Computers and telecommunications products have become so entrenched in our daily lives that if a network is interrupted for even a short time, the interruption can, at best, inconvenience users and, at worst, cripple a business for days, weeks or longer. Because today’s complex computing and networking systems are so crucial to everyday life, businesses need uninterruptible power supply (UPS) systems to ensure a continuous supply of high-quality power.
A white paper released in 2003 by Venture Development Corp., a technology market research and strategy company, further emphasizes the importance businesses are placing on UPS systems. The paper, titled “2003 Power Protection Market Intelligence Program,” revealed that the North American UPS market was $1.7 billion in 2003 and should reach $2 billion by 2006.
Simply put, a UPS system is a set of power controls with a backup store of energy. Often thought of as only a source of power during power outages, statistics show that less than 10 percent of power problems are caused by complete power failures and 98 percent of those power failures last less than two minutes. Therefore, in most cases, the UPS is not called upon to provide 100 percent power to the network. The number one job of the UPS is to manage a system’s electrical faults, not its electrical outages. System electrical faults include breaker trips, voltage sags, power surges, frequency variation, waveform distortion, spikes and noise.
UPS Topologies
There are three UPS topologies: offline, line-interactive and online/double conversion. Offline is the simplest and most often used in small applications - 100 W to 1,000 W. Under normal operating conditions, AC power from the utility passes straight through the UPS to the critical load. The offline UPS is inexpensive and the efficiency is very high in normal operation, but it is limited in its ability to correct for sags, frequency variations and other common distrubances. Offline UPS products are typically used in homes or for powering individual computer workstations running noncritical applications that require only outage protection.
Line-interactive topology resembles the offline product, but inserts a transformer or inductor in series between the utility power source and the load. This inline inductor enables the UPS inverter to “interact” with incoming power and provide a measure of power conditioning to the load. Like the offline UPS, the line-interactive products can be inexpensive and efficient because they only support the entire critical load during power disturbances, and only for short durations. Inline products are also used on small applications ranging from 500 W to 4,000 W.
The premium UPS topology is the true online or double-conversion product. This configuration completely isolates the critical load from the incoming AC input power. The critical load is always being supplied by the output inverter, which is always being supplied from the internal DC bus. This design is mature and well understood. According to Kevin Stoll, vice president of marketing at Liebert Corp., a provider of mission-critical power and cooling technology, online topology has been applied successfully to every imaginable application, but is commonly used in larger applications - 500 W to 5 MW.
Types of UPS Energy Storage Batteries
Batteries remain the most widely used power source for UPS systems and, according to most industry experts, will likely continue to be through the end of this decade. Batteries are inexpensive and can provide from five minutes to three or four hours of energy. However, batteries require a fair amount of maintenance and have short life spans. Because batteries contain sulfuric acid and lead, disposal is a problem. And the biggest drawback is the inability to predict the end of a battery’s life and thus, exactly when it will fail.
Stationary lead acid batteries are the most common of the batteries used for UPS applications. There are two major types of lead acid batteries. One type is the flooded battery, often called a vented or wet cell battery. Because vents allow gases to escape during recharging, these batteries lose electrodes that are immersed in liquid electrolytes and the batteries must be “topped up” periodically, adding to the maintenance burden. Although most flooded lead acid batteries have a design life of 20 years, their actual life span varies widely based on application, load and environment.
The second type of lead acid battery is the valve regulated lead acid (VRLA) battery. Except for a pressure release valve, these batteries are sealed and do not require as much maintenance as flooded batteries. The electrolytes are immobilized either in a gel or an absorptive separator and no liquid is present. These batteries only expel gases when they are overpressurized - caused by either overcharge or a too high ambient temperature. They are available with a five-, 10- or 20-year design life, but like flooded batteries, their actual life span varies widely based on the same variables.
Although lead acid batteries are the technology of choice for UPS systems, Valence Technology Inc. is leading the charge in alternative battery options by offering Saphion technology, a safe, high-performance, rechargeable and environmentally friendly lithium-ion battery technology. The technology uses a phosphate material that is different than most lithium-ion batteries. Valence Technology touts its system, the K-Charge, as a high-energy, smart, maintenance-free UPS solution. The K-Charge system is smaller and weighs about one-third less than conventional lead acid battery systems. It is also able to provide twice the run time of traditional VRLA systems and is cheaper to own, according to Valence Technology.
In addition to lithium-ion, nickel cadmium, polymer, sodium, sulfur and nickel metal hydride batteries are all being studied as alternatives to lead acid batteries.
In spite of their popularity in UPS systems many companies are actively searching for alternatives to batteries because of their unpredictable end of life. The most promising alternatives are flywheels, ultracapacitors and fuel cells.
Flywheels
Flywheels are simply rotating disks that store kinetic energy, which is available as a backup power source when needed. There are two main types of flywheels: high-speed flywheels, which are made from carbon fiber/composite and spin fast - 50,000 to 70,000 rpm, and lower speed flywheels, which are made from steel and spin much slower - 2,000 to 4,000 rpm.
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According to Liebert’s Stoll, flywheels typically provide 10 to 30 seconds of backup power to ride through short interruptions and enable transition to a backup generator. A single flywheel can be sized to support UPS systems up to 300 kVA. Multiple flywheel systems can operate in parallel to support larger systems.
“Flywheel technology represents a viable alternative to battery backup in some applications,” said Stoll. Flywheels are much more reliable than batteries and have lower maintenance and life-cycle costs. Compared to batteries, flywheels have less environmental impact, fewer temperature restrictions and a smaller footprint. However, they do have a higher initial cost and cannot provide backup power as long as batteries can.
Ultracapacitors
The ultracapacitor is another option that is being investigated for UPS systems. Ultracapacitors can provide short-term bridge power for one to 60 seconds, enough time to allow a network to be transferred to a secondary power source or shut down in a controlled, orderly fashion. Ultracapacitors are also capable of sustaining power quality during momentary voltage sags or power surges.
Ultracapacitors store energy via electrostatic charge. They are small and lightweight and can be quickly and easily recharged. In addition, they have a small footprint, a long shelf life without performance degradation and they will operate efficiently and reliably in a wide temperature range (-40 F to 149 F). Perhaps the biggest advantage of ultracapacitors is that there is little or no maintenance cost associated with their use as a UPS.
Maxwell Technologies, a developer and manufacturer of innovative, cost-effective energy storage and power delivery solutions, is the frontrunner in developing the ultracapacitor as a viable option for safe and reliable backup power. “Ultracapacitors’ burst power capabilities and longevity make them a preferred energy storage and power delivery option for many applications,” said Richard Smith, Maxwell’s executive vice president of strategic business development.
On the downside, however, ultracapacitors are still considered by many to be new technology, and, although they hold promise, their initial cost is much higher than the cost of a comparable UPS battery system. Standardization is fundamental to ultracapacitors becoming a viable option for commercial energy storage in industrial and telecom applications.
Fuel Cells
Fuel cells have received much attention in the past few years. “Money is being poured into fuel cell research and development at several different levels. GM, the State of Ohio and the Federal government, just to name a few, are working on fuel cell development,” said Stoll. “However, for ride-through power, fuel cells haven’t reached commercial viability.”
Fuel cell technology is not new, the basic concepts underlying fuel cell performance and operation go back more than 150 years. The primary reason fuel cells have not been highly utilized for energy generation has always been their high costs relative to alternative methods.
The most common type of fuel cell is the proton exchange membrane (PEM), in which the membrane is composed of exotic, expensive metals; thus one of the reasons for fuel cells’ high cost. PEM fuel cells also have short lives (4,000 to 6,000 hours or about 6 months of continuous operation). In addition, a fuel cell’s transient load response is low, meaning it takes about 30 seconds for the load to go from zero to 100 percent; therefore, for UPS applications, fuel cells must be used with another source of power that can bridge that 30 second gap.
Choosing a UPS System
When choosing the correct UPS system, reliability should be the number one factor. In other words, how important is it for the UPS to do its job if the network fails? For entities like the New York Stock Exchange and investment firms Goldman Sacs and J.P. Morgan, the UPS system must perform flawlessly whenever it is called upon. Network system failure for even a few seconds could mean the loss of millions of dollars and irreplaceable information for these entities. On the other hand, for the small consulting firm or temporary staffing agency down the street, network system failure is typically nothing more than a nuisance and any lost data in most cases can be reconstructed.
The second factor to consider is availability. How often is the UPS going to be providing power, and what level of power conditioning can the UPS provide? The third consideration should be cost. In addition to the purchase price of the UPS system, installation costs, operating costs and maintenance costs should also be considered. Battery replacement costs, in particular, will vary by type of UPS, depending upon how frequently the UPS has to call on the battery to condition power.
As Table 1 illustrates, there are four tiers of UPS customers, each with different reliability and availability requirements.
UPS systems are an integral, important part of today’s networking environment. The demand for “no fail” power has never been higher, nor has the cost of unreliable power. As companies continue to develop UPS system technologies, new options to fit most any scenario will soon be available. While batteries will likely continue to be the cornerstone of UPS systems for the near future, other technologies are finding their way into the UPS market.