Flywheel Technology


The Basics

Flywheel systems store kinetic energy (energy of mass in motion) by constantly spinning a compact rotor in a low-friction environment.

A flywheel’s stored kinetic energy is proportional to the mass of its rotor, the square of its radius, and the square of its rotational speed (RPM).

Ek = K ⋅ mass ⋅ radius2 ⋅ RPM2

All About Balance

Developing the optimal flywheel for a given application requires carefully balancing numerous factors. Increasing the rotational speed of the flywheel, for example, increases stored energy, but also increase the stress on the flywheel, requiring the use of stronger and more expensive material for the rotor. Similarly, a heavier or larger diameter wheel will increase energy storage, but perhaps with an unacceptable tradeoff in system size or transportation and installation costs.

Active Power’s CleanSource flywheels balance weight, size, speed, stress, and material strength to support a stored energy and cost profile optimized for our target markets in data center, health care, and industrial and manufacturing applications.


CleanSource Flywheel

Active Power’s CleanSource flywheel technology integrates a motor, flywheel energy storage, and generator into an elegant and economical solution.


For 99% of its operating life, the flywheel machine is a motor. Roughly 1 kW of input power is used to keep the rotor spinning at 7,700 RPM.

Energy Storage.

As an energy storage system, the flywheel rotor spins at a constant speed to store kinetic energy.


When short-term backup power is required because utility power is lost, the flywheel becomes a generator: inertia allows the rotor to continue spinning and the kinetic energy is converted to electricity. Power electronics in the system extract that energy and discharge it as electric power to support the load.


Magnetic Unloading

The flywheel rotor is partially supported by magnetic unloading technology. This technology unloads a majority of the flywheel’s weight from the field-replaceable mechanical bearing cartridge, extending its operational life.

7,700 RPM

The single-piece forged 4340 steel rotor spins at 7,700 RPM (584 MPH tip speed) which stores up to 10.5 MJ of energy.

Rough Vacuum

A vacuum pump evacuates the chamber, reducing the drag on the rotor to improve efficiency and extend system life. The air pressure within the housing is held under 2 mTorr.


XT Series Flywheel

(250 kW for 25 seconds)

Used in CleanSource XT275 UPS

The HD Series Flywheel

(675 kW for 15 seconds) Used in the CleanSource HD675 UPS

4.4" high; 25.5" in diameter 600 lbs rotating mass 6.2 MJ of stored energy


12.5" high; 25.5" in diameter 1700 lbs rotating mass 10.5 MJ of stored energy