Low-tech materials, high-tech design: a new spin on flywheels

22 January 2001

EPRI has estimated that power disturbances may cost US industry $30 billion per year and the power quality business has been estimated to be worth around $11 billion per year. This is prompting increasing interest in flywheel energy storage systems, particularly those that can avoid the high costs associated with some of the more exotic technologies.

Over the last decade, the power quality market has seen the emergence of several new energy storage technologies that address immediate delivery of energy to critical loads following the interruption of utility power. These alternatives to traditional electrochemical batteries include such exotic technologies as supercooled electromagnets and quick-start engines augmented by hydraulic or pneumatic energy storage. More recently, there has been increasing interest in flywheels, with their inherent simplicity.

Flywheel technologies range from high-tech composite wheels operating at ultrahigh rotational speeds to the more traditional steel wheels coupled to existing rotating machines.

With its CleanSource flywheel technology Active Power of Austin, Texas, United States, is aiming to combine the most attractive features of modern and classic flywheel designs. The strategy is essentially to use a low flywheel rotational speed - generally much less than 10 000 rpm - and low-tech materials, but in a high-tech design, which can deliver up to 500 kW of power while occupying a floor space of less than 10 square feet.

The traditional steel flywheel technology has several advantages over high-tech machines using composites. For example steel is familiar, predictable and relatively cheap. Steel wheels tend to be heavier, so rotational speeds can be lower, which helps keep the design simple (eg allowing use of conventional bearings). Among the major disadvantages are low energy/power density. Also use of a separate flywheel coupled to a motor/generator (as opposed to the integrated systems of modern designs) requires multiple bearing sets, which can reduce reliability and increase maintenance costs.

High tech, high costs

As a result of continuous developments in composites, flywheels are now available that operate at rotational speeds in excess of 100 000 rpm, with tip speeds in excess of 1 000 m/sec.

But the advantages afforded by modern composites do not come without cost. The ultrahigh rotational speeds that are required to store significant kinetic energy in these systems virtually rule out the use of conventional mechanical bearings, so generally magnetic bearings are used, which require sophisticated computer control. Also aerodynamic drag losses force most high-speed flywheels to operate in a partial vacuum, which complicates the task of dissipating heat.

The integrated flywheel and motor/generator normally used in the high speed systems is usually a rotating-field design, with the magnetic field supplied by rare earth permanent magnets. Since the specific strength of these magnets is typically just fractions of that of the composite flywheel, they must spin at much lower tip speeds; in other words, they must be placed very near the hub of the flywheel. This compromises the power density of the generator. An alternative is to mount them closer to the outer radius of the wheel, but contain their inertial loads with the composite wheel itself. This forces the designer of the high speed machine to either derate the machine speed, or operate closer to the stress limit of the system, thus compromising safety.

Nevertheless, the high-speed high-tech route can provide a compact, lightweight flywheel battery, which needs little maintenance, suffers insignificant degradation from multiple discharges, and exhibits minimal sensitivity to operating temperature.

However, current component costs drive system prices to levels that make competing with existing energy storage technologies exceedingly difficult, except in perhaps the most esoteric applications. These costs practically eliminate the option of operating the wheels with reasonably large factors of safety. Thus, some sort of inertial containment system becomes necessary to minimise collateral damage from a failed flywheel. Such failure can occur for many reasons, including crack growth from material flaws undetected at manufacture, excessive shock loads in the installed environment, and magnetic bearing failure. The cost and complexity of providing sufficient containment further reduces the competitiveness of this technology.

Getting the best of both worlds

The fact that the power quality market has inherently different requirements from the electric vehicle, utility load-levelling and satellite control markets prompted Active Power to investigate potential design hybrids of classical and high tech that achieve high performance at competitive cost. The CleanSource family of flywheel batteries, using a high-power-density integrated motor/generator/flywheel operating at relatively low speeds (eg 7700 rpm), is the result of this effort.

Ride-through requirements for the power quality industry fall into two main categories:

* enough time to power the load until standby generator startup (~10-45 sec);

* enough time to power the load through the vast majority of events (~5 sec).

The CleanSource design takes maximum advantage of a flywheel’s natural capacity to produce high power in a compact space for a relatively short period. By designing the product for applications that require relatively short power delivery times, the system aims to deliver what the application demands at the lowest possible cost. The CleanSource system covers a broad range of power classifications with single or paralleled systems to extend power and/or runtime as needed.

The high-power, low-loss capability of the CleanSource system is the result of a proprietary generator technology, use of a vacuum enclosure, and a combination of magnetic and conventional mechanical bearings. The partial vacuum reduces aerodynamic losses and noise, while the hybrid bearing system minimises bearing losses and extends service life. For example, the standby losses of a 480 kW Active Power unit are under 3.5 kW, a standby efficiency that is significantly higher than traditional electrochemical battery strings.

Given the power quality market’s minimal concern with installed system weight for a particular level of stored energy, Active Power took a conservative design approach to produce a well-engineered and inherently safe system. Instead of “pushing the envelope” of rotational speed with esoteric materials, the CleanSource product incorporates rotors machined from a solid block of forged high-strength steel.

The motor, generator and flywheel rotor functions are integrated into one single piece of solid forged steel. The integration and use of the full length of the rotor as active generator results in lower costs and higher power densities. The field coil provides current to magnetise the teeth of the steel rotor which rotates past the copper coils imbedded in the armature to generate power. As the rotor slows during a discharge, the field is increased to raise the magnetism of the rotor teeth, thereby compensating for the speed loss which in turn keeps the voltage constant until approximately 80 per cent of the rotor energy is consumed. The field circuit also serves as the magnetic bearing, unloading a large portion of the rotor weight, which greatly extends the life of the mechanical bearings.

Multiple accelerometers feed an onboard microprocessor to allow monitoring of shaft vibration. The microprocessor periodically performs spectral analysis of the acceleration signals to detect any unusual signal components. A number of thermistors detect local operating temperatures within the unit and the associated electronics cabinet. The potential for runaway motoring is addressed through automatic shutdown upon loss of any system-critical component. In addition, conservative trip levels either issue “caution” or “shutdown” signals in the event of any unusual vibration or temperature readings. They are designed for much longer service intervals and extreme operating environments than typical electrochemical battery installations and are claimed to be the first competitive alternative to batteries in cost, footprint, and efficiency. Currently, the technology is available in a variety of delivered power ratings up to 250 kW per wheel, providing 40 kW for 120 seconds/480 kW for 12.5 seconds.

The technology can be applied to larger systems, and the individual devices can be placed in arbitrarily large arrays of parallel units to offer flexibility in energy storage, peak power rating, and ride-through time.

Active Power’s CleanSource flywheel system is targeted to three primary markets: continuous power; power quality improvement; and battery isolation and redundancy (see diagrams, left).

Continuous power

In many circumstances, the end user of critical power has a strong desire to eliminate the requirement for electrochemical batteries due to environmental restrictions, maintenance concerns and/or limited space. If the power quality configuration includes a standby engine/generator for long-term protection, a flywheel energy storage system may be well suited for providing power until the start and synchronisation of the genset. Active Power also provides a means to provide the starting power for the engine to achieve a truly battery-free continuous power solution.

Power quality improvement

Due to the fact that the vast majority of power quality events have a duration of only a few seconds, some power users have the opportunity to improve the quality of their power in all but long-term outage situations with minimal cost and space outlays. Batch or process manufacturing sites with a history of short-term power glitches or sags (which have remained unprotected due to the high costs or space requirements of traditional energy storage) are ideal applications for high-power flywheel systems.

Battery isolation and redundancy

One of the primary determinants of electrochemical battery life is the number of times the cell is discharged, the battery life being inversely proportional to the number of discharge events. A flywheel battery is an effective means of protecting a chemical battery string.

CAT UPS systems

As well as its own CleanSource DC systems, which are designed as a replacement or complement to lead–acid batteries in UPS applications, Active Power’s flywheel technology is also being used in the new range of jointly developed battery-less UPS systems being offered by Caterpillar, on which EPRI has collaborated.

The Cat UPS system, which is claimed to achieve the smallest footprint in the industry, provides power from the flywheel when it senses a power disturbance and in the case of long term outages or brownouts achieves uninterrupted transition to a standby generator set. The CAT UPS has the ability to send a start signal and provide redundant starting power to a standby genset. Features also include e-mail notification and remote monitoring via the Internet.

Caterpillar is claiming an efficiency of 97 per cent for its UPS, compared with 90-94 for conventional UPS systems.

For 60Hz applications the following Cat UPS devices are now available: the UPS 250 (250 kVA/250 kW), which was the original machine, introduced in July 2000, the UPS 300 (300 kVA/240 kW), the UPS 600 (600 kVA/480 kW) and the UPS 900 (900 kVA/720 kW). For 50 Hz, the devices available are: UPS 250 (250 kVA/200 kW), UPS 500 (500 kVA/400 kW) and the UPS 750 (750 kVA/600 kW).

EPRI worked with Duke Energy to install and demonstrate the first, 250 kW, Caterpillar flywheel UPS system, at carpet-maker Shaw Industries plastic filament extrusion facility in Charlotte, NC, USA.

Duke Power identified Shaw Industries as a customer who was losing production through sag and momentary outage events. In one case the plant was out of action for eleven hours following a sag that tripped all four extrusion lines. Remedial action included sending all available employees to the shop floor to work sequentially on each of the four extruder lines. Once one extruder was running the next was worked on. This was a painstaking process because each line produced multiple separate filaments of plastic that had to be threaded by hand through the machine.

The cost of a power quality event at the extruder reached far beyond the direct production loss to include the cost of scrap material, the indirect labour cost, and the potential purchase of competitors’ material. PQ mitigating devices such as uninterruptible power supplies, ferroresonant transformers, surge arresters and contactor coil-hardening devices had already been fitted, but this only made it more clear that an energy-storage solution was required to enable the process line to ride through sag and momentary outage events successfully.

To document the site conditions fully, monitoring instruments were connected to the drive panel board. These measurements were gathered for a year before the new flywheel system was placed in service. It was noted that if the extruder system stopped due to a sag, about 64 bobbins of partially-filled product would be wasted. This material could not be recycled because of the work hardening of the strip during the process.

Once installed, the flywheel demonstrated a very fast response by reacting to the switching event of a power factor correction capacitor located at the local substation. Subsequently, on 6 January 2000, the flywheel system also caught a 17 cycle sag and protected the extruder process line. It is worth noting that the three other extruders in the same building were tripped by this sag event causing hours of lost production. It is interesting too, that this duration of sag has been identified in EPRI studies as the most prevalent event on radial feeder systems.

This installation at present only uses about one third of the flywheel’s rated capacity and so measurements of efficiency give lower than the nameplate information. This will change as the load is increased by the connection of another extruder line.

Based on the electrical monitoring and 1999 plant maintenance records, Extruder Line No 1 was stopped on average 30 times per year due to power quality events. The line was down on average 45 hours. This downtime represents $15 000 dollars of lost product that could be saved with an energy storage device. Each time an outage occurs, the restart time is 1.5 hours and absorbs the labour of at least two operators. At the time of system trip, all the material on partially filled bobbins (less than half full) is scrapped.

This represents on average two hours of production, as a full roll takes four hours to produce. With the process line out of action, potentially the company has to purchase material from a competitor to ensure that production is maintained in the carpet backing weaving plant.

Plans are in place to improve the efficiency of Extruder Line No 2 so it can be added to the flywheel. When this additional line is added, the savings will increase by $36 000, to a total of $51 000 annually.

December 2000 saw the first shipments by Active Power to Caterpillar of the 600 kVA and 900 kVA UPS machines. Among the initial applications will be advanced data centres in Maryland and California.

Recent applications

Active Power’s flywheel technology is now in use at a number of sites, including data centres, web hosting and broadcasting facilities, semiconductor plants and other industrial

installations, with users including ABC (New York facility), AEP (protection for corporate offices in Ohio), Fairview Hospital, Cleveland, Micron Technologies, Idaho, and Community First Bank, Fargo, North Dakota.

Among other recent applications is backup duty for the STMicroelectronics advanced semiconductor fabrication facility in Rousset, France. Fifty one 240 kW flywheel systems are being supplied through a Caterpillar dealer. They will be used in conjunction with power electronics supplied by MGE UPS Systems to protect four 2500 kVA transformers. The 0.018 micron wafer fabrication facility needs near-perfect power for optimal product quality and plant productivity. Previously, in October 1999, MGE UPS Systems had formed a partnership with Active Power to market advanced power quality technology, along with Southern Company.

The flywheel technology has also been integrated into an existing Liebert UPS system at a Comcast cable and Internet hub facility in the United States. An Active Power CS 200 CleanSource flywheel system was installed by Constellation Energy Source, a wholly owned subsidiary of Baltimore Gas & Electric, in a demonstration project undertaken in collaboration with EPRI. The aim was to improve the overall reliability of the UPS and extend the life of the existing battery string. Under the old configuration, in the event of large load changes, the existing UPS would switch to battery while the site’s gas engine generator stabilised. Now the flywheel supports the UPS load, saving the batteries for emergency backup.

In November 2000, Emerson Electric subsidiary, Liebert, which is a worldwide reseller of Active Power products, ordered 36 CleanSource 240 kW flywheels for use in a data centre.

Around the same time Active Power announced it was expanding its manufacturing facilities tenfold to meet demand for the CAT UPS and its own CleanSource DC system.

Meanwhile, at the low power end of the market, Active Source is developing a flywheel-based system for telecom applications, designed to provide 6 kW of back-up power for 8 hours.

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