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Like batteries, ultracapacitors are energy storage devices. They use electrolytes and configure various-sized cells into modules to meet the power, energy, and voltage requirements for a wide range of applications. But batteries store charges chemically, whereas ultracapacitors store them electrostatically. Currently, ultracapacitors are more expensive (per energy unit) than batteries.

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.

How an Ultracapacitor Works

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.

An ultracapacitor can be viewed as two nonreactive porous plates, or collectors, suspended within an electrolyte, with a voltage potential applied across the collectors. In an individual ultracapacitor cell, the applied potential on the positive electrode attracts the negative ions in the electrolyte, while the potential on the negative electrode attracts the positive ions. A dielectric separator between the two electrodes prevents the charge from moving between the two electrodes. Diagram 2 depicts an ultracapacitor, its modules, and an ultracapacitor cell.

Schematic of an ultracapacitor module, module schematic, and an ultracapacitor cell.

Once the ultracapacitor is charged and energy stored, a load (the vehicle's motor) can use this energy. The amount of energy stored is very large compared to a standard capacitor because of the enormous surface area created by the porous carbon electrodes and the small charge separation (10 angstroms) created by the dielectric separator. However, it stores a much smaller amount of energy than does a battery. Since the rates of charge and discharge are determined solely by its physical properties, the ultracapacitor can release energy much faster (with more power) than a battery that relies on slow chemical reactions.

Many applications can benefit from ultracapacitors, whether they require short power pulses or low-power support of critical memory systems. Ultracapacitors can be primary energy devices for power assist during acceleration and hill climbing, as well as for recovery of braking 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 DC/DC power electronics, 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 reacceleration.

Learn about the energy storage research and development that's being performed at NREL to improve batteries and ultracapacitors for advanced vehicles.