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How Do Flywheel Energy Storage Systems Work?

Flywheel Energy Storage Systems turn electrical energy into kinetic energy by accelerating a wheel very fast and harvesting energy later by slowing it down. 

  • Pros: high power density, long lifetime, high efficiency, carbon-free
  • Cons: mechanical stress, material durability limits, shorter discharge times

FESS Interior Design

  • A flywheel contains a dual-function electrical motor to store and generate energy
  • When discharging energy rotor is turned on and slows down the motor
  • The rotor acts as a generator and outputs electrical energy
  • Bearings are put in place to resist rotational forces to maintain their position
    • Problem: requires high-performance lubricants and frequent maintenance
    • Solution: magnetic bearings are used so that there’s no friction
  • The enclosure acts as a container for the FESS
  • The power electronic converter converts kinetic energy into electrical energy
  • Limitations
    • Speeding up the flywheel increases the centrifugal force (F = mw2r) which strains the material → high tensile strength in the material is required
    • Resistance of the material limits the amount of energy able to be stored

Rotational Energy Equation: K = 0.5Iw2 

  • K = Rotational Energy
  • I = Rotational Inertia
  • w = Angular Velocity
  • Rotational mass is proportional to rotational inertia

Low-Speed vs. High-Speed FESS

  • Low-Speed FESS: made up of steel
    • Spin at up to 10,000 RPM
    • Housed in a traditional smaller scale mechanical enclosure
  • High-Speed FESS: made up of carbon/fiberglass (high strength-to-weight ratio)
    • Spin at up to 100,000 RPM
    • Housed in a vacuum utilizing magnetic bearings to minimize friction


  • FESS costs on average $1,500-$6000/kWh (2016)
  • Largely due to installation costs with very few maintenance costs
  • Li-ion costs on average $200-1,260/kWh (2016)
  • Hybrid FES/Li-ion Systems can potentially reduce costs

Engineering FESS from Scratch

Koko Xu

Lover of international cuisines and Class B chess player.