Originally featured on
Jul 2

Written by:
7/2/2009 10:00 AM  RssIcon

ups-flywheel.jpgIn this article, we’re going to examine a totally environmentally safe, aka “green,” solution to UPS needs. The focus will be on what are called rotary UPS or flywheel UPS systems. These solutions differ greatly in how they operate, but have unique benefits to both the user and the environment.

In this series, we’ve assumed that the UPS system would be driven by a bank of batteries and backed up by a generator. As explained previously, both battery and generator solutions are not particularly environmentally friendly. They pollute.

The downside of batteries

Let’s look briefly at some drawbacks of using batteries in your UPS system. In my research for this series, one statement from the Pentadyne white paper “Battery-Free UPS energy storage” kept coming back to mind. It says, “The first 10 seconds matter 100 percent of the time. The next 10-plus seconds matter less than 1 percent.” As broadcast control rooms increasingly resemble data centers, the importance of continual power was emphasized. If your control room loses power for even 10 seconds, how long will you be off the air? Ten seconds? No, you’ll be off much longer than that. The time to reboot servers, production control switchers and automation systems will be measured in tens of minutes — at best! And that’s assuming you didn’t lose data when the lights went out.

A study of utility power interruptions by the Electrical Power Institute says that 98.7 percent of all brief voltage sags, outages and other disturbances last less than 10 seconds. Fully, 84 percent last less than two seconds. This means that if you only rely on a generator for backup power, it’s needed for just over 1 percent of the power disturbances.

However, before you ditch the genset, note that a study by the Aperture Research Institute says that if gensets aren’t up and powering a facility’s cooling system within 60 seconds, servers will crash because of thermal overload. (See Acknowledgements 1.) This means there’s no such thing as 15 minutes of UPS battery backup time. If your genset doesn’t start in less than one minute, kiss your resume goodbye.

While battery-sourced UPS systems have a long life and are still considered the backbone of most auxiliary power systems, they still have other drawbacks like maintenance and disposal.

A study conducted by the R&E Advanced Technologies Team of MGE UPS Systems details a study on commonly-used UPS batteries. The study looked at 8000 records concerning batteries and UPS systems. The data showed that in a 240-cell battery system (such configurations are common in large UPS installations), five cell failures were experienced in the first year (2 percent of 240 cells), nine (4 percent) in the second year, 48 (20 percent) in the third and 84 (35 percent) in the fourth year. Because valve relief lead acid (VRLA) batteries predominately fail open circuit, the failure of any cell means the entire battery system fails. When that happens, the backup capability becomes zero. (See Acknowledgements 2.)

Then there is the cost of replacing batteries. For a UPS system in the 800kW range, the total cost of replacement batteries over a 10-year period may be $250,000. The MGE study recommends replacing VRLA batteries every three years.

The replacement cost for labor and materials of a six-minute-capacity VRLA battery system with a 10-year prorated warranty is about $60,000. And, you’ll need multiple interim cell replacements every year between replacements of the complete battery system. Expect those service calls to cost about $1000. (See Acknowledgements 3.)

If these numbers aren’t enough to dissuade you from using batteries, let’s see what it could cost when it comes time to dispose of those batteries.

Lead-acid battery recycling

As if you couldn’t guess, there are plenty of federal, state and local regulations regarding the disposal of hazardous materials. (See, it's not easy being green.) The bottom line is when it comes to lead-acid batteries, you can’t dispose of them — period. They have to be recycled.

The only legally acceptable method of getting rid of them is through an approved recycler. In 1996, Congress passed a law called the Battery Act, giving the EPA enforcement authority to penalize anyone improperly disposing of batteries. The civil penalty is only $10,000 per violation. However, if someone becomes injured or made sick through your actions, the potential liability is unlimited. You may recall the movie, “Erin Brockovich,” which illustrated how the Hinkley Compressor Station in Hinkley, CA, was sued and finally penalized $333 million for polluting local ground water.

Here’s how to protect yourself.

The EPA considers you the owner and responsible party for all the batteries you purchase or own. To get that monkey off your back, you must take certain steps.

Contract the entire disposal process with an EPA approved third party. Be sure you obtain a certificate of recycling, complete with:

• Contractor name and address

• Battery type and quantity disposed

• Date of removal

• Date of disposal

• Location and number of the EPA certified recycling facility.

Without the above certificate, you can later be held responsible for any damages caused by the batteries you “thought” you’d gotten rid of.

Evergreen UPS

There is another way to provide backup power without all the hassles of EPA permits, disposal rules and the hazards of batteries. It’s to use a flywheel-based UPS system. These systems use the kinetic energy stored in a spinning wheel to provide the seconds of time needed to start a generator. In addition, they have other benefits.

Flywheel systems don’t suffer the wear out from repeated cycling as do batteries. And, they can deliver hither peak currents. Flywheel UPS technology can typically achieve efficiencies of 97 percent as opposed to the 93 percent efficiency from a battery-based UPS system. There are other benefits from using flywheel systems.

They require less space than equivalent battery systems. There’s no ventilation or hazardous gas issues. Finally, flywheel systems have long life spans and provide good ROI.

Interested? Let’s look inside a flywheel UPS.

The storage capacity comes from the spinning flywheel. The physics of such a solution are particularly interesting. A 12in flywheel weighing 23lb and spinning at 100,000rpm will develop 3kWh of energy. However, do a little math, and you’ll discover the rim velocity of the flywheel will be more than 3,500mph or almost five times the speed of sound! The centrifugal force at the rim will be 1,700,000G, requiring a material tensile strength of 500,000psi. (See Acknowledgements 4.) To operate at such velocity (not typical, by the way) requires that the flywheel be supported on magnetic bearings and housed inside a vacuum enclosure. Clearly, building such a UPS isn’t your standard DIY project.

One of the most-often mentioned criticisms of flywheel technology is that it can’t provide the length of back uptime that batteries are able to provide. While a flywheel system easily can provide up to about 15 seconds of backup power, a similar-sized battery chain may be able to provide 15 minutes of protection. However, as one engineer noted, “If you can’t get your backup generator started in 15 seconds, you probably won’t be able to get it started in 15 minutes either.”

Reliability of flywheel systems is good. According to Pentadyne, its flywheel systems have amassed 4 million hours of operation with a MTBF of 1 million hours, which is more than 100 years. Comparable battery-based UPS systems have an MTBF of 10,000 hours or about once per year.

Typical operating system

070209_fig1.jpgFlywheel UPS systems can be portable or permanently mounted. For data and video applications, solutions may come in rack-sized enclosures. A schematic is shown in Figure 1 to the right.

The flywheel system outputs a DC voltage much like that from a battery string. Output performance is voltage- and time-dependent. Typical operating parameters for a Pentadyne VSS+ are shown in Figure 2 to the left. 070209_fig21.jpg

Mechanical bearings induce drag proportional to the square of the shaft speed. Therefore, a magnetic levitation system is used to fully isolate the flywheel rotating components from friction-inducing attachments. There are no brushes or communicators to cause drag. Also, because no roller bearings are used, maintenance is lower.

070209_fig3.jpgThe schematic for one axis of a magnetic levitation system is shown in Figure 3 to the right. The system works by measuring rotor shaft displacement via a capacitive sensor. If the distance between the sensor and shaft needs adjustment, the current to the appropriate electromagnet is changed to maintain the proper displacement. Each X, Y and Z axis is controlled with separate circuits. This particular Pentadyne unit uses five independent circuits: radial upper X, Y; lower X Y and axial Z.

Now the cost

Technology has a cost, and flywheel UPS solutions are no exception. Expect to pay one-and-a-half to two times more for a similar-sized VRLA battery system. However, cost recovery may require as little as three years.

Over a 20-year life space, the cost-savings can range around $100,000 or about three times the initial system cost.

Unlike battery systems, flywheel solutions require little maintenance. Maintenance may be limited to oil and/or capacitor replacement, both of which are inexpensive. Compare that to the ongoing costs of replacement, disposal, temperature control and other factors required for batteries, and flywheel solutions look increasingly favorable.

There are several vendors with mature solutions. See the "Resources" section at the end of this article for more information.


1 Battery-free UPS energy storage, Pentadyne

2 “Making industrial UPS protection financially attractive,” ECM

3 Pentadyne VSS+ manual

4 “The spin on flywheels,” Distributed energy

Other resources:

Active Power


Vycon Energy



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