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

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Power electronics have won this competition in all the high-speed wheels on the market.
However several high-power, low-speed systems use electromechanical couplings to
isolate the shaft and effectively couple an accelerating and decelerating flywheel with a
constant-speed generator. Whether electronic or mechanical the main function of these
devices is to allow energy to be taken from the wheel before its frequency and power
output drop below usable levels. In fact the low-end (i.e., end-of-discharge) cutout speed
at which the flywheel is considered discharged is primarily dependent on the current
carrying capability of the electronics (or electromechanical coupling) and the size of the
load. For example, most flywheels have output current proportional to load and inversely
proportional to speed. This means a lighter load can go to a lower speed before the
system cuts out on maximum current. The flywheel system can actually deliver 1.5 to 2
times more energy at light load than high load as can be seen in performance curves.
While for different reasons, in this feature the flywheel is similar to a conventional leadacid
battery.
When power electronics are used the variable frequency, ac output of the flywheel
alternator is simply rectified providing a dc voltage and current. From this point a power
electronic inverter may be used to recreate ac at the desired waveform, frequency and
voltage. So the function of the power electronics is to couple the fixed-frequency ac
electrical grid with the variable-speed flywheel and also to invert, regulate, and provide
wave shaping for the ac electrical output of the system. By reversing the process the
power electronics are also able to draw power from the ac line connection, and drive the
flywheel motor to spin up and recharge the wheel. In some cases a power electronics
system recharges the flywheel and can use energy drawn from the electric grid. The most
common power electronic systems use two matching bi-directional or 4-quadrant
converters to carry out all of the functions described here.
Controls and Instrumentation
Flywheel systems require some controls and instrumentation to operate properly.
Instrumentation is used to monitor critical variables such as rotor speed, temperature, and
alignment. Rotor speed and alignment are also often controlled variables, through active
feedback loops. The latter is especially important for systems with magnetic bearings,
and most magnetic systems have complex controls to reduce precession and other
potentially negative effects on the rotor.
In many systems, other instrumentation is used to monitor performance or design
parameters related to failure modes. In some composite flywheel systems, for example,
instrumentation is used to measure deformation of the rotor over time, alerting operators
when the rotor shape indicates possible failure in the future.
System Packaging
Nearly all-modern flywheel systems have some type of containment for safety and
performance enhancement purposes. This is usually a thick steel vessel surrounding the
rotor, motor-generator, and other rotational components of the flywheel. In the event of
catastrophic failure, the containment vessel would stop or slow parts and fragments,
preventing injury to bystanders and damage to surrounding equipment.
Flywheels Page 9