08-05-2013, 02:47 PM
Flywheel Energy Storage
Flywheel Energy.pdf (Size: 1.1 MB / Downloads: 216)
Executive Summary
Flywheels have been around for thousands of years. The earliest application is likely the potter’s wheel. Perhaps the most common application in more recent times has been in internal combustion engines. A flywheel is a simple form of mechanical (kinetic) energy storage. Energy is stored by causing a disk or rotor to spin on its axis. Stored energy is proportional to the flywheel’s mass and the square of its rotational speed. Advances in power electronics, magnetic bearings, and flywheel materials coupled with innovative integration of components have resulted in direct current (DC) flywheel cantly increasing battery life. A flywheel could also be used alone for applications where longer-term backup capability is not required or economically justified. Variations In general, flywheels can be classified as low speed or high speed. The former operate at revolutions per minute (rpm) measured in thousands, while the latter operate at rpm measured in the tens of thousands. As noted above, doubling the rpm quadruples the stored energy, all else equal, so increasing rpm significantly increases the energy density of a flywheel. Operating at higher rpm necessitates fundamental differences in design approach. While low-speed flywheels are usually made from steel, high-speed flywheels are typically made from carbon or carbon and fiberglass composite materials that will withstand the higher stresses associated with higher rpm. Higher rpm also creates greater concern with friction losses from bearings and air drag. High-speed flywheels universally employ magnetic bear-ings and vacuum enclosures to reduce or eliminate the two sources of friction. Magnetic bearings allow the flywheel to levitate, essentially eliminating frictional losses associated with conventional bearings. While some low-speed flywheels use only conventional mechanical bearings, most flywheels use a combination of the two bearing types.
Where to Apply
DC flywheel energy storage systems are
generally more reliable than batteries,
so applicability is mostly an issue of
cost-effectiveness. Batteries will usually
have a lower first cost than flywheels,
but suffer from a significantly shorter
equipment life and higher annual operation
and maintenance expenses.
What to Avoid
UPS batteries are sized to provide
backup power for periods measured
in minutes. The period ranges from
about 5 minutes up to around 1 hour,
but is commonly about 15 minutes. A
period of 15 minutes, more or less, is
generally presumed adequate to allow
an orderly shutdown of equipment
Conclusion
Flywheels appear poised to replace or
supplement batteries as a backup power
supply in UPS systems. Six companies
currently offer DC flywheel energy
storage products. Another half dozen
or so are developing products they
expect to bring to market within the
next few years. Still others offer products
where the flywheel is an integral
part of the UPS system rather than
being a direct substitute for batteries.
Flywheel Energy.pdf (Size: 1.1 MB / Downloads: 216)
Executive Summary
Flywheels have been around for thousands of years. The earliest application is likely the potter’s wheel. Perhaps the most common application in more recent times has been in internal combustion engines. A flywheel is a simple form of mechanical (kinetic) energy storage. Energy is stored by causing a disk or rotor to spin on its axis. Stored energy is proportional to the flywheel’s mass and the square of its rotational speed. Advances in power electronics, magnetic bearings, and flywheel materials coupled with innovative integration of components have resulted in direct current (DC) flywheel cantly increasing battery life. A flywheel could also be used alone for applications where longer-term backup capability is not required or economically justified. Variations In general, flywheels can be classified as low speed or high speed. The former operate at revolutions per minute (rpm) measured in thousands, while the latter operate at rpm measured in the tens of thousands. As noted above, doubling the rpm quadruples the stored energy, all else equal, so increasing rpm significantly increases the energy density of a flywheel. Operating at higher rpm necessitates fundamental differences in design approach. While low-speed flywheels are usually made from steel, high-speed flywheels are typically made from carbon or carbon and fiberglass composite materials that will withstand the higher stresses associated with higher rpm. Higher rpm also creates greater concern with friction losses from bearings and air drag. High-speed flywheels universally employ magnetic bear-ings and vacuum enclosures to reduce or eliminate the two sources of friction. Magnetic bearings allow the flywheel to levitate, essentially eliminating frictional losses associated with conventional bearings. While some low-speed flywheels use only conventional mechanical bearings, most flywheels use a combination of the two bearing types.
Where to Apply
DC flywheel energy storage systems are
generally more reliable than batteries,
so applicability is mostly an issue of
cost-effectiveness. Batteries will usually
have a lower first cost than flywheels,
but suffer from a significantly shorter
equipment life and higher annual operation
and maintenance expenses.
What to Avoid
UPS batteries are sized to provide
backup power for periods measured
in minutes. The period ranges from
about 5 minutes up to around 1 hour,
but is commonly about 15 minutes. A
period of 15 minutes, more or less, is
generally presumed adequate to allow
an orderly shutdown of equipment
Conclusion
Flywheels appear poised to replace or
supplement batteries as a backup power
supply in UPS systems. Six companies
currently offer DC flywheel energy
storage products. Another half dozen
or so are developing products they
expect to bring to market within the
next few years. Still others offer products
where the flywheel is an integral
part of the UPS system rather than
being a direct substitute for batteries.