Battery energy-storage-system

UNESCOREGIONALOFFICEFOR SCIENCEANDTECHNOLOGYFOR EUROPE(ROSTE)
1262iA DORSODURO - VENICE, ITALY 30123 - TEL. 041-5225535- FAX 041-5289995

BATTERY ENERGY STORAGE
SYSTEMS
D. PAVLOV, G. PAPAZOV and M. GERGANSKA
UNESCO Regional Office for Science and Technology for Europe
(ROSTE)

KThe authors are responsible for the choice and presentation of the facts
contained in this book and for the opinions expressed therein, which are
not necessarily those of UNESCO and do not commit the Organization..

Preface
In an attempt to make the power industry more effective, a new
trend in electric power production has witnessed intense develop-
ment during recent years, that of energy storage. Several options
have been considered for this purpose, one of them being the bat-
tery energy storage system. B o t h classical lead-acid batteries, as well
as new advanced types of batteries are being used. A number of
demonstration battery energy storage plants and facilities have been
designed and built, and are now subjected to testing. It has become
general practice for experts in the power industry, and battery re-
searchers andmanufacturers to meet at joint conferences to exchange
information and opinions on the problems of energy storage. It is now
opportune to siirrimarize the results and experiences so far acquired
in tlie design arid utilization of battery energy storage systems.
In 1954, Elsevier in Amsterdam issued the book entitled “Power
Sources for Electric Vehicles” edited by B.D. McNicol and
D.A..J. Rand, which presented a comprehensive survey of the cur-
rent knowledge in the field.
Motor car transport is being increasingly adopted, since it is an
important and indispensable element of the normal functioning of
every modern social community. It has, however, a serious environ-
mental impact in that it causes considerable air pollution in large
cities and densely populated areas.
Development and large-scale commercialization of electric vehi-
cles has become oiie of the greatest challenges of the late 20th cen-
tury. However, the electrocliemica1power sources used for propulsion
of these vehicles cannot yet meet the challenge. Annual international
i

conferences o n the problems of electrochemical power sources show
that more effort is being placed on broad-spectrum investigations in
the field. Accumulated theoretical knowledge and practical experi-
ence o n battery energy storage systems for electric vehicle applica-
tions should now be analyzed and evaluated.
T h e Regional Office for Science and Technology for Europe
(ROSSE) at the United Nations Educational, Scientific and Cultural
Organization (UNESCO) entrusted us w i t h the task of carrying out
an overview of the current status and future perspectives of battery
energy storage systems for applications in the power industry and in
transport, w i t h the purpose of attracting wider public attention to
the problems of these systems.
T h e current status and the problems confronting battery en-
ergy storage systems for the power industry are presented by
Prof. DrSci. D. Pavlov, and for electric vehicle applications, by
Dr. G. Papazov. The English version of the text was provided by
Mrs. M. Gerganska. All three of us work at the Central Laboratory
of Electrochemical Power Sources, Bulgarian Academy of Sciences,
Sofia, Bulgaria.
If we have achieved, even in part, the aims envisioned by
UNESCO for this book, and if our efforts contribute, though mod-
estly, t o the development of battery energy storage systems, we will
be most satisfied.
D. Pavlov, G. Papazov, M. Gerganska
May 1991, Sofia, Bulgaria
11

Contents
Preface ................................................ i
Chapter 1
BATTERY ENERGY S T O R A G E S Y S T E M S
FOR THE POWER I N D U S T R Y
.......................................
1
. Introduction 1
1.1. The four basic elements of every national electric
power system ......................................... 1
1.2. Power industry and its problems ........................
1.2.1. Energy, power and response time ..................
1.2.2. Quality of energy supply systems .................
3
3
5
1.2.3. Ecological problems and the development
of power industry ................................ 6
2. Electric energy storage ........................... 7
2.1. Pumped Hydroelectric Energy Storage Systems (PHESS) .. 8
2.2. Compressed-Air Energy Storage Systems (CAESS) ....... 9
2.3. Superconducting Magnetic Energy Storage Systems
(SMESS) ............................................. 11
2.4. Battery Energy Storage Systems (BECS) ................ 13
2.4.2. Basic principles of battery operation ...............15
2.4.1. The revival of battery energy storage systems ...... 13
2.4.3. Some advantages of battery energy storage systems . 15
2.5. Choosing the right option for electric energy storage ...... 17
...
111

2.4.2. Basic principles of battery operation ...............15
2.4.3. Some advantages of battery energy storage systems . 15
2.5. Choosing the right option for electric energy storage ...... 17
3. Batteries for energy storage .in operation and
under development ................................ 19
3.2. Sodium/Sulfur Batteries ............................... 20
3.2.1. Principles of cell operation ....................... 20
3.2.2. Design of sodium/sulfur cells ..................... 23
3.1. Development projects for battery energy storage systems .. 19
3.2.3. Specification and test results for battery modules
and pilot plant of the Japanese “Moonlight Project” 24
3.3. Zinc/Bromine Batteries ................................ 28
3.3.1. Reactions and principles of cell design arid operation 28
3.3.2. Chemistry and electrochemistry of the zinc/bromine
cell ............................................ 31
3.3.3. Battery system design ........................... 33
3.3.4. Characteristics of zinc/bromirie batteries ........... 33
3.4. Zinc/Chlorine batteries ................................ 37
3.4.1. Fundamentals of zinc/chlorine batteries ............ 37
3.4.2. Battery design .................................. 39
3.4.3. Battery characteristics ........................... 40
iv

5.2. Lead-acid battery energy storage systems (LABESS)
in operation by 1990 throughout the world ............... 63
6. Lead-acid battery energy storage systems for
load levelling ...................................... 67
6.1. System structure ...................................... 67
systein ............................................... 68
6.2.1. Plant layout .................................... 68
6.2.2. The battery ..................................... 69
6.2.3. Power conditioning system ....................... 76
6.2.5. Equipment energy losses ......................... 78
6.2. Chino 10 hgW/40 MWh lead-acid battery energy storage
6.2.4. Facility monitoring and control system ............ 78
6.2.6. Economics of Chino LABES Plant ................ 79
7. LABESS for instantaneous (spinning) reserve
and frequency control applications .............. 80
7.1. Island networks ....................................... 80
7.2. The BEWAG 8.5/17MW Lead-Acid Battery Energy
Storage Plant ......................................... 81
7.2.1. System frequency response having given rise t o
the construction of the BEWAG LABESplant ..... 81
7.2.2. System design and characteristics ................. 82
8. Lead-acid battery energy storage systems for
peak shaving ....................................... 86
8.1. What is peak shaving? ................................. 86
transport network ..................................... 91
8.2. Johnson Controls 300 1<W/6001ïWh LABES Facility ...... 88
8.3. Lead-acid battery energy storage systems in the railway
V

9. Valve-regulated lead-acid batteries for b a t t e r y
energy storage systems ........................... 94
10. Strategic advantages of BES systems ............ 96
References ......................................... 98
Chapter 2
ENERGY S T O R A G E S Y S T E M S F O R ELECTRIC
V E H I C L E S
1.
2.
3.
4.
5.
6.
7.
vi
M o t o r vehicles and environmental p o l l u t i o n ... 103
Specification of energy storage systems for
electric vehicles ................. ........... .. ... .. 106
Charge a n d capacity o f batteries for electric
vehicles ......................... ............ . ...... 122
Types o f cycles o f electric vehicle batteries .... 128
Requirements t o t h e construction and manu-
facturing technology o f batteries for EV energy
storage systems ................................... 139
Specification o f operating energy storage
systems for electric vehicles ..., . .. ........ ...... 1.12
L y r i c a l epilogue .............. .... ............. .... 144
References .. , ................ ............... . ..... 145

Chapter 1
BATTERY ENERGY STORAGE
S Y S T E M S FOR THE POWER INDUSTRY
D. PAVLOV
1. Introduction
1.1. T h e four basic elements of every national electric
power system
Production of electric energy is the basic pillar for normal func-
tioning of every modern social community and a guarantee for its
progress. It is organized in an electric power system comprising three
basic elements:
electric
power plants: thermal power plants fired by coal or nuclear fuel, gas-
fired steam plants, oil- or gas-fired combustion turbines, hydroelectric
plants, etc.
b) Electric power distributing systems including transformer fa-
cilities, transmission trunk lines and distribution lines t o every cus-
tomer.
c) Consumers of electric power and energy. These are users in
industrial, transport, agricultural and telecommunication contexts,
and people in their day-to-day life, administrative buildings, etc.
The electric power produced by the generating utilities is deliv-
ered through the transmission/distribution system t o the consumers
for utilization. The consumers’ demand for electric power varies
cyclically during day and night, as well as within the week and the
seasons.
a) Electric power and energy generating utilities, i.e.

Demand. This is the rate at which electric energy is delivered to
the consumer, measured in kW (kilowatts) integrated over a specific
time interval (15 min) [l].
Figure 1 shows an example of a daily customer demand profile.
A baseload level of demand is introduced. The power capacity for
meeting this demand level is generated and maintairied by thermal
power plants fired by low-cost fuels such as coal or nuclear fuel. To
be economically effective, baseload generating units should operate
at a minimum capacity of 500-1000 MW and under constant load.
I I
. . , , ,
I 3 5 ' 7 ' 9 ' 1 1 ' i 3 ' 1 ' ' 1 7 ' 1 9 21 23
DCoal mGas steam @Gas turbine Bottery
Fig. 1. Example of customer energy demand curve for a working day [il.
Hour of day
During the night (hours O to 6), the demand decreases to about
15-30% below the baseload level. The daytime demand is signifi-
cantly higher than the baseload level. I t is served by gas-fired steam
plants. They burn natural gas or oil which are more expensive fu-
els than coal. There are two peaks in the daytime demand profile
related t o the increased energy consumption for the transportation
of people from home t o the working place and back, as well as for
increased household needs. Peak power is generated by gas-fired
turbines utilizing relatively expensive fuel, and also by hydroelectric
2

power plants. The ratio of actual t o peak power demand over a given
period is called load factor.
There is an intrinsic contradiction in each power supply system
between producers and consumers of electricity. To be efficient,
power plants should operate at constant load. T h e customers’ de-
mand, o n the other hand, undergoes cyclic fluctuations. This leads
t o inefficient utilization of the generating capacities. A possible so-
lution t o this problem is the involvement of a new element in the
energy system.
Electric energy storage. At night, when energy demand is low,
generated electric energy is stored in appropriate facilities, and is
delivered to meet peak-hour demands duringthe day. Thus, low-cost
fuel power plants work at maximum load during the night and store
the generated energy to sell it at increased cost during peak demand
periods. The introduction of this fourth element inthe electric power
system makes its operation more efficient. This not only brings about
considerable savings of expensive fuels such as gas and oil, but also
improves the load factor of the power generating facilities.
1.2. Power i n d u s t r y and i t s problems
1.2.1. Energy, power and response time
It has been established that the different forms of motion (me-
chanical,thermal,electromagnetic,gravitational, chemical, etc.) are
converted into one another following definite quantitative ratios. To
allow measuringof the various forms of motion by a unified measuring
unit, the term energy has been introduced. The electrical energy
is determined from the product of the voltage and the quantity of
charge that passes through an electrical device (load).
The electrical
power is determined from the product of voltage and current.
T h e work done per unit time is called power.
3

In thermal power stations, the chemical energy accumulated in
coal, crude oil or natural gas is transformed by burning (oxida-
tion of the hydrocarbons) into heat (high-temperature, high-pressure
steam), which sets in motion a turbine, whereby the thermal energy
is transformed into mechanical. The turbine shaft is connected to the
shaft of an electric generator. On rotation, this common shaft drives
the rotor of the generator as a result of which the mechanical energy
is converted into electrical energy. It is evident that, to obtain elec-
trical energy from coal, several processes of energy conversion have
t o occur.
In an electrochemical power source, a battery in particular, this
energy transformation path is much shorter. In this case, through
electrochemical reactions of oxidation and reduction proceeding on
the surface of the two electrodes, the chemical energy is directly
transformed into electrical power.
Conversion of one type of energy into another requires a certain
time period. The time needed for an energy-generating system to
change its power from a value (A) to another value (B) is called
response (transition) time (Fig. 2).
/ i
ition time _J
Time
Fig. 2. Power curve showing the change of power from level A t o level B.
4

A thermal power plant needs tens of minutes t o change from one
power level to another, while for a battery, the response time is of
the order of millionths of a second. For an energy utility t o meet all
load fluctuations, it should dispose of a system of power plants with
various response times rangingfrom milliseconds t o hours.
1.2.2. Quality of energy supply systems
The quality of an electric power supply is determined by the
available reserve capacity at the energy utility. Figure 3 illustrates
the distribution of the electric-system capacity expressed by a typical
weekly load curve of an electric utility.
Generation for load í No storage)
100
2.
Generation for load í No storage)
Baseload
Mon Tue Wed! Thul Fri 1 Sat Sun 1
Generation for load íWHh storage)
Mon I Tue IWed I Thu I Fri I Sat I Sun I
I I IReserve
Baseiood energy to S i O M g t
Peaking energy from storage
Fig. 3. Typical weekly load curve for an electric utility [il.
The energy system should have 15-20% of reserve power available
t o be able to meet any customer demand. If there is no or insufficient
reserve capacity andthe load level exceeds the power generation level,
5

a decline in voltage at the consumer side will appear which would
upset the normal operation of the users’ machines and electrical de-
vices or even cause them t o fail. For this reason it is essential for
the normal functioning and development of each social community t o
have reliable nationalandlocal electric power systems w i t h capacities
exceeding the actual energy demand by at least 15%.
Unfortunately, however, only rich and advanced modern countries
possess such high-grade energy systems. T h e power systems of most
countries inthe world have capacities that only just meet their energy
demands, and in some cases are simply inadequate. This hunger for
electricity is very often a limiting factor for the economical and social
development of a country.
1.2.3.
indust r y
Ecological problems and the development of power
The electric energy needs of the population, industry, agriculture,
transport, etc. increase every year, and the claims for high-quality
electric power become ever more demanding in relation to the in-
creasing automation and computerization of the national economy.
Previously, these needs were met by expanding the capacities of all
types of electric power generating facilities. Operation of these facil-
ities, however, is based on the combustion of coal, oil and gas, which
is accompanied by harmful gas emissions of COZ, SO2 and others.
The increased content of SO2 in the atmosphere has led to the forma-
tion of acid rains causing enormous damage t o the agricultural crops
and the forests. The accumulated CO2 in the air might bring about
considerable climatic changes b o t h on a regional and global scale
(the so-called “greenhouse effect”). Thus the rapid development of
power industry has added comfort to society and its individuals, but
it has posed very serious ecological problems of a national, regional
and global nature. In response to these processes, various organi-
6

zations and social movements are being founded whose activities of
environmental control and protest actions begin t o have a significant
impact o n the policy of state governments and of companies engaged
in electric power production. The efforts of these movements, com-
binedw i t h the wisdom of a number of state governments, have led t o
the adoption of dead-line terms for decreasing the harmful gm con-
tents in the atmosphere, especially those of SO:! and COL,in order
to restrict possible environmental damage.
Solutions are being sought in several directions:
First, in reducing SO2 emissions by building up special facilities
at electric power plants for purification of exhaust gases. This
method has an undoubtedly beneficial effect o n the environmental
aspect of electric energy production, but it involves rather expensive,
complicated and not fully efficient procedures leading to increase in
energy and power costs.
Second, in buildingup a system of energy storage plants which
haw a considerable impact on the efficiency of energy utilities as well
as significant cost benefits.
Third, in thc sphere of electric power consumption, all techno-
logical processes of major energy consumers have been revised w i t h
regard to power consumption, and the most energy-consuming pro-
cedures replaced by new technologies with lower power demands.
The basic problems and the development trends of energy storage
will be discussed in the chapters to follow.
2. Electric energy storage
During the last few decades, several options for electric energy
storage have been devised. Many countries have started programs
aimed at development of energy storage technologies. The basic prin-
ciples of some of these options, that have found successful application,
will be outlined below.
7

2.1.
(PHESS)
Pumped Hydroelectric Energy Storage Systems
Figure 4 presents the schematic of such a system.
Fig. 4. Schematic of a PumpedHydroelectric Energy Storage System.
These energy storage units require two large water reservoirs lo-
cated at different heights, so that water fall is possible. During pe-
riods of low demand, the excess power is utilized t o pump water
from the lower reservoir and transfer it to the upper one. At peak
demand periods, the pumped storage plant acts as a hydroelectric
power plant thus adding capacity to the energy system. This storing
option is cost-effective if used only 5 t o 8 hours in the peaking range.
Its response time is of the order of 5 to 10 minutes.
T h e above energy storage technology has been in use for over 50
years now. At present, there are about 35 pumped storage plants
in operation all over the world with a total capacity of 25,000 MW.
This energy storage option is most appropriate for countries with
mountainous relief. Construction of these plants requires from 8 to
10 years and is often associated with considerable environmental im-
8

pact. Pumped hydroelectric energy storage systems are cost-effective
if they are designed for power units of over 1000 MW.
In Italy, for example, pumped storage plants supply 14% of the
net power capacity. In Japan, they amount to about 10% of the
national net capacity, while for France, Germany and the UK, this
figure is 6%, and 3% for the USA. At the moment, more than 200
pumped storage plants are under construction worldwide. Conse-
quently, by the beginning of the next century, this energy storage
option will become an important element of many national electric
power systems.
2.2.Compressed-Air Energy Storage Systems (CAESS)
A compressed-air storage plant uses inexpensive off-peak energy
to drive the motor of a compressor for compressing air that is stored
in a salt cavern located deep underground or in large hard rock cav-
erns. During peak demand periods, gradual release of pressure is
performed and the air coming up to the surface is heated by burn-
ing oil or gas, and is then expanded through expansion turbines
that drive the rotor of an electric current generator. Compressor
motor and generator are combined in one machine. During air com-
pression, the rnotor/generator is connected to the compressor and
decoupled from the turbine. During electric current generation, the
motor/generator is disconnected from the compressor and coupled to
the turbine. Compressed-air storage units burn only one third of the
fuel used by conventional combustion turbines to produce the same
amount of electricity. This leads to a two-third reduction in the en-
vironmental pollution caused by the combustion process of turbines
which are usually located in urban areas. Ways have been sought for
optimization of the system operation, such as return of the heat re-
leased during air compression back to the energy system. This energy
storage option is cost-effective if operated at a power above 25 MW.

For every hour of electric current generation, 1.7 ki of air compression
are needed. The response time is about 10 minutes. Efficiency of air
compression is 65-75%. The starting period is 20 to 30 minutes. As
rcgards the security aspects, measures should be provided against
leakage of compressed air. The service life of air-compressed storage
plants is about 30 years.
A block diagram of such a system is presented in Fig. 5.
Fig. 5. Block diagram of a Compressed-Air Energy Storage System [2].
Compressed-air storage technology was first devised in Germany,
and since 1978 a 290 MW, four-hour capacity unit has been in op-
eration in Huntorf. The plant uses two salt caverns, and storage
efficiency of over 80% is reported. The cost of unit power is about
425 $ kW-’. A 30-year operational life of the plant is expected.
Commercial operation of the German CAES plant has shown that
this type of energy storage option is sufficiently reliable.
10

Compressed-air storage plants have a negligible environmental
impact, and can be built within 2 to 5 years. They arc: fit,ted w i t h
modified combustion turbines of routine production. This technology
can find application only in countries with natural deep underground,
hard rock or salt caverns.
At present, several demonstration compressed-air energy storage
plants are being built: in the USA, Alabama (110 MW, 26-hour
capacity), in the USSR (1050 MW, 10-hour capacity, thrce-unit plant
with salt,cavern storage), in Israel (300 h'lW, lo-hour capacity, three-
unit plant), etc. The Italian conipany ENEL hasst,artedconstruction
of modular mini-units of 25 and 50 MW, arid 10-hour capacity, using
aquifer storage.
2.3. Superconducting Magnetic Energy Storage
System (SMESS)
There is a theoretical and a technical option to store electrical en-
ergy as such, without converting it into other forms. This is possible
owing to the ability of some substances to become superconducting
at extremely low temperatures.
Because of the conductor's electrical resistance at anibient tem-
perature, part of the electrical energy is lost in the form of heat
emission (joule losses). These losses can be compensated by adding
new quantities of electricity t o the power supply network.
At extremely low temperatures, some alloys and ceramic materi-
als achieve superconducting properties, i.e. they lose their electrical
resistance. When direct current is fed into an electric circuit of su-
perconductors, the current will circulate endlessly along the closed
ring without energy losses. When an energy demand appears, the
requested electrical power can be drawn from that closed ring.
Large-scaleinvestigations are presently being performed aimed at
devisinga technology for the production of superconductingmagnetic
11

Fig. 6. Schematic of a Superconducting Magnetic Energy Storage
System [2].
energy storage plants. A block diagram of such a plant is presented
in Fig. 6.
T h e heart of this storage system is the electromagnetic supercon-
ducting coil. The latter operates on direct current. Charging of the
electromagnetic coil w i t h electricity from the ac generating utility is
accomplished via a two-way converter. A refrigeration system main-
tains the temperature of the electromagnetic coil at a very low fixed
value. Operation of the electromagnetic coil, converter and refrig-
erator is monitored and controlled by a controller. Such an energy
storage plant should be sited near a substation where the transformer
converts the high voltage energy from the utility network t o appro-
priate low voltage power. The response time of this type of storage
system for switching between charging and discharging is about 20
milliseconds. The ac-ac efficiency is 90% or more.
The experimental SMES systems so far set up operate at ex-
tremely low temperatures -269°C (4 K), the temperature of liquid
helium) and the coil-wire used is made of NbTi and NbSn alloys.
12

With the discovery of ceramic high-temperature semiconductors, it
can be expected that superconducting magnetic storage plants will
be constructed that are capable of operating at the temperature of
liquid nitrogen (-196°C). Since the technology for liquid nitrogen
production is well advanced and cost-effective, the expenses for con-
struction and maintena.nceof the refrigeration system will be reduced
significantly.
The current density in superconductive wires may reach ex-
tremely high values as the conductor exerts no electrical resistance
leading to joule losses. This allows the wire cross-section t o be de-
creased more than five times with respect to copper wires used at
ambient temperature. This will change substantially the existing
classical electric power system.
In Japan, an energy storage project is being developed known
as the “Moonlight Project”. The power of the Japanese super-
conducting magnetic storage system is 1000 MW, energy density
is 12 Wh kg-’, storage efficiency SO-SO’%, storage utilization rate
approx. 75%. The system will be used for daily and weekly en-
ergy storage. Underground bed rocks are required for construction
of this system. Location possibilities are restrictcd, because anti-
magnetic measures are needed for environmental protection. Protec-
tion against superconductive material degradation is also necessary.
A 10 MW, two-hour capacity SMESC pilot plant has been devel-
oped inthe USA.The refrigeration system is based on liquidhelium.
2.4. Battery Energy Storage Systems (BESS)
2.4.1. The revival of battery energy storage systems
At the beginning of this century, electric power supply for indus-
trial and domestic needs was provided by dc generators arid battery
facilities operating under floating charge conditions. During this pe-
13

riod, batteries proved to be a diverse and flexible means of solving
the load factor problem.
During the 1930s, an expansion of ac technologies for electric
power generation, transmission and distribution applications was
noted and, very soon, the dc battery system was abandoned and
hence also the storage of energy as an element of the power system.
In the 1960s, a powerful reliable and cost-effective static recti-
fier was devised. Nuclear power plants were equipped w i t h large
stand-by lead-acid battery storage facilities ensuring their reliability
by supplying reserve power and energy. New and innovative elec-
tric power applications in industry and every-day life brought about
radical changes in the profile of the daily, weekly and seasonal de-
mand ciirves. To enhance the operational efficiency of electric power
utilities, energy storage units were introduced. At first, pumped hy-
droelectric energy storage plants were used for that purpose, and
later, the old lead-acid battery storage systems were revived. They
were based on totally new conversion, management and control tech-
nologies.
A t the end of the 1970s, for the first time, BEWAG-AG decided
to install the Battery Storage Facility in West Berlin under a test
program in order to collect the necessary operational and technical
information. It started operation in July 1981.
Within the “Moonlight” energy storage project, a 1 MW/4 MWh
load-levelling battery plant started operation in 1986 in Tatsumi,
Japan.
In July 1988, the largest battery plant for load-levelling (10 MW/
40 MWh) was set in operation in the USA, at Chino, California.
Since then, many technologically advanced countries throughout
tlie world have started large-scale research and test programs aimed
at the introductionof battery energy storage systems intheir national
ccoiioinies and public services.
14

2.4.2. Basic principles of battery operation
When two appropriately chosen electrodes are immersed in the
respective electrolyte and a direct electric current from an external
source flows between them, electrochemical reactions proceed on the
electrode surfaces during which inactive substances are transformed
into electrochemically active ones. This process is called charging
of the electrochemical power source or the battery. As a result of
these reactions, electrical energy is converted to chemical and an
electromotive force is created between the two electrodes. When
the electrodes are interconnected via a load, under the action of
this electromotive force, electrocheniical reactions proceed on the
electrode surfaces in an opposite direction to the reactions during
charge. This process of current generation is called discharge. The
battery can endure thousands of charge-discharge cycles. However,
parallel t o the reversible processes of charge and discharge, certain
low rate irreversible processes also take place that limit batt,ery cycle
life.
During off-peak periods, the battery is charged from the electric
power utility via a converter. The latter converts the alternating
current into dc. During discharge, the direct current generated in
the battery is transformed by the converter iiit,o alternating current
and the latter is delivered through the tramfornier to the utility for
meeting energy demands. Operation of the converter arid the battery
are monitored and controlled by a controller,
2.4.3. Some advantages of battery energy storage systems
In the process of developnient of the new generation of BEC sys-
tems, lead-a,cid batteries were widely used, which allowed the latter
to exhibit a number of useful advantages leading to significant cost
beneíits. The following ecoiioniical features of lead-acid battery stor-
age systems were demonstrated.
15

a) Modular design. Construction of BES plants is realized on a
modular basis, i.e. through connectingof the individualbattery cells,
in parallel and/or in series, various configurations could b e obtained
for any desired voltage, power or ampere-hour capacity. This allows
BESS construction to be accomplished instages according t o demand
needs.
b) Short construction terms. All BESS elements are produced
at the factories within a few months only andthen the actual building
of the BES plant is reduced to installing, assembling and testing of
these elements in a working system.
c) Small environmental impact. Battery energy storage sys-
tems are basically closed systems. No other materials are consumed
except water, and hence n o air and environmental pollution is caused.
They are quiet and can be located near, and even in, housing city
areas.
d) High level of recycling of the materials employed a
n the
batteries. At the end of a battery's service life, many of the materials
used for its manufacture can be regenerated.
From the above, it follows that the revival of BES systems during
the 1980s is a normal process based on the rapid progress of electric
power industry.
To b e economically effective, BES systems should meet the fol-
lowing challenging performance requirements:
o 30 years of service life
o 75% ac-ac efficiency
o Unit power cost about 400-700 $ kW-'
e 5 hour discharge.
Many projects are being implemented at present worldwide aimed
at achieving the above parameters.
16

2.5. Choosing the right option for electric energy
storage
When deciding o n the type of off-peak energy storage system to
adopt, the following considerations should be taken into account:
o profile of the 24-hour demand cyclogram
o available budget and its possibilities
0 national topographical peculiarities
o environmental aspects and limitations.
According t o an EPRI investigation, there are a great number
of hard rock caverns in the USA, which suggests dominating im-
portance of off-peak energy storage through compressed-air storage
plants. Second in importance are battery energy storage systems,
and pumped hydroelectric storage facilities come third.
In Italy, owing to its pronounced mountainous relief, pumped
hydroelectric energy storage systems have proved t o be most cost-
effective, and hence this country, together w i t h Japan, occupy the
leading positions in the construction of this type of storage plant o n
a worldwide basis.
EPRI in the USA have carried out an analysis of the economics
of the various options for energy storage. The results of these evalu-
ations are summarized in Table 1.
An analysis of the results indicates that battery storage systems
and superconducting magnetic storage systems are more appropri-
ate for use when there are peak demands w i t h a duration of 2 t o
3 hours. Pumped hydroelectric storage plants and compressed-air
storage systems may cover efficiently peak power demand needs of
up to 10 hours. They can also function as intermediate load power
systerns.
17

Table 1. Estimated costs for energy storage technologies [2]
~ ~
Technology Power Energy Hours of Total
related related storage cost
$kW-' $kWh-' $ kW-'
Compressed-air Small module
Large module
Pumpedhydro Conventional
Underground
(2000 M W )
Battery Lead-acid
(10 M W )
Advanced
(10 M W )
(25-50 M W )
(110-220 MW)
electric (500-1500 M W )
Supereconducting (Target)
magnetic (1000 MW)
575 5 10 625
415 1 10 425
1000 10 10 1100
1040 45 10 1490
125 170 3 635
125 1
O0 3 425
150 275 3 975
The unit power costs are highest for pumped hydroelectric plants,
owing to the large capital costs for construction of the facilities. With
regard to the unit energy storage costs, this type of storage option is
considerably cost-effective. Total costs ($ kW-') for compressed-air
and battery storage units show similar values. The above rating of
the various energy storage options will, of course, vary for the various
countries, depending on their specific economic and technological
conditions and requirements.
18

3. Batteries for energy storage ~ in opera-
tion and under development
3.1. Development projects for battery energy storage
systems
T h e “Moonlight Project” is the nickname of an RSrD prograin
for energy storage in Japan. The nanie “nioonlight” was selected t o
imply the analogy betweenthe nioon that does not sliine with its own
light but reflects the sun’s light, arid the energy storage batteries that
do not “generate” their own energy but dispatch the stored electric
power produced by another source.
T h e “Moonlight Project” includes six progranis, one o f which is
the Advanced Battery Electric Power Storage Systeni. Sonie of the
basic functions of this system will be discussed below.
T h e Electric Power ResearchInstitute inthe USA lias beeii carry-
ing out research and developirieiit activities within the Energy Stor-
age Program since 1972. T h e profirani incliirlcs dcvclopIricrit of lead-
acid and advanced batteries.
Bothprograms are haced o n almost the sanie electrochemical sys-
tems. First, the lead-acid battery has been chosen as a basic chemical
power source commercially available. R&D activities are aiming at
adapting, both from a constructional arid technological point of view,
this 100-year old battery t o the requirements of energy storage. Sec-
ond, new electrochemical power sources aie being investigated and
developed, such as: sodium/sulfur, zinc/chloride, zinc/broniide and
redox/Aow batteries.
Japan’s project is targeted at devising a demonstrational model
of a battery energy storage system with the following parameters:
o power output ~ 1MW
o charge time - 8 h
o discharge time ~ 8 h
19

o overall energy efficiency - min. 70% (at ac input/output)
o service life ~ min. 10years (2000 cycles)
Batteries should conform t o all environmental standards [4].
Analogous specifications have been adopted by the American
10 MW demonstration battery storage plant.
Several demonstration and testing battery storage facilities using
lead-acid batteries have been built and are in operation in other
countries in the world. The testing results of these units will be
discussed later. The basic properties and characteristics of the so-
called advanced batteries will be described first.
Sodium
NO
3.2. Sodium/Sulfur batteries
3.2.1. Principles of cell operation
The sodium/sulfur cell consists of a negative electrode (cathode)
of molten sodium (Na) and a positive electrode (anode) of molten
sulfur separatkd by a beta-alumina (P-Al20,) ceramic ion-conductive
membrane. Through this membrarie, only sodium ions can pass, but
not electrons or sulfur ions. A block diagram of a sodium/sulfur cell
is given in Fig. 7.
Ij-oiumino Sodium
polysulfide
S.NOS, .
( X . 3 i 5 )
Fig.
20
7. Block diagram of a sodium/siilfur celi [5].

The reactions that proceed in the cell can be expressed by the
following ecpiation:
Discharge
2Na+2S + Na& ( x = 3-5)
Charge
During discharge, metallic sodium of the negative electrode is ion-
ized to positive sodium ions (Nat) and electrons are released. The
sodium ions pass through the beta-alumina membrane and reach the
positive sulfur electrode. The electrons released o n the negative elec-
trode flow through an external circuit, pass through a load whereby
certain useful electric work is done, and reach the sulfur electrode.
There, they are bonded to the sulfur atoms and form sulfur ions
(S2-). These react, with the sodium ions giving sodium polysulfide
(NazS,). There is a voltage of 2 V between the sodium and the sulfur
electrodes. Under the act,ion of this electromotive force, the above
reactions procecd and electrons move from the sodium t o the sulfur
electrode doing some work.
During charge, reverse processes take place. In this case, electric
energy shoiild he introduced into the cell to enable proceeding of the
reverse processcs. Under the action of an external -oltage applied t o
the cell, electrons from the polysulfide electrode move back, through
the external circuit, to the sodium electrode. As a result, the sulfur
ions of the polysulfide molecule (Y)
are transformed into sulfur
atoms, and the released Na+ ions pass through the beta-alumina
membrane and are bonded to the electrons forming sodium atoms.
For the battery to operate, a temperature of about 350°C should
be maintained. In this way, both sodium and sulfur are kept in the
liquidstate and the resistance of the beta-alumina membrane is very
low.
Figure 8 shows the voltage curves duringcharge and discharge of
the battery, as a function of the composition of sodium polysulfide.
21

0 i O 2 0 3 0 0 4 0 5 0 7 0 8 0 9 0 1 0 0
S Discharge composition, O/. Na253
Fig. 8. Cell voltage us. sodium polysulfide composition during charge and
discharge [6].
The voltage characteristics depend on the temperatiire and the
composition of sodium polysulfide. Figure 9 illiistra2tc:s tlic changcs
in the open circuit voltage as a function of anodc coinpositioii.
300'C
A 330T
0 360.C
390'C
0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1
Anode composition (mole ratio of sulfur)
Fig. 9. Open circuit voltage as a fiinrtioii of molar ratio of siilfur iiisodium
polysulfide [5].
22

When the molar ratio of sulfur t o polysulfide falls below 0.7, the
cell voltage begins to decline.
T h e sodium/sulfur battery i s hermetically sealed and completely
maintenance-free. There are no side reactions during charging and
discharging. It is free of self-discharge. The state of charge can
be easily monitored by measuring the amounts of electricity (Ah)
charged and discharged.
A
r
s s
* N
.
l
1
3.2.2. Design of sodium/sulfur cells
Schematic of a sodium/sulfur cell is given in Fig. 10.
Insulolion ring
(olumina ceramics)
Anode cose í iron 1
Anode (sulfur.
graphite (elt )
i(cilumino ceramics)
Solid electrolyte tube
-Colhode
metollic
( sodium,
liber)
LCothode tube (copper)
Fig. 10. Principal design of sodiurn/sulfur unit cell [5]
The beta-alumina membrane is a cylindrical tube w i t h a bottom
at one end. Sodium is filled inside this tube, and sulfur and sodium
23

polysulfide in a metal cylindrical case outside tlic tiihc. At the up-
per opening of the beta-alumina tube, the anodc, the cathode and
the metallic case are connected arid welded t,ogct,lic,rwith insiilat-
ing ceramics in between. A graphite niat is inserkd in tlic anodic
space aimed at improving sulfur and polysulfide clcctric conductiw
ity, whilst in the cathodic space, a stainless steel fibcr is placed t o
conduct the current. This fiber retains sodium and is also iiiknded
t o prevent release of sodium in the everit of brcakagc of the beta-
alumina tube, thus having an iinportant safety role. Tlie nieta1 case
surface is platedwith chroniiunito prevent corrosioniiiidcr tlie action
of sulfur and sodium polysulfide.
Na/S cells are arranged in a tlierrnoinsulatcd c a e with a iiiaiii-
tained constant temperature of 350°C. Before operation, the batt,ery
is pre-heated gradually t o 350°C, arid only after that does tlie normal
operation of charging and discharging start. Further heating by the
heaters is barely needed since the h t t e r y gciierates r c x t i o n heat.
3.2.3. Specification and test results for battery modules and
pilot plant of the Japanese “Moonlight Project”
A 50 kW/400 kWh sodium/sulfiir battery module has been pro-
T h e output capacity of the plant is 8000 kW1i (1000 kWh x 8
duced by the YUASA company in Japan.
hours). Output voltage is 1000 V dc, output current 1011A dc.
T h e technical results from testing of various configurations of the
above battery type are summarized in Table 2.
T h e sodium/sulfur bat,tery plant is located at the Tatsumi sub-
station 77 close to the Tatsiimi lead-acid battery energy storage test
plant. By the end of 1990, the 1 M W / 8 MWh sodium/sulfur pi-
lot plant was half completed and operation of 500 1iW output was
started in November 1990.
24

Table 2. Technical results from testing of a sodium/sulfur battery produced
by YUASA Battery Co. Ltd.
10 kW class pilot modules [7]
Energy density - per footprint
- p e r volume
- per weight
Starting time
Stopping t i m e
Response to load change
Discharge ~ 6 h overall eff.
~ 4 h overall efT.
Voltage variation - on charging
Energy consumption in standing
~ hot standing
- cold standing
~ on discharging
50 kW class battery modules [SI
Voltage
Current
output
Capacity
Current density
Electrode area
Module composition
External dimensions
Energy efficiency ac-ac
Energy density ~ per weight
- per volume
- per footprint
52.4 kWh m-2
26.8 kWh m-3
42.6 Wh kg-'
1 s
1 s
10% 1.2 ins-'
76.S%
73.6%
11.8%
4.2%
19.4% day-'
0%: day-'
200 v
250 A
50 kW
400 kW11
50.5 m.4 an-'
495 cm2
(7s x lop) x 1Gs = 1120 cells
Width = 2.5 m Length = 2.3 m
Height = 2.8 m Weight = 12.8 t
76.6%
31.1 Wli kg-'
17.5 1tWh mP3
48.5 kWh in-'
25

Table 2. (Continued)
1 MW class pilot plant [8]
Beta-aluminatubes:
outer diameter
length
weight
specific resistivity
tube resistivity
fracture strength
Capacity of the cell
Battery output capacity
Output voltage
Current
ac-dc converter
Conversion efficiency
Output transformer
Number of cells
Buildingarea
Charge/discharge efficiencies
dc-dc efficiency (including
aux. power consumption)
ac-ac efficiency
beta-alumina doped with Li20
68 mm
450 mm
3.6 kg
< 4.5 R cm at 350°C
8 m
> 200 MPa
> 300 Ah
8 MWh (1MW - 8 h)
ac 6.6 kW dc 1.0 kV
dc 1kA
self-commutated 1200kVA
up to 96%
ac reactor 240 kVA
26,880 cells
800 m2(total for 2 floors)
approx. 87%
86%
approx. 76%
The batteries for this project were made by YUACA Battery Co.
Ltd. in collaboration with NGK Spark Plug Co. Ltd. The power
conditioning system was manufactured by Toshiba Corporation.
Figure 11shows a bird’s-eyeview of a 1MW sodium/sulfur pilot
plant consisting of twenty 50 kW battery modules.
Sodium/sulfur batteries have high charge and discharge efficien-
cies with no loss of energy during storage. The batteries are compact
with high storage energy density. Individual Na/C batteries sub-
jected to testing have undergone 1500 cycles already.
26

Environmental proieciion equiprneni
Machinery room
Fig. 11. Bird’s-eye view of 1MW sodium/sulfur pilot, plant [7].
Basic trends in the design of Na/S batteries are targeted at opti-
mization of specific energy and efficiency, and at preventing reactant
leakage from the cells. Safety is the key t o practical use of these
batteries and of course to extending their life, too.
Whether or not this type of battery will hold an important posi-
tion in the energy storage system, will depend on its service life, o n
the competitive power of its price to that of the lead-acid battery,
o n the simplicity and cost-effectiveness of its production technology
yielding reliability of the end product, and easy and inexpensive op-
eration and maintenance of the battery. These are all questions that
await answers in the near future.
27

3.3. Zinc/Bromine batteries
3.3.1. Reactions and principles of cell design and operation
During the Franco-Prussian war, French balloonists flying over
the Prussian lines illuminated their maps by means of strange prim-
itive static batteries containing zinc and bromine. This is the first
known historical evidence of the invention and practical use of a
zinc/halogen battery. It should be recalled that the lead-acid bat-
tery was also devised by a Frenchman. I t would be fair t o say that
duringthe last century, France was a pioneer inthe discovery and de-
velopment of new electrochemical power sources. Early zinc/bromine
batteries had probably proved inefficient in performance and main-
tenance, and were therefore forgotten until the 1970s. As a result
of the Arab oil embargo, interest in electrochemical power sources
increased greatly worldwide, and the question of devising new ad-
vanced battery systems gained momentum. Zito, Magnetti-Marelli
and E x ~ o r i
Research and Engineering (ER&E) were attracted by the
high electromotive force provided by Zn/Br batteries and started
design and development work o n this type of battery. Magnetti-
Marelli developed an electrolyte circulating battery design, whereby
the performance of both the zinc and the bromine electrodes was
enhanced. ER&E combined the ideas of circulating electrolytes, liq-
uid bromine complexing agents and the use of low-cost conductive
plastic electrodes. At first, 20 to 60 kWh batteries for e!ectric ve-
hicles were manufactured and, on testing, these showed encouraging
performance parameters. Later on, zinc/bromine batteries were also
developed for load-levelling applications.
Operation of Zn/Br systems is based o n the following reactions:
ne Zn2++ 2e-
L
-
-
T -external circuit
Brz+ 2e- + 2Br-
28

W h e n the reactions proceed in the direction from left to right,
battery discharge occurs, whilst in the reverse direction, charging of
the battery is accomplished.
T h e theoretical electromotive force of this battery is 1.83 V, but
since complexing agents are involved, the open circuit ce11voltage is
lower, namely 1.76 V. T h e theoretical energy density is 436 Wh kg-',
while the practical one is only 65 Wh kg-'. Other characteristics
are: peak power 95 W kg-', depth of discharge loo%, and energy
efficiency 60-65% [9]. The zinc/bromine battery operates at ambient
temperature. Althoughit is an electrochemical system usingaqueous
electrolyte, no decomposition of water is observed during charge.
This system faced a serious problem related to the rapid self-
discharge caused by the property of bromine to dissolve readily in
zinc bromide electrolytes andto diffuse to the zinc electrode osidizing
it. To avoid this process, it was necessary to remove bromine from
the zinc electrode, and to divide the anodic and cathodic sections of
the cell by a separator. Thus, a cell design vas developed in which
the bromine compartment of the cell was connected by nieans of a
tube to a storage compartment for collecting the evolved bromine.
T h e electrolyte, forced by a pump, circulated between the electrodes
and the storage compartment. This design principle lias led to a
significant decrease in battery self-discharge, but lias not eliniiiiated
it completely. A schematic of the zinc/bromine system is 1)reseIited
in Fig. 12. Atomic bromine formed on the electrode is indicated by
the dots in the figure.
This type of battery construction has solved anotlicr problem of
zinc/bromine batteries as well. that of the zinc electrode. In the
early static battery design, zinc deposited on tlie clectrode iii the
form of non-uniformdendritic plating. Dendrites sonidmes grew
across to the bromine electrode aiid caused short circuits in the cells.
By introducing electrolyte circulation also in tlie zinc half-cell, tlie
zinc deposit, formed over the electrode surface duriiig charge, becaine
29

more uniform and shortages were eliminated. Complete discharge is
required for every charge-discharge cycle to equalize the zinc distri-
bution over the negative electrode surface.
Cathode loop Anode loop
Zn deposit
Fig. 12. Schematic of a circulating electrolyte Zn/Br battery [9].
The double circulation loop battery design also proved benefi-
cial for the thermal management of the cell. In static zinc/bromine
batteries, the reaction heat was accumulated in the cell. With the
introduction of electrolyte circulation, the temperature of the elec-
trolyte becomes controllable and thermal homogeneity of the whole
electrochemical system can be achieved.
Circulating electrolyte batteries require various auxiliaries t o con-
trol battery operation. Obviously, their efficiency will influence the
power consumption for actual cell operation and hence affect overall
battery efficiency.
The power of an electrochemical power source is, as a rule, propor-
tional to the electrodes’ surface area, while its capacity is determined
by the amount of active materials that takes part in the electrochem-
ical processes. Through the adoption of the bromine circulation loop
and the storage compartment, the quantity of this active mass com-
30

ponent grows significantly. The zinc circulation loop also increases
the volume of the electrolyte and hence the quantity of zinc ions.
Nevertheless, zinc h
a
s remained the capacity limiting active mate-
rial. Batteries designed for high-power applications comprise a large
number of zinc electrodes with relatively thin zinc plating deposited
o n their surface. When high battery capacity is needed, the thickness
of the Zn electrodes is increased.
The cathodic half-cell is divided from the anodic one by means of
a microporous polyethylene separator with pore radius smaller than
1 pm. Such separators are commercially available and are widely
used inlead-acid battery manufacture. The separator displays barrier
properties w i t h respect to bromine diffusion, but it is permeable for
the solution ions that carry the electric charges between the two
electrode sections of the cell.
Consequently, the
problem of material resistance to oxidation is of primary importance
for this battery. Fortunately, most of the commercially available
and relatively low-cost plastic and carbon materials meet the above
requirements, which will make the large-scale production of these
batteries feasible and cost-effective from an engineering point of view.
However, improvement of the overall battery construction reliability
is necessary to eliminate all hazards of explosion or bromine leakage
in the atmosphere.
Bromine is a strong metal-corrosive agent.
3.3.2.Chemistry and electrochemistry of the zinc/bromine cell
During charge of the zinc/bromine cell, bromine evolved at the
electrode associates w i t h the bromide ions and dissolves easily in the
solution of zinc bromide:
nBr2+Br- -
3 BrGn+l) (where n = 1,2 or 3)
31

Trihromitle ions (Br:) are formed first, then pentabromide (Br;)
and eventually heptabromide (Br;). When the concentration of
bromine rises significantly, it is evolved as a separate liquid phase
collected in a special bromine storage compartment. In spite of this,
however, bromine concentration in the solution remains relatively
high and hence considerable self-discharge proceeds.
A second nieans of bromine storage has also been applied, i.e. as
a complexed form. In this case, complexing agents are added to the
electrolyte, that react with bromine forming a separate phase which
is collected through precipitation in the storage reservoir. Various
complexing agents have been used, e.g. quaternary ammonium ions,
n-ethyl, n-methyl-morpholinium bromide oil, etc. A possible elec-
trolyte composition of a discharged zinc/bromine cell is: zinc bromide
3 M, quaternary ammonium bromide 1 M, KC1 4 M as supporting
electrolyte [g].
During discharge of the zinc/bromine cell, the valve between the
Br2 complex storage department and the circulation loop is opened.
Brz complex is mixed with the solution in the circulation stream and
is pumped t o the bromine electrode where i t is reduced to bromine
ions. At the other electrode, zinc is oxidized to zinc ions. These
react w i t h the bromine ions forming zinc bromide. These processes
are accompanied by release of the electrical energy accumulated by
the cell during charging.
The charge/discharge voItage characteristics of the cell are shown
in Fig. 13. Duringcharge, a slight linear increase in cell voltage is ob-
served. During discharge, the voltage decreases very slowly at the be-
ginning, and as a result of complete exhaustion of the zinc resources,
the cell voltage drops rapidly at the end of discharge. Coulombic
efficiency of the zinc/bromine cell is about 80% (i.e. 20% is inef-
ficient). The inefficiency is mainly due t o electrode self-discharge.
Other sources of inefficiency are the energy losses for driving the
32

electrolyte pumps, as well as for supplying power t o the battery con-
trol and management system and to its microprocessor.
Discharge
L
O 60 120 I80 U0
Tlme. min
Fig. 13. Charge/discharge curves for a zinc/broniine cell [9].
3.3.3. Battery system design
ZincJbromine batteries have been designed in two niodifications,
w i t h monopolar and w i t h bipolar electrodes. These two battery de-
signs are shown in Fig. 14.
Bipolar orrongement Monopolor orrangements
Electrolyte Manifold Eledrdyte
t
-
-
t t t t t t
O E 2E3E4ESE
EMF olong mnifoId=(n-l)E.
where n = No, of cells ond
E = call voltogc
EMFolong monifoid O os all electrodes
of the some polarity ore connecled
Fig. 14. Bipolar and monopolar stack designs for Zn/Br systems [lo]
33

Inthe monopolar version, there are separate positive and negative
electrodes, and all monopolar electrodes are connected in parallel. I
i
i
this way, high battery power and low voltage are achieved.
T h e bipolar battery design uses electrodes which have one zinc
and one bromine side. These bipolar electrodes are connected in
series, only the end electrodes being monopolar. Cell-to-cell current
flows from the entire electrode surface and is carried through its
thickness. The output is high voltage and low current.
Inenergy storage batteries, a definite number of cells with bipolar
electrodes are arranged in series. These strings are connected in
parallel forming a module w i t h the desired voltage, capacity and
power.
3.3.4. Characteristics of zinc/bromine batteries
Table 3 presents the specifications of a 10 ltW/SO 1tWh battery
used for testing. This is a reduced-size model of the 50 kW/400
kWh battery module for the 1 MW pilot plant wliose parameters
and testing results are also given in the table. These batteries were
developed by Meidensha Electric Manufacturing Co. Ltd. in Japan
within the framework of the “Moonlight Project”.
T h e 1 MW battery energy storage plant is installed o n the
premises of the Imajuku Substation in the Western part of Fukuoka
City. It was constructed from November 1989 through September
1990. Operation tests started in December 1990 and will continue
till March 1992. This is the largest Zn/Br battery in the world.
Interesting and useful testing results are expected.
34

Table 3. Results from testing of Zn/Br battery modules produced by M e i -
densha Electric Manufacturing Co., Ltd, Japan. T h e tests were performed
at the Government Industrial Research Institute, Osaka [7].
10 kW battery [7]
Configuration (24 cells in series x
3 series in parallel)
x 4 in parallel = 360
1600 cm2 x 13 mA cm-'
1.67 V x 166 Ah18 h
Unit cells
Open circuit voltage 43.8 V
Maximumcharging voltage 50.0 V
Charging power 12.7 kW
Discharging power 10.0 kW
~ per volume
- p e r weight
Dimensions - width 1.37 m
- depth 1.59 m
- height 1.67 m
33.6 kWh m-'
14.9 kWh m-3
29.1 Wh kg-'
Starting time 1 s
Self-discharge rate 7.4%
Stopping t i m e 1 s
Change time charge-discharge 1 s
Change t i m e discharge-charge 1 s
Response to load change 10% 0.9 m s
Discharge - 6 h capable eff.
- 4 h capable eff.
Energy consumption in standing 0%
Test cycle life
68.3%
66.5%
19.9%
- on discharge 15.5%
appr. 550 cycles
End of battery life was due t o carbon plastic electrode degradation
35

50 kW class battery module [8]
Voltage
Current
Output power
Capacity
Module configuration
External dimensions ~ width
- length
~ height
Weight
Energy efficiency ac-ac
~ per volume
- per weight
100 v
500 h
50 kW
400 kWh
(30 cells in series x
x 2 in series = 1440 cells
3.9 m
1.6 m
3.1 m
16 tons
73.2%
63.1 kWh m-2
20.7 kWh mP3
25.0 Wh kg-'
Imajuku Energy Storage Test Zn/Br Plant [SI
Output capacity
Output voltage
current
ac-dc converter
Output transformer
Number of batteries
Number of ceils
Submodule battery
output power
weight
dimensions - width
- depth
- height
Buildingarea
4.4 MWh (1MW ~ 4 h)
ac 6.6 kV; dc 1.1kV
self-commutated 1000 kVA
self-cooling 1200 kVA
24 submodules (series)
30 cells (scrics)
x 24 stacks (parallel)
x 24 submodules (series)
= 17,280 cells
dc 1k A
23 kW
8 tons
1630 mm
1520 mm
3150 mm
735 m2
36

3.4. Ziric/Chlorine batteries
3.4.1. Fundamentals of zinc/chlorine batteries
The electrochemical reactions o n which operation of this type of
battery is based are as follows:
Z n e Zn2++ 2e-
L
-
T -external circuit
C12+ 2e-F=+ 2C1-
During battery discharge, the above reactions proceed from left
to right, and in the opposite direction during charge. The theoretical
voltage of the Zn/Cl- cell is 2.12 V. It is higher thanthat of the Zn/Br
cell. Since no complexing agents are used, the open circuit voltage
of the Zn/ClL cell has a value equal t o the theoretical one. The
theoretical energy density is 465 Whkg-' against 60 t o 80 Wh kg-' in
the practical circuit depending on cell design. T h e depth of discharge
is 96% [9].
Zn/C12 batt,eries are similar t o Zn/Br ones. However, bromine
and chlorine differ in chemical and physical properties. At ambient
temperature, chlorine is gaseous, while bromine is a reddish-brown
liquid. Consequently, different methods for storage of the two halo-
gens in the cell reservoirs should be used. This leads t o substantial
differences in design of the two types of zinc/halogen cells.
Chlorine is slightly soluble in zinc chloride solutions. For this
reason, during charge, chlorine is evolved in the form of bubbles that
leave the electrolyte forming a gaseous phase. This gas should be
collected and stored in an appropriate manner, t o be fed back into
the solution and reduced t o chlorine ions at the electrode, when elec-
tric current is delivered by the battery. So far, there have been two
methods for chlorine storage in use. In the first method, chlorine is
compressed untilliquefication and is stored as liquid chlorine at pres-
sures of 70-80 psig. When electric current is to be generated by the
37

1.75v cut aff
740 cm-2
coulûmblc
N
1 voltoge -
'u 2.18 v
E 1 -
10- Charge coubmk+
Discharge -
jg---~~LcE2--~
- FUII pwer
-
-
-
-
7
?
.-
540

20- Usable
2 0 ( ( 1 1 1 1 I l I l I I I
The cell is charged at a voltage of 2.25 V. Duringdischarge, the
cell voltage is kept at the 1.9 V level for a long period of time. T h e
discharge is carried down to a cut-off voltage of 1.75 V. Voltaic effi-
ciency is about 88%. Voltage losses are primarily due to poor elec-
trolyte conductivity between the electrodes. Coulombic efficiency is
about 87%, capacity losses being due primarily t o the self-discharge
caused by chlorine diffusion towards the zinc electrode. As a rule,
n o separator is used in this type of battery. The net electrochemi-
cal efficiency is decreased, because part of the energy is utilized for
supplying power t o the auxiliaries: gas and electrolyte pumps, the
inert rejection system, the hydrogen recombination system and the
cooling system
2.4
2.0
-1.6 u
-
-1.2 k
--cl8 %
0.4
38

3.4.2. B a t t e r y design
EDA’s (Energy Development Association, USA) Zn/Clp battery
design [5] is based on the use of graphite electrodes, a single chlorine
circulating loop and cooling of the electrolyte and the gaseous phase
to form C12(H20),. A schematic of EDA’s Zn/C11 battery is shown
in Fig. 16.
e
I
heat exchanger
Fig. 16. Schematic of tlie circulating zinc/chlorine battery [9].
The battery is composed of an electrochemical module (electrode
stack) and electrolyte of zinc chloride solution w i t h added potas-
sium chloride (to improve electrolyte conductivity). Using tubes and
pumps, an electrolyte circulation loop is formed. Zinc is deposited o n
the cathode. For a uniform plating to be formed, the current density
should be in the range from 20 t o 45 mA crnp2, and zinc loading be-
tween 90-300 mAh crnv2. Chlorine evolved on the anode is removed
from the stack and pumped into the hydrate storage reservoir. To
allow formation of chlorine hydrate, prior t o mixing with chlorine
the electrolyte is cooled by a refrigeration system. In the hydrate
reservoir, an ice-lile slurry of chlorine hydrate is stored.
39

Duringbattery discharge, the cooled hydrate slurry froiri tlie storc
is passed through a heat exchanger, where chlorine liydrntc is tlcconi-
posed. The chlorine-rich stream is then pumped to the anode wliere
chlorine is reduced to chloride. On the negative electrode, zinc is os-
idized to zinc ions that react w i t h chloride ions giving zinc cliloride.
In the course of these processesi the electrical energy stored during
charge is liberated, i.e. doing work.
T h e chlorine electrode is made of a porous graphite material. Its
surface is activated. The chlorine-containing solution passes through
the pores of the graphite electrode, whereby chlorine is transformed
to chloride ions (“flow-through electrode”). (In tlie zinc/bromine
cell, the bromine electrode is of the “flow-by” type.) The rate of
the chlorine-rich electrolyte stream deterniines the rate of battery
discharge.
Self-discharge of the zinc/chlorine battery is caused 1)y the reac-
tion between zinc and chlorine. Electrolyte circulatioii aims at faster
removal of chlorine from the electrode st,acliand hencc rcdiiciiig the
self-discharge.
Zinc/chlorine batteries tend to release hydrogen, because they
operate with acidic solutions. Zinc corrodes in acidic electrolytes
evolving hydrogen. This hydrogen should be bonded t o chlorine, via
ultraviolet irradiation, for example. In this way, potential hazards of
explosions in the chlorine storage reservoir are avoided.
Zinc/chlorine battery design is usually basedon bipolar electrode
stacks.
3.4.3. Battery characteristics
Design and development of a zinc/chlorine battery associated
with the Energy Storage Project in Japan was carried out by the
Furukawa Electric Co. Ltd. This company has developed 1, 10 and
40

50 kW battery configurations. The test results for 1kW and 10 kW
battery modules, before completion of the cycle life tests, are pre-
sented in Table 4.
After a critical analysis of the technological, performance and
economical parameters of the above battery modules, further de-
velopment of zinc/chlorine batteries was interrupted. This type of
battery poses serious environmental hazards since chlorine is a toxic
gas.
Table 4. Some results f r o m testing of 1 kW and 10 kW Zn/Clz battery
modules produced by FurukawaElectric Co. Ltd. T h e tests were performed
at t h e Government Industrial Research Institute, Osaka [7].
1 kW battery
Configuration
Unit cell - voltage
- capacity
Open circuit voltage
Charging power
Discharging power (8 h rate)
Coulombic efficiency
Voltaic efficiency
Energy efficiency
Self-discharge rate
~ initial energy efficiency
after t w o weeks
self-discharge rate
- after four weeks
self-discharge rate
30 cells in series x
2 series in parallel
2.0 v
75 Ah16 h rate
63.0 V
1.41 kW
1.01 kW
84.0%
83.0%
70.5-76%
71.1%
68.7%
3.4%
67.9%
4.5%
41

10 kW battery
Module
Unit ceiis
Open circuit voltage
Maximum charging voltage
Charging power
Discharging power (8 h)
Energy efficiency
- overall efficiency
- coulombic efficiency
- voltage efficiency
- aux. power efficiency
- per volume
- per weight
Self-discharge rate
Starting time
Stopping time
Change time charge-discharge
Change time discharge-charge
Response t o load change
Discharge - 6h capable eff.
Energy consumption on standing
- 4 h capable eff.
- on discharging
- hot standing
- cold standing
(24 cells in series x
x 2 in parallel = 96
2800 cm2 x 22 mA
1.95 V x 495 Ah18 h
50.9 V
60.0 V
14.9 kW
11.6 kW
65.7%
86.5%
90.2%
93.4%
33.6 kWh m-2
14.9 kWh m-3
29.1 Wh kg-'
4.5%
2 min
1 s
2 min
77 min
10% 0.9 ms-'
60.5%
60.6%
7.5%
24.5%
0.7%
0%
42

4. Lead-acid batteries
4.1. Some history
In 1860, Gaston Plante presented to the French Academy of Sci-
ences a 9-cell battery (composed of lead and lead dioxide electrodes
immersed in H2S04 solution andseparated by rubber tapes) and a re-
port entitled “Nouvelle pile secondaire d ’une grande puissance”.
This report was the birth certificate of the lead-acid storage battery.
‘(D’une grande puissance” -what wisdom andforesight shown by
Plante so many years ago! Today, over 400 million cars worldwide
have engines driven by high-power lead-acid batteries.
During the period 1880--1900, lead-acidbatteries found their first
practical application in the early power stations. They were used
as a stand-by source of energy and power. With the progress of
indust,ry and of dc-electroenergetics, production and usage of lead-
acid batkrics as energy storage facilities gained increasing popularity
to reach, in 1930, large-scale commercialization. In most towns in
Germany, such as Berlin, Munich, Hamburg, Leipzig, Stuttgard and
Bremen, large lead-acid battery storage facilities were in operation.
The largest battery storage unit was in Berlin. It had a capacity of
66,500 1iWha,ndwas capable of delivering 186 MW of electric power
within 30 minutes. The city of Chicago was supplied w i t h electricity
by dc generators and large leacl-acid batteries owned by the Common
Wealth Edison Company.
With the development of ac technologies for electric power gen-
eration and distribution in the 1930s, the dc battery system was
abandoned, to be revived again during the 1980s.
At present, a number of lead-acid battery energy storage facili-
ties in various countries worldwide are under construction or in the
demonstration and/or actual operation stage. We will discuss the
technical and economical aspects of lead-acid battery energy storage
43

technologies in the next sections o n the basis of knowledge, experi-
ence and information obtained so far.
4.2. Electrochemistry of the lead-acid battery
T h e basic reactions that proceed in the lead-acid battery and
determine its electromotive force (emf) are:
+
- Pb+H,SO, PbS04+2Ht+2e-
PbOz +2H+ +HzS04 +2e- +PbS04 +2 H z 0
T P L +- external circuit
These reactions together w i t h the corresponding charge/discharge
curves of the cell are presented in Fig. 17.
DISCHARGE
O io 20
Time. h
CHARGE
+
j
-
e- . + Z
Recti1ier
f - '
Positive plate Negalive plate
7
I=3.2 A
2.m
io 20
Time, h
Fig. 17. A scheme of the charge and discharge reactions proceeding in the
lead-acid cell and the corresponding voltage transients.
44

Calciilatcd tlicrrriod~namically,
the voltage between the lead sul-
fate and t l i e lead dioxide electrodes in the cell is 2.040 V, but the
open circuit voltage is iisually taken as 2.0 V (rated voltage). The
tlieoretical specific energy of the cell is 170.2 Wh kg-’. To trans-
form the lead-acid cell into a practical power source, several design
requirements must be rnet.
Lead and lead dioxide active materials are b o t h porous. Part
of the active mass acts as a conductive sl<elet,on, and another part
(30 to 55%) participates in the reactions leading t o generation and
acciiiiiiilatioii of cncrgy. T h e active materials are fixed in lead-based
grids that are chemically resistant t o H2SOS solution. T h e positive
and ncga2tive plates are separated by microporous separators that
arc iori-permeable and chemically resistant to H2S04, 0 2 and HL.
T h e cell iises approximately 36% H2S04 solution as electrolyte. The
positive aiitl iiegative plates are interconnected in semi- blocks with
terniinal posts protriidirig from the cell. T h e plates of the lead and
the lead dioxitle seini-bloclis together with the separators and the
electrolyte hetween thein form the active block. In it, all processes
occur, eiicrgy is accumulated and electric ciirreiit is generated during
discharge. Ahove the active block, there is a space containing a
certain amoiiiit of HLSO4 soliition (upper reservoir). Below tlie active
block, another free space is available where the shedded active niass
is collected to avoid short-circuits between the plates.
At tlie end of charging, decomposition of water takes place and
H
2 and 0 2 gases are evolved. The cell is provided with a valve
as an outlet for these gases. Since gas evolution is associated with
water consurnption, an equivalent amount of water must be added
periodically t o the cell iiiorder t o maintain the required electrolyte
conccritratjioii. Water is added through the outlet. T h e cells are
mounted in a plastic container fitted with a cover. T h e cells arejoined
in series with lead connectors that may b e situated over the cover, or
pass through the cell partitions (“through-the-wall’’ arrangement).
45

The construction of a coiiventional present-day SLI lmttcry is
shown in Fig. 18.
Terminai posts Iniercell Post stmp
comedor
Fig. 18. Exploded drawing of a pasted-plate lead-acid battery.
4.3. Electrical characteristics of lead-acid batteries
Discharge curves. When electric current flows through the cell,
the close circuit voltage depends on both the direction and magnitude
of the current, and o n cell temperature. Figure 19 presents a set
of discharge voltage curves for a 12 V/100 Ah battery at 25°C for
various discharge currents.
Discharge proceeds within a given period of time, after which the
voltage begins to decrease rapidly. Since deep discharges have an
adverse effect upon battery performance, a limit is set for the end-
of-discharge voltage (U, ~ final or cut-off voltage). When the time
of discharge is between 1 and 20 hours, U, = 1.75 V. For shorter
discharges, U, = 1 V. The mean discharge voltage (Ud) is shown
w i t h a dotted line. This value is used for the calculation of battery
energy and power.
46

4loA
528A
1 2 4 6 8 1
0 12 14 16 1
8 20 22 24 26 28 30
Time, min
Fig. 19. Discharge voltage curves for a 12 V/100 Ah (22 h rate) starter
battery [12].
Capacity. The capacity ( C d ) of a battery is determined by the
quantity of electricity that can be delivered during discharge at con-
stant current until the final discharge voltage is reached.
The time (t) needed for reaching the final discharge voltage is
marked o n the abscissa and is known as the rate of discharge. In-
ternational and domestic standards require the capacity to be deter-
mined by a discharge w i t h a current at which the battery reaches
U, = 1.75 V at 20°C after 20 or 5 h (
C
2
0 or CS). This capacity is
known as the rated capacity. Under normal operating conditions,
the battery should not be discharged beyond 80% of the rated ca-
pacity. This capacity is known as working capacity.
T h e relationship between capacity and discharge current is ex-
pressed by the empirical equation formulated by Peukert in 1898
and widely accepted:
where K and n are constants. According t o Peukert, n = 1.30,
while K depends on the temperature, the HzS04 concentration and
the design of the battery.
47

12V11OOAh SLI battery
I I I I I
h 20minlOrnin 5 min lmin 1
I I I I l I I
O ~ l O O 2 O O 300 400 6001.A
Rate oí discharge (1) or current (I1
Fig. 20. Capacity 'us.
12 V/100 Ah starter battery [13].
discharge current (I)
or discharge rate (t) of a
The relationship between capacity andcurrent is shown inFig. 20.
Energy. The energy (Ed) delivered by the battery during dis-
charge under constant current conditions is equal t o the product of
the mean voltage of the battery multiplied by its capacity. Figure
21 presents the dependence of energy and mean voltage on discharge
current. When the discharge current increases, the energy delivered
by the battery is decreased. Therefore, in battery energy storage
plants, discharge of batteries should be carried out with moderate
currents, i.e. 3-10 h discharge rate. The delivered energy under
routine operation is usually 80% of the rated value.
Power. The power of a battery is the energy delivered per unit
time. The value per unit weight or volume is known as specific power
of the battery. Figure 22 presents the power us. current dependence.
When the current increases, power is also augmented. Therefore, in
order to deliver high power, batteries are designed t o be discharged
at heavy currents. As the capacity decreasesw i t h increase of current,
the discharge time will decrease rapidly.
48

@
12VI Kx) Ah SLI battery
20h 20minX)min Smin Imin t
I I I I I I
Current, A
Fig. 21. (a) Average discharge voltage vs. discharge current; (b) energy vs.
discharge current for a 12 V/100 Ah battery [13].
100 200 300 400 Kx) c
Current, A
O
Fig. 22. Power vs. discharge current of a 12 V/100 Ah starter battery [13].
49

Cycle life. The service life of a battery is the number of
charge/discharge cycles obtained duringlaboratory bench tests. The
battery must attain a given number of cycles before its capacity is
reduced t o 80% of the rated value. The real life of a battery may be
longer or shorter than that experienced under laboratory conditions.
During practical use, the battery is subjected t o other life-limiting
factors that are not taken into consideration in the laboratory tests.
Current test procedures are aimed at maximum simulation of real
operating conditions.
4.4. Charging characteristics
That part of the current utilized for the formation of lead and
lead dioxide during battery charge is called charge acceptance. The
remaining current is consumed for water decomposition. Figure 23
shows the charge acceptance of a battery vs. its state-of-charge.
g 20
o 20 40 60 80 loo120
5 0
Charge ;
,
,
Rated capacity
Fig. 23. Charge acceptance of a traction battery ws. i t s state-of-charge
at 30°C [14].
The data show that almost the entire amount of charging elec-
tricity is used for the transformation of PbS04 t o Pb and PbOz until
a 60~75%
state-of-charge is reached. At this stage of battery charg-
50

ing with a current of 0.1 A per Ah, the cell voltage reaches 2.35 V
and gas evolution starts. After that, water decomposition proceeds
simultaneously w i t h the charging reactions. T h e charge acceptance
is graduallyand continuously reduced. Cell voltage is increased from
2.35 t o 2.50 V. T h e cell is completely charged. During the next
charging stage, water decomposition and self-discharge are the main
processes that take place. The battery is overcharged.
These stages can be clearly identified by galvanostatically charg-
ing a battery which has previously been subjected t o three different
depths of discharge (50, 75 and 100% DOD). After these discharges,
the efficient charge stage acquires three different durations. Figure 24
shows the changes in cell voltage during charge. The chargingcurrent
was 0.1 A per Ah.
2.8 -50 per cent 75 per cent
dischorqed
Time
20
Fig. 24. Changes in celi voltage during charge, following three discharge
runs to different depths [15].
T h e gas evolution voltage is 2.35-2.40 V per cell at 75% state-of-
charge.
The following parameters are used t o define charging regimes:
o Initial and final charging voltage (2.1 t o 2.4-2.7 V per cell).
o Initial gas evolution voltage (2.35 t o 2.40 V per cell).
o Charging current during the efficient charge stage (0.3 t o
0.1 A Ah-' or I c h = 30-10% C, A).
51

o Current at the beginning of gas evolution (I:h
= 0.07 A Ah-'
o Finalcharging current (ICh = 0.01-0.03 A Ah-' or I!h = 1-3%
o Upper charge temperature limit (45-50'C).
or I
,
h
= 7% C
5 A).
c
5 A).
T h e duration of charge must be short, the energy and power
efficiencies must attain maximumvalues, the irreversible processes in
the active masses and the grids must not be enhanced, thus ensuring
long service life of the battery.
There are several charging regimes in use for energy storage plant
batteries which meet the above requirements: a) controlled current-
voltage charging method, b) tapered charging method, and c) pulsed
charging method [13].
The specific charging method for each battery is usually pre-
scribed by the battery manufacturer.
4.5. Effect of electrolyte stratification
During discharge, the concentration of the acid in the cell de-
creases, and during charge it increases. Concentration gradients are
formed in the cell between the solution above the active block and
that between the plates. The formation of concentration gradients in
the active block of a 400 Ah battery was studied during cycling w i t h
169 mA Ah-' at 100% DOD until a cut-off voltage of 1.7 V/cell was
reached [16]. Figure 25 shows the concentration changes during four
consecutive cycles at a charge/discharge ratio of 1.02.
Stratification of the acid is enhanced as the number of cycles is
increased. This indicates that the concentration changes accumulate
duringcycling. The difference between the acid concentration at the
top and the bottoni of the active block was used as a measure for the
extent of stratification. In the above studies this reached 0.15 sg.
52

Charge time. h
Fig. 25. Electrolyte stratification measured during battery charging t o 2%
overcharge and after 100% DOD [16].
As HzS04 is an active material, stratification will affect cell ca-
pacity. It has been established that the capacity decreases by l%
for
each 0.01 sg unit of stratification. This capacity loss depends on the
DOD and the charge/discharge ratio. With increase in overcharge,
the extent of stratification (and hence the capacity loss) is dimin-
ished. To eliminate fully the capacity losses due to strat'ification,
the battery should be subjected to 15% overcharge. Since the ac-
tive block is a compact assembly, during overcharge the evolved gas
will exert a pumping action which will transfer the dense acid at the
lower half of the active block to the top, thus enhancing the equal-
ization of the acid concentration. Intensified gas evolution, however,
will lead t o greater water consumption and hence heavier battery
maintenance, on the one hand, and will increase the corrosion of the
positive grids, o n the other hand. That is why the use of devices
for forced electrolyte circulation is recommended. These are usu-
ally small air-lift pumps that let an air flow into the cell to stir the
electrolyte throughout the charge cycle.
53

4.6. Charge-discharge energy efficiency
Figure 26 presents a typical charge-discharge curve for a lead-acid
cell. The quantity of electricity consumed for charging is about 15%
greater than that of the discharge.
I l
Charge or discharge S
Fig. 26.
discharge [17].
Dependence of the cell potential o n the state-of-charge or
Since the charge and discharge were conducted at the same cur-
rent, the difference AE, between the areas situated below the charge
and discharge curves gives the energy losses. To improve the energy
efficiency, this area AE, should be made as small as possible. This
can be achieved by reducing the polarization of the positive and neg-
ative battery plates and decreasing the ohmic drop in the electrolyte
(separator) duringcharge and discharge as well as by decreasing the
duration of overcharge.
T h e highest energy losses are related to battery overcharge. If
the latter is eliminated, however, the process of conversion of lead
sulfate to lead and lead dioxide will not proceed fully and the battery
will be gradually sulfated. Besides, a stratification of the electrolyte
occurs also leading t o battery capacity drop.
54

Investigations have been conducted w i t h cells whose electrolyte
hac been agitated using a special device for admitting air t o stir the
electrolyte. This device is shown in Fig. 27.
Air
II
Fig. 27. Electrolyte agitator [17]
In this device, air is introduced by a blower into the inner tube
of a double-wall cylinder from the top. This air lifts the electrolyte,
while ascending as bubbles, through the gap between the inner and
outer tubes. The electrolyte is sucked from a hole at the lower part
of the outer tube and ejected from a hole at the upper part of the
same tube.
The obtained results have shown that: first, battery charge with-
out overcharge is possible whereby no plate sulfatization proceeds
when the electrolyte is stirred thoroughly t o prevent stratification;
second, an equalizing charge w i t h 25% overcharge should be per-
formed after every 30 cycles t o prevent coarsening of the lead sulfate
crystals accumulated in the plates.
55

Figure 18 presents the capacity/cycle number dependences for a
200 Ah battery subjected to 100% charge without electrolyte stir-
ring, 120% charge without external agitation but with intense gas
evolution stirring the electrolyte, and finally w i t h outer electrolyte
agitation and equalizing overcharges conducted after every 30 cycles.
U< LO- agitation)
Cycies
Fig. 28. Influence of overcharge and electrolyte stirring o n cycle life [17].
As can be seen from the figure, when lead-acid batteries are
subjected to charge-discharge cycling w i t h 100% state-of-charge and
electrolyte stirring to prevent stratification, and equalizing charges
are carried out periodically, corrosion of the grids and softening of
the active materials can be strongly suppressed and life performance
markedly improved. Under the above conditions, not only is the life
of the battery extended, but its charge-discharge energy efficiency is
improved as well.
56

4.7. Methods for reducing water losses
Maintenanceof lead-acid batteries consists primarily inperiodical
refilling the cells w i t h water. Water is lost from lead-acid batteries
through the following processes:
e Electrolysis of water during overcharge of the battery
o Self-discharge under open-circuit conditions
o Evaporation of water
Manual filling up of the cells t o a constant electrolyte level is very
laborious and time-consuming, especially in a battery energy storage
plant. Therefore, major challenges in battery servicing are to find
a way to reduce water losses and to replace manual refill methods.
The following methods have been proposed:
a) Single-point (common point) watering. This system is used
for fill up of conventional batteries w i t h excess electrolyte above the
active block, i.e. flooded batteries. In these batteries, a water
addition system is fitted that can automatically adjust the electrolyte
level in each cell of the battery. This watering system comprises:
devices to monitor the level of the electrolyte in the cells and to
stop the flow of water when the electrolyte reaches a previously
adjusted level;
a device for escape of evolved oxygen and hydrogen gases from
the cell to eliminate explosion hazards.
Finally, the design should avoid electrical shorting between the cells
through the common filling system.
These systems have found wide application instand-by energy fa-
cilities for power plants, post offices, cultural and commercial centers
using stand-by lead-acid batteries. A common point refill system is
utilized in many battery energy storage plants. Operating experience
of batteries w i t h the above refill system in various modifications has
shown that it is not always sufficiently reliable and safe.
57

b) Catalytic plug recombination of hydrogen and oxygen. De-
signed t o recombine hydrogen and oxygen t o water that is brought
back into the cell. Instead of a cell valve, a catalytic plug is included
that enables the following reactions t o proceed:
2H2 +O2 + 2H2OVapOUr +114 kcal
H~Ovapour+ HZOiiquià +9.7 kcal
T h e first reaction requires a stoichiometric ratio between the
evolved H
2 and 0 2 , and is accompanied by the release of a great
amount of heat. The latter causes the cell temperature t o rise as a
result of which the reaction rate is increased. If left uncontrolled,
this could lead t o an explosion. Various designs of catalytic plugs
have been used t o avoid this hazard.
Metals from the platinumgroup are used as catalysts for the re-
combination. Carbon, alumina or asbestos wool are usually used
as catalyst carriers. The major disadvantage of this water recycling
method is the high price of the catalyst materials, which h
a
s re-
stricted large-scale application of the method.
During charge-dis-
charge operation, hydrogen and oxygen are often evolved in non-
stoichiometric amounts. That is why the efficiency of the catalytic
plug is reduced. A method has been proposed suggesting that two
auxiliary catalytic electrodes are fitted in the cells. They are pre-
sented in Figure 29.
On one auxiliary electrode, the reaction of oxygen reduction t o
OH- ions proceeds. For this purpose, the oxygen electrode is con-
nected through an appropriate diode t o the lead electrode of the cell
(Fig. 29a). On the other catalytic electrode, hydrogen is oxidized to
hydrogen ions (Fig. 29b). To ensure the right potential for the elec-
trochemical reaction of hydrogen oxidation, the hydrogen electrode is
connected through a proper electronic device (diode and resistor) to
58
c) Closed oxygen and hydrogen cycles.

I I L J
p
m
2 Pb Pb02 Pb
Fig. 29. Schematic of cells with auxiliary electrodes [is].
the lead dioxide electrode of the cell. The reactions proceeding at the
two auxiliary electrodes are catalyzed by metals from the platinum
group. This makes the method very expensive and hence its applica-
tion is strongly restricted. It has recently been found that tungsten
carbide displays similar catalytic properties to those of platinumw i t h
respect to hydrogen andoxygen reactions [19]. Consequently, interest
in this method has increased of late.
d) Valve-regulated recombinant lead-acid batteries. During
charge, the following reactions take place in the cell:
positive
plates
PbS04+2H20 +PbO2 +2H+ +H2S04 +2e-
HzO -;O2 +2H+ +2e-
negative
plates
PbS04 +2H+ +2e- +Pb +H2S04
2H+ +2e- -H
2

Figure 30 shows the relationship between the charge acceptance
of positive and negative plates and the time of charge.
20
2.6
2.4 2
c
I
22
2.o
O 1 2 3
Chorge tlrne, h
Fig. 30.
v
u
s
. time [2O].
Charge acceptance of positive and negative plates at 40°C
When the positive plate reaches a 60--70% state-of-charge, a re-
action of oxygen evolution starts and its rate is increased with time.
This leads to an equivalent decrease in positive plate charge accep-
tance. The negative plate is charged w i t h 100% charge acceptance
until about 95% state-of-charge. After that, hydrogen evolution
starts. This difference inthe behaviour of the H
2 and 0 2 reactions has
been exploited in the oxygen cycle. Oxygen evolved first, is brought
to the negative plate, where it oxidizes the lead in the grid. Thus,
on the one hand, the negative plate is kept discharged and hence
evolution of hydrogen is prevented, and on the other hand, oxygen
itself reacts in the cell. The basic problem w i t h this type of oxygen
cycle battery was t o prevent oxygen from being lost from the active
block. A solution was found in immobilizing the H2S04 electrolyte
between the plates. This was achieved by:
60

o Using gelled electrolyte. The cracks formed in the gel act as
channels for the transport of oxygen. This technology was devised
by the German company Sonnenschein for small lead-acid batteries
(several Ah), and then further developed for large stationary batter-
ies [21]. The U
S company .Johnson Controls Inc. lias enhanced this
technology [22,23] and is now manufacturing gelled-electrolyte bat-
teries for both traction and stationary applications, and for energy
storage systems as well.
o Using absorptive glass mat separators. The electrolyte in
this case is absorbed by the glass fibre mat that has the property
of absorbing H2S04and water, and adsorbing oxygen. Oxygen is
retained inthe glass mat separator and flows from the positive to the
negative plate, while H2S04and water participate in the reactions
in the cell. Gates in the USA [24,25] and I’UASA in Japan [25] were
the first t o employ this technology in small batteries, and later in
stationary batteries. Now these batteries are under large-scale tests
for energy storage applications.
The problem of oxygen retention between the plates lias found an
adequate technical solution, but even so, over time, ccrtain amounts
of hydrogen are evolved at a low rate, which accumulate above the
electrode stacks in the cells. To avoid explosion, a valve is fitted that
lets the gas out and controls the pressure in the cell. These batteries
are maintenance-free and are called valve-regulated batteries.
61

5. Lead-acid batteries in the battery energy
storage system (BESS)
5.1.
systems
Functions of lead-acid battery energy storage
After the revival of interest in energy storage systems, the first
batteries to be used for that purpose were flooded lead-acid batteries
of the traction/industrial type. Early battery storage plants were
installed for demonstration purposes to validate the feasibility of de-
sign, investment , operational and maintenance costs of the system;
to demonstrate operational and economic benefits; t o establish BESS
applications with higher efficiency; and to test feasibility of BESsys-
tems w i t h power utilities. Technical, technological and economical
information has been gathered over 2-3 years.
Analysis of the testing results of these demonstration BES plants
shows that battery energy storage systems can improve the opera-
tional efficiency and the cost-effectiveness of the electric power system
by providing the following functions.
a) Load-levelling: off-peak battery charging and on-peak dis-
charging, which leads first t o improvement of the load factor of base-
load generating units, and second to reduction of energy costs by
storing cheap energy at night and selling it at higher cost duringthe
daytime peak demand hours.
b) Peak-shaving: Often major utility customers need instanta-
neous delivery of peak electric power for meeting technological needs.
The lead-acid battery, owing to its low internal resistance and very
short response time, is capable of dispatching considerable power
within several milliseconds. Through charging of the battery from
the energy utility, electric power is concentrated inthe battery. This
power can be delivered through high-current discharge of the bat-
tery (Fig. 23) when a high peak demand appears. This method of
energy storage allows electric utilities to save capital investments for
62

expansion of the generating facilities and, on the other hand, offers
utility customers the cost benefit of reducing their expenses for peak
demand charges.
c) Load-following: When the energy demand exceeds the lo-
cal electric system power level, the voltage of the system begins to
decline. The battery storage unit takes over part of the load by dis-
charging electricity, thus enhancing the stability of the power supply.
d) Frequency control and spinning reserve: To ensure normal
operation of the customers' electrical devices and machines, stable
ac frequency should be provided. Overloading of the electric power
system may cause the frequency of the supplied current t o fluctu-
ate. Storage batteries may play the role of spinning reserve and,
through discharging, compensate for such frequency distortions and
thus maintain the frequency of the local power system within the
desired limits.
Utility
lines charging
(off peak)
'
Battery
dixhorging
(on p k )
1
r--,-----
Fig. 31. Schematic of a customer-owned LABESS [il.
63

Some of the operational LABES systems are listed in Table 5.
Table 5. L A B E S systems in operation by 1990 throughout the world [27].
Company Size In service Application
Utility operated
Berliner Kraft und Licht
(BEWAG), Berlin, FRG 14 MWh
Kansai Electric Power Co. Ltd
Tatsumi, Japan 4 MWh
17 MW
1MW
Southern California Edison Co.
Chino, CA, U S A 40 MWh
10 MW
Customer operated
Elektrizitätswerk 400 kW
Hammermiihle 400 kWh
Selters, FRG
Hagen Battcric AG 500 kW
Socst, FRG 7 MWh
Crescent Electric 500 kW
Membership Corporation 500 kWh
Statesville, NC, U S A
Delco Remy. Division 300 kW
of General Motors 600 kWh
Muncie, IN, USA
Vaal Reefs Exploration 4 MW
South Africa
Johnson Controls, Inc. 300 kW
H u m b o l d t Foundry 600 kWh
Milwaukee, WI, USA
and M i n i n g Co. 7 MWh
1986
1986
1988
1980
1986
1987
1987
1989
1989
frequency regul.
spinning reserve
demonstration
multi-purpose
test program
demonstration
multi-purpose
test program
load-levelling
peak-shaving
load-levelling
peak-shaving
load-levelling
peak-shaving
peak-shaving
peak-shaving
emergency power
peak-shaving
load-levelling
64

The table shows that when battery storage plants (LABESP)
are installed at the side of the electric power generating utilities,
they have a power of over 1 MW. The power of lead-acid battcry
storage facilities (LABESF) at the customer side is of the order of
300 t o 500 kW. The only exception is the L A B E S facility in South
Africa which not only serves for peak-shaving purposes, but also for
emergency power supply.
Customer operated lead-acidbattery storage facilities are utilized
primarily for peak-shaving and load-levelling. All thcse applications
bring immediate financial profits to the customers, and make theni
less dependent on the power supply utilities in peak deniand periods.
Thus sufficient stability of the technological processes is guaranteed.
The use of lead-acid battery storage plants by electric power gen-
erating utilities is aimed not only at levelling the electric loads, but
also at improving the quality of the energy delivered by the utilities
t o the customers. LABES systems can function a
.
? spinning reserves;
provide instantaneous fast power reserve; regulate frequency, voltage
damp-out, subsynchronous oscillations andother system instabilities.
Which option to choose for BEC plant location - before or after
the meter?
If the power demand is not very high and a number of customers
could be grouped on a territorial principle, siting LABES plants on
the utility side of the meter is advisable. The storage unit should be
installed close t o the substation at i t s low-voltage stage. LABESP
capacity should be properly rated to meet the local peak power de-
mand.
For major energy consumers having high peak power demands,
e.g. commuter railroads and stations, metropolitan train or subway
systems, foundries, large administrative or commercial centers, etc.,
it would be more cost-effective and associated w i t h smaller power
lossest o install the LABECfacility o n the customer side of the meter.
65

Battery charging could be accomplished at any off-peak intervals
during the day and the night.
Spindler [ l ]has performed a cost analysis of the utilization of
LABEC plants by major electric power consumers in the USA. The
investment pay back periods have also been estimated. The obtained
results are summarized in Table 6.
Table 6. Economic analysis of selected cases of LABESS application on the
customer side [il.'
Customer / Demand Converter Battery Capital Payback
Application charge size size cost period
$ kW-' M W / a c M W h / a c $M years
Commuter railroad 16.24 5.8 5.6 2.48 2.2
Steel manufacture 11.88 3.5 2.3 1.46 3.5
Copper alloys plant 13.50 0.9 0.7 0.53 3.5
Truck part plant 13.82 0.5 0.7 0.56 4.3
Chemical manufacture 9.87 0.4 0.5 0.32 6.2
~ ~~~ ~ ~ ~~
Based on a battery cost of 260 $ kWh-', converters at 130 $ kW-', and
balance of plant, 140 $ kWh-I, operating 250 days yr-l.
It can be seen that with commuter and metropolitan railroads, as
well as steel and alloys production using electric furnaces, customer
LABEC plant construction is economically effective.
In both cases, applied before or after the meter, battery energy
storage brings significant profits to the electric utility company.
66

6 . .Lead-acid battery energy storage systems
for load-levelling
6.1. System structure
This consists of the following basic components:
0 lead-acid battery
o ac/dc power conversion system
o facility monitoring and control system.
(converter, transformer,
dc and ac switchgear)
The electric energy is supplied by a utility distribution network
through an ac switchgear t o the high-to-low-voltage transformer.
Then an ac-dc converter follows. Through a dc switchgear, the cur-
rent is fed into the battery t o charge it. Operation of all these LABES
plant components is managed by a monitoring and control system.
During battery discharge, direct current is generated which passes
through a dc-ac converter and is then delivered t o the customers to
meet their demand, or after increasing its voltage in a transformer,
is fed back into the utility distribution line.
Figure 32 gives a schematic of a lead-acid battery energy storage
system.
Facility monitoring
dc 1 oc
-I- Charge
-1 -Discharge
Fig. 32. Schematic of electric utility battery energy storage system [il.
67

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