Flywheels: A Power Protection Alternative
For some control applications, a brief power outage or brownout is an inconvenience. However, for the majority of computer-controlled processes, even a momentary glitch in the power supply is completely unacceptable, as well as costly. The traditional means of guaranteeing a continuous power supply has been the use of battery-based uninterruptible power systems (UPSs) along with engine gen-sets.
For some control applications, a brief power outage or brownout is an inconvenience. However, for the majority of computer-controlled processes, even a momentary glitch in the power supply is completely unacceptable, as well as costly. The traditional means of guaranteeing a continuous power supply has been the use of battery-based uninterruptible power systems (UPSs) along with engine gen-sets. While UPSs are a proven solution, their lead-acid batteries are slow to recharge, require constant maintenance, are temperature sensitive, and pose a heavy environmental cost. Proper disposal—governed by strict environmental regulations —is also an issue.
Battery manufacturers state that battery life can be maintained for at least four years if they are kept at a constant temperature of 75 °F and experience no excessive cycling. This can be problematic for facilities that experience frequent short duration power interruptions. Every time the batteries are called upon in such instances, the battery cycling degrades the overall life of the battery. In addition, most batteries for UPSs are connected in series, which means than one dead cell in the string can render the string useless.
An alternative solution—the flywheel
Acting as a mechanical battery, the flywheel stores kinetic energy in a high-speed rotating group and converts it back into electrical energy to support critical loads. Providing ride-through time to bridge power to the backup generators, the flywheel system is a space saving, near-zero maintenance, and affordable means of guaranteeing constant power for mission-critical installations. A 2006 Federal Technology Alert by the U.S. Department of Energy stated, “Flywheels appear poised to replace or supplement batteries as a backup power supply in UPS systems. ...Although the initial cost of a flywheel is typically greater than batteries it would be replacing or supplementing, its longer life and simpler maintenance will often result in lower life-cycle costs.”
Clean energy storage
Flywheel technology stores kinetic energy in a quiet, spinning disk to provide a reliable and predictable source of dc power. With recent advances that have made it more compact and able to support higher power applications, flywheel technology has emerged as a reliable, environmentally friendly power protection solution that stores energy mechanically instead of chemically, thereby enhancing dependability and reducing the carbon footprint.
Designed for high power, short-duration applications, a flywheel system can replace lead-acid batteries by working like a dynamic battery that stores energy kinetically by spinning a mass around an axis. Electrical input spins the flywheel rotor up to speed, and a standby charge keeps it spinning 24/7 until called upon to release the stored energy (see cut-away image). Proven technology used in the flywheel allows the flywheel hub—a high-speed permanent magnet motor/generator with contact-free magnetic bearings —to levitate 100% and sustain the rotor during operation. This configuration allows the rotor hub to spin with no metal contact, eliminating bearing wear, bearing oiling or greasing, or maintenance. As a result, no bearing replacements are required for the life of the flywheel.
Unlike traditional batteries, the flywheel can charge and discharge at high rates for countless cycles without degradation throughout its 20-year life. The amount of energy available and its duration is proportional to its mass and the square of its revolution speed. In the flywheel world, doubling mass doubles energy capacity, but doubling rotational speed quadruples energy capacity: E = kMω² (k depends on the shape of the rotating mass; M represents the mass of the flywheel; and ω is the angular velocity).
When used in conjunction with a UPS system, flywheels can provide uninterrupted DC ride-through power and voltage stabilization during brief utility power disruptions and brownout situations, preserving the battery array for use in longer-term outages. Most backup generators require six to 10 seconds to come on-line and connect with the UPS via the automatic transfer switch. Some flywheel units can provide up to 300 kW of instant ride-through power and voltage stabilization for more than 20 seconds (or other combinations of power and time), which is more than enough time for the majority of electrical disturbances. As a plus, flywheel units can be paralleled for additional power capacity, run-time, and/or redundancy.
Normally the sizing of UPSs and flywheels is done based on actual load. Most engineers size a UPS at 30-40% larger than the actual load to allow for growth. Once the UPS is sized, the flywheel needs to be sized to the UPS. All UPS ratings are based on kVA and kW numbers, the rating used for power applications is the kW rating. When this kW number is established, it will be labeled as the full load kW rating. For example: A 275 kVA UPS rating with a power factor (pf) rating of .9 equates to 248 kW of UPS power. Since most loads are sized to UPS at an approximately 80% load factor, along with inverter efficiency of 96%, this equates to 207 kwb. Therefore, this is the rating used to size the flywheels to assure proper power rating and proper amount of run time requirement. To make it easier to size flywheels, most flywheel manufacturers supply customers with run-time charts that match kVA with run time (see Run Time Specification Chart graphic).
As illustrated in the Run Time Specification Chart, by using two flywheels, Model 1 will achieve 26.6 seconds of run time and by using two flywheels, Model 2 will achieve 28.6 seconds of run time. In either case, the flywheel exceeds the goal of meeting a 20-second run time requirement as a minimum. This provides a solution that fits most facilities’ needs and ample time to transfer to an engine-genset if a longer power outage occurs.
Return on investment
When comparing the life-cycle cost of batteries with the life-cycle cost of flywheels, it’s clear which technology has a longer cost savings over the life of the technology. This return on investment (ROI) is a method used by most businesses to justify the dollars spent in purchase decisions. What most engineers have discovered is that the flywheel has been favored over batteries due the cost savings of an ROI in three to four years. However, it is important to know that the purchasing decision is not necessarily an either-or option since the flywheel can be used with or without batteries.
When used with batteries, the flywheel is the first line of defense against damaging power glitches because it absorbs all short-duration discharges. As a result, the flywheel reduces the number and frequency of discharges, which shorten battery life. When the flywheel is used with the UPS and no batteries, the system will provide instant power to the connected load exactly as it would with a battery string. However, if the power event lasts long enough to be considered a “hard” outage (rather than just a transient outage), the flywheel will hand off to the facility’s engine-generator.
Frank DeLattre is president of Vycon. He can be reached at firstname.lastname@example.org
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