The need for modular energy storage systems - technologies that can allow large loads or even whole facilities to ride through brief power sags or outages - has surged in the power quality marketplace, giving rise to a host of new offerings.  The options include low- and high-speed flywheels, improved chemical batteries, superconducting magnetic energy storage, and ultracapacitors, as well as new systems that incorporate several of these devices or even combine them with distributed generation technologies.

The market opportunity for these technologies is significant. Brief power outages lasting a few seconds or less are becoming increasingly costly to businesses as more sensitive electronic equipment becomes commonplace in commercial and industrial operations.  Such outages cost facilities an average of $4,000 to $11,000 each, although many end users suffer much greater losses. For example, one semiconductor manufacturer reports that a single five-second outage could cost the company $12 million in lost production alone - the equivalent of its entire annual electricity bill.  But for all end users, the effects of outages can be even broader when scrapped product, damage to equipment, and the labor required to get things running again is considered.

Emerging energy storage technologies offer significant enhancements over chemical battery-based uninterruptible power supply systems - the dominant technology in what is now a $2 billion annual market in the United States.  The new options hold the promise of much longer life, lower maintenance requirements, and no hazardous materials to cope with.  To be successful, however, the first cost of these energy storage technologies will have to come down, and developers will need to find effective market channels and learn how to identify customers with the right mix of outages and outage- related costs to justify the purchase of the new ride-through systems.

There is little dispute that power disturbances are costly to business.  One oft-cited study has estimated that power interruptions cost U.S. industries $13.3 to $25.6 billion per year in lost production, not including scrapped product, damage to equipment, or labor required to get things going again.  These "secondary" expenses increase the business cost of momentary outages by over 58 percent, and studies have shown that they can add an estimated 30 percent to the cost of long power outages (those lasting four hours of more).

Until recently, the only practical way for energy users to store electrical energy was the chemical battery - in particular, the familiar valve-regulated lead-acid (VRLA) battery and flooded lead- acid batteries.  In fact, the market for uninterruptible power supplies (UPS's) based on lead-acid batteries amounts to more than $2 billion per year in the U.S. alone.

UPS's based on chemical batteries enjoy this huge market for a reason.  They are modular and easily mass-produced.  The presence of chemical batteries in every one of the hundreds of millions of automobiles plying the roads around the world has served to make them one of the most ubiquitous - and most thoroughly field-tested - technologies in the world.  Despite this success, chemical batteries do have a number of drawbacks, detailed below, that have spurred competing technologies to challenge their supremacy in the energy storage marketplace.

A number of weaknesses in common chemical-battery technology include:

HIGH MAINTENANCE REQUIREMENTS.  Lead-acid batteries must be regularly inspected for heat buildup and acid leaks.  They also need to be tested for reserve capacity, since there is no easy way to monitor a chemical battery's storage level.

LIMITED CALENDAR LIFE.  Most chemical batteries must be replaced, regardless of the application, at least every six to seven years. This presents an eventual replacement and disposal cost that must be factored into project economics.  The U.S. Environmental Protection Agency places requirements on battery manufacturers to dispose of spent batteries; some battery salvagers reportedly remove old batteries "free of charge" because of the valuable lead and other metals that can be recovered.

TEMPERATURE SENSITIVITY.  Chemical batteries are also sensitive to temperature extremes.  Battery life is cut in half for every 15 degrees Fahrenheit above 77 F to which a chemical battery is exposed.  In the hottest regions of the U.S., batteries may have to be replaced every year, and even in the best of climates, standard warranties only cover a five-year period.

HAZARDOUS MATERIALS.  Chemical batteries typically contain lead and other materials that require special handling and disposal procedures.  End users must provide for containment of any acid spills from damaged, overheated, or overfilled batteries.  Disposal costs could become a critical problem if newer regulations increase the steps required to legally dispose of these batteries.

LOW SYSTEM ENERGY DENSITY.  Although a single chemical battery has a relatively high energy density, researchers have found that when batteries are stacked together, it may lead to uncontrolled heating of some cells.  Because of this, batteries must be spaced apart to allow for cooling, which increases the overall space required for an installation.

LIMITED DEEP DISCHARGE.  When an application requires only shallow discharges that use a third or less of a battery's full capacity, it may be possible to complete 10,000 or more charge/discharge cycles.  But when lead-acid batteries are regularly "deep cycled" to 80 percent of more of full discharge, their life is typically limited to less that 600 charge/discharge cycles.  In addition, just a few hundred deep discharges can reduce the energy storage capacity of a lead-acid battery to about half of its original capacity.

LIMITED ENERGY DELIVERY AT HIGHER POWER LEVELS.  Lead-acid batteries are also affected by the rate at which they are discharged, with higher power levels reducing the amount of energy delivered.  For example, a string of batteries capable of delivering a power level of 1 megawatt for, say, 30 minutes, will not be able to deliver 2 megawatts for 15 minutes.  The duration of ride-through coverage and the energy level supplied will be reduced at the higher power level, and the capabilities of the string of batteries will continue to decline as the batteries age.

The limitations and frailties of lead-acid batteries have been known for many decades, but the fact that these energy storage devices still dominate the modular energy-storage market is testimony to their versatility and usefulness.  Chemical batteries are likely to play a strong role in this market for many years to come, even though emerging storage technologies promise to make many of the disadvantages of chemical battery-based systems a thing of the past.

The new energy storage technologies are similar in function but strikingly different in architecture, ranging from low-speed flywheel and improved chemical battery systems to cutting-edge superconducting magnetic energy storage (SMES) devices, ultracapacitotrs, and high-speed flywheel systems.  All of them are aimed at instantaneously providing sufficient power to allow sensitive processes and, in many cases, whole facilities, to ride through power sags.

These technologies promise improved performance over existing lead- acid chemical batteries, longer life spans (10 to 20 years or more), and longer maintenance intervals.  They also will offer immunity to high temperatures (up to 140 F), include few or no hazardous materials, and provide simplified means of measuring the amount of stored energy available.

Just a few years ago, most of the enthusiasm for high-speed flywheel technology was for automotive applications, but it has now become increasingly clear that the leading early adopter for high- speed flywheel energy storage technologies will be the telecom industry, which is likely to embrace the new options as battery replacements.  The opportunities for alternative technologies should continue to grow as more UPS systems are sited in places where it is hard to maintain the temperatures that batteries need for long life.  One estimate puts the potential number for remotely sited telecom flywheel systems at 100,000 units. Naturally, the economics for such installations will depend greatly on how frequently the existing chemical batteries would have had to be replaced.

The third - and potentially the most lucrative - market for the new modular energy-storage technologies will be for load leveling and distributed storage.  The devices could be charged either off-peak or via alternative sources such as wind or solar power, and the energy could then be discharged gradually at times of peak demand.

This may well turn out to be the granddaddy of all energy storage market segments.  Estimates of its potential size run as high as $1 trillion worldwide.  For this market to become a reality, however, the new technolgies will have to migrate from their current topology as high-power, low-capacity (around 1 to 5 kWh) systems, to units that deliver lower power but offer at least four to five times more energy storage than current systems.

Tribology Systems, Inc. (TSI) uses its own high-precision ceramic bearings as the basis for its flywheel energy storage systems.  The company reports that its solid-lube bearings can survive in a vacuum without maintenance for up to 20 years.  TSI is also working with a manufacturer of carbon-fiber composite flywheel rims, a company that makes motor/generators, and an electronic controller firm to bring its systems to market.  Its target applications include backup power for the telecommunicatinos industry, load leveling, and backup and energy storage for renewable energy systems.

The basic design includes a patented mounting hub for the flywheel that eliminates mass shift of the composite rim at all speeds, a proprietary controller that matches the flywheel to any required energy source and load, and a self-contained bearing cooling system.  TSI touts its ceramic bearings as a lower-cost alternative to the active magnetic bearings that many companies use.  The overall design has only a half-dozen mechanical parts, which should help make mass production cost-competitive.

TSI has tested a prototype unit at its own facilities at speeds up to 28,000 rpm.  Another 0.15-kWh unit has been sent to Bellcore for testing at up to 50,000 rpm.  A million-dollar cost-sharing contract from the U.S. Defense Advanced Research Projects Agency (DARPA) helped TSI design and test the containment systems for the flywheels to prevent harm to surrounding personnel and property in the event of a catastrophic failure.  So far, no specific military applications have been announced.  TSI has also been working with regional Bell Telephone Operating Companies to develop requirements for the telecommunications-oriented backup power units that Bellcore will be purchasing for beta testing.  TSI has reportedly quoted Bellcore on costs for delivery of 1 million of the units.