Merriam-Webster’s Dictionary defines a flywheel as a device that stores kinetic energy or uses kinetic energy for braking. A flywheel like that made by Active Power (NASDAQ: ACPW) could be just what your datacenter needs if you’re worried about a loss of power.
Sure, you’ve got a diesel generator (or five) in the basement, but generators typically require several seconds to start up — at best. And when you’re talking about mission-critical applications and data, those seconds may make a sizable difference to your datacenter’s operations.
“In the event of an outage, the flywheel responds immediately without interruption,” said Martin Olsen, Active Power’s vice president of business development.
“The standby diesel and the power line connect at the automatic transfer switch (ATS),” Olsen explained. “When the ATX gets the signal and sees it has no line voltage, it switches over to the generator within milliseconds. Because the flywheel is constantly spinning, using an uninterruptible power supply (UPS)
The flywheel is powered by the utility electricity and keeps spinning after the power is cut. It only needs to provide power for a few seconds, as the generator turns on. The 15 seconds of power it provides as it spins down is more than sufficient to cover the time it takes to crank up a diesel generator, Olsen claimed.
600 pounds of steel spinning like an LP
The flywheel is 600 pounds of steel sitting on top of a magnetic coil in a vacuum designed to reduce friction. The flywheel rests on a core of metal bearings, which is the one component that wears out over time, Olsen said.
The vacuum is created by pumping out the air in the cylinder that holds it.
The flywheel itself is kept spinning by utility power and receives no more power during a blackout. 1,200 Watts of power will keep it spinning at 7,700 RPM, which is a low-speed flywheel, Olsen said.
He added that competing flywheel manufacturers spin magnetic bearing flywheels at up to 60,000 or even 80,000 RPM using a lighter composite material. “Composite materials are not as well understood as steel,” Olsen said.
Olsen said that the famous power
failure at 365 Main St. in San Francisco used a competitor’s flywheels, not Active Power’s.
“A medium-speed or low-speed flywheel is similar to flywheels used in other places such as vehicles and aerospace,” he added.
He said that the steel wheel doesn’t wear out, but the bearing cartridge is replaced every three years.
Typical configurations use at least two flywheels, so one is spun down, the bearing replaced, and then powered up again. The procedure is then repeated with the other flywheel, he said.
“We never replace the 600 pound flywheel,” he said. “We plan on a 20-year life for the whole system.”
Next page: Competing with batteries.
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Competing with batteries
Olsen said that flywheels compete with batteries. Batteries must be carefully monitored.
He admitted that flywheels also have to be carefully monitored, and that Active Power’s system track 150 telemetry points, but he said that when the flywheel’s spinning, you know it can deliver the power, whereas it can be difficult to be certain that batteries can provide the power they’re rated for.
Olsen said that Active Power flywheels use less space than a battery-powered UPS and waste less energy. Batteries are 92 percent efficient, but flywheels are 98 percent efficient, he said. Flip the numbers around, and batteries waste eight percent of the power they store, while flywheels only waste two percent, making them four times better by this metric, Olsen said.
Wasting less is more green, he added. The overall solution has one-fourth the carbon footprint of a comparable battery-based solution, Olsen said.
He said that flywheels are more reliable than batteries — seven times less likely to fail, he claimed — but admitted that datacenter owners have expressed some skepticism about them.
“In the beginning, the biggest concern was that we deliver 15 seconds of power and batteries provide 15 minutes. Some customers are still concerned about getting their generator started within 15 seconds, but that has never been a problem and never will be a problem because generators are federally mandated to start in five to eight seconds for critical environments, such as life support for a hospital, and generator manufacturers don’t make different kinds of generator,” Olsen said.
“Some people want the extra time in case they need to attempt to start the generator manually,” Olsen added. “But if the generator doesn’t start the first time … the issue is likely that there’s no fuel or bad batteries or water in the lines from condensation.”
“Sometimes the emergency stop button is still set to manual,” Olsen said.
“People press the emergency stop button because they don’t want the generator to start while they’re doing maintenance and sometimes they forget to press it again.”