Batteries Be Gone

Flywheel-based uninterruptible power supply an alternative worth considering


As a parent of young children, I abhor battery-operated toys, and batteries in particular. We constantly have to buy these infernal energy sources, and battery technology seems positively stone age when compared to high-tech advances in other areas. Conventional batteries wear out quickly, and rechargeable batteries degrade with each recharge.

I felt the same way about batteries when I worked as an engineer at a power plant. We had a large battery room that reeked with foul odors, a veritable seething cauldron of malevolent chemical reactions. We avoided the dreaded room at all costs and hoped the batteries would operate as designed when needed. We longed for a better solution, and I suspect most process plant personnel feel the same way about their battery rooms.

Until recently, battery rooms filled with lead-acid batteries have been a necessary evil as a backup power source for most process plants. Most plants get their primary power from the utility grid, and backup power is provided by an uninterruptible power supply (UPS) fueled by batteries.

UPSs usually provide relatively small amounts of long-term backup power to control system electronics or larger amounts of power for a short amount of time until diesel generators can be brought on-line.

Flywheel technology originally developed for hybrid electric vehicle applications recently has been commercialized as a viable substitute to batteries in UPSs. Basic operation is simple and intuitive.

A small amount of power is provided to spin a flywheel. Power can be drawn as needed from the flywheel when voltage disturbances or outages occur. A flywheel can supply a small amount of power for a relatively long period of time or a larger amount of power for a shorter duration.

For example, Pentadyne ( sells a flywheel energy storage device that can support a 120 kW UPS load for 20 sec., or an 8 kW load for 5 min.

Batteries share many of the same power provision characteristics as flywheels but present a host of operational and maintenance issues. According to a recent white paper authored by Caterpillar (, lead-acid battery UPSs have proven unreliable and expensive for a variety of reasons.

Although batteries are typically warranted for 20 years pro rata, they actually must be replaced every four to five years because they wear out much more quickly with repeated use. Battery leaks and spills are common, creating a safety hazard and potential damage to the environment.

UPS batteries require a large amount of floor space to house a dedicated air conditioning system, eyewash stations, and a separate battery room. Installation and maintenance are expensive due to the requisite spill containment systems, special floor coatings, and the myriad of maintenance tests that must be conducted. Even when maintenance is performed appropriately, there is no way of knowing if an aging battery will survive a full discharge, concludes the Caterpillar white paper.

The solution for many process plant applications could be a flywheel-based UPS. These systems are much lighter and take up much less floor space than battery systems. The aforementioned Pentadyne system is only 25 in. wide by 33 in. deep by 71 in. high and weighs only 1,300 lbs. A comparable battery-powered UPS would need more than twice the floor space and would weigh over 3,000 lbs.

Flywheel UPSs have a useful service life in excess of 20 years with minimal maintenance. Maintenance of the Caterpillar UPS consists of changing bearings once every two to three years. The Pentadyne UPS uses active magnetic bearings that require little or no maintenance.

These systems are impervious to high cycling or temperatures under 105°.

Eliminating batteries from the system also eliminates most environmental and safety issues.

Recovery after use is extremely rapid, typically less than 30 sec. Flywheel UPSs have virtually no restrictions on the number of energy discharge cycles. Unlike battery UPSs, flywheels do not wear out more quickly with repeated discharges.

Flywheels have been around for a long time, so it is somewhat surprising the technology has taken so long to be applied to UPSs. One of the main factors restricting flywheel use has been low efficiency due to excessive rotational friction losses.

Modern flywheel UPSs operate at standby efficiencies as high as 99.7% by applying a number of high-tech advances. The flywheel itself is constructed of strong yet lightweight fiber composites that can be spun at much higher rates than steel flywheels.

Active magnetic bearings eliminate mechanical losses. The entire flywheel assembly operates in a vacuum to minimize losses due to atmospheric friction. Vacuum is maintained by advanced molecular vacuum pumps that maintain the system at a vacuum of 10-4 torr.

Standby losses--that is, the power required to keep the flywheel spinning at full speed at no load--are only 300 W for the Pentadyne UPS (0.25% of rated power output).

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