Handbook of Energy Storage for Transmission or ... - W2agz.com

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Description
Introduction
Discovered by Henrich Helmholtz in the 1800s, electrochemical capacitors were first
practically used in 1979 for memory backup in computers and are now manufacturer by
many companies. Electrochemical capacitors are distinguished from other types as
“double-layer capacitors 1 .” Manufactured products have also been given names including
“super,” “ultra,” “gold,” “pseudo,” as well as “electric double-layer” capacitors.
Double layer electrochemical capacitors differ from other types by having capacitance
and energy density values several orders of magnitude larger than even the largest
electrolytic-based capacitor. They are true capacitors in that energy is stored via
electrostatic charges on opposing surfaces, and they can withstand a large number of
charge/discharge cycles without degradation. They are also similar to batteries in many
respects, including the use of liquid electrolytes, and the practice of configuring various
size cells into modules to meet power, energy, and voltage requirements of a wide range
of applications.
The first products were rated at two to five volts and had capacitance values measured in
fractions of a Farad to several Farads. Although early applications were primarily
computer memory backup, the technology has evolved to larger scale applications.
Today’s devices range in size up to hundreds of thousands of Farads at low voltage and,
in some applications, systems voltages (multiple series-connected capacitors) are above
600 V. The technology has grown into an industry with an annual sales estimated to be
$100 million. It is poised for rapid growth in the near future with higher energy and
higher voltage devices suitable for power quality and advanced transportation
applications. With the advent of distributed power generation, capacitors are being
considered for fuel cell and micro-turbine load inrush support, and for leveling
fluctuating energy flow from natural sources like wind turbines or solar.
Capacitor Fundamentals
A capacitor is a device used for storing electrical charge. There are three distinct types of
capacitors: electrostatic, electrolytic, and electrochemical, see appendix for a description
of each type. The simplest capacitor is a parallel-plate electrostatic. It has two conductors
of area A separated by a distance t. The region between the plates is usually filled with
air, paper or other dielectric material, which increases the stored energy in the device.
The charge, Q, that is stored in the device, is proportional to the voltage applied to the
conductors. This proportionality constant is the capacitance. The capacitance C is equal
to the dielectric constant times the area divided by the separation.
1 There is some uncertainty within the industry on the exact name for capacitors with massive storage
capability. This is in part due to the many names of products by different manufacturers, but also due to the
relative newness of the industry and recent advances. An electrochemical capacitor commonly stores
energy through non-faradic processes (electrostatic). However, faradic processes (electron transfer due to
chemical or oxidation state changes) can and do occur. Because both processes can occur, the generic term
electrochemical is more appropriate than double-layer electrochemical capacitor, which also excludes the
mixed-metal-oxide capacitor technology. In general, this report uses the generic term electrochemical
capacitor as suggested by A. Burke and endorse by B. Conway and J. Miller.
Electrochemical Capacitors 8