Environmental Engineering Reference
In-Depth Information
(BEVs) or for high pulse power applications (i.e. hybrid electric vehicles). In the case
of energy optimized systems, the cycle life is quantified at 80% DOD. Note that a
lead-acid battery fabricated as a thin metal foil (TMF) structure is not suited to EV
applications, and in general, neither is the electronic double layer capacitor (EDLC)
nor, more generally, the ultra-capacitor. Energy capacity multiplied by deep cycling
capability (0.80) multiplied by cycles to wear out gives a metric of energy-life. For
example, if a vehicle consumes 250 Wh/mi on a standard drive cycle, and it uses a Li
ion traction battery (listed in Table 10.3) that is capable of 43,200 Wh/kg of energy
throughput, and if a battery life of 8 years (100,000 mi) is specified, then a range
of 172.8 mi/kg can be anticipated. To meet the range goal requires some 578.7 kg of
battery. If the average consumption increases to 400 Wh/mi, then a battery mass of
926 kg is necessary.
Battery systems for hybrid vehicles are optimized for shallow cycling (1%
to perhaps 4% of capacity per event) and have comparably higher cycling. In
Table 10.3 the hybrid system cycling capability has been projected from its low DOD
cycling capability to a comparative value had its DOD been 80% on a log-log
plot. This said, the metric of energy-life of the hybrid vehicle, advanced battery
technologies, will be typically more than double that of EV batteries. Now, it can be
seen from Table 10.3 that ultra-capacitors (EDLCs) are capable of ten times
the energy-life of even pulse power optimized advanced batteries. The temperature
application range of ultra-capacitors is also much better than that of battery systems.
Hence, there is resurgence of interest in ultra-capacitors as energy buffers in hybrid and
fuel cell applications. The next section expands on the topic of ultra-capacitor ESS.
10.2 Capacitor systems
Table 10.1 listed the ideal conventional capacitor as storing 4.3
10
3
J/kg (1.2 Wh/kg)
in its electric field. Practical conventional capacitors, of course, are not capable of even
this amount of energy storage. A conventional capacitor achieves high capacitance
by winding great lengths of metal foil plates separated by a dielectric film. The voltage
rating is determined by the dielectric strength (V/m) and its thickness. An ultra-
capacitor works differently. Instead of metal electrodes separated by a dielectric (sheet
or film) that facilitates charge separation across its thickness, an ultra-capacitor
achieves charge separation distances on the order of ion dimensions (~10
˚
). The
ultra-capacitor's charge separation mechanism, or double layer capacitor model, was
described by Helmholtz in the late 1800s. In Figure 10.18 the construction of an ultra-
capacitor is shown to consist of carbon (AC) film electrodes that are impregnated with
conductive electrolyte and laminated on current collector metal foils. Positive and
negative foils with this carbon mush have an electronic barrier or separator that is
porous to ions between them, generally of 50-80% porosity.
The electrolyte materials commonly used in ultra-capacitors are propylene car-
bonate (PC) or with acetonitrile (AN, i.e. 10-20% by mass, but generally 75%
by volume) and the quaternary salt tetraethylammonium tetrafluoroborate (TEATFB,
5-15% by mass and 25% by volume), AC (10-20% by mass) and the remainder the
