Flywheels: reducing energy risk

For some electricity consumers, ensuring continuous supply is a top priority. Keith Field of
Pentadyne Power Corp explores how flywheels can meet the needs of such consumers and
offer substantial benefits.

While for the majority of domestic and industrial users, a brief power outage or brown-out is just an inconvenience, there are a substantial number of organisations, for which even a momentary lapse in power supply is completely unacceptable. Examples include hospitals, data centres, casinos, manufacturing and other industrial sites, airports, television stations and military facilities.

The traditional means of guaranteeing a continuous power supply has been the use of lead-acid batteries. Although a tried and tested solution, such batteries come with a heavy environmental cost. They also require continual and expensive maintenance, are heavy and are slow to recharge. Disposal is also an issue, as most battery manufacturers state that battery life can be maintained as advertised for at least four years if, and only if, they are kept at a constant 75°F (requires air conditioning) and experience no excessive cycling. When one considers the nature of the chemicals used in the batteries, this situation is clearly not ideal. In terms of reliability, lead-acid batteries are not an optimal solution. Most batteries for uninterruptible power systems (UPS) are arranged in strings and it can take only a single dead cell in a string to render the entire string inactive. Fortunately for those requiring a UPS solution, there is now a proven alternative to batteries in the form of flywheels.

Acting as a mechanical battery, a flywheel power system 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 interruptible power for mission-critical installations.

Unlike traditional approaches to flywheel technology, Pentadyne has opted to store the bulk of the energy captured by the flywheel in the form of rotational velocity, which is measured in rotations per minute (rpm). Previously, the bulk of the kinetic energy had been stored in the form of the flywheel’s mass. However, such an approach severely limits the maximum rotational velocity of the flywheel, reducing its energy density.

Figure 1: A cross-section schematic of a Pentadyne flywheel. A number of innovations
combine to offer extreme energy efficiency and reliability, while at the same
requiring minimal maintenance.

To give some idea of the energy density made possible by this approach, an array of five Pentadyne flywheels provides 11.4 seconds of power for a 100 per cent loaded 1000kVA UPS (or twice that at a more reasonable 50 per cent loading), then does a smooth hand-off to the emergency generator. The flywheels occupy half the space of a valve regulated battery set, or less than a quarter that of wet cells.
Pentadyne’s flywheels are made from carbon fibre and achieve speeds of up to 54,000rpm. As they are magnetically levitated and operate in a sealed vacuum (see Figure 1), standby power losses can be kept to an absolute minimum. In fact, while on standby, a typical Pentadyne system consumes only 300W, that’s around one tenth of that consumed by low velocity heavy flywheels currently on the market. Despite the high speeds involved, the design of the unit as a whole prevents dangerous transfer of kinetic energy to the cabinet in the highly unlikely event of a flywheel coupling failure.

Flywheel maintenance is infrequent and inexpensive. Pentadyne flywheels are designed to operate for more than 50,000 hours (which translates to six years continuous operation), before filter capacitors need to be replaced. The use of magnet levitation eliminates any need for drag inducing bearings. A maintenance-free vacuum sleeve on the rapidly spinning central shaft maintains the factory-sealed vacuum. Pentadyne systems are designed with a life-span of 20 years in mind, during which, a battery UPS system would result in the dumping of 15,000lb of lead and hundreds of gallons of sulphuric acid.
In terms of economics, flywheels are extremely competitive, Figure 2 is a cost comparison between the Pentadyne VSS+ dc, a steel flywheel and a VRLA battery string at a 190kW/225KVA load. The capital cost of the flywheel is 1.5 times that of the batteries, but this is compensated by the need to replace strings of batteries at least five times in 20 years, based on the optimistic life expectancy of four years. The cost of maintenance is 80 per cent less and of cooling and ventilation is essentially zero. In the medium term, the initial cost of the flywheel is recovered within 18 months, beyond this point the flywheel saves hundreds of thousands of dollars.

Figure 2: Cost comparison between VRLA batteries, steel and
carbon fibre (CF) flywheels, over a 20-year period.

Flywheel UPS for the pharmaceutical industry

When it comes to manufacturing pharmaceuticals, stringent adherence to quality assurance is of paramount concern. Precise quality control processes ensure products are manufactured to the most exacting specifications and dosages. This critical requirement demands that reliable power is maintained throughout the production process and that all parameters are monitored and recorded to assure the proper product manufacture. For one of the world’s top three multi-billion dollar pharmaceutical companies, the importance of its corporate data centres processing production and inventory data across its worldwide operations is so critical that any loss of power – even for a moment – is simply unacceptable, given the disruption that such an event would cause.

A better solution

Over time, this particular pharmaceutical manufacturer’s UPS systems were ageing. The need for a better solution was clear to the facility management staff, but space for power equipment on the plant floor was very limited and there was no room for cooling equipment. Facilities personnel had plenty of experience with both wet cells and valve regulated lead-acid (VRLA) batteries. VRLA batteries were unacceptable as they demonstrated low reliability. Wet cells had somewhat better reliability, but required plenty of valuable floor space, dedicated HVAC for cooling and explosive gas venting, and very frequent maintenance and testing. Clearly, neither of these lead-acid battery choices seemed worthwhile. Adding limited, unconditioned floor space to the mix just made matters worse.

Fortunately, this world-class manufacturer was able to cure its ills with an advanced DC flywheel power system from Pentadyne that is fully compatible with UPS systems from leading manufacturers. Since the pharmaceutical company required high power – but for only a short duration to assure smooth transfer to a backup generator – the flywheel systems were an ideal solution for their particular application. A single Pentadyne flywheel system can deliver 190kW for 10 seconds (or longer durations at lower loads). The unique design allows the units to be easily paralleled when higher power, longer run time or redundancy are needed – just like battery cabinets for quick and easy expansion. In this case, three paralleled systems accommodated a ride-through time of 20 seconds for their 400-kVA UPS (assuming an unrealistic 100 per cent loading) before transferring to the diesel generator.

Real-world verification

The facilities manager expressed a firm interest in the flywheel-based solution, but indicated that he would welcome a witness test to build up his level of confidence into this innovative approach. The UPS manufacturer performed real-world testing at their state-of-the-art facility in the US Midwest to show the UPS and three flywheels in action. The conclusive tests proved convincing and installation was given the go ahead. Once in place, a one-time test after the start-up service was performed. Once the flywheel-based UPS was online, the UPS rectifier was disabled briefly through software controls to confirm that each flywheel unit would transition to discharge mode and support the load. Then full load was applied and the main grid supply circuit-breaker was opened to disconnect the power. The flywheel UPS supported the load seamlessly and it has ever since. While virtually all UPS users have found lead-acid batteries a bitter pill to swallow, this particular multinational brand has discovered a cure.

Flywheels for NASA

The National Aeronautics and Space Administration (NASA) is no stranger to flywheel technology, having employed flywheel energy storage and altitude control for long-term mission requirements in space. Now NASA has brought this tried and tested technology down to Earth for their mission-critical systems and laboratories. Needing a replacement for the batteries used in their UPS, NASA sought a commercially-available solution that would be cleaner, more compact and less costly than problematic lead-acid batteries. Already well-versed in both the disadvantages of battery-only backup and the advantages of mechanical energy storage, NASA’s Glenn Research Center (GRC) selected the Pentadyne clean energy storage flywheel to be the first line of defence in its UPS system. It’s been doing the job there 24/7 since early 2004.

Power wheels

With accumulated masses of data and research, NASA’s mission-critical systems and laboratories rely heavily on continuous power. A long-time user of lead-acid batteries for energy storage, NASA was intent on finding a solution to replace the batteries in its UPS. The flywheel would instantaneously bear the brunt of all voltage sags and other power disturbances. NASA’s own R&D years ago led to the design and fabrication of a solar panel-charged flywheel intended for the International Space Station. “More than 85 per cent of the energy put into the wheel comes out,” said Ray Beach, team leader for the GRC’s flywheel development. Good performance, but no match for the typical 99.85 per cent energy efficiency of the vastly more affordable Pentadyne flywheel system.

The flywheel ensures that critical loads at the GRC remain isolated from power quality problems. A generator handles longer duration outages. The resulting continuous power quality system protects the laboratories from power disturbances while eliminating the use of batteries and their costly maintenance.

Measuring a mere 25 x 33 x 71 inches, the flywheel met the project’s size requirement and was compatible with NASA’s existing UPS and generator. Upon testing, Glenn researchers found that the flywheel-based UPS solution performed flawlessly, kinetically storing energy in its composite rotating group when the DC voltage is above the normal set level, and instantly supplying proper voltage when the DC bus drifted lower.

When grid supply was lost completely, the flywheel provided safe, immediate and reliable ride-through power until the generator launched. Once the generator took the load, the flywheel recharged rapidly, ready to protect the load from any further disturbances.

In addition to surpassing UPS batteries in performance and reliability, the flywheel sports a compact design. There are no poisonous materials used in the system; eliminating toxic disposals into the Earth’s ecosystem and aiding NASA’s mission of space exploration.

Above and beyond

The GRC’s Pentadyne system has been responding to an average of more than 50 power disturbances per year, most of them very brief. According to independent utility studies, 99 per cent of all power disturbances last less than 10 seconds. The flywheel-based UPS solution also delivers energy-saving current adjustment and an extended lifecycle with extremely low standby consumption. Installed in March of 2004, the flywheel has consistently exceeded the project’s objectives.

Flywheels are an ideal solution for data centres and hospitals for whom
uninterruptable power is an absolute necessity.

Call Henry Inc is the company that manages facility operations at the GRC and other government facilities. In July of 2006, Call Henry Supervisor Scott Marabito said that – since March of 2004 – the flywheel “handled 125 short duration outages! This is great! The energy to keep the flywheel running is substantially less than what would have been necessary to charge up batteries.”

In summary, carbon fibre flywheels offer a cost-effective and environmentally-friendly alternative to traditional lead-acid batteries and even steel flywheels, offering higher performance and reliability. Given the need to replace batteries regularly, switching to a flywheel system makes economic sense.

Keith Field is vice-president of marketing at Pentadyne Power Corp. He has more than 20 years of experience in the electronics industry. For more information on flywheel UPS
solutions and Pentadyne’s products, visit www.pentadyne.com. Alternatively email: sales@pentadyne.com