Lead acid batteries have a history of providing energy storage in telecom and datacom applications, where they offer anywhere from several minutes to several hours of backup power. Sometimes, the batteries are used to enable a safe shutdown of the system. In other cases, they “bridge” to a generator, fuel cell or other source of long-term backup power.
As short-term energy-storage devices, lead acid batteries are attractive because of their low cost. However, their reliability is often a concern because of the conditions under which they typically operate. In many systems, the batteries experience mostly short-term disruptions that last a few seconds. The repeated hits for pulses of energy tend to degrade battery reliability and shorten operating life.
Two emerging technologies address this problem. Flywheel-based systems use large spinning disks to store energy mechanically and then convert that energy back to electrical energy as needed. Meanwhile, ultracapacitors (also known as supercapacitors or electrochemical double-layer capacitors) provide a means of storing energy electrically. Both approaches supply pulses of power reliably and may be used with or in place of batteries. However, flywheels and ultracapacitors tend to serve different applications because of their varying performance capabilities, size, maintenance needs and cost.
That said, it's worth noting the current progress being made in ultracapacitor development since these components have the potential to impact a wider range of applications. They are currently being applied in consumer, industrial, transportation and other commercial applications. From digital cameras to fuel cells to trucks and trains, the list of products incorporating ultracapacitors is continually growing.
One interesting new use is in the telecom industry, where an ultracapacitor-based energy source seeks to replace some of the lead acid batteries currently in use. The Powercache energy-storage module developed by Maxwell Technologies (San Diego) combines ultracapacitors with electronics to create a device that mimics the operation of a 48-V battery. By providing several seconds of backup power, Powercache overcomes the battery reliability issues. It's not meant to replace every 48-V battery in telecom — just those found in installations where uptime is paramount and where the Powercache unit can bridge to a long-term power source (for more on this product, see “Data Points” on page 10).
Nevertheless, this telecom product illustrates the compelling advantages of the underlying ultracapacitor technology — decades of maintenance-free operation and the ability to recharge quickly and handle successive disturbances. The cost of Powercache is two to three times that of the 48-V battery it replaces. But considering that the battery may be replaced three or four times over the life of the equipment, the added cost of the new product may be justified.
Although Powercache isn't meant to replace the lead acid battery found in the ubiquitous “green box” that powers the phone lines going to the home, its impact could eventually be felt there. As its production ramps up, the cost of Powercache is expected to reach price parity with the lead acid battery by 2008. At that point, it may show up in the green box, where it would enable the use of a smaller cheaper, lead acid battery that would also last longer.
The steady decline in ultracapacitor cost — dropping by half every two years — is one of the forces driving their commercialization. The other driver is the steady increase in capacitance per volume that results from advances in chemistry and packaging. For example, applications that formerly relied on a 2.5-V 450-F cell with a volume of 192 cc will soon be able to substitute a 54-cc-sized 350-F cell from Maxwell Technologies. The new unit will come in the common D-cell battery package and cost less than half of the 450-F model.
As in the battery world, high-volume applications are likely to dictate the package types available to most designers. Nevertheless, the inherent scalability of ultracapacitor technology should enable the development of a range of packaging and performance options suitable to numerous applications.