Factsheet to accompany the report “Pathways for energy storage in the UK”
Sodium-sulphur battery
Application/markets
Brief description of technology
The cells require insulation to operate at high
temperatures, as lack of insulation would lead to
unacceptable thermal losses on the battery pack.
Therefore stationary / utility applications on
reactive timescales of seconds and minutes are the
main area of interest for the battery technology.
This application increases renewable energy
generation penetration, and assists with utility
supply load levelling and peak load shaving etc.
Sodium-sulphur (Na-S) batteries are a type of hightemperature, molten metal battery that was
introduced in the 1970s. They belong to a family
of batteries known as sodium beta batteries; the
negative electrode is made of liquid sodium while
a solid ceramic material of beta-alumina (a type of
aluminium oxide) serves as the electrolyte (Figure
1).
In a sodium sulphur battery, the positive electrode
is made from molten sulphur. In order that the
electrodes be kept in their molten state, and to
enable the reaction mechanism whereby sodium
ions are passed through the electrolyte to combine
with the sulphur, these batteries must be operated
and regulated at a high temperature, typically
300°C. This is not as prohibitive as it sounds; if the
batteries are cooled then self-discharge is made
negligible as a chemical reaction cannot take place,
and maintaining the high temperature during
operation is easily achieved by harvesting the
natural heating effect from high current
conduction during operation.
Advantages/disadvantages
Na-S batteries can have high cycle efficiency
typically in the range 75-90% [3]. They have a high
energy density and their long term durability is
good, with a pulse power capacity over six times
the continuous rating. In addition, Na-S batteries
have good cycling flexibility (different depths of
discharge do not unduly affect the lifetime) and
exhibit low maintenance requirements [4].
However, difficulties in making megawatt-scale
batteries include thermal management and safety,
along with electrical networking, involved in
series-parallel cell configurations, and overall cell
reliability [4].
Current status
The current technology status is commercial,
however manufacture has currently been halted
by the major supplier, NGK.
Figure 1: Cut-away of a Sodium Sulphur Cell [1]
Technical/economic data
A 2011 white paper for the U.S. government
estimated a power subsystem cost of $350/kW
and an energy storage subsystem cost of
$350/kWh with a round-trip efficiency of 75% over
a lifetime of 3000 cycles [2]. In their critical review
of 2009, Chen et al. estimate the energy and
power density of Na-S batteries to be 150-240
Wh/kg and 150-230 W/kg respectively [3] (see
Table 1).
A Japanese pilot project in the early 1990’s
brought a 1 MW, 8 MWh NaS system to the proofof-concept stage in a utility load-levelling
application before being discontinued. Since the
early 2000’s Japan has invested considerably in the
technology, which has been pioneered by
manufacturer NGK, and Japan now has 20 MW of
daily peak shaving capacity spread over 30 sites
[3]. A 2009 report by Japanese manufacturer NGK
stated that they had installed 11MW of Na-S plant
in the USA and Germany, and had over 55 MW
planned in various countries for load levelling,
wind, and solar applications [5]. In the UK, large
installations include 1 MW of storage at Lerwick
power station, Scotland.
As of October 2011, manufacturer NGK has
requested that all their Na-S batteries be taken
offline while they investigate a fire incident, and
have temporarily ceased production [7].
1
Factsheet to accompany the report “Pathways for energy storage in the UK”
Energy Density
150240Wh/kg[3]
150-230
W/kg[3]
Typical Rated
Capacity
(MW)
Nominal
Duration
0-34[6]
0.05-8[3]
Secondshours [3]
Seconds-8
hrs [6]
Cycle
Efficiency
[%]
75[2]
75-90[6]
Energy Cost
[$/kWh]
350[2]
300-500[3]
Power Capacity
Capital Cost
[$/kW]
Typical
Life
350[2]
1000-3000[3]
5-15 years[6]
10-15years[3]
3000 cycles[2]
2500 cycles[3]
2500-4500 cycles[6]
Table 1: Technical and economic data for Na-S batteries
Time to commercialisation and R&D needs
NGK have not yet published the results of their
investigation into the fire at their 2 MW Mitsubishi
Materials Corporation installation, but have
advised all customers to stop using their Na-S
batteries while the investigation is on-going [7].
Future research must therefore address this
serious safety concern, before commercialisation
can continue.
Safety, security, environmental and public
perception issues
The high operating temperature may be a cause
for concern in some applications, but for large
non-mobile grid installations it is easier to mitigate
the danger. The Mitsubishi plant fire took eight
and a half hours to bring under control and a
further two weeks before it was extinguished,
raising significant safety concerns, as it was the
third such incident since 2005. Sodium burns or
explodes on contact with water, which also makes
it critical that it be protected from moisture. This
attribute, coupled with the high operating
temperature and the fire incidents result in a
negative public image which must also be
addressed if the technology is to be accepted.
www.sciencedirect.com/science/article/pii/S100200710800381X
[4]
B. Dunn, H. Kamath, and J.-M.Tarascon,
“Electrical energy storage for the grid: A battery of
choices,” Science, vol. 334, no. 6058, pp. 928–935,
2011. [Online]. Available: http://www.sciencemag.org/content/334/6058/928.abstract
[5]
A. Okimoto, “Nas battery application,”
NGK Insulators Ltd., Asia Pacific Partnership 6th
Renewable Energy and Distributed Generation
Task Force Meeting, April 2009.
[6]
M. Beaudin, H. Zareipour,
A. Schellenberglabe, and W. Rosehart, “Energy
storage for mitigating the variability of renewable
electricity sources: An updated review,” Energy for
Sustainable Development, vol. 14, no. 4, pp. 302 –
314, 2010. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S0973082610000566
[7]
S. Sakabe, “Nas battery fire incident and
response,” NGK INSULATORS, LTD, News Release,
October 2011.
References
[1]
Figure: Nas battery. [Online]. Available:
http://commons.wikimedia.org/wiki/File:NaS_battery.png
[2]
S. Schoenung, “Energy storage systems
cost update: A study for the doe energy storage
systems program,” Sandia National Laboratories,
Technical Report, April 2011.
[3]
H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li,
and Y. Ding, “Progress in electrical energy storage
system: A critical review,” Progress in Natural
Science, vol. 19, no. 3, pp. 291 – 312, 2009.
[Online]. Available: http://-
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