Flywheels and small mechanical stores: Science and technology
How spin speed stores energy, and why rotors leak it
What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.Flywheels store energy in a fast-spinning rotor and deliver it in seconds to minutes. They are durable and made of common materials, but lose charge quickly, so they suit grid frequency control rather than shifting energy for hours. Springs and hydraulic accumulators are niche cousins used mainly in machines.
Mechanism
FavourableA motor spins a heavy rotor up to speed; to discharge, the same machine acts as a generator and slows the rotor. Stored energy rises with the square of speed, so modern units spin fast inside vacuum enclosures, often on magnetic bearings, to cut drag and friction: Beacon Power's composite rotors run at about 16,000 rpm, Calnetix/Vycon steel units at 14,000 to 36,750 rpm and Active Power's steel units at 7,700 rpm. Springs store energy as elastic strain and hydraulic accumulators as gas compressed behind a piston or bladder; both release it in seconds and are used in vehicles, cranes and industrial hydraulics rather than on the grid.3
Beacon Power composite rotor about 16,000 rpm3 Calnetix/Vycon steel rotor 14,000-36,750 rpm3 Active Power steel rotor 7,700 rpm3Material scarcity and toxicity
FavourableRotors are steel or carbon-fibre composite, with copper windings, magnets and vacuum housings. No electrolyte or toxic chemicals are involved. Carbon fibre stores more per kilogram at material level (300 to 350 Wh/kg for high-grade fibres against about 50 Wh/kg for steel), but a whole composite rotor prototype reached only 11.7 Wh/kg and 5.6 Wh/kg with its housing. Steel is cheaper (about USD 1 per kg against roughly USD 31 to 102 per kg for carbon fibres) and simple to recycle. High-speed magnetic bearings, used in China's 30 MW plant, may contain rare-earth magnets.123
Material-level specific energy steel about 50 Wh/kg; carbon fibre 300-350 Wh/kg3 Composite prototype, whole system 5.6 Wh/kg3 Material cost steel about USD 1/kg; carbon fibre USD 31-102/kg3Degradation and ageing
MixedThere is no chemical fade, so flywheels tolerate many deep cycles a day, which is exactly what frequency regulation demands; Beacon Power quotes a life of 175,000 cycles (developer rating). Ageing comes from bearing wear, rotor fatigue and vacuum leakage. The main weakness is standing loss: IRENA reports self-discharge as high as 15 percent per hour, while Helix Power claims a steady-state loss below 1 percent of rated power (developer claim), so energy is held for minutes rather than hours.34
Cycle life, Beacon Power 175,000 cycles (developer rating)3 Self-discharge (IRENA) up to 15% per hour4 Steady-state loss, Helix Power below 1% of rated power (developer claim)3Variants compared1235
| Variant | Example | Typical role | Notes |
|---|---|---|---|
| Carbon-fibre flywheel | Beacon Power: 100 kW / 25 kWh units, 15 min; 20 MW plants in New York and Pennsylvania | Grid frequency regulation | Light, fast rotors in vacuum |
| Long-duration steel flywheel | Amber Kinetics: 8 kW / 32 kWh unit, 4 h | Solar shifting, various | Prototyped |
| Short-duration steel flywheel | Active Power, Piller, Vycon: 125-675 kW, about 15 s | Uninterruptible power supplies | Commercial |
| Flywheel-battery hybrid | Torus (Utah) | Demand response and backup for businesses | 70 MW demand response memorandum with PacifiCorp's Rocky Mountain Power, since expanded to up to 500 MW (company) |
| Magnetic-levitation flywheel array | Dinglun, Shanxi, China: 120 units, 30 MW | Grid frequency regulation | World's largest flywheel plant (2024) |
| Springs and hydraulic accumulators | Vehicles, cranes, industrial presses | Seconds of peak power, energy recovery | No grid-scale examples found |
Sources
- 'World's largest' 30MW flywheel energy storage project connects to grid in China Energy-Storage.news, 2024
- Milestones for flywheel, lithium battery grid-scale projects POWER Magazine, 2011
- A review of flywheel energy storage systems: state of the art and opportunities (Li and Palazzolo, Journal of Energy Storage 46, 103576) Elsevier (Journal of Energy Storage), 2022
- Electricity Storage and Renewables: Costs and Markets to 2030 IRENA, 2017
- Torus, Rocky Mountain Power sign MOU for up to 70 MW of demand response Utility Dive, 2025