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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.
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. |
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.
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