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

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energy storage systems capable of rapid cycling. Such systems require uniform voltage
among the many capacitor cells in series-connected strings for reliable operation. This
motivates increased emphasis on cell temperature uniformity and efficient heat removal
from cells. Although charge/discharge efficiency is generally high for capacitors, they
nevertheless dissipate energy, which can cause excessive internal temperature rise
without appropriate heat removal techniques.
An important issue related to the creation of reliable high-voltage strings of cells is cell
uniformity. So reducing manufacturing variability is certainly important. Improving
control of the production process is an ongoing effort for many companies according to
recent reports. Still another issue in capacitor design relates to product cost reduction.
For example, Maxwell has reported on their cost-reduction program. They are
developing spiral-wound cell construction capability using particulate-carbon electrode
materials pasted on current collectors. This is in contrast with the carbon cloth used with
a manual, accordion-fold design.
Development thrusts in electrode materials include examining the performance of various
activated carbons to find lower-cost materials. Some new carbon materials are being
implemented. Several companies are attempting to find replacements for activated
carbon cloth material, which is much more expensive than the particulate carbon,
especially particulate materials that have a natural origin. Other electrode materials that
have been investigated include metal-oxides of ignoble elements, ones having good
performance without associated high-costs typically found in the platinum group metals.
There has been some development activity using nano-structured materials, both for
carbons in symmetric double layer capacitors, and in the pseudocapacitor electrode of an
asymmetric capacitor.
The third major development thrust has been with the electrolyte. Work has been
reported on using polymer electrolytes for both aqueous and non-aqueous designs. Also,
there has been some effort to find replacement materials for the acetonitrile-based
electrolytes used in many type II products. The performance of these electrolytes is very
good but its use creates concerns because of toxicity and safety issues.
The thrusts for the asymmetric capacitor activity have expanded from the nickel
oxyhydroxide/carbon system to other systems including a lead oxide/carbon system and a
MnO 2
/carbon system. Reports of device performance using these other material systems
are most encouraging. A major advantage of these systems is low materials cost. Yet
another system that has been described in several papers recently is a lithium-titanate
electrode in combination with a carbon electrode and an organic electrolyte. This design
offers higher voltage than can be obtained in present symmetric organic electrolyte
capacitors, and it is referred to as a type IV electrochemical capacitor.
Yet another design that has been described in the literature is a graphite/carbon capacitor.
This type IV capacitor relies on charge intercalation in the graphite of one electrode and
double layer charge storage on activated carbon in the other electrode. The electrolyte
for this system is an organic solvent with a lithium salt. This particular system has an
operating voltage approaching 4 V. None of these advanced devices are commercially
available at this time.
Electrochemical Capacitors 33