U.S. Department of Energy - Energy Efficiency and Renewable Energy
Vehicle Technologies Office
Like batteries, ultracapacitors are energy storage devices, and essential in HEV and PHEV propulsion. They use electrolytes and configure cells into modules to meet the power, energy, and voltage requirements for a wide range of applications and appliances. But batteries store charges chemically, whereas ultracapacitors store them electrostatically. Currently, ultracapacitors are more expensive (per energy unit) than batteries, but they can withstand hundreds of thousands of charge/discharge cycles without degrading.
Ultracapacitors are true capacitors in that energy is stored via charge separation at the electrode-electrolyte interface, and they can withstand hundreds of thousands of charge/discharge cycles without degrading. They provide quick bursts of energy.
An ultracapacitor, also known as a double-layer capacitor, polarizes an electrolytic solution to store energy electrostatically. Though it is an electrochemical device, no chemical reactions are involved in its energy storage mechanism. This mechanism is highly reversible, and allows the ultracapacitor to be charged and discharged hundreds of thousands of times.
Many applications can benefit from ultracapacitors, especially HEVs and PHEVs. Ultracapacitors can be the primary energy source during acceleration and hill climbing, as well as for recovery of braking energy because they are excellent at providing quick bursts of energy. Using an ultracapacitor in conjunction with a battery combines the power performance of the former with the greater energy storage capability of the latter. It can extend the life of a battery, save on replacement and maintenance costs, and enable a battery to be downsized. At the same time, it can increase available energy by providing high peak power whenever necessary. However, the combination of ultracapacitors and batteries requires additional power electronics, such as a DC/DC converter, which would increase the cost of the vehicle.
The use of ultracapacitors for regenerative braking can greatly improve fuel efficiency under stop-and-go urban driving conditions. Only ultracapacitors can capture and store large amounts of electrical energy (generated by braking) and release it quickly for the next acceleration.