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

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observed with increased temperature are fully reversible if the temperature is not
excessive. Self-discharge rates increase dramatically with temperature and often establish
a practical upper operating temperature limit. Correspondingly, product life decreases at
high temperatures since mechanisms responsible for the leakage current are often
chemical side-reactions.
Such undesired chemistry in type II capacitors results from electrochemically active
impurities that were originally present in the package (water, for instance), and new
impurities that are created during capacitor operation due to electrolyte decomposition or
arise from permeation into the package through seals. One common method to
counteract the elevated leakage current levels and thus increase operating life of type I
and II cells is to reduce the average voltage applied to a cell. This reduces the effective
energy density of the capacitor but can substantially increase operating life.
Exceptional low-temperature performance can usually be expected in all electrochemical
capacitors. This is possible because, unlike batteries, reaction kinetics do not limit the
charge or discharge rate of an electrochemical capacitor. Instead, the limit is usually
established by the electrolyte conductivity. Thus, capacitors can operate with good
performance at very low temperatures. Generally, but not always, aqueous electrolyte
electrochemical capacitors (types I and III) have the least change in performance at low
temperatures compared with room-temperature values.
Combining Cells into Modules
Unlike conventional electrostatic and electrolytic capacitors, electrochemical capacitors
are inherently low voltage devices. The maximum voltage of a single cell in a
commercial product is 2.7 V. Thus, to meet the 600- to 800-V requirements of a utility
application, hundreds of cells are series-connected and a dc-to-dc boost converter may
also be employed.
Failure of just one cell in a series string can lead to failure of the entire storage system. A
cell can fail as an open circuit or as a short circuit. The most common failure is an open
circuit. Of course, if the failure is an open circuit, the entire system will stop working.
On the other hand, if a single cell short circuits, then other cells in the string will
experience higher voltage, which may stress them. This stress could lead to accelerated
aging of those remaining and premature failure of another cell, and so on. Thus, one cell
failure in this scenario could start a cascade situation where the entire string of cells
would rapidly become a short circuit.
For long life, each cell in a series-string must remain below its maximum voltage rating
under all conditions, which includes charge/discharge as well as float operation. The
three key parameters affecting the cell voltage are variability in capacitance, internal
resistance, and leakage current. Each of these parameters can lead to voltage imbalance
among cells in a string. Thus, the construction of the cell, and its normal variability, will
affect the reliability of a high-voltage string.
Cell Over Voltage in a Series String
Preventing cell over voltage is particularly critical for type I and II symmetrical
capacitors. When gas is generated due to over voltage in a symmetric electrochemical
capacitor there is no means for recombination and pressure rises inside the package.
Some small capacitors have crimp seals for pressure relief that can vent small amounts of
Electrochemical Capacitors 17