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Wireless Systems Design

Technologies Look To Bring Backup Power Out Of Flux

Across the wireless industry, a confluence of factors has made the upgrade of backup power a top priority. Networks are straining to handle the burgeoning number of products and the skyrocketing demand for them. At the same time, networks must provide the reliability that consumers expect. In 2003, the blackouts on the East Coast of the U.S. revealed the extent of this pressure. Not surprisingly, carriers are striving to optimize every aspect of their networks including backup power. The problem is that they need entirely new backup-power solutions to handle the harsh conditions of the outside-plant (OSP) environment.

Traditional technologies, such as lead-acid batteries and engine-generator sets, have long provided reliable backup power inside central-switching offices. As wireless networks extend into remote outdoor environments, however, the traditional solutions work less effectively. Valve-regulated lead-acid (VRLA) batteries are short-lived, overly sensitive to temperature, and too heavy for many outdoor applications (like rooftops). In addition, they're unable to keep up with the energy demands of OSP wireless technology. For their part, engine-generator sets produce combustion emissions. They're also noisy and maintenance dependent. With wireless sites scattered across wide geographic areas, preventive maintenance becomes a serious issue.

Recently, researchers have made advances in adapting traditional technologies to the OSP environment. Yet they may still be at a long-term disadvantage in the face of the alternatives—some of which use battery technology. The most promising option is the lithium-ion battery. These batteries have expected lifetimes of more than 10 years in extreme environments. They offer substantial weight and space savings over both lead-acid and nickel-cadmium storage systems. They also fulfill many OSP requirements by including no ventilation requirements, better cycling characteristics, and a more flexible form factor.

Unfortunately, lithium-ion batteries carry a significant downside. At 8X to 10X the cost of VRLA batteries, they require substantially more capital outlay than many carriers are prepared to spend. Another troubling factor is that lithium-ion batteries face an uncertain path of innovation and cost reduction. Most high-current research has focused on automotive applications, in which sales volume may speed development. But technology that's designed for mobile use may not translate directly or easily into stationary applications. In addition, lithium is highly flammable. The risk of fire adds to liability concerns and replacement costs.

The proton-exchange-membrane (PEM) fuel cell is an alternative to the lithium-ion battery. In this system, hydrogen fuel directly generates DC power. The only byproduct is water. Such a process makes fuel cells particularly adaptable to the OSP environment, as they match the strengths of the new batteries. Fuel cells for backup applications are designed for reliable operation in a very broad temperature range. The systems provide immediate—and as necessary extended—response to power interruptions. In addition, their general light weight and small footprint make them suitable for rooftop locations. The clean process produces zero emissions and little noise. Plus, the units are easily monitored and controlled with remote automated systems.

In conjunction with these benefits, fuel cells promise to generate cost savings. Their initial unit cost is roughly half to one-third of the cost of lithium batteries. Although they're still more expensive than VRLA batteries, fuel cells carry a lower life-cycle cost. They also boast lower maintenance needs and longer life. Companies like Plug Power have been designing fuel cells specifically for stationary applications. In fact, several major carriers have announced plans to implement fuel-cell technology as part of their ongoing infrastructure strategies.

Some fuel-cell manufacturers also are designing systems that comply with the Network Equipment Building Standards (NEBS). Among other things, NEBS approval indicates that a product will endure operation in harsh environmental conditions while protecting people from injury and the network from outages. It assures providers that fuel cells can be reliably integrated into the network.

Other certification developments are providing further assurance. A growing number of systems have been UL-certified to ANSI Z21.83, a U.S. national product standard for fuel cells. Several fuel-cell manufacturers are working closely with rating agencies like CSA-International and UL.

Of course, fuel cells also face challenges. The logistics of fuel supply is chief among them. Think of the refilling of tanks via drop-off fueling and the concerns surrounding the siting of hydrogen. Several companies have undertaken field projects to resolve these issues. As they are deployed more extensively, fuel cells may provide reliable service for customers and lower costs for carriers. Yet it's unlikely that they'll dominate the backup-power landscape. In that landscape, several technologies will probably find appropriate niches based on application needs.

TAGS: Automotive
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