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Place for flywheels in your UPS?

Editorials  By - John Dix, Network World, 07/17/06

Keeping data centers up and running during power outages traditionally has required banks of batteries to feed gear while generators are spooled up. There is an alternative, however.

Pentadyne today will announce the VSS+, a 190-kilowatt flywheel system that sidesteps many battery shortcomings.

Batteries require ongoing maintenance to ensure they can still hold a charge. They also require air conditioning, are heavy, eat up valuable data center space and have to be replaced every two to four years.

Pentadyne's flywheel system, says CEO Mark McGough, requires virtually no maintenance for its 20-year life expectancy, always has the same kinetic energy when the wheel is spinning at the prescribed revolutions per minute (rpm), has no special thermal requirements, and is lightweight and takes up only 5 feet of floor space.

The downside is the VSS+ can generate power for only 12.5 seconds.

McGough says that's enough to run roughly 100 blade servers while generators are coming online, and he says more than 98% of power problems are sags and interruptions lasting less than 2 seconds. As many as eight VSS+ systems can be ganged in parallel to address different power needs.

Flywheels have been around a long time, but Pentadyne's advantage is in the wheel's material. Kinetic energy equals mass multiplied by velocity squared.

"If you double the size of the mass, you get twice the energy," McGough says. "But if you double the velocity, you get four times the energy."

Metal flywheels can't be spun at high speeds for fear of catastrophic failure, so the only way to get more energy out of them is to increase their weight, which requires more energy to operate. Pentadyne's wheel is made of carbon fiber and can be spun at 50,000 rpm.

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