U.S. Department of Energy - Energy Efficiency and Renewable Energy

Distributed Energy Program

Supercapacitors

Capacitors are electronic devices. Conventional capacitors have enormous power but store only tiny amounts of energy. Batteries can store lots of energy but have low power — they take a long time to be charged or discharged. Supercapacitors offer a unique combination of high power and high energy.

How It Works

Batteries are charged when they undergo an internal chemical reaction. They deliver the absorbed energy, or discharge, when they reverse the chemical reaction. In contrast, when a supercapacitor is charged, there is no chemical reaction. Instead, the energy is stored as a charge or concentration of electrons on the surface of a material.

Advantages

Supercapacitors are capable of very fast charges and discharges, and apparently are able to go through a large number of cycles without degradation.

Applications

Supercapacitors found their first application in military projects — for example, starting the engines of battle tanks and submarines or replacing batteries in missiles. Common applications today include starting diesel trucks and railroad locomotives, actuators, and in electric/hybrid-electric vehicles for transient load leveling and regenerating the energy of braking. NASA has used 30 large supercapacitors in its turbo-electric city bus.