Supercapacitors are ideal when a quick charge is needed to fill a short-term power need; whereas batteries are chosen to provide long-term energy. Combining the two into a hybrid battery satisfies both needs and reduces battery stress, which reflects in a longer service life. Such batteries are being made available today in the lead acid family.
Supercapacitors are most effective to bridge power gaps lasting from a few seconds to a few minutes and can be recharged quickly. A flywheel offers similar qualities, and an application where the supercapacitor competes against the flywheel is the Long Island Rail Road (LIRR) trial in New York. LIRR is one of the busiest railroads in North America.
To prevent voltage sag during acceleration of a train and to reduce peak power usage, a 2MW supercapacitor bank is being tested in New York against flywheels that deliver 2.5MW of power. Both systems must provide continuous power for 30 seconds at their respective megawatt capacity and fully recharge in the same time. The goal is to achieve a regulation that is within 10 percent of the nominal voltage; both systems must have low maintenance and last for 20 years. (Authorities believe that flywheels are more rugged and energy efficient for this application than batteries. Time will tell.)
Japan also employs large supercapacitors. The 4MW systems are installed in commercial buildings to reduce grid consumption at peak demand times and ease loading. Other applications are to start backup generators during power outages and provide power until the switch-over is stabilized.
Supercapacitors have also made critical inroads into electric powertrains. The virtue of ultra-rapid charging during regenerative braking and delivery of high current on acceleration makes the supercapacitor ideal as a peak-load enhancer for hybrid vehicles as well as for fuel cell applications. Its broad temperature range and long life offers an advantage over the battery.
Supercapacitors have low specific energy and are expensive in terms of cost per watt. Some design engineers argue that the money for the supercapacitor would be spent better on a larger battery. Table 4 summarizes the advantages and limitations of the supercapacitor.
|Advantages||Virtually unlimited cycle life; can be cycled millions of time High specific power; low resistance enables high load currents Charges in seconds; no end-of-charge termination required Simple charging; draws only what it needs; not subject to overcharge Safe; forgiving if abused Excellent low-temperature charge and discharge performance|
|Limitations||Low specific energy; holds a fraction of a regular battery Linear discharge voltage prevents using the full energy spectrum High self-discharge; higher than most batteries Low cell voltage; requires series connections with voltage balancing High cost per watt|
Table 4: Advantages and limitations of supercapacitors.