D
Technical Data and Applications of
WIMA Double-Layer Capacitors
Construction Principle
The construction principle of a Double-
Layer Capacitor can be described as a
plate capacitor where the most important
aim is to obtain electrodes with an extre-
mely large surface. For this purpose activa-
ted carbon is ideally suited, as it allows to
achieve capacitance values of up to 100 F/
g of active mass of the electrode. The elec-
trolyte, the conductive liquid between the
electrodes is a conducting salt dissolved in
an aqueous or organic solvent which per-
mits to apply voltages of 2.5 V.
the solvent - even through the separator
film. This phenomenon represents the main
reason for the limited voltage capability
of 2.5 V only and the steep decrease of
capacitance versus frequency exhibited by
Double-Layer Capacitors.
Cascaded SuperCap Modules
Several SuperCap cells can be built up
to enormous capacitances of the desired
voltage by means of series or parallel
connection (cascade). When cascading
SuperCaps, the voltage of single cells
must not exceed 2.5 V (decomposition of
the electrolyte!) Hence, series connections
need in any case to be balanced since a
possibly slightly different aging of the indi-
vidual cells due to temperature may over
time cause deviating capacitances and
thus different voltage drops at the cell. The
balancing will be factory-mounted into a
module. This can be made passively and
in a cost-efficient way by simple resistors
in those cases where additional losses
as bypass current through the balancing
resistors can be tolerated by the appli-
cation. Alternatively, an active balancing
can be made by keeping each cell at a
certain voltage by means of a reference
source. That means if the comparator
circuit detects a commencing overload of
any cell individual discharge is initiated by
a bypass resistor. Except the leakage cur-
rent of the cells there are no considerable
losses created during active balancing.
Active balancing.
Comparator compares voltage at the
capacitor by a reference voltage and
switches in order to discharge through a
bypassing resistor until overvoltage has
declined.
Operational Life
For physical reasons it is unavoidable that
Double-Layer Capacitors are subjected to
aging which follows the logarithmic depen-
dence of voltage applied and ambient
temperature (Arrhenius behaviour) that can
be observed with other components, too.
However, continuous studies have shown
that WIMA products exhibit a significantly
improved behaviour in terms of life time
being achieved by a laser-welded, herme-
tically sealed construction of the cells in
metal cases which makes penetration from
outside impossible; they cannot dry up and
can withstand a certain thermal expansion
movement. Only by this innovation one can
consider the component being suitable for
long-year maintenance-free application.
Construction principle of the WIMA
Double-Layer Capacitor
The actual double-layer consists of ions
which, when voltage is applied, attach
to the positive or negative electrode cor-
responding to their opposite poles and
thus create a dielectric gauge of a few
Angstrom only. This results in a very high
capacitance yield caused by the very huge
surface of the electrode in accordance
with the formula
C =
e
x Surface
Distance
To visualise this, the internal surface of a
Double-Layer Capacitor would cover a
football pitch.
A permeable diaphragm acting as a
separating layer and called separator
avoids short-circuit between the two elec-
trodes and considerably influences the
characteristics of the capacitor. Charge or
discharge of the Double-Layer Capacitor
is combined with the transformation of the
layers in the electrical field and thus with
the movement of the charge carriers in
11.08
Passive balancing.
Without resistors: U reciprocal-effect to C - thus locale overvoltage easily can occur
With resistors:
U proportional-effect to R - thus voltage is fixed
111
D
Technical Data and Applications of
WIMA Double-Layer Capacitors
When properly treated WIMA SuperCaps
have a service life beyond 10 years and
can easily sustain more than 500.000 char-
ge/discharge cycles. The efficiency is far
higher than 90%.
Application Examples
In general Double-Layer Capacitors are
applied for voltage support, for saving or
for replacing conventional battery or char-
ger solutions. The typical application is the
quick supply of several 100 A to 1000 A in
the direct current field.
Slip Control in Wind Power
In large-scale wind turbine systems, slip
controllers are used to control the rotation
speed by altering the angle of the rotor
blades. The drives are mains-indepen-
dent and if electrically controlled use the
energy stored in batteries or double-layer
capacitors. These storage devices have to
meet stringent requirements. During winter
time the temperatures in the wind tower
top housing often reach around -40° C,
and during summer time they may easily
go up to more than +60° C during operati-
on. The current of 200 A necessary for the
breakaway torque of e. g. a 3 kW motor
presents big problems to batteries due to
the ambient conditions described. Their
short life time and frequently necessary
maintenance renders them unsatisfactory.
However, when properly dimensioned,
modern SuperCap solutions enable a
maintenance-free usage of the electrical
storage device of minimum 10 years.
Start of Micro-Turbines, Fuel Cells or
Diesel-Electric Generator working as
Power Set
For micro-turbines driven with natural gas
for generation of electrical energy on oil
platforms, in part also for gas pumping
stations, in sensible areas like hospitals
and huge factories the use of SuperCap
modules to replace conventional starter
batteries (by experience needing replace-
ment every 2 to 3 years) is the optimum
choice. Usually about 300 kJ of electrical
energy at a system voltage of 240 V are
needed for a turbine start-up time of 10
to 20 s.
When starting special micro-turbines or for
bridging during start of a fuel cell working
as emergency power supply, generally a
few 100 kJ of electrical energy are required
for a system start time of approx. 10 to
20 sec. The stored energy time is approxi-
mately 20 s. Due to the system voltage of
48 V, 22 cells of 1200 F are cascaded in a
Life time expectancy for WIMA SuperCaps
module to achieve the setpoint voltage in
order to replace a battery block.
For start-up of generators for energy
supply of autonomous telecommunication
stations which are located decentrally
in a tight network but supplied with fuel
the new double-layer capacitors would
provide a solution. Right now tests are run
with 14 V series connections (70 to 100 F)
which should render a maintenance-free
service. After three starting processes in a
sequence their energy with 300 to 500 A
each flowing (depending on the size of the
motor) is used up. The now running gene-
rator, however, immediately supplies them
with electrical energy again.
Starting huge Railway, Naval or Truck
Motors
The start of V16 or V24 cylinder motors
(6000 kW), e. g. for generator drives of
diesel-electric trains or start of a naval
diesel engine requires considerably high
currents. 1300 A are quite usual which can
be covered by capacitor units of 450 to
600 F at 28 V. Frequently the crankshaft is
turned by two starters on both sides (e. g.
7 kW each with a positive switch off after
9 s for 2 min), in order to avoid torsion
of the huge mass. The low total internal
resistance of less then 3 m¸ which is
beyond reach for batteries the capacitor
solution is outstanding.
Recuperation of Braking Energy
In times of resource shortage of fuel the
highest possible recuperation of braking
energy is a challenging aim. While recup-
eration in electric train drives or in hybrid
busses is already practiced since long, for
non-mains connected vehicles the energy
recuperation to the on-board battery has
only be realized to the extent of few per
cent. The basic reason is the charge cur-
rent limitation of batteries where the recu-
perable energy is obtained at very high
currents in a scope of milliseconds. If for
example 1 ton shall be decelerated from
100 km/h to 0 km/h 400 kJ are released,
for 10 tons it is ten times as much. So far
no suitable high-energy storage devices
were available (guideline values: 500 A
to 1000 A). This is the domain of the new
SuperCaps since in the foreseeable future
even most modern battery systems will not
be in a position to cope with such energy.
11.08
Advantages in Comparison with
other Energy Storage Solutions
WIMA SuperCaps are showing foIlowing
advantages in comparison with other ener-
gy storage solutions:
” Low internal resistance (less than 1/10
of what a usual battery exhibits)
” Release of high currents (10 to 100
times more than batteries)
” Maintenance-free operation
” No risk of damage due to complete
discharge of the component
”
igh life expectancy
H
” Usage in isolated systems, e. g. inac-
cessible areas, is unproblematic
” Comparatively low weight
WIMA Double-Layer Capacitors are par-
ticularly suitable in applications where
high and even highest currents - not in
pure AC operation - occur. By combining
the advantage of conventional capacitors
as fast suppliers of electricity with that of
batteries as notable energy reservoirs the
SuperCap represents the link between bat-
tery and conventional capacitor.
Standard SuperCap Battery
Capacitor
Capacitance
<
1
m
F/cm
2
1000 000
m
F
p er Surface
(1 F/cm
2
)
Energy-
<
0.01 Wh/kg
<
10 Wh/kg 100 Wh/kg
density
<
0.1 kW/kg
>
1 kW/kg 0.1 kW/kg
Power-
density
112
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