Flywheel Energy

Part of Energy Storage & Batteries

A spinning flywheel stores kinetic energy that can be released as mechanical or electrical power — ideal for smoothing intermittent power sources and delivering short bursts of high power.

Why This Matters

The flywheel is one of the oldest energy storage devices in human history — the potter’s wheel and the millstone both use flywheel principles. A heavy wheel spinning at speed stores significant kinetic energy and can release it as useful mechanical work. Connected to a generator, a flywheel converts mechanical storage to electrical storage and back with high efficiency and zero degradation.

For a rebuilding civilization, flywheels solve a specific problem that chemical batteries handle poorly: rapid power bursts. Starting a large electric motor, operating a forge blower for a brief intense heat, or running a welding arc for seconds at a time — all these demand power faster than a battery can safely deliver. A flywheel charged by a modest continuous generator can discharge through these loads without battery damage.

Flywheels also smooth the output of intermittent mechanical power sources. A water wheel whose rotation varies with current speed, or a windmill that surges and lulls, produces uneven torque. Adding a heavy flywheel to the shaft averages out this variation, delivering steady rotation to a connected machine or generator.

Physics of Flywheel Storage

Stored energy: E = ½Iω², where I is the moment of inertia (kg·m²) and ω is angular velocity (radians per second).

Moment of inertia for a disk: I = ½mr², where m is mass and r is radius.

Practical calculation: A cast iron disk flywheel, 60 cm diameter, 10 cm thick:

• Volume = π × 0.3² × 0.1 = 0.0283 m³
• Mass = 0.0283 × 7,200 (iron density) = 204 kg
• I = ½ × 204 × 0.3² = 9.18 kg·m²
• At 300 RPM (31.4 rad/s): E = ½ × 9.18 × 31.4² = 4,527 J ≈ 1.26 Wh

This is modest — about equal to a small AA battery — but the power can be delivered in a fraction of a second if needed, enabling very high instantaneous power output.

Scaling up: Energy scales with mass and the square of both radius and speed. Doubling the radius (to 120 cm) with the same speed stores 16× more energy. Doubling the speed stores 4× more energy. High-speed flywheels are far more energy-dense but require better bearings and stronger materials to avoid catastrophic failure.

Speed limits: The stress in a spinning disk is proportional to ω²r². The maximum safe speed is limited by material tensile strength. For cast iron: maximum peripheral velocity ~100 m/s. For steel: ~150 m/s. Exceeding this causes the flywheel to burst with catastrophic energy release.

Construction

Casting a flywheel:

1. Make a sand mold from a wooden pattern — a flat disk with a central hub
2. Cast from gray cast iron (melts at ~1200°C, achievable in a cupola furnace)
3. Machine the bore of the hub to fit the shaft, and turn the outer rim to balance
4. Static balance test: mount on a horizontal shaft and let it rest — the heavy side goes to the bottom; remove material from heavy side or add to light side until it rests at any angle

Alternative fabrication:

• Stack steel plates and bolt them together — easier than casting
• Concrete flywheel with embedded steel reinforcement for low-speed applications (potter’s wheel, grinding stone)
• Lead-filled steel drum for very dense, compact storage

Shaft and bearings:

• Heavy flywheels require robust bearings — journal bearings (shaft turning in a lubricated sleeve bore) or ball bearings
• Self-aligning journal bearings machined from bronze or cast iron, lubricated with tallow or oil, can support hundreds of kilograms of shaft weight
• Bearing alignment is critical — misalignment causes rapid wear and vibration

Housing: Enclose the flywheel in a protective case. A flywheel failure at high speed throws debris with lethal energy. Even at low speed, a flywheel can grab clothing or limbs.

Integration with Generators

Motor-generator flywheel set: An electric motor drives the flywheel during periods of surplus power. When power is needed, the now-spinning flywheel drives a generator. Efficiency 80–90% per cycle (motor efficiency × generator efficiency, less bearing losses).

Direct mechanical coupling: A flywheel on a water mill shaft smooths torque variations. A belt drive connects the flywheel to the mill machinery. The flywheel acts as a low-pass filter — absorbing brief torque surges and filling torque gaps.

Clutch and engagement: For controlled power delivery from flywheel to load, a friction clutch allows engagement at any speed. Engage gradually to avoid shock loading. A dog clutch (positive engagement) is simpler but requires speed matching before engagement.

Practical Applications

Motor starting: Charge flywheel with a small motor over 30–60 seconds, then engage clutch to spin up a large motor quickly. Avoids the current surge that would otherwise damage the power supply.

Forge blower: A hand-cranked flywheel stores energy between crank strokes, delivering steady airflow to the forge rather than pulsed. Large millstone-style flywheels were traditional on lever-action bellows for this purpose.

Short-run machinery: Lathes, grinders, and drill presses used intermittently. Spin up flywheel, engage work, disengage when done. The flywheel provides smooth, sustained power through the cut.

Emergency power pulse: Charge flywheel slowly from any available source; discharge through generator in seconds to provide brief electrical power (starting a radio, charging a capacitor bank, powering lights).