Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-12-20
2004-11-30
Nguyen, Tran (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S074000, C310S090000
Reexamination Certificate
active
06825588
ABSTRACT:
This invention pertains to flywheel energy storage systems used for prevention of power interruptions, and more particularly a flywheel uninterruptible power supply for reliably storing several kilowatt-hours of energy at low cost. The flywheel system uses a solid steel alloy cylinder for a flywheel, which is supported for high-speed rotation using simple passive radial magnetic bearings. Use of a solid flywheel construction with a restricted diameter, steel alloy composition and a fracture mechanics design approach allows for increased operating speed and achievement of energy densities far above those previously obtained with large steel flywheels. This increased energy storage capability makes the flywheel commercially viable for use in back up power applications, significantly reducing the cost of the total flywheel system. A process for manufacture of the flywheel cylinders is disclosed which provides quality assurance of flywheel integrity.
BACKGROUND OF THE INVENTION
Flywheel uninterruptible power supplies are becoming recognized as potentially viable economic alternatives to electrochemical batteries for prevention of power interruptions to critical loads. Electrochemical batteries used in these applications, and valve regulated lead acid batteries in particular, have many undesirable traits. The life of batteries is short, typically between 1 to 7 years depending on the environment and use. They require frequent periodic maintenance and inspection, are subject to thermal degradation and can fail unpredictably. Lead acid batteries and other types as well are also environmentally noxious. However, lead acid batteries are relatively inexpensive. Flywheel systems are thought to have promise to eliminate the disadvantages of batteries with the expectation of achieving 20 year lives with minimal or no maintenance, temperature insensitivity, previously unachievable reliability while being environmentally benign.
Flywheel uninterruptible power supplies use a rotating flywheel to store energy kinetically. A high-speed flywheel stores electrical energy in the rotating inertia of a flywheel. An attached motor/generator is used to accelerate and decelerate the flywheel for storing or retrieving energy. Flywheels can be either constructed of metal or of high strength composite materials. The flywheel can be supported for rotation on mechanical bearings, magnetic bearings or a combination. To reduce the losses from aerodynamic drag, the housing surrounding the flywheel can be maintained at a low pressure, or for slower flywheels it can be filled with a gas of small molecule size such as helium. Many designs of motor/generators exist and can be employed. Motor/generators can also be made as separate components.
Flywheel systems can be divided into two basic categories based on their desired function: power ride-through and energy back-up. A ride-thru system is typically designed for discharging a high level of power for a short duration of time until an auxiliary energy generating means such as a generator set can be brought online. Discharge times range from 10 seconds to about 2 minutes with power levels of up to several hundred kilowatts. Applications for ride-through flywheel uninterruptible power supplies include computer data centers and also critical manufacturing operations such as semiconductor processing. In marketing, ride-through flywheel systems can demand high prices because flywheel systems should have reliability and longevity advantages, and electrochemical batteries are inherently unsuitable and perform very poorly with repeated high power discharges.
The second category of flywheel systems, energy back-up, are used to provide power to support the load for the duration of a power interruption, until the utility power can be restored. Discharge times can be as much as 8 hours or more and the power levels are typically only a few kilowatts or less. Energy storage capacity though is large with multiple kilowatt-hours of storage. Promising applications for these systems are in telecommunications, for maintaining service reliability for telephone, cable TV, wireless and the Internet. Energy back-up flywheel uninterruptible power supplies are marketed based on their energy storage capacity, and because of the low power level, they compete with batteries primarily based on the increased longevity, higher reliability, and lower maintenance requirements. The more difficult cost targets for large energy back-up flywheel systems therefore make minimizing the cost per stored energy extremely important. The potential market for this application is enormous, so there has been considerable interest in developing flywheel energy back-up systems that would satisfy the industry requirements, all to no avail until now.
There are fundamentally two types of flywheel energy storage systems: low speed industrial steel flywheel systems and high speed composite flywheel systems. Commercial flywheel uninterruptible power supplies employing large steel flywheels currently operate with maximum tip speeds of only about 200 to 250 meters per second. The stored energy is proportional to the square of the tip speed and thus energy storage per flywheel size and weight is limited for flywheels with tip speed limited to 250 m/s. Strength and safety concerns have been factors that cause manufacturers to limit the operational speeds to 250 m/s or less. Small diameter steel flywheels can develop higher strengths due the fabrication attributes of the reduced size. For example, small diameter steel hubs for use inside composite energy storage flywheels have been laboratory tested to higher speeds. However to date, commercial operation of large diameter steel energy storage flywheels has been limited to relatively low speeds.
For efficiently storing large amounts of energy, especially in cost sensitive applications such as energy back-up, composite flywheels are commonly considered necessary. Composite flywheels can store large amounts of energy per weight due to the high strength capability of the constituent fibers such as glass and carbon. They can also be made of large diameter size while still having the maximum strength due to the strength being added by the already high strength fibers being wound into the rim. Composite flywheels have been very expensive in the past, however the price in recent years has been dramatically reduced due both to a drop in the price of carbon fiber and also the development of new more economical commercial processes.
Despite the benefits of increased energy storage capability with composite flywheels, they do have several undesirable traits. Composite materials outgass considerably due to the polymer matrix. The copious outgassing makes the maintainability of the vacuum in the surrounding enclosure difficult. Composite materials also experience a reduction in strength from the volumetric addition of resin with the high strength fibers and also some reduction occurs due to the ability of the matrix material, typically an epoxy, to translate the load from one fiber to others. A further reduction in strength results from the winding angle as fibers are wound onto the flywheel rim during manufacture and especially when multiple tows are used to make very large flywheels at low cost. When winding the fibers with the resin to form a composite flywheel rim, void flaws can be introduced into the parts. Despite these reductions in strength, composite material flywheels still have very high fiber direction strengths and for which the reductions can easily be accounted.
Unfortunately, composite material flywheels can exhibit some troublesome attributes that include poor temperature performance as well as creep and stress rupture, Most polymer matrix composite flywheels have low temperature capability, meaning that the epoxy matrix becomes soft at a relatively low temperature. The matrix loses its ability to optimally translate load between fibers with a relatively small increase in operating temperature. Because the radial strength is much lower than the hoop strength in filamen
Campbell David R.
Gabrys Christopher W
Neary J. Michael
Nguyen Tran
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