Electronic Design

Hot Air + Flywheel = A Bridge Supply With Longer Endurance

Bridge power is an interesting niche in the power-supply world because of the way it continually turns to outside-the-box technology. Bridge supplies are a subset of uninterruptible power supplies (UPSs) for data centers and telecommunications central offices (COs). They're only called upon to supply power for the seconds or minutes that elapse between a utility failure and the time the normal UPS is fully online, or until power is restored.

The battery bank mandated for all telephone COs represents the classic bridge supply. Batteries have two problems, though—the cost of replacement and the uncertainty about how often replacement is necessary.

Battery manufacturers may claim a 10-year lifespan. Battery-based UPS providers often say five. And, data-center managers generally figure on two to three years to be conservative. The problem is the lifetime-shortening wear that batteries endure when they have to deal with frequent power outages of short duration.

This is where bridge supplies start to get interesting. The first new-generation bridge supplies, from Pentadyne and Active Power, stored energy in flywheels instead of batteries. Then, ultra-capacitors came along to challenge flywheels for energy storage (see "Bridge-Power Supplies Pack 22 2700-F Ultracaps" at www.electronicdesign.com, ED Online 8970).

The latest wrinkle, from Active Power, pairs the flywheel with a turbine powered by compressed air routed through a heat exchanger that raises the inlet air temperature to maximize turbine efficiency (see the figure). This extends the time that the bridge supply can stay online. An additional benefit is that the exit air, having expanded, now has a temperature of 59°F. So, it's not a burden on the data-center cooling system.

The voltage on the dc bus in the figure is nominally held at 540 V. When a utility outage causes the bus voltage to sag below a programmable threshold, flywheel discharge is initiated to support the dc bus. Then, below a programmable flywheel speed, a command initiates turbine discharge. The flywheel speed threshold is selected to provide as much delay as possible prior to initiating a turbine discharge while still allowing ample reserve flywheel capacity to ensure that the turbine can accelerate to rated speed and is carrying full load.

During turbine operation, the flywheel speed is maintained low enough so that it has headroom to absorb energy from a step unload until turbine inlet pressure controls can be adjusted. The flywheel serves as a speed control device for the turbine because its response is relatively fast compared to the response of the pressure-regulating valves.

As for the operation of the valves, at the beginning of a turbine discharge, the TSU has stored a lot of heat. So a relatively small amount of air goes through the TSU, and most is directed around it. As the TSU transfers its heat to the compressed air and cools, a higher percentage of the air goes through it and less is bypassed.

For Active Power's CoolAirDC system, the maximum output power is 85 kW and the maximum run-time at full load is 15 minutes. Its estimated lifespan is 20 years.

Active Power

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