A flywheel power system is used in power conditioning and uninterruptable power supply (UPS) applications to provide short-term electricity to a system to either "ride out" brownouts, power dips, and short-term blackouts which may not cause the average household more than passing irritation, but they can cause significant loss to a server farm or a hospital.

The energy is stored in a spinning flywheel, either a huge, heavy one spinnning slowly or a light, strong one spinning quickly (the best type.) Either way, they are able to provide the kilowatts of power required for the 20 or 30 seconds needed to determine if the power interruption is going to be long enough to justify starting a power generator or turning on a fuel cell.

They are intended to replace batteries in such backup power applications because a battery backup able to provide the kilowatts of power needed to support vital systems for any useful length of time takes the form of racks and racks of heavy, expensive, and dangerous lead-acid cells that need to be maintained and monitored constantly to ensure that they are always ready, and require special environmental concerns for toxic outgassing and acid leakage control.

To add insult to injury, there is an initial dip in voltage output when the batteries first take a load, which means that for true uninterruptible power, they have to actually be part of the circuit at all times, which adds more wear-and-tear to the equation. (Even if they are just held ready, they deteriorate, but if they are used constantly, they deteriorate even faster.) Even then, every few years you have to throw them all away (hopefully to be recycled.)

A flywheel system has many advantages over batteries, in that there is no special environment required, they can be recharged as fast as they are discharged, and are "always on". For example, the VSS-120 Voltage Support Solution flywheel power system is able to provide 120 kilowatts for 20 seconds, is the size of a refrigerator, and can be placed in the same room as the operators and equipment. The core of the VSS-120 is a multi-layer carbon fiber composite flywheel with a titanium hub attached to a nickel alloy shaft. Three sets of magnetic bearings (five-axis) levitate the entire assembly for high rotational speeds without friction, lubrication, or maintenance, and an integrated pump maintains vacuum to eliminate aerodynamic drag over the flywheel’s lifetime of 15 years without the use of external mechanical parts.

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