IEEE Power & Energy Magazine - September/October 2017 - 25

Rechargeable batteries are today's most widespread
electrical energy storage devices and store electrical energy
in the form of chemical energy.
is required to start the operation, whereas it is usually not
needed during the charge and discharge processes, as internal
heat can be generated by electrochemical reactions. The entire
cell needs to be arranged in blocks for good heat conservation
and also encased in a vacuum-insulated box. The electrolyte
is an electrical insulator, allowing only sodium ions, rather
than electrons, to pass through it. In the course of discharge
operation, sodium ions stemming from oxidation reactions at
the sodium/beta alumina interface move through the electrolyte and merge with the sulphur to form sodium pentasulphide
(Na 2S5). Because Na 2S5 is immiscible with the remaining
sulphur, a two-phase liquid mixture appears in the cathode.
When the available free sulphur is completely consumed, single-phase sodium polysulphides (Na2S5−x) are gradually generated with increasing sulphur content. The reversed process
corresponds to charge operations.
The intriguing potential of the NaS battery comes from
its ability to provide high energy density (150-240 Wh/kg)
and round-trip efficiency (75-90%), long lifetime (2,500-
4,000 cycles), and deep, fast discharge. The power density of
an NaS battery is much higher than its lead-acid and VRB
counterparts but relatively lower compared to NiMH and Liion batteries. In addition, its ability to work at high temperatures allows operation within some hot, harsh environments.
These advantages make NaS batteries suitable for stationary energy storage. Over the past decade, NaS batteries have
played an important role in supporting the power system and
renewable energy generation, specifically for wind farms and
solar generation plants. Data from Navigant Research demonstrate that, up until 2014, NaS batteries had dominated the
grid-related, utility-scale energy storage market, contributing
to load leveling, emergency power supply, and UPS applications. For example, Japan has demonstrated the use of NaS
batteries at over 190 sites, with greater than 270 MW of stored
energy. The largest installation is a 34-MW/245-MWh NaS
battery unit applied to wind stabilization in Northern Japan.
Utilities in the United States have also deployed substantial
NaS battery storage for peak shaving, backup power, and
firming wind capacity, among other key applications. In 2010,
the world's largest NaS battery station, with 4-MW power for
up to 8 h, was built in Presidio, Texas.
Although the NaS battery is a market leader with welldeveloped technology, care must always be taken with its
use. This is because pure sodium is hazardous and will spontaneously burn if exposed to air and moisture. If the beta
alumina ceramic electrolyte is broken, the molten sodium
and sulphur are directly mixed, incurring short-circuits and
september/october 2017

exothermic reactions. Even when no gas is produced and no
explosion induced, the battery temperature can rise up to
2,000 °C, which is quite dangerous. A relevantly negative
example is the Japan Tsukuba plant fire incident experienced
by NGK Insulators in 2011.

Redox Flow Battery
The modern RFB was developed by the U.S. National Aeronautics and Space Administration in the 1970s. The first was
an iron-chromium RFB that exhibited dramatic capacity
decay due to the active ions' cross--contamination between the
positive and negative solutions, along with excessive hydrogen
generation at the negative electrode during charging. Since
then, various categories of RFBs have been developed, such
as the polysulphide/bromine flow battery, the all-VRB, and
the zinc/bromine hybrid flow battery.
Among these RFBs, the VRB has proved the most promising for two main reasons. First, both positive and negative
electrolytes in the VRB employ the same element, i.e., vanadium. Hence, when capacity decay occurs by active ions'
crossover, electrolytes can be remixed to recover the capacity.
Second, because the standard potential of redox couples at the
negative electrode exceeds the hydrogen evolution potential,
no excessive hydrogen is generated.

Voltage

Beta Alumina Tube
Na2S5-x

+
e

Molten Na

Na+

Molten Sulfur

figure 3. The schematic of an NaS battery during discharge operation.
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2017