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EPRI Proprietary Licensed Material magnetic bearings. These bearings would enable much smaller standby losses than even HTS bearings, and would greatly increase the reliability and simplify the control systems of high-speed flywheels. Composite Materials Another area of intense interest to flywheel developers and manufacturers is the use of new composite materials in flywheel design. The term “composite material” is used to describe materials which have complex microstructures, often consisting of several materials in combination, and which defy classification by composition, crystal structure, or physical properties. The key point is that composites have a higher tensile strength and are lighter than steel. The expected benefit in flywheel applications is an order of magnitude increase in the practical wheel speed. In the case of flywheels, the most common composite materials under consideration are graphite fiber composites and glass fiber composites. Both these materials are typically composed of fine fibers in a parallel arrangement, held together by a binder. This arrangement produces low-weight materials with very high tensile strength - often greater than that of steel - in the direction of the fibers. In flywheel applications, the lighter weight reduces the hoop and radial stresses within a spinning rotor at a given radial velocity, and the higher tensile strength allows a much higher maximum stress before yield. These effects combine to allow composite rotors to achieve much higher rotational speeds (and therefore, higher energy storage potential) than steel rotors. At present, the specific research with respect to flywheels is generally dedicated to characterization, adaptation and qualification of new materials to a rotary application. Composite materials do not act linearly. As a result they tend to be much more difficult to characterize and understand than materials such as steel. The adaptation of such materials to rotary applications can also be difficult. For example, fiber composite rotors cannot be cut out of a block of existing material; they must be constructed in such a way that fibers are wound around the circumference of the rotor, to absorb maximum stress. For these reasons, research into new rotor materials can be costly and time-consuming. Research Activities There are three major directions in flywheel rotor material research. The first is towards stronger, lighter materials, which allow higher rim speeds and energy density. The second is towards cost reduction of composite rotors. The third is towards more effective and more repeatable manufacturing techniques, producing safer and more reliable rotors. In general, all the material developments involve these three directions to some degree. The Composites Technology Center at Pennsylvania State University in University Park, PA has concentrated its efforts in flywheel technology on the development of lighter, stronger, and cheaper composite rotors. Penn State investigators are working on perfecting a carbon filament winding system, which will improve the strength of rotors while allowing the use of cheaper carbon materials. They have also been involved in investigations into the use of exotic materials such as carbon filaments, carbon nanotubes, and higher temperature materials, all of which can improve flywheel performance in the future. Flywheels Page 22
EPRI Proprietary Licensed Material The University of Texas Center for Electromechanics in Austin, TX has focused on modeling and characterization as steps towards the development of a significant prototype project, a 2MW, 480MJ flywheel for the Advanced Locomotive Propulsion System (ALPS). University of Texas investigators have made significant progress into software modeling of the stress and dynamics characteristics of composite rotors. They have also developed new non-destructive characterization tools to understand performance of new materials and geometries. They are now investigating other effects on high-speed flywheels, such as the effect of thermal conditions, particularly in a nearvacuum environment, and behavior under non-standard conditions such as magnetic bearing failure. Applications Flywheels have inherent appeal due to the sheer simplicity of storing kinetic energy in a spinning mass. For decades piston engines have used this concept to smooth power output and operation. In short-duration electrical applications they are competitive with traditional energy-storage technologies such lead-acid batteries. Advances in power conversion technologies coupled with advances in flywheel designs have paved the way for new applications for flywheel-based energy storage. Today flywheel systems are capable of supplying megawatts for a few seconds with very fast response and low energy losses. This section describes two primary applications related to utility transmission and distribution operations that are likely to benefit from emerging flywheel (or other) energy storage technologies. The first application is energy storage for voltage support or grid stabilization. The second is energy storage as a supplemental source of current to serve power demand of highly fluctuating loads. A third application is mentioned here for completeness, but is not further developed. The application is bridging power for uninterrupted power to critical substation loads. This is somewhat rare in that most substation critical loads, except telecom loads, are supported by station batteries. In this application the energy storage is usually configured to provide short-term bridging power that carries the load from loss of primary power until a standby generator can be started, or until, recovery of the primary source. Voltage Stabilization Support to the Electric Grid Problem Description Need for system stability, in both central and distributed power systems, as well as the specific functions of reactive power supply and frequency regulation support, are considered here. The reactive power control application solves the problem of VAR control to maintain power flow and voltage stability. The frequency regulation application solves the problem of controlled injection needed to regulate system frequency. The later is particularly relevant for improving stability of relatively weak areas in the T&D. These applications are cited as ancillary services in FERC order 888 1996, and will eventually carry a location dependent market value in a restructured utility situation. Flywheels Page 23
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