Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-05-23
2003-06-24
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C074S572200, C523S468000, C528S120000, C528S122000, C528S123000
Reexamination Certificate
active
06583528
ABSTRACT:
This invention pertains to high performance composite flywheels, and more particularly a flywheel with a rim and integral hub for a high speed, large energy storage flywheel energy storage system with significantly reduced costs. The flywheel includes a two ring, standard modulus carbon fiber rim that is press-fit together and pressed over a tapered solid steel hub. The flywheel rim rings are filament wound carbon fiber in an epoxy resin matrix that allows increased operating temperature and radial strength of the rims with simultaneous manufacturing ease and use of low cost constituent materials.
BACKGROUND OF THE INVENTION
Modern flywheel energy storage systems convert back and forth between a spinning flywheel's rotational energy and electrical energy. A flywheel energy storage system includes a flywheel, a motor/generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor/generator converts energy between electrical and mechanical, and the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low pressure enclosure to minimize aerodynamic losses that would occur from operation in the atmosphere, while low speed systems can be operated at atmosphere.
Several types of flywheel designs are used for energy storage; they can be classified into three groups based on their design attributes: low performance industrial, high performance industrial and aerospace. Low performance industrial flywheels are normally constructed of steel and all the energy stored in the flywheel is stored in the rotational inertia of the low cost spinning steel flywheel. The performance of existing steel flywheels in terms of tip speed and energy per unit weight is low, primarily because they are generally limited to tip speeds around 200 m/sec or less. However, the advantages of low performance industrial (steel) flywheels are that they are relatively simple and low cost.
The second group of energy storage flywheels, high performance industrial, uses composite materials for increased flywheel performance. Filament wound glass fibers in conjunction with low cost standard modulus carbon fibers in an epoxy matrix are typically used to form the energy storage rim. The flywheels are typically designed so that the rim stores most of the flywheel's energy, usually more than 90%. Therefore, the goal of designers has been to minimize the cost of the rim by using the lowest cost materials and manufacturing techniques for construction of the rim. However, the use of the lowest cost fibers for the rim results in an undesirable attribute: a rim with significant radial growth when spun to high speed. The large growth is due to the lower elastic modulus of the low cost fibers. For example, glass fiber has an elastic modulus of about 10-13 million pounds/square inch (msi) compared to the elastic modulus of standard modulus carbon fiber of about 30-39 msi. A lightweight hub is used to couple the rim to the shaft. The hub must be designed to match the substantial growth of the inner periphery of the low cost composite rim, so that the rim will not grow radially away from and become separated from the hub, but instead remain connected to the hub at high speed. Most flywheel designs use machined aluminum hubs and less frequently hubs made from composite materials. Some hubs contain thin bending elements that allow the hub to grow with the rim. Metallic hubs are typically made from aluminum instead of steel because of the higher coefficient of thermal expansion. This higher coefficient of thermal expansion facilitates shrink fit installation of the hub and allows for approximately twice as much precompression of the hub so that it can follow the large rim growth at high speed. In some designs, the space available inside the flywheel rim inner diameter can be used for integral placement of the motor/generator or bearing system.
The design goal for high performance industrial flywheels is improvement over the performance of steel flywheels while maintaining relatively low cost. These high performance flywheels have higher tip speeds than the old low performance industrial flywheels, ranging from 500 to 1000 m/sec. The benefits of this higher rotation speed capability include storage of more energy, reduced bearing loads due to lighter flywheel weight, and the use of smaller or more powerful motor/generators due to the higher operating frequency.
Aerospace flywheels are designed for use in satellites and other space applications including space station power storage. The high launch costs for lifting objects into space make the design goals of this group of flywheels unique compared with industrial flywheels. The relative importance of the cost of the actual flywheel is insignificant compared to its performance, that is, the energy to weight and energy to size ratios. These flywheels are constructed of mostly if not all composite materials. Designs however focus on using the highest strength, more expensive intermediate modulus carbon fibers (40-50 msi). In many cases, the designs also concentrate most of the material comprising the flywheel to close to the outer diameter. This increases the inertia and energy storage of the flywheel while minimizing its weight. A radially preloaded press-fit construction in conjunction with use of the highest strength carbon fibers further allows for higher tip speeds, usually over 1000 m/sec, hence more energy storage per unit weight. Hubs, if metallic, are made small to reduce the flywheel weight. Flywheels used for defense applications such as mobile rail gun compulsators have many of the same or similar design and cost objectives and should be included in the aerospace category.
Of the composite material flywheels, various designs have been proposed and constructed, dating back to the 1970's. Many designs for these high-speed energy storage flywheels included filament wound composite rings which are made of either glass and or carbon fibers in an epoxy matrix. These rings are usually wound with the fibers in the hoop direction and sometimes with a small amount of axial direction reinforcement added. Radial direction fibers are not added with filament winding. Such filament wound rings have the inherent advantage of very high hoop direction strengths, which are needed to match the very high hoop stresses generated during rotation. One drawback to the use of hoop wound composite rings for the rim portion of a high-speed flywheel is the inherently low radial tensile strength resulting from the absence of fiber reinforcement in that direction. It is therefore desirable to construct flywheels that operate in radial compression instead of radial tension. Because the radial direction stresses in a rotating filament wound ring are controlled by the non-dimensionalized radial thickness of the ring (ratio of ID to OD), such rings must be made radially thin to reduce the radial tensile stresses generated. Because a single ring must be made very thin (ratio of ID to OD≈0.8) so that it does not fail at a prematurely low rotational speed, the ring becomes less effective for energy storage.
To increase the effective ring thickness and hence the energy storage capacity of composite rims, a common approach for high performance industrial flywheels is to construct the rim of several different material rings. The rim is designed with the material having the lowest ratio of elastic modulus to density (hereinafter referred to as “specific stiffness”) at the inner diameter. This is usually glass fibers. The material used in the rim rings surrounding the inner ring is progressively higher specific stiffness materials (usually standard modulus carbon fibers) toward the outer diameter. Materials with higher specific stiffness will grow less than materials with lower specific stiffness when subjected to high-speed rotation. Therefore, the hybrid material composite rim with radially graduated specific stiffness precludes generation of unacceptable radial tensile stresses during r
Indigo Energy, Inc.
Mullins Burton S.
Neary J. Michael
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