Flywheel promises 50,000-hour MTBF and near-zero maintenance.
It spins in absolute silence at 25,000 to 52,000 rpm, floating in a vacuum nearly as a devoid of molecules as outer space. Yet, charged within this 12-inch-diameter flywheel is a 190-kW jolt of power.
The thought of it strikes Dirk Freeman as almost eerie, “like levitation,” he says. A former technology consultant to Longmont Broadcasting, now with Blair Media, Freeman recently bought one for KDEN/Telemundo’s multimillion-dollar digital TV transmission facility near Denver, CO. “It’s like something out of science-fiction or the supernatural,” he marvels.
Actually, it’s a condensed carbon-fiber composite flywheel. What makes this extraordinary power density work, is a combination of light weight, at only 25 pounds; a nearly airless environment, which eliminates the heat and wear of its extremely rapid spinning; and a frictionless, bearing-free harness of magnetic-levitation technology.
When Freeman first read about the wheel, made by Pentadyne Power Corp. of Chatsworth, CA, near Los Angeles, he decided he’d someday have to get one.
In time he found uninterruptible power supply (UPS) manufacturer Toshiba, who markets the Pentadyne VSS+DC clean-energy system with their cutting-edge G8000 UPS generator. And, in 2007, he also located an apt home for one at KDEN/Telemundo, thus making this client the first anywhere to apply the fascinating flywheel as a backup for digital broadcast transmission technology (although a number of broadcasters were already using Pentadynes with other UPS packages).
For its part, KDEN/Telemundo is now assured that its signals—beaming from a site in the middle of nowhere, some distance from Denver—will never stop, at least not due to a local outage or voltage sag.
Frictionless = Ultimate Low-Maintenance
For all its 160-kW energy, the flywheel’s power comes in just a burst, which lasts, says Freeman, “at full load, from 17 to 21 seconds”—very brief, yet enough time for the UPS to start the ignition on an integrated Cummins diesel.
Next, as the backup power revs to accept the transmitter’s protected load, the flywheel transitions back to standby, then recharges in seconds. The load never even senses the interrupting blip in its high-quality, conditioned power.
Freeman does concede that, in this kind of application, earlier-generation heavy steel flywheel systems, or lead-acid batteries, “do work as well.” However, the new ultra-light flywheel design offers several practical benefits, which make it far more desirable. Above all, gone from the scene, now, are the burdens of lead battery cells maintenance. These typically entail quarterly visits and frequent monitoring, incur high-operating costs, give only relatively low reliability, and impose a significant burden for physical floorspace. In the case of KDEN/Telemundo, battery packs would have meant adding a 100-square-foot room, with an indoor environment kept, “isolated, cooled, and temperature-controlled” at 25°C (72°F),” Freeman figures.
By contrast, the new 5.7-square-foot, 71-inch tall flywheel cabinet operates comfortably in the same wide range (i.e., 0°C–40°C / 32°F–122°F) as the other UPS components.
And, overall, when comparing the demands of either a battery set or the massive steel-disk flywheels which use metal bearings, Freeman’s Pentadyne promises much more power, in less space, with minuscule maintenance.
And, operationally, since its installation in mid-2007, he reports, “We have repeatedly tested it, including putting it through a simulated crowbar event on the transmitter.” (This test directs tens of thousands of volts instantly to ground, to protect the transmission gear’s inductive output tubes, one of which can cost as much as a Pentadyne flywheel.) Through all the tests and real-world conditions, nothing has yet fazed the system.
Bye-Bye to Batteries?
At sites having even greater power loads, the benefits of eliminating batteries grow even more attractive.
By way of illustration: in one high-end three-phase UPS system offered by Liebert subsidiary Emerson Network Power, in order to get a 540-V internal DC voltage rail, notes Emerson’s power systems senior product manager Bill Campbell, “You need what amounts to two 240-cell battery blocks in series or, if using 12-V jars [like car batteries]… 40 of them in series.”
Campbell notes that Emerson/Liebert (well-recognized as a leading UPS vendors) has sold most of the 300-plus flywheels that Pentadyne has shipped from its Chatsworth factory over the last five years, and has been delighted to be able to offer this alternative.
The scores of batteries that Campbell describes for one UPS system, easily mounts much higher at larger sites and data-centers, and can run into hundreds, and even thousands, of cells. For instance, the 500-bed Sparrow Hospital in Lansing, MI, has previously needed “truckloads” of batteries, notes Jon Harris, who oversees the hospital’s electrical services. On a 1,000-amp circuit, he says, “If I want to ride through an [outage] period of 30 seconds to a minute, I have to put in ... a huge number of batteries, with all their weight and space.”
Of course, space in a hospital is in short supply and expensive. In 2006, Sparrow upgraded to a frictionless carbon-fiber flywheel Liebert FS and Liebert 610 UPS. For Harris, a load that had formerly required a scattered array of more than 100 batteries is now being protected with a single centralized backup.
As for the dollar-and-cents, although the six Liebert FS carbon fiber flywheels at Sparrow did cost more initially, Harris estimates that they should pay for themselves in just three to five years. In fact, the payback curve has been rapidly improving against initial estimates, due to a steep run-up in the cost of lead for batteries.
At any rate, after recovering its investment, Sparrow Hospital anticipates getting a total of more than half a million dollars’ worth of avoided battery costs, over the 20-year flywheel life.
Gone too will be battery-mandated space cooling chores, health and safety risks, environmental controls, and hazardous materials disposal headaches.
Meanwhile, the benefit of regaining several hundred square feet of floor space will be immediate.
Following its initial order, hospital managers have recently bought a dozen more Liebert UPS systems to back up the cardiology, neurology, catheterization, magnetic resonance, computerized tomography departments, and an IT center.
Hybrid UPS Power Preserves Batteries
Given the wheel’s technical advances, this next point will seem paradoxical, and the “bye-bye batteries” scenario looks a bit premature; it turns out that a second major role for the flywheels comes in complementing rather than replacing batteries.
Configured as the primary storage device, flywheels mesh with the DC bus of the UPS, supporting its voltage, and thus isolating the load from any incoming faults. This protects the batteries from any duty, thereby avoiding the discharge/recharge wear-and-tear, which accelerates their deterioration. Campbell explains: “If you, as a user, are in an environment that experiences many short-term outages, you’re just chewing-up your batteries. So, you can put a flywheel in parallel with the battery. Let the flywheel handle all the short-term outages, and let the batteries be there for the long term,” especially if there is no generator.
About one-third of Emerson’s Pentadyne-equipped UPS customers do this hybrid arrangement, he notes.
But again: Why keep the batteries at all? Primarily, their longer energy reserve adds a bit of surety and flexibility. As noted above, flywheels deliver a brief 10-to-60 seconds of runtime. Within this window, a properly maintained generator can power-up and synchronize with no worries. But, where grid outages are frequent, retaining batteries as a second reserve will bring peace of mind.
Such a hybrid describes a Pentadyne-based configuration designed in mid-2007 by Power Innovations, of Linden, UT, for the City of San José Airport. Critical safety and security circuits at terminal building total about 50 kilovolt amperes, notes Shashi Naik, an electrical engineer and project manager for the city. Previously, batteries had been the mainstay backup for this site, proximate to a portion of the PG&E grid, which had suffered frequent, if brief, outages before recent upgrades. The grid faults were giving the batteries a grinding workout, which necessitated added wet-nursing from maintenance and too-frequent replacements. During outages, emergency loads were draining batteries quickly, “and the whole system had a trickle charge—so that also adds complication,” Naik says.
That previous backup, “was a plain old UPS about 20 years old,” he says, “and so we decided to replace it with a new one. And then we thought: ‘Why not have a stored energy device that would help us to work on the short outages and prevent any load on the batteries?’”
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Photo: Pentadyne |
| Critical flight operations are kept blip-free by a flywheel at San Jose International Airport. |
Power Innovations recommended retaining the current battery inventory, but shielding the cells from such overuse, by integrating a 190-kW Pentadyne-made Liebert FS flywheel.
“So, the maintenance folks will be very happy,” says Naik.
If an outage lasts more than 55 seconds, and the generator hasn’t synched in time, batteries may still come into play, which is a good reason for keeping them. However, in general, he says, “the load doesn’t demand that the batteries immediately pump-in the power. So, that itself saves a lot of time—having the flywheel as the first line of defense. Eventually, batteries will start draining. But there is no sudden load on the batteries, because the flywheel, near the end of its rotation, gives a gentle, gradual handoff of the load.”
Under the new array, the batteries should suffer only minor wear, if any, and will last much longer.
Technical Considerations
PI’s Chief Executive Officer Robert Mount, who recommended the Pentadyne wheel to Naik, amplifies: “Most of the vendors that are out there do a flow charge on the batteries. So the batteries have an accelerated dehydration on them, and have a very difficult method for monitoring whether batteries are good.” All in all this means “batteries are a very weak link,” he says. “The people who are spending thousands on UPS systems really don’t have the assurance that if the power leaves, the UPS will
really work.”
Some of the drawbacks of batteries can be overcome by adding enhanced electronics and sensors. The latter can monitor status round-the-clock. Periodic battery tests can be programmed to evaluate condition, even daily. Rectifier inputs can be made to range broadly, having the effect, he says, of “constantly exercising the plates.” The wear of a flow-charge can also be reduced by replacing it with a “cycle-charge” that will be activated under certain defined conditions, to reduce battery dehydration.
All these advanced options should, he suggest, be considered as part of a backup solution. But they’re not typically available on any off-the-shelf UPS.
Other factors to weigh in the selection process are UPS programmability and a site’s electrical capacity: Can it support both a bank of batteries and a flywheel?
Power Innovations also touts the flywheel’s value in cushioning the negative effects of mediocre power quality, where that’s important. When having a lot of fluctuations in power—i.e., common voltage sags and harmonics if for only a split—or a few seconds—it can be disruptive to certain highly sensitive equipment, says Mount. The chemical reactions of batteries don’t handle this well. “Whenever you put load on the battery, they have the ‘knee’ where voltage goes down a little bit and then comes up. The neat thing about the flywheel is the energy is not ‘soft;’ it’s ‘hard,’” he says. This, he adds, means that if the voltage sags, or if the backup energy is needed, “you’re not going to have the quick dip” as with batteries.
In batteries’ favor, though, as noted earlier, they’re sometimes retained as a safety cushion against the flywheel’s limited 10-to-20-second burst, sometimes perceived as “cutting things close.”
In truth, though, as Campbell points out, even if 20 seconds seems short subjectively, in a properly maintained UPS, it’s quite adequate “and “nothing unusual to do.” In fact, every US medical facility is required to be able to switch to backup in just 10 seconds.
Freeman reports that his new UPS near Denver makes the transition in ten to 12 seconds; and when grid power returns, phasing-back to the mains “requires twelve to eighteen seconds to get fully online and locked in phase.” Meanwhile, his single Pentadyne flywheel and 225-kilovolt-ampere Toshiba G8000 UPS provide more than enough time and power to cover these gaps. In any case, the present 20-second span can easily be boosted considerably if needed, he adds.
This is possible because Pentadyne-made Liebert FS flywheels are parallelable, Campbell explains, enabling an increase in the time window and/or a boosting in the power output. “We’ve paralleled up to eight at a time for well over a megawatt, without a problem—and could go higher,” he says.
Not Gambling with Power Surety
Cache Creek Casino Resort in rural Brooks, CA, north of Sacramento provides several more examples of how the flywheel can provide additional flexibility and reliability.
Project manager and electrical specialist Tim Horton notes that the site has historically suffered outages, which sometimes lasted hours, and thus UPS reliability is all-important. Casino slot machines and table games are, he says, “very critical loads for us and must be on uninterruptible power; every voltage event or outage comes at great inconvenience to all.”
Horton’s first UPS, which was purchased four years ago and is still in use, relies on a large steel rotary flywheel. Unfortunately, not long after the casino’s opening in 2004, the wheel had a run of bad luck, blowing bearings and losing a breaker.
So, a quest began to find, as he puts it, “the side ‘B’ for the casino’s A/B power need—a nice, clean, uninterruptible power for the gaming floor,” one giving adequate kilovolt-amperes and spin-time to allow for the backup generator to start, “and to handle uneven power with swells and sags.”
As he searched the marketplace, Horton did keep the door open for steel flywheels, and he evaluated two vendors’ models; however, phone calls to other casino operations for references revealed that, “They were having terrible trouble,” just as he’d had, with bad bearings and motherboards. Service techs were always busy fixing them.
Ultimately, based on positive past experiences with Liebert, he opted for its 610 model UPS; and, for the energy storage plant, he integrated five 225-kW Pentadyne carbon fiber flywheels and two battery banks. The total came to 1,000 kilovolt-amperes, for a plus-one redundancy cushion of about 40%.
In the new system, a bidder’s quote for delivering these five high-performance flywheels surprisingly beat out the rival bid for a single, enormous (room-filling), steel-wheel-equipped UPS.
All in all, then, it was a highly attractive proposition. And, so far at least, the new system has worked out beautifully.
First, says Horton, the flexibility in having five wheels means that the failure of one (unlikely though that may be) will cause just minor inconvenience and merely a fractional loss of backup. “The generator will still synch within eight seconds,” he says, and the slot machines will keep spinning.
Second, electrical maintenance, whether scheduled or otherwise, can occur without the necessity of powering-up the generator, as was necessary before.
Third, the five-cabinet modularity will allow for the shifting of resources around the property, from one quadrant to another, as needs dictate. On this score, for even more versatility, Horton recently purchase a Liebert integrated static switch, “so now I can move loads around automatically, in case of a failure, or for the sake of maintenance, and also in the event of outages,” he says. “I can switch from one unit to another and never have downtime… [The switch] has given us redundancy. I can keep the floor up and our customers happy during the phase-in of the new equipment,” which was occurring in mid-2007. “It’s an amazing switch,” he sums up, and says he’s in the process of ordering more to manage the entire property.
Looking ahead, with modular flywheels, Horton says, “You can series the flywheels together as much as you need to back your load. We stopped at five—giving us four-plus-one—but left future space in the system for added load… if we need more time.” Neither kind of flexibility was possible before. According to Horton, size, flexibility, price, operational ease, and state-of-the-art technology “made this a much easier proposition for us. We are very satisfied so far. We’re having beautiful luck with them, and excellent service.”
Cost and Payback
Horton preferred not to discuss the project’s total cost figures, except to note, again, that, of the five-flywheels, UPS came in, surprisingly, as the low bid, in addition to being the preferred solution technically.
At the San José Airport, Naik reports that his department spent about $50,000 on a UPS, and another $50,000 for the Pentadyne flywheel as the power source. Given the wheel’s dramatically lower maintenance need, its 6-year mean-time-between-failure rating, its ability to prolong battery life, and the prospect of enhanced overall energy surety, he says, “It was not a tough sell.” In fact, more flywheels will be on order for the city’s new airport terminal, now under construction.
A Pentadyne-equipped UPS will typically cost more, Campbell concedes, but the difference compared to a lower-first-cost battery plant will be recovered within a few years. Thereafter, over the system life cycle a customer will be “far better off,” he says, as a Pentadyne flywheel is much less expensive to operate and maintain, and provides exceptional uptime.
Remarkable MTBF, and the extraordinarily light wear-tear and maintenance, are due to: the absence of any bearings; the elimination of the need for costly overhaul; the avoidance of scheduled downtime expense; and the elimination of a mechanical pump otherwise required to maintain a high vacuum, as Pentadyne’s Johnny Gonzales, vice president of sales, points out.
Keith Field, Pentadyne’s marketing vice president, adds that the flywheel’s operating costs are dramatically lower than those of earlier-generation steel flywheels. “The standby draw of the entire flywheel system is only 275 watts,” he says. Without any bearing and pump maintenance intervals to worry about—each costing several thousand dollars and hours of downtime— “the Pentadyne system,” says Field, “is probably the highest uptime availability energy storage product on the market.” The only recommended service is replacement of a set of capacitors (as is typical of any UPS), requiring one hour every six years, at a cost of a few hundred dollars.
Toshiba’s Greg Mack, who is business unit manager for the company’s UPS division, has found that, “Sites that have few power problems and no step loads, like data centers, get return on investment by the time of what would be the first battery string replacement: about three or four years.” In the medical equipment sector, he adds, “Payback is much faster.” Certain medical equipment “creates havoc for batteries,” wearing them out as often as “every 12 to 18 months, due to the frequent cycling,” he says. All in all, a compact flywheel for hospitals, in lieu of batteries is “a perfect solution.”
Flywheel Tech: The ‘Revolution’
Pentadyne’s Field summarizes by offering a brief perspective on the flywheel’s physics and early development, early in this decade.
Previous-generation steel flywheels, introduced in the 1990s, had relied on a massive puck weighing half a ton and measuring several feet in diameter, he notes. At that size, rolling on bearings, it could spin at 7,000 rpm.
However, such great mass can theoretically be reduced without a loss of power, if speed can be increased. Indeed, “Doubling rpm quadruples the energy capacity,” he says.
That principle spurred Pentadyne to develop a smaller, lighter wheel “that spins seven times faster,” he says. The end product coming from Pentadyne’s R&D shop was a mere 25-pound carbon-fiber cylinder— “about size of a racing tire on a go cart,” as one customer puts it.
Besides using mag-lev technology, “the other big breakthrough,” Field continues, ‘is the systems’ powerful internal vacuum, which reduces aerodynamic drag.”
This is accomplished with a vacuum sleeve on the rotating shaft, eliminating the need for mechanical pump, and reducing maintenance even further. End result: no friction, no heat, no energy loss, and very high power density.
From this, a patented synchronous-reluctance motor-generator produces the high power. It all adds up to a floating flywheel, which boasts, he says, “an extremely energy-efficient design.”