Devices and methods for increasing energy and/or power density in composite flywheel energy storage systems

Abstract

A flywheel formed of a composite material having fibers, oriented substantially in a circumferential direction around the flywheel, embedded in a matrix material. The flywheel having an inner surface, an outer surface, and a thickness therebetween and defining an axis of rotation. A plurality of load masses are distributed circumferentially on the inner surface at a longitudinal segment along the axis. A rotation of the flywheel about the axis with a rotational velocity generating hoop stress in the fibers in the circumferential direction and through-thickness stress is generated in the matrix material in a radial direction. Each load mass produces a force on the inner surface operative to reduce the maximum through-thickness stress in the matrix material as the flywheel rotates about the axis. The rotational velocity otherwise sufficient to produce structural failure of the matrix material produces structural failure of the fibers and not the matrix material.

What is claimed is: 
     
         1 . An apparatus comprising:
 a hollow cylindrical flywheel for a motor/generator,   the flywheel being formed of a composite material including a matrix material and fibers oriented at least in part in a circumferential direction around the flywheel embedded in the matrix material,   the flywheel having a longitudinal axis of rotation, a radially inner surface, a radially outer surface, and a radial thickness between the inner surface and the outer surface,   rotation of the flywheel about the axis of rotation generating hoop stress in the fibers in the circumferential direction and through-thickness stress in the matrix material in the radial direction, the material properties of the fibers and the matrix material being such that rotation of the flywheel about the axis of rotation at a first rotational velocity sufficiently high to produce structural failure of the flywheel produces failure of the matrix material in the radial direction and not failure of the fibers in the circumferential direction, such rotational velocity being a first rotational velocity; and   a plurality of load masses distributed circumferentially around, and coupled to, the inner surface of the flywheel at a longitudinal segment along the axis of rotation, rotation of the flywheel causing each load mass from the plurality of load masses to produce a radially outwardly directed force on the inner surface of the flywheel, the radially outwardly directed force acting to reduce a maximum through-thickness stress in the matrix such that a second rotational velocity greater than the first rotational velocity and sufficiently high to produce structural failure of the flywheel in the longitudinal segment produces failure of the fibers in the circumferential direction and not failure of the matrix material in the radial direction.   
     
     
         2 . The apparatus of  claim 1  wherein each load mass is formed of a permanent magnetic material. 
     
     
         3 . The apparatus of  claim 1  wherein each load mass is formed of an inert material. 
     
     
         4 . The apparatus of  claim 1  wherein the plurality of load masses is a first plurality of load masses and the longitudinal segment is a first longitudinal segment, and further comprising a second plurality of load masses distributed circumferentially around, and coupled to, the inner surface of the flywheel at a second longitudinal segment along the axis of rotation, rotation of the flywheel causing each load mass to produce a radially outwardly directed force on the inner surface of the flywheel, the force acting to reduce the maximum through-thickness stress in the matrix such that the matrix material does not fail in the radial direction at the second longitudinal segment at the second rotational velocity. 
     
     
         5 . The apparatus of  claim 4  wherein each load mass in the first plurality of load masses is formed of a permanent magnetic material and each load mass in the second plurality of load masses is formed of a permanent magnetic material. 
     
     
         6 . The apparatus of  claim 4  wherein each load mass in the first plurality of load masses is formed of a permanent magnetic material and each load mass in the second plurality of load masses is formed of an inert material. 
     
     
         7 . The apparatus of  claim 4 , wherein each load mass in each of the first plurality of load masses and the second plurality of load masses is formed of a permanent magnetic material and has a magnetic polarity, the magnetic polarities of the load masses in each of the first plurality of load masses and the second plurality of load masses alternating circumferentially around the flywheel, each load mass in the first plurality of load masses being disposed in axial alignment with, and having its magnetic polarity oriented in the same direction as, a corresponding load mass in the second plurality of load masses, the first plurality of load masses being axially spaced from the second plurality of load masses to define an axial stator gap therebetween having a longitudinal gap length, and further comprising a stator having a plurality of conductive windings disposed thereon, and having an axial thickness less than the longitudinal gap length, the stator being disposed in operative relationship to the flywheel such that at least a portion of the windings are disposed axially between the first plurality of load masses and the second plurality of load masses in the stator gap such that rotation of the flywheel relative to the stator produces a flow of electrical current in the windings. 
     
     
         8 . The apparatus of  claim 1  wherein each load mass has a circumferential dimension, an axial dimension, and a radial dimension, and wherein each dimension of every load mass is substantially the same. 
     
     
         9 . The apparatus of  claim 1  wherein each load mass is configured to maintain its structural integrity when the flywheel is rotated at the second rotational velocity. 
     
     
         10 . The apparatus of  claim 1  wherein the plurality of load masses is a first plurality of load masses, the longitudinal segment is a first longitudinal segment, and the flywheel has an axial length, and further comprising a plurality of additional pluralities of load masses, each additional plurality of load masses distributed circumferentially around, and coupled to, the inner surface of the flywheel at a respective additional longitudinal segment along the axis of rotation, the first longitudinal segment and the plurality of additional longitudinal segments distributed substantially uniformly along the axial length of the flywheel, rotation of the flywheel causing each load mass in each additional plurality of load masses to produce a radially outwardly directed force on the inner surface of the flywheel, the force acting to reduce the maximum through-thickness stress in the matrix such that the matrix material does not fail in the radial direction at any of the additional longitudinal segments at the second rotational velocity. 
     
     
         11 . The apparatus of  claim 1 , wherein each load mass in the plurality of load masses is formed of a permanent magnetic material and has a magnetic polarity, the magnetic polarities of the load masses alternating circumferentially around the flywheel, and further comprising a stator having a plurality of conductive windings disposed thereon, the stator being disposed in operative relationship to the plurality of load masses such that rotation of the flywheel relative to the stator produces a flow of electrical current in the windings. 
     
     
         12 . The apparatus of  claim 11  wherein:
 each load mass in the first plurality of load masses and the second plurality of load masses has a radial dimension, the radial dimensions of all of the load masses in the first plurality of load masses and the second plurality of load masses is substantially equal to a first radial dimension, 
 each load mass in the first plurality of load masses and the second plurality of load masses has a density, the densities of all of the load masses in the first plurality of load masses and the second plurality of load masses is substantially equal to a first density, 
 each of the first plurality of load masses and the second plurality of load masses producing a pressure on the inner surface of the flywheel upon rotation of the flywheel at the first velocity substantially equal to a first pressure, 
 
       and further comprising a third plurality of load masses distributed circumferentially around, and coupled to, the inner surface of the flywheel within in the axial stator gap,
 each load mass in the third plurality of load masses having a radial dimension, the radial dimensions of the third plurality of load masses being substantially equal to a second radial dimension less than the first radial dimension, the difference between the first radial dimension and the second radial dimension defining a radial extent of the stator gap within which the at least a portion of the windings is disposed, 
 each load mass in the third plurality of load masses having a density, the densities of the third plurality of load masses being substantially equal to a second density greater than the first density, 
 the third plurality of load masses producing a pressure on the inner surface of the flywheel upon rotation of the flywheel at the first velocity substantially equal to the first pressure.