7.1 Flywheel energy storage: principles and components

Flywheel energy storage harnesses rotational kinetic energy to store and release power quickly. Key components include a rotor, bearings, containment system, and power electronics. These work together to efficiently convert electrical energy to mechanical energy and back.

The principles behind flywheel energy storage involve rotational kinetic energy and moment of inertia. Energy density and self-discharge rate are crucial factors in flywheel performance, with high-speed composite flywheels offering better energy storage capabilities than low-speed steel ones.

Flywheel Components

Rotor and Bearings

Top images from around the web for Rotor and Bearings

• 

  Kinetic energy recovery system - Wikipedia View original

  Is this image relevant?

• 

  Energy storage - Wikipedia View original

  Is this image relevant?

• 

  Rotational Kinetic Energy: Work and Energy Revisited | Physics View original

  Is this image relevant?

• 

  Kinetic energy recovery system - Wikipedia View original

  Is this image relevant?

• 

  Energy storage - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Rotor and Bearings

• 

  Kinetic energy recovery system - Wikipedia View original

  Is this image relevant?

• 

  Energy storage - Wikipedia View original

  Is this image relevant?

• 

  Rotational Kinetic Energy: Work and Energy Revisited | Physics View original

  Is this image relevant?

• 

  Kinetic energy recovery system - Wikipedia View original

  Is this image relevant?

• 

  Energy storage - Wikipedia View original

  Is this image relevant?

1 of 3

• 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
  • Mechanical bearings (ball bearings, roller bearings) are used in low-speed flywheels
  • Magnetic bearings (active magnetic bearings, superconducting magnetic bearings) are used in high-speed flywheels to reduce friction and wear

Containment and Power Electronics

• Vacuum enclosure 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
• Motor/generator converts electrical energy to kinetic energy during charging and kinetic energy back to electrical energy during discharging
  • Permanent magnet synchronous machines (PMSM) and induction machines (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
  • Bidirectional inverter 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 ($E_k$) is the energy stored in a rotating object and is given by the equation: $E_k = \frac{1}{2} I \omega^2$
  • $I$ is the moment of inertia, which depends on the mass and shape of the rotor
  • $\omega$ 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: $I = \frac{1}{2} m r^2$, where $m$ is the mass and $r$ is the radius
  • For a thin-walled cylinder: $I = m r^2$, 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