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

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A type IV electrochemical capacitor is currently not available in a commercial product,
however there are active research programs directed toward development of such devices.
These devices use an asymmetric design with an organic electrolyte. This combination
provides the opportunity for the faradaic-pseudocapacitive charge storage with the higher
operating voltage afforded by the organic electrolyte. For example, the design could
mate an electrostatic electrode with a faradaic pseudocapacitive electrode that operates by
intercalation, similar to one electrode in a lithium ion battery. Or, there could be charge
storage in an electrochromic polymer such as a polythiophene. There are many faradaic
electrode materials that can be used with the double layer electrode, again using a large
capacity ratio as previously described to obtain high cycle life.
Double-Layer Technology Comparisons
Table 2 compares some general properties of each type capacitor. Type IV products are
not described because of their present early state of development. As listed, type I
products have low to moderate energy density, type II products have moderate to high
energy density, and type III products have high to very high energy density. Power
performance can be very high for type I products because of the use of high-conductivity
aqueous electrolytes. Type II products can be high in power, and type III products,
depending on optimization, can be low to high.
Cycle life can be high for all types of capacitors. Self-discharge rates for type I and II
designs are generally low because they use balancing resistors. These resistors are
included to help maintain voltage uniformity in series-strings of cells. The self-discharge
rate of the type III capacitor is very low, usually less than a commercial lead-acid battery.
Temperature performance is excellent for type I and type III designs because of the low
freezing points of the sulfuric acid or potassium hydroxide solutions used for the
electrolyte. The low temperature performance of type II capacitors depends intimately on
the exact solvent used in the electrolyte and cell design details. Performance can be good
to excellent.
Packaging of the different type products varies considerably. Two of the commercial
type I capacitor products use bipolar construction, which involves sealing a stack of cells
using a potting material around the stack perimeter. The stack is then placed within an
epoxy or metal package. Type II products invariably are well sealed, often using a
hermetic design that involves welded metal packaging with glass-to-metal seals. Because
this package is completely sealed, it usually contains a rupture valve that is designed to
burst at a specified overpressure condition. This is used to prevent the cell from
exploding due to internal gas generation during abuse situations. The use of a rupture
valve in the hermetic packages should be mandatory for safe operation of these devices.
The type III product is a single cell design with a plastic package similar to that of an
aircraft nickel cadmium battery. The cell is not hermetically sealed, but has a resealable
safety valve to permit gas release during severe over-voltage conditions.
Voltage balance for a series string of capacitor cells can involve active or passive
systems. A passive system is generally a parallel string of resistors attached to the
capacitor string at each cell. The active systems include cell voltage monitoring and in
some cases forces individual cells to charge or discharge and bring voltage uniformity to
the string. Type III electrochemical capacitors have natural voltage balancing when
connected in a series string. This is due to several reasons, one being that the device can
Electrochemical Capacitors 12