---
title: "Short note: Flywheel energy storage"
subject: "Energy Environment and Society"
chapter: 5
key: "semester_7/energy_environment_and_society:flywheel-storage"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/flywheel-storage
site: BCT Engineering
---

# Short note: Flywheel energy storage

Energy Environment and Society · Chapter 5: Energy Storage

- Asked in: year not given
- Video: [Can Spinning Store Energy? Let’s Explore Flywheels | Explore A Lab: Tour 3](https://www.youtube.com/watch?v=qw85rIgpffM) — Future Energy Systems
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/flywheel-storage
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-5

## Question

Write a short note on flywheel energy storage.

## Short answer

Flywheel energy storage uses a high-speed rotor in a vacuum to store kinetic energy. It delivers very fast power response, high efficiency (85–95%), and over 100,000 cycles, but has low energy capacity and high standby losses.

## Answer

A **flywheel energy storage system (FESS)** stores energy as rotational kinetic energy in a spinning rotor (mass). When extra electricity is available, a motor accelerates the flywheel; when energy is needed, the same machine works as a generator and slows the flywheel down.

Energy stored: E = ½ I ω², where I = moment of inertia and ω = angular speed. Since E rises with the square of speed, high speeds (20,000–50,000 rpm) are used with strong, light rotors (carbon fibre).

#### Components
Rotor (flywheel), motor/generator, magnetic or low-friction bearings, vacuum chamber (to reduce air friction) and power electronics.

```text
 Grid <-> [Power electronics] <-> [Motor/Gen]
                                      |
                              [Flywheel in vacuum]
```

#### Advantages
- High power density, very fast response (milliseconds), efficiency of 85–95%.
- Very long life (> 100,000 cycles), little maintenance, no chemical hazard, and wide temperature range.

#### Disadvantages
- Low energy capacity: it can supply power only for seconds to minutes; self-discharge due to friction (standby losses).
- High initial cost and safety concerns (rotor failure), and requires containment.

#### Applications
Frequency regulation and power quality in the grid, UPS in data centres, regenerative braking in trains and buses, smoothing wind and solar power, and spacecraft.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A flywheel stores E = 1/2 I w^2. Why are very high speeds used?
   A. Energy is proportional to speed, so it is needed for linear gain
   B. Energy rises with the square of speed, so higher speed stores much more energy
   C. It reduces the moment of inertia needed to zero
   D. It eliminates the need for a motor
   Answer: B. Since E is proportional to w^2, doubling speed quadruples energy; strong light rotors such as carbon fibre withstand high rpm. (Key point: Energy formula and speed)

2. How does a flywheel system charge and discharge?
   A. A motor speeds up the flywheel to store energy; the same machine as generator slows it to release energy
   B. A pump raises water to a reservoir
   C. Ions collect on electrodes
   D. Chemical reaction in a battery
   Answer: A. A motor/generator accelerates the rotor when surplus power exists and acts as a generator when energy is needed. (Key point: Operation)

3. Why is the rotor placed in a vacuum chamber with magnetic bearings?
   A. To raise the rotor mass
   B. To reduce air friction and bearing losses, lowering standby losses
   C. To generate hydrogen
   D. To cool the grid
   Answer: B. Friction causes self-discharge; vacuum and low-friction bearings reduce it. (Key point: Vacuum chamber)

4. What is the main disadvantage of flywheel storage?
   A. Very short cycle life
   B. Slow response
   C. Low energy capacity, supplying power only for seconds to minutes
   D. Toxic chemicals
   Answer: C. Flywheels have high power but low energy capacity, so suit frequency regulation and short backup rather than long storage. (Key point: Limitation)

## Related in Energy Environment and Society

- [Energy storage technologies and challenges](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aenergy-storage-technologies-challenge) — Energy Environment and Society, chapter 5, asked 3×
- [Forms of energy storage](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aenergy-storage-forms-four) — Energy Environment and Society, chapter 5
- [Hydrogen production and storage](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Ahydrogen-production-storage) — Energy Environment and Society, chapter 3
- [Short note: Pumped storage hydropower](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Apumped-storage-hydropower) — Energy Environment and Society, chapter 5

## Related in other subjects

- [Flywheel energy relation and rim sizing](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Aflywheel-energy-fluctuation-derivation) — Theory of Mechanism and Machine II, chapter 2
- [Flywheel size from turning moment areas](https://www.bctengineer.com/question.md?key=semester_7%2Ftheory_of_mechanism_and_machine_ii%3Aflywheel-energy-areas-numerical) — Theory of Mechanism and Machine II, chapter 2
- [Mechanical and thermal energy storage](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aenergy-storage-technologies) — Power Plant Design, chapter 1
- [RoR at 40% exceedence; storage regulation](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-40-percent-exceedence-storage-regulation) — Hydropower Engineering, chapter 3