A flywheel stores energy as rotational kinetic energy — the faster it spins and the more mass it has distributed at its rim, the more energy it stores. This principle is used in everything from Formula 1 KERS systems to grid-scale energy storage.
Flywheel Energy Formula
Stored energy: E = ½ × I × ω², where I is the moment of inertia (kg·m²) and ω is angular velocity (rad/s).
For a solid disk: I = ½mr². For a rim (mass concentrated at the edge): I = mr². A rim stores twice the energy of a disk with the same mass and radius — which is why flywheels concentrate mass at the perimeter.
Engineering Applications
Formula 1 KERS: A 700 mm diameter, 5 kg carbon-fiber flywheel spinning at 60,000+ RPM stores about 4 MJ of energy. Deployed for 6.7 seconds, it provides an 80 hp boost.
Grid energy storage: Large flywheel systems (250 kg, composite material, 30,000 RPM) store 25–50 kWh. Used for frequency regulation and UPS systems. Advantages: 20+ year lifespan, no chemical degradation, instant charge/discharge.
Space applications: NASA uses flywheels for energy storage on satellites. In vacuum with magnetic bearings, they can spin for years with minimal losses.
Material Limits
The energy density of a flywheel is limited by material strength — specifically the specific strength (strength-to-density ratio). Carbon fiber composites have the highest specific strength, enabling the fastest spin rates.
Maximum energy per unit mass ≈ K × σ/ρ, where σ is tensile strength, ρ is density, and K is a shape factor (0.5 for a rim). Steel: ~200 kJ/kg. Carbon fiber: ~1,000 kJ/kg. For comparison, lithium batteries store ~500–1,000 kJ/kg (140–280 Wh/kg).
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