Abstract:
A geometric design of ES generator/motor electrodes mounted on the inner surface of a fiber-composite rotor is provided. The electrode configuration is able to sustain very high g levels. The rotor may be formed of carbon-fiber wound an top of an inner E or S-glass fiber composite core. The electrode design provides the needed area to satisfy the power requirements of the storage system and utilizes a stacked wedge-like electrode array that bath solves the high-g problem and results in a doubling or tripling of the electrode area, relative to that of electrodes that conform to the inner cylindrical surface of the rotor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 62/207,341 tiled “Improved Design for Electrostatic Generator/Motor Electrodes Located on the Inner Surface of an EMB Rotor,” filed Aug. 19, 2015, incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The United States Government has rights in this invention pursuant to Contract No. DF AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Field of the Invention 
         [0004]    The present invention relates to electromechanical battery (EMB) electrode design, and more specifically, it relates to means for overcoming the high g load on the rotor electrodes of an EMB. 
         [0005]    Description of Related Art 
         [0006]    In the design of EMB flywheel energy storage modules utilizing high strength fiber composite rotors and electrostatic generator/motors, the electrodes of such modules must withstand very high centrifugal forces. This situation is particularly evident for small rotors (e.g., diameters of 10 cm or less) that may operate at speeds in excess of 200,000 RPM. In such modules, the centrifugal force at the inner surface of the rotor may be of in excess of a million g, implying that an item weighing only 1 gram on the inner surface of the rotor will have an equivalent weight of over 1 metric ton. Since even the lightest electrode structure for the rotor can be expected to weigh tens of grams, its equivalent “weight” can be of order 10 to 100 metric tons. This centrifugal force will be exerted on the inner surface of the rotor. A simple and practical solution to this high-g load problem is desirable. A means of increasing the rotor electrode area by a substantial factor relative to the area of the inner surface of the rotor is also desired since increasing the electrode area of an E-S generator results in a proportionate increase in power output from the generator. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention pertains to the geometric design of E-S generator/motor electrodes mounted on the inner surface of a fiber-composite rotor. Particularly in small EMB rotors, the centrifugal g-forces are very high so that the E-S rotor electrode design must be such as to be able to sustain these forces. In addition, if the rotor is fabricated from carbon-fiber composite, its electrical conductivity would interfere with the proper operation of the E-S generator/motor electrodes. Since carbon-fiber rotors are commonly wound on top of an inner E or S-glass fiber composite core, mounting the electrodes on the inner surface both can solve the high-g problem and the carbon-fiber conductivity problem. The issue then is to achieve the needed area to satisfy the power requirements of the storage system. This problem is most evident in vehicular applications. This invention pertains to a design that employs a stacked wedge-like electrode array that both solves the high-g problem and results in a doubling or tripling of the electrode area, relative to that of electrodes that conform to the inner cylindrical surface of the rotor. The invention can be used in flywheel storage systems or other rotating machinery, including for vehicular or stationary use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0009]      FIG. 1A  is a schematic section drawing of an EMB rotor comprising a carbon fiber wound on top of an inner E- or S-glass fiber composite core with rotor electrode support rings on the inside surface of the rotor. 
           [0010]      FIG. 1B  is a schematic section drawing of an electrically non-conducting EMB rotor with rotor electrode support rings on the inside surface of the rotor. 
           [0011]      FIG. 1C  is a schematic section drawing of an EMB rotor comprising a carbon fiber wound on top Of an inner E- or S-glass fiber composite core, where the inner core has been machined to form slots. 
           [0012]      FIG. 1D  is a schematic section drawing of an electrically non-conducting EMB rotor where its inner surface has been machined to form slots. 
           [0013]      FIG. 2  is a schematic section drawing of a stator electrode end. 
           [0014]      FIG. 3A  shows a rotor section of  FIG. 1  and shows an upper stator section and a lower stator section where each stator includes arrays of the electrodes of  FIG. 2 . 
           [0015]      FIG. 3B  shows an axial view of the stator of  FIG. 3A   
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    In the design of the fiber-composite rotors for EMBs, high-strength fibers are embedded in an epoxy matrix. The fibers used include &amp;glass and the higher-strength S-Glass, basalt fibers, and carbon fibers such as IMS65 and the highest strength carbon fiber, T1000 (tensile strength 7 Gpa, i.e., about 1 million psi). In the context of this invention, it is important to note two circumstances. First, it should be noted that carbon fiber composites are electrically conducting and this fact must be considered when designing the electrode system for an E-S generator/motor to be mounted on the rotor. The second circumstance relates to a rotor design practice that is commonly employed when designing and constructing carbon-fiber composite rotors. That is, in filament winding of the rotor, the innermost composite layers are made using S-glass instead of carbon fiber. This two-layer design is employed because carbon-fiber composites are highly sensitive to the presence of point loads, while S-Glass composites are much less sensitive. Centrifugal stresses within the S-glass composite are transferred in part to the high-modulus carbon-fiber composite so that the S-glass is not stressed above its safe working limit. 
         [0017]    The first circumstance implies that when the outer part of the rotor body is composed of carbon-fiber composite, its electrical conductivity precludes the location of the conducting strips required for the electrodes on the outer surface of the rotor. In such a case, the conductivity of that part of the rotor surface lying between the strips would greatly vitiate the performance of the E-S generator. Thus, for carbon fiber composite rotors to be made compatible with the use of surface-mounted rotor electrodes, these electrodes must be located on the inner surface of the rotor, and that inner part of the rotor must be non-conducting, i.e., it must be composed of glass, or basalt fiber composite. 
         [0018]    The next issue faced by the designer is to insure that the electrode area is Sufficiently large to generate the power required by the application. If the application of the EMB is the bulk storage of energy, with hours-long charging and discharging times, then simple designs can be used. An example would be to use vertical strips of metal foil or metallic coating on the inner surface of the rotor. However if the peak power demands are substantially higher, as they would be in vehicular applications, for example, then a higher electrode area would likely be required. The invention thus addresses two issues: (1) Dealing with high-g centrifugal force fields, and (2) Increasing the electrical capacitance and the maxi/min capacity ratio of the E-S electrode system. 
         [0019]    The basic concept of the new system is illustrated schematically in the section views shown in  FIGS. 1 and 2 . 
         [0020]      FIG. 1A  shows a sectional view of hollow cylindrical rotor  10 , which is formed of a non-conducting core  12  within a carbon fiber composite  14 . The inner stir face of the rotor  10  has been configured to include multiple vertically spaced cells  16 , bounded on their upper and lower surfaces by wedge-shaped fiber-composite separator rings  18 . One exemplary embodiment of the rings is formed of G-10 glass and another is formed of fiberglass. Other embodiments are possible. Thus, a configuration of cells is achieved by assembling a stack of fiber-composite rings of the proper shape within the inside surface of the glass or basalt fiber-composite inner portion of the rotor. In this embodiment, the separators are made wedge-shaped instead of being constructed as constant-thickness thickness in order to minimize the compressive stresses that will occur at their bases, however, other shapes are within the scope of the invention. Note that is can be beneficial to make the tips of the rings  18  to be rounded rather than points pointed tips tend to have a lower breakdown voltage. Likewise, the troughs of the cells can benefit from having rounded corners rather than sharp corners. This same principle applies to all of the electrode configurations described herein. Keeping these stresses within their limiting value will also determine the maximum value of the ratio of the outer radius of these rings to their inner radius  FIG. 1B  is a sectional view that shows a configuration where rotor  11  is conductive and where the inner non-conducting core of  FIG. 1A  is omitted. 
         [0021]    An alternate configuration of cells can be achieved by machining slots on the inner surface of the rotor as shown in the sectional view of  FIG. 1C . In this configuration, the inner surface is machined in an electrically non-conducting core  20  around which is wound a carbon fiber composite  22 . Note that the core  20  can beneficially have all rounded or beveled corners to increase the electrical breakdown voltage.  FIG. 1D  is a sectional view that shows another rotor configuration having slots machined on the inner surface. In this embodiment, the entire rotor  30  is formed of electrically non-conducting material. Note that the core  20  can beneficially have all rounded or beveled corners to increase the electrical breakdown voltage. As much as possible, the contour of the peaks and troughs should be uniform. 
         [0022]      FIG. 2  shows a side sectional view of a blade  40  of the stator. To achieve a high max/min capacity ratio and also to enhance the voltage-holding ability of the electrode system, a tapered cross-section metal electrode  42  is employed. This electrode is then completely sheathed by a high dielectric constant polymer  44 , e.g., polyester fiber composite (K=10 at E-S generator frequencies). The max/min ratio for this electrode structure depends upon the structure geometric parameters but can be expected to be 3:1 or greater. Although the figure shows the blade as having corners, it is beneficial if the edges that face the rotor electrodes are rounded or beveled. It is beneficial to have a rounded top, rather than a flat top. 
         [0023]    The new conductor array is can be incorporated into a “balanced circuit” system (See  FIG. 15  in U.S. Pat. No. 7,834,511 incorporated herein by reference). In using this circuit, the rotor electrodes operate in a “virtual ground” situation. That is, the stator is divided into an upper (“plus” charged) section and a lower (“minus” charged) section. See  FIG. 3A , described below. The ES generator capacitor is thus divided into an upper and lower hall, with the rotor electrodes, which run the full length of the rotor, completing the circuit. These electrodes could be made, for example, of vertical strips of metal foil bonded to the inner corrugated surface of the rotor cell structure. Their positioning would be such as to match the periodicity of the stator electrode system, by incorporating an azimuthal gap between them with the same periodicity. To optimize the max/min capacity ratio, the gaps between the rotor conductor strips might be made wider than the width of the strips themselves. 
         [0024]    More specifically,  FIG. 3A  shows a sectional view of one side of the rotor  50  which includes the separator rings  52  on which is laid down a conducting strip  53 . In this embodiment, the rotor is electrically non-conductive. The figure also shows an upper series of stator electrode blades  54  and a lower series of stator electrode blades  56 . The upper series of blades  54  are in electrical contact with conductive strip  58 , which is attached to stator section  60 . The lower series of blades  56  are in electrical contact with conductive strip  62 , which is attached to stator section  64 . Notice the gap  66  between stator section  60  and stator section  64 . If the upper and lower stator sections are formed of electrically conductive material, the conductive blades  54 ,  56  can be in direct contact with stator sections  60 ,  64  and conductive strips  58  and  62  can be omitted. Although the conducting strip  53  is shown as having a pointed peak, it is beneficial if the peak is rounded. Likewise, although the blades  54  and  56  are shown as having flat tops, it is beneficial if they are rounded. Although the blades appear to have non-uniform sizes and dimensions, it is beneficial if they all are as identical as possible.  FIG. 3B  shows an axial view of the stator of  FIG. 3A  and shows rotor  60 , strip  58  and blades  54 . For clarity, a balanced circuit similar to the one shown in  FIG. 15  of U.S. Pat. No, 7,834,513 is provided. A source  80  of positive voltage is connected to inductor  82  and resistor  84  and to conductive strip  58 . A source  86  of negative voltage is connected to inductor  88  and resistor  90  and to conductive strip  62 . Note that it is beneficial if the conducting strip  53  is a plating rather than a foil. Such plating can be formed by vapor deposition. In this embodiment, a connection is made from a node  85  between resister  84  and strip  58  to a node  91  between resistor  90  and strip  62 . The connection from node  85  is made to a capacitor  94  and to a load  96  to a capacitor  98  to node  91 . Based on the teachings herein, those skilled in the art will understand that other circuits are useable with the configuration of  FIG. 3A . Assembly of this and similar embodiments can be accomplished by building or stacking the device from one end of the rotor. For example, starting at the bottom of the rotor  50  with reference to the page, a first ring can be attached to the rotor. Then, a blade is attached to the stator. The assembly process is continued up the rotor. The rotor electrode can be deposited by rotating the rotor such that is does not directly line up with the stator electrodes. It is possible to first fabricate the stator and its blades and then connect the rotor sections around the stator. Similarly, the rotor can be prefabricated and the stator assemble inserted in sections within the inner radius of the cylindrical rotor. Other assembly procedures will be apparent to those skilled in the art based on this disclosure. It is also possible to tithe rotor blades to be rings with non-conducting material located along the longitudinal axis of the stator on its outside wall such that there are conducting blades corresponding to the spacing of the rotor electrodes. 
       Example Calculations 
       [0025]    To demonstrate aspects of the performance of the new electrode geometry, a calculation of the ratio of the capacitor area with the new configuration located on the inner surface of a rotor with a given inner radius to that for electrodes mounted directly on that inner surface. The number of cells of the example was that shown in  FIG. 1A . For this case the area ratio was 2.5:1. Higher values could be obtained, if need be, by increasing the number of cells. In addition, a calculation was made of the compressive stress at the base of the wedges in a small rotor with an inside radius of 0.035 m., operated at 250,000 rpm. The calculated stress was 293 MPa, well below the maximum compressive stress value cited in the literature for glass-fiber composite (e.g., G-10 fiber-glass composite, 448 MPa). 
         [0026]    Thus, a new electrode configuration has been described, one designed for installation on the inside of the rotor of an EMB, and aimed at the twin objectives of operation at high centrifugal g levels at the same time increasing the electrode an as needed, for example, in vehicular applications. The results of calculations showing the improved performance were also given. 
         [0027]    The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.