Flywheel Power System Beefs Up Reliability For UPSs

A backup power source that can complement or in some instances replace conventional battery-backed uninterruptible power supplies (UPSs) is possible thanks to a novel flywheel power system. When used with a battery-backed UPS, Pentadyne's Voltage Support Solution (Model VSS-120) improves UPS reliability by handling the short power line disturbances that produce coup de fouet, or battery "whiplash." Such an effect degrades battery reliability and operating life.

The flywheel system also provides several minutes of dc backup power, which is sufficient to protect against most power line disturbances. Consequently, VSS-120 may operate as a standalone UPS.

The VSS-120 delivers 2400 kWs (kilowattseconds) of energy to support loads ranging from 120 kW for 20 seconds up to 10 kVA for five minutes. Output is 500 to 800 V, and the system can be recharged at the same rate it discharges. On average, only 120 W of energy maintains operation during idle.

The unit consists of a flywheel module, a power-conversion module, a motor and bearing controller module, and a user interface (see the figure). This equipment is housed in a 25- by 33- by 71-in. cabinet that weighs approximately 1000 lb. These measurements represent dramatic reductions over battery-backed UPSs.

Obtaining the same capacity with batteries would require a cabinet weighing over 3000 lb with a footrprint that's twice as wide as the Pentadyne unit. Whereas batteries require a controlled environment, the VSS-120 can operate from -20° to 50°C, allowing for outdoor installations. The flywheel system also can handle vibrations—up to 5 G of vertical loading and 3 G of horizontal loading,

Maintenance is minimized, since the flywheel module is designed to operate 15 years without any major service. The other system components have been chosen for similar long life. In contrast, UPS batteries may need to be replaced after just a few years of service. The VSS-120 even claims advantages over existing flywheel power systems. It weighs less than comparable products as a result of the flywheel's high-speed operation. In Pentadyne's product, the flywheel itself weighs just 51 lb versus 600 lb for existing flywheels that can generate about the same energy. Those larger flywheels rotate at about 7700 RPM, while Pentadyne's spins at 60,000 RPM.

The VSS-120 will become generally available in the first quarter of 2004, with pricing starting at $44,000. Early units will arrive in the second half of this year at higher prices. The company expects pricing to drop to $25,000 in two to four years.