2.5.1 Double-layer capacitors (DLC)
Electrochemical double-layer capacitors (DLC),
also known as supercapacitors, are a technology
which has been known for 60 years. They fill
the gap between classical capacitors used
in electronics and general batteries, because
of their nearly unlimited cycle stability as well
as extremely high power capability and their
many orders of magnitude higher energy
storage capability when compared to traditional
capacitors. This technology still exhibits a large
development potential that could lead to much
greater capacitance and energy density than
conventional capacitors, thus enabling compact
designs.
The two main features are the extremely high
capacitance values, of the order of many thousand
farads, and the possibility of very fast charges
and discharges due to extraordinarily low inner
resistance which are features not available with
conventional batteries.
Still other advantages are durability, high reliability,
no maintenance, long lifetime and operation
over a wide temperature range and in diverse
environments (hot, cold and moist). The lifetime
reaches one million cycles (or ten years of operation)
without any degradation, except for the solvent
used in the capacitors whose disadvantage is that
it deteriorates in 5 or 6 years irrespective of the
number of cycles. They are environmentally friendly
and easily recycled or neutralized. The effi ciency is
typically around 90 % and discharge times are in
the range of seconds to hours.
They can reach a specifi c power density which is
about ten times higher than that of conventional
batteries (only very-high-power lithium batteries
can reach nearly the same specifi c power density),
but their specifi c energy density is about ten times
lower.
Because of their properties, DLCs are suited
especially to applications with a large number of
short charge/discharge cycles, where their high
performance characteristics can be used. DLCs
are not suitable for the storage of energy over
longer periods of time, because of their high selfdischarge
rate, their low energy density and high
investment costs.
Since about 1980 they have been widely applied
in consumer electronics and power electronics.
A DLC is also ideally suited as a UPS to bridge
short voltage failures. A new application could be
the electric vehicle, where they could be used as
a buffer system for the acceleration process and
regenerative braking