Valve-regulated lead-acid (VRLA) batteries, commonly used in UPS systems, are subject to a variety of life-shortening influences. Temperature variations of as little as 10°C can reduce the service life by 50%. Cycling (discharging and recharging) similarly reduces the life expectancy of VRLA batteries.
Because overall system reliability, either in terms of continuous uptime or managed process shutdown, is the purpose of the UPS, these battery problems are of great concern. As recommended by the UPS manufacturers, so-called "10-year" batteries are commonly replaced every three to five years. In addition, it's common to find UPSs configured with redundant battery strings as further protection against failure.
Improvements in flywheel dc power-system technologies are making inroads into mitigating or even eliminating the reliability concerns associated with VRLA batteries. Advanced flywheel systems can provide tens of seconds or even minutes of ride-through.
To mitigate the effects of cycling, a flywheel power system can be operated in parallel with a battery string. This would handle all short-term demands caused by voltage sags and intermittent outages and leave the batteries to handle longer-term outages. Completely replacing the VRLA batteries is possible when the flywheel's energy storage capacity is sufficient for orderly process shutdown or transfer to a standby engine-generator.
Flywheel power systems using a variety of technologies and implementations are available. They range from large, heavy, steel flywheels spinning at relatively low speeds to lighter, smaller carbon composite flywheels spinning at very high speeds.
The economics of flywheel power systems are attractive. Even though the first cost of these flywheel power systems, at comparable power levels, is about twice the cost of a typical VRLA battery string, the operational costs and total life-cycle costs have proven to be dramatically lower than for VRLAs.