MAGNETICALLY COUPLED FLYWHEEL
FIELD OF THE INVENTION
[0001] The present invention relates to flywheels. More particularly, the present
invention relates to a magnetically coupled flywheel.
BACKGROUND OF THE INVENTION
[0002] Electrical energy storage systems are of use both to producers and consumers of
electrical energy. Storage systems for electrical energy may include storage batteries or
other chemistry-based storage systems, capacitors or other electrically-based storage
systems, thermal storage, or mechanical energy storage systems. Mechanical energy
storage systems may include gravity-based storage systems, or inertial systems. Inertial
systems may include flywheel systems.
[0003] A typical flywheel system consists of a flywheel in the form of a rotating mass
that shares a common shaft with a rotor of a motor/generator unit. The rotating mass
may include a material that is sufficiently dense and strong to effectively store the
energy while remaining intact and operation. For example, the rotating mass may
include steel, a composite material, or a combination of such materials. The
motor/generator unit functions as a motor during a charge phase of the system and as a
generator during a discharge phase.
[0004] During a charge phase, electrical power to be stored by the flywheel is provided
to the motor/generator unit from a generating system or from an electrical grid in the
form of an electrical current. The motor/generator unit then functions as an electrical
motor. The current causes the rotor of the motor to generate a positive torque that
provides angular acceleration to increase the rotational velocity, and thus the rotational
kinetic energy, of the flywheel. The flywheel reaches a desired rotational velocity at
which the flywheel is storing a desired quantity of energy. (Since rotational kinetic
energy is proportional to the square of the angular velocity, flywheels are typically
designed to spin at high speed.) The charge phase then ends.
[0005] After the charge phase, a store phase (typically longer than the charge phase)
may begin. During the store phase, the motor/generator unit may be disconnected from
any external electrical circuit, being thus placed in an idle mode. Thus, rotational inertia
of the flywheel causes the flywheel to continue to rotate, storing the energy as rotational
kinetic energy of the flywheel. During the store phase, various frictional forces may act
to slow the speed of rotation of the flywheel and to cause loss of the stored energy.
Therefore, during the store phase, current may be provided intermittently and for brief
periods to the motor/generator unit to restore lost energy.
[0006] When power is to be extracted from the flywheel system, a discharge phase is
entered. During a discharge phase, energy that is stored in the flywheel is converted to
electrical power and made available for use (e.g., via an electrical power grid). The
motor/generator unit is connected to an external electrical circuit and functions as a
generator. Rotation of the flywheel turns the rotor of the generator and generates
electrical power while applying a decelerating or braking force to the flywheel. The
discharge phase may continue until there is no longer a need for the stored energy. The
flywheel system may then revert to the store phase. In other cases, the rotational
velocity of the flywheel may be reduced during the discharge phase to less than a
minimum velocity (e.g., below which the system is no longer capable of generating
usable electrical power). In this case the system may enter a wait phase.
[0007] During the wait phase, the flywheel may be stopped or may be rotating at a
minimal velocity. The motor/generator is disconnected from external circuits and placed
in an idle mode. The wait phase may continue until electrical energy is available to
charge the flywheel again.
[0008] For an energy producer, energy storage enables provision of electrical power to
the electrical grid at a constant rate. For example, the rate of generation of electricity
using renewable sources such as solar, wind, or tidal power may vary as the power
source varies. Thus, at times when electrical power production exceeds demand, excess
produced energy may be stored. On the other hand, at times when demand for electrical
power exceeds production, the stored energy may be provided to the electrical grid for
use by consumers. Similarly, energy storage may enable electrical power production at a
constant rate, regardless of momentary demand. Thus, electrical power may be
generated without a need for (e.g., fuel based) generators that are operated only when
demand is high (and may produce more carbon or pollutants than the generators that are
operated constantly).
[0009] Similarly, a storage system may be used by a consumer to save energy costs. For
example, the cost of electrical power from the grid may vary periodically. An electric
power rate structure may charge more for electrical power during peak demand hours
and less during off-peak hours (e.g., a rate during peak hours may be triple the rate
during off-peak hours). A consumer with an energy storage system may thus buy
electrical power during off-peak hours and use the saved energy during peak demand
hours.
[0010] As compared with other energy storage techniques, systems, or methods, a
flywheel provides some advantages. For example, the number of charge/discharge
cycles is virtually unlimited, limited only by the wear of the mechanical parts. The
amount and frequency of required maintenance may thus also be low as compared with
other systems. Flywheel systems may also be relatively insensitive to environmental
factors such as temperature changes. A flywheel system does not require use of
hazardous materials, does not emit harmful gasses, and components of the system may
be recyclable at the end of the useful life of the system.
SUMMARY OF THE INVENTION
[0011] There is thus provided, in accordance with an embodiment of the present
invention, a stabilization system for a rotating load, the system including: a mechanical
bearing to continuously support a shaft of the rotating load so as to hold the shaft at a
substantially fixed axis of rotation; a magnetic stabilization assembly including a
plurality of electromagnets arranged around the shaft; a control circuitry for controlling
a resultant magnetic field generated by the electromagnets such that the magnetic field
acts on a ferromagnetic element of the shaft to reduce imbalance forces acting on the
shaft.
[0012] Furthermore, in accordance with some embodiments of the present invention,
the ferromagnetic element includes a rotor ring.
[0013] Furthermore, in accordance with some embodiments of the present invention,
the system includes a sensor to sense a vibration of the shaft, the control circuitry being
configured to control the resultant magnetic field so as to minimize the sensed vibration.
[0014] Furthermore, in accordance with some embodiments of the present invention,
the control circuitry includes an H-bridge or a power amplifier to drive the
electromagnets to generate a desired magnetic field.
[0015] Furthermore, in accordance with some embodiments of the present invention,
the control circuitry is configured to compensate for a previously measured variation in
a dimension of a mechanical component.
[0016] Furthermore, in accordance with some embodiments of the present invention,
the rotating load includes a flywheel for storing energy.
[0017] Furthermore, in accordance with some embodiments of the present invention,
the mechanical bearing includes a bearing selected from a group of bearings consisting
of a metal ball bearing, a hybrid ball bearing, and a ceramic ball bearing
[0018] There is further provided, in accordance with some embodiments of the present
invention, a flywheel energy storage system including a flywheel within an evacuable
enclosure, the flywheel including a core rotatable about an axis of rotation and a
plurality of rods, a proximal end of each rod being attached to a periphery of the core,
the rods extending substantially radially with respect to the axis of rotation.
[0019] Furthermore, in accordance with some embodiments of the present invention,
the rods are attached to the periphery of the core in a staggered pattern.
[0020] Furthermore, in accordance with some embodiments of the present invention,
each proximal end is attached to the periphery of the core by holder that is configured to
hold the proximal end by a mechanism selected from a group of holding mechanisms
consisting of a press fit, a self-locking wedge, high shear-stress glue, and a collapsible
ferrule.
[0021] Furthermore, in accordance with some embodiments of the present invention, a
distal end of a rod of said plurality of rods is weighted.
[0022] Furthermore, in accordance with some embodiments of the present invention, a
rod of the plurality of rods includes fiberglass.
[0023] Furthermore, in accordance with some embodiments of the present invention, a
rod of the plurality of rods includes a bundle of fibers wrapped around a column.
[0024] There is further provided, in accordance with some embodiments of the present
invention, a flywheel energy storage system including: a DC bus; a plurality of
flywheels; a plurality of motor/generator units, each motor/generator unit being
rotatably coupled to a flywheel; a plurality of controller/inverters, each
controller/inverter being electrically coupled to a motor/generator unit and to the DC
bus; and a central controller to control each controller/inverter so as to set a discharge
rate for each of the flywheels when its motor/generator unit is operating in a discharge
mode, and to increase a voltage level of a voltage signal generated by the
motor/generator unit in the discharge mode.
[0025] Furthermore, in accordance with some embodiments of the present invention, a
controller/inverter includes an H-bridge circuit.
[0026] Furthermore, in accordance with some embodiments of the present invention,
the central controller is configured to control a controller/inverter of the plurality of
controller/inverters to operate in a discharge mode while concurrently controlling
another controller/inverter of the plurality of controller/inverters to operate in a charge
mode.
[0027] There is further provided, in accordance with some embodiments of the present
invention, a flywheel energy storage system for storing electrical energy, the system
including: a flywheel with a rotatable mass and a shaft, the flywheel being enclosed
within an evacuable enclosure, the shaft supported by bearings on opposite sides of the
rotatable mass; and an electric motor/generator unit having a stator and a rotor, the rotor
being fixed to the shaft in a cantilevered manner within the enclosure and being
magnetically coupled to the stator, the stator being located outside of the enclosure.
[0028] Furthermore, in accordance with some embodiments of the present invention, a
distance between the rotor and the stator is adjustable.
[0029] Furthermore, in accordance with some embodiments of the present invention,
the stator is configured to couple to each of a plurality of rotors.
[0030] Furthermore, in accordance with some embodiments of the present invention,
the flywheel includes lead enveloped in a shell that includes carbon fiber.
[0031] Furthermore, in accordance with some embodiments of the present invention,
the flywheel includes a plurality of glass fibers, each fiber being at least partially
wrapped around a column of a plurality of columns that are arranged in a circular
pattern that is centered on an axis of rotation of the flywheel, such that each fiber
extends substantially radially outward from the axis when the flywheel rotates.
[0032] Furthermore, in accordance with some embodiments of the present invention,
the flywheel includes a structure with an eccentric mass distribution that is rotatable to
adjust a balance of the flywheel.
[0033] Furthermore, in accordance with some embodiments of the present invention,
section of the enclosure between the rotor and the stator includes glass.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] In order to better understand the present invention, and appreciate its practical
applications, the following Figures are provided and referenced hereafter. It should be
noted that the Figures are given as examples only and in no way limit the scope of the
invention. Like components are denoted by like reference numerals.
[0035] Fig. 1 schematically illustrates a flywheel energy storage system with a flywheel
magnetically coupled to a rotor of a motor/generator unit, in accordance with an
embodiment of the present invention.
[0036] Fig. 2 schematically illustrates sharing of a single motor/generator unit by a
plurality of flywheel units of a flywheel energy storage system, in accordance with an
embodiment of the present invention.
[0037] Fig. 3 schematically illustrates a flywheel energy storage system with magnetic
reduction transmission, in accordance with an embodiment of the present invention.
[0038] Fig. 4 schematically illustrates a flywheel with a shell construction, in
accordance with an embodiment of the present invention.
[0039] Fig. 5A schematically illustrates a brush-like flywheel that includes radially
projecting rods, in accordance with an embodiment of the present invention.
[0040] Fig. 5B schematically illustrates a staggered arrangement of rods of the brush
like flywheel shown in Fig. 5A.
[0041] Fig. 6 schematically illustrates operation of a rod holder of the brush-like
flywheel shown in Fig. 5A.
[0042] Fig. 7 schematically illustrates operation of a collapsible ferule to hold a
projecting rod.
[0043] Fig. 8 schematically illustrates a variant of the brush-like flywheel shown in Fig.
5A, in which the radially projecting rods are weighted.
[0044] Fig. 9 schematically illustrates structure of a cassette of a glass fiber brush-like
flywheel, in accordance with an embodiment of the present invention.
[0045] Fig. 10 illustrates a technique to form a fiber bundle for the cassette shown in
Fig. 9.
[0046] Fig. 11 schematically illustrates an open-frame motor/generator unit, in
accordance with an embodiment of the present invention.
[0047] Fig. 12 schematically illustrates a flywheel cluster, in accordance with an
embodiment of the present invention.
[0048] Fig. 13A schematically illustrates an active magnetic balancing system for a
flywheel of a flywheel energy storage system, in accordance with an embodiment of the
present invention.
[0049] Fig. 13B shows a top view of the active magnetic balancing system shown in
Fig. 13A.
[0050] Fig. 13C schematically illustrates control of the active magnetic balancing
system shown in Fig. 13A using a three-phase H-bridge.
[0051] Fig. 13D schematically illustrates control of the active magnetic balancing
system shown in Fig. 13A using power amplifiers.
[0052] Fig. 14A is a schematic illustration of a flywheel balancing assembly for a
flywheel energy storage system, in accordance with an embodiment of the present
invention.
[0053] Fig. 14B is a side view of the flywheel balancing assembly shown in Fig. 14A.
[0054] Fig. 15 schematically illustrates a flywheel energy storage system that includes
an array of flywheel units, in accordance with an embodiment of the present invention.
[0055] Fig. 16 schematically illustrates a flywheel energy storage system that is directly
connected to a renewable energy DC bus, in accordance with an embodiment of the
present invention.
[0056] Fig. 17 schematically illustrates a flywheel energy storage system that includes a
constant voltage DC bus, in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0057] In the following detailed description, numerous specific details are set forth in
order to provide a thorough understanding of the invention. However, it will be
understood by those of ordinary skill in the art that the invention may be practiced
without these specific details. In other instances, well-known methods, procedures,
components, modules, units and/or circuits have not been described in detail so as not to
obscure the invention.
[0058] Embodiments of the invention may include an article such as a computer or
processor readable medium, or a computer or processor storage medium, such as for
example a memory, a disk drive, or a USB flash memory, encoding, including or storing
instructions, e.g., computer-executable instructions, which when executed by a
processor or controller, carry out methods disclosed herein.
[0059] In accordance with embodiments of the present invention, a flywheel energy
storage system includes a flywheel. The flywheel, as well as any shaft, axle, or other
rotating component that is rotatable together with the flywheel, is enclosed within a
vacuum enclosure. Air, or other gaseous or fluid contents of the vacuum enclosure, may
be evacuated to form a vacuum within the vacuum enclosure.
[0060] Enclosing the flywheel within vacuum that is formed within an evacuated
vacuum chamber may be advantageous. Operating the flywheel in a vacuum minimizes
air drag on rotating components of the flywheel.
[0061] In accordance with embodiments of the present invention, some or all
components of a motor/generator unit of the flywheel energy storage system are located
outside of the vacuum enclosure. For example, the entire motor/generator unit may be
located outside of the vacuum enclosure. As another example, a stator of the
motor/generator unit may be located outside of the vacuum enclosure, while a rotor of
the motor/generator unit is located within the vacuum enclosure.
[0062] Placement of the motor/generator unit outside of the vacuum enclosure may be
advantageous as compared with a system in which the motor/generator unit is also
enclosed within a vacuum enclosure. For example, placement of the motor/generator
unit outside of the vacuum may enable a motor/generator unit to be movable among an
array of enclosed flywheels. Thus, the movable motor/generator unit may be coupled at
different times to different flywheels in the array. In this manner, by reducing the
number of required motor/generator units, cost of a multiple flywheel system may be
reduced (relative to a system that requires a dedicated motor/generator unit for each
flywheel). Furthermore, placement of the motor/generator unit outside of the vacuum
enclosure may simplify cooling of the motor/generator unit (e.g., by enabling
convection or conductive cooling).
[0063] In accordance with embodiments of the present invention, the flywheel is
coupled to the motor/generator unit via magnetic coupling. Magnetic coupling of the
flywheel to the motor/generator unit may be advantageous. Magnetic coupling
eliminates any requirement for vacuum sealing about the flywheel shaft that could
introduce mechanical friction on the shaft.
[0064] Magnetic coupling may include coupling a rotating first magnetic element of the
flywheel that rotates together with the flywheel to a second magnetic element that
rotates together with the rotor of the motor/generator unit. In this case, the rotor of the
motor/generator unit is located outside of the vacuum enclosure. Magnetic coupling in
this case may enable the motor to stop rotating when in an idle mode. For example, the
first and second magnetic elements may be moved apart to decouple the rotations. This
may reduce wear of the rotating components and bearings of the motor/generator unit,
as well as reducing losses caused by eddy currents and by hysteresis within the iron core
of the stator.
[0065] Magnetic coupling may include an electromagnetic interaction between a rotor
of the motor/generator unit that rotates together with the flywheel (e.g., at the end of a
shaft of the flywheel inside the vacuum enclosure) with a stator of the motor/generator
unit that is located outside of the vacuum enclosure.
[0066] The mass of the flywheel may be greater than 50 kg. More specifically, the mass
may be in the range of 100 kg to 200 kg.
[0067] Fig. 1 schematically illustrates a flywheel energy storage system with a flywheel
magnetically coupled to a rotor of a motor/generator unit, in accordance with an
embodiment of the present invention.
[0068] Flywheel energy storage system 90 includes a flywheel unit 100 and a
motor/generator unit 200.
[0069] Flywheel unit 100 includes flywheel 110. Flywheel 110 includes a mass that is
rotatable about flywheel shaft 140. Flywheel shaft 140 is supported by bearings 120.
For example, bearings 120 may include ball bearings made, e.g., of steel or of ceramic
materials, magnetic bearings, a combination of magnetic bearings and ball bearings, or
another type of bearing. Flywheel shaft 140 and flywheel 110 are rotatable at a high
angular velocity within vacuum enclosure 150.
[0070] Flywheel shaft 140 is provided with flywheel magnetic coupling plate 130 at an
end of flywheel shaft 140 that is proximal to motor/generator unit 200. Flywheel
magnetic coupling plate 130 is mounted in a cantilevered manner to flywheel shaft 140.
As used herein, cantilevered attachment or mounting of an object (e.g., a coupling plate
or rotor) to a shaft indicates that the object is supported only by the shaft, without any
support on the side of the object opposite the side from which the shaft extends.
Flywheel magnetic coupling plate 130 may include one or more magnets, or a
ferromagnetic material that is attracted to a magnet on motor/generator magnetic
coupling plate 210 of motor/generator unit 200. The magnets may include permanent
magnets or electromagnets. The magnets may be embedded or enclosed within flywheel
magnetic coupling plate 130 or may be mounted to a surface of flywheel magnetic
coupling plate 130. Alternatively or in addition to being mounted at an end of flywheel
shaft 140, a magnetic coupling plate may be mounted directly to flywheel 110 or to
another component of flywheel unit 100 that rotates with the same rotational velocity as
flywheel 110.
[0071] Flywheel magnetic coupling plate 130 may be operated to function as part of a
magnetic coupling to motor/generator unit 200. For example, flywheel magnetic
coupling plate 130 may be coupled to motor/generator magnetic coupling plate 210 of
motor/generator unit 200. Motor/generator magnetic coupling plate 210 may include
one or more magnets, or a ferromagnetic material that is attracted to a magnet on
flywheel magnetic coupling plate 130. Motor/generator magnetic coupling plate 210
may be configured to be rotatable with a rotor of motor/generator 220. Flywheel
magnetic coupling plate 130 may be positioned close to (e.g., within a few tenths of a
millimeter of) coupling cover 160 of vacuum enclosure 150. Similarly, motor/generator
magnetic coupling plate 210 may be positioned close to (e.g., within a few tenths of a
millimeter of) coupling cover 230 of motor/generator unit enclosure 250. High-torque
magnetic coupling devices are known in the art and are commercially available.
[0072] All or part of coupling covers 160 and 230 may be constructed of a magnetically
susceptible material that enables magnetic coupling between flywheel magnetic
coupling plate 130 and motor/generator magnetic coupling plate 210. For example,
coupling cover 160 or 230 may include aluminum, glass, plastic, or another
magnetically susceptible material.
[0073] Motor/generator magnetic coupling plate 210 may be connected to shaft 215 of
motor/generator unit 200. Shaft 215 may be connected (directly or indirectly, e.g., via a
transmission 240) to a rotor of motor/generator 220.
[0074] In accordance with some embodiments of the present invention, motor/generator
unit 200 may be enclosed within motor/generator enclosure 250. Motor/generator
enclosure 250 may be sealed so as to isolate motor/generator 220 from the ambient
environment. In this case, motor/generator enclosure 250 may be evacuated or filled
with a rarified gas (e.g., air).
[0075] In accordance with other embodiments of the present invention, motor/generator
220 and motor/generator magnetic coupling plate 210 (and transmission 240) may be
open to the ambient atmosphere. Exposing motor/generator 220 to the atmosphere may
enable use of standard components in motor/generator unit 200. Furthermore, Exposing
motor/generator 220 to the atmosphere may simplify cooling of motor/generator 220
(e.g., using standard air or liquid convection or conduction techniques) and may
eliminate any need for handling outgassing of components as could be required in a
sealed enclosure.
[0076] Flywheel energy storage system 90 may be provided with a mechanism to
enable gradual engagement or disengagement of flywheel magnetic coupling plate 130
with motor/generator magnetic coupling plate 210. For example, the gradual
engagement/disengagement mechanism, as represented by engagement/disengagement
movement 260, may enable movement of motor/generator magnetic coupling plate 210
toward or away from flywheel magnetic coupling plate 130. The
engagement/disengagement mechanism may align the axis of rotation of
motor/generator magnetic coupling plate 210 with the axis of rotation of flywheel
magnetic coupling plate 130 while a separation distance between the plates is too large
to enable magnetic coupling (e.g., during a store or wait phase).
Engagement/disengagement movement 260 may be applied to gradually shorten the
separation distance. As the distance is reduced slowly, motor/generator magnetic
coupling plate 210 gradually engages flywheel magnetic coupling plate 130 while one
of the plates is rotating at high speed. For example, a speed of rotation of a rotor of
motor/generator 220 may be controlled to match a rotational speed of flywheel magnetic
coupling plate 130 during, or prior to, engagement.
[0077] As another example, at the start of a charge phase, motor/generator magnetic
coupling plate 210 may be rotating faster than flywheel magnetic coupling plate 130.
On the other hand, at the start of a discharge phase, flywheel magnetic coupling plate
130 may be rotating faster than motor/generator magnetic coupling plate 210. Gradual
engagement causes the rotational velocity of the more slowly rotating plate to gradually
increase until the rotational velocities of the two plates are approximately equal.
[0078] Alternatively or in addition, the rotor of motor/generator 220 (and
motor/generator magnetic coupling plate 210) may be angularly accelerated (e.g., by
motor operation of motor/generator 220) to a rotation velocity that is close to that of
flywheel magnetic coupling plate 130. Engagement/disengagement movement 260 may
then be operated to reduce the separation distance between motor/generator magnetic
coupling plate 210 and flywheel magnetic coupling plate 130 until the plates engage one
another.
[0079] Placement of motor/generator unit 200 outside of flywheel vacuum enclosure
150 may enable use of different motor/generator units 200 with a single flywheel unit
100. For example, different motor/generator units may be distinguished from one
another by different gear ratios (e.g., a high ratio reduction gear for high speed
operation of flywheel 110, and a low reduction ratio or no gear at all for lower speed
operation of flywheel 110).
[0080] Placement of the motor/generator unit outside of flywheel vacuum enclosure 150
may enable use of separate motors or generators in place of a single device with both
motor and generator capabilities. Each separate motor or generator device may be used
during the appropriate phase of operation of a flywheel energy storage system. Such
separate motors and generators may be less expensive and simpler than a single device
with capabilities of both.
[0081] In accordance with some embodiments of the present invention, flywheel unit
100 may include magnetic thrust bearings. The magnetic thrust bearings are configured
to counter any axial forces that are applied to flywheel shaft 140 during operation of
flywheel energy storage system 90. For example, the magnetic thrust bearings may
operate on magnetic plate 170. Magnetic plate 170 is attached to an end of flywheel
shaft 140 that is distal to motor/generator unit 200. Magnetic plate 170 may be
permanently magnetized (e.g., is made of or includes soft iron or another ferromagnetic
material). Magnetic plate 170 may rotates at the same high rotational velocity as
flywheel 110. Therefore, the permanent magnets of magnetic plate 170 may be
encapsulated so as to prevent disintegration or rupture of magnetic plate 170 when
spinning at high speed. The encapsulation may be made from carbon fiber composite or
from nonmagnetic metal.
[0082] The magnetic thrust bearings may include permanent magnets 181 that are
located on rear plate 165 of flywheel vacuum enclosure 150. Permanent magnets 181
may include ring magnets that are arranged concentrically about the longitudinal axis of
flywheel shaft 140, or block or cylindrical magnets that are arranged in a circular
pattern.
[0083] Permanent magnets 181 are configured to repel the magnets on magnetic plate
170. Thus, when magnetic plate 170 is placed at the bottom end of a vertically oriented
flywheel shaft 140, permanent magnets 181 and magnetic plate 170 cooperate to at least
partially support the weight of flywheel 110 (and thus reduce stress on bearings 120).
Similarly, in an inverted system (e.g., where gravity may tend to increase the distance
between magnetic plate 170 and permanent magnets 181), permanent magnets 181 may
be configured to attract the magnets (or a ferromagnetic material) on magnetic plate
170.
[0084] In accordance with some embodiments of the present invention, adjustment
device 180 may be provided to enable fine adjustment of the equilibrium position of
flywheel shaft 140. For example, such fine adjustment may enable compensation for
changes in the effective weight of flywheel 110 with regard to bearings 120. For
example, when motor/generator magnetic coupling plate 210 engages flywheel
magnetic coupling plate 130, an axial force may be exerted on flywheel shaft 140.
Adjustment device 180 may include a magnet (e.g., permanent magnet or
electromagnet). For example, the magnet of adjustment device may be configured to
repel or attract magnetic plate 170 (depending on the configuration of flywheel energy
storage system 90). Adjustment movement 190 may be applied to adjustment device
180 to adjust the force that is exerted on magnetic plate 170 so as to counteract a force
that is exerted by motor/generator magnetic coupling plate 210 on flywheel magnetic
coupling plate 130. Adjustment movement 190 and engagement/disengagement
movement 260 may be controlled by single controller that is configured to coordinate
the movements with one another (and thus maintain a constant axial force on flywheel
shaft 140).
[0085] In accordance with some embodiments of the present invention, a flywheel
energy storage system may include a plurality of flywheel units, each enclosed in a
separate vacuum enclosure. Placement of the motor/generator unit outside of flywheel
vacuum enclosure 150 may enable moving a single motor/generator unit 200 from one
flywheel unit to another. Thus a single motor/generator unit 200 may be shared by
several of the flywheel units. The cost of such a system may thus be reduced.
[0086] For example, a flywheel unit with a steel flywheel rotor, e.g., with a mass of
about 400 kg and rotating at a speed of about 15,000 revolutions per minute (rpm) may
store about one kilowatt-hour (kWh) of energy. If greater energy- storage capacity is
required, then multiple flywheel units may be provided.
[0087] Fig. 2 schematically illustrates sharing of a single motor/generator unit by a
plurality of flywheel units of a flywheel energy storage system, in accordance with an
embodiment of the present invention.
[0088] Multiple flywheel energy storage system 300 includes a plurality of flywheel
units lOla-lOlc, shown as arranged in a single row (more than one row of flywheel
units may be included). Each row of flywheel units lOla-lOlc is provided with a single
motor/generator system 310, controlled by system controller 325. Controller 325 is
configured to operate in accordance with programmed instructions.
[0089] For example, controller 325 may be configured to wait for a command from a
main controller. When the command is received, controller 325 causes movement
control system 315 to connect the motor/generator unit 200 to flywheel unit 101a. An
indication may be received that motor/generator unit 200 is correctly placed and has
engaged successfully flywheel unit 101a. Controller 325 then operates motor/generator
unit 200 in a motor mode to accelerate the flywheel of flywheel unit 101a. Controller
325 monitors the speed of one or both of the motor/generator and the flywheel. When
the monitored speed is within a pre-determined threshold of the target speed, power to
motor/generator unit 200 is discontinued. Controller 325 the causes movement control
system 315 to disengage the motor/generator unit 200 from flywheel unit 101a.
[0090] Once movement control system 315 indicates successful disengagement, and if
more of flywheel units 101b or 101c are to be energized, e.g., flywheel unit 101c,
controller 325 may cause movement control system 315 to move motor/generator unit
200 to flywheel unit 101c (as indicated by motor/generator unit 200'). The preceding
process is then repeated until all flywheel units are spinning at a target angular velocity
(and multiple flywheel energy storage system 300 is storing quantity of energy equal to
its full capacity), or power is to be extracted from the system.
[0091] When the energy stored in multiple flywheel energy storage system 300 is
approximately equal to the system's capacity, motor/generator unit 200 may be
disengaged from all of flywheel units lOla-lOlc. Motor/generator unit 200 may be
positioned at a standby location. Alternatively or in addition, motor/generator unit 200
may remain near one of flywheel units lOla-lOlc but in an idle mode. Thus, if power is
to be provided from the flywheel, motor/generator unit 200 may be placed in a
generator mode, thus providing uninterrupted power supply (UPS) functionality.
[0092] When power is to be extracted from the multiple flywheel energy storage system
300 array, controller 325 may cause movement control system 315 to position a
motor/generator unit 200 to engage one of flywheel units lOla-lOlc (e.g., 101a).
Motor/generator unit 200 engages the flywheel unit (e.g., 101a) and operates in a
generator mode.
[0093] When engaging the flywheel unit (e.g., 101a), a rotor of motor/generator unit
200 accelerates to the speed of the flywheel of the flywheel unit (e.g., 101a). Electrical
power is then generated. The electrical power varies in frequency and amplitude as the
rotation of the flywheel decelerates. Thus, signal conditioning may be provided prior to
feeding the generated power into a power grid. For example, motor/generator unit 200
may be associated with power converter 320. Power converter 320 may be incorporated
into, mounted on, or located near motor/generator unit 200.
[0094] The output signal of power converter 320 may be at a level that enables the
signal to be connected to a common power bus. The common power bus may aggregate
electrical power that is generated by a plurality of motor/generator units of a multiple
flywheel energy storage system 300.
[0095] In accordance with another embodiment of the present invention, separate motor
and generator units may be provided. The motor or generator unit may be moved
separately throughout an array of flywheel units by a movement control system.
[0096] In accordance with another embodiment of the present invention, a
motor/generator unit may be provided with a variable transmission (e.g., a gear box).
The variable transmission may be controlled to reduce or increase a difference in
rotational velocity between the motor/generator unit and the flywheel of a flywheel unit.
This may enable use of high-efficiency or low-cost commercially available devices that
are not capable of operating at the high speed at which the flywheel is rotating.
[0097] In accordance with another embodiment of the present invention, flywheel units
are arranged in two or more arrays. This may allow for continuous provision of energy
during operation of a multiple flywheel energy storage system. For example, the
motor/generator unit of one array may be engaged and operating while the
motor/generator unit of the other array is disengages. Alternatively or in addition, the
system can be constructed that at least two motor/generator units are movable
throughout a single array of flywheels units. Flywheel modules may be arranged in a
linear, circular, curved, or other arrangement, or in the form of a two- or threedimensional
matrix.
[0098] In accordance with an embodiment of the present invention, a transmission (e.g.,
transmission 240 as shown in Fig. 1) may be utilized to enable the flywheel to rotate at a
higher angular velocity and exerting a low torque to engage a motor/generator unit
whose rotor is rotating at a lower angular velocity and with a large torque. For example,
a rotor of a motor/generator unit may rotate at 6,000 rpm while engaging a flywheel that
is rotating at 60,000 rpm or more. Such transmissions are known in the art and are
commercially available. The transmission may include mechanical components (e.g.,
gears) or may be based on magnetic interactions.
[0099] In accordance with an embodiment of the present invention, various
motor/generator units of a flywheel energy storage system may be provided with
different transmissions 240. Each transmission 240 may provide a different transmission
ratio. For example, a motor/generator unit having a transmission 240 that provides a
transmission ratio of 1:3 could be used when the angular velocity of the flywheel is less
than about 15,000 rpm. Another motor/generator unit may include a transmission 240
that provides a transmission ration of 1:6 may be used for flywheel rotations for up to
60,000 rpm or more. Other transmission ratios may be used.
[00100] In accordance with some embodiments of the present invention, a
magnetic coupling between a flywheel unit and a motor/generator unit may provide
function of a reduction gear or transmission.
[00101] Fig. 3 schematically illustrates a flywheel energy storage system with
magnetic reduction transmission, in accordance with an embodiment of the present
invention.
[00102] Magnetic pinion 410 is mounted on flywheel shaft 430 of flywheel unit
102. Thus, magnetic pinion 410 rotates together with flywheel 110. Cap 162 of vacuum
enclosure 152 is placed near (e.g., within a few tenths of a millimeter) magnetic pinion
410 and is made of a magnetically susceptible material. Thus, magnetic pinion 410 may
be magnetically coupled laterally to magnetic gear 420 of motor/generator unit 202.
Magnetic gear 420 may be connected, via shaft 215 to a rotor of motor/generator unit
202. Engagement/disengagement movement 260 may be operated to cause magnetic
gear 420 to magnetically engage magnetic pinion 410.
[00103] Magnetic gear 420 may typically have a larger radius than magnetic
pinion 410. Thus, when magnetically coupled with one another, flywheel 110 may
rotate faster than the rotor of motor/generator unit 202. Thus, the magnetic coupling
between magnetic gear 420 and magnetic pinion 410 may provide the function of a
reduction gear.
[00104] Other configurations that provide both magnetic coupling and reduction
transmission are possible. For example, the magnetic gear of the motor/generator unit
may have an annular configuration within which magnetic pinion 410 may rotate.
[00105] The maximum angular velocity at which a flywheel may safely spin may
depend on the material from which the flywheel is constructed (e.g., the tensile strength
of the material) and its structure. For example, a flywheel that is constructed from steel
may be limited to a maximum angular velocity of about 20,000 rpm. A flywheel rotor
may be constructed using composite materials (e.g., having a lower density than steel
but a much higher tensile strength). A flywheel constructed using a composite material
may have a maximum angular velocity of about 50,000 rpm or more.
[00106] Carbon fibers having high tensile strength have been used to form a
flywheel rotor assembly. Such flywheels have been known to fail by a laminate
disintegration mechanism due to the low sheer and tensile strength of an epoxy adhesive
matrix used to bond layers of the carbon fibers.
[00107] In accordance with an embodiment of the present invention, a flywheel
rotor may be constructed using carbon fibers in which carbon nano-tubes are added to
an adhesive epoxy resin. The nano-tubes may increase the interlaminate strength of the
composite material, and may thus enable the rotor to operate at high rotational velocities
without disintegration.
[00108] In accordance with an embodiment of the present invention, a flywheel
rotor may be constructed with a low-density outer shell with high tensile strength that
envelopes a high-density inner core having lower tensile strength. Such a flywheel may
be rapidly rotated without causing disintegration of the high-density material.
[00109] Fig. 4 schematically illustrates a flywheel with a shell construction, in
accordance with an embodiment of the present invention.
[00110] Flywheel 112 includes an outer shell 510 that envelopes an inner shell
520. Outer shell 510 and inner shell 520 surround inner core 530.
[00111] Outer shell 510 is constructed to have a high tensile strength. For
example, outer shell 510 may include a carbon fiber composite material having a high
tensile strength by inclusion of carbon nano-tubes in the epoxy resin used to bond the
material. Thus, outer shell 510 may withstand the very high tensile stress caused by high
speed rotation of flywheel 112.
[00112] Inner shell 520 may include a high density material. The high density
material in inner shell 520 may be included to increase the moment of inertial of
flywheel 512.
[00113] For example, high density material in inner shell 520 may include lead.
Lead may be cast using centrifugal casting into outer shell 510 which serves as a cast
mold. (As opposed to traditional centrifugal casting, the cast mold is not removed after
casting.) In this manner, lead, which exhibits a relatively low tensile strength but has a
high density, may be utilized to increase the mass (and thus the moment of inertia) of
inner shell 520 and of flywheel 112. The composite material in surrounding outer shell
510 prevents the lead in inner shell 520 from disintegrating during high-speed rotation
of flywheel 112. Other combinations of materials may be used.
[00114] Due to the high cost of carbon fiber, a flywheel may be designed to limit
the quantity of high tensile strength material that is incorporated into the flywheel while
providing a sufficiently large moment of inertia. The design may also reduce the risk of
catastrophic failure of the flywheel during high-speed rotation. In accordance with an
embodiment of the present invention, the flywheel may include radially projecting rods.
[00115] Fig. 5A schematically illustrates a brush-like flywheel that includes
radially projecting rods, in accordance with an embodiment of the present invention.
[00116] Brush-like flywheel 550 includes flywheel core 551 from whose
periphery projecting rods 554 extend radially. (Extending from the periphery is used
herein to exclude a configuration in which both ends of a single rod that is inserted into
a core extend outward on different sides of the core.) For example, flywheel core 551
may be made of steel or another dense material.
[00117] Projecting rods 554 may include fiberglass. Fiberglass, although
exhibiting high tensile strength (S-glass has a specific strength that is greater than that
of carbon fiber), is deformable (exhibiting large strain when subjected to a stretching
force, being characterized by a low specific modulus relative to carbon fiber and many
other materials). Therefore, dimensions of an enclosure that encloses brush-like
flywheel 550 may be sufficiently large to accommodate stretching of projecting rods
554.
[00118] For example, the rod may be subjected to a pultrusion process in which
the glass fiber bundles are immersed in a matrix material and pulled through a heated
die. The glass fibers may include a large number of micro-fibers, for example, each
having a diameter in a range of 7 mih to 20 mih. This pultrusion process creates a highdensity
high-strength rod that has an axial structure (all fiber bundles being aligned
essentially parallel to the rod axis). The matrix material is of a thermosetting nature.
Thus, the matrix material is cured during its passage through the heated die. The rigidity
of the resulting rods is substantially increased by the pultrusion process. Thus, the
pultrusion processing may enable use of relatively low cost materials, such as S-glass or
E-glass, that could not be used otherwise.
[00119] Interlayer shear strength of the rod construction may be increased by the
addition of carbon nano-tubes into the matrix material. An optimal type and quantity of
nano-tubes to be added may be determined by testing. For example, carbon nano-tubes
may make up 0.03% by weight of the matrix material. In some cases, measured rod
strength has been found to increase by 20%-30 % as a result of addition of carbon nanotubes.
[00120] For example, projecting rods 554 made out of fiberglass may each have a
diameter as large as about 100 mm, or a typical value of about 12 mm, or another
diameter.
[00121] Projecting rods 554 may be attached to the periphery of the flywheel core
in a staggered pattern or arrangement. A staggered arrangement may increase
uniformity of stress distribution and reduce maximal stress in flywheel core 551.
[00122] Fig. 5B schematically illustrates a staggered arrangement of rods of the
brush-like flywheel shown in Fig. 5A. In the staggered arrangement shown, projecting
rods 554 (viewed head on) that extend from the periphery of flywheel core 551 are
arranged in rows 553a-553c. Rows 553a-553c are staggered with respect to one another
such that, for example, row 553b is shown as laterally displaced with respect to row
553a and 553b. Other staggered arrangements of projecting rods 554 on flywheel core
551 are possible.
[00123] Brush-like flywheel 550 is operated in an evacuated vacuum enclosure
(e.g., flywheel vacuum enclosure 150 as shown in Fig. 1). Operation in an evacuated
enclosure eliminates or reduces aerodynamic drag on projecting rods 554. During
rotation of brush-like flywheel 550 about its axis, projecting rods 554 are subjected to
uniaxial loading due to the centrifugal force and their length increases. For example,
each projecting rod 554 may extend by 1-2% of its length (such that each projecting rod
554 is about half a meter long, the diameter of brush-like flywheel 550 may increase by
one or two centimeters) when rotating at full speed. Therefore, the vacuum enclosure is
designed with sufficient diameter to prevent the distal tips of projecting rods 554 from
coming into physical contact with the enclosure.
[00124] Projecting rods 554 are connected to the periphery of core 551 by rod
holders 552. For example, a proximal end of rod holder 552 may be attached (e.g.,
screwed or glued into, or otherwise secured) to core 551, e.g., into a tapped hole on core
551 (the tapped hole and rod holder 552 are typically designed to withstand centrifugal
forces on a projecting rod 554 held by a rod holder 552). A proximal end of each rod
holder 552 may be screwed into a tapped hole in core 551, or otherwise secured to core
551, after a projecting rod 554 is inserted into, and held by, a distal end of rod holder
552. Alternatively or in addition, a rod holding structure may be incorporated into core
551.
[00125] Fig. 6 schematically illustrates operation of a rod holder of the brush-like
flywheel shown in Fig. 5A.
[00126] In accordance with some embodiments of the present invention, a
projecting rod 554 may be attached to a rod holder 552 by press fit. In this case, a
proximal end (or all of) projecting rod 554: is cooled (e.g., by liquid nitrogen or another
coolant) so as to reduce its diameter. The cooled end is inserted into an accurately
machined cavity of rod holder 552. When the end of projecting rod 554 warms, the
proximal end of projecting rod 554 expands and fills the cavity. Projecting rod 554 is
thereafter held in base by friction with the walls of the cavity.
[00127] Alternatively or in addition, a cavity 558 of rod holder 552 includes
wedge-shaped spaces, as shown in Fig 6. The spaces may be filled with glue, such as a
high shear-stress glue, or a matrix material as a proximal end of projecting rod 554 is
inserted into cavity 558. When the material hardens, the material and cavity 558 serve
as a self-locking wedge mechanism that prevents the projecting rod 554 from being
pulled out of cavity 558 by centrifugal forces.
[00128] Alternatively or in addition, a projecting rod 554 may be held to a cavity
in a rod holder 552 or in core 551 by a collapsible ferule.
[00129] Fig. 7 schematically illustrates operation of a collapsible ferule to hold a
projecting rod.
[00130] Collapsible ferule 560 includes a machined (or otherwise formed) metal
part with a conical external shape. The internal shape of collapsible ferule 560 is shaped
to a profile that is designed to create a desired load profile on projecting rod 554.
Initially, when projecting rod 554 is inserted into ferule 560, the internal diameter of
ferule 560 is greater than the diameter of projecting rod 554. Ferule 560, with projecting
rod 554 inserted, is pushed into conical hole 562. The cone angle of conical hole 562
matches the cone angle of the external surface of ferule 560. When ferule 560 with
inserted projecting rod 554 is pushed into conical hole 562, ferule 560 is pressed and
collapses onto proximal end 554a of projecting rod 554. Continued pushing into conical
hole 562 continues to press ferule 560 onto projecting rod 554 until a desired pressure is
attained, forming constricted neck 554b on projecting rod 554. For example, if the outer
diameter of a fiberglass projecting rod 554 is about 12 mm, then the diameter of
constricted neck 554b may be reduced by about 0.1 mm.
[00131] Use of ferule 560 may apply circularly uniform pressure on the outer
surface of projecting rod 554, thus avoiding mechanical failure of outer fibers of
projecting rod 554 which could result in reduced pull strength. The ferule inner profile,
relating the inner diameter of ferule 560 to its insertion distance, and the desired
pressure may be calculated by taking into account the effects of Poisson's effect
contraction on the rod diameter resulting from exertion of the high pull force. The
desired pressure profile on projecting rod 554 may be calculated to minimize the
combined (von Mises) stress on projecting rod 554. For example, at constricted neck
554b of projecting rod 554, pull forces may be very high. Thus, at constricted neck
554b, pressure forces should be sufficiently low to avoid high von Mises stress. The
pressure profile should increase gradually in accordance with the decrease in the pull
force due to the friction forces on the outer surface of projecting rod 554.
[00132] A brush-like flywheel construction may be advantageous. For example,
such a construction is unlikely to catastrophically fail. If one of projecting rods 554
were to fail or disintegrate, the centrifugal forces would throw the resulting debris to be
thrown outward toward the walls of a vacuum enclosure that encloses the rotor. Since
brush-like flywheel 550 typically includes hundreds of projecting rods 554, the kinetic
energy of a single failed rod is relatively low. Therefore, a requirement for reinforced
housing or for some other safety features may be reduced or eliminated. In addition,
safety factors that are used in determining operating parameters may be relaxed
somewhat relative to other types of flywheels. A flywheel system incorporating brush
like flywheel 550 may be equipped with imbalance detectors that could sense any
imbalance caused by a detached projecting rod 554. Upon detection of such a failure,
braking may be applied to brush-like flywheel 550, or brush-like flywheel 550 may
otherwise be brought to a gradual halt. Furthermore, since brush-like flywheel 550 is
constructed primarily out of glass and metal, components may be recyclable and no use
of any hazardous material is required.
[00133] In accordance with some embodiments of the present invention, a weight
may be added to a distal end of each projecting rod of a brush-like flywheel. Addition of
such weights increases the moment of inertia of the flywheel.
[00134] Fig. 8 schematically illustrates a variant of the brush-like flywheel shown
in Fig. 5A, in which the radially projecting rods are weighted.
[00135] Weighted brush-like flywheel 570 includes projecting rods 554 with end
weights 572 added to the distal ends of the rods. Each end weight 572 may be connected
to a projecting rod 554 using one or more of the attachment techniques discussed above
for attaching projecting rods 554 to core 551 or to rod holders 552, or using another
attachment technique. End weights 572 may be constructed out of a dense material, e.g.,
steel or another material.
[00136] Attachment of end weights 572 to the distal ends of projecting rods 554
may be advantageous. For example, the increase in moment of inertia may increase the
quantity of energy that may be stored for a given angular velocity without increasing the
length of each projecting rod 554. Alternatively, attachment of end weights 572 may
enable shortening each projecting rod 554, thus decreasing the lateral dimensions of
weighted brush-like flywheel 570 relative to a brush-like flywheel without end weights.
Alternatively, weighted brush-like flywheel 570 may be spun at a slower speed than a
brush-like flywheel without end weights to store a similar quantity of energy.
[00137] In accordance with an embodiment of the present invention, a brush-like
flywheel may include projecting glass fibers that act as projecting rods. The flywheel
may be constructed out of stacked assemblies, each herein referred to as a cassette.
[00138] Fig. 9 schematically illustrates structure of a cassette of a glass fiber
brush-like flywheel, in accordance with an embodiment of the present invention.
[00139] A plurality of flywheel cassettes 580 may be stacked to form a single
brush-like flywheel with projecting rods in the form of projecting fibers. For example,
the fibers may include glass. When stacked, the flywheel cassettes 580 are all centered
about and mounted to a common central shaft 588.
[00140] Flywheel cassette 580 includes two plates 582 (only one plate is shown)
sandwiching columns 584. For example, the plates 582 may be constructed out of a
dense material, such as steel or another material.
[00141] Columns 584 extend from one plate 582 to the other. Columns 584 are
arranged in a circular pattern that is concentric with plate 582 and with the axis of the
flywheel. Columns 584 may be attached to plates 582 using screws, or using another
suitable attachment mechanism or technique.
[00142] Glass fibers making up fiber bundles 586 are each partially (or fully)
wrapped about each column 584. For example, fiber bundles 586 may each include a
plurality of extremely thin fiber glass fibers, e.g., with a typical diameter of 7 microns to
20 microns. The fibers extend symmetrically and by an equally amount (e.g., by a
typical distance of about 15 cm) from either side of column 584.
[00143] Fibers of a fiber bundle 586 may be glued together at contact region 586a
where the fiber bundle 586 bends around a column 584.
[00144] When the flywheel rotates, centrifugal forces cause fibers of each fiber
bundle 586 to extend radially outward from the axis of the flywheel. The centrifugal
forces act approximately equally on both extending ends of fiber bundle 586. Thus, the
effect of the centrifugal forces essentially tends to hold fiber bundles 586 in place. A
fiber bundle 586 thus wrapped around a column 584 may be advantageous of an
arrangement where glue or another holding method is depended upon to withstand or
overcome the centrifugal forces. Use of techniques whereby projections, such as fibers
or rods, pass through the core, although balancing the centrifugal forces, are limited as
to the attainable density of the projections.
[00145] Since the tensile strength of very thin fiber glass fibers is typically very
high (e.g., much higher that the tensile strength of ordinary glass rods), a flywheel
including a plurality of cassettes 580 may be rotated at very high speeds without
reaching the maximal tensile limit of the fibers.
[00146] Use of projecting fiber bundles 586 may be advantageous. For example,
the likelihood of catastrophic failure of the flywheel is reduced. Since the flywheel may
include millions of individual fibers, the kinetic energy of each fiber is relatively very
low.
[00147] Fig. 10 illustrates a technique to form a fiber bundle for the cassette
shown in Fig. 9. A standard filament winding machine (not shown) may be applied to
wind fibers 590 around two columns 584. Columns 584 may be held in place by a
suitable fixture or holder (not shown). For example, the distance between the columns
584 may be typically equal to approximately 30 cm. Fibers 590 may be made of
fiberglass, steel music wire, or any other high tensile-strength fiber. After fibers 590 are
wound around the two columns 584, fibers 590 are glued to form bundles in the region
where fibers 590 are wrapped around columns 584. After the glue cures, fibers 590 are
cut, typically along midline 592 between columns 584. Thus, two U-shaped bundles are
formed (about each of columns 584).
[00148] Alternatively or in addition, fibers 590 may be soaked in a matrix
material prior to winding. After winding, fibers 590 may be cut and formed into bundles
(e.g., each bundle having a typical diameter of about 12 cm). The bundles may then be
cured (e.g., thermally or at room temperature, depending on the matrix material).
[00149] In accordance with embodiments of the present invention, a rotor of the
motor/generator unit is mechanically coupled to a shaft of the flywheel unit within a
vacuum enclosure. The stator of the motor/generator unit is located outside the vacuum
enclosure. Such an arrangement is herein referred to as an open-frame motor/generator
unit.
[00150] Fig. 11 schematically illustrates an open-frame motor/generator unit, in
accordance with an embodiment of the present invention.
[00151] Open-frame flywheel energy storage system 600 includes flywheel 601
enclosed within vacuum enclosure 150. Flywheel 601 may include a brush-like flywheel
as shown, or another configuration of a flywheel.
[00152] Rotor 604 of open-frame motor/generator unit 602 is mechanically
attached, in a cantilevered manner (with no bearing or other support of rotor 604 other
than shaft 142), to shaft 142 of flywheel 601. Rotor 604 is housed within cap 162 of
vacuum enclosure 150. (Cap 162 may be in the form of a curved dome.) Stator 606 of
open-frame motor/generator unit 602 is located outside of cap 160. Cap 162 is made of
a magnetically susceptible material (e.g., a glass composite material such as Kevlar® or
fiberglass, or another material) so as to enable magnet coupling between rotor 604 and
stator 606. For example, rotor 604 and stator 606 may be separated by a typical distance
of 3 mm, or another distance.
[00153] Stator 606 may be connected to an inverter of a high- voltage (HV) direct
current (DC) bus, or other suitable circuitry.
[00154] Use of open-frame motor/generator unit 602 in a flywheel energy storage
system may be advantageous. For example, enclosing rotor 604 within vacuum
enclosure 150 may minimize atmospheric drag on rotor 604. Bearings 120 that support
flywheel 601 also support rotor 604, thus avoiding the cost of additional bearings. Stator
606 may be removed from the remainder of open-frame flywheel energy storage system
600 when no charging or discharging is taking place. Such removal may reduce eddy
current losses caused by rotor 604 rotating within the stator 606. Stator 606, being
located outside of vacuum enclosure 150, may be cooled by natural convection or by
forced convection (e.g., by a blower, fan, or pump). No electrical connections are
required between components within vacuum enclosure 150 and circuitry outside of
vacuum enclosure 150. A single stator unit may be shared with a plurality of flywheel
units by an automatic movement system.
[00155] In accordance with some embodiments of the present invention, a
plurality of individual flywheel units may be coupled to one another to form a flywheel
cluster. Each flywheel cluster may be coupled to a single motor/generator unit. Each
individual flywheel unit may be designed for a particular maximum rotation velocity.
Coupling the flywheel units together may enable increasing the quantity of energy that
is stored, without requiring modification of the design (e.g., flywheel or bearings) of the
flywheel units. Furthermore, the energy may be stored or extracted without the cost of
additional motor/generator units.
[00156] Fig. 12 schematically illustrates a flywheel cluster, in accordance with an
embodiment of the present invention.
[00157] Flywheel cluster 1000 includes a stack of flywheel units 90. Each
flywheel unit 90 includes a flywheel 1002 (which may be a brush-like flywheel, as
shown, or another type of flywheel) that is individually supported and secured by a set
of bearings 120 within its vacuum enclosure 150. Each flywheel unit 90 is permanently
coupled to an adjacent flywheel unit 90 of flywheel cluster 1000 by magnetic coupling
1004. Thus, all flywheel units 90 of flywheel cluster 1000 rotate in tandem at a single
rotational velocity.
[00158] A flywheel unit 90a at one end of flywheel cluster 1000 is coupled to
motor/generator unit 204. For example, motor/generator unit 204 may include an openframe
motor/generator unit as shown. As another example, flywheel 1002 of flywheel
unit 90a may be magnetically coupled to motor/generator unit 204. As another example,
motor/generator unit 200 may be mechanically coupled to flywheel unit 90a and
enclosed within vacuum enclosure 150 of flywheel unit 90a.
[00159] In accordance with embodiments of the present invention, a flywheel
unit may include an active magnetic bearing. For example, use of active magnetic
bearings may enable long-life and reliable support of the flywheel in the vacuum
environment, where use of air bearings is precluded. Use of active magnetic bearings
typically includes use of another set of conventional bearings ("landing bearings")
during transportation and initial activation of the flywheel unit. The active magnetic
bearings need to be powered at all times. Any interruption of supplied power would
necessitate use of the landing bearings.
[00160] A typical active magnetic bearing system includes a shaft position sensor
(e.g., based on eddy current or capacitive sensors) to monitor the position of the rotating
shaft that is being stabilized. A rotor spins with the shaft (may be mounted to the shaft
or may be identical with the shaft). Inductive actuators are used to attract the rotor or
shaft being stabilized. Control and drive circuitry controls operation of the inductors in
accordance with the sensed position of the shaft.
[00161] In accordance with embodiments of the present invention, a flywheel or
other rotating load may be supported by mechanical bearings. A stabilization system
may be provided to balance the rotating load. For example, the stabilization system may
include an active magnetic balancing system.
[00162] Fig. 13A schematically illustrates an active magnetic balancing system
for a flywheel of a flywheel energy storage system, in accordance with an embodiment
of the present invention. Fig. 13B shows a top view of the active magnetic balancing
system shown in Fig. 13A.
[00163] Flywheel active magnetic balancing system 900 is configured to stabilize
shaft 940 of flywheel 920. Although flywheel active magnetic balancing system 900 is
described herein as applied to a flywheel system for energy storage, flywheel active
magnetic balancing system 900 may be applied to stabilize any rotating load whose
shaft is supported by mechanical bearings.
[00164] Shaft 940 is continuously held in place by ball bearing assembly 910. For
example, ball bearing assembly 910 may include a metal ball bearing, a ceramic ball
bearing, a hybrid ball bearing assembly, or another mechanical bearing. Flywheel 920
may be balanced to a high degree (e.g., as required by ISO 1940 class Gl). However,
radial forces on ball bearing assembly 910 may limit the bearing's lifetime and may
create severe vibrations during operation. The radial forces may contribute to internal
friction and may cause overheating of ball bearing assembly 910, reducing operational
efficiency of a flywheel unit that includes flywheel 920.
[00165] Flywheel active magnetic balancing system 900 includes one or more
sensors 950. Sensors 950 may include vibration or force sensors. Sensors 950 are
mounted on the stationary side of ball bearing assembly 910, or in close proximity to it.
Each sensor 950 may give a fast and accurate reading of imbalance forces operating on
the ball bearing assembly 910 in a particular direction. Sensors 950 may be located
sufficiently far from inductive actuators 960 to prevent inductive actuators 960 from
influencing readings by sensors 950.
[00166] A magnetic stabilization assembly that includes a plurality of (e.g., three)
electromagnets is controllable to create a resultant magnetic field that reduces
imbalance forces acting on shaft 940. Each electromagnet is included in an inductive
actuator 960. Inductive actuators 960 are mounted on a stationary (non-rotating)
structure. Inductive actuators 960 may be operated to attract rotor ring 970. Rotor ring
970 is mounted on shaft 940, or may be incorporated into or may be identical with shaft
940. For example, rotor ring 940 may be made from stacked layers of electrical steel
(e.g., such as is used in transformer cores). The use of thin silicone steel (e.g., of 0.2
mm thickness) can contribute to reduced eddy current losses.
[00167] Control unit 980 may include a processor or other control circuitry. For
example, control unit 980 may include an application-specific integrated circuit (ASIC).
Control unit 980 may be configured to receive a signal that is indicative of vibration or
force from sensors 950. An algorithm may be applied to the signals to calculate how
inductive actuators 960 are to be driven in order to minimize or reduce the sensed forces
that act on ball bearing assembly 910.
[00168] Use of flywheel active magnetic balancing system 900 may be
advantageous. For example, stress and losses by ball bearing assembly 910 may be
reduced significantly, thus increasing reliability, service life, and time between
maintenance for ball bearing assembly 910. Operation of flywheel 920 is not solely
dependent on operation of the magnetic bearings, since temporary failure of the
magnetic bearings would enable continued operation while only temporarily increase
the load on ball bearing assembly 910. Geometrical stability of the shaft (which may be
a major concern with conventional magnetic bearings) is ensured by ball bearing
assembly 910. Changes in flywheel balance, either dynamically (as the rotation speed
changes) or over time (due to creep), may be compensated continuously. Control unit
980 may be configured to provide information about any sensed imbalance or creation
of vibrations. Such provided information may be utilized to avoid catastrophic failure
events.
[00169] Control of flywheel active magnetic balancing system 900 may differ
from control of a magnetic bearing system. In a typical magnetic bearing system, shaft
position is measured and corrected. However, in flywheel active magnetic balancing
system 900, shaft 940 is fixed in space by ball bearing assembly 910 and the magnetic
field generated by inductive actuators 960 exerts a force on rotor ring 970 (and on shaft
940). Ball bearing assembly 910 provides sufficient stiffness such that exerted forces do
not cause significant movement or deflection of shaft 940. Thus, sensors 950, which
include vibration or acceleration sensors, are used in flywheel active magnetic balancing
system 900. Sensor 950 measures vibration caused by imbalance of shaft 940. This
vibration may be described by a sinusoidal functional shape, where the phase of the
sinusoidal function is determined by the position of sensor 950 relative to a vector that
describes the rotational imbalance. When flywheel active magnetic balancing system
900 is not functional, imbalance forces are countered by ball bearing assembly 910.
When flywheel active magnetic balancing system 900 is functional, the magnetic field
exerts forces to counter the imbalance (and the radial forces on ball bearing assembly
910 are minimized). It may be noted that the vibrations that are measured by sensor 950
are in principal the same whether the radial load is handled by ball bearing assembly
910 or by flywheel active magnetic balancing system 900.
[00170] Control of flywheel active magnetic balancing system 900 may include
controlling inductive actuators 960 to create a rotating force vector with the same
rotation speed as that of shaft 940 (synchronous force vector). For example, the driving
frequency of the coil excitation (in case of three inductive actuators 960 placed at 120
degree intervals) may be half of the rotational speed of shaft 940. (For example, the
force exerted by each of inductive actuators 960 may be proportional to the square of
the current in that inductive actuator 960. Thus, when the current is described by a sine
wave, the frequency of the exerted force is double that of the current.) The phase angle
is varied over 360 degrees while the vibrations are measured by sensor 950. A
maximum sensed vibration is indicative that the magnetic force vector is in phase with
the mechanical imbalance vector. The phase of the rotating magnetic force vector is
then changed by 180 degrees (opposite the mechanical imbalance) while its amplitude is
varied from zero to a preset maximal value. When the amplitude is zero, all the
imbalance is handled by ball bearing assembly 910 alone. As the amplitude of the
magnetic force vector is increased, the force on ball bearing assembly 910 is reduced.
When the force on ball bearing assembly 910 reverses direction, the optimal amplitude
is indicated. This procedure may be repeated from time to time to enable compensation
for changes in flywheel balance.
[00171] Alternatively or in addition, the phase to be applied may be calculated
using measurements from two sensors 950 sensing vibrations mounted with a 90 degree
separation about the flywheel axis. A balancing calculation algorithm that is executed
by controller 980 may be configured to determine desired phase angle for the balancing
signal.
[00172] Fig. 13C schematically illustrates control of the active magnetic
balancing system shown in Fig. 13A using a three-phase H-bridge.
[00173] Inductive actuators 960 are driven by a three-phase H-bridge 981 (similar
H-bridges used in motor controllers). Three-phase H-bridge 981 is driven by controller
982. Controller 982 is configured to provide standard three-phase control of inductive
actuators 960. Controller 982 is additionally configured to modify the phase of the three
sine wave signals that drive inductive actuators 960 so as to modify the phase of the
rotating resultant force vector. Thus, the phase may be controlled to minimize vibrations
or flywheel rotor imbalance as detected by sensor 950.
[00174] Controller 982 is furthermore configured to compensate for geometric
variation of mechanical components due to various tolerances in the production and
assembly of the inductive actuators 960 and rotor ring 970. Such manufacturing
tolerances (e.g., with typical magnitudes of about 0.1 mm) could cause variations in the
resultant force (which is dependent on the reciprocal of the square of the distance
between inductive actuator 960 and rotor ring 970). Thus, even when high-end
machining and wire cutting techniques are used, tolerances of ±0.1 mm to ±0.01mm
could be present. Since the distance between rotor ring 970 and magnetic actuators 960
may be about 0.3 mm, such tolerances could lead to considerable variance in the force
and could require compensation, even if the actuators themselves and the driving
current were to be perfectly accurate.
[00175] Lookup table (LUT) 983 may include corrections to the drive signals that
are based on geometrical reference data 984. Geometrical reference data 984 may be
measured during production may be and can be utilized by controller 982, together with
LUT 983 and with rotation data provided by encoder 952, to facilitate calculation of the
correction. Thus, rotating force vector may be correctly generated constantly to
compensate for variation in dimensions due to manufacturing and assembly tolerances.
[00176] Fig. 13D schematically illustrates control of the active magnetic
balancing system shown in Fig. 13A using power amplifiers.
[00177] Inductive actuators 960 are fed by power amplifiers (POAMP) 986. Each
power amplifier 986 may be individually controlled. Thus, each phase may be
individually controlled. LUT 983 may be utilized in controlling power amplifiers 986 so
as to compensate for geometrical tolerances provided by the geometrical reference data
984. More than three inductive actuators may be individually controlled, enabling
additional flexibility in correcting individual drive signals and increasing the accuracy
of the rotating force vector.
[00178] In accordance with an embodiment of the present invention, a flywheel
rotor may include an automatic balancing system. Wireless control may be utilized to
change a configuration of an eccentric structure on the flywheel shaft in order to adjust
the balancing of the flywheel (without use of slip ring or other contact-based
communication between a controller and balancing mechanism).
[00179] Fig. 14A is a schematic illustration of a flywheel balancing assembly for
a flywheel energy storage system, in accordance with an embodiment of the present
invention. Fig. 14B is a side view of the flywheel balancing assembly shown in Fig.
14A.
[00180] Flywheel balancing assembly 1200 may be mounted on a flywheel rotor.
Flywheel balancing assembly 1200 may be controlled by controller 1240. Controller
1240 may be stationary (not rotating). Controller 1240 may control operation of
flywheel balancing assembly 1200 in accordance with sensed signals from imbalance
sensors.
[00181] Flywheel balancing assembly 1200 includes two worm gears 1210. Each
worm gear 1210 includes eccentric borehole 1215 that is aligned parallel to the axis of
worm gear 1210. Eccentric borehole 1215 causes worm gear 1210 to have an eccentric
mass distribution that may be adjusted to balance the flywheel by rotation about that
longitudinal axis of worm gear 1210.
[00182] Each worm gear 1210 may be rotated by rotation of a worm screw 1220
by operation of motor assembly 1230. Worm screw 1220 and motor assembly 1230 also
rotate together with the flywheel rotor. (Placement of two worm screws 220 and two
motor assemblies 1230 in a symmetric manner about the flywheel rotor may avoid
introducing imbalance in the flywheel.) Each motor assembly 1230 may be controlled
by a signal generated by controller 1240 to rotate clockwise or counterclockwise by a
controlled rotation angle. When no command signal is generated by controller 1240,
worm gears 1210 self-lock and there is no movement of worm gears 1210.
[00183] A control signal may be transmitted wirelessly from a stationary
controller 1240 to a motor assembly 1230 that rotates together with the flywheel. For
example, an optical signal may be generated by controller 1240, e.g., by a light emitting
diode (LED), diode laser, or other device. The optical signal may be detected by a
photo-sensor (e.g., photovoltaic cell) that is mounted on the rotating components of
flywheel balancing assembly 1200. Transmitted commands may be distinguished from
one another by transmitter location (e.g., angular or radial encoding), by wavelength of
the optical signal, or by another characteristic of the optical signal.
[00184] An electromagnetic signal may be generated by controller 1240. A motor
assembly 1230 may be provided with a inductor to enable inductive powering of motor
assembly 1230.
[00185] In accordance with embodiments of the present invention, a flywheel
energy storage system may include a plurality of flywheel units arranged in one or more
arrays (e.g., as shown in Fig. 2). The flywheel energy storage system may be controlled
to determine a rate at which a motor/generator unit is storing energy in a flywheel unit
or is extracting energy from the flywheel unit. The controlling may be such that a rate of
energy storage or extraction is kept a substantially constant level, even when a
motor/generator unit is disconnected from all flywheel assemblies (e.g., is being moved
from one flywheel assembly to another). The controlling may be coordinated with a
smart grid that is configured to determine a level of power that is to be provided by, or
to be stored in, the flywheel energy storage system.
[00186] Fig. 15 schematically illustrates a flywheel energy storage system that
includes an array of flywheel units, in accordance with an embodiment of the present
invention.
[00187] Flywheel array energy storage system 303 includes a plurality of
flywheel units 103 arranged in a plurality of flywheel groups 313 (e.g., four flywheel
groups 313). Each flywheel group 313 is provided with a motor/generator unit 200 and
an associated power controller 326. The motor/generator unit 200 and power controller
326 of a flywheel group 310 is configured to couple with any flywheel unit 103 of that
flywheel group 310. Power controller 326 is configured to control its associated
motor/generator unit 200 store or provide electrical power at a determined rate. For
example, an electrical power storage or supply rate may range from 0.5 kW to 15 kW
(or another range). Motor/generator units 200 are connected to central inverter 330.
Central inverter 330 is connected to alternating current (AC) mains power grid 390.
During typical operation, each motor/generator unit 200 may operate at a typical power
level. For example, at a typical power level of about 10 kW per motor/generator unit
200, a flywheel array energy storage system 303 with four flywheel groups 313 may
operate at a total power level of about 40 kW.
[00188] The power level of each motor/generator unit 200 and of central inverter
320 is controlled by local controller 340. Local controller 340 is configured to operation
of the entire flywheel array energy storage system 303.
[00189] Local controller 340 may be in communication with one or more remote
controllers 360. Communication with remote controller 360 may take place via smart
grid network 350, or via another network or communications channel.
[00190] Alternatively or in addition, local controller 340 may be in
communication with site controller 370. Site controller 370 may be configured to
manage energy flow at a renewable energy generation site or another energy generation
and storage facility. (Site controller 370 may manage multiple local controllers 340. At
a small site, local controller 340 may function as a site controller.)
[00191] Movement control system 316 may be operated to move a
motor/generator unit 200 from one flywheel unit 103 to another. During the movement,
the power to or from the flywheel group 313 with which that motor/generator unit 200
is associated may be interrupted. Local controller 340 may be configured to reduce or
minimize effects of the interruption.
[00192] Local controller 340 may be configured to operate so as to reduce or
eliminate effects of the interruption in one flywheel group 313. For example, prior to
and during movement, the power level of other flywheel groups 313 of flywheel array
energy storage system 303 may be gradually increased while the power level of that
flywheel group 313 is gradually decreased to zero.
[00193] For example, in a flywheel array energy storage system 303 with four
flywheel groups 313 may have a nominal power level of 10 kW per flywheel group 313.
Prior to movement, power of the flywheel group 313 in which the movement is to take
place may be reduced to 5 kW. Concurrently, the power levels of two other flywheel
groups 313 are increased to a compensating power level of 12.5 kW. Next, the power
level of the flywheel group 313 in which movement takes place is reduced to zero,
while the power of the other three flywheel groups 313 is increased to 13 kW, 14 kW
and 14 kW, respectively. The modified power levels are maintained until the
motor/generator unit 200 of that flywheel group 313 is coupled to another (or the same)
flywheel unit 103. At this point, the power level may be gradually changed until the
power levels of all flywheel groups 313 are restored to their original nominal levels
(e.g., 10 kW).
[00194] Local controller 340 may be configured to ensure that only one
motor/generator unit 200 of only one flywheel group 313 of flywheel array energy
storage system 303 is being moved (or being prepared to be moved) a any given time.
For example, local controller 340 may be configured (e.g., by programmed instructions)
to enable limited flexibility with regard to the overall energy storage limits of flywheel
array energy storage system 303. For example, a flexibility margin (e.g., of about 1
kWh) would enable one motor/generator unit 200 to remain connected to a particular
flywheel unit 103 until another movement (and coupling) of another motor/generator
unit 200 is complete. Any variation in power level of flywheel array energy storage
system 303 may be communicated (e.g., via smart grid network 350 or otherwise) to
remote controller 360 or to site controller 370.
[00195] Local controller 340 may be configured to change power levels of
flywheel groups 313 so as to enable disengaging a motor/generator unit 200 from a
flywheel unit 103 so as to reduce the operating hours of that motor/generator unit 200.
[00196] Typically, a flywheel array energy storage system is configured to
operate at high DC voltage (e.g., as high as 1000 V, or more typically at about 400 V).
For example, a typical solar cell array may include a DC bus that is created by
aggregating the output of several few solar panels in order to create a high-voltage and
high-current bus. The bus is connected to an inverter that converts the output to
synchronized alternating current (AC) voltage that is fed to an AC mains power grid. In
some cases, an active power management unit may be used in order to optimize power
transfer from each solar array to the central bus.
[00197] Typically, and as described above, a flywheel energy storage system may
interface to the AC mains power grid. The flywheel energy storage system may store
energy from the grid, or supply stored energy to the grid. The multiple energy
conversions involved may affect the efficiency of the process.
[00198] In accordance with an embodiment of the present invention, a flywheel
energy storage system may be directly connected to a renewable energy direct current
(DC) bus. In this manner, the number of power conversions may be reduced.
[00199] Fig. 16 schematically illustrates a flywheel energy storage system that is
directly connected to a renewable energy DC bus, in accordance with an embodiment of
the present invention.
[00200] Renewable energy flywheel energy storage system 400 is designed to
store energy from renewable energy generating devices. Renewable energy generating
devices, such as wind turbine 402 or solar cell array 404, are connected to high voltage
DC bus 435. High voltage DC bus 435 may operate at a variety of possible DC
voltages. For example, the DC voltage of high voltage DC bus 435 may range from
400V to 1000 V. The DC voltage may be fed into main inverter 450. Main inverter 450
is configured to convert DC voltage to an AC one- or three-phase voltage that is
synchronized to phase and frequency of AC mains power grid 390.
[00201] In some cases, wind turbine controller 411 and solar cell controller 421
may be in communication with system controller 460. Wind turbine controller 411 and
solar cell controller 421 may be configured to monitor operation wind turbine 402 and
solar cell array 404, respectively. For example, wind turbine controller 411 and solar
cell controller 421 may report to system controller 460 the current supply level at which
energy is supplied by wind turbine 402 or solar cell array 404. Wind turbine controller
411 and solar cell controller 421 may report any detected malfunction of wind turbine
402 or solar cell array 404 that could reduce power that is supplied or that is forecasted
to be supplied.
[00202] According to an embodiment of the present invention, high voltage DC
bus 435 is connected to energy routing unit 440. Energy routing unit 440 is configured
to function as a managed energy router. Energy routing unit 440 may direct power from
renewable sources to flywheel storage DC bus 431. The directed power is fed into
inverter units 432 which drive the DC motors of motor/generator units 200.
Alternatively, energy routing unit 440 may direct energy supplied by inverter units 432
via flywheel storage DC bus 431 to be fed to main inverter 450. Alternatively, energy
routing unit 440 may direct power from high voltage DC bus 435 to main inverter 450.
[00203] System controller 460 may be configured to control energy routing unit
440 in accordance with a programmed decision system. Alternatively or in addition,
system controller 460 may be configured to issue commands based on information
received from other entities or remote controllers, e.g., via the smart grid network 350
or via other viable means of communications.
[00204] When the voltage of flywheel storage DC bus 431 is essentially the same
as that of high voltage DC bus 435, then flywheel storage DC bus 431 may be
connected directly to high voltage DC bus 435 without energy routing unit 440. Flow of
energy to and from a flywheel unit may be managed by inverter 432 and flow of energy
into main inverter 450 may be managed by an inverter controller. In this manner, the
simplification of the system may avoid the cost and losses due to additional
components, and system reliability and resilience may be improved.
[00205] Use of renewable energy flywheel energy storage system 400 may be
advantageous. Overall efficiency may be improved by avoiding multiple power
conversions (since DC current is converted to AC current only when being fed into AC
mains power grid 390). Energy routing unit 440 may increase flexibility of the system
by enabling increasing power to main inverter 450 when there is a temporary decrease
in the power supplied by wind turbine 402 or by solar cell array 404. Energy routing
unit 440 may route excess energy to be stored by the flywheel storage system. For
example, regulations may limit the electrical power that is fed into AC mains power
grid 390. Thus, surplus energy is utilizable at a later time. Connection of all units to a
single high voltage DC bus 435 and a central system controller 460 may enable
individual control of each flywheel unit.
[00206] In accordance with an embodiment of the present invention, a flywheel
energy storage system may include a constant voltage DC bus. A constant voltage DC
bus may avoid use of a master-slave configuration, where a failure or fault in the master
unit could result in failure of the entire system to operate.
[00207] Fig. 17 schematically illustrates a flywheel energy storage system that
includes a constant voltage DC bus, in accordance with an embodiment of the present
invention.
[00208] Constant DC voltage flywheel energy storage system 1300 includes a
plurality of flywheel units 1310. Each flywheel unit 1310 is associated with
motor/generator unit 1320 and controller/controller/inverter unit 1330.
[00209] For example, an associated flywheel unit 1310 and motor/generator unit
1320 may include an open frame flywheel/rotor within a vacuum enclosure, and a
outside of the vacuum enclosure, as shown. Other configurations of flywheel units and
motor/generator units may be used (e.g., a motor/generator unit whose rotor is
magnetically coupled to a flywheel unit within a vacuum enclosure, or a
motor/generator unit that is mechanically coupled to a flywheel unit and that is also
enclosed within the vacuum enclosure).
[00210] Components of constant DC voltage flywheel energy storage system
1300 may be controlled a local system controller 1370 via control bus 1350.
Alternatively or in addition, local system controller 1370 may be controlled by global
controller 1390 via network 1380, which controls Components of constant DC voltage
flywheel energy storage system 1300 via control bus 1350.
[00211] Each controller/inverter unit 1330 is connected to high voltage DC bus
1340. High voltage DC bus 1340 may connect to AC mains power grid 390 via
rectifier/inverter 1360. In some cases, high voltage DC bus 1340 may be connected to
one or more renewable energy sources via renewable energy power bus 1345. The DC
voltage of high voltage DC bus 1340 may be kept constant. For example, the DC
voltage of high voltage DC bus 1340 may have a value of approximately 650 V, or
another value.
[00212] During a charge phase of operation, controller/inverter unit 1330
operates in a charge mode to convert DC current for high voltage DC bus 1340 to an
AC voltage. For example, a DC bus voltage of 650 V may be converted to an AC
voltage with amplitude of about 300 V for a flywheel rotational velocity of 20,000 rpm,
or having an amplitude of about of 600 V for a flywheel rotational velocity of 40,000
rpm. The AC voltage is fed to stator coils of motor/generator unit 1320 to create a
torque to accelerate the flywheel of flywheel unit 1310.
[00213] During a discharge phase of operation, an AC voltage describable by a
sine wave is induced within the stator coils of motor/generator unit 1320 operating in a
discharge mode. The amplitude of the induced AC voltage is of similar magnitude to the
amplitude of the AC voltage that is fed into the stator coils during the charge phase
(e.g., 300 V with 20,000 rpm and 600 V with 40,000 rpm). Controller/inverter unit 1330
includes bridge circuit and pulse-width modulation (PWM) controller 1335. For
example, bridge circuit and PWM controller 1335 may include an H-bridge circuit and a
PWM control circuit. Controller/inverter unit 1330 utilizes the inductance of
motor/generator unit 1320 to step up the voltage signal for feeding into high voltage DC
bus 1340 in a current- limited mode. The current- limited mode causes the voltage of the
output of controller/inverter unit 1330 to increase to a voltage level that enables current
flow from controller/inverter unit 1330 to high voltage DC bus 1340 at a predetermined
current level. With such current-limited operation, multiple inverter units 342 may
concurrently feed power into a single high voltage DC bus 1340 without mutual
interference. Local system controller 1370 may control a set point of current to be fed
by controller/inverter unit 1330 to DC bus 1340. Thus, the rate of discharge of energy of
a flywheel of each flywheel unit 1310 is controlled by local system controller 1370.
[00214] A constant DC voltage flywheel energy storage system 1300 that
includes a constant voltage DC bus may be advantageous. For example, charging and
discharging of multiple flywheel units 1310 (with the corresponding controller/inverter
units 1330 operating in charge or discharge mode, respectively) that are connected to a
single high voltage DC bus 1340 may be performed concurrently. A single
controller/inverter unit 1330 supports both charging and discharging functions. Direct
connection of high voltage DC bus 1340 to renewable energy power bus 1345 is
enabled.
[00215] Constant DC voltage flywheel energy storage system 1300 may enable
increased functional flexibility and resilience. For example, failure of an individual
controller/inverter unit 1330 (except due to a short circuit) would not affect proper
operation of the inverter units 1330. In contrast, a system configured for sequential
operation, e.g., a system in which different flywheel units would provide power at
different voltages, would limit power levels to that of an individual flywheel unit 1310.
CLAIMS
1. A stabilization system for a rotating load, the system comprising:
a mechanical bearing to continuously support a shaft of the rotating load so as to hold
the shaft at a substantially fixed axis of rotation;
a magnetic stabilization assembly including a plurality of electromagnets arranged
around the shaft;
a control circuitry for controlling a resultant magnetic field generated by the
electromagnets such that the magnetic field acts on a ferromagnetic element of
the shaft to reduce imbalance forces acting on the shaft.
2. The system of claim 1, wherein the ferromagnetic element includes a rotor ring.
3. The system of claim 1, comprising a sensor to sense a vibration of the shaft, the
control circuitry being configured to control the resultant magnetic field so as to
minimize the sensed vibration.
4. The system of claim 1, wherein the control circuitry comprises an H-bridge or a
power amplifier to drive the electromagnets to generate a desired magnetic field.
5. The system of claim 1, wherein the control circuitry is configured to compensate
for a previously measured variation in a dimension of a mechanical component.
6. The system of claim 1, wherein the rotating load comprises a flywheel for
storing energy.
7. The system of claim 1, wherein the mechanical bearing comprises a bearing
selected from a group of bearings consisting of a metal ball bearing, a hybrid ball
bearing, and a ceramic ball bearing.
8. A flywheel energy storage system comprising a flywheel within an evacuable
enclosure, the flywheel including a core rotatable about an axis of rotation and a
plurality of rods, a proximal end of each rod being attached to a periphery of the core,
the rods extending substantially radially with respect to the axis of rotation.
9. The system of claim 8, wherein the rods are attached to the periphery of the core
in a staggered pattern.
10. The system of claim 8, wherein each proximal end is attached to the periphery of
the core by holder that is configured to hold the proximal end by a mechanism selected
from a group of holding mechanisms consisting of a press fit, a self-locking wedge, high
shear-stress glue, and a collapsible ferrule.
11. The system of claim 8, wherein a distal end of a rod of said plurality of rods is
weighted.
12. The system of claim 8, wherein a rod of said plurality of rods comprises
fiberglass.
13. The system of claim 8, wherein a rod of said plurality of rods comprises a
bundle of fibers wrapped around a column.
14. A flywheel energy storage system comprising:
a DC bus;
a plurality of flywheels;
a plurality of motor/generator units, each motor/generator unit being rotatably coupled
to a flywheel of said plurality of flywheels;
a plurality of controller/inverters, each controller/inverter being electrically coupled to a
motor/generator unit of said plurality of motor/generator units and to the DC
bus; and
a central controller to control each controller/inverter so as to set a discharge rate for
each of the flywheels when its motor/generator unit is operating in a discharge
mode, and to increase a voltage level of a voltage signal generated by the
motor/generator unit in the discharge mode.
15. The system of claim 14, wherein a controller/inverter of said plurality of
controller/inverter comprises an H-bridge circuit.
16. The system of claim 14, wherein the central controller is configured to control a
controller/inverter of said plurality of controller/inverters to operate in a discharge mode
while concurrently controlling another controller/inverter of said plurality of
controller/inverters to operate in a charge mode.
17. A flywheel energy storage system for storing electrical energy, the system
comprising:
a flywheel including a rotatable mass and a shaft, the flywheel being enclosed within an
evacuable enclosure, the shaft supported by bearings on opposite sides of the
rotatable mass; and
an electric motor/generator unit having a stator and a rotor, the rotor being fixed to the
shaft in a cantilevered manner within the enclosure and being magnetically
coupled to the stator, the stator being located outside of the enclosure.
18. The system of claim 17, wherein a distance between the rotor and the stator is
adjustable.
19. The system of claim 17, wherein the stator is configured to couple to each of a
plurality of rotors.
20. The system of claim 17, wherein the flywheel comprises lead enveloped in a
shell that comprises carbon fiber.
21. The system of claim 17, wherein the flywheel comprises a plurality of glass
fibers, each fiber being at least partially wrapped around a column of a plurality of
columns that are arranged in a circular pattern that is centered on an axis of rotation of
the flywheel, such that each fiber extends substantially radially outward from the axis
when the flywheel rotates.
22. The system of claim 17, wherein the flywheel includes a structure with an
eccentric mass distribution that is rotatable to adjust a balance of the flywheel.
23. The system of claim 17, wherein a section of the enclosure between the rotor
and the stator comprises glass.