Energy_Storage_Technologies.pdf

Mechanical bearings on a number of system

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mechanical bearings on a number of system, significantly reducing or eliminating the frictional losses, and therefore the self-discharge. To determine how much energy a particular device may hold, refer to the equation for the kinetic energy of a spinning mass: E = 1 2 2 , where I is the moment of inertia and for a solid rotating disc is defined as I = 1 2 mr 2 ( m is mass of the disc, and r is the radius of the disc), and ω is the rotational velocity. For a solid rotating disc the above equation becomes: E = 1 4 m ( ) 2 . This implies that by increasing the maximum speed of the disc the energy capacity is more greatly increased than by increasing the mass of the disc [2]. Variations These devices are categorized into low-speed and high speed designs. Low Speed Most low-speed designs are 10,000 rpm or less, and are typically made of extremely heavy steel discs. The shaft is either vertical or horizontal, and may have mechanical or magnetic bearings. High Speed High-speed designs operate above 10,000 rpm, some upwards of 100,000 rpm. Because of the speeds, and associated fatigue failure risks, stronger materials are required, including composites of graphite or fiberglass and therefore also require magnetic bearings and a vertical shaft [2]. System Design Considerations Since flywheels will not necessarily be turning at the correct speed to allow the generator to produce a 60Hz waveform, the output must be processed to resolve this issue. Typically a flywheel will decrease in speed as it is discharging energy, and so to account for this, the AC power out of the generator is typically converted to DC then back from DC to a 60 Hz AC waveform through the use of power electronics, ensuring a consistent output waveform without requiring the flywheel to always be spinning at the right speed. Another design consideration is that a source of loss is the fluid that the device rotates in. If the rotor is surrounded by air, or another highly viscous fluid, that will be an additional source of friction. Many times the system is enclosed in a vacuum to further reduce such frictional losses. Lastly, in sizing a system, power and energy capacity can be treated as essentially independent. Power capacity is determined by the power conversion system, and the motor and generator, while the energy capacity is determined by the flywheel mass and speed. Many flywheels are designed to provide high power output for short periods, on the order of 5 to 50 seconds [2]. Operation & Maintenance The component that requires the most maintenance would be the bearings. Typically magnetic bearings are complex systems requiring some care to operate and maintain. Since these devices have hazardous failure modes inspecting these devices for signs of fatigue is critical for preventing catastrophic failure.
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