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

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usually designed for high-power output, while high-speed composite rotor systems can be
designed to provide either high power or high-energy storage.
When designed for power, where electric power conversion is adequately sized,
flywheels can deliver relatively high kW for a short period of time. Most power flywheel
products presently available provide from 100 to 500kW for a period of time ranging
between 5 and 50 seconds. The power capability of flywheel systems can be far larger
than these commercial systems, however. The largest flywheel built to date is an 8000MJ
system built by the Japan Atomic Energy Research Institute (JAERI) for use in fusion
energy research. This system uses a steel wheel to deliver up to 340MW for as long as
30 seconds.
Energy and Efficiency
Energy depends on mass and rotational velocity. High RPM, sometimes considered a
measure of technical sophistication, is only part of the equation for high energy. For
flywheels, the important parameter is rotational velocity or rim surface speed, which is
circumference times RPM. For example, a small 0.5 kWh flywheel has a relatively small
diameter rotor and may spin at 100,000 RPM; whereas a heavier 6-kWh flywheel has a
bigger diameter rim and maintains the same rim surface speed at only 20,000 RPM.
Therefore flywheel systems designed for high energy as opposed to high power tend to be
larger diameter, taking advantage of weight and increased rim speed. However this
advantage is physically limited to a speed of about 2 km/second by practical material
strengths.
Round-trip efficiency and standby power loss become critical design factors in energy
flywheel design since losses represent degradation of the primary commodity provided
by the storage system (energy). However, they are largely irrelevant in power flywheel
design (although standby losses could be an “operating cost” factor in comparison with
other power technologies that have significantly lower losses). For these reasons, energy
flywheels require more advanced technologies than power flywheels. These energy
flywheels usually have composite rotors enclosed in vacuum containment systems, with
magnetic bearings. Such systems typically store between 0.5 and 10 kWh. The largest
commercially available systems of this type are in the 2-6 kWh, with plans for up to 25
kWh.
Round-trip efficiency for energy flywheels is between 70 and 80%. The standby losses
are very small, typically less than 25W per kWh of storage and in the range 1 – 2% of the
rated output power.
Calendar and Cycle Life
The nature of flywheel systems means that there is at least one moving part, the rotor
itself. As might be expected, the most important life-limiting parts are the bearings on
which the rotor rests. Continuous operation of a flywheel, even if it is not cycled, will
eventually lead to deterioration of these bearings. Some designers have attempted to
mitigate this life-limiting issue by either augmenting or entirely replacing mechanical
bearings with magnetic bearings.
Flywheels generally exhibit excellent cycle life in comparison to other energy storage
systems. Most developers estimate cycle life in excess of 100,000 full charge-discharge
Flywheels Page 11