Environmental Engineering Reference
In-Depth Information
TABLE 6.2
Properties and Primary Limitations of Sodium-Sulfur System
Characteristic
Comments
Advantages
Potential low cost relative to other
advanced batteries; high cycle
life; high energy; good power
density; flexible operation; high
energy efficiency; insensitivity to
ambient conditions; state-of-
charge identification
Inexpensive raw materials, sealed, no-maintenance
configuration; liquid electrodes; low-density active
materials; high cell voltage; cells functional over wide
range of conditions (rate, depth of discharge,
temperature); > 80% due to 100% Coulombic efficiency;
reasonable resistance; sealed high-temperature systems;
high resistance at top of charge; straightforward current
integration due to 100% Coulombic operation
Limitations
Thermal management; safety; seal
and freeze-thaw durabilities
Effective enclosure required to maintain energy
efficiency and provide adequate stand time; reaction
with molten active materials must be controlled; cell
hermeticity required in corrosive environment due to
use of ceramic electrolyte with limited fracture
toughness that can be subjected to high levels of
thermally driven mechanical stress
applications. Candidate uses include several that are associated with elec-
tric power generation and distribution (utility load leveling, power qual-
ity, and peak shaving) and some that involve powering motive devices
(electric cars, buses, and trucks and hybrid buses and trucks) and space
power (aerospace satellites). The uses related to electric power are col-
lectively referred to as stationary applications to differentiate them from
the motive applications. Sodium-sulfur technology was introduced
in the mid-1970s and has advanced through a variety of designs since
then.
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The advantages and limitations of such systems are summarized
in Table 6.2.
From the time of its discovery through the mid 1990s, NaS system was
among the leading candidates believed to be capable of satisfying the needs
of a number of emerging energy storage applications that had very promis-
ing markets. The one application, however, that generated the most interest
centered on powering EVs. Based on the large size of the potential market
in combination with the inherent environmental advantages, many private
companies and government organizations invested heavily in technology
development. Significant advancements were made and because of accept-
able performance, durability, safety, and manufacturability, at least four
automated pilot-production facilities were built and operated by the mid
1990s. However, during this same period, government and industry realized
that the public (especially in the United States) would not purchase pure
