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2.2.1.4 SUPERCAPS
Electrochemical double-layer capacitors (also referred to as ult-
racaps or supercaps) have a large surface area and a low thick-
ness of the dielectric. Supercaps are particularly characterized
by a significantly improved power consumption and power output
(gravimetric power density to 20,000 W/kg), and high efficiency
(cycle efficiency 98 percent at 80 percent DoD), as compared with
batteries. Also they have a high level of cycle stability (approx.
500,000 cycles at 80 percent DoD). Disadvantages are high costs
and a low energy density (gravimetric energy density of approx.
4 Wh/kg) [see Frost & Sullivan (2009); Tübke (2011), for example].
Supercaps could represent an interesting extension of existing
Li-Ion energy storage devices and offer „stop-and-go“ support,
however, in general only for brief strong charge processes (recup-
eration) and discharge processes (boost function), that offload the
battery. However the high power density could support implemen-
tation in mild hybrids. Due to the low energy density, systems to
date are not suitable for providing energy storage for pure electric
vehicles.
2.2.1.5 REDOX FLOW BATTERIES
The process in redox flow batteries is based on the principle of
storing chemical energy in the form of dissolved redox pairs in
external tanks. Power is generated in a separate power module.
During discharge, the electrode is supplied continuously with the
dissolved material from the storage tank that must be converted;
the resulting product is routed into another storage tank. When
charging, the pump direction of the electrolyte is reversed. A dis-
tinction is made between redox flow batteries, with two fluid elec-
troactive components, and hybrid flow batteries with one fluid and
one solid electroactive component. Vanadium, vanadium bromide
and polysulfide bromide can be used as electrolytes for redox flow
batteries. On the other hand, zinc-bromine or cerium-zinc is used
for the hybrid-flow battery. The electrolytes are usually carbon-
based or made of graphite, partly in the form of a felt. Usually a
Nafion (112, 125) or polystyrene sulfonic acid membrane is used
as the separator. Depending on the metal pairing of the electroly-
te, different voltage levels are achieved, from approx. 1.0 to 2.2 V
(graphite electrodes with aqueous electrolyte at max. 1.7 V) [Tüb-
ke (2011)].
Current development objectives include, in particular, new elec-
trolyte systems for higher energy densities, but also electrode
optimization for increased power, as well as reduction of system
and maintenance costs, for example, through new membranes.
The energy capacity is essentially determined by the tank size for
the electrolyte solution and this can be easily scaled. Efficiency
is as high as 80 percent. The system offers a flexible structure
(separation of energy storage device and converter) and is cha-
racterized by a long service life and a high level of cycle stability
(> 10,000). Likewise, low maintenance effort, fast response time
(μs-ms), good overcharge and deep discharge tolerance, and no
self-discharge, are exhibited. However, the aqueous electrolytes
also result in a low energy density, as well as high costs in the
high-energy range [Tübke (2011)]. Energy density depends on the
solubility of the redox pairs and is currently at approx. 70 Wh per
liter of electrolyte fluids for a vanadium bromide combination.
In the target power range, redox flow batteries offer promising de-
velopment potential for better exploitation of grid capacities and
avoiding bottlenecks through distributed, grid-integrated power
storage. Thus they are particularly interesting for stationary ap-
plications.
8 The Federal Institute for Materials Research and Testing, has performed technical safety experiments on the basis of the UN Manual of Tests and Criteria for Transport of Dangerous Goods.
The object of the experiments was to determine whether the technology is safe at extreme climate and air pressure fluctuations, electrical short circuits, overload or wrong polarity, and with
strong mechanical influences, such as vibration, impact, and collision. Likewise, the range test performed by DEKRA was passed; DEKRA tested the charging and discharging of the battery,
determination of tractive resistance and maximum speed, as well as the general technical safety of the vehicle [Pudenz (2011)].