The typical outage or voltage dip rarely lasts more than a few seconds. A properly sized UPS will provide that much ride-through.

Do battery-based UPSs give you the blues? Perhaps you should consider the advantages of a flywheel UPS. In some applications, this battery-free UPS can supplement a battery-based UPS — or even be used in place of that UPS.

The battery blues

When you think of UPSs, you probably think of batteries. After all, most UPSs chemically store energy in battery cells. Unfortunately, batteries can cause many complications. Batteries require extensive maintenance, considerable expertise, and lots of floor space — all of which is expensive. Because of the time required to build a wet cell battery, orders have long lead times. And thanks to Internet-fueled demand, lead times can be an entire year.

To further aggravate matters, batteries fail well before their rated backup time in today's cost-cutting, maintenance-averse world. The battery failure causes shown in the sidebar, on page PQ18, are preventable — but only through the use of resources most owners don't provide. Batteries work fine if management commits to maintaining them, but typically management will cut costs at any price. That's the great weakness of batteries — they are not items you can buy and forget.

Fortunately, battery-free UPSs (such as flywheels) are an option. Unlike batteries, the flywheel UPS uses a mechanical means of storing energy. Let's consider some applications for flywheel UPS technology.

Applications for a flywheel UPS

Many people measure backup quality in minutes — they think the more, the better. However, this metric is unrealistic for most applications. The typical outage or voltage dip rarely lasts more than a few seconds. Thus, you need just a few seconds of ride-through per event. A properly sized UPS will provide just that.

Today's flywheel UPSs use a single conversion design and don't have long backup times. So, on first examination, it appears the flywheel UPS isn't suitable for a data center. Let's look at why this assumption is shaky.

Suppose you get 10 voltage dips or outages in a typical week. That means 10 hits to your batteries. Anyone familiar with batteries knows they last much longer when you don't use them. A cell degrades with each use — no matter how small. That's why you can use it only so many times before it wears out.

We know the rated life spans of batteries are optimistic at best. Five years is a typical limit to a cell's life. Thus, you will take your battery out of service at least once every five years, in addition to normal maintenance. However, the flywheel UPS has a service life of 20 years.

A flywheel UPS (placed ahead of your batteries) eliminates most hits your batteries take and increases cell life significantly. The resultant longer replacement cycle reduces purchase costs and saves your battery capacity for when you really need it. Plus, you have the flywheel UPS online during cell changeout.

Do the cost savings absorb the cost of the added flywheel UPS? That depends on many factors, and in some cases you may not justify the cost. But such an application is certainly worth exploring, especially if you risk downtime during each cycle of battery replacement.

Here's another application to consider. The Achilles' heel of standby generator systems is the battery. By using a flywheel UPS to start your generator, you eliminate the cause of most generator system failures. Now let's look at some other advantages that flywheels provide.

Known ride-through

What's the real capacity of a battery system? Given the rampant inattention to battery maintenance, few owners can estimate their available battery ride-through time within 50%. But by using a flywheel-based UPS, you know the exact speed of the flywheel and the amount of energy it stores. So, you know the available ride-through time.

Temperature insensitivity

Chemical storage is a temperature-sensitive proposition. Batteries lose capacity as temperature rises. But, it gets worse. In the interest of saving square footage, many owners stack cells three and four tiers high. This creates several problems, including a temperature gradient between each tier. That gradient makes it impossible to set up a proper float charge for the whole battery.

A flywheel UPS can operate at temperatures as high as 40°C. In models currently on the market, the form factor eliminates the temptation to stack one on top of the other. But even if you did stack these, the temperature gradient would be irrelevant.

Ventilation not required

The flywheel UPS doesn't produce gas, whereas UPS batteries do. A wet-cell battery requires frequently changed, relatively cool air. Without adequate ventilation, a buildup of combustible gases produces an unacceptable situation. While valve regulated lead acid (VRLA) batteries have been hyped as “maintenance-free,” they are actually more maintenance-intensive than wet cells. They also require some degree of ventilation.

Low space requirements

Batteries require huge rooms for housing — and that means ample floor space. For example, the five-minute battery of a 400kW UPS requires about 200 sq ft. This translates into 200 sq ft of support space for every 2000 sq ft of raised floor. System redundancy can increase the floor space requirement to 400 sq ft per 2000 sq ft of raised floor. So, nearly 20% of the floor space produces no revenue because of the basic UPS equipment. You need space for ancillary equipment (e.g., air handlers) that accompanies the UPS, too. A comparable flywheel UPS uses about a fourth of the floor space a battery uses, and it doesn't require special air handling.

Reduced weight

A flywheel UPS weighs about a fourth as much as a comparable battery system. The floor loading of a battery requires structural strengthening that may be prohibitively expensive, especially in a high-rise.

Reduced lead times

The lead time for obtaining a battery now runs six to 12 months — depending on what cells you buy and what kind of battery you configure. Flywheel UPS lead time is a fraction of this, at present.

Toxic-waste reduction

The EPA requires you to document every step of battery disposal, from de-installation to receipt of the batteries at a certified recycling facility. The high maintenance requirements of batteries, especially VRLAs, expose maintenance people to dangerous voltages and toxic materials. To avoid exposure to lead dust, you must use special equipment and procedures to clean lead-plated connection bars and battery posts. Batteries also require acid spill containment and spill recovery procedures. Flywheel energy storage systems require very little maintenance and do not involve hazardous materials or toxic substances.

Flywheel in your future?

No single technology can solve all problems, and flywheel technology is no exception. Its weaknesses include a high initial cost and short ride-through. These weaknesses are no problem for some applications, but prohibit the flywheel UPS from replacing a battery-based UPS in others. In those cases, you can often build a strong economic and operational case for using a flywheel UPS to complement and enhance your battery backup. Only a thorough analysis of your system and your company's priorities will tell you which way to go.

Why Batteries Fail Battery cells fail for many reasons, including:

Incorrect battery design

Most designs result in cells stacked too high, too close together, or even facing the wrong direction.

Incorrect floating charge

Owners fail to consider key variables when deciding on the floating charge. For example, cells arranged in tiers will have a different charge requirement for each tier.

Incorrect cell maintenance metrics

Internal cell resistance has a proven high accuracy for determining weak cells, yet most owners rely on hygrometer readings instead. Hygrometer readings are largely irrelevant to determining battery condition. Most owners feel they are saving money by using a cheaper test than ensuring their batteries work. Premature failure is inevitable.

No attention to intercell connectors

There is no excuse for a battery ever to have heavily corroded intercell connectors. Yet, such corrosion seems to be the rule, not the exception.

Untrained maintenance personnel

A huge number of case histories document poor battery-maintenance practices that proper training would have prevented. Some of these practices center on workmanship, basic testing procedures, and even knowing what to look for.