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A flywheel device contains a rotary flywheel that spins at speeds of 37,000 RPM, converting electrical energy into stored kinetic energy. In a UPS application, if a power outage occurs, the flywheel converts the kinetic energy into DC power and sends it to the UPS, which supplies it to the facility as AC power.
"Traditionally, flywheels have used mechanical bearings to rotate the flywheel device," explains Frank DeLattre, CEO of VYCON. "But we've developed a magnetic bearing technology that allows us to produce a higher energy density, which makes it highly effective as a UPS power source."
Cycle Life and Lifespan: Flywheels don't have a cycle life. Unlike batteries, their energy storage level does not diminish with repeated use. You can discharge and recharge a flywheel thousands of times, and the run-time performance will always stay the same. VYCON estimates the lifespan of its flywheels to be about 20 years.
• Discharge Rate and Recharge Time: A flywheel normally discharges its entire capacity in 15 to 20 seconds. With UPS applications, this may be sufficient to keep the data center operational during the 10 to 12 second changeover to backup power. VYCON's flywheels are normally set to recharge in 15 minutes, but are capable of a two-minute recharge.
"Kinetic energy storage is based primarily on the formula: mass times speed squared," DeLattre explains. "Our magnetic bearing allows us to levitate and rotate the spinning flywheel rotor at higher speeds. By doubling the speed of the flywheel, we can quadruple the amount of energy it produces."
• Size and Weight: Flywheel systems typically have a comparable size footprint to battery systems. They weigh far less than lead acid and nickel zinc batteries, and are competitive in weight with lithium-ion batteries.
"Our standard flywheel cabinet weighs about 1,800 pounds, and occupies a 30-by-30-inch footprint,” says DeLattre, who calls it the smallest footprint of any type of UPS energy storage system. "A single flywheel cabinet provides up to 400 kW of power."
• Cooling Requirements: VYCON's flywheels are capable of operating at temperatures up to 40 C under normal conditions. If the flywheel cabinet is de-rated (i.e., reduced to 300 kW), it can operate at temperatures up to 50 C.
• Maintenance Requirements. Traditional flywheels have high maintenance requirements, since the mechanical ball bearings used to rotate the flywheel must be replaced every year. However, the magnetic flywheel system eliminates bearing maintenance and replacement needs.
"As long as the flywheel is rotating, you know it's producing energy," says DeLattre. "You don't need to test them every few months to make sure they're working. Our flywheels only require routine annual maintenance."
• Initial Cost vs. Total Cost of Ownership. The initial costs of VYCON's flywheel systems are almost equal with those of lead acid batteries. A total cost of ownership analysis should figure in savings through reduced cooling and maintenance costs, and eliminating the need to replace batteries.
• Safety Concerns: Unlike batteries, flywheels are incapable of achieving thermal runaway.
Comparing Uninterruptible Power Supply (UPS) Energy Storage Options
UPS Energy Storage Option 1: Lead-Acid Batteries
UPS Energy Storage Option 2: Lithium-Ion Batteries
UPS Energy Storage Option 3: Nickel-Zinc Batteries
UPS Energy Storage Option 4: Flywheels