专利汇可以提供Axially free flywheel system专利检索,专利查询,专利分析的服务。并且A flywheel energy storage system that allows high-speed operation with use of mechanical rolling element bearings for radial support of a vertical axis flywheel while they allow it to be mechanically free or unrestrained in the axial direction. Magnetic bearings are used to carry the flywheel weight axially. The axial unconstraint by the mechanical bearings allows the flywheel to freely grow or shrink in axial length from Poisson Effect contraction that occurs when rotating to very high speeds and stress levels as well as from thermal expansions from motor/generator heating or other sources, and also insures that the magnetic bearing carries all of the flywheel weight, thus greatly extending the life of the mechanical bearings. Radially compliant elements mechanically in series with the mechanical bearings provide for a lower radial stiffness, which allows the flywheel to traverse its rigid body resonance at a low speed. Above that speed, the flywheel spins about its mass center instead of the geometric center and radial loading on the bearing becomes much lower.,下面是Axially free flywheel system专利的具体信息内容。
Accordingly, it is expressly intended that all these embodiments, species, modifications and variations, and the equivalents thereof, are to be considered within the spirit and scope of the invention as defined in the following claims, wherein I claim:1. A flywheel energy storage system having a flywheel that is supported by a combination mechanical and magnetic bearing system for rotation about a substantially vertical axis inside a container with low drag atmosphere, and is accelerated with a motor and decelerated with a generator for storing and retrieving energy, comprising:rolling element mechanical bearings for providing radial support for said flywheel while leaving said flywheel mechanically unrestrained in the direction of said vertical axis, said rolling element mechanical bearings being located at top and bottom ends of said flywheel;radially compliant elements mechanically in series with said rolling element mechanical bearings for reducing radial stiffness of said radial support; anda magnetic thrust bearing system for providing axial support for said flywheel;wherein said flywheel is magnetically supported axially and is mechanically unrestrained axially by said mechanical bearings.2. A flywheel energy storage system as described in claim 1 wherein:said magnetic thrust bearing system is comprised of one or more rotating permanent magnets attached to the rotating flywheel that are in close proximity to one or more stationary permanent magnets such that the rotating and stationary magnets are in axial repulsion.3. A flywheel energy storage system as described in claim 2 wherein:said rotating permanent magnets are in the form of a ring that is an assembly of multiple individual magnets around said ring circumference.4. A flywheel energy storage system as described in claim 2 wherein:said magnetic bearing system includes two bearings located at both a top and a bottom ends of said flywheel.5. A flywheel energy storage system as described in claim 2 wherein:said flywheel is constructed primarily of steel.6. A flywheel as described in claim 5 wherein:said flywheel rotates in normal fully-charged operation at less than 25 krpm.7. A flywheel energy storage system as described in claim 2 wherein:said motor and generator uses an air core armature.8. A flywheel energy storage system as described in claim 7 wherein:said flywheel is constructed primarily of steel, said motor and generator comprises a field coil that generates flux through protrusions in said flywheel and said flux induces alternating current in said air core armature as said flywheel rotates.9. A flywheel energy storage system as described in claim 1 wherein:said magnetic thrust bearing system is comprised of one or more active controlled electromagnetic coils that control the axial position of said flywheel.10. A flywheel energy storage system as described in claim 9 wherein:said actively controlled electromagnet coil functions as part of a permanent magnet biased active magnetic thrust bearing.11. A flywheel energy storage system as described in claim 9 wherein:said flywheel is constructed primarily of steel.12. A flywheel energy storage system as described in claim 1 wherein:said radially compliant elements comprise one or more radial springs.13. A flywheel energy storage system as described in claim 12 wherein:said radial springs rotate with said flywheel.14. A flywheel energy storage system as described in claim 12 wherein:said radial springs do not rotate with said flywheel but have a fatigue life of greater than 5 billion cycles of radial deflection equal to the radial distance between the mass center and the geometric center of said flywheel.15. A flywheel energy storage system as described in claim 12 wherein:said radial springs comprise one or more quill shafts.16. A flywheel energy storage system as described in claim 1 wherein:said rolling element mechanical bearings comprise tandem preloaded bearing pairs.17. A flywheel energy storage system as described in claim 1 wherein: said rolling element mechanical bearings on the top and bottom of said flywheelare each single bearings, andat least one resilient element engaged with said rolling element mechanical bearings for preloading said rolling element mechanical bearings, said resilient element having a axial stiffness lower than the stiffness of said magnetic thrust bearings for providing axial support for said flywheel.18. A flywheel energy storage system as described in claim 1 wherein:said motor and generator uses an air core armature.19. A flywheel energy storage system as described in claim 1 wherein:said flywheel is mechanically unrestrained for axial movement by an axial sliding connection between two surfaces between said flywheel and said container.20. A flywheel energy storage system as described in claim 1 wherein:said flywheel is mechanically unrestrained for axial movement by a low axial stiffness spring between said flywheel and said container.21. A flywheel energy storage system as described in claim 1 wherein:said radial support allows said flywheel to traverse its cylindrical rigid body critical resonance at a speed that is less than 25% of the normal fully charged operation speed.22. A method for storing and retrieving energy in a flywheel energy storage system comprising:supporting a flywheel about a substantially vertical axis by a combination mechanical and magnetic bearing system and accelerating and decelerating said flywheel with an attached motor and generator;providing radial support for said flywheel with mechanical rolling element bearings located at the top and bottom ends of said flywheel wherein radially compliant elements are coupled mechanically in series with said rolling element mechanical bearings for reducing radial stiffness of said radial support; and providing axial support for said flywheel with one or more magnetic thrust bearings, wherein said flywheel is mechanically unrestrained to move axially by said mechanical bearings.23. A flywheel energy storage system comprised of a flywheel that is supported by a combination mechanical and magnetic bearing system for rotation about a substantially vertical axis inside a container with low drag atmosphere and is accelerated and decelerated for storing and retrieving energy through use of a motor/generator, comprising:rolling element mechanical bearings for providing radial support for said flywheel, said rolling element mechanical bearings being located at the top and bottom ends of said flywheel;radially compliant elements mechanically in series with said rolling element mechanical bearings for reducing radial stiffness of said radial support; andone or more magnetic thrust bearings for providing axial support for said flywheel;whereby said flywheel has an axial position that is controlled by said one or more magnetic thrust bearings.
This is related to U.S. Provisional Application No. 60/316,559 filed on Aug. 30, 2001 and entitled “Axially Free Flywheel System”.
This invention pertains to a flywheel energy storage system and more particularly to a flywheel system having a flywheel supported by a combination of an axial magnetic bearing and compliant radial rolling element mechanical bearings, with the flywheel being mechanically unrestrained in the axial direction. The unrestrained axial support allows for Poisson Effect contraction and thermal expansions during operation and simultaneously greatly extends the mechanical bearing life by reducing or eliminating axial loading. The radial compliance in the mechanical bearing support permits high-speed supercritical operation and reduced radial bearing loads.
BACKGROUND OF THE INVENTION
Flywheel power supplies have emerged as an alternative to electrochemical batteries for storing energy, with many advantages including higher reliability, longer life, lower or no maintenance, higher power capability and environmental friendliness. Flywheel power supplies store energy in a rotating flywheel that is supported by a low friction bearing system inside a chamber. The chamber is usually evacuated to reduce losses from aerodynamic drag. A motor/generator accelerates the flywheel for storing energy, and decelerates the flywheel for retrieving energy. Power electronics maintain the flow of energy in and out of the system and can instantaneously prevent power interruptions, or alternatively can manage peak loads.
One way to support a flywheel for rotation at high speeds is with rolling element mechanical bearings such as ball bearings. The life of mechanical bearings is strongly influenced by the loads that these bearings must carry. To extend the life of flywheel systems using mechanical bearings, a magnetic bearing can effectively be used in combination with the mechanical bearings for the purpose of reducing the load on the mechanical bearings. In this arrangement, the flywheel typically rotates about a vertical axis and the mechanical bearings provide radial support while the magnetic bearing carries much of the flywheel's weight axially. One such flywheel energy storage system is shown in FIG.
1
. The flywheel system
30
includes a steel flywheel
31
that rotates inside an evacuated vessel
32
. The interior of the vessel
32
is a chamber
41
maintained at a vacuum for reduction of aerodynamic drag. The flywheel
31
, in this design, has an integrated motor/generator
33
that accelerates and decelerates the flywheel through cooperation with the outer diameter of the flywheel
31
. The motor/generator
33
includes an outer laminated stator portion
34
, motor/generator coils
42
, stator flux return path
35
and a field coil
36
for operation. The flywheel
31
is supported for rotation on upper and lower rolling element mechanical bearings
37
and
39
. These bearings
37
,
39
are mounted in fixed upper and lower mounts
38
and
40
. Load on the bearings
37
,
39
is reduced through the use of an axial magnetic bearing
43
, shown as an annular electromagnet with electromagnetic coil
44
. The magnetic bearing
43
supports a majority of the weight of the flywheel
31
while allowing some desired amount of loading on the bearings
37
,
39
. The electromagnet is controlled either by using strain gauges, not shown, on the support structure that sense and control the bearing loading through use of a closed loop controller, or simply by use of a constant current power supply.
Unfortunately, this configuration of mechanical and magnetic bearing system is not optimal for allowing rotation to very high-speeds for efficiently storing large amounts of energy. There are several drawbacks. The rigid radial support provided by the mechanical bearings would cause the rigid body critical speeds to be encountered at a relatively high speed if the flywheel were operated to high speeds. Encountering these resonances at a high speed would impart severe loading on the bearings that would reduce their life and potentially be dangerous. Operation at high speeds, but below the rigid body critical speeds, causes high bearing loading from any flywheel imbalances because the flywheel is being forced to spin about a geometric center rather than the mass center.
Another major problem with operating a flywheel system to high speeds is that the loads on the bearings can significantly increase due to dimensional changes in the flywheel. The effect of high speed rotation is illustrated in FIG.
2
. The dimensions of a flywheel
50
are shown at
52
for zero speed and at
51
for high speed rotation. The effect of the centrifugal stress is that the outer diameter expectedly grows by a radial increment
53
from the radial and hoop stresses. This growth does not change the bearing loading. The secondary result from this growth is that the flywheel shrinks by an axial increment
54
from Poisson ratio contraction. For axially thick flywheels, the shrinkage can be as much as 0.050 inches. Such a large length change will not only drastically load the mechanical bearings against each other but can also cause them to fail before achieving full speed. The flywheel can also expand and contract axially from temperature changes in the flywheel or surrounding structure. Heating from high power motor/generators is one potential cause for added bearing loads.
The increased bearing loading drastically affects the life of mechanical bearings. Bearing life is generally a cubic function of the load, so that a doubling of the load will decrease the life by a factor of eight. Further compounding the shortening of life from the increased axial loading is that for angular contact bearings, axial loading can be as much as 35 times more fatiguing to the bearing than an equivalent size radial load. This sensitivity varies based on the contact angle, number of balls, ball diameter and the axial thrust load applied.
Besides problems of axial bearing loading that occurs between the bearings during operation, use of a mechanical strain gauges to measure axial loading at a single bearing is not as sensitive as desirable for removing almost the entire bearing axial load, especially if the flywheel support structure is rigid. Likewise, applying a constant current to the coil cannot provide sufficiently accurate axial load removal for maximum reliable operating life.
The construction of flywheel systems that support a flywheel with mechanical bearings can also suffer significant damage from shipping and handling. The system shown in
FIG. 1
has no power during shipping and hence the ball bearings must carry the full flywheel weight. The strain gage and load cell can become damaged and plastically deformed from impact loadings, especially if they were designed to be sensitive enough to maintain very low axial bearing loading. The bearings of this as well as other design systems could be easily damaged from impact loads such as simply setting the system down during transportation handling. The force generated from an impact can be several times the weight of the flywheel and can cause the balls to Brinnell indent the bearing races or cause the bearings to shift in position.
SUMMARY OF THE INVENTION
This invention provides a flywheel energy storage system that allows high-speed operation with use of mechanical rolling element bearings for flywheel support. The mechanical bearings provide radial support for a vertical axis flywheel but they allow it to be mechanically free or unrestrained in the axial direction. One or more magnetic bearings are used to carry the flywheel weight axially. The axial unconstraint by the mechanical bearings allows the flywheel to freely grow or shrink in axial length from Poisson Effect contraction that occurs when rotating to very high speeds and stress levels as well as from thermal expansions from motor/generator heating or other sources. Excessive axial loads applied from the bearings on each other are thereby prevented. The axial mechanical freedom also insures that the magnetic bearing carries all of the flywheel weight, thus dramatically extending the mechanical bearing lives. The life of rolling element bearings is generally a cubic function of the applied loads and axial loading on commonly used angular contact bearings is many times more fatiguing to the bearings than radially applied loads. Eliminating the axial loading from the flywheel greatly extends the bearing lives. Tandem multiple preloaded angular contact bearings can be used for the mechanical bearings. These bearing sets share the loads between several bearings, extending life, and are manufactured with the desirable minimum axial preload for longest term reliable operation. The axial preload is accurately built-in and does not change as the flywheel is rotated. Alternatively, the bearings can each be single bearing pairs that are preloaded using springs with a stiffness that is lower than the magnetic bearing. Therefore, the bearings maintain their near designed preload despite the axial position of the flywheel from the magnetic bearing support or changes in the flywheel dimensions.
The axial magnetic bearings can use permanent magnets, attached to the flywheel, that are arranged to be in vertical repulsion with stationary cooperating permanent magnets. This provides a completely passive axial magnetic bearing system. In another embodiment of the invention, the axial magnetic bearing uses an actively controlled electromagnetic coil. The coil is controlled using either flywheel axial acceleration or, more preferably, a position sensor. The coil can be used in a simple electromagnet or in a permanent magnet biased thrust actuator for higher lift force and/or lower power consumption. The use of an active magnetic bearing does not require magnets on the flywheel and has the potential for higher speed rotation. It also can be lower in cost, and not suffer from any demagnetization effects.
The high speed capability of flywheels in accordance with the invention is further facilitated by using radially compliant elements mechanically in series with the mechanical bearings. Providing for a lower radial stiffness allows the flywheel to traverse its rigid body resonance at a low speed. Above that speed, the flywheel spins about its mass center instead of the geometric center and radial loading on the bearing becomes much lower. The power loss from rotation can also be reduced. The flywheel can then smoothly and easily operate to higher speeds. Balance requirements for the flywheel can be significantly reduced, reducing costs and extending mechanical bearing life. In one embodiment, the radial support allows the flywheel to traverse its cylindrical rigid body critical resonance at a speed that is less than 25% of the normal fully charged operation speed.
The use of the radially compliant elements or springs with the mechanical bearings also has the effect of helping the flywheel to rotate stably while having axially free sliding connections to the bearings. Although not completely free, due to friction, the flywheel is essentially mechanically unrestrained by the mechanical bearings, so that it can move axially. Use of sliding joints where energy can be lost from frictional damping in the rotating object is well known in the field of high speed machinery to potentially cause problems with nonsynchronous rotordynamic whirl and is usually avoided. However, rotor whirl can be avoided if the foundation stiffness is made sufficiently low for the mass of the rotating object. In this case, the radially compliant elements or springs that allow the flywheel to spin above the rigid body resonance also help keep the system stable with the reduced stiffness.
In one aspect of the invention, the radially compliant elements are placed between the mechanical bearings and the flywheel such that they rotate with the flywheel. The result is that above the rigid body resonance, the radial springs simply deflect and the flywheel rotates about its mass center Because the springs rotate with the flywheel, the springs do not cycle with each revolution. The life of the springs in the flywheel systems are thus increased for longer-term reliable operation. The radially compliant element can be a radial spring such as a tolerance ring or alternatively a quill shaft.
In another embodiment, the radial springs do not rotate with the flywheel but have a fatigue life of greater than 5 billion cycles of radial deflection equal to the radial distance between the mass center and the geometric center of said flywheel. This provides for at least 1 year of continual rotation at 10,000 rpm. An even higher cycle life such as ten to twenty times higher is even more preferable to preferably last the life of the system. Besides reducing the rigid body resonances, the low radial stiffness can potentially allow the flywheel to spin smoothly through other vibration modes that may exist depending on the system construction.
The invention also makes the flywheel system significantly less prone to damage during shipping, handling and installation. The axially unrestrained condition of the flywheel in the mechanical bearings prevents the flywheel from axially impact loading the bearings with the weight of the flywheel when the system is set down. The radial compliant elements in series with the mechanical bearings prevent damage from radial impact loading. The result of the invention is a much more robust flywheel system employing mechanical bearings, a system that accounts for the axial flywheel dimension changes, a system that can rotate to higher speeds for storing more energy and a system that maximizes the life of the mechanical bearings by elimination of axial loading from the flywheel simultaneously with greatly reduced radial loading.
The invention provides for greatly increased mechanical bearing life of the flywheel system. To extend the bearing life even further, the flywheel can be designed to operate with a slower rotational speed, reducing the bearing fatigue cycles incurred. In one aspect of the invention, the flywheel is constructed primarily from steel instead of composite materials to provide a lower operating speed. The flywheel can also preferably be constructed with an increased diameter so its normal fill speed operation is at less than 25 krpm. Implementing a motor/generator that has an air core armature can also further reduce bearing loads. The air core armature provides high efficiency while reducing or eliminating non-circumferential force generation during operation. dr
DESCRIPTION OF THE DRAWINGS
The invention and its many attendant advantages will become more clear upon reading the following description of the preferred embodiments in conjunction with the following drawings, wherein:
FIG. 1
is a schematic elevation of a prior art flywheel energy storage system with a mechanical and magnetic bearing system;
FIG. 2
is a sectional elevation of a solid cylindrical flywheel illustrating the dimensional effects on the flywheel when rotated to high speed;
FIG. 3
is a schematic elevation of a flywheel energy storage system in accordance with the invention;
FIG. 4
is a schematic elevation of another configuration of flywheel energy storage system in accordance with the invention;
FIG. 5
is a bar chart comparing the mechanical bearing life versus flywheel diameter for use with the invention;
FIG. 6
is a schematic elevation of another flywheel energy storage system in accordance with the: invention;
FIG. 7
is a schematic elevation of another flywheel energy storage system in accordance with the invention;
FIG. 8
is a schematic drawing of another flywheel energy storage system in accordance with the invention;
FIG. 9
is a schematic elevation of another flywheel energy storage system in accordance with the invention;
FIG. 10
is a schematic elevation of another flywheel energy storage system in accordance with the invention;
FIG. 11
is a schematic elevation of another flywheel energy storage system in accordance with the invention; and
FIG. 12
is a schematic elevation of another flywheel energy storage system in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning to the drawings, wherein like reference characters designate identical or corresponding parts,: and more particularly to
FIG. 3
thereof, a flywheel energy storage system
60
in accordance with the invention is shown having a flywheel
61
supported for rotation about a substantially vertical axis of rotation inside a sealed container
62
. The inside of the container
62
is a chamber
63
preferably maintained at a vacuum or other low-drag atmosphere for reduction of aerodynamic drag. In systems where drag is less of a concern, the low drag atmosphere can be a small molecule gas such as helium instead of a vacuum, with the result of potentially lower costs but higher drag loss.
The flywheel
61
is radially supported with upper and lower mechanical bearing supports
64
and
68
engaging upper and lower flywheel shafts
67
and
71
. The bearing supports
64
,
68
use rolling element bearings
65
and
69
to allow low loss rotation. The bearings can be individual ball bearing sets or tandem multiple preloaded angular contact bearing sets to share the radial loading and insure adequate and accurate preload for long term operation, as shown. The bearings preferably use the minimum preloading amount, which is a function of the bearing size. Ceramic hybrid bearing sets, ceramic balls in metal or steel races, are preferred for highest speed and longest life operation. They can be lubricated with grease, oil or more preferably a dry lubricant such as molybdenum disulfide to prevent contamination of the vacuum in the chamber
63
. The rolling element bearings
65
,
69
are mechanically in series with radial compliant elements
66
,
70
for reducing the radial stiffness of the bearing supports
64
,
68
. In this configuration, the compliant elements, which can be radial springs, are located between the mechanical bearing sets
65
,
69
and the support housings
72
,
73
and thus are stationary. The radial compliant elements are preferably constructed from metal for long life in vacuum, and for high thermal conductivity. They can be made from numerous constructions including metal mesh, or tolerance rings such as those sold by USA Tolerance Rings Inc.
The weight of the flywheel is borne axially by upper and lower magnetic bearings
74
and
75
. The upper and lower flywheel shafts
67
and
71
slide axially freely (with some friction) inside the rolling element bearings
65
,
69
such that the mechanical bearings do not carry appreciable axial loading from the flywheel. Some very small loading is transferred to the bearings from axial friction with the flywheel shafts
67
,
71
, however this amount would be negligible compared to the weight due to the sliding interface. Because the axial loading is removed from the mechanical bearings, their lives can be greatly extended. The life of rolling element bearings is generally a cubic function of the applied loading such that a reduction of the bearing loads by half results in an increase in life by a factor of eight. Further adding to the benefits of axial load elimination of the invention is that axial loads are typically many times more fatiguing to common angular contact bearings than radial loads. In some cases, depending on the bearing size and configuration, axial loads can be as much as
35
times more fatiguing to the bearing than an equivalent value radial load.
In the flywheel system shown in
FIG. 3
, the magnetic bearings
74
,
75
are passive. They have permanent ring magnets
76
and
79
, or an annular array of magnets, attached to or rotating with the flywheel
61
, arranged in axial repulsion with stationary ring magnets
78
and
80
in close axial proximity. The magnetic bearings can be employed at either the top or bottom of the flywheel, or at both ends. As shown, the top magnetic bearing
74
includes a thrust disk
77
attached to and rotating with the top flywheel shaft
67
for attachment of the rotating ring magnet
76
. Because high power magnets such as NdFeB are mechanically weak in tensile strength, difficulty may be encountered in reinforcing the magnets for rotation to high speed, especially if the flywheel is very heavy and requires ring magnets of very large diameter and surface area to generate sufficient axial force. The magnet ring attached to the flywheel may be an annular array made from an assembly of individual magnet pieces around the circumference. The use of pieces prevents development of excessive hoop stresses that could otherwise cause the magnet to fail mechanically. Gaps between magnet pieces that develop when rotated to high speed are very small and thus are not magnetically problematic.
Magnets with axial magnetization are generally easier to manufacture and are lower in cost than radially magnetized magnets. Several designs of axial repulsion magnetic bearings can be constructed and used with the invention The magnetic bearings generate axial repulsive forces between the flywheel and the rest of the system. They can also generate some radial destabilizing forces that tend to push the flywheel radially off center and radially load the bearings. Care should be taken in the exact system design (magnet dimensions) to insure that these forces remain small for the entire allowable radial deflection of the upper and lower bearing supports
64
,
68
.
The mechanical rolling element bearings
65
and
69
provide radial support for the flywheel while the magnetic bearings carry the axial weight of the flywheel. The connection between the flywheel shafts and the mechanical bearings is unconstrained axially such that thelmagnetic bearings essentially carry all of the flywheel weight. This greatly extends the lives of the mechanical bearings by eliminating detrimental axial loading. The sliding connection also serves a second significant purpose by preventing dimensional changes in the flywheel from adding loading to the mechanical bearings. As a flywheel is rotated to very high speeds and stress levels, the outer diameter grows radailly. However, the large radial and hoop direction stresses cause the flywheel to shrink axially due to Poission Ratio contradiction of the flywheel material. If the bearings were rigidly connected to the flywheel, excessive bearing loading and possible bearing failure could result. The amount of contraction depends on the axial thickness of the flywheel and the operating stress levels, however contractions on the order of 0.050 inches are possible. The axial loading from dimensional changes can also result from temperature changes in the flywheel from motor/generator heating and or from temperature changes to the surrounding flywheel container.
The radial bearings are in series with compliant elements
66
and
70
such that the upper and lower radial support each have a low stiffness. The low radial stifffness allows the flywheel to operate above the rigid body resonance at a relatively low speed. Above the resonance, the flywheel spins about its mass center instead of geometric center and thus radial loads are significantly reduced, extending the bearing life. Having the transition through the resonance at low speed, such as less than 1000 rpm for some high speed systems, prevents encountering the resonance at a high speed which would cause extreme bearing loading and can possibly fail the bearings. Operating supercritically can also lower the bearing drag. In one embodiment, the radial support preferably allows the flywheel to traverse its cylindrical rigid body critical resonance at a speed that is less than 25% of the normal fully charged operation speed. Because the radial compliant elements are stationery, they are cycled as the flywheel rotates due to the offset between the mass center and geometric center of the flywheel. This distance is typically small at less than a few thousandths of an inch, especially for a balanced flywheel. In one embodiment of the invention, radial springs have a fatigue life of greater than 5-75 billion cycles of radial deflection equal to the radial distance between the mass center and the geometric center of said flywheel. This allows the flywheel system to last 1-15 years while rotating at 10 krpm. A longer fatigue life is preferable and can be attained by careful design and manufacturing. Small radial spring deflections resulting from a smaller offset between the mass center and geometric center of the flywheel, and from radial spring design improvements, can result in significantly longer fatigue lives. A thin metal spring or metal mesh that does not undergo significant bending stress can accomplish this.
The lower radial stiffness from the compliant elements also facilitates stable rotation to high speed. Sliding joints on rotating objects, which can dissipate energy through friction, are thought to be best avoided in rotating machinery design as they can lead to nonsynchronous rotordynamic whirl problems. However, rotor whirl is a function of the foundation flexibility. The lower radial stiffness imparted from having radial compliant elements in series with the bearings also helps the stability of the flywheel system despite having sliding connections.
During shipping, handling and installation of the flywheel system, the invention is much more robust and resistant to damage than prior art flywheel systems that use mechanical bearings. The axial unrestrained connection between the flywheel and mechanical bearings prevents impact damage to the bearings from the weight of the flywheel, especially when the system is set down hard. The radial compliant elements in series with the mechanical bearings also prevent damage from radial impact loading. The passive axial magnetic thrust bearings increase in repulsive force as the axial gap between ring magnets decreases, and will prevent contact in most shipping situations. In the event that the unit is dropped during shipment, the end of the shaft can be designed to contact the bottom of the housing or an impact absorbing bumper before the magnets make contact, to prevent damage to the magnets.
In operation, the flywheel
61
is accelerated and decelerated for storing and retrieving energy through an attached motor/generator
81
. Many designs of motors and generators exist and could be used with the invention. The motor/generator is preferably a brushless design for long life. The motor/generator
81
shown is a brushless permanent magnet synchronous motor/generator type. The motor/generator
81
uses magnets
82
attached to the lower flywheel shaft
71
. The magnets
82
cooperate with a surrounding laminated stator
83
for electrical/mechanical energy conversion.
Another flywheel system
90
, shown in
FIG. 4
, includes a flywheel
91
mounted on radial bearings
94
and
98
, and an axial magnetic bearing
102
for rotation about a vertical axis inside a container
92
having an evacuated internal chamber
93
. The flywheel is accelerated using a motor/generator
109
using magnets
110
attached to the flywheel shaft
108
that magnetically interact with a surrounding stator
111
to accelerate the flywheel
91
to store energy, and decelerate the flywheel to recover energy. The flywheel
91
is radially supported by the upper and lower mechanical bearing supports
94
and
98
. The bearing supports
94
,
98
use preloaded tandem pair angular contact bearings
95
,
99
. Radial compliant elements
97
and
101
may be used in tandem with the bearings
95
and
99
such that the compliant elements rotate with the flywheel
91
. The result of having the compliant elements rotate with the bearings is that, above the rigid body resonance speed, they do not continue to be cycled and just simply deflect to a stable deflected position to allow rotation about the mass center. This provides a much longer life for the compliant elements. If the compliant elements
97
,
101
are low cost tolerance rings, the rings may be selected to minimize the axial friction between the flywheel shafts
107
,
108
and the tolerance rings to reduce the potential for any axial loading from the flywheel
91
being transferred to the bearings
95
,
99
. However, some friction is required to overcome the drag torque in rotating the mechanical rolling element bearings. Placing the radial compliant elements such that they make up the sliding connection provides the benefit of rotational friction in the axial unrestraint.
The magnetic bearing
102
for axial levitation of the flywheel
91
uses an electromagnetic coil
104
to provide stable axial levitation. The coil in this case is part of a simple stationary electromagnet
103
located at the top of the flywheel
91
. The flywheel
91
is constructed of steel and thus the electromagnet
103
exerts an upward force on the flywheel
91
. Control for the coil power is provided by an controller (not shown) using feedback from an axial sensor
106
that senses the flywheel axial acceleration or more preferably simply the flywheel axial position. Other magnetic bearing control schemes using the electromagnetic coil
104
for also sensing the flywheel position are known and could alternatively be employed.
Because attractive magnetic bearings inherently generate unstable tilting moments that would tend to tilt the flywheel, the electromagnet
103
and the axial end of the flywheel
105
can be curved step-wise to reduced generation of such moments. A tilting moment exerted on the flywheel
91
by the magnetic bearing would cause extra radial loading on the radial mechanical bearings
95
,
99
and thus would be undesirable. The use of an active magnetic bearing as shown in this configuration can reduce costs by not requiring permanent magnets although the costs of a control system is required, and it can potentially allow higher rotational speeds by not having rotating magnet loading. The life of rolling element bearings, such as ball bearings, is greatly influenced by the loads that the bearings must carry. Larger bearings can carry larger loads, however they can be rotated only at lower speeds. They also have inherently higher losses, which may or may not be problematic depending on the application and system design. The stress and hence material utilization of the material in a flywheel is related to the square of the peripheral speed and not the rotational speed. Because it is desirable to most effectively utilize a flywheel and also to have the longest bearing life, it is preferable to have a high peripheral speed with a low rotational speed. This is made possible by using flywheels that have a large diameter. It is further facilitated by using metal or steel flywheels that have lower tip speed capabilities than composite material flywheels. A comparison of mechanical bearing life versus flywheel diameter for solid steel flywheels used with the invention is shown in FIG.
5
. All of the flywheels are operating with the same stress level and tip speed, 500 m/sec. All bearings are subject to 40 lbs radial loading and zero added axial loading using tandem preloaded angular contact bearing pairs all preloaded with the minimum axial preloads. The bearings are ceramic hybrids using dry lubricant. As shown, an 8 inch diameter flywheel rotates at 47 krpm, a 16 inch diameter flywheel rotates at 23.5 krpm and a 24 inch diameter flywheel rotates at 15.7 krpm. The bearing life for a slower rotating flywheel is longer than for a faster rotating flywheel due to a lower number of cycles per interval of time as expected. This is a linear relationship. However, a secondary effect has also been found to take effect. The slower rotational speed of a larger flywheel also allows a larger bearing set to be used. The larger bearing for the same radial load dramatically increases the life of the mechanical bearing as shown. An L
1
life corresponds to 99% survival rate of the bearings for that loading and life. The life of the 24 inch diameter bearing is shown to be a smaller increment higher than the 16 inch diameter flywheel as compared to the difference between the 16 inch and 8 inch flywheels. This is because the 16 inch and 24 inch flywheels are both shown using the same size bearings and the extended life is merely the result of the slower rotational speed. Other effects such as maintaining bearing lubrication must also be considered in the bearing life and other types of bearings and lubrication methods can be applied. However, larger diameter and steel flywheels can dramatically increase the mechanical bearing life with the invention. In another embodiment of the invention a flywheel constructed of steel is used with diameter greater than length for longer mechanical bearing life. Preferably, the flywheel rotates at 25 krpm or less in normal operation. Solid steel disks allow maximum energy storage by having equivalent radial and hoop stresses, however steel ring flywheels rotate with even lower tip speeds and could also be employed with the invention.
Another flywheel system
110
, shown in
FIG. 6
, has a steel flywheel
111
with diameter greater than length, used for storing energy. The flywheel
111
rotates about a substantially vertical axis inside a container
112
having an internal evacuated chamber
113
. The flywheel is accelerated and decelerated using a brushless motor/generator
128
, in this case, of the reluctance type. The flywheel shaft
115
contains a radial gear rotor
129
that cooperates with a surrounding stator
130
to form the motor/generator
128
. A separate motor and generator could also be employed. The flywheel
111
is radially supported using upper and lower ball bearing sets
116
and
117
. Series connected radially compliant elements
118
and
119
are attached between the inner race of the bearings
116
,
117
and the flywheel shafts
114
and
115
such that they rotate with the flywheel
111
. In this case, another aspect of the invention is also illustrated. The axial unrestrained connection between the flywheel
111
and mechanical bearings
116
,
117
is provided not through sliding connections but through low axial stiffness springs which in this case are also the radial compliant elements
118
and
119
. A magnetic bearing
122
carries the axial weight of the flywheel, and illustrates yet a further aspect of the invention. The magnetic bearing
122
is a permanent magnet biased thrust bearing having an electromagnetic coil
125
for axial force generation and is controlled using a position sensor
127
. A thrust disk
126
attached to the upper flywheel shaft
114
provides a target for axial force generation. A steel yoke
123
provides a path for the flux from the electromagnetic coil
125
, and permanent magnets
124
amplify the force. The benefits of using permanent magnet bias include higher lifting force capability and lower power consumption as well as more linearized response.
Another flywheel energy storage system
140
, illustrated in
FIG. 7
, uses a steel disk flywheel
141
that rotates inside a container
142
having an internal chamber
143
maintained at a vacuum The flywheel is accelerated and decelerated using a brushless motor/generator
162
having permanent magnets
163
attached to an upper shaft
144
and cooperating with a surrounding fixed stator
164
for energy conversion. The flywheel
141
is radially supported using upper and lower bearing supports
146
and
150
. The bearing supports use tandem preloaded angular contact bearing pair sets
147
and
151
for rotation. Radially compliant elements
149
and
153
reduce the radial stiffness. Liner tubes
165
and
166
can be used to allow free axial sliding between the flywheel shafts
144
,
145
and the compliant elements
149
,
153
.
In this configuration, the magnetic bearing uses a permanent magnetic bearing
154
to carry the weight of the flywheel
141
. The permanent magnetic bearing
154
drives flux through the flywheel
141
and returns through a yoke
157
to be magnetically efficient. A lower magnetic bearing
155
, which is active, controls the flywheel axial position. The active magnetic bearing
155
uses an electromagnetic coil
160
and steel poles
159
to provide a downward force that balances the force from the upper magnetic bearing
154
. A position sensor
167
provides feedback and an upper axial stop
166
prevents damage to the sensor when the system
140
is inoperative. The magnetic bearings in this configuration illustrate another aspect of the invention. The magnetic bearings
154
,
155
provide radial centering stiffness or support as well as axial support. As explained previously, axial magnetic bearings tend to generate unstable tilting moments that try to tilt the flywheel. However in this case, the magnetic bearings
154
,
155
generate some stable radial centering stiffness along with the axial force and unstable tilting moments. The radial centering stiffness of the upper magnetic bearing
154
works against the unstable tilt moment from the lower magnetic bearing and vice-versa. The passive radial centering stiffnesses are the result of putting annular grooves
158
and
161
in the faces of the flywheel
141
. The annular poles
157
,
159
and a ring magnet
156
tend to line up radially with the rotor poles created by the grooves
158
,
161
and hence generate a positive radial stiffness. Other designs of passive radial magnetic bearings exist and could also be used. The passive radial stiffness reduces radial mechanical bearing loads that would otherwise result from the tilting moments of the upper and lower magnetic bearings
154
,
155
. They can also reduce loads imparted from external sources that would be carried by the mechanical bearings
154
,
155
. However, the mechanical rolling element bearings
146
,
150
primarily carry the radial loading. Sources of radial loading include system tilt, earthquakes, unbalance, etc.
Another flywheel energy storage system
170
, shown in
FIG. 8
, has a steel flywheel
171
with diameter greater than length. The flywheel
171
rotates inside an evacuated chamber
173
within a container
172
. In this configuration, the flywheel is supported by upper and lower mechanical bearing supports
174
and
175
having mechanical bearing sets
176
and
179
with outer races that rotate with the flywheel
171
. The mechanical bearings
176
,
179
are attached to the flywheel
171
through the radial compliant elements
177
and
180
that also rotate with the flywheel. Upper and lower shafts
178
and
181
are stationary. The axial position of the flywheel
171
is maintained by upper and lower active magnetic bearings
182
and
185
. The magnetic bearings use annular electromagnetic coils
184
and
187
along with yokes
183
and
186
to generate axial force, and an axial position sensor
188
provides feedback. During shipping and handling, the active electromagnetic bearings
182
,
185
would not be operable. The flywheel bearing
179
could rest on a stop, not shown, on the shaft
181
or alternatively the flywheel could be mechanically axially supported, not through the lower bearing
179
, to prevent damage. The flywheel
171
could be made to rest on the lower shaft
181
or yoke
186
. Passive axial permanent magnet repulsion bearings on the other hand provide the benefit of axial support and free rotation without the use of power. A motor/generator
189
uses a laminated stator ring
190
, motor/generator coils
193
, a flux return ring
191
and an annular field coil
192
to provide energy conversion and voltage regulation.
The invention is applicable not only for flywheel systems employing steel flywheels, but could also be used with higher speed composite flywheels, as shown in
FIG. 9
, wherein another flywheel energy storage system
200
is shown having a high speed composite flywheel
201
that rotates inside an evacuated chamber
203
within a container
202
. The composite flywheel
201
is made of a hoop wound glass fiber/epoxy ring
204
inside a hoop wound carbon fiber/epoxy ring
205
. Other configurations of composite flywheels could also be used with the invention. The flywheel is attached to a central tube
207
by a high elongation hub
206
that allows the flywheel
201
to grow radially with speed. Upper and lower mechanical bearing supports
209
and
212
support the flywheel radially. The supports include rolling element bearings
210
and
213
attached to radial compliant elements
211
and
214
for reduced radial stiffness. The mechanical bearings
210
,
213
rotate about a stationary central shaft
208
. The axial weight of the flywheel is supported by a combination of active and passive magnetic bearings
216
and
226
. The active magnetic bearing
216
provides control through use of an electromagnetic coil
218
and yoke
217
that act upon a ferromagnetic target
219
attached to the hub
206
. A position sensor
220
provides feedback. A passive magnetic bearing
226
using a permanent magnet ring
215
that acts on the central tube
207
reduces the force requirements of the active magnetic bearing
216
but cannot lift the entire weight of the flywheel
201
.
The rolling element bearings in this configuration illustrate another aspect of the invention. Because the axial loading is essentially eliminated from the mechanical bearings, the mechanical bearings
210
,
213
can be roller bearings. Roller bearings offer even higher radial load capability and life for a given radial load. They are also available in ceramic hybrid form for maximum speed capability.
The motor/generator
221
illustrates another aspect of the invention by using an air core design to reduce the radial destabilizing forces generated. The motor/generator
221
uses permanent magnets
223
on the bore of an outer tube
222
to create a radial magnetic field in cooperation with the central tube
207
. Air core stator coils
224
attached to a support
225
provide for power conversion. Other designs or air core motor/generators could also be used. Likewise, an axial gap motor/generator could also be used to prevent generation of radial motor/generator destabilizing forces that would add to the radial loads carried by the mechanical bearings.
Another flywheel energy storage system
230
in accordance with the invention, shown in
FIG. 10
, has a steel disk flywheel
231
supported for rotation about a vertical axis in an evacuated chamber
233
inside a container
232
to reduce aerodynamic drag. The flywheel
23
1
is supported radially by upper and lower rolling element bearings
234
,
235
. An axial magnetic bearing
244
supports the weight of the flywheel
231
by using a rotating permanent ring magnet
245
or annular array of magnets that axially repels a stationery permanent ring magnet
246
. The radial support for the flywheel
231
uses tandem preloaded angular contact bearing pairs
236
,
239
that are mounted to the container
232
. The radial compliant elements or springs in this configuration comprise upper and lower quill shafts
238
,
241
. The quill shafts
238
,
241
are connected to inner ends of shafts
237
,
240
that are radially supported in the bearings
236
,
239
, but are axially unrestrained and can slide inside the bearings
236
,
239
. To prevent potentially excessive radial displacement, limit tubes
242
,
243
are provided. The limit tubes
242
,
243
limit displacement by contacting the bearings or the housing.
The axial unrestraint by the radial support
234
,
235
along with the use of a permanent magnet repulsive bearing
244
is that the temperature effects do not effect the loads or life of the bearings
236
,
239
. Permanent magnets have a temperature coefficient whereby they tend to have reduced flux density with elevated temperature. The repulsive axial magnetic bearing
244
results in a levitation height that changes with temperature and the mechanical bearings are unaffected due to the unrestraint or free sliding connection.
The flywheel
231
is accelerated and decelerated using an attached motor/generator
247
. The motor/generator
247
uses an air core armature
250
, which reduces or eliminates generation of destabilizing forces that must be carried by bearings
234
,
235
,
244
. The motor/generator
247
uses a steel disk
247
′ having an annular array of multiple circumferentially spaced protrusions
248
around the circumference that act as poles and form an air gap
249
, in which the air core armature
250
is supported by a support tube
252
. A concentric annular field coil
251
, supported on in inner circumference of the air core armature
249
, generates flux that travels in a flux path through the flywheel
231
and steel disc
247
′, is focused into axial rays by the protrusions
248
, and jumps the air gap
249
through the air core armature
250
, inducing an alternating current in the air core armature
250
as the flywheel
231
rotates. The field coil
251
provides a simple and effective method for power regulation. Raising or lowering the current to the field coil
251
for a given speed of the flywheel
231
can control the back emf of the motor/generator
247
.
Another flywheel energy storage system
290
, shown in
FIG. 11
, includes a flywheel
291
having multiple piece construction, and supported for rotation about a vertical axis in an evacuated chamber
253
inside a sealed container
292
. The flywheel
291
is constructed from two steel rotor portions
254
,
255
that are connected together by a stainless steel magnetic insulator
256
. The rotor portions
254
,
255
have multiple, circumferentially spaced protrusions
273
,
274
around the outer circumference that face an armature airgap
275
that is created between the two rotor portions
254
,
255
. An annular air core armature
276
is located in the armature air gap
275
. Field coils
277
,
278
generate flux that travels in a flux path
281
to and from the flywheel
291
through steel poles
279
,
280
, across the air gap
275
and through the air core armature
276
. The flux induces alternating current voltage in the armature
276
as the flywheel
251
rotates. Wires
282
for the armature
275
and the field coils
277
,
278
exit the sealed container
292
through a vacuum tight feedthrough
285
and connect to power electronics
283
and a power buss
284
. The vacuum in the chamber
253
is maintained by a vacuum connection
286
to an external vacuum pump
287
.
The flywheel
291
is supported radially by upper and lower mechanical bearings
257
,
258
and an axial magnetic bearing
269
supports the weight of the flywheel
291
. The axial magnetic bearing
269
uses rotating permanent magnets
270
that axially repel stationery permanent magnets
271
. The radial supports
257
,
258
comprise single rolling element mechanical bearing sets
259
,
264
that are attached to stationery mounts
261
,
265
. Radially compliant elements or tolerance rings
260
,
266
connect the bearings
259
,
264
to the flywheel shafts
263
,
268
and allow for axial sliding. Because rolling element bearings typically require some small amount of preload to prevent ball skidding and bearing damage, axial springs
262
,
267
provide axial preload against the flywheel
291
and essentially against each other. To maintain proper preload despite changes in the potential axial position of the flywheel, the preload springs
262
,
267
preferably have a lower stiffness than the axial magnetic bearing
269
. The stiffness is preferably much lower so that axial displacement of the flywheel
291
from temperature changes to the magnetic bearing
269
do not change the bearing preload by more than a pound or so. Typical required preloads can be as low as 5 pounds for bearings that radially support a flywheel weighing several hundred pounds, but they vary depending on the many factors of the system design. Other arrangements of preload springs and bearing locations could also be utilized in accordance with the invention. For instance, the preload springs could be mounted stationery and the mechanical bearings could be made to slide axially in the stationery mounts. The benefit of using single bearing pairs is reduced costs and lower drag losses, however they provide lower radial load capacity.
Another flywheel energy storage system
300
, shown in
FIG. 12
, has a steel flywheel rim
301
that rotates in an evacuated chamber
303
inside a sealed container
302
. The flywheel rim
301
is comprised of two rim pieces
304
,
305
that are assembled to create an armature airgap
308
. The rim
301
has an annular array of multiple circumferentially spaced protrusions
307
that face the armature air gap
308
and form poles of a motor/generator
309
. A field coil
310
generates flux that travels in a flux path
312
through the rim pieces and across the air gap
308
, inducing alternating current voltage in an air core armature
311
located in the armature air gap
308
, as the flywheel
301
rotates. Wires
325
from the motor/generator
309
exit the container
302
through a sealed connection
326
. A field controller
327
controls the current to the field coil
310
to control the induced voltage in the armature
311
. A rectifier
328
rectifies power from the armature
311
to a DC buss
330
when generating power and an inverter
329
produces synchronous alternating current to drive the motor/generator
309
as a motor for storing energy.
The rim
301
is connected to a central tube
313
through the use of a hub
306
. The flywheel
301
is supported radially by upper and lower mechanical bearings
314
,
315
, and the weight of the flywheel
301
is supported by an axial magnetic bearing
322
. The flywheel
301
rotates about a stationery central shaft
318
. Single ball bearing sets
316
,
319
slide on the central shaft
318
and connect to the central tube
313
through radial springs
317
,
320
. The bearings
316
,
319
are axially preloaded by a single central spring
321
on the center shaft
318
.
A vacuum pumping port
311
allows for pulling of an initial vacuum
303
and for sealing the container
302
. An internal getter pump
332
maintains the vacuum
303
for the life of the flywheel system
300
.
Obviously, numerous modifications and variations of the preferred embodiment described above are possible and will become apparent to those skilled in the art in light of this specification. Different attributes of all of the different disclosed configurations can be interchanged and are not intended to be exclusive for use with the other elements and attributes of a particular system configuration. Many functions and advantages are described for the disclosed preferred embodiments, but in many uses of the invention, not all of these functions and advantages would be needed. Therefore, I contemplate the use of the invention using alternate or fewer than the complete set of noted components, features, benefits, functions and advantages. Moreover, several species and embodiments of the invention are disclosed herein, but not all are specifically claimed, although it is intended that all be covered by generic claims. Therefore, it is my intention that each and every one of these species and embodiments, and the equivalents thereof, be encompassed and protected within the scope of the following claims, and no dedication to the public is intended by virtue of the lack of claims specific to any individual species.
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