Environmental Engineering Reference
In-Depth Information
less than the cost of an advanced battery such as nickel-metal hydride (NiMH) or
lithium ion (Li ion), then the combination would make a good business case: the
ultra-capacitor delivers all peaking power and the VRLA the continuous power.
However, with the cost of power electronics still at $0.14/W the complete system is
too expensive at high power. However, when lead-acid batteries are used in mild
hybrid vehicles the economics are somewhat better, but life and warranty remain
issues.
4.4.2 Nickel-metal hydride
The previous section has shown that an NiMH system can far surpass a lead-acid
battery system in energy and power density, plus have an energy-lifetime that is
nearly seven times longer. In today's market the NiMH battery is the preferred high
cycle life energy storage medium. One serious drawback, as Table 4.15 shows, the
NiMH system does not respond well in cold temperatures.
NiMH secondary battery systems are far superior to lead-acid systems and
even VRLA in terms of turnaround efficiency and cycle life. At issue is their
exorbitant cost of approximately $30/Ah in an 18-cell module. This is more than 20
times the cost of VRLA in a similar rated module, except for cycle life. In a mild
hybrid vehicle application an NiMH battery system may be rated 26 Ah at 42 V,
whereas its alternative VRLA would be rated 104 Ah at 42 V. The difference is due
to the fact that NiMH can deliver four times the energy of a VRLA for the same Ah
rating, because it can be cycled through much deeper SOC swings. VRLA batteries
must be maintained near 80% SOC or higher to ensure adequate life, whereas the
NiMH can be designed for operation at 50% SOC.
4.4.3 Lithium ion
Plastic lithium ion technology has the potential to significantly impact vehicle
integration issues currently impeding the application of hybrid powertrains. Plastic
lithium ion provides packaging flexibility, reduced mass and low maintenance. It is
promoted as an emerging technology having potential to meet all energy and power
needs, manufacturer cost targets and packaging requirements of the vehicle inte-
grator (also the manufacturer). This technology is sometimes referred to as lithium-
polymer (LiPo). As with conventional lithium ion, the LiPo is a 'rocking chair'
electrochemistry because the lithium ions move back and forth through the elec-
trolyte without undergoing chemical change. While the lithium molecules move
back and forth across the electrolyte, the electrons are released to do the same in the
external circuit. Because the electrode material in a LiPo structure undergoes a
reversible change during oxidation, no chemical reorganization need take place so
there should be little degradation of the cell. Hence, the LiPo has the potential of
long operating life. However, LiPo is a thin film technology so its durability in
automotive harsh environments could be problematic. Specifics of LiPo technology
are tabulated in Table 4.16.
