Handbook of Energy Storage for Transmission or ... - W2agz.com

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Friction and Energy Losses
In any real flywheel system, there are forces that act against the spinning wheel, causing
it to slow down and lose energy. These forces arise from friction between the rotor and
surrounding environment, between the rotor bearing and its support, and from the stresses
and strains within the rotor itself. In addition to these energy losses through friction, the
minute stress differentials within the spinning rotor and induced magnetic currents in the
motor/generator can also cause energy losses.
The mechanical bearings, which support the flywheel rotor, are a significant source of
friction. Many developers have introduced magnetic bearings into the flywheel system,
which remove load from mechanical bearings and reduce frictional losses.
The fluid surrounding the rotor is also a source of frictional loss. At higher speeds, this
loss can be very large indeed. Most developers have addressed this problem by enclosing
the rotor within a vacuum or low-viscosity fluid.
Thermal Effects
The energy lost during rotation is transformed into heat, which raises the temperature of
the flywheel rotor. If heat accumulates it must somehow be removed to prevent damage
to the rotor and other components. Material considerations will define a maximum
temperature for the rotor. One way to reduce heat is to limit the operating speed of the
flywheel system so that the steady-state temperature of the rotor is within a safe margin
of the maximum temperature. This speed limitation will also reduce the energy density
of the flywheel system.
The answer to this problem has been low-loss bearing technology, which has kept
thermal effects from being a limiting factor in most practical flywheel systems. Vacuum
containment and magnetic bearings can significantly reduce friction, and therefore reduce
the amount of heat that must be removed. The trade-off is that they also can make it
difficult to remove the heat that remains. In flywheels with bearing enhancements,
thermal energy normally leaves the rotor only through radiation, sometimes requiring
special heat removal methods within the enclosure.
Some manufacturers have chosen to include active cooling systems in their products,
through the use of a low viscosity gas in the containment system. Some investigators
have suggested hydrogen cooling, similar to the technique used for large electric
generators.
Subsystem and Components
A flywheel has several critical components. These components will be discussed in
further detail in the following subsections (See Figure 3).
• Rotor – a spinning mass that stores energy in the form of momentum
• Bearings – pivots on which the rotor rests
• Motor-Generator – a device that converts stored mechanical energy into electrical
energy, or vice versa
Flywheels Page 4