Protecting computing continuity is serious business. According to the META/Gartner Group, the average cost of service interruption tops $300,000 per hour. For specialized financial, manufacturing or other critical services, the cost of a momentary voltage sag can run into the millions. Strategic Research Group determined downtime cost for a typical brokerage firm can exceed $6 million per minute.

Not only does properly managing the infrastructure’s power management system play a critical role in the overall reliability of the enterprise, the power management components you select to protect your infrastructure will directly translate to the number of nines (99.xxx%) you achieve in uptime reliability.

The typical approach to mitigate power anomalies in enterprises has been to install one or more uninterruptible power supplies (UPSs), banks of lead-acid batteries, power management software, backup generators, automatic transfer switches and other supporting power devices. Based on real-world failures, managers are scrutinizing the weakest point in the power certainty scheme: lead-acid batteries. Frequent and costly battery maintenance and replacement, temperature control issues, space constraints, fire hazard permitting, toxic and explosive gas emissions, hazardous material handling and disposal concerns and, most importantly, high failure rates and poor predictability are driving contractors as well as facility and IT managers to consider other viable approaches to ensure business continuity.

One of the technologies that has caught the eye of many UPS users is flywheel energy storage systems.

Flywheel technology has been around for decades, but only in the last several years has it reached true commercial viability for protecting the enterprise. This clean energy storage technology is solving sophisticated power problems that challenge data centers every day. Flywheels used with double-conversion UPS systems provide reliable mission-critical protection against transients, harmonics, voltage sags and spikes, outages and other damaging power disturbances. The latest generation of UPS flywheel systems is delivering better energy, space and environmental efficiencies over first-generation flywheel products, as well as enormous long-term cost, uptime and dependability advantages over problematic battery-based systems.

The Spin on Commercially Available Flywheel Energy Storage

Today, there are two types of flywheels for UPS applications. The first to market, which is still sold today, is a relatively simple design that yields much higher power density (smaller footprint) than VRLA batteries (which are cheaper and more compact than wet-cell types). Using a 600-pound steel “puck,” it replaces UPS battery strings, eliminating the need for HVAC, hazmat and safety issues, and a whole lot of floor space. The system can deliver 250 kW to protected loads for a good 13 seconds (longer for lower outputs), providing the right amount of time to start and switch over to the UPS’s backup generator.

A few years ago, a Los Angeles company developed the next-generation flywheel, a system that is lighter, faster, smaller and much more energy efficient than first-generation flywheels. Using a fast-spinning, light but stronger-than-steel carbon-fiber-composite cylinder, these new generation flywheel systems are integrated with major UPS brands and are sold by industry leaders such as Liebert/Emerson Network Power, Toshiba, Socomec Sicon and EHWA. These newer models deliver a lighter, more compact and near maintenance free energy storage solution compared to slow, heavy flywheels. (fig. 1)

A flywheel system works like a dynamic battery that stores energy kinetically by spinning a mass about 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.

The amount of energy available and its duration is proportional to its mass and the square of its revolution speed. First-generation flywheels rely primarily on mass to store energy Using a large mass, however, imposes floor loading and mounting issues, and limits maximum safe rotational speed, hampering energy density (the amount of energy stored versus the footprint of the device). In the flywheel world, doubling mass doubles energy capacity, but doubling rotational speed squares energy capacity.

With physics on its side, Pentadyne Power Corporation broke the mold a few years ago by offering a high-speed, carbon-fiber-composite flywheel system. Instead of utilizing the mass as a primary source of energy, it takes advantage of rotational speed. The higher the speed, the smaller the mass needed for any given energy output. Smaller mass and innovative technology combined to make full magnetic levitation of the spinning mass a possibility. This assures silent, vibration-free operation and eliminates the need for bearings that require annual service as well as complete replacement every two or three years.

Both heavy and light flywheels spin in a vacuum chamber to reduce aerodynamic drag. Most use a continually running vacuum pump to maintain that vacuum, but the lightweight carbon-fiber offering has a factory-sealed vacuum and employs a patented molecular vacuum sleeve integrated with its center shaft. The fast rotation of the shaft allows helical grooves in the maintenance-free sleeve to maintain high vacuum. It also eliminates the frequent oil changes and semi-annual pump replacements.

Power Quality and Reliability

Battery-based UPS systems provide high-quality, seamless ride-through power to protected loads in the event of poor power quality on the utility feed or a complete power failure. A typical double-conversion UPS uses a string of batteries connected to the DC bus between the rectifier and the inverter. In most cases, 10 seconds or less are needed to bridge the gap between utility failure (collapse of the AC bus) and genset hand-off. Predictability of battery performance, however, is a big issue known only too well to veteran UPS users. There’s really only one truly reliable way to know state of charge and what will happen when a load is suddenly applied to a string of batteries: crossing of one’s fingers. Power management systems provide a good guess, but the only way to know when batteries will save the day is when they are called on for real-world duty. Like an old string of holiday lights with one bad bulb, one dead cell in a battery string renders the entire string inoperable. Batteries are wired in series and their expected MTBF is little more than 2,000 hours: Truly the weakest link of any UPS system.

Battery manufacturers state that UPS battery life can be maintained as advertised if, and only if, they are temperature controlled to a constant 72° - 75°F and experience no excessive cycling, i.e., they’renot used.

The major reasons batteries fail (as cited by battery manufacturers) are attributable to heat or cold, poor maintenance, corrosion, loose connectors and ripple current. However, the primary source of battery failure is quite simply their usage. Every time the battery is used in a short discharge activity or “cycling,” it experiences the “coup de fouet” or “whiplash” effect, rendering it significantly less capable of operating properly when most needed.

On the upside, flywheel energy storage systems operating in parallel with battery strings can eliminate the coup de fouet and virtually all cycling. According to the Electric Power Research Institute (EPRI), a well-respected organization formed primarily of utilities, 80 percent of all utility power anomalies/disturbances last less than two seconds, and 98 percent last less than 10 seconds. Thus, a flywheel’s power reliability is an excellent substitute for batteries in bridging the time to the backup generator handoff. For data centers – whether freestanding or corporate IT facilities -- where continuous power is mandatory, flywheel bridging to the genset handoff, with or without batteries, is the ideal solution. In industrial applications where temperature control is problematic, and at hospitals where floorspace is at a premium, flywheel bridging to the genset is an ideal solution that eliminates batteries from the equation. (Fig.2)

A flywheel operating in parallel with batteries “hardens” them, increasing longevity and reliability since the flywheel will absorb all short-term anomalies/disturbances, leaving the batteries untouched. In this configuration, the only time the batteries would be called upon would be in the event of a genset failing to start. Unfortunately, the few minutes of battery time would be of little use in such a scenario, which is why many facilities simply eliminate problematic batteries altogether. Although there is often concern about this, the IEEE Gold Book (appendix L, table XII) acknowledges that this is exceptionally uncommon. Compiling six separate studies encompassing 25,000 critical (such as hospital generators) and non-critical (rarely tested) emergency gensets, IEEE reports that 99.5 percent start on the first attempt. The leading cause of that 0.5 percent failure, as it turns out, are the genset batteries.

Utility Power Reliability Getting Worse

National and local utility studies are signaling that electric grid reliability is in decline and that power disturbances will become more frequent. On the other side of the coin, increased use of microprocessors in a large variety of power, control, processes and manufacturing equipment boosts sensitivity to common electric grid power fluctuations.

Flywheel Vs. Batteries

Weaning oneself away from the love-hate relationship with batteries is no easy task. Batteries are a cheap and known commodity, and experienced users are fully aware of their significant shortcomings. Some users even refuse to monitor or test their battery strings, since doing so negatively impacts performance and may hasten a cell failure that will make the entire string useless when called upon to do real-world duty. Yet the gamble is taken and batteries continue to be installed, along with requirements that include:

• Frequent and costly maintenance;
• Excessive HVAC needs, which introduce further expense and failure issues;
• Expensive monitoring;
• Large footprint;
• Massive weight, which limits placement flexibility;
• Slow recharge;
• Frequent individual cell replacement;
• Second string redundancy to halve failure odds;
• Fire hazard permits, employee safety and OSHA requirements for toxic chemicals and explosive gases;
• Environmental and disposal issues;
• Separate replacement battery storage requirements; and
• An overarching “cross-your-fingers” approach to power reliability.

Replacement with a flywheel reverses the negative impact of each of the above, resulting in:

 

• Infrequent, low-cost maintenance (near none with carbon fiber flywheels);
• No temperature control or ventilation requirements;
• Built-in precise monitoring and diagnostics;
• Minimal footprint/high power density;
• Low weight enables siting flexibility anywhere in the building;
• Rapid recharge functionality (symmetrical discharge/recharge with carbon fiber flywheels);
• Flywheel module not replaced for at least 20 years;
• Redundancy due to poor reliability is not applicable;
• No hazmat or dangerous emissions;
• No disposal issues;
• No separate storage room required; and
• A reliable strategy for power reliability.

Life Cycle Costs

While a flywheel system costs about 1.5 to 2 times as much as a similarly sized monitored 5-minute VRLA battery bank (the cheapest available), it recovers its cost rapidly, usually in two to three years. Thereafter, the flywheel solution becomes an ongoing source of both power security and cost-containment. Over a 20-year design lifespan, the cost savings are in the range of $100,000 per unit deployed – enough to repay the cost of the flywheel three times or more. The cost savings are vastly lower due to the greatly reduced maintenance and replacement needs. For the lowest-maintenance model on the market, recommended hardware service is just a quick, low-cost capacitor replacement once every six years (52,560 hours). That’s it for the entire 20-year design life. For the same period, battery strings would need to be replaced five times based upon an optimistic life expectancy of four years. In addition to entire string replacements, there would likely be a very high number of individual cell replacements, monitoring system costs, temperature control and ventilation expenses, and costs related to battery disposal, space utilization, fire-hazard permitting, hazardous-materials handling, stored replacement cells, acid spill containment, inspections, OSHA compliance, and – the most costly of all – lack of reliability and resulting downtime.

Energy efficiency is increasingly important and that was one area where most flywheels fall short. Standby electric usage by steel flywheels is about 3,000 watts. With the generational leap made by maglev carbon fiber technology, energy efficient high-speed models use one-tenth that amount – a mere 300 watts – which is comparable or less than the float charge of a similar sized battery array. With the elimination of the temperature control and ventilation needs of batteries, energy efficiency of flywheel energy storage is vastly better than that of battery arrays.

Throughout the U.S., and around the world, data centers (IDCs and corporate IT), broadcasters, hospitals, laboratories, airports, manufacturers, military facilities and other sites are hardening their battery strings, or eliminating them altogether, by applying flywheel energy storage to their UPS systems. Data center design expert Robert McFarlane of leading technology systems specifier Shen, Milsom & Wilke has said, "I'm starting to consider recommending a combination of UPS and flywheels."

Giving up old habits, especially bad ones, definitely takes intestinal fortitude. However, those who have taken the plunge are glad they did. John Smith of the NetAlliant data center in Chattanooga, Tennessee, said, “We take power protection seriously, I hate batteries, they’re nothing but trouble.” His flywheel system has endured the vagrancies of more than 100 power disturbances a year, all without missing a beat. Smith – and hundreds of others – are sold on the reliability and performance of their battery-free UPS system and have no plans to ever look back.