Patent Publication Number: US-2007114795-A1

Title: Flywheel system with synchronous reluctance and permanent magnet generators

Description:
This application is a continuation of U.S. application Ser. No. 11/251,394 filed Oct. 14, 2005, which claims priority from U.S. Divisional application Ser. No. 10/863,868 filed Jun. 7, 2004 now U.S. Pat. No. 7,109,622, which claims priority from U.S. Provisional Application 60/476,226 filed Jun. 6, 2003. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to the electromechanical arts and energy storage systems. In particular, the present invention relates to flywheel systems used for energy storage and conversion.  
      2. Description of Related Art  
      Flywheel energy storage systems have provided a mechanical energy storage solution for hundreds of years as evidenced by the potter&#39;s wheel. Such systems differ in many respects from modern-day flywheel energy storage solutions. More recent design imperatives including high power density and electric power outputs have led to lightweight, high-speed flywheels operating in evacuated chambers and driving a similarly high-speed electric generator.  
      A typical application for today&#39;s flywheel is to provide electric power to an electric network for a brief period of time, as might be needed when an electric power outage occurs. Such applications require that the flywheel operate in a stand-by mode, fully charged and ready to convert its mechanical energy into electrical power to support the electrical network when network supply voltage droops.  
      To the extent that a protracted power outage occurs and the flywheel&#39;s usable electric output is depleted by the external electric network, the flywheel&#39;s internal electrical loads may be deprived of the electric power required to complete a normal flywheel shutdown. Critical loads internal to the flywheel system may include electric and electronic controls.  
      Supplying electric loads internal to the flywheel system during coast down presents a particular problem when the flywheel&#39;s electric generator has a minimum operating speed as is typical of inductive generators. Here, another source of electric power will be needed during some portion of the coast down period.  
     SUMMARY OF THE INVENTION  
      Now, in accordance with the invention, there has been found a flywheel system that provides electric power to critical loads during coast down despite the absence of an external power source. A flywheel mass supported by electromagnetic bearings is rotatably coupled to a motor-generator for exchanging mechanical power with the motor generator. Further, the flywheel mass is rotatably coupled to a backup generator for converting mechanical energy from the flywheel mass into electrical power for providing electrical power to the electromagnetic bearings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention is described with reference to the accompanying drawings that illustrate the present invention and, together with the description, explain the principles of the invention enabling a person skilled in the relevant art to make and use the invention.  
       FIG. 1  is a diagram showing modules included in the flywheel backup power supply system constructed in accordance with the present invention.  
       FIG. 2  is a diagram showing elements included within the power electronics module of the flywheel backup power supply system of  FIG. 1 .  
       FIG. 3  is a diagram showing elements included in the flywheel module of the flywheel backup power supply system of  FIG. 1 .  
       FIG. 4  is a diagram showing regimes included in the operation of the flywheel backup power supply system of  FIG. 1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  shows the flywheel system  100  of the present invention. It includes the flywheel module  102 , power electronics module  104 , and electrical network  106 . In the flywheel module, a first rotatable coupling  120  interconnects the flywheel mass  112  with the motor-generator  116  and a second rotatable coupling  118  interconnects the flywheel mass with the backup generator  110 . At least one electromagnetic bearing  114  provides rotatable support for the flywheel mass.  
      The power electronics module  104  is interconnected to sources and consumers of electric power including the backup generator  110 , the motor-generator  116 , and the electrical network  106 , and at least one electromagnetic bearing  114 .  
      A first electrical circuit  122  conducts electric power unidirectionally as shown by flow arrow  128  from the backup generator  110  to the power electronics module  104 . A second electrical circuit  124  conducts electric power unidirectionally as shown by flow arrow  130  from the power electronics module to at least one electromagnetic bearing  114 . A third electrical circuit  126  conducts electric power bi-directionally as shown by the opposed flow arrows  132 ,  134  between the motor-generator  116  and the power electronics module. A fourth electrical circuit  108  conducts electric power bi-directionally as shown by the opposed flow arrows  136 ,  138  between the power electronics module and the electrical network  106 .  
      Flywheel system  100  charging includes absorption and storage of mechanical energy by increasing the rotational speed and hence kinetic energy of rotating elements within the flywheel module  102  including the flywheel mass  112 . Flywheel system charging takes place when the electrical network  106  supplies electric power as shown by flow arrow  138  and the motor-generator  116  consumes electric power as indicated by flow arrow  132  while functioning as an electric motor.  
      Flywheel system  100  discharging includes releasing mechanical energy by decreasing the rotational speed and hence kinetic energy of rotating elements within the flywheel module  102  including flywheel mass  112 . Flywheel discharging takes place when the electrical network  106  consumes electrical power as shown by flow arrow  136  that is supplied by the motor-generator  116  as shown by flow arrow  134  while the motor-generator functions as an electric generator.  
       FIG. 2  shows a first embodiment of the flywheel system including selected elements of the power electronics module  200 . The power electronics module  104  includes three electric power converters. The first converter  202  is a uni-directional AC-to-DC converter, the second converter is a bi-directional AC-to-DC converter  204 , and the third converter is a bi-directional DC-to-DC converter  206 . The power electronics module also includes an internal DC bus  222 , an external DC bus  108 , and a fifth circuit  224 .  
      As mentioned above, the first, second, and third circuits interconnect the power electronics module  104  and the flywheel module  102 . What follows is a description of selected sources and users of electric power flowing in these circuits.  
      The first circuit  122  interconnects a backup generator AC output  240  as indicated by flow arrow  208  to a first converter AC input  264 . The fifth circuit  224  connects a first converter DC output  242  to an external DC bus tap  244  on the external DC bus  108 . Electric power users including the electrical network  106  and the third converter  206  thereby receive backup power from their respective interconnections  246 ,  248  with the external DC bus.  
      The second circuit  124  interconnects an electromagnetic bearing electric power input  266  to an internal DC bus tap  268  on the internal DC bus  222 . Electric power flows from the internal DC bus to the electromagnetic bearing(s)  114  as shown by flow arrow  130 . The internal DC bus may receive electric power from the motor-generator  116 , the electric network  106  or the backup generator  110 . When the motor-generator is providing electric power, the motor-generator electrical connection  256  is an AC output interconnected to a second converter AC connection  254  by the third circuit  126 . Power flows from a second converter DC connection  252  as indicated by flow arrow  134  to the internal DC bus. When the electric network is providing electric power, an electric network connection  246  is a DC output interconnected to a third converter external DC connection  248  by external DC bus  108 . Power flows from a third converter internal DC connection  250  as indicated by flow arrow  214  to the internal DC bus. When the backup generator is providing electric power, power flows as described above from the backup generator to the external DC bus and thereafter to the internal DC bus.  
      Turning now to electric power flows associated with charging and discharging the flywheel system  100 , the motor-generator  116  may function either as an electric motor or as an electric generator. During charging the motor-generator functions as an electric motor. During discharging, the motor-generator functions as an electric generator.  
      During charging, the electric network  106  provides DC power to the third converter  206  via external DC bus  108  as indicated by flow arrow  138 . The converter adjusts the voltage to a level suitable for interconnection with the internal DC bus  222  and transfers electric power as indicated by flow arrow  214  to the internal DC bus. The second converter  204  takes power from the internal DC bus, synthesizes an AC output indicated by flow arrow  132 , and transfers power to the motor-generator via third circuit  126 . The AC output is suitable for powering the motor-generator  116  for accelerating the flywheel  360  (see  FIG. 3 ).  
      During discharging, the second converter  204  receives electric power from the motor-generator  116  via third circuit  126  as indicated by flow arrow  134 . The converter adjusts the voltage to a level suitable for interconnection with the internal DC bus  222  and transfers electric power to the internal DC bus. The third converter  206  takes power from the internal DC bus and adjusts the voltage as required for interconnection with the external DC bus  108 . Flow arrows  216  and  136  indicate transfer of electric power from the second converter to the electrical network via the external DC bus.  
      It should be noted that although the electrical network  106  is interconnected to the external DC bus  108 , a person of ordinary skill in the art will recognize that the electrical network may include electrical sources and loads having electrical characteristics that differ from those of the external DC bus. Auxiliary electric power converters  230  provide for interconnecting such sources and loads to the extent they are present in the network.  
      It should also be noted that while output  242  of first converter  202  may be processed by third converter  206  as shown in  FIG. 2 , in a second embodiment, a fourth unidirectional DC-to-DC electric power converter (not shown) might be used to interconnect first converter output  242  with the electromagnetic bearings  114 . In this embodiment, the fourth converter adjusts the voltage level at first converter output  242  to accommodate the requirements of the electromagnetic bearings.  
       FIG. 3  shows selected flywheel module elements  300 . Rotating elements include the flywheel shaft  346  and the flywheel mass  112 . Stationery elements include the flywheel housing  358 , first and second electromagnets  306 ,  318 , and first and second electric stators  308 ,  342 . The flywheel  360  includes the flywheel mass  112  and the flywheel shaft  346 . The flywheel shaft includes first and second sections  310 ,  314 . The flywheel shaft shares a common axis of rotation  322  with and is attached to the flywheel mass.  
      The flywheel  360  has integrated features including antifriction bearings  354 ,  356  and electromagnetic bearings  324 ,  328 , a backup AC generator  110 , and a synchronous reluctance AC motor-generator  116 . The sections that follow provide details relating to these features.  
      Antifriction bearings  354 ,  356  provide rotatable support to the flywheel  360  at low flywheel speeds. The flywheel utilizes first and second touchdown bearing shafts  302 ,  320  mated with respective first and second antifriction bearings  354 ,  356  for rotatable support. The antifriction bearings support both radial and thrust loads. The touchdown bearing shafts extend outwardly from respective opposing ends of the flywheel shaft and share a common axis of rotation  322  with the flywheel shaft  346 . The first and second antifriction bearings  354 ,  356  are fixed to respective first and second flywheel housing parts  362 ,  364 .  
      Electromagnetic bearing(s)  114  provide rotatable support to the flywheel  360  at higher flywheel speeds when the flywheel is no longer supported by the antifriction bearings  354 ,  356 , but now relies on at least one electromagnetic bearing for support. Here, first and second electromagnetic bearings  324 ,  328  are shown. The first electromagnetic bearing  324  is proximate to the first shaft section  310  and includes an electromagnet  306  attached to the first flywheel housing part  362  and an adjacent ferromagnetic portion  304  that is integral with the flywheel shaft  346 . The second electromagnetic bearing  328  is proximate to the second shaft section  314  and includes an electromagnet  318  attached to the second flywheel housing part  364  and an adjacent ferromagnetic portion  316  that is integral with the flywheel shaft  346 .  
      Each of the ferromagnetic portions of the shaft  304 ,  316  includes a respective plurality of thin ferromagnetic laminates  332 ,  336  having electrical insulation interposed between adjacent laminates. These laminated ferromagnetic structures increase the effectiveness of the electromagnetic bearings by reducing eddy current losses. In particular, eddy currents induced in the ferromagnetic portions by the electromagnets result in I 2  R heating losses. The thin ferromagnetic laminates reduce the magnetic flux in (results in smaller induced voltage) and the conductivity of (smaller conductive cross-section) each ferromagnetic laminate. The result is a reduction in eddy current losses by a factor of approximately 1/n 2  where n is the number of lamella in a ferromagnetic portion.  
      The backup generator  110  is a variable speed permanent magnet AC machine. It includes a first electrical stator  308  adjacent to a flywheel shaft permanent magnet portion  330 . The flywheel shaft permanent magnet portion is in the first flywheel shaft section  310  and includes a permanent magnet  348  integral with the flywheel shaft  346 .  
      Since a permanent magnet generator is self-exciting, the backup generator generates electric power as long as the flywheel  360  is rotating even if no external source of electric power is available. The backup generator therefore provides electric power to the electromagnetic bearings  324 ,  328  when operation of the electromagnetic bearing(s) is desirable and when no other electric power source is available to operate the electromagnetic bearing(s). As a person of ordinary skill in the art will recognize, the power produced by the backup generator may be used to power electric loads internal or external to the flywheel system  100 .  
      The motor-generator  116  is any inductive AC machine known to ordinary persons of skill in the art such as a wound-rotor or a reluctance type machine. The motor-generator includes a second electrical stator  342  and a rotor  344 .  
      In an embodiment, the motor-generator  116  is a variable speed synchronous reluctance AC machine. Here, the motor-generator includes a second electrical stator  342  adjacent to a flywheel shaft reluctor portion  344 . The reluctor portion is in the second flywheel shaft section  314  and includes a plurality of ferromagnetic reluctor poles  352  integral with the flywheel shaft  346 .  
      While functioning as an electric motor, the motor-generator  116  transfers torque  332  to the flywheel shaft  346  increasing the rotational speed of the flywheel  360 . While functioning as an electric generator, the motor-generator transfers torque from  366  the flywheel shaft reducing the rotational speed of the flywheel.  
      Since the motor-generator is not self-exciting, it produces electric power only when induced electric currents magnetize the rotating reluctor poles  352 . An externally excited stator  342  that is magnetically coupled with the reluctor portion induces such currents. Therefore, the motor-generator  116  cannot generate electric power unless there is a source of electric power external to the motor-generator. The power electronics  104  may provide the excitation power; however, when the flywheel  360  speed falls below a minimum value, useful generation of electric power by the motor-generator ends.  
       FIG. 4  is a graph  400  that illustrates the charging, charged, and discharging cycle of the flywheel system  100 . The vertical axis  402  represents rotational speed of the flywheel mass  112  in revolutions per minute (RPM). The horizontal axis  404  represents time.  
      Starting from a stand-still and during pre-liftoff  406 , flywheel system charging begins when the motor-generator  116  functions as a motor, applying an accelerating torque  332  to the flywheel shaft  346  as electrical power is converted to mechanical motion. As the flywheel  360  speed increases, the electromagnetic bearing(s)  324 ,  328  operate during speed range S 1  to substantially disengage the touchdown bearing shafts  302 ,  320  from the antifriction bearings  354 ,  356 ; this is termed “liftoff”  408 .  
      During the post-liftoff period  410 , the flywheel  360  is accelerated to the maximum speed range S 4 . Upon reaching speed range S 4 , the flywheel is fully charged  412 . Prior to discharging, the motor-generator cycles on and off as required to maintain flywheel speed within speed range S 4 . This cycling is required to recover speed decay resulting from friction and other losses in the system.  
      While charging  424  and cyclically while charged  412 , electrical power from the electrical network  106  is conducted in the direction of flow arrow  214  via external DC bus  108 , the third converter  206 , the internal DC bus  222 , the second converter  204 , and the third circuit  126  to the motor-generator  116 . Electric power supplied to the internal DC bus by the electrical network also powers the electromagnetic bearing(s)  114  via second electrical circuit  124  as indicated by flow arrow  130 .  
      The discharging period  426  begins when the motor-generator  116  functions as a generator, applying a retarding torque  366  to the flywheel shaft  346  and converting the energy of mechanical motion into electric power. During this process, the rotational speed of the flywheel  360  is reduced. As the speed decreases from speed range S 4  to speed S 3 , the motor-generator generates electric power. Note that similar flywheel discharging occurs when the electrical network&#39;s external power source  232  is interrupted: In this case, the power flow to the electrical network  106  indicated by flow arrow  258  stops and the electrical network becomes dependent on the flywheel system for delivery of electric power via external DC bus  108  as indicated by flow arrow  136 .  
      During the initial discharging period  414 , electrical power from the motor-generator  116  is conducted in the direction of flow arrow  134  via the third circuit  126 , the second converter  204 , the internal DC bus  222 , the third converter  206 , and the external DC bus  108  to the electrical network  106  as indicated by flow arrow  136 . Electrical power supplied to the internal DC bus by the motor-generator also powers the electromagnetic bearings  324 ,  328  via the second circuit  124  as indicated by flow arrow  130 .  
      When the flywheel  360  reaches the minimum motor-generator speed S 3 , the synchronous reluctance (inductive) motor-generator  116  is no longer able to provide enough electric power to operate the electromagnetic bearing(s)  324 ,  328 . During the subsequent backup power speed regime  416 , the backup generator  110  provides sufficient electric power to operate the electromagnetic bearings. The backup generator also provides electric power for other electrical loads that may be necessary to the safe shut-down of the flywheel. As one who is skilled in the art will recognize, backup generator power is available via external DC tap  244  and internal DC tap  268  for powering critical loads whether they be internal or external to the flywheel system  100 .  
      When touchdown  418  occurs in speed range S 2 , the electromagnetic bearing(s) are no longer needed and the antifriction bearing shafts  302 ,  320  are once again supported by respective antifriction bearings  354 ,  356 .  
      During the backup power speed regime  416 , electrical power from the backup generator  110  flows as indicated by flow arrow  208  via the first converter, the fifth circuit  224 , the external DC bus  108 , the third converter  206 , the internal DC bus  222 , and the sixth circuit  124  to the electromagnetic bearings  114  as indicated by the flow arrow  130 . Here, the third converter is included in the power flow path to accommodate the backup generator&#39;s variable voltage output that rises and falls with the speed of the flywheel  360 .  
      Post-touchdown  420  begins when the speed of the flywheel  360  falls below speed range S 2 . This regime is the final portion of the discharging process  426 . If a source of external power is not available, the flywheel will come to rest.  
      While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.