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A Different Spin on Power Quality Protection

Key Points
  • For power quality protection, flywheels offer a cost-effective alternative to battery systems.
  • Flywheels use the inertia of a spinning wheel to store and generate power.
  • Flywheels are durable and require little maintenance, lowering their total cost of ownership.

FlywheelComputers and other sophisticated electronic equipment are sensitive to power interruptions, even for a very short duration. To protect equipment, many facilities rely on battery-operated, uninterruptible power supplies (UPS). Flywheel technology, however, may offer a cost-effective and reliable alternative.

No chemical energy required

Battery systems use chemical energy; flywheels store kinetic (motion) energy by using the inertia of a spinning wheel. Because storage capacity is largely determined by wheel size and speed, a flywheel with a small mass can store a significant amount of energy if the wheel has a large diameter and it's spinning very fast.

Flywheels only provide energy storage for about 15 seconds. However, when properly integrated and maintained, standby generators can reliably support the critical load in 10 seconds or less, providing adequate switchover time. The capacity of a flywheel is dependent on load, according to Active Power, Inc. For example, at 100 percent load, standard ride-through time is 14 seconds, while at 25 percent load, a flywheel can supply up to 52 seconds of backup power.

There are different design approaches to flywheel technology:

  • Rotary systems control the energy supply and condition it in terms of frequency and waveform stability.
  • Battery-free systems act as a substitute for the battery; additional electronics condition and deliver the energy.

Steel flywheels with mechanical bearings have a wheel speed of 7,000 to 9,000 revolutions per minute (rpm). Flywheels based on active magnetic bearings and composite materials are lighter and faster (30,000 rpm); they're often sold as standalone energy storage systems. They also minimize standby power losses.

Comparing battery and flywheel systems

Flywheels have a number of advantages over battery systems. They're rarely affected by temperature fluctuations, take up less space, are more durable and have lower maintenance requirements. The following table provides a detailed comparison of these two technologies:

 Flywheel Battery UPS 
Maintenance Low  Battery replacements and more frequent capacitor replacements 
Reliability  Very high  High, but battery failures do occur 
Footprint  Small  Larger, due to battery storage racks 
Waste disposa No hazardous materials  Serious issue with lead disposal 
Ride-through time  15 seconds  15 minutes 
Recharge time  Minutes  Hours 

Flywheel systems typically have a higher initial purchase price, but battery UPS systems require assembly and wiring to the UPS panel, which can increase installation costs.

Getting the right fit 

Flywheel UPS systems are recommended for three-phase critical power applications requiring the highest efficiency and reliability—above 100 kVA (kilovolt-ampere) and 480 VAC (voltage). Generally, battery UPS systems are a better fit for:

  • Single-phase 120/208 VAC applications
  • Smaller, three-phase applications
  • Extended run-time operations of five minutes or more, where there's no backup generator available
  • Facilities with backup generators that may not start reliably

Although they may cost more, flywheel systems are rugged, environmentally friendly and expandable—all of which help to lower the total cost of ownership. 


Image source: U.S. Department of Energy