Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-06-22
2001-04-03
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S090500, C310S156030, C310S266000, C074S572200
Reexamination Certificate
active
06211589
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a flywheel system suitable for storing energy when demand for energy is low, and from which energy can be retrieved when energy demand increases. In addition, the invention relates to systems for maintaining power quality and providing uninterruptable power sources. More particularly, the invention relates to magnetic bearings and motor/generators for use with energy storage flywheels.
BACKGROUND OF THE INVENTION
In general, power generation equipment operates most efficiently under steady-state conditions that allow design engineers to optimize operating conditions. To the extent that power generation equipment operates outside these optimum design conditions, energy efficiency generally declines. For example, if a power plant must be turned down to produce a lower amount of power, then flue gas energy recovery systems may become less efficient, resulting in an overall decrease in conversion of energy to useful electrical energy. On the other hand, when a power generating plant operates at above its designed capacity, inefficiencies may result from factors such as incomplete combustion and the inability of energy recovery systems to recover a high proportion of incremental heat produced.
Although energy-efficient operation of power generation equipment requires a steady-state load, demand for power varies cyclically throughout a day, and also varies seasonally. Thus, it is not often feasible to operate power plants at optimum levels of efficiency, since exactly corresponding demand does not occur for any great length of time. Energy generation systems are also subject to faults, which cause voltage and phase changes and outages, which are not acceptable to many users. These users must have an assurance of power quality and/or an auxiliary energy storage or generation system to prevent outages.
Efforts have been made to “smooth” the demand for energy from power plants to facilitate steady-state power generation equipment operation. Some of these efforts have focused on auxiliary power generation equipment that may be operated when demand is high and shut down, or “turned down”, when demand is low. These auxiliary units are not only expensive but are usually also inefficient, since they also do not operate at their optimum load levels but at varying levels, depending upon demand. Other efforts have focused on energy storage. Examples of such energy storage systems include, for example, the use of batteries to store electricity or the use of pumped storage systems. The pumped systems utilize excess power generated during low power demand periods to pump water to an elevated storage position, thereby imparting potential energy to the water. When demand for energy increases, the water is released from storage and flows to a lower elevation, releasing potential energy, which is typically converted, via turbines, to kinetic energy and subsequently to electricity.
It has also been proposed that excess energy could be stored in large flywheels that are caused to rotate at very high speeds, thereby storing energy as kinetic energy. However, these flywheel energy storage concepts present several challenging issues. A flywheel that rotates on a mechanical bearing will generally suffer relatively high energy losses due to bearing friction. Thus, the ratio of output energy from the flywheel to input energy (a measure of overall efficiency) is often relatively low so that such systems are usually commercially unattractive.
Magnetic bearings have been proposed for a variety of flywheel designs. However, these bearings also suffer significant drawbacks. Permanent or electromagnets do not provide lateral stabilizing forces to hold a rotating flywheel in position. Thus, electromagnets with complex and low efficiency servosystems are required for lateral stability. Also, magnets in a motor/generator are often arranged so that, when the wheel rotates at high speed and components undergo radial expansion, the gap between the magnets and the field coils increases, thereby decreasing efficiency—an undesirable effect.
Also, a flywheel rotating at high speed generates high radial and hoop stresses in the wheel structure. And, the higher the rate of rotation, the greater these forces become. At some point, hoop stresses, which exceed radial stresses, may cause a failure of wheel materials with potentially devastating results. To avoid this eventuality, expensive high strength materials must be used. This high cost discourages the use of flywheels, since it is desirable to use a lowest cost method of energy storage.
A flywheel that is used to store energy may be expected to rotate within a normal operating range of frequencies or rotational speeds related to the highest and lowest amounts of energy stored. Generally, it is undesirable that the flywheel have a critical frequency, which sets up a resonance condition, within this operating range of speeds. However, materials and mechanical designs frequently make it difficult, if not impossible, to entirely eliminate critical frequencies within the normal operating range of the flywheel. This impairs the operating flexibility of the flywheel since it is undesirable to operate through the critical frequency as a normal condition of use.
In order for flywheels to become commercially attractive for use as energy storage devices, the flywheels must be relatively inexpensive to produce, able to store a commercially useful amount of energy, without risk of self-destruction due to radial or hoop stresses, must have a critical frequency that is outside the range of operating conditions, and should have efficient bearings and motor/generators to minimize energy losses.
SUMMARY OF THE INVENTION
The invention provides a flywheel system for storage of energy from a power source during periods of low energy demand and subsequent discharge of this energy during periods of greater energy demand. The flywheel can also be used to provide a local source of energy during a supply outage, thereby ensuring an uninterruptable power source, or as a power quality device to maintain voltage and phase stability.
The flywheel of the system includes a circular composite ring having a radial width limited to a proportion of the radius of the ring to reduce internal radial stresses to below the transverse capability of the circumferentially wound high strength fibers. Thus, preferably the radial width is less than about 30% of the outside radius of the ring. The composite material of the ring includes circumferentially wound unidirectionally high strength fibers consolidated together in a matrix of thermoplastic resin. Because the ring is circumferentially wound, the high strength fibers are oriented to counteract hoop stresses that act on the ring when the flywheel rotates. High strength to weight metallic glass is an attractive alternative material for the flywheel.
The flywheel ring is supported by at least one spoke that extends along a diameter of the ring, one end of the spoke being fixedly attached to a location on the ring, and the other end of the spoke fixedly attached to a diametrically opposite location on the ring. The spokes of the ring are also fabricated from a composite that can include high strength fibers, a substantial proportion of which are aligned along a longitudinal axis of the spoke, that are consolidated together in a matrix of a thermoplastic resin. When the spokes are attached to the ring, the spokes are configured to have a degree of flexibility such that, when the flywheel rotates at increasing speed, and the ring is caused to expand radially outward under centrifugal force, the spokes grow radially to match the growth of the ring, without subjecting the ring to significant radial force at points of spoke attachment to the ring. Radial growth of the spoke is achieved under the action of centrifugal forces by a combination of bending deformation of the spoke, as it straightens from its initial “drooped” static configuration into a more nearly planar radial configuration, and axial elongation of the spoke. Al
Ahlstrom Harlow G.
Barton John R.
Chapman Michael
Garrigus Darryl F.
Luhman Thomas S.
Johnson Kindness PLLC
Mullins Burton
O'Connor Christensen
The Boeing Company
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