|Fig. 1: An early example of a flywheel in the White and Middleton Engine. (Source: Wikimedia Commons)|
Flywheels have long been used to store energy in the form of rotational kinetic energy. While past applications of the flywheel have used conventional mechanical bearings that had relatively high losses due to friction, the development of magnetic bearings constructed using High Temperature Superconductors (HTSC) has greatly decreased the losses due to friction and increased efficiency immensely. While even modern application of magnetic bearings face the same safety problems that were faced at the turn of the century, the increased efficiency of magnetic bearings as well as their limited environmental harm make flywheels a promising modern energy storage device.  Further continued developments in HTSC and design improvements in hybrid bearings may see the development of economical flywheel systems for various applications in the near future.
The flywheel is a very basic conceptual machine that takes advantage of the conservation of energy by storing energy in the form of rotational kinetic energy. A basic flywheel is a device that has a large moment of inertia and by spinning around only one axis is used to store rotational energy
From the simple equation we see that the energy capacity of such a storage device relies on the moment of inertia of the wheel as well as the angular velocity. Modern flywheel applications utilizing high-Tc superconductor bearings and operating in vacuum can reach rpms between 23,000-40,000 with a maximum usable storage energy of 300 W h.  These modern applications face the same safety issues of the previous incarnations, though the improvements of mechanical bearings improve the efficiencies immensely.
Flywheel systems have various advantages, such as, long lifetimes, high energy density and large maximum power outputs. More advanced systems can accelerate up to speed in mere minutes, quicker than other forms of energy storage. Further, the modern FES applications, have a very limited cost on the environment, with zero CO2 emissions. 
|Fig. 2: G2 Flywheel Module. An example of a modern application of HTSC Flywheel system. The casing and refrigeration units are seen. (Courtesy of NASA)|
Different flywheel applications make use of either mechanical bearings or magnetic bearings. Magnetic bearings are much more attractive as they greatly reduce losses due to friction. Further magnetic bearings are able to operate in vacuum and lead to even better efficiency. The magnetic bearing support the rotor load through magnetic levitation rather than through any mechanical process. The unique property that superconducting material blocks the magnetic field from its interior means that it possesses complete diamagnetic properties and provides frictionless and stable levitation. While the refrigeration costs of Low Temperature Superconductors hurt the economic feasibility of FES, the development of High Temperature Superconductors, that operate in the 100-150 °K range.  With the development of high temperature superconductors, the cost associated with refrigeration go down significantly. Further since the importance of the HTSC lies in the restoring force that it offers due to flux pinning, the structural strength of the superconductor is less important than in superconducting transmission cable applications, where the fragile ceramic HTSC need to be upgraded. In FES applications, HTSC powders can be formed into any shape as long as the flux pinning is strong.
While the HTSC is important, they often cannot provide the lifting forces needed for large rotors, thus in most current applications a hybrid system with a permanent magnet that provides the lifting force and a superconductor that provides the restoring force is used.
As progress is made the HTSC field, the refrigeration cost associated with FES applications will continue to drop, improving their ability to compete economically.
FES systems have already been applied in the realms of transportation, space satellites, and uninterruptable power supplies. In the transportation realm, flywheels are being explored as possible replacements for batteries in electric cars. Further flywheels have been used to support major computing networks as they have limited maintenance needs and can provide load leveling for large battery systems.
With the improved developments of higher temperature superconductors, the feasibility of FES systems as possible replacements for chemical batteries continues to get better. While the superconductors themselves often drive much of the cost in superconductor applications, given that the HTSC need not be structurally rigid brings down this limitation. Rather in the case of FES, much of the limitations comes from the refrigeration units required to cool the materials down to their superconducting limit. As the Tc of superconductors increase, the economic viability of using flywheels for electric cars gets better.
At the same time though, the cost of the materials themselves may continue to be a limiting factor. Further, the safety issues that were present even at the turn of the century are still there today. With a wheel spinning at very high rpms, if the stability of the rotor breaks, we are talking about a tremendous release of kinetic energy. Therefore until further developments are made in terms of casing and protection, it seems that we will not be seeing many applications of FES that cannot be safely secured against mechanical failure and flywheel explosion.
© Tim Haefele. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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