Environmental Engineering Reference
In-Depth Information
The area given by (10.25) is enormous. To put this into perspective we divide
by 10
4
m
2
/ha and obtain 11.86 ha! Or, in English units, this amounts to some
30 acres. In other words, the acreage of a small farm rolled up into a small canister
with a volume of 0.6 L.
10.2.1 Symmetrical ultra-capacitors
The ultra-capacitor just described is a symmetrical type because both of its
electrodes are composed of the same porous carbon matrix ingredients. Capaci-
tance is purely a double layer effect, there is no ionic or electronic transfer as in
electrochemical cells, only polarization. Unlike an electrochemical cell that
functions by virtue of the Faradic process of ionic transfer, an ultra-capacitor is a
non-Faradic process that is simply charge separation and no electronic transfer. In
a conventional capacitor the energy storage effect is purely a surface phenom-
enon, so most of the materials used are there for structure, not for energy storage
[15,16]. Ultra-capacitors, however, achieve phenomenal surface area for rather
finite plate areas by having porous electrodes that are dense with crevices and
pores. A battery makes the best use of available materials because the electrode
mass contributes in a Faradic process to the energy storage task. However,
the Faradic process involves ion transfer, so there are transport delays and
time dynamics to contend with. Capacitors and ultra-capacitors have very fast
pulse response times because only stored charge is removed or restored at the
interfaces rather than reactions occurring in the bulk electrode material. By
extension, this means that ultra-capacitors have cycle life orders of magnitude
greater than electrochemical cells. It is not unreasonable to expect an ultra-
capacitor to provide several million cycles in use. The Nissan production Condor
capacitor-hybrid truck, a commercial 4 ton load capacity, 7 L diesel series electric
hybrid with twin 55 kW ac synchronous motors, relies on a 583 Wh, 346 V ultra-
capacitor module. A commercial truck such as this is designed for urban stop-go
driving with a durability target of 600,000 km of driving over which it is expected
to encounter 2.4 M braking cycles. The reason for using an ultra-capacitor is to
store regenerated braking energy and deliver it to the electric drive system for
vehicle launch and acceleration. Moreover, Nissan Motor Co. [17] claims a 50%
improvement in fuel consumption and CO
2
reduction of 33% with its capacitor-
hybrid system. A passenger car hybrid would target roughly one-third of the
mileage and stop-go events as the delivery truck, or 200,000 km of driving and
800,000 stops [18,19].
Energy and power density of ultra-capacitors is of utmost importance. Energy
density, for example, translates into the ability of the storage system to source
vehicle power demands for protracted lengths of time - tens of seconds instead of
just fractions of a second. Power density provides an indication of how well the
ultra-capacitor can deliver pulse power when needed. The classical relation
between energy and power density is the Ragone plot in which a collection of data
points are plotted with specific energy density (Wh/kg) on the ordinate and power
density (W/kg) on the abscissa. Each point on the Ragone plot is the result of a
