Flywheel UPS, not batteries, for data center company's backup power

For its backup power, data center colocation company Terremark will switch to flywheel-based UPS systems from batteries -- a move in search of better total cost of ownership.

Data center colocation company Terremark Worldwide Inc. will install almost 10,000 kilovolt-amps' (kVA) worth of data center backup power with flywheel-based uninterruptible power supply (UPS) systems rather than batteries at a sprawling new data center campus in Culpeper, Va.

The Miami-based Terremark, with colocation centers on four different continents, is building the campus on a 30-acre site about 90 minutes southwest of Washington, D.C. The site will include five 50,000-square-foot data center buildings and three additional administrative buildings, with the first data center scheduled to go live in June.

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For Terremark, the somewhat unconventional switch to flywheel-based UPS systems over battery-powered ones afforded the opportunity to save precious data center square footage and, over time, money as well.

Going it alone with flywheels
Terremark is no stranger to employing flywheel-based UPSs, which have emerged as a viable alternative to batteries for short-term data center backup power. In all but a few West Coast data centers that the company has purchased, Terremark uses only flywheel UPS systems. Flywheels can handle short outages of up to 15 seconds, and provide ride-through power until the data center's primary utility power returns or the backup diesel generators come online.

With the increased reliability of generators, I think we will see more interest in flywheels.
Robert McFarlane,
president, Interport Financial DivisionShen Milsom & Wilke Inc.

For the Culpeper site, Terremark bought 11 900 kVA UPS flywheel systems, which are made by Active Power Inc.. The company bought them in a package from Caterpillar that includes backup diesel generators.

Terremark's flywheel-only approach stands in contrast to other companies that employ flywheel UPSs to complement battery-powered systems. Flywheel vendors such as Active Power and Pentadyne Power Corp. say that flywheels can prolong the life of batteries by handling power blips that barely register on a radar but nonetheless drain battery juice. Robert McFarlane, president of the Interport Financial Division of New York-based Shen Milsom & Wilke Inc. and a data center power expert, says companies use both means to increase their backup power and reduce the number of batteries. Meriter Hospital, a nonprofit hospital in Madison, Wis., uses a mix.

Flywheel space and cost savings
But Terremark has gotten comfortable enough with flywheels that it's willing to forgo batteries altogether. Stewart said the technology presents Terremark with several advantages over batteries.

One is data center space savings. "One nice thing about the flywheel is it uses a lot less space than long strings of batteries," said Ben Stewart, Terremark's senior VP of facility engineering. Case in point: Terremark's 200,000-square-foot Miami data center. Aside from IT equipment, the facility holds a 6 MW generator and 6 MW worth of flywheel UPS. If the company were running only battery UPSs, the batteries wouldn't fit in 2,000 square feet.

And while Terremark anticipates cost savings over time, flywheel technology is more expensive up front. Stewart said a 900 kVA flywheel unit sells for a little more than $300,000 but "compares very closely to the static UPS equivalent" -- that is, batteries. But because batteries need to be replaced every three to five years, a cost Stewart estimated at about $100,000 for each replacement, "the total cost [of flywheels] is a lot lower. It essentially lasts forever."

Plenty of power margin
Using a flywheel to generate electricity is nothing new; the technology has been around for decades.

The primary power source jump-starts the flywheel spinning. This builds kinetic energy based on the mass of the flywheel and the speed at which it rotates, which can be as fast as 54,000 rotations per minute. When the power cuts out -- even for a second or two -- the flywheel releases the built-up kinetic energy back into the data center until primary power returns or a backup generator turns on.

Modern flywheels are encased in vacuum cylinders that minimize friction, allowing the wheel to spin longer and generate more energy.

A main criticism of flywheels is they don't provide power for as long as a string of batteries. While flywheels can provide ride-through backup power for about 15 seconds, strings of batteries can provide it for as long as 15 minutes, depending on how long the battery string is. But Stewart said it doesn't really matter – if you can't get a generator running in 15 seconds, you probably can't get it running in 15 minutes either.

"You've got plenty of margin on the flywheel," Stewart said, adding that even with some European-made flywheels that Terremark runs elsewhere, which provide only six seconds of ride-through, "we've never had a missed start."

Stewart is partial to Active Power flywheels because a 900 kVA model has three 300 kVA flywheels inside, so if they need to replace one, the other two can still provide power.

Flywheel UPSs becoming mainstream
Though McFarlane has yet to work on a project that installs only flywheel-based UPSs, he said there is "more interest because they've definitely improved." He said they're more durable now because of engineering improvements such as magnetic bearings in vacuum cylinders that minimize friction, and, therefore, wear and tear. In his experience, most companies use them in conjunction with batteries.

According to McFarlane, one sign that flywheels have become mainstream is that the power and cooling giant Liebert Corp. now resells them. Since battery-based UPSs have been de rigueur for data centers for decades, Liebert's endorsement of a flywheel product -- it resells Pentadyne flywheels -- is a significant step.

"With the increased reliability of generators, I think we will see more interest in flywheels," McFarlane said.

Let us know what you think about the story; email Mark Fontecchio, News Writer. You can also check out our Server Specs blog.

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