AAWSAP DIRD, Ultracapacitors as Energy and Power Storage Devices, November 2010

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Chapter 1: Concept
Overview
Ultracapacitors, also known as
supercapacitors, hybrid capacitors,
electrochemical capacitors, electrochemical
double-layer capacitors, or ultracaps, are
energy-storage platforms that offer energy
storage capable of extremely rapid charge
and discharge rates. Ultracapacitors also
have the ability to be cycled hundreds of
thousands of times. The phenomenal
charge and discharge capabilities make
them ideal for supporting volatile memory
and computing applications, energy
efficiency/capture processes, and high­
power applications. Ultracaps store their
charge in the electrical double layer
between the electrode and the electrolyte.
Charge storage is a physical mechanism
rather than a chemical phase change, so
these are theoretically capable of cycling an
infinite number of times. 2 One of the
Figure 1. Ultracaps in Various Configurations.'
disadvantages of ultracaps can be a high
self-discharge rate. 3 The charge and discharge voltage output for an ultracap is a
sloping linear curve, which allows for straightforward and accurate state-of-charge
monitoring.4 The real advantage of an ultracapacitor is the ability to deliver or accept
bursts of power in a short time.
Ultracaps were introduced in 1966 and found initial use 12 years later as backup power
devices for volatile memory and clocks. Over the last 30 years, numerous advances
have been made that have led to many uses of ultracaps, from transportation to
portable electronics and more. Ultracaps are becoming more affordable as activated
carbon electrodes and manufacturing improvements have driven costs down. Several
companies now make ultracapacitors to fill a broad spectrum of applications. Figure 1
shows a number of ultracaps with various capacities to fill a range of functions.
Transportation, microelectronics, and aerospace markets are some of the many areas
where ultracaps have become enablers. Advanced materials and improved cell designs
will lead to improvements in both power density and energy density. The improved
performance is expected to make ultracaps important components for efficient power
leveling and high-power receiving and delivery. The unique signature of ultracapacitors
must be understood for aerospace and military applications.
As an electrochemical capacitor, ultracapacitors store energy within the electric double
layer formed at the interface between the electrode and electrolyte. In a conventional
capacitor, the energy is stored by moving charge carriers from one plate to another,
and the charge separation creates a potential. Voltage differentials in a conventional
capacitor are dependent upon the dielectric material separating the plates. In the
ultracap, the electrical double layer is the separation of charge in a vanishingly thin gap
between two plates. Figure 2 depicts the components and basic design of the ultracap,
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