A flywheel banks energy simply as the spin of a heavy disc, charging and discharging unlimited cycles with no chemical battery inside it to degrade, which makes it well suited for smoothing short-term power fluctuations from an intermittent source like wind or hydro rather than for bulk storage. Energy scales with the square of the angular speed, so doubling a flywheel’s RPM quadruples the energy it stores, and shifting that same mass from a solid disc out to the rim doubles the stored energy again for an identical mass and radius — which is exactly why bicycle wheels, pottery wheels, and industrial flywheels all concentrate their mass at the edge. A 500 kg, 1.2-meter steel-rim flywheel spinning at 1,500 RPM stores about 2,221 kJ (617 Wh), roughly what a 12 V, 50 Ah battery holds, while a 2,000 kg industrial-scale unit at 1,000 RPM stores about 10,966 kJ (3,046 Wh) and the rim of either is stressed close to the limit of ordinary steel.
A flywheel stores energy as rotational kinetic energy — spin a heavy disc up to speed and it holds that energy until you extract it. Flywheels are mechanical batteries with unique advantages: unlimited charge/discharge cycles, no chemical degradation, buildable from basic materials, and they can deliver large bursts of power on demand. For a post-collapse village with intermittent power from wind or hydro, a flywheel smooths out short-term fluctuations and bridges gaps.
Principles of Kinetic Storage
The Physics
Stored energy in a flywheel:
E = 1/2 × I × w²
Where:
- E = energy (joules)
- I = moment of inertia (kg·m²) — depends on mass and mass distribution
- w = angular velocity (radians/second)
Energy scales with the square of speed — doubling RPM quadruples stored energy. This makes speed more important than mass.
Rim Mass vs Solid Disc
Moment of inertia depends on where the mass is located:
- Solid disc: I = 1/2 × m × r² (half the mass times radius squared)
- Rim-weighted (mass concentrated at the edge): I = m × r² (full mass times radius squared)
A rim-weighted flywheel stores twice as much energy as a solid disc of the same mass and radius. This is why bicycle wheels, pottery wheels, and industrial flywheels all concentrate mass at the rim.
Practical Energy Storage
| Flywheel | Mass | Radius | Max RPM | Stored Energy | Rim speed |
|---|---|---|---|---|---|
| Cast iron disc, 50 cm | 100 kg | 0.25 m | 1,500 | 38.6 kJ (10.7 Wh) | 39 m/s |
| Steel rim, 80 cm | 150 kg | 0.40 m | 2,000 | 526 kJ (146 Wh) | 84 m/s |
| Heavy steel rim, 1.2 m | 500 kg | 0.60 m | 1,500 | 2,221 kJ (617 Wh) | 94 m/s |
| Industrial-scale, 2 m | 2,000 kg | 1.00 m | 1,000 | 10,966 kJ (3,046 Wh) | 105 m/s |
For scale, a 12 V, 50 Ah battery holds 12 × 50 = 600 Wh, close to the 617 Wh of the 1.2 m steel-rim wheel in the third row (2,221 kJ ÷ 3.6 = 617 Wh), though the battery gives it up over hours and the wheel in minutes.
The cast iron disc is solid (I = ½ × m × r²); the other rows are rings (I = m × r²). The steel rows run the rim at 55 to 86 MPa (density × rim speed squared), which is under the safety factor of 8 that Flywheel Construction recommends for the weakest mild steel; the 105 m/s wheel would be unsafe in cast iron. See Burst Energy Calculation.
Flywheels are best suited for short-duration storage (minutes to hours) and power smoothing rather than bulk energy storage. For daily/weekly storage, batteries or thermal storage are more practical.
Flywheel Construction
Cast Iron or Steel Disc
The simplest approach — a heavy, solid disc on a shaft:
- Material: Scrap flywheel from an engine (already balanced!), brake rotors bolted together, or a custom-cast iron disc
- Shaft: A solid steel shaft press-fit or keyed through the center hub
- Balance: A flywheel that vibrates at speed will destroy its bearings and potentially break free. See balancing section
- Maximum safe speed: 100-150 m/s is close to the burst speed of weak cast iron, not a working speed. Run cast iron at no more than about 42 m/s at the rim (a safety factor of 8 on stress), which for a 50 cm diameter disc is about 1,600 RPM. See Cast Iron & Steel Disc
Concrete Flywheel
An accessible alternative where metal is scarce:
- Cast a reinforced concrete disc (50-100 cm diameter, 15-30 cm thick)
- Embed steel rebar or wire mesh for tensile reinforcement
- Cast a steel hub in the center
- Limitation: Concrete has very low tensile strength (1.7-2.8 MPa vs 345+ MPa for mild steel). Maximum safe speed is much lower — about 10 m/s at the rim
- Best used as a large, slow smoothing flywheel; at 10 m/s a 1 m diameter disc turns at 191 RPM, and the energy it holds is small however heavy it is
- A 1-meter, 500 kg concrete flywheel at its 10 m/s limit (191 RPM) stores 12.5 kJ (3.5 Wh); at 400 RPM it would store 55 kJ but with a rim speed of 20.9 m/s and a safety factor of only 2. See Concrete Flywheel
Rim-Weighted Design
Maximize energy storage per kg:
- Build a light hub from welded steel plate or tubing (spokes)
- Attach a heavy steel rim — thick-walled pipe, rim from a truck wheel, or a custom-welded ring
- The spokes must withstand the centrifugal force pulling the rim outward
- This design stores 2x more energy than a solid disc of the same total weight
Balancing
An unbalanced flywheel at speed is extremely dangerous — the cyclic forces can shatter bearings, bend shafts, and break the flywheel free:
- Mount the flywheel on its shaft between support bearings
- Spin slowly by hand — observe where it settles (heavy side goes to the bottom)
- Add weight to the opposite side (drill and bolt small weights) or remove material from the heavy side (drill holes)
- Repeat until the flywheel shows no preferred resting position (static balance)
- For high-speed operation, dynamic balance is also needed — spin the flywheel and check for vibration at multiple speeds
Bearings & Friction
Bearing friction determines how quickly the flywheel loses its stored energy (“self-discharge rate”).
Ball & Roller Bearings
- Best option for low friction and long life
- Salvage from industrial equipment, vehicles, or machinery
- A flywheel on quality ball bearings can coast for hours with minimal loss
- Grease regularly (every 100-500 hours of operation)
Journal Bearings (Low-Tech)
- A polished shaft running in a lubricated sleeve (bronze, babbitt metal)
- Higher friction than ball bearings but can be made from scratch
- Require constant oil supply (gravity drip feed or wick oiler)
- The flywheel decelerates faster — suitable for short-term storage only
Reducing Windage
Air resistance (windage) slows the flywheel:
- A smooth, rounded rim profile has less drag than a flat-sided disc
- Enclosing the flywheel in a housing reduces air turbulence
- For high-speed flywheels, a partial vacuum in the housing dramatically reduces windage (requires a sealed housing and hand-operated vacuum pump)
Coupling to Generator
Motor-Generator Set
The most common approach:
- Use a single electric motor/generator coupled to the flywheel shaft
- To store energy: run the motor to spin up the flywheel
- To extract energy: the flywheel spins the generator, producing electricity
- The same machine serves both functions (motor when powered, generator when driven)
- A car alternator, treadmill motor, or industrial motor all work
Clutch & Gearing
If the flywheel spins at a different RPM than the generator wants:
- Use a belt drive with variable pulley sizes to match speeds
- A clutch allows disconnecting the flywheel when not storing or extracting energy (reduces bearing wear)
- Vehicle clutch assemblies (from manual transmission vehicles) can be adapted
Variable Speed Considerations
As the flywheel discharges, it slows down, and generator output voltage drops in proportion to speed:
- For battery charging: voltage drops below charging threshold before the flywheel is fully discharged (typically only 75% of stored energy is usable)
- For direct AC output: an inverter with variable input voltage is needed
- Gear ratio optimization: Select gearing so the flywheel’s useful RPM range matches the generator’s efficient operating range
Safety & Containment
Burst Energy
A flywheel failure at speed is catastrophic — fragments fly outward with enormous kinetic energy, like shrapnel:
- A 100 kg, 0.25 m radius cast iron disc at 1,500 RPM contains 38.6 kJ
- A 500 kg, 0.6 m radius steel ring at 1,500 RPM contains 2.2 MJ — the energy equivalent of about 0.53 kg of TNT, and about five times that if it is driven to its burst speed
Take containment seriously.
Containment Design
Options for containing a burst:
- Underground pit: The simplest and most effective. Dig a pit, place the flywheel below ground level. In a failure, fragments hit the earth walls. Cover with a heavy metal or timber lid with a shaft hole
- Steel containment ring: A thick steel cylinder around the flywheel with at least 10 cm clearance, anchored to a concrete pad. No wall thickness has been sourced; the 15-25 mm once given here is not proven adequate (see Housing)
- Sand-filled housing: An inner steel shell surrounded by packed sand in an outer shell. Sand absorbs fragment energy
- Concrete bunker: For large flywheels — thick concrete walls all around, with the shaft exiting through a labyrinth seal
Speed Limiting
Never exceed the designed maximum speed:
- Centrifugal governor: A mechanical speed limiter that disconnects the drive or applies a brake at maximum RPM
- Electrical speed detection: If using a motor-generator, monitor output frequency (proportional to RPM) and cut drive at maximum
- Painted reference marks: Paint a mark on the flywheel and use a strobe or ear to estimate RPM (the pitch of the hum is proportional to speed)
Rule of thumb: Design maximum operating speed at 50% of calculated burst speed. This provides a 4x safety factor on stress (stress scales with speed squared).
See Also
- micro-hydro-turbine — Consistent power source to charge the flywheel
- advanced-wind-turbine — Intermittent source that benefits most from flywheel smoothing
- district-heating — Another form of energy storage (thermal)
When to Use Flywheels vs Batteries
| Criteria | Flywheel | Lead-Acid Batteries |
|---|---|---|
| Cycle life | Unlimited | 300-1,500 cycles |
| Self-discharge | High (hours) | Low (weeks-months) |
| Best duration | Seconds to minutes | Hours to days |
| Power density | Very high | Moderate |
| Energy density | Low | Moderate |
| Materials needed | Steel, bearings | Lead, acid, plastic |
| Maintenance | Bearing replacement | Water topping, terminal cleaning |
| Degradation | None (mechanical wear only) | Chemical degradation over time |
Use a flywheel when: You need to smooth rapid power fluctuations (wind gusts, load spikes), provide burst power for starting motors, or buffer between a variable source and a sensitive load.
Use batteries when: You need to store energy for hours or days (overnight from solar, calm days from wind), or when the total energy requirement is more than a few hundred watt-hours.
The ideal system combines both: Flywheel handles second-to-minute variations, batteries handle hour-to-day storage. The flywheel reduces cycling stress on the batteries, extending their life significantly.