Patent Publication Number: US-2007103009-A1

Title: Method and Structure for Integrated Energy Storage Device

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
      This application claims priority to provisional patent application Ser. No. 60/732,449; filed on Oct. 31, 2005; commonly assigned, and of which is hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION  
      A flywheel is an electromechanical battery that stores energy mechanically in the form of kinetic energy. Flywheels store energy very efficiently and energy density compared with chemical batteries. In addition to energy density, flywheel energy storage devices also offer several important advantages over chemical energy storage. The rate at which energy can be exchanged into or out of the battery is limited only by the motor-generator design. Therefore, it is possible to withdraw large amounts of energy in a far shorter time than with traditional chemical batteries. It is also possible to quickly charge flywheel devices.  
      Flywheel energy storage devices are not affected by temperature changes as chemical batteries nor do they suffer from the memory effect. Moreover, they are not as limited in the amount of energy they can hold. They have long life and are environmental friendly without toxic/heavy chemical. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored.  
      Conventional flywheel energy storage devices are intricate electromechanical control systems. They are complex and costly to construct and maintain. Furthermore, high performance flywheels deploy expensive composite materials which outgas and affect device performance. The composite materials have limited energy storage/weight ratio due to relatively low tensile strength. As a result, commercially available flywheel energy storage devices are expensive and bulky with large footprint, and have not been adopted widely in industrial applications and almost no presence in commercial and residential applications.  
      Thus, there is a need in the art for methods and apparatus for fabricating an integrate flywheel device with high energy storage/weight ratio, small form factor, and low cost for commercial and residential applications.  
     SUMMARY OF THE INVENTION  
      The present invention relates to a method and device for fabricating an integrated flywheel device using semiconductor materials and IC/MEMS processes. Conventional flywheels deploy high tensile strength and light weight carbon composite materials to achieve high energy storage/weight ratio. Single crystal silicon has higher tensile stress than carbon composites and is relative light weight. With high energy storage/weight ratio and no defects, single crystal silicon is an ideal material for flywheel and can operate at much higher speed than conventional flywheel.  
      The integrated silicon flywheel is operated by electrostatic motor and supported by electrostatic bearings, which consume much less power than magnetic actuation in conventional flywheel energy storage systems.  
      The silicon flywheel device is fabricated by IC and MEMS processes to achieve high device integration and low manufacturing cost. The silicon flywheel and MEMS motor is formed by Deep Reactive Ion Etch (DRIE). Permanent magnetic material is deposited using methods such as sputter, evaporation, Physical Vapor Deposition (PVD), pulsed laser deposition, etc. Planar coils are fabricated by deposition, electroplating, photo lithography and etch.  
      To minimize energy loss due to friction, high vacuum is desirable in a flywheel device. For the integrated silicon flywheel, high vacuum can be achieved using hermetic bonding methods such as eutectic, fusion, glass frit, SOG, anodic, covalent, etc.  
      To achieve large energy capacity, an array of silicon flywheels is fabricated on a single substrate, and multiple layers of flywheel energy storage devices are stacked. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a simplified top-view diagram illustrating components of an integrated flywheel energy storage device according to one embodiment of the present invention.  
       FIG. 2  is a simplified cross section diagram illustrating components of an integrated flywheel energy storage device according to one embodiment of the present invention.  
       FIG. 3  is a simplified cross section diagram illustrating assembled integrated planar flywheel energy storage device according to one embodiment of the present invention.  
       FIG. 4  is simplified diagrams illustrating an array configuration of integrated flywheel energy storage devices according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      According to the present invention, techniques for manufacturing objects are provided. More particularly, the invention provides a method and device for fabricating an integrated flywheel device using semiconductor materials and IC/MEMS processes. As illustrated in Prior Art diagrams, a conventional flywheel energy storage device has a flywheel member coupled to a permanent magnet of a motor/generator. When storing energy, the motor spins the flywheel to high speed converting electrical energy to kinetic energy. When releasing energy, the flywheel spins the generator converting kinetic energy back to electrical energy.  
       FIG. 1  is a simplified top-view diagram illustrating components of an integrated flywheel energy storage device according to one embodiment of the present invention. As illustrated, the integrated flywheel device is configured similar to an electrostatic micromotor. The flywheel  101  is actuated by the stator electrodes  103  and spins at high speed. With active feedback (capacitance sensing), 6 Degree Of Freedom (DOF) of the flywheel can be controlled and flywheel is levitated and suspended from the substrate  105 . The flywheel device is fabricated on a single crystal silicon substrate using MEMS and IC processes.  
       FIG. 2  is a simplified cross section diagram illustrating components of an integrated flywheel energy storage device according to one embodiment of the present invention. As illustrated, the device consists of four substrates: flywheel substrate  201 , control and generator substrate  203 , top housing substrate  205 , and bottom housing substrate  207 . The control and generator substrate consists of flywheel levitation control electrodes  209  and Copper coil winding  211 . Flywheel resting supporting structures  213  are formed on the housing substrates. A permanent magnet  215  is attached to the flywheel  101 . The four substrates are bonded and the chamber enclosed is hermetically sealed  217 . Bonding and hermetically sealing methods include: Eutectic, Fusion, Glass frit, SOG, Anodic, Covalent, etc. Inside the chamber is a high vacuum  219  where the flywheel spins in high speed without aerodynamic friction losses.  
      The flywheel sits on the resting support structures  213  when system is off. During operation, the flywheel is levitated by the control electrodes  209  via electrostatic force and active position feedback, which function as electrostatic bearings. The stator electrodes  103  spin the flywheel to maximum speed converting electrical energy to kinetic energy. During discharging, the generator is turned on and electricity is generated in the Copper coil winding via interaction with the permanent magnet.  
       FIG. 3  is a simplified cross section diagram illustrating assembled integrated planar flywheel energy storage device according to one embodiment of the present invention. As illustrated in A-A zoomed-in view, a permanent magnetic film  301  is deposited onto the flywheel surface and planar coil  303  is formed on the generator substrate. The permanent magnetic film is coupled to the planar coil via electromagnetic interaction thru vacuum gap  305 .  
      The flywheel sits on the resting support structures  213  when system is off. During operation, the flywheel is levitated by the control electrodes  209  via electrostatic force and active position feedback, which function as electrostatic bearings. The stator electrodes  103  spin the flywheel to maximum speed converting electrical energy to kinetic energy. During discharging, the generator is turned on and electricity is generated in the planar coils  303  via interaction with the permanent magnet film  301 .  
      The permanent magnetic material is selected from Neodymium-iron-boron (NdFeB), Samarium Cobalt (SmCo), etc. Deposition methods include: Sputter, Evaporation, Physical Vapor Deposition (PVD), pulsed laser deposition, etc. The plan coil material is selected from Copper, Nickel, etc. Fabrication methods include: Sputter, Evaporation, Physical Vapor Deposition (PVD), electroplating, photo lithography, and etch.  
       FIG. 4  is a simplified diagrams illustrating an array configuration of integrated flywheel energy storage devices according to one embodiment of the present invention. As depicted in the top view, an array of integrated flywheel energy storage devices are fabricated on a single substrate for larger capacity according to one embodiment of the present invention. According to another embodiment of the present invention, multiple layers of flywheel energy storage devices are stacked as shown in the side view diagram. Each storage device is individually operated and controlled.  
      It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.