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

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Electrolytic capacitors are generally constructed in a spiral wound configuration. Besides
aluminum, these capacitors can be made using tantalum or niobium. Electrolytic
capacitor technology today provides devices rated up to 600 V. These capacitors are
widely used in filtering applications (dc) because of their relatively low cost, high energy
density, and low power dissipation. These are the upright tubular capacitors that are
commonly seen in power supplies.
The third distinct type of capacitor is designated as electrochemical, and has been
referred to as electric double layer ultra capacitors. Electrochemical capacitors store
energy by charge separation at the interface between a solid electrode and an electrolyte.
Individual capacitor cells operate at low voltage (< 2.7 V) compared to electrostatic or
electrolytic capacitors. What makes them interesting is that electrochemical capacitors
can have much higher energy densities than other types of capacitors. Thus, they can
deliver the energy over longer times than electrostatic and electrolytic capacitors of the
same physical size. A common application of small electrochemical capacitors is to
provide power for computer memory backup during power outage.
The electrochemical capacitor, in its simplest form, is comprised of two double layer
charge storage surfaces in series, i.e. two electrostatic capacitors in series. The double
layer charge storage surface is formed at the interface between a conductor and an
electrolyte when a voltage is imposed across them. Essentially there is an increase in the
electrolyte ion concentration, with a change in electrolyte ion orientation, near the surface
of the electrode. Charge separation at this interface occurs over a very short distance,
~10 angstroms. Thus, very large capacitance values, on the order of 100 Farads/gram of
material, can be obtained with the use of a high surface area conductor like activated
carbon. Although the double layer phenomenon has been known for more than 100
years, the first practical device was created in the late 1960’s.
Ideal Capacitor and ESR
An ideal capacitor is a fundamental circuit element and has no resistive or inductive
components. In reality, the first order model of an actual capacitor is a series
combination of an inductor, a resistor, and a capacitor. The series resistance and
inductance are intrinsic to the construction of the capacitor and are dependent on the
design characteristics of the device, such as its geometry and physical size. Note that
series resistance is also referred to as the equivalent series resistance, ESR.
The series-RLC circuit has a characteristic frequency f o , the self-resonance frequency, at
which the magnitude of the impedance is a minimum. This self-resonant frequency
occurs when the inductance and the capacitance balance each other to produce an
impedance equal to the series resistance value. This frequency occurs at f o = 1/ 2π√(LC).
At frequencies below f o , capacitive behavior is dominant; above this frequency inductive
behavior is dominant. The series resistance can be measured precisely at frequency f o .
The self-resonant frequency of electrostatic capacitors can be in the MHz or higher range,
depending on the size and design of the device. The self-resonance frequency for
electrolytic capacitors is generally in the range of kHz to tens of kHz because the
capacitance is much larger and the inductance is also higher, making the resonance
frequency lower. For large electrochemical capacitors, the self-resonant frequency is
Electrochemical Capacitors 57