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Backup Power for the Wireless Tower

New solutions may fill the gap left by the failure of traditional sources

We've all heard about the potential use of fuel cell technology in mobile phones and laptop computers, but they may be arriving faster than you think in other parts of the cellular network.

Behind every smoothly running wireless network is an increasingly fragile power infrastructure. The dizzying deployment of new wireless products - and the skyrocketing demand for them - are straining the capacity of networks, even as reliability and quality requirements remain deep into the nines. These conditions call for wireless networks that are optimized in every respect, including backup power. Yet in the scramble to meet these demands, one fact has become increasingly clear: the traditional solutions for backup power, valve-regulated lead acid (VRLA) batteries, come up short in wireless's outside plant (OSP) environment.

That leaves wireless operators searching for alternatives (even as they try to make do with the traditional solutions for now). The challenge is to find new backup power sources that satisfy multiple requirements - reliability, flexibility, and durability - at a cost that makes sense for cost-conscious companies. Fortunately, several candidates have emerged.

Why Traditional Solutions Won't Work
In traditional indoor settings, like a central network switching office, traditional solutions provide reliable backup power. Flooded lead acid batteries typically achieve useful life spans of 20 years. Maintenance techniques for these and other widely used sources, such as engine-generator sets, are proven and easily implemented. Onsite personnel can monitor loads, equipment condition, and provisioning requirements for these indoor solutions.

The key word, however, is indoor. As wireless networks deploy their equipment and backup power sources in remote outdoor environments, the traditional solutions work far less effectively. VRLA batteries have proved short-lived, overly sensitive to temperature, too heavy for many outdoor applications (like rooftops), too laden with environmental issues, and unable to keep up with the increasing energy and duration demands of OSP wireless technology. With wireless sites scattered across wide geographic areas, preventive maintenance for all these units becomes a serious issue. Some of these shortcomings can have dramatic consequences: the short operating life of batteries, for instance, has been implicated in the failure of backup power to wireless networks during last year's blackout.

Over the last few years, researchers have made some advances in adapting the traditional technologies to the OSP environment. Even so, alternative technologies may gain the advantage in the race to serve wireless networks in the new distributed landscape.

New Batteries Emerge
Among these alternatives are several that also use battery technology. Even the most promising among them, however - the lithium-ion battery - is not without its hurdles.

Lithium-ion batteries have expected lifetimes of more than 10 years in extreme environments. They offer substantial weight and space savings over both traditional lead-acid and nickel-cadmium storage systems. Their other benefits - no ventilation requirements, better cycling characteristics, and more flexibility of form factor - fulfill many OSP requirements.

Unfortunately, those distinct advantages come at a cost: currently at 8-10 times the expense of VRLA batteries, lithium-ion batteries require a higher initial capital outlay than many companies are prepared to make. As with other innovations, cost will come down as the technology matures, but network administrators are under pressure to come up with solutions now. Moreover, the scarcity of the elements used in the battery will likely keep the cost somewhat high over the long term.

Just as troubling is the uncertain path of innovation and cost reduction that lithium-ion batteries will take. Most high-current research has focused on automotive applications, particularly hybrid and electric vehicles, where the potential sales volume may accelerate development. However, technology designed for mobile use may not translate directly, or even easily, into stationary applications. Even on the automotive front, several key issues remain to be resolved, among them load response and the need for intelligent systems to charge and maintain the batteries.

One other disadvantage may prove troublesome for wireless networks as well: lithium's high flammability. The risk of fire only adds to liability concerns and replacement costs.

The Promise of Fuel Cells
Yet another option is the proton-exchange membrane (PEM) fuel cell. In this system, hydrogen fuel directly generates DC power, with water as the only by-product. Such a process makes fuel cells particularly adaptable to the OSP environment even as they carry 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, while their generally lightweight, small footprint makes them suitable for rooftop locations. The clean process produces zero emissions and little noise, and the units are easily monitored and controlled with remote automated systems.

Topping all these benefits is the cost savings generated by fuel cells. Initial unit cost runs roughly half to one-third that of lithium batteries. While still more expensive than VRLA batteries, fuel cells carry a lower life cycle cost, with lower maintenance needs and longer life.

The technology has drawn widespread support within government agencies, from the Department of Defense to the National Institute for Standards and Technology. Just as important, companies like Plug Power have been designing fuel cells specifically for stationary applications, avoiding the uncertain path of innovation taken by the lithium-ion battery. And stationary fuel cells already have a track record in the field, as such established utilities as the Long Island Power Authority have installed them to run as both a parallel and a backup power source.

Several challenges remain to be worked out. Chief among them is the logistics of fuel supply, specifically the refilling of tanks via drop-off fueling, and the corresponding concerns surrounding the siting of hydrogen.

Into the Future
As with most technologies, further innovation will help wireless operators at two key points: driving down costs and increasing reliability. Rather than one technology achieving a dominant position in OSP backup power, it is likely that several will find appropriate market niches based on unique application needs.

Whatever the details, there is clearly great potential for workable solutions to provide lower costs for network operators and reliable service for their customers. Such a win-win scenario will allow wireless communications to move ahead with confidence in the years to come.

More Stories By Eoin Connolly

Eoin Connolly is product manager for Plug Power Inc.

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