7.1 Flywheel energy storage: principles and components
3 min read•august 7, 2024
harnesses rotational to store and release power quickly. Key components include a , , containment system, and . These work together to efficiently convert electrical energy to mechanical energy and back.
The principles behind flywheel energy storage involve rotational kinetic energy and . and are crucial factors in flywheel performance, with offering better energy storage capabilities than low-speed steel ones.
Flywheel Components
Rotor and Bearings
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Rotor stores kinetic energy in its rotation and is made of high-strength materials (steel, composite materials) to withstand high rotational speeds
Rotor shape is designed to maximize moment of inertia for a given mass and volume, which increases energy storage capacity
Bearings support the rotor and minimize friction losses during rotation
(, ) are used in low-speed flywheels
(, ) are used in high-speed flywheels to reduce friction and wear
Containment and Power Electronics
surrounds the rotor to minimize air drag and reduce self-discharge losses
Enclosure is typically made of high-strength materials (steel, composite materials) to contain potential rotor failures
converts electrical energy to kinetic energy during charging and kinetic energy back to electrical energy during discharging
(PMSM) and (IM) are commonly used motor/generator types in flywheel systems
Power electronics control the flow of energy between the flywheel and the electrical grid or load
converts AC to DC during charging and DC to AC during discharging
Voltage and frequency regulation ensure compatibility with the connected electrical system
Flywheel Energy Storage Principles
Rotational Kinetic Energy and Moment of Inertia
Rotational kinetic energy (Ek) is the energy stored in a rotating object and is given by the equation: Ek=21Iω2
I is the moment of inertia, which depends on the mass and shape of the rotor
ω is the angular velocity of the rotor
Moment of inertia (I) quantifies an object's resistance to rotational acceleration and is determined by the mass distribution relative to the axis of rotation
For a solid cylinder: I=21mr2, where m is the mass and r is the radius
For a thin-walled cylinder: I=mr2, which maximizes moment of inertia for a given mass
Energy Density and Self-Discharge Rate
Energy density is the amount of energy stored per unit mass or volume of the flywheel
High-speed flywheels (made of composite materials) can achieve energy densities of 100-200 Wh/kg
Low-speed flywheels (made of steel) typically have energy densities of 5-30 Wh/kg
Self-discharge rate is the rate at which a flywheel loses stored energy over time due to friction and other losses
Flywheels with magnetic bearings and vacuum enclosures can achieve self-discharge rates of less than 1% per hour
Flywheels with mechanical bearings and no vacuum enclosure may have self-discharge rates of 5-20% per hour