Power Electronics

New Spin on Energy Storage May Reshape UPSs

A few months back, I discussed one vendor's efforts to replace lead-acid batteries used in telecom applications with ultracapacitors (“Ultracapacitors Challenge Batteries in Telecom,” Power Electronics Technology, April 2004.) As I noted, lead-acid batteries are considered the weak link in many backup power systems, because their reliability and longevity are degraded by transient power demands and environmental factors. Two energy-storage technologies — ultracapacitors and flywheels — have emerged to address battery limitations.

Although ultracaps are suited to lower power applications (those up to a few kilowatts), flywheel-based energy-storage devices have been making their mark in applications requiring hundreds of kilowatts. As with ultracaps, flywheel power sources have been providing several seconds of backup power, which is sufficient for ride-through to a generator, fuel cell or other long-term power source. But a recent announcement by Active Power, the Austin-Texas-based manufacturer of flywheel-based energy-storage devices and UPSs, suggests flywheel technology is being extended to provide longer-duration backup power. The company is currently commercializing a battery-free technology that will deliver 20 kWh of backup power.

Active Power just marked a milestone in product development by creating a prototype capable of delivering 80-kW output for 15 min. (See “Battery-Free Energy-Storage System Doubles Output” in DataPoints, page 10.) The 80-kW prototype represents one version of a product that's slated to ship this year. Another model aimed at cell towers and other telecom systems will deliver 10 kW for 2 hr. Such developments could have wide-ranging implications for the design of UPSs, power systems and their intended applications.

Active Power's energy-storage device is meant to replace the battery cabinet required in certain UPSs. Although the company isn't saying how its new prototype works, it has hinted that the technology combines flywheel-based energy storage with other mature technology — not ultracaps, fuel cells or microturbines — using novel packaging techniques. Although the products should match battery cabinets in terms of size, cost and required electrical performance, the technology could improve the reliability of UPSs by an order of magnitude and also offer other advantages.

For example, the energy-storage technology will not suffer the degradation in operating life or loss of capacity that batteries experience over temperature. As with existing flywheel products, the battery-free storage devices will operate from 0°C to 40°C with no degradation in performance or operating life. Even these limits may be subject to extension, because the lower limit is currently dictated by the LCD's inability to work at colder temperatures, while the upper limit ties back to OSHA's safety requirements.

In terms of possible impact on the UPS, several factors must be considered. Whereas batteries are sensitive to noise and transients, the new energy-storage technology is said to be immune to these disturbances. Consequently, it may be possible to eliminate some of the protection circuitry found in battery-based UPSs. The company has also suggested that in large systems of 100 kVA or more, it may be possible to co-package the energy-storage device with the UPS, something typically not done with 100-kVA UPSs.

Also, while battery output voltage varies as the battery is discharged, the output of the battery-replacement technology remains steady up until the time the energy source is depleted. With a better-regulated source feeding the UPS, gains in conversion efficiency and other performance parameters may be possible. It may even be possible to tailor the power-conversion circuitry within the battery-free energy-storage device to produce a regulated 48-V bus directly, eliminating the batteries and the rectifier in some telecom applications.

Although Active Power's technology may provide one path to obtaining these benefits, it's possible that other energy-storage technologies will seek to supplant batteries in UPSs. Should that occur, power supply and power system designers will need to take a fresh look at existing UPS and power system designs to be sure they've taken full advantage of the new energy sources in their designs.

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