GE Global Research
______________________________________________________________
Sodium-Metal Halide Batteries in DieselBattery Hybrid Telecom Applications
Job Rijssenbeek, Herman Wiegman, David Hall,
Christopher Chuah, Ganesh Balasubramanian and
Conor Brady
2011GRC699, August 2011
Public (Class 1)
Technical Information Series
Technical Report Abstract Page
Title
Sodium-Metal Halide Batteries in Diesel-Battery Hybrid Telecom Applications
Author(s)
Job Rijssenbeek
Herman Wiegman
David Hall
Christopher Chuah†
Ganesh Balasubramanian†
Conor Brady†
Component
Chemical Energy Systems Laboratory, Niskayuna
Report
Number
2011GRC699
Date
August 2011
Number
of Pages
04
Class
Public (Class 1)
Phone
1 518-387-5335
1 518-387-7527
1 518-387-5686
-
Key Words: battery; sodium metal halide; hybrid; fuel savings
Abstract: GE is commercializing the DurathonTM* sodium-metal halide battery for
stationary power and electrified transportation applications. Attributes of the technology,
which include high energy capacity, long cycle life, safe failure modes, and compatibility
with extreme operating environments, make it particularly suitable for applications such
as stationary back up power in areas where the electrical grid is unavailable. In off-grid
or weak-grid telecom applications, cell towers that rely on backup generators and
batteries can operate more fuel-efficiently by utilizing Durathon batteries instead of lead
acid batteries for energy storage, reducing fuel costs and associated emissions. This
manuscript describes the battery technology, performance in hybrid telecom applications
and demonstrates the value Durathon batteries bring to off-grid and weak-grid telecom
customers.
Manuscript received August
, 2011
†GE Energy Storage
*Trademark of General Electric Company
Sodium-Metal Halide Batteries in Diesel-Battery
Hybrid Telecom Applications
Job Rijssenbeek, Herman Wiegman, David Hall
Christopher Chuah, Ganesh Balasubramanian
GE Global Research
One Research Circle
Niskayuna, NY 12309 USA
job.rijssenbeek@research.ge.com
GE Energy Storage
1 River Rd., Bldg 2
Schenectady, NY 12345 USA
Conor Brady
BETA R&D, GE Energy Storage
Lancaster Court, Lancaster Park
Newborough Road, Needwood
Burton-on-Trent, DE13 9PD, UK
Abstract— GE is commercializing the DurathonTM* sodium-metal
halide battery for stationary power and electrified transportation
applications. Attributes of the technology, which include high
energy capacity, long cycle life, safe failure modes, and
compatibility with extreme operating environments, make it
particularly suitable for applications such as stationary back up
power in areas where the electrical grid is unavailable. In offgrid or weak-grid telecom applications, cell towers that rely on
backup generators and batteries can operate more fuel-efficiently
by utilizing Durathon batteries instead of lead acid batteries for
energy storage, reducing fuel costs and associated emissions.
This manuscript describes the battery technology, performance
in hybrid telecom applications and demonstrates the value
Durathon batteries bring to off-grid and weak-grid telecom
customers.
Keywords- battery; sodium metal halide; hybrid; fuel savings
I.
INTRODUCTION
Telecom operators in areas where grid power is unavailable
(off-grid) or only intermittently available (weak-grid) have
relied on diesel generators to power their Base Transceiver
Stations (BTS). While inexpensive to install, the escalating cost
of diesel fuel, and its delivery to remote locations, has driven
the search for alternative solutions with lower total cost of
ownership. Short of installing new energy sources such as
photovoltaics or wind turbines, which are often quite
expensive, fuel usage can be dramatically reduced by use of a
diesel-battery hybrid power system. In this scenario, a long
cycle life battery is used to share the load with the diesel
generator. The latter is used to power both the BTS and charge
the battery at a higher efficiency than if powering the BTS
only. Once the battery is charged, the generator is turned off
and the battery sustains the BTS load. Using optimized
protocols, fuel savings of up to 50% can be achieved with a
reduction in generator hours of up to 70% over conventional
diesel operation. A similar mode of operation can be employed
in situations where grid power is intermittently available. The
battery is used to ride through shorter outages after which the
off-grid mode described above is used until grid power is
restored. Whether in off-grid or weak-grid situations, the
reduced fuel consumption directly impacts the operational
expenditures of telecom sites and cuts greenhouse gas
emissions. Furthermore, combining locally available power
sources such as photovoltaics and/or wind turbines with
batteries can completely eliminate the need for a diesel
generator in these situations. General Electric‟s DurathonTM
sodium metal halide battery technology has a unique
combination of high energy density, long cycle life and low
cost that makes it especially well-suited for hybrid telecom.
This manuscript describes the NaMx technology, lab-scale
data, fuel savings models and field validation.
II.
SODIUM METAL HALIDE BATTERY TECHNOLOGY
The sodium metal halide technology excels in applications
requiring high energy density (i.e., long support times) and
long cycle life, even in extreme environments. In the last few
years, GE has refined the technology such that it is now
sufficiently developed for application in heavy-duty
transportation and stationary power quality applications. The
new technology (NaMx) is superior to the incumbent lead acid
batteries in performance, and is expected to have the lowest life
cycle cost of any battery technology for these applications,
including hybrid telecom.
First conceived in the late 1970‟s, the sodium metal halide
cell consists of a solid metal and sodium chloride positive
electrode that is separated from a liquid sodium negative
electrode by a ceramic ”-alumina solid electrolyte (BASE)
tube (Figure 1a) [1-3]. It operates at approximately 300 C, a
temperature at which the sodium and catholyte (NaAlCl4) are
molten and the ionic conductivity of the BASE is high. On
*Trademark of General Electric Company
1
with no impact on performance, unlike lead-acid systems,
which must be derated or suffer drastically shortened lifetimes.
The Durathon battery may be cooled to ambient
temperature, thereby freezing the sodium electrode, and it will
maintain its state of charge. With no parasitic reactions or selfdischarge, the battery has an indefinite shelf life in the frozen
state.
The Durathon battery is not only robust but also “smart”,
with capability for remote diagnostics and health monitoring. It
includes an integrated Battery Management System (BMS) that
handles thermal management, protection functions and
prognostics. The BMS communicates with the BTS‟s
controller to manage the hybrid operation.
Figure 1. (a) Cell schematic highlighting main components of the NaMx
cell. (b) Pictorial representation of the charging reaction, which consumes
NaCl to form NiCl2 from the β"-alumina ceramic separator inward.
charge, the metal, iron and/or nickel in this study, in the
positive electrode (cathode) is oxidized (1) and combines with
chloride ions dissolved in the liquid electrolyte to form a metal
dichloride (2). The electrons extracted in reaction 1 travel
through the external circuit. Meanwhile, sodium chloride
dissolves in the electrolyte to replenish the chloride ions (3).
The sodium ion diffuses via the liquid electrolyte and then
through the ceramic electrolyte. It is reduced to metallic
sodium by electrons from the circuit upon entering the negative
electrode (anode) compartment (4). The net charging reaction
forms sodium metal in the anode and metal chloride in the
cathode (5) (Figure 1b).
M  M2+ + 2e-
(1)
M + 2Cl  MCl2
(2)
NaCl  Na+ + Cl-
(3)
Na + e  Na
(4)
2+
+
-
-

Net:
2NaCl + M  2Na + MCl2
(5)
The open circuit voltage (OCV) of such cells is 2.58V for
M = Ni and 2.35V for M = Fe. Among the attractive features of
the NaMx technology are: high energy density (120 Wh/kg;
320 Wh/L at the battery pack level); long lifetimes under
repeated cycling; insensitivity to extreme temperature
environments; overcharge and over-discharge tolerance;
battery resilience to multiple cell failures; and good safety
performance under catastrophic mechanical impact [4].
Cell operation at elevated temperatures is beneficial
because the insulated battery case makes the performance
insensitive to the environmental temperature and battery
cooling (if needed) can easily be accomplished using ambient
air. No special cooling medium is required for NaMx batteries.
Under active cycling conditions like those found in hybrid
telecom applications, less than 1kWh of heater energy per day
is required to maintain the battery at operating temperature.
Thus Durathon batteries are tolerant to extreme temperatures
These qualities make Durathon batteries well-suited for
industrial applications like hybrid telecom, uninterruptible
power supplies, utility peak shaving, and electric fleet vehicles.
GE is completing a state-of-the-art battery manufacturing
facility to produce Durathon batteries for these markets. Startup
is expected in late 2011. This manuscript will focus specifically
on the hybrid telecom application.
III.
HYBRID TELECOM OPERATION
The GSMA estimates that by 2012 there will be 640,000
off-grid diesel generator powered base stations consuming
$14.6B in fuel [5]. For various reasons, the generators are
frequently oversized compared to the BTS load leading to
inefficient operation and poor fuel utilization. By coupling the
generator with batteries, the generator can be run more
efficiently while the batteries charge, and then turned off while
the batteries discharge to support the BTS. Fuel savings can
reach 50% or more, not including the ancillary benefits of
fewer fuel deliveries, and reduced generator run-time and
maintenance. These are especially significant benefits for
remote installations. Fuel savings are driven by the ratio of
recharge time to discharge time. More battery run time per day
translates into greater energy delivered per day (kWh/day) by
the battery and thus greater fuel savings. The NaMx technology
can recharge at high rates (greater than C/3) without threat of
thermal run-away or affecting cycle life. This enables the use
of more rectifier modules in the cell tower power system which
in turn increases the diesel generator loading during recharge
operation. In lab testing the appropriate metric is energy
delivered per day (Wh/day). For the NaMx technology, various
modes of operation are possible depending on the sizing of the
load, generator and other system requirements. Generally,
recharge/discharge time ratios of 1/2 or better are realized (e.g.,
4-hour charge, 8-hour discharge).
In summary, Durathon reduces operating expenses in
hybrid telecom applications by

Reducing fuel consumption

Reducing generator run time and maintenance

Eliminating air conditioning for battery

Providing long battery life (>5 years expected)
2
provide a good approximation to the performance of the
production version slated to be commercialized in late 2011.
Lab data (Fig. 2-3) from single cells and strings supports long
life under various duty cycles, and energy delivered of over
250 Wh/cell/day. For a typical 1.2kW BTS, this translates into
50% fuel savings (over diesel only operation). In these tests the
cells/strings are cycled under prescribed conditions reflective
of how they would be used in the field. Cycling limits such as
time, voltage, current, and/or capacity (amp-hours) are used to
determine the end of charge/discharge. In the field, this control
would be handled by the BMS in communication with the BTS
control hardware. Energy delivered (Wh/cell/day) is the
discharge energy delivered per cell per total cycling time
(charging time plus discharging time) in days and, given
system details, translates directly to daily fuel savings. This can
be calculated for each cycle.
Figure 2. Representative examples of the performance (in Wh/day)
demonstrated by Durathon cells and strings on various hybrid telecom
duty cycles and modes of operation. The specific cycling conditions
(dictated by the characteristics of the battery design and the base station
requirements) determine the energy throughput and the associated fuel
savings. The data in blue are from a single-cell test, while the other data
are from 10-cell strings. Blue, red and green traces are running in mode 1
and yellow trace is running in mode 2.
IV.
As of this writing, the experiments shown in Figs. 2 and 3
remain on test demonstrating excellent stability over long term
cycling. The NaMx technology does not suffer from the sudden
loss of performance after ~20% capacity loss experienced by
lead acid batteries. Therefore extrapolated lifetimes of many
years in the hybrid telecom application are eminently feasible.
Note that these lifetimes should be achievable under any
ambient temperature conditions.
LAB-SCALE TECHNOLOGY DEMONSTRATION
GE has aggressively developed new cathode formulations,
cell materials of construction, and cell and battery designs to
best serve the energy storage needs of a variety of markets,
ranging from hybrid locomotives to stationary back-up power.
These efforts have yielded extremely long lived and high
throughput battery designs for hybrid telecom applications
(amongst others advances). Depending on the technology
readiness, tests are performed at the single cell, 10-cell string
or full battery level. These data show great promise for the
hybrid telecom application.
Cathode formulation, cell design, duty cycle and battery
size all affect performance. Each also constrains the others and
an optimal configuration is selected for a particular application.
All the data herein were collected on pre-commercial cells that
V.
OPERATING EXPENSES SAVINGS MODEL
A system model of the BTS site was merged with an
equivalent circuit model of the NaMx battery so that combined
system studies and value assessments could be made. This
combined „battery + system‟ model is instrumental in
optimizing control settings to generate the greatest value to the
customer over a defined financial period. The combined model
accounts for:

Generator rating and fuel burn characteristics

Rectifier power limitations and efficiency rating

Battery behavior and terminal voltage characteristics

Air conditioning load and impact to generator
operation

Effects of grid availability
Lab data such those in Figs. 2 and 3 were used to validate
Figure 3. Example of the performance (in Wh/day) and long life
demonstrated by an early Durathon variant. Data are from a 10-cell string
running in mode 1 operation.
Figure 4. Model-predicted cell performance (blue) compared to
experimental data (grey) for various use profiles.
3
Diesel Fuel Consumed (L/day)
40
35
35.5
32.0
30
25
20.8
20
20.5
19.3
16.3
15
10
5
0
Diesel
Only
Diesel +
VRLA
(New;
Mode 1)
Diesel + Diesel + Diesel + Diesel +
VRLA
NaMx
NaMx
NaMx
(Aged; (Design 1; (Design 1; (Design 2;
Mode 1) Mode 1) Mode 2) Mode 2)
Figure 5. Comparison of the model-predicted generator daily fuel usage
for a 1.2 kW BTS powered by a diesel generator only (15 kVA; grey), by
the diesel and VRLA batteries (600Ah, as-installed and after 3 years of
operation; oranges), and by the diesel and NaMx batteries (165Ah, asinstalled for two designs and two modes of operation; blues).
the battery performance model (Fig. 4). The good agreement
between the equivalent circuit battery model and experiment
enables a high fidelity value calculator of customer fuel savings
under a wide range of use conditions. Furthermore, this system
model also offers the ability to compare the value of different
battery technologies and modes of operation (Fig. 5). This
system level approach enables GE to provide an individualized
solution based on a customer‟s unique situation and goals.
VI.
FIELD VALIDATION
Field demonstrations of Durathon batteries are currently
underway in India at rural revenue generating BTS sites, in
both off-grid and weak-grid situations. Initial demonstrated
fuel savings to the customer are extremely attractive (Fig.6).
These sites were originally outfitted with valve-regulated lead
acid (VRLA) batteries that were run in a timer & voltage cutoff mode of operation. The baseline fuel consumption
measurements are shown below for the 3-year old VRLA
batteries which achieved ~4 hours of discharge operation per
day. After installation of Durathon batteries, fuel usage
decreased dramatically. Two modes of operating the batteries
were tested, showing up to 50% reduction in fuel consumption
relative to the aged VRLA operation. The longer cycle life of
Durathon batteries is expected to maintain the fuel savings
benefits versus lead acid for years. These field trials are
continuing.
An ancillary benefit to the integrated BMS is that it enables
remote monitoring of the battery, allowing the customer to
Figure 6. Comparison of the measured daily fuel usage for a BTS
powered by a diesel generator and aged VRLA batteries at a customer site
(orange bar) versus after installing Durathon batteries (blue bars) tested
under two different modes of operation.
verify the fuel savings, detect and manage faults, and schedule
preventative maintenance. Detailed alerts and status updates
can be sent to a maintenance center, or even as a text message
to a mobile device. Beyond the status of the battery, these
communications can relay information about the entire BTS
site.
VII. CONCLUSIONS
Durathon batteries show long life and excellent customer
value in hybrid telecom applications. GE‟s Durathon battery
factory is nearing completion (End of 2011) and will initially
focus on stationary power quality and industrial transportation
markets where the NaMx technology shines. GE is committed
to sustained technology development meaning customers can
expect better performance, longer life and even greater value as
the technology is further refined. Durathon has the potential to
be a game changing technology for BTS operators who face the
dual pressures of increasing fuel prices and increasing demand
for coverage in remote locations.
REFERENCES
[1]
[2]
[3]
[4]
[5]
J.L Sudwoth, J. Power Sources 100 (2001) 149.
C.-H. Dustman, J. Power Sources 127 (2004) 85.
X. Lu, G. Xia, J. P. Lemmon, Z. Yang, J. Power Sources 195 (2010)
2431.
D. Trickett, “Current Status of Health and Safety Issues of
Sodium/Metal Chloride (Zebra) Batteries”, NREL Tech Report, 1998.
D. Taverner, “Green Power for Mobile Community Power”, GSM
Association, White Paper, 2010.
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