Editor’s note: “Storing That Power” is a seven-part series detailing technologies capable of reserving power obtained from renewable sources. Read each week to learn more about pumped hydroelectric, industrial-scale batteries, flow batteries, flywheels, compressed air energy storage, gravel batteries and molten salt.

Some methods of power storage use chemicals to store an electric charge. Others use various ways of applying physics to create potential energy storage that can be tapped to produce power when it is needed. One of the most intriguing methods of potential energy power storage is flywheel power storage.

Other potential energy storage systems use static potential energy — in which the physical medium essentially sits still, waiting to be released — like the body of water in a pumped hydro system waiting to be released through the turbines to produce electricity. But in flywheel power storage, the energy is in a carefully balanced flywheel spinning at many thousand revolutions per minute. A flywheel can almost instantaneously be tapped to release power when it is needed.

Flywheel power storage units are large cylinders roughly the size of a small car, 7 feet (2.13 meters) in diameter and weighing roughly one and a half tons (1360 kilograms). The flywheels are sealed inside a vacuum chamber and are supported on frictionless magnetic bearings, so that there are virtually no losses once the flywheels are spinning. They spin at up to 16,000 RPM, meaning the outer rim of the flywheel is moving at about 1,500 miles per hour (2,400 kilometers per hour). The magnetic bearings allow the flywheel to float inside its enclosure without any physical contact or wear, leaving the flywheel with a life expectancy of at least 20 years.

A single flywheel unit is able to store and deliver 25 kWh of extractable energy, and can operate through thousands of charging and discharging cycles. The sealed vacuum and frictionless environment makes flywheel storage systems low-maintenance and long-lasting, and the system does not require the use of potentially hazardous chemicals.

Because flywheel power storage is a rather industrial system, it does not require any particular location characteristics, and is well suited for urban and rural installations. The flywheels are typically installed with the cylinders buried in the ground to provide additional protection in the case of a possible mechanical failure, which could result in shrapnel from a disintegrating flywheel shooting off in all directions at high speeds.

As a segment of the energy storage market, flywheel storage suffered a significant setback when Beacon Power, one of the leaders in the field, was forced to file for bankruptcy near the end of 2011. Despite this setback, Beacon was acquired by another company and is continuing to develop additional power storage facilities using their flywheel technology.

The system efficiency of a flywheel energy storage system is around 85 percent. This efficiency, combined with its fast response time, also makes flywheels an excellent system for providing uninterruptible power supply for locations with power-critical needs like data centers and hospitals. Although the initial capital costs for these systems are higher than other battery systems, the flywheels take up less space and need much less maintenance than battery systems, which makes them cost-effective over their lifespan.

The largest flywheel storage facility is a 20 megawatt plant at Stephentown, New York which is connected to the New York state grid.