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

EPRI Proprietary Licensed Material
capacitor than for a typical lead-acid battery design since they have minimal chemical
reactions for charge storage. Capacitors generally can be charged at any rate provided
overheating does not occur. This means that higher power chargers can be effectively
used for capacitors since they can be charged in seconds to minutes, not hours. Similarly,
their discharge rate can be high and is only limited by the series resistance of the
capacitor. However, high-rate charge and discharge, particularly with cycling, can lead
to internal heating of the capacitor, which without dissipation, can lead to overtemperature
conditions and system failure as described previously. Shorting an
electrochemical capacitor generally does not cause damage provided maximum
temperatures are not exceeded. Type III and IV capacitors generally cannot be left in a
shorted state without damage. Also they have a minimum operating voltage before
damage may occur.
Health, Safety, and Environmental Issues
Safety issues can be grouped into several categories. One relates to electrical, a second to
chemical, and a third to fire and explosion hazards. Electrical hazards are similar to those
of batteries, not any better and not any worse. Hazards from chemical burns and
chemical exposures can be similar to some batteries. Fire hazard is essentially
nonexistent for type I and III products, which have aqueous electrolyte. For type II
capacitors, fire hazard should be similar to some organic electrolyte batteries. An
unknown safety related issue arises because acetonitrile is contained in the electrolyte of
some large type II capacitors (see discussion under Chemical Hazards about acetonitrile).
This situation has not been fully evaluated for potential problems it may create in larger
scale utility or automotive applications.
To consider these issues, it is helpful to identify the exact materials used in each type of
capacitor. Large type I capacitors use potassium hydroxide electrolyte, carbon
electrodes, and generally nickel or steel current collectors or conductive polymer bipolar
plates. Packages are generally steel or epoxy. The Elit and the ECOND companies make
capacitors using this construction.
Type II electrochemical capacitors use carbon electrodes, paper or polymer separators,
aluminum current collectors, and usually an acetonitrile solvent containing an ammonium
salt for the electrolyte. Manufacturers of large type II capacitors include Maxwell,
Panasonic, NESS, and EPCOS.
Type III electrochemical capacitors use nickel-oxyhydroxide positive electrodes, carbon
negative electrodes, potassium hydroxide electrolyte, polyethylene case, polymer
separator, and module packages generally of steel or a polymer. ESMA is the
manufacturer of commercial products of this type.
Type IV devices are under development. They use carbon for one electrode and various
types of battery electrodes for the second electrode. Electrolytes typically are various
salt-containing organic solvents including acetonitrile-based solutions in some cases.
Electrical Hazards
Series-strings of the electrochemical capacitor cells often have voltages at lethal levels.
These systems are similar to any voltage source with respect to electrical operating
safety. Electrochemical capacitor systems are capable of delivering very high currents,
higher than comparable lead-acid battery systems for instance, which can cause severe
Electrochemical Capacitors 20