A flywheel energy storage machine stores electricity by using it to spin a heavy rotor at very high speed. The energy is held as rotational kinetic energy, the same kind of energy found in a spinning wheel or turbine. Flywheels matter in renewable energy systems because solar panels and wind turbines can change output quickly.
A flywheel can smooth those changes by absorbing extra power and returning it within seconds.
Key Facts
- Rotational kinetic energy: E = 1/2 Iω^2
- Moment of inertia for a solid disk: I = 1/2 MR^2
- Angular speed conversion: ω = 2πf, where f is rotations per second
- Power is the rate of energy transfer: P = E/t
- A flywheel stores more energy when its moment of inertia or angular speed increases.
- Low-friction bearings and vacuum housings reduce energy losses during storage.
Vocabulary
- Flywheel
- A rotating mass designed to store energy as rotational kinetic energy.
- Rotor
- The spinning part of a flywheel machine that contains most of the stored energy.
- Moment of inertia
- A measure of how strongly an object resists changes in its rotational motion.
- Angular speed
- The rate at which an object rotates, usually measured in radians per second.
- Motor-generator
- A device that can use electricity to spin the flywheel and can also convert the flywheel's motion back into electricity.
Common Mistakes to Avoid
- Treating flywheel energy as chemical battery energy is wrong because a flywheel stores energy mechanically in motion, not in chemical bonds.
- Forgetting to square angular speed in E = 1/2 Iω^2 is wrong because doubling the spin speed makes the stored energy four times larger.
- Using rotations per minute directly as ω is wrong because the energy formula needs angular speed in radians per second.
- Assuming a flywheel can store energy forever is wrong because friction, air drag, electrical losses, and bearing losses slowly reduce stored energy.
Practice Questions
- 1 A flywheel has a moment of inertia of 8.0 kg m^2 and spins at 300 rad/s. How much rotational kinetic energy does it store?
- 2 A flywheel delivers 120,000 J of energy to a power system in 4.0 s. What average power does it supply?
- 3 A solar farm briefly produces more power than the grid needs. Explain why a flywheel energy storage machine is useful for this situation and why it is especially good for short bursts of power.