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Determination of Moment of Inertia & Radius of Gyration of Flywheel (Part – IV) 

The expense of refrigeration led to the early dismissal of low temperature superconductors for use in magnetic bearings. High-temperature superconductor (HTSC) bearings however may be economic and could possibly extend the time energy could be stored economically. Hybrid bearing systems are most likely to see use first. HTSC bearings have historically had problems providing the lifting forces necessary for the larger designs, but can easily provide a stabilizing force. Therefore, in hybrid bearings, permanent magnets support the load and HTSC are used to stabilize it. The reason superconductors can work well stabilizing the load is because they are good diamagnets. In hybrid-bearing systems, a conventional magnet levitates the rotor, but the high temperature superconductor keeps it stable. If the rotor tries to drift off center, a restoring force due to flux pinning restores it. This is known as the magnetic stiffness of the bearing. Rotational axis vibration can occur due to low stiffness and damping, which are inherent problems of superconducting magnets, preventing the use of completely superconducting magnetic bearings for flywheel applications.

Since flux pinning is the important factor for providing the stabilizing and lifting force, the HTSC can be made much easier for FES than for other uses. HTSC powders can be formed into arbitrary shapes so long as flux pinning is strong. An ongoing challenge that has to be overcome before superconductors can provide the full lifting force for a FES system is finding a way to suppress the decrease of levitation force and the gradual fall of rotor during operation caused by the flux creep of SC material.

Parasitic losses such as friction, hysteresis and eddy current losses of both magnetic and conventional bearings in addition to refrigerant costs can limit the economical energy storage time for flywheels. However, further improvements in superconductors may help eliminate eddy current losses in existing magnetic bearing designs as well as raise overall operating temperatures. Even without such improvements, however, modern flywheels can have a zero-load rundown time measurable in years.

Advantages of Flywheel: Flywheels store energy very efficiently (high turn-around efficiency) and have the potential for very high specific power compared with batteries. Flywheels have very high output potential and relatively long life. Flywheels are relatively unaffected by ambient temperature extremes.

 Disadvantages of Flywheel: Current flywheels have low specific energy. There are safety concerns associated with flywheels due to their high speed rotor and the possibility of it breaking loose and releasing all of it's energy in an uncontrolled manner. Flywheels are a less mature technology than chemical batteries, and the current cost is too high to make them competitive in the market.

Compiled by Samar Das