shoulder  48  of the upper drive shaft segment  24   a.    
     The axial position of the bearing support flange  43  with respect to the magnetic thrust bearing assembly  26  fixes the axial position of the upper drive shaft segment  24   a  of the rotor  25 , and maintains the substantially constant gap  30  between the top surface  25   a  of the rotor  25  and the lower face  26   d  of the magnetic thrust bearing assembly  26 . The gap  30  is determinative to applying the correct lifting force to the rotor  25  and reducing the axial loading to the rolling element bearing set  42 . The gap  30  may be adjusted by placing shims (not shown) at surface  51  to raise the bearing support flange  43 , thereby lifting the rotor  25  and decreasing gap  30  to apply a greater magnetic lifting force. 
     The lower mechanical bearing assembly  47   b,  shown in  FIG. 5 , acts between the lower drive shaft segment  24   c  and the housing bottom plate  33 . The lower mechanical bearing assembly  47   b  has a pair of rolling element bearing sets  42 , 42  contained within a removable bearing cartridge  42   a  to facilitate the quick and easy replacement of worn or damaged bearing assemblies. The two rolling element bearing sets  42 , are preferably of the same general type and construction as the upper mechanical bearing sets (although they may be of a smaller size due to the lesser mechanical loading), i.e., they are both preferably ceramic angular contact ball bearing sets, and most preferably very high speed, super precision hybrid ceramic bearing sets. 
     The cartridge  42   a  of lower mechanical bearing assembly  47   b  further includes bearing support flange  53  having a bearing recess  90 . Lower drive shaft segment  24   c  has a shoulder  89  to locate the bearings  42  axially. A bearing preload cap  54  is secured by, bearing preload screw  32 , to the lower drive shaft  24   c.  The bearing preload cap  54 , and bearing, preload screw  32  axially restrain the inner races of each of the rolling element bearing sets  42 , 42  and apply a preload to the rolling element bearing sets  42 , 42  by compressing the inner races between an end surface  58  of the bearing preload cap  54  and the lower bearing shoulder  89  of the lower drive shaft segment  24   c.  The outer races  42   c  of the rolling element bearing sets  42  are unrestrained axially inside the bearing recess  90  of lower mechanical bearing assembly  47   b.  This allows the lower drive shaft segment  24   c  of the rotor  25  to move axially as the rotor  25  contracts axially at high speed due to Poisson Ratio effects. This also allows for axial movement due to temperature induced expansion and contraction in both the rotor  25  and the first housing  21 , whilst maintaining the gap substantially constant. 
     The bearing support flange  53  is fixed to base plate  33  of the first housing  21  by way of machine screws  56 . The lower mechanical bearing assembly  47   b  also preferably comprises lower bearing cover  55 , which provides, with the assistance of resilient gasket or O-ring  57 , vacuum tight sealing of the lower mechanical bearing assembly  47   b.  as well as provides a point to mechanically support or lock the rotor  25  against axial vibration or movement during, for example, installation or shipping. A jack screw  57  is inserted in a threaded hole  40  formed for this purpose in the lower bearing cap  55  to engage a socket  32   a  formed in the head of the bearing preload screw  32 . The jack screw  57  supports the rotor both axially and radially when engaged in the socket to inhibit transient loads being applied to the bearing assemblies  47 . 
     In order to minimize the wear on the mechanical bearing assemblies and in order to minimize friction as the flywheel  24  is rotating, it is preferable, but not essential, for the magnetic thrust bearing assembly  26  to support substantially the entire weight of the flywheel  24 . More specifically, it is preferable for the magnetic thrust bearing assembly  26  to support at least 90% of the flywheel  24 &#39;s weight, and more preferably between about 95% and 100% of the flywheel  24 ′s weight. In an ideal situation, the preferred embodiment, as illustrated, the magnetic thrust bearing assembly  26  is capable of supporting substantially 100% of the flywheel&#39;s weight. The axial location provided by the upper bearing assembly  47   a,  maintains the gap  30  constant, even if the magnetic bearing assembly  26  provides a lift greater than the weight of the rotor. 
       FIG. 8  illustrates the flux path generated by the magnetic thrust bearing assembly  26  of  FIGS. 2 through 4 . As illustrated in  FIG. 8 , the flux field  62  is ovoid/circular. However, in three dimensional representations of the energy storage system  20 , the magnetic flux path is torroidal in shape. As previously discussed, the downwardly depending perimeter skirt portion  61  helps shape the magnetic field and thus contributes to the inherent stability of the rotor  25  while the rotor  25  is rotating during operation of the energy storage system  20 . The annular backing plate  26   b  and downwardly depending perimeter skirt portion  61  create a flux field  62  that holds substantially the entire weight of the rotor  25 ,  FIG. 8  illustrates the magnetic flux substantially penetrating the rotor  25  to lift same, and to a lesser extent penetrating the annular backing plate  26   b  and downwardly depending perimeter skirt portion  61 . The non-magnetic spacer ring  26   c  inhibits migration of the flux field from the magnet  26   a  and facilitates the establishment of the compact magnetic loop. The non-magnetic wall  28  of the housing  21  also does not interfere with the flux path to enhance the lifting capacity of the magnetic bearing assembly  26 . In a preferred embodiment the permanent magnet occupies approximately 60% of the area of the end face  25  indicated at A1, and 40% of the area is the skirt indicated at A2. Other area ratios may be adopted with a ratio of 30% the permanent magnetic and 70% the skirt up to 70% of the permanent magnet and 30% the skirt. Use of backing plate in this manner allows for 40% less magnetic material and provides 4× the lifting force of the magnets alone. Stray flux is contained, directed into the rotor face and prevented from curving back down to the rotor sides and causing a significant drag torque on the system. Additionally, utilizing the large available upper annular surface area of the rotor facilitates the use of lower strength, bonded magnetic materials. These materials are lower cost and easily formable compared to sintered magnets. 
     It is preferred that zero electrical energy is required to be drawn from the power source to which the energy storage system  20  is connected to support the weight of the flywheel  24 . This is achieved through the use of permanent magnetic material in the construction of the annular permanent magnet  26   a.  Thus no energy is consumed by the magnetic thrust bearing assembly  26  in supporting the weight of the flywheel  24 . Moreover, as the magnetic thrust bearing assembly  26  is mounted to the first housing  21 , the weight of the flywheel  24  is supported by attractive forces of the magnetic thrust bearing assembly  26 , which is itself supported by the cylindrical outer wall  28  of the first housing  21 , which is, in turn, supported by the base plate  33  of the first housing  21 . 
     In the preferred embodiment illustrated in  FIGS. 1 through 5 , the energy storage system  20  is made more efficient by minimizing the frictional forces which might otherwise act directly on the rotor  25  as it rotates. Accordingly, the rotor  25  should not come into contact during rotation with the any of the internal surfaces projecting into the first housing  21 , including the lower face  26   d  of the magnetic thrust bearing assembly  26 . To this end, it has been described above how the gap  30  between the top surface  25   a  of the rotor portion  25  and the lower faces  26   d  and  85  of the annular permanent magnet  26   a  and the downwardly depending perimeter skirt portion  61 , respectively, are maintained. To the same end, a minimum clearance gap  70  is at all times defined between the outer circumferential edge  25   c  of rotor  25  and the internal surface  82  of first housing  21 . Similarly, the components within the first housing  21  are shaped and otherwise dimensioned to maintain at all times a minimum clearance gap  75  between the lower surface  25   b  of the rotor  25  and the upper internal surface  98  of the base plate  33 . 
     To further reduce and substantially eliminate drag forces acting on the rotor  25  during operation (i.e., while the flywheel  24  is rotating), it is desirable to reduce windage losses on the rotating components by drawing at least a partial vacuum within at least the first housing  21 , and preferably within both the first housing  21 , and second housing  22 . To this end, it is preferred to seal both the first  21  and second  22  housings to atmosphere by, for example, the placement of resilient gaskets or O-rings  86 , 57  in operative sealing relation around all mating joints of the components of the two housings  21 , 22 , including, without limitation, between the wall components  27 , 28  and  33  of the first  21  and second  22  housings, and between the bearing preload cap  54  and the bearing support flange  53 , as best seen in  FIGS. 2, 4 and 5 . 
     A vacuum source, such as a conventional vacuum pump  91 , is preferably connected by flexible tubing or the like to the interior volume of the first housing  21  by connection to, for example, a vacuum port  87  attached to, or formed in, for example, the base plate  33 , so as to be in fluid communication with the gaps  30 , 70  and  75 , thereby to allow for the drawing of at least a partial vacuum within the first housing  21  upon operation of the vacuum pump. 
     It is also preferable, though not essential, to operatively connect a vacuum source, being preferably the same vacuum source mentioned in the previous paragraph, but optionally being a second vacuum source (not shown), to the second housing  22  to also create an at least partial vacuum in the second housing  22 , thereby to reduce frictional, losses that would otherwise occur upon rotation of components of the motor/generator  72 . A particularly preferred manner of introducing such an at least partial vacuum initially created in the first housing  21  into the second housing  22  without the need for a second vacuum source, is by providing for a vacuum passageway to be established between the first housing  21  and second housing  22  when assembled together as shown in the figures. As seen in  FIG. 4 , a vacuum passageway  187  extends in fluid communication through the coupling shaft  34 , the key  34   b  and the keyway  34   b , around the inner races  42   b  of the two rolling element bearing sets  42  of the upper mechanical bearing assembly  47   a,  downwardly past the inner radial surface of the bearing support flange  43 , to connect with a radial channel  50 . Channel  50  surrounds the basal connection point of the upper drive shaft segment  24   a  to the rotor  25 . The radial channel is itself in fluid connection with the gap  30 . In this manner, the vacuum, source operatively connected to the first housing  21  is also operatively connected to the second housing  22  through vacuum passageway  187  upon mounting of the second housing  22  atop the first housing  21 . 
     The vacuum pump  91  is preferably energized from electricity drawn from the electrical power grid to which the energy storage system  20  is connected during its charging phase, but may, or may not, depending upon design choice, be energized from electricity supplied by the motor/generator  72  during periods when the electrical grid is not available to supply such electrical energy. In either case, the sealing of the first  21  and second  22  housings should ideally, but not essentially, be designed and built to sustain said at least partial vacuum over the full design period of rotation of the rotor  25  during de-energization of the motor/generator  72 , so as to minimize drag forces acting on the rotor  25  during such periods. To minimize energy consumption, the vacuum pump  91  may be controlled to switch off when a partial vacuum is drawn with a check valve  92  to inhibit leakage in to the housing  20 . 
     The motor/generator  72  is connected to an external electrical power source so as to enable the motor/generator  72  to draw electrical energy from an electrical power source, such as an electrical power grid, when the connection is energized. The motor/generator  72  draws electrical energy from the electrical power grid in order to drive rotation of the rotor  25 . The driving of the rotor  25  by the motor/generator  72  effectively converts the electrical energy inputted into the system into kinetic energy that is stored in the rotation of the rotor  25  of the flywheel  24 . The kinetic energy stored in the rotation of the rotor  25  is thus stored in the energy storage system  20  for reconversion to electrical energy and release of the electrical energy during rotation of the motor/generator by the flywheel  24 , when the connection is de-energized. 
     According to the preferred embodiment illustrated, the second housing  22 , having the motor/generator  72  mounted therein, is releasably mounted atop the first housing  21 . The modular construction of the energy storage system  20  allows the charge/discharge power used and generated by it to be readily altered without redesigning or disassembling the entire system by increasing/decreasing the motor/generator  72  size on any given energy storage system  20 .  FIG. 3  illustrates the motor/generator  72  being connected to the upper drive shaft segment  24   a  in a releasable manner through coupling shaft  34  as described above. The second housing  22  is connected to the first housing  21  in a releasable manner by bolts passing through the flange  64  and in to the annular backing plate  26   b,  It will be noted that the coupling does not affect the positioning of the bearing assembly  47   a,  thereby maintaining the required clearance between the rotor  25  and the magnetic bearing assembly  26 . By virtue of the releasable coupling of the motor/generator  72  to the upper drive shaft segment  24   a  and the releasable coupling of the second housing  22  (in which the motor/generator is mounted) to the first housing  21 , the energy storage system  20  is effectively constructed or assembled in a modular manner so as to facilitate the replacement of worn or damaged parts, or the interchanging of motors/generators having a particular desired power rating in order to more effectively or efficiently store and discharge electricity in accordance with a predetermined criteria. The modular nature of the preferred embodiment illustrated in  FIG. 3  facilitates varying the ratings or power specifications of the motor/generator once the flywheel energy storage system has been manufactured. It is also preferable, but not essential, that the second housing  22  and the motor/generator  72  mounted therein are readily removable and interchangeable without the need for disassembly of the first housing  21  or any of the structures contained therewithin. Accordingly, modular construction of the energy storage system  20  as illustrated and described herein allows the charge/discharge power ratings of the energy storage system  20  to be readily altered or customized by increasing/decreasing the motor/generator size or type on any given energy storage system  20 . This flexibly allows an energy storage system  20  having the same flywheel stored energy capacity (e.g. 20 kWH) to be utilized either for Long Duration, Low Power (e.g. Peak Shifting/Time of Use) or Short Duration, High Power (e.g. Voltage Support) applications with only quick and easy swapping out of a different motor/generator unit mounted within interchangeable second housings. 
     In the preferred embodiment illustrated in  FIGS. 1 through 5 , the motor/generator  72  shown is an induction type motor/generator  72 . More particularly, the preferred motor/generator  72  illustrated is preferably a three-phase induction type unit, which is comprised of a rotor  74 , press fit onto the coupling shaft  34 , and a stator winding  76 , pressed into the inside circumference of the cylindrical outer wall  22   a  of the second housing  22 . 
     As illustrated in  FIGS. 1 through 4 , the motor/generator  72  is preferably liquid cooled, such that the second housing  22  also preferably includes a coolant jacket comprised of a main coolant channel  80  encircling the outer surface of the cylindrical outer wall  22   a  of the second housing  22 , said main coolant channel  80  being enclosed on its outer periphery by a removable outer shell  88 . O-ring seals  81  assist in sealing the removable outer shell  88  to the cylindrical outer wall  22   a  of the second housing  22 . Coolant flows into ingress port  38 , passes through the main coolant channel  80 , and then outward through egress port  39 . The coolant flow can be via an external pump, or natural convection (in which case the ingress  38  and egress  39  ports are beneficially reversed from the arrangement shown) in order to the remove waste heat from the second housing  22  and the stator winding  76 . 
     Electrical cable connections to the motor/generator  72  are preferably made through the top plate  35  at port  41 , which port should be made vacuum tight around such connections by rubber grommets, O-ring seals and the like (not shown). 
     It will be appreciated that the rotor  25  is, as shown in the figures, solid and comprised of high strength steel. At least a portion of the rotor  25  must be ferromagnetic in order to interact with the magnetic thrust bearing assembly  26 . Preferably, at least an upper portion of the rotor opposite the bearing assembly  26  is magnetic, and, as a further preference, the entire rotor  25  is ferromagnetic. It may preferable in some embodiments of the energy storage system  20  for the rotor  25  to have a mass between about 1,000 kg and 5,000 kg with 3,000 kg a preferred mass. 
     In operation, power is supplied to the rotor/generator  72  which applies a torque to accelerate the rotor  25 . It is preferable, but not essential, that the motor/generator  72  be capable of rotating the rotor  25  at high speed, between about 10,000 and 20,000 RPM. As the rotor  25  accelerates, it stores the energy supplied by the rotor/generator  72  as kinetic energy. Upon attainment of the maximum speed, the electrical power may be disconnected. In a typical implementation, the maximum rotation speed of the rotor  25  is obtained within  2  hours of the electrical connection to the motor/generator  72  being energized by the power grid. It also be preferable, but not essential, that such high speed rotation of the rotor  25  continue for at least  6  hours following the electrical connection to, the power grid being de-energized. If the power is disconnected, or if additional electrical energy is required by the grid, the motor/generator is switched to a generating mode and the energy stored in rotor  25  drives the generator and supplies electrical power. In some embodiments, the storage capacity of the energy storage system  20  is approximately 20 kWh. The energy storage is a function of the weight of the flywheel and the speed at which the flywheel  24  is rotated. During rotation the gap  30  is maintained by the bearing assembly  47   a.  Changes in axial dimensions, due to thermal changes or dynamic forces, is accommodated in the lower bearing  47   b  which may slide axially relative to the end plate  33 . The flux path described in  FIG. 8  ensures the rotor  25  is maintained axially by the magnetic bearing and accordingly, the axial loads in the bearings  47   a,    47   b  are reduced. 
     Because of the relationship between an energy storage system&#39;s  20  energy storage limitations and an energy storage systems&#39;  20  inherent size and weight, it may be advantageous and preferable in some applications to use, or otherwise require the use of a plurality of smaller energy storage systems  20  in favour of a lesser number of large energy storage system  20  constructed according to the preferred embodiment. An array of relatively smaller energy storage systems  20  allows for users to store a greater amount of energy in the form of kinetic energy whilst maintaining ease of deployment and greater flexibility to accommodate for electrical power requirements of different scales in particular applications. In such situations, it may be preferable that the array of energy storage systems be controlled by a common control unit. Further, it may be even more preferable that the common control unit controls the electrical energy draw and the release of energy from each of the energy storage systems  20  in the array of energy storage systems. For some commercial embodiments, it may be preferable to have an array of energy storage systems having a collective energy output of at least 500 kWh. 
     In this regard,  FIG. 9  illustrates an array  100  of energy storage systems  120 ,  220 ,  320 , and  420  being contained within a collective container  101 . 
       FIG. 10  illustrates an array or a plurality of collective containers  101 ,  201 ,  301 ,  401  each of which contains an array of energy storage systems  120 ,  220 ,  320 , etc. 
       FIG. 11  illustrates an array of domed vaults  102 ,  202 ,  302 , and  402 . Each of the vaults is above grade and houses an energy storage system  120  therewithin. Similarly,  FIG. 12  illustrates in section an array of concrete vaults  102 ,  202 ,  302 ,  402 , and  502 . Each of the vaults  102 ,  202 ,  302 ,  402 , and  502  may be located below-grade, and each houses an energy storage system  120 ,  220 ,  320 , etc., respectively. 
     The provision of the flywheel support with one of the bearing assemblies axially locating the shaft and the other bearing permitting the drive shaft to float axially facilitates alternative configurations of rotor. As shown in  FIG. 13 , the rotor  25  is formed with ancillary rotor discs,  125  spaced along the drive shaft  24   a.    
     Each of the discs  125  has an upper face  127  directed toward a respective permanent magnet thrust bearing  126  which is located within the housing  21 . Upper bearing assemblies  147  axially locate the rotor  25  with a lower bearing assembly  147  radially permitting relative axial movement. 
     The discs  125  are formed from a ferromagnetic material and the thrust bearings  126  have a similar configuration to the thrust bearing shown in  FIG. 4 , with an annular permanent magnet and a surrounding skirt overlapping the discs. 
     The magnetic thrust bearings attract respective ones of the discs  125  to support the mass of the rotor  25 , as described above. 
     It will be appreciated that the array of discs  125  may be formed on the lower drive shaft  24   c  to support the rotor from beneath by attraction. 
     Various other modifications and alterations may be used in the design and manufacture of the energy storage system according to the present invention without departing from the spirit and scope of the invention, which is limited only by the accompanying claims. For example, separate and apart from the use of the liquid cooling means illustrated in the Figures, the second housing  22  could additionally be fabricated with external cooling fins for convective or forced air cooling to the ambient atmosphere.