UNESCOREGIONALOFFICE FOR SCIENCEAND TECHNOLOGY FOR 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 t o make the power industry more effective, a new
t r e n d in electric power production has witnessed intense development during recent years, that of energy storage. Several options
have been considered for this purpose, one of t h e m being the battery 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 t o testing. I t has become
general practice for experts in the power industry, and battery researchers and manufacturers t o meet at j o i n t conferences t o exchange
information and opinions o n the problems of energy storage. It is now
opportune t o siirrimarize the results and experiences so far acquired
in t l i e design arid utilization of battery energy storage systems.
In 1954, Elsevier in Amsterdam issued the book entitled “Power
Electric Vehicles” edited by B.D. M c N i c o l and
D.A..J. Rand, which presented a comprehensive survey o f the cur-
Sources for
rent knowledge in the field.
M o t o r car transport i s being increasingly adopted, since it is an
important and indispensable element of the n o r m a l functioning of
every modern social community. I t has, however, a serious environmental impact in t h a t it causes considerable air pollution in large
cities and densely populated areas.
Development and large-scale commercialization o f electric vehicles has become oiie of the greatest challenges o f the late 2 0 t h cen-
tury. However, the electrocliemica1 power sources used for propulsion
of these vehicles cannot yet meet the challenge. Annual international
i
conferences o n t h e problems of electrochemical power sources show
that more effort is being placed o n broad-spectrum investigations in
t h e field. Accumulated theoretical knowledge and practical experience o n battery energy storage systems for electric vehicle applications should now be analyzed and evaluated.
T h e Regional Office for Science and Technology for Europe
(ROSSE) at the U n i t e d 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 t o
t h e problems of these systems.
T h e current status and the problems confronting battery energy storage systems for the power industry are presented by
Prof. D r S c i . 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 i f our efforts contribute, though modestly, t o the development of battery energy storage systems, we w i l l
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
F O R THE POWER I N D U S T R Y
1. Introduction .......................................
1.1. The four basic elements of every national electric
power system
1
.........................................
1
1.2. Power industry and i t s problems ........................
1.2.1. Energy, power and response t i m e . . . . . . . . . . . . . . . . . .
1.2.2. Quality of energy supply systems . . . . . . . . . . . . . . . . .
1.2.3. Ecological problems and the development
of power industry ................................
2
.
Electric energy storage
...........................
3
3
5
6
7
2.1. Pumped Hydroelectric Energy Storage Systems (PHESS) . .
2.2. Compressed-Air Energy Storage Systems (CAESS) .......
2.3. Superconducting Magnetic Energy Storage Systems
(SMESS) .............................................
2.4. Battery Energy Storage Systems (BECS) . . . . . . . . . . . . . . . .
8
9
11
13
2.4.1. T h e revival of battery energy storage systems . . . . . . 13
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
...
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
3.1. Development projects for battery energy storage systems . . 19
...............................
20
3.2.1. Principles of cell operation .......................
20
.....................
23
3.2. Sodium/Sulfur Batteries
3.2.2. Design of sodium/sulfur cells
3.2.3. Specification and test results for battery modules
and pilot plant of the Japanese “Moonlight Project”
3.3. Zinc/Bromine Batteries
................................
24
28
3.3.1. Reactions and principles of cell design arid operation 28
3.3.2. Chemistry and electrochemistry of the zinc/bromine
cell ............................................
3.3.3. Battery system design
...........................
3.3.4. Characteristics of zinc/bromirie batteries
3.4. Zinc/Chlorine batteries
3.4.2. Battery design
37
. . . . . . . . . . . . 37
..................................
39
...........................
40
3.4.3. Battery characteristics
iv
33
. . . . . . . . . . . 33
................................
3.4.1. Fundamentals of zinc/chlorine batteries
31
5.2. Lead-acid b a t t e r y energy storage systems (LABESS)
in operation by 1990 throughout the world . . . . . . . . . . . . . . . 63
.
Lead-acid battery energy storage systems for
load levelling ......................................
6.1. System structure ......................................
6
67
67
6.2. Chino 10 hgW/40 MWh lead-acid battery energy storage
...............................................
....................................
T h e b a t t e r y .....................................
Power conditioning system .......................
Facility monitoring and control system ............
Equipment energy losses .........................
Economics of Chino LABES Plant . . . . . . . . . . . . . . . .
systein
6.2.1.
6.2.2.
6.2.3.
6.2.4.
6.2.5.
6.2.6.
Plant layout
68
68
69
76
78
78
79
7 . L A B E S S for instantaneous (spinning) reserve
and frequency control applications
..............
7.1. Island networks .......................................
7.2. T h e BEWAG 8.5/17MW Lead-Acid B a t t e r y Energy
Storage Plant .........................................
80
80
81
7.2.1. System frequency response having given rise t o
the construction of the BEWAG LABES plant
7.2.2. System design and characteristics
8
. . . . . 81
. . . . . . . . . . . . . . . . . 82
.
Lead-acid battery energy storage systems for
peak shaving .......................................
8.1. What i s peak shaving? .................................
86
86
8.2. Johnson Controls 300 1<W/600 1ïWh LABES Facility . . . . . . 88
8.3. Lead-acid battery energy storage systems in t h e railway
transport network .....................................
91
V
9. Valve-regulated lead-acid b a t t e r i e s f o r b a t t e r y
e n e r g y storage systems
. . . . . . . .. . . . . . . . . . . . . . . . .. . 94
10. S t r a t e g i c advantages of BES systems
References
.. . . . . . . . . . . 96
.... . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Chapter 2
ENERGY S T O R A G E S Y S T E M S F O R ELECTRIC
VEHICLES
1. M o t o r vehicles and e n v i r o n m e n t a l p o l l u t i o n
. . . 103
2.
Specification of e n e r g y storage systems f o r
e l e c t r i c vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
3.
C h a r g e a n d capacity o f b a t t e r i e s f o r e l e c t r i c
vehicles .. ..... .... .. .. ... . ... .. . ... .. ... . . . . . . .. .. . 122
4.
T y p e s o f cycles o f e l e c t r i c vehicle b a t t e r i e s
5.
R e q u i r e m e n t s t o t h e c o n s t r u c t i o n and m a n u f a c t u r i n g t e c h n o l o g y o f b a t t e r i e s f o r EV e n e r g y
storage systems ...................................
139
6.
Specification o f o p e r a t i n g e n e r g y storage
systems f o r e l e c t r i c vehicles . .. , . . . . . . .... . .. .. .. 1.12
7 . L y r i c a l epilogue
References
vi
. . . . 128
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
. . , . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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 o f every national electric
power system
Production of electric energy is the basic pillar for n o r m a l functioning of every modern social community and a guarantee for i t s
progress. It is organized in an electric power system comprising three
basic elements:
a) Electric power and energy generating utilities, i.e. electric
power plants: thermal power plants fired by coal or nuclear fuel, gasfired steam plants, oil- or gas-fired combustion turbines, hydroelectric
plants, etc.
b) Electric power distributing systems including transformer facilities, transmission trunk lines and distribution lines t o every customer.
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.
T h e electric power produced by the generating utilities is delivered through t h e transmission/distribution system t o the consumers
for utilization. The consumers’ demand for electric power varies
cyclically during day and night, as well as w i t h i n t h e week and the
seasons.
Demand. T h i s is the rate at which electric energy is delivered t o
t h e 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 o f 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
DCoal
5 ' 7 ' 9 ' 1 1 ' i 3 ' 1 ' ' 1 7 ' 1 9 21
Hour of day
mGas steam
@Gas turbine
23
Bottery
Fig. 1. Example of customer energy demand curve for a working day
[il.
D u r i n g the night (hours O t o 6), the demand decreases t o about
15-30% below t h e baseload level. The daytime demand is significantly higher than the baseload level. I t is served by gas-fired steam
plants. They burn natural gas or o i l which are more expensive fuels 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. T h e r a t i o 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. T o b e efficient,
power plants should operate at constant load. T h e customers’ demand, o n the other hand, undergoes cyclic fluctuations. T h i s leads
t o inefficient utilization of t h e generating capacities. A possible sol u t i o n 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 t o meet peak-hour demands during the day. Thus, low-cost
fuel power plants work at maximum load during the night and store
the generated energy t o sell it at increased cost during peak demand
periods. The introduction of this f o u r t h element in the electric power
system makes i t s operation more efficient. T h i s n o t o n l y brings about
considerable savings of expensive fuels such as gas and oil, but also
improves t h e 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
I t has been established t h a t the different forms o f m o t i o n (mechanical , thermal, electromagnetic , gravitational, chemical, etc.) are
converted i n t o one another following definite quantitative ratios. T o
allow measuring of the various forms of m o t i o n by a unified measuring
unit, the term energy has been introduced. T h e electrical energy
is determined f r o m the product of the voltage and the quantity of
charge that passes through an electrical device (load).
T h e work done per unit t i m e is called power. T h e electrical
power is determined f r o m the product of voltage and current.
3
In thermal power stations, the chemical energy accumulated in
coal, crude o i l or natural gas is transformed by burning (oxidat i o n o f t h e hydrocarbons) i n t o heat (high-temperature, high-pressure
steam), which sets in m o t i o n a turbine, whereby the thermal energy
is transformed i n t o mechanical. T h e turbine shaft is connected t o the
shaft of an electric generator. On rotation, this common shaft drives
the r o t o r of the generator as a result of which the mechanical energy
is converted i n t o electrical energy. It is evident that, t o obtain elect r i c a l energy f r o m 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 o f oxidation and reduction proceeding o n
the surface of t h e t w o electrodes, the chemical energy is directly
transformed i n t o electrical power.
Conversion o f one type of energy i n t o another requires a certain
t i m e period. T h e time needed for an energy-generating system t o
change i t s power f r o m a value (A) t o another value (B) is called
response (transition) time (Fig. 2 ) .
i
/
ition time
_J
Time
Fig. 2. Power curve showing t h e change of power from level A t o level B.
4
A thermal power plant needs tens of minutes t o change f r o m one
power level t o another, while for a battery, the response time is of
the order of millionths of a second. For a n energy utility t o meet a l l
load fluctuations, it should dispose of a system of power plants with
various response times ranging f r o m milliseconds t o hours.
1.2.2. Quality of energy supply systems
T h e quality o f 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.
100
Generation for load í No storage)
2.
Baseload
Sun
1
Generation for load íWHh storage)
Mon I Tue I Wed I Thu I Fri I Sat I Sun
I
Mon
Tue Wed! Thul Fri 1 Sat
I
I
IReserve
Baseiood energy to S i O M g t
Peaking energy from storage
Fig. 3. T y p i c a l w e e k l y load c u r v e for an electric utility
[il.
T h e energy system should have 15-20% of reserve power available
t o b e able t o meet any customer demand. I f there is n o or insufficient
reserve capacity and the load level exceeds the power generation level,
5
a decline in voltage at the consumer side w i l l appear which would
upset the n o r m a l operation of the users’ machines and electrical devices or even cause them t o fail. For this reason it is essential for
t h e n o r m a l functioning and development of each social community t o
have reliable national and local electric power systems w i t h capacities
exceeding the actual energy demand by at least 15%.
Unfortunately, however, o n l y r i c h and advanced modern countries
possess such high-grade energy systems. T h e power systems of most
countries in the world have capacities t h a t only j u s t meet their energy
demands, and in some cases are simply inadequate. T h i s hunger for
electricity is very often a l i m i t i n g factor for the economical and social
development o f a country.
1.2.3.
Ecological problems and the development of power
indus t r y
T h e 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 t o the i n 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 facilities, however, is based o n the combustion of coal, o i l and gas, which
i s accompanied by harmful gas emissions of COZ, SO2 and others.
T h e increased content of SO2 in the atmosphere has led t o the format i o n of acid rains causing enormous damage t o the agricultural crops
and the forests. T h e accumulated CO2 in the air might b r i n g about
considerable climatic changes b o t h o n a regional and global scale
(the so-called “greenhouse effect”). Thus the rapid development of
power industry has added comfort t o society and i t s individuals, but
it has posed very serious ecological problems of a national, regional
and global nature. In response t o these processes, various organi6
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-
bined w i t h the wisdom of a number o f state governments, have led t o
the adoption o f dead-line terms for decreasing the harmful gm contents in the atmosphere, especially those of SO:! and COL, in order
t o 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. T h i s
method has an undoubtedly beneficial effect o n the environmental
aspect of electric energy production, but it involves rather expensive,
complicated and n o t fully efficient procedures leading t o increase in
energy and power costs.
Second, in building up a system of energy storage plants which
haw a considerable impact o n the efficiency of energy utilities as well
as significant cost benefits.
Third, in t h c sphere of electric power consumption, a l l technological processes of m a j o r energy consumers have been revised w i t h
regard t o power consumption, and the most energy-consuming procedures replaced by new technologies w i t h lower power demands.
The basic problems and the development trends o f energy storage
w i l l be discussed in the chapters t o 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. T h e basic principles of some of these options, that have found successful application,
w i l l be outlined below.
7
2.1.
P u m p e d Hydroelectric Energy Storage Systems
(PHESS)
Figure 4 presents the schematic of such a system.
Fig. 4. Schematic of a Pumped Hydroelectric Energy Storage System.
These energy storage units require two large water reservoirs located at different heights, so that water fall is possible. D u r i n g periods of low demand, t h e excess power is utilized t o pump water
f r o m the lower reservoir and transfer it t o t h e upper one. At peak
demand periods, the pumped storage plant acts as a hydroelectric
power plant thus adding capacity t o the energy system. T h i s storing
option is cost-effective i f used only 5 t o 8 hours in the peaking range.
I t s response t i m e is of the order of 5 t o 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 a l l over the world w i t h a t o t a l capacity of 25,000 MW.
T h i s energy storage option i s most appropriate for countries w i t h
mountainous relief. Construction of these plants requires f r o m 8 t o
10 years and is often associated w i t h considerable environmental im-
8
pact. Pumped hydroelectric energy storage systems are cost-effective
i f 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 t o about 10% o f t h e
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. Consequently, by the beginning o f the next century, this energy storage
o p t i o n w i l l become an i m p o r t a n t 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
t o drive the motor of a compressor for compressing air that is stored
in a salt cavern located deep underground or in large hard r o c k caverns. D u r i n g peak demand periods, gradual release of pressure i s
performed and the air coming up t o the surface i s heated by burn-
i n g o i l or gas, and is then expanded through expansion turbines
that drive the r o t o r of an electric current generator. Compressor
m o t o r and generator are combined in one machine. During air compression, t h e rnotor/generator i s connected t o the compressor and
decoupled f r o m the turbine. During electric current generation, the
motor/generator is disconnected f r o m the compressor and coupled t o
the turbine. Compressed-air storage units burn only one third of the
fuel used by conventional combustion turbines t o produce the same
amount o f electricity. T h i s leads t o a two-third reduction in the environmental 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 released during air compression back t o the energy system. T h i s energy
storage option is cost-effective i f operated at a power above 25 MW.
For every hour of electric current generation, 1.7 k i of air compression
are needed. The response t i m e is about 10 minutes. Efficiency of air
compression is 65-75%. T h e starting period is 20 t o 30 minutes. As
rcgards the security aspects, measures should b e provided against
leakage of compressed air. T h e service life o f 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 operation in Huntorf. The plant uses two salt caverns, and storage
efficiency of over 80% is reported. T h e 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 i s sufficiently reliable.
10
Compressed-air storage plants have a negligible environmental
impact, and can be built w i t h i n 2 t o 5 years. They arc: fit,ted w i t h
modified combustion turbines of routine production. T h i s technology
can find application only in countries w i t h n a t u r a l deep underground,
h a r d 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
w i t h salt, cavern storage), in Israel (300 h'lW, l o - h o u r capacity, threeunit plant), etc. T h e Italian conipany ENEL has st,arted construction
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 i s a theoretical and a technical option t o store electrical energy as such, w i t h o u t converting it i n t o other forms. T h i s is possible
owing t o the ability of some substances t o become superconducting
at extremely low temperatures.
Because of the conductor's electrical resistance at anibient temperature, p a r t of the electrical energy is lost in t h e f o r m o f 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 materials achieve superconducting properties, i.e. they lose t h e i r electrical
resistance. W h e n direct current is fed i n t o an electric circuit o f superconductors, the current w i l l circulate endlessly along the closed
r i n g w i t h o u t energy losses. W h e n an energy demand appears, t h e
requested electrical power can be drawn f r o m t h a t closed ring.
Large-scale investigations are presently being performed aimed at
devising a technology for the production of superconducting magnetic
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 superconducting coil. T h e latter operates o n direct current. Charging o f the
electromagnetic coil w i t h electricity f r o m the ac generating utility is
accomplished via a two-way converter. A refrigeration system maintains the temperature of the electromagnetic coil at a very low fixed
value. Operation of the electromagnetic coil, converter and refrigerator 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 f r o m the utility network t o appropriate low voltage power. T h e 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.
T h e experimental SMES systems so far set up operate at extremely low temperatures -269°C (4 K), the temperature of liquid
helium) and the coil-wire used i s made of NbTi and NbSn alloys.
12
With the discovery of ceramic high-temperature semiconductors, it
can be expected that superconducting magnetic storage plants w i l l
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, t h e expenses for construction and maintena.nce o f the refrigeration system will b e reduced
significantly.
The current density in superconductive wires m a y reach extremely high values as t h e conductor exerts n o electrical resistance
leading t o joule losses. T h i s allows the wire cross-section t o be decreased more than five times w i t h respect t o copper wires used at
ambient temperature. T h i s w i l l change substantially the existing
classical electric power system.
In Japan, an energy storage project is being developed k n o w n
as the “Moonlight Project”. The power of the Japanese superconducting magnetic storage system is 1000 MW, energy density
is 12 Wh kg-’, storage efficiency SO-SO’%, storage utilization rate
approx. 75%. T h e system w i l l be used for daily and weekly energy storage. Underground bed rocks are required for construction
of this system. Location possibilities are restrictcd, because antimagnetic measures are needed for environmental protection. Protect i o n against superconductive material degradation is also necessary.
A 10 MW, two-hour capacity SMESC pilot plant has been developed in the USA. T h e refrigeration system is based o n liquid helium.
2.4. B a t t e r y Energy Storage Systems ( B E S S )
2.4.1. The revival of battery energy storage systems
At t h e beginning o f this century, electric power supply for industrial and domestic needs was provided by dc generators arid battery
facilities operating under floating charge conditions. D u r i n g this pe13
riod, batteries proved t o 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 t h e storage of energy as an element of t h e power system.
In the 1960s, a powerful reliable and cost-effective static rectifier 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. N e w and innovative elect r i c power applications in industry and every-day life brought about
radical changes in the profile of the daily, weekly and seasonal de-
m a n d ciirves. To enhance the operational efficiency of electric power
utilities, energy storage units were introduced. At first, pumped hydroelectric energy storage plants were used for t h a t purpose, and
later, the old lead-acid battery storage systems were revived. T h e y
were based o n t o t a l l y new conversion, management and control technologies.
A t t h e end of the 1970s, for the first time, BEWAG-AG decided
t o install the B a t t e r y Storage Facility in West Berlin under a test
program in order t o collect the necessary operational and technical
information. I t 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, t h e 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 w o r l d have started large-scale research and test programs aimed
at the introduction of battery energy storage systems in their national
ccoiioinies and public services.
14
2.4.2. Basic principles of battery operation
When t w o appropriately chosen electrodes are immersed in the
respective electrolyte and a direct electric current f r o m a n external
source flows between them, electrochemical reactions proceed o n the
electrode surfaces during which inactive substances are transformed
i n t o electrochemically active ones. This process is called charging
of t h e electrochemical power source or t h e 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 v i a a load, under the action of
this electromotive force, electrocheniical reactions proceed o n the
electrode surfaces in an opposite direction t o the reactions during
charge. T h i s process of current generation is called discharge. T h e
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.
D u r i n g off-peak periods, the battery is charged f r o m the electric
power utility via a converter. The latter converts the alternating
current i n t o dc. During discharge, the direct current generated in
the battery is transformed by t h e converter iiit,o alternating current
and the latter i s delivered through the tramfornier t o the utility for
meeting energy demands. Operation o f t h e converter arid the battery
are monitored and controlled by a controller,
2.4.3. Some advantages of battery energy storage systems
In t h e process of developnient of the new generation of BEC systems, lead-a,cid batteries were widely used, which allowed t h e latter
to exhibit a number of useful advantages leading to significant cost
beneíits. The following ecoiioniical features of lead-acid battery storage systems were demonstrated.
15
a) Modular design. Construction of BES plants is realized on a
modular basis, i.e. through connecting of the individual b a t t e r y 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 t o b e accomplished in stages according t o demand
needs.
b) Short construction terms. All BESS elements are produced
at the factories w i t h i n a few months only and then the actual building
of t h e BES plant i s reduced t o installing, assembling and testing of
these elements in a working system.
c) Small environmental impact. Battery energy storage systems are basically closed systems. No other materials are consumed
except water, and hence n o air and environmental pollution is caused.
T h e y are quiet and can be located near, and even in, housing c i t y
areas.
d) H i g h level of recycling of the materials employed an the
batteries. At the end of a battery's service life, many of t h e materials
used for i t s manufacture can b e regenerated.
F r o m the above, it follows t h a t the revival of BES systems during
the 1980s i s a normal process based o n the rapid progress of electric
power industry.
To b e economically effective, BES systems should meet t h e following challenging performance requirements:
30 years of service life
o 75% ac-ac efficiency
o Unit power cost about 400-700 $ kW-'
e 5 hour discharge.
o
Many projects are being implemented at present worldwide aimed
at achieving the above parameters.
16
2.5. Choosing t h e right option for electric energy
storage
W h e n deciding o n the type of off-peak energy storage system t o
adopt, the following considerations should be taken i n t o account:
demand cyclogram
and i t s possibilities
o profile o f t h e 24-hour
o available budget
national topographical peculiarities
o environmental aspects and limitations.
0
According t o a n EPRI investigation, there are a great number
of hard rock caverns in the USA, which suggests dominating importance 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 t o i t s pronounced mountainous relief, pumped
hydroelectric energy storage systems have proved t o be most costeffective, and hence this country, together w i t h Japan, occupy t h e
leading positions in the construction of this type of storage plant o n
a worldwide basis.
EPRI in the USA have carried o u t an analysis of the economics
of the various options for energy storage. T h e results of these evaluations are summarized in Table 1.
An analysis of the results indicates that battery storage systems
and superconducting magnetic storage systems are more appropriate for use when there are peak demands w i t h a duration of 2 t o
3 hours. P u m p e d hydroelectric storage plants and compressed-air
storage systems may cover efficiently peak power demand needs of
up to 10 hours. T h e y can also function as intermediate load power
systerns.
17
Table 1. Estimated costs for energy storage technologies [2]
~
~
Technology
Compressed-air
Pumped hydro
electric
Battery
Power Energy Hours of Total
related related storage cost
$ k W - ' $kWh-'
$ kW-'
Small module
(25-50 M W )
Large module
(110-220 MW)
Conventional
(500-1500 M W )
Underground
(2000 M W )
Lead-acid
(10 M W )
Advanced
(10 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
1O0
3
425
150
275
3
975
T h e unit power costs are highest for pumped hydroelectric plants,
owing t o the large capital costs for construction of the facilities. With
regard t o 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. T h e above rating of
the various energy storage options will, o f course, vary for the various
countries, depending o n their specific economic and technological
conditions and requirements.
18
3 . Batteries for energy storage
t i o n and under development
~
in opera-
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. T h e nanie “nioonlight” was selected t o
imply the analogy between the nioon that does n o t sliine with i t s o w n
light but reflects the sun’s light, arid the energy storage batteries that
do n o t “generate” their o w n 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 B a t t e r y Electric Power Storage Systeni. Sonie o f the
basic functions o f this system will b e discussed below.
T h e Electric Power Research Institute in t h e USA lias beeii carrying out research and developirieiit activities within the Energy Storage Program since 1972. T h e profirani incliirlcs dcvclopIricrit o f leadacid and advanced batteries.
Both programs are haced o n almost t h e sanie electrochemical systems. First, the lead-acid battery has been chosen as a basic chemical
power source commercially available. R&D activities are aiming at
adapting, b o t h f r o m a constructional arid technological p o i n t o f view,
this 100-year o l d battery t o t h e requirements o f energy storage. Second, new electrochemical power sources aie being investigated and
developed, such as: sodium/sulfur, zinc/chloride, zinc/broniide and
redox/Aow batteries.
Japan’s project i s targeted at devising a demonstrational model
of a battery energy storage system with t h e following parameters:
1 MW
8h
discharge t i m e 8 h
o power o u t p u t
o charge t i m e
o
~
-
~
19
min. 70% (at ac input/output)
min. 10years (2000 cycles)
o overall energy efficiency
o service life
~
-
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. T h e testing results of these units will be
discussed later. The basic properties and characteristics o f the socalled advanced batteries w i l l be described first.
3 . 2 . Sodium/Sulfur b a t t e r i e s
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. T h r o u g h this membrarie, only sodium ions can pass, but
A block diagram of a sodium/sulfur cell
is given in Fig. 7.
not electrons or sulfur ions.
Sodium
NO
Ij-oiumino
Sodium
polysulfide
Fig. 7. Block diagram of a sodium/siilfur celi [5].
20
.
S.NOS,
(X.3i5)
The reactions that proceed in the c e l l can be expressed by the
following ecpiation:
Discharge
2Na+2S
+
Na&
( x = 3-5)
Charge
D u r i n g discharge, metallic sodium of the negative electrode is ion-
ized t o 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 electrode flow through an external circuit, pass through a load whereby
certain useful electric work i s done, and reach t h e sulfur electrode.
There, they are bonded t o the sulfur atoms and f o r m sulfur ions
(S2-). These react, w i t h the sodium ions giving sodium polysulfide
(NazS,). There i s 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 f r o m the sodium t o the sulfur
electrode doing some work.
During charge, reverse processes take place. In this case, electric
energy shoiild he introduced i n t o the cell t o enable proceeding of the
reverse processcs. Under the action of an external \-oltage applied t o
the cell, electrons f r o m t h e polysulfide electrode move back, through
t h e external circuit, t o the sodium electrode. As a result, the sulfur
ions of the polysulfide molecule (Y)are transformed i n t o sulfur
atoms, and the released Na+ ions pass through the beta-alumina
membrane and are bonded t o the electrons forming sodium atoms.
For the battery t o operate, a temperature o f about 350°C should
be maintained. In this way, b o t h sodium and sulfur are kept in the
liquid state and the resistance of the beta-alumina membrane is very
low.
Figure 8 shows the voltage curves during charge and discharge of
the battery, as a function o f 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].
T h e voltage characteristics depend on the temperatiire and t h e
composition of sodium polysulfide. Figure 9 illiistra2tc:s tlic changcs
in the open circuit voltage as a function of anodc coinpositioii.
300'C
A
0
330T
360.C
390'C
0.60 0.65 0.70 0.75 0.80 0.85 0.90
Anode composition (mole ratio of sulfur)
0.95
1
Fig. 9. Open circuit voltage as a f i i n r t i o i i of molar r a t i o of s i i l f u r iiis o d i u m
polysulfide [5].
22
W h e n the molar r a t i o of sulfur t o polysulfide falls below 0.7, the
cell voltage begins t o decline.
T h e sodium/sulfur battery i s hermetically sealed and completely
maintenance-free. There are n o side reactions during charging and
discharging. I t is free o f self-discharge. T h e state of charge can
be easily monitored by measuring the amounts o f electricity (Ah)
charged and discharged.
3.2.2. Design of sodium/sulfur cells
Schematic of a sodium/sulfur cell is given in Fig. 10.
A
r
i
Insulolion ring
(olumina ceramics)
Anode cose í iron 1
Anode (sulfur.
graphite (el t )
Solid electrolyte tube
(cilumino ceramics)
ss
*
N
.
l
-Colhode ( sod ium ,
metollic liber)
L C o t h o d e tube (copper)
1
Fig. 10. Principal design of sodiurn/sulfur unit cell [5]
T h e beta-alumina membrane is a cylindrical t u b e w i t h a b o t t o m
at one end. Sodium is filled inside this tube, and sulfur and sodium
23
polysulfide in a metal cylindrical case outside t l i c tiihc. At the upper opening o f the beta-alumina tube, the anodc, the cathode and
the metallic case are connected arid welded t,ogct,lic,r with insiilat-
ing ceramics in between. A graphite niat is i n s e r k d in t l i c 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. T h i s fiber retains sodium and is also i i i k n d e d
t o prevent release o f sodium in the everit of brcakagc of the betaalumina tube, thus having an iinportant safety role. T l i e nieta1 case
surface i s plated with chroniiuni t o prevent corrosion i i i i d c r tlie action
o f sulfur and sodium polysulfide.
Na/S cells are arranged in a tlierrnoinsulatcd c a e with a iiiaiiitained constant temperature o f 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
p i l o t plant o f t h e Japanese “Moonlight Project”
A 50 k W / 4 0 0 kWh sodium/sulfiir battery module has been produced by the YUASA company in Japan.
T h e o u t p u t capacity of the plant is 8000 kW1i (1000 kWh x 8
hours). Output voltage i s 1000 V dc, o u t p u t current 1011 A dc.
T h e technical results f r o m testing o f various configurations of the
above battery t y p e are summarized in Table 2.
T h e sodium/sulfur bat,tery plant is located at the Tatsumi substation 77 close to t h e Tatsiimi lead-acid battery energy storage test
plant. By the end o f 1990, the 1 M W / 8 MWh sodium/sulfur pil o t plant was half completed and operation of 500 1iW o u t p u t 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 p i l o t modules [7]
Energy density
- per
footprint
volume
- per weight
-per
Starting time
Stopping t i m e
Response t o load change
Discharge 6 h overall eff.
4 h overall efT.
Voltage variation - on charging
on discharging
Energy consumption in standing
hot standing
- cold standing
~
~
~
~
52.4 kWh m-2
26.8 kWh m-3
42.6 Wh kg-'
1s
1s
10% 1.2 ins-'
76.S%
73.6%
11.8%
4.2%
19.4% day-'
0%: day-'
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
~
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 M W class pilot plant [8]
Beta-alumina tubes:
outer diameter
length
weight
beta-alumina doped w i t h Li20
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
Building area
Charge/discharge efficiencies
dc-dc efficiency (including
aux. power consumption)
ac-ac efficiency
68 mm
450 mm
3.6 kg
< 4.5 R c m at 350°C
8 m
> 200 MPa
> 300 Ah
8 MWh (1 MW - 8 h)
ac 6.6 kW dc 1.0 kV
dc 1 kA
self-commutated 1200 kVA
up t o 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.
L t d . in collaboration w i t h NGK Spark P l u g Co. L t d . The power
conditioning system was manufactured by Toshiba Corporation.
Figure 11 shows a bird’s-eye view of a 1 MW sodium/sulfur pilot
plant consisting of twenty 50 kW battery modules.
Sodium/sulfur batteries have high charge and discharge efficiencies w i t h no loss of energy during storage. The batteries are compact
w i t h high storage energy density. Individual Na/C batteries subjected t o testing have undergone 1500 cycles already.
26
Environmental proieciion equiprneni
Machinery room
Fig. 11. Bird’s-eye view of 1 MW sodium/sulfur pilot, plant [7].
Basic trends in the design of Na/S batteries are targeted at optimization of specific energy and efficiency, and at preventing reactant
leakage f r o m t h e cells. Safety is the key t o practical use of these
batteries and o f course t o extending their life, too.
Whether or not this type of battery w i l l h o l d an i m p o r t a n t posit i o n in the energy storage system, w i l l depend on i t s service life, o n
the competitive power of i t s price t o that of the lead-acid battery,
o n t h e simplicity and cost-effectiveness of i t s production technology
yielding reliability of the end product, and easy and inexpensive operation 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
D u r i n g the Franco-Prussian war, French balloonists flying over
the Prussian lines illuminated their maps by means of strange primi t i v e static batteries containing zinc and bromine. T h i s i s the first
k n o w n historical evidence of the invention and practical use of a
zinc/halogen battery. I t should be recalled that the lead-acid battery was also devised by a Frenchman. I t would b e fair t o say that
during the last century, France was a pioneer in the discovery and development of new electrochemical power sources. Early zinc/bromine
batteries had probably proved inefficient in performance and maintenance, and were therefore forgotten until the 1970s. As a result
of the A r a b o i l embargo, interest in electrochemical power sources
increased greatly worldwide, and t h e question of devising new advanced battery systems gained momentum. Zito, Magnetti-Marelli
and E x ~ o rResearch
i
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 t y p e of battery. MagnettiMarelli developed an electrolyte circulating battery design, whereby
the performance of b o t h the zinc and the bromine electrodes was
enhanced. ER&E combined the ideas of circulating electrolytes, liquid bromine complexing agents and the use o f low-cost conductive
plastic electrodes. At first, 20 t o 60 kWh batteries for e!ectric vehicles were manufactured and, o n 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-
T- -L
Brz+
28
2e-
+
2Br-
-
external circuit
W h e n the reactions proceed in t h e direction from l e f t t o right,
b a t t e r y discharge occurs, whilst in t h e reverse direction, charging of
t h e b a t t e r y i s accomplished.
T h e theoretical electromotive force of this battery i s 1.83 V, but
since complexing agents are involved, t h e open circuit ce11 voltage is
lower, namely 1.76 V. T h e theoretical energy density i s 436 Wh kg-',
while t h e practical one is only 65 Wh kg-'. O t h e r characteristics
are: p e a k power 95 W kg-', d e p t h of discharge loo%, and energy
efficiency 60-65% [9]. T h e zinc/bromine b a t t e r y operates at ambient
temperature. Although i t is an electrochemical system using aqueous
electrolyte, no decomposition of water is observed during charge.
T h i s system faced a serious problem related t o t h e rapid selfdischarge caused by t h e p r o p e r t y of bromine to dissolve readily in
zinc bromide electrolytes and to diffuse t o t h e zinc electrode osidizing
it. To avoid t h i s process, it was necessary to remove b r o m i n e from
t h e zinc electrode, and t o divide t h e anodic and cathodic sections of
t h e cell by a separator. Thus, a cell design \vas developed in which
t h e bromine compartment of t h e cell was connected by nieans o f a
t u b e t o a storage compartment for collecting t h e evolved bromine.
T h e electrolyte, forced by a pump, circulated between the electrodes
and t h e storage compartment. T h i s design principle lias l e d t o a
significant decrease in b a t t e r y self-discharge, but lias n o t eliniiiiated
it completely. A schematic of the zinc/bromine system i s 1)reseIited
in Fig. 12. A t o m i c bromine formed on t h e electrode i s indicated by
t h e dots in t h e figure.
T h i s t y p e of b a t t e r y construction has solved anotlicr p r o b l e m of
zinc/bromine batteries as well. that of t h e zinc electrode. In the
early static battery design, zinc deposited on t l i e clectrode iii t h e
form of non-uniform dendritic plating. Dendrites s o n i d m e s grew
across to t h e bromine electrode aiid caused short circuits in t h e cells.
By introducing electrolyte circulation also in t l i e zinc half-cell, t l i e
zinc deposit, formed over the electrode surface duriiig charge, becaine
29
more u n i f o r m and shortages were eliminated. Complete discharge is
required for every charge-discharge cycle t o equalize the zinc distrib u t i o n over t h e negative electrode surface.
Cathode loop
Anode loop
Zn deposit
Fig. 12. Schematic of
a
circulating electrolyte Zn/Br battery [9].
T h e double circulation loop battery design also proved beneficial for the thermal management of the cell. In static zinc/bromine
batteries, the reaction heat was accumulated in the cell. With t h e
introduction of electrolyte circulation, the temperature of the electrolyte becomes controllable and thermal homogeneity of the whole
electrochemical system can be achieved.
Circulating electrolyte batteries require various auxiliaries t o cont r o l battery operation. Obviously, their efficiency will influence the
power consumption for actual cell operation and hence affect overall
battery efficiency.
T h e power of an electrochemical power source is, as a rule, proportional t o the electrodes’ surface area, while i t s capacity is determined
by the amount of active materials that takes part in the electrochemical processes. T h r o u g h the adoption of the bromine circulation loop
and the storage compartment, the quantity of this active mass com-
30
ponent grows significantly. T h e zinc circulation loop also increases
the volume of t h e electrolyte and hence the quantity of zinc ions.
Nevertheless, zinc has remained the capacity limiting active material. Batteries designed for high-power applications comprise a large
number of zinc electrodes with relatively thin zinc p l a t i n g deposited
o n their surface. W h e n high battery capacity is needed, the thickness
of the Zn electrodes is increased.
T h e cathodic half-cell is divided f r o m the anodic one by means of
a microporous polyethylene separator with pore radius smaller t h a n
1 pm. Such separators are commercially available and are widely
used in lead-acid battery manufacture. T h e separator displays barrier
properties w i t h respect t o bromine diffusion, but it is permeable for
the solution ions that carry the electric charges between the two
electrode sections of the cell.
Bromine is a strong metal-corrosive agent.
Consequently, the
problem of material resistance t o oxidation is of primary importance
for this battery. Fortunately, most o f the commercially available
and relatively low-cost plastic and carbon materials meet the above
requirements, which w i l l make t h e large-scale production of these
batteries feasible and cost-effective f r o m an engineering point of view.
However, improvement of the overall battery construction reliability
is necessary t o eliminate a l l hazards of explosion or bromine leakage
in the atmosphere.
3.3.2. Chemistry and electrochemistry of the zinc/bromine cell
D u r i n g charge of the zinc/bromine cell, bromine evolved at t h e
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
h i g h 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 t o the
electrolyte, t h a t react w i t h bromine forming a separate phase which
i s 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 electrolyte composition of a discharged zinc/bromine cell is: zinc bromide
3 M, quaternary ammonium bromide 1 M, KC1 4 M as supporting
electrolyte [g].
D u r i n g discharge of the zinc/bromine cell, the valve between the
Br2 complex storage department and the circulation loop is opened.
Brz complex is mixed w i t h the solution in the circulation stream and
i s p u m p e d t o the bromine electrode where i t is reduced t o bromine
ions. At the other electrode, zinc is oxidized t o 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.
T h e charge/discharge voItage characteristics o f the cell are shown
in Fig. 13. During charge, a slight linear increase in cell voltage i s observed. D u r i n g discharge, the voltage decreases very slowly at the beginning, and as a result of complete exhaustion of the zinc resources,
the cell voltage drops rapidly at the end of discharge. Coulombic
efficiency o f the zinc/bromine c e l l is about 80% (i.e. 20% is inefficient). T h e inefficiency is mainly due t o electrode self-discharge.
Other sources o f inefficiency are the energy losses for driving the
32
electrolyte pumps, as well as for supplying power t o t h e b a t t e r y cont r o l and management system and t o i t s microprocessor.
Discharge
L
O
60
I80
120
U0
Tlme. min
Fig. 13. Charge/discharge curves for
a
zinc/broniine c e l l [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 designs are shown in Fig. 14.
Bipolar orrongement
Monopolor orrangements
Electrolyte
Manifold
Eledrdyte
t
-
t t t t t t
O E 2E3E4ESE
EMF olong m n i f o I d = ( n - l ) E .
where n = No, of cells ond
E = call voltogc
EMF olong monifoid O os all electrodes
of the some polarity ore connecled
Fig. 14. Bipolar and monopolar stack designs for Zn/Br systems
[lo]
33
In t h e monopolar version, there are separate positive and negative
electrodes, and all monopolar electrodes are connected in parallel. Iii
t h i s 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 f r o m the entire electrode surface and is carried through i t s
thickness. T h e o u t p u t is h i g h voltage and low current.
In energy 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. T h i s is a reduced-size model of the 50 k W / 4 0 0
kWh battery module for the 1 MW p i l o t 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 o f Fukuoka
City. I t was constructed f r o m November 1989 through September
1990. Operation tests started in December 1990 and will continue
till M a r c h 1992. T h i s i s the largest Zn/Br battery in the world.
Interesting and useful testing results are expected.
34
Table 3. R esul t s from testing of Zn/Br battery modules produced by M e i densha Electric Manufacturing Co., Ltd, Japan. T h e t e s t s were performed
at t h e Government Industrial Research I n s t i t u t e ,
Osaka [7].
10 kW battery [7]
Configuration
Unit cells
Open c i r c u i t voltage
Maximum charging voltage
Charging power
Discharging power
(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
43.8 V
50.0 V
12.7 kW
10.0 kW
33.6 kWh m-'
14.9 kWh m-3
29.1 Wh kg-'
1.37 m
1.59 m
1.67 m
footprint
per volume
- p e r weight
Dimensions - width
- depth
- height
Starting time
1 s
Self-discharge rate
7.4%
1 s
Stopping t i m e
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.
68.3%
- 4 h capable eff.
66.5%
Voltage variation - on charging
19.9%
- on discharge
15.5%
0%
Energy consumption in standing
Test cycle life
appr. 550 cycles
End of battery life was due t o carbon plastic electrode degradation
Energy density
- per
~
35
Table 3. (Continued)
50 kW class battery module [8]
Voltage
100 v
Current
Output power
500 h
50 kW
400 kWh
(30 cells in series x
24 series in parallel)
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-'
Capacity
Module configuration
External dimensions
~
width
- length
~
height
Weight
Energy efficiency ac-ac
Energy density - per footprint
per volume
- p e r weight
~
I m a j u k u Energy Storage Test Z n / B r Plant
Output capacity
Output voltage
current
ac-dc converter
Output transformer
Number of batteries
Number of ceils
[SI
4.4 MWh ( 1 M W
4 h)
6.6 kV; dc 1.1k V
dc 1 k A
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
~
ac
Submodule battery
output power
weight
dimensions
- width
- depth
- height
Building area
36
23 kW
8 tons
1630 mm
1520 mm
3150 mm
735 m2
3.4. Ziric/Chlorine batteries
3.4.1. Fundamentals of zinc/chlorine batteries
T h e electrochemical reactions o n which operation of this t y p e of
battery is based are as follows:
Z n e Zn2++
T-L
2e-
C12+
2C1-
2e-F=+
-
external circuit
D u r i n g battery discharge, the above reactions proceed f r o m left
t o right, and in the opposite direction during charge. T h e theoretical
voltage o f the Zn/Cl- cell is 2.12 V. It is higher than that of the Zn/Br
cell. Since n o complexing agents are used, the open circuit voltage
of the Zn/ClL cell has a value equal t o the theoretical one. T h e
theoretical energy density is 465 Wh kg-' against 60 t o 80 Wh kg-' in
the practical circuit depending o n cell design. T h e d e p t h of discharge
i s 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 t w o halogens in the cell reservoirs should be used. T h i s leads t o substantial
differences in design of the t w o types of zinc/halogen cells.
Chlorine is slightly soluble in zinc chloride solutions. For this
reason, during charge, chlorine is evolved in the f o r m o f bubbles that
leave the electrolyte forming a gaseous phase. T h i s gas should b e
collected and stored in an appropriate manner, t o b e fed back i n t o
the solution and reduced t o chlorine ions at the electrode, when elect r i c current is delivered by the battery. So far, there have been t w o
methods for chlorine storage in use. In the first method, chlorine is
compressed until liquefication and is stored as liquid chlorine at pressures of 70-80 psig. W h e n electric current is to be generated by the
37
N
'u
cut aff
1 1.75vvoltoge
2.18 v
E
1
7
----7?
\
40
5 40
j g --- - ~ ~ L c E 2 - - ~
.-
FUII
cm-2
pwer
-1.2
coulûmblc
Usable
2010-
20
- 2.4
2.0
--1.6 -u
Charge
(
(
1
- 0.4
coubmk+
Discharge
1
1
1
I
l
I
k
--cl8 %
l
I
I
I
T h e cell is charged at a voltage of 2.25 V. During discharge, the
cell voltage is kept at the 1.9 V level for a long period of time. T h e
discharge is carried down t o a cut-off voltage of 1.75 V. Voltaic efficiency is about 88%. Voltage losses are primarily due t o poor electrolyte conductivity between the electrodes. Coulombic efficiency is
about 87%, capacity losses being due primarily t o the self-discharge
caused by chlorine diffusion towards t h e zinc electrode. As a rule,
n o separator is used in this type of battery. The n e t electrochemical 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
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 o n the use of graphite electrodes, a single chlorine
circulating loop and cooling of the electrolyte and t h e gaseous phase
t o f o r m C12(H20),. A schematic of E D A ’ s Zn/C11 battery is shown
in Fig. 16.
e
I
heat exchanger
Fig. 16. Schematic of t l i e c i r c u l a t i n g zinc/chlorine b a t t e r y [9].
T h e battery is composed of an electrochemical module (electrode
stack) and electrolyte of zinc chloride solution w i t h added potass i u m chloride ( t o improve electrolyte conductivity). Using tubes and
pumps, an electrolyte circulation loop is formed. Zinc is deposited o n
the cathode. For a uniform plating t o be formed, the current density
should be in t h e range f r o m 20 t o 45 mA crnp2, and zinc loading between 90-300 mAh crnv2. Chlorine evolved o n the anode is removed
f r o m the stack and pumped i n t o 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 o f chlorine hydrate is stored.
39
D u r i n g battery discharge, the cooled hydrate slurry froiri t l i e storc
is passed through a heat exchanger, where chlorine liydrntc i s tlcconiposed. T h e chlorine-rich stream is then pumped t o the anode wliere
chlorine is reduced to chloride. On the negative electrode, zinc is osidized t o 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 i s liberated, i.e. doing work.
T h e chlorine electrode i s made of a porous graphite material. I t s
surface is activated. T h e chlorine-containing solution passes through
the pores of t h e graphite electrode, whereby chlorine is transformed
t o chloride ions ( “flow-through electrode”). (In t l i e zinc/bromine
cell, the bromine electrode i s of the “flow-by” type.) T h e rate of
the chlorine-rich electrolyte stream deterniines the rate of battery
discharge.
Self-discharge of the zinc/chlorine battery i s caused 1)y t h e react i o n between zinc and chlorine. Electrolyte circulatioii aims at faster
removal of chlorine f r o m the electrode st,acli and hencc rcdiiciiig the
self-discharge.
Zinc/chlorine batteries tend t o release hydrogen, because they
operate w i t h acidic solutions. Zinc corrodes in acidic electrolytes
evolving hydrogen. This hydrogen should be bonded t o chlorine, v i a
ultraviolet irradiation, for example. In this way, potential hazards of
explosions in the chlorine storage reservoir are avoided.
Zinc/chlorine battery design is usually based o n bipolar electrode
stacks.
3.4.3. B a t t e r y characteristics
Design and development of a zinc/chlorine battery associated
with the Energy Storage Project in Japan was carried o u t by the
Furukawa Electric Co. Ltd. T h i s company has developed 1, 10 and
40
50 kW battery configurations. The test results for 1 kW and 10 kW
b a t t e r y modules, before completion of t h e cycle life tests, are presented in Table 4.
A f t e r a critical analysis of the technological, performance and
economical parameters of the above b a t t e r y modules, further development of zinc/chlorine batteries was interrupted. T h i s t y p e of
b a t t e r y poses serious environmental hazards since chlorine is a toxic
gas.
T a b l e 4. Some results f r o m testing o f 1 kW and 10 kW Zn/Clz b a t t e r y
m o d u l e s p r o d u c e d by Furukawa E l e c t r i c Co. Ltd. T h e tests were p e r f o r m e d
at t h e G o v e r n m e n t I n d u s t r i a l Research I n s t i t u t e , Osaka [7].
1 kW battery
Configuration
30 cells in series x
Unit c e l l - voltage
2 series in p a r a l l e l
2.0 v
C o u l o m b i c efficiency
75 Ah16 h r a t e
63.0 V
1.41 kW
1.01 kW
84.0%
V o l t a i c efficiency
83.0%
E n e r g y efficiency
70.5-76%
- capacity
O p e n c i r c u i t voltage
Charging p o w e r
D i s c h a r g i n g p o w e r (8 h rate)
Self-discharge r a t e
~
initial energy efficiency
a f t e r t w o weeks
self-discharge r a t e
- after
f o u r weeks
self-discharge r a t e
71.1%
68.7%
3.4%
67.9%
4.5%
41
Table 4. (Continued)
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
Energy density - per footprint
- p e r volume
- per weight
Self-discharge rate
Starting t i m e
Stopping time
Change time charge-discharge
Change time discharge-charge
Response t o load change
Discharge - 6h capable eff.
- 4 h capable eff.
Voltage variation - o n charging
- o n discharging
Energy consumption on standing
- hot standing
- cold standing
42
(24 cells in series x
2 series in parallel)
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%
4. Lead-acid batteries
4.1. Some history
In 1860, Gaston Plante presented t o the French Academy o f Sciences a 9-cell battery (composed of lead and lead dioxide electrodes
immersed in H2S04 solution and separated by rubber tapes) and a rep o r t entitled “Nouvelle pile secondaire d ’une grande puissance”.
T h i s report was the birth certificate o f the lead-acid storage battery.
‘(D’une grande puissance” - what wisdom and foresight shown by
Plante so many years ago! Today, over 400 m i l l i o n cars worldwide
have engines driven by high-power lead-acid batteries.
D u r i n g t h e period 1880--1900, lead-acid batteries found their first
practical application in the early power stations. T h e y 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 leadacid b a t k r i c s as energy storage facilities gained increasing popularity
t o 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.
T h e largest battery storage unit was in Berlin. It h a d a capacity of
66,500 1iWh a,nd was capable of delivering 186 MW of electric power
w i t h i n 30 minutes. T h e c i t y 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 generation and distribution in t h e 1930s, the dc battery system was
abandoned, t o b e revived again during the 1980s.
At present, a number of lead-acid battery energy storage facilities in various countries worldwide are under construction or in the
demonstration and/or actual operation stage. W e w i l l discuss the
technical and economical aspects of lead-acid battery energy storage
43
technologies in the n e x t sections o n the basis of knowledge, experience 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 i t s electromotive force (emf) are:
+
PbOz
+ 2H+ + HzS04 + 2e- +PbS04 + 2 H z 0
T
Pb+H,SO,
-
P
L
+- external circuit
PbS04+2Ht+2e-
These reactions together w i t h the corresponding charge/discharge
curves of the cell are presented in Fig. 17.
DISCHARGE
.
f+
-j - '
e-
CHARGE
+ Z
Recti1ier
Positive plate
Negalive plate
7
I= 3.2 A
2.m
O
io
20
Time. h
io
20
Time, h
Fig. 17. A scheme of t h e charge and discharge reactions proceeding in t h e
lead-acid cell and t h e corresponding voltage transients.
44
Calciilatcd tlicrrriod~namically,the voltage between t h e lead sulfate and t l i e lead dioxide electrodes in the cell i s 2.040 V, but the
open circuit voltage i s iisually taken as 2.0 V (rated voltage). T h e
tlieoretical specific energy of the cell is 170.2 Wh kg-’. To transf o r m the lead-acid cell i n t o a practical power source, several design
requirements must b e rnet.
Lead and lead dioxide active materials are b o t h porous. Part
o f the active mass acts as a conductive sl<elet,on, and another part
(30 t o 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 t o H2S04, 0 2 and HL.
T h e cell iises approximately 36% H2S04 solution as electrolyte. T h e
positive aiitl iiegative plates are interconnected in s e m i - blocks with
terniinal posts protriidirig f r o m the cell. T h e plates of the lead and
the lead dioxitle seini-bloclis together with the separators and the
electrolyte hetween thein f o r m the active block. In it, all processes
occur, eiicrgy i s accumulated and electric ciirreiit i s generated during
discharge. Ahove the active block, there is a space containing a
certain amoiiiit of HLSO4 soliition (upper reservoir). Below t l i e active
block, another free space is available where t h e shedded active niass
is collected t o avoid short-circuits between t h e plates.
At t l i e end o f charging, decomposition of water takes place and
H2
and
0 2
gases are evolved.
T h e cell i s provided with a valve
as an outlet for these gases. Since gas evolution i s associated with
water consurnption, an equivalent amount of water m u s t be added
periodically t o the cell iii order t o maintain the required electrolyte
conccritratjioii. Water is added through t h e outlet. T h e cells are
mounted in a plastic container fitted with a cover. T h e cells are joined
in series with lead connectors that may b e situated over t h e cover, or
pass through the cell partitions ( “through-the-wall’’ arrangement).
45
T h e construction of a coiiventional present-day SLI l m t t c r y i s
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,
t h e close circuit voltage depends o n b o t h the direction and magnitude
o f 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 w i t h i n a given period of time, after which t h e
voltage begins t o decrease rapidly. Since deep discharges have an
adverse effect u p o n battery performance, a limit is set for t h e endof-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) i s shown
w i t h a dotted line. T h i s value is used for the calculation of battery
~
energy and power.
46
4lo A
528A
1 2
4
6 8 10 12 14 16 18 20 22 24 26 28 30
Time, min
Fig. 19. Discharge voltage curves for
a 12 V/100
Ah (22 h rate) s t a r t e r
battery [12].
Capacity. T h e capacity ( C d ) of a b a t t e r y is determined by the
quantity o f electricity that can be delivered during discharge at constant current until the final discharge voltage is reached.
T h e t i m e ( t ) needed for reaching the final discharge voltage i s
marked o n the abscissa and is known as the rate of discharge. International and domestic standards require the capacity t o be determined by a discharge w i t h a current at which t h e battery reaches
U, = 1.75 V at 20°C after 20 or 5 h (C20 or CS). T h i s capacity is
k n o w n as the rated capacity. Under n o r m a l operating conditions,
the battery should n o t be discharged beyond 80% of the rated capacity. T h i s capacity is k n o w n as working capacity.
T h e relationship between capacity and discharge current i s expressed 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, t h e HzS04 concentration and
t h e design of the battery.
47
12V11OOAh SLI battery
I
I
I
I
I
lmin
h 20minlOrnin 5 min
I
I
I
I
O ~ l O O 2 O O 300
l
400
I
1
I
6001.A
Rate oí discharge (1) or current (I1
Fig. 20.
C a p a c i t y 'us.
discharge c u r r e n t ( I )or discharge r a t e (t) of a
12 V/100 Ah s t a r t e r b a t t e r y [13].
T h e relationship between capacity and current is shown in Fig. 20.
Energy. T h e energy (Ed) delivered by the battery during discharge under constant current conditions is equal t o the product of
the mean voltage of the battery multiplied by i t s capacity. Figure
21 presents the dependence of energy and mean voltage o n 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 o u t w i t h moderate
currents, i.e. 3-10 h discharge rate. T h e delivered energy under
routine operation is usually 80% of the rated value.
Power. T h e power of a battery is the energy delivered per unit
time. T h e value per unit weight or volume is k n o w n as specific power
of the battery. Figure 22 presents the power us. current dependence.
W h e n the current increases, power is also augmented. Therefore, in
order t o deliver h i g h power, batteries are designed t o be discharged
at heavy currents. As the capacity decreases w i t h increase of current,
the discharge time will decrease rapidly.
48
@
1 2 V I Kx) Ah SLI battery
20h 20minX)min Smin
I
I
I
t
Imin
I
I
I
Current, A
Fig. 21. (a) Average discharge voltage vs. discharge c u r r e n t ; (b) energy vs.
discharge c u r r e n t for a 12 V/100 Ah b a t t e r y [13].
100 200 300
400
Kx)
c
O
Current, A
Fig. 22. P o w e r vs. discharge c u r r e n t of a 12 V/100 Ah s t a r t e r b a t t e r y [13].
49
Cycle life.
T h e service life o f a battery is the number of
charge/discharge cycles obtained during laboratory bench tests. T h e
battery must attain a given number of cycles before i t s capacity is
reduced t o 80% of the rated value. T h e real life o f a battery may be
longer or shorter t h a n that experienced under laboratory conditions.
During practical use, the battery is subjected t o other life-limiting
factors that are n o t taken i n t o consideration in the laboratory tests.
Current test procedures are aimed at maximum simulation of real
operating conditions.
4.4. Charging characteristics
That p a r t of the current utilized for the formation o f lead and
lead dioxide during battery charge i s called charge acceptance. T h e
remaining current is consumed for water decomposition. Figure 23
shows the charge acceptance of a battery vs. i t s state-of-charge.
g
20
5 0
o
20
40
60
Charge
Rated capacity
;,
80
loo120
Fig. 23. Charge acceptance of a t r a c t i o n b a t t e r y ws. i t s state-of-charge
at 30°C [14].
T h e data show that almost the entire amount o f charging elect r i c i t y is used for the transformation of PbS04 t o Pb and PbOz until
a 6 0 ~ 7 5 %state-of-charge is reached. At this stage o f 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 gradually and continuously reduced. Cell voltage is increased f r o m
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 t h e m a i n
processes that take place. The battery is overcharged.
These stages can be clearly identified by galvanostatically charg-
ing a b a t t e r y which has previously been subjected t o three different
depths of discharge (50, 75 and 100% DOD). After these discharges,
t h e efficient charge stage acquires three different durations. Figure 24
shows the changes in cell voltage during charge. T h e charging current
was 0.1 A per Ah.
2.8 - 50 per cent
20
75 per cent
dischorqed
Time
Fig. 24. Changes in celi voltage d u r i n g charge, f o l l o w i n g three discharge
runs t o different depths [15].
T h e gas evolution voltage is 2.35-2.40 V per cell at 75% state-ofcharge.
T h e following parameters are used t o define charging regimes:
o
I n i t i a l and final charging voltage (2.1 t o 2.4-2.7 V per cell).
o
I n i t i a l 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
or
I,h= 7% C5 A).
o
c
5
(I:h
= 0.07 A Ah-'
Final charging current (ICh = 0.01-0.03 A Ah-' or
I!h
= 1-3%
A).
o Upper charge temperature limit
(45-50'C).
T h e duration o f charge must be short, the energy and power
efficiencies must attain maximum values, the irreversible processes in
the active masses and the grids must n o t 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 currentvoltage charging method, b) tapered charging method, and c) pulsed
charging m e t h o d [13].
T h e specific charging method for each battery is usually prescribed by the battery manufacturer.
4.5. Effect o f electrolyte stratification
During discharge, the concentration of the acid in the cell decreases, and during charge it increases. Concentration gradients are
formed in the cell between the solution above the active block and
that between the plates. T h e 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 r a t i o of 1.02.
Stratification of the acid is enhanced as the number of cycles is
increased. This indicates that the concentration changes accumulate
during cycling. The difference between t h e acid concentration at the
t o p and the b o t t o n i 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. E l e c t r o l y t e s t r a t i f i c a t i o n measured during b a t t e r y charging t o 2%
overcharge and a f t e r 100% DOD [16].
As HzS04 is an active material, stratification w i l l affect c e l l capacity. It has been established that the capacity decreases by l%
for
each 0.01 sg unit of stratification. T h i s capacity loss depends on the
DOD and the charge/discharge ratio. With increase in overcharge,
t h e extent of stratification (and hence the capacity loss) i s diminished. To eliminate fully t h e capacity losses due t o strat'ification,
the battery should be subjected t o 15% overcharge. Since the active block is a compact assembly, during overcharge the evolved gas
will exert a pumping action which w i l l transfer the dense acid at the
lower half of the active block t o the top, thus enhancing t h e equalization of the acid concentration. Intensified gas evolution, however,
w i l l lead t o greater water consumption and hence heavier battery
maintenance, on the one hand, and w i l l increase the corrosion of the
positive grids, o n the other hand. That is w h y the use o f devices
for forced electrolyte circulation is recommended. These are usu-
ally small air-lift pumps t h a t let an air flow i n t o 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. T h e quantity o f electricity consumed for charging i s about 15%
greater than that o f the discharge.
I
Charge or discharge
F i g . 26.
l
S
Dependence of t h e cell p o t e n t i a l o n t h e state-of-charge or
discharge [17].
Since the charge and discharge were conducted at the same current, 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. T h i s
can be achieved by reducing t h e polarization o f the positive and negative battery plates and decreasing the ohmic drop in the electrolyte
(separator) during charge and discharge as well as by decreasing the
duration of overcharge.
T h e highest energy losses are related t o battery overcharge. If
the latter is eliminated, however, t h e process of conversion of lead
sulfate t o lead and lead dioxide w i l l n o t proceed fully and the battery
will b e 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. T h i s device is shown in Fig. 27.
Air
II
Fig. 27. Electrolyte agitator [17]
In this device, air i s introduced by a blower i n t o the inner t u b e
of a double-wall cylinder f r o m the top. T h i s air l i f t s t h e electrolyte,
while ascending as bubbles, through the gap between the inner and
outer tubes. T h e electrolyte is sucked f r o m a hole at the lower part
of the outer tube and ejected f r o m a hole at the upper part o f the
same tube.
T h e obtained results have shown that: first, battery charge w i t h o u t 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 performed 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 t o 100% charge w i t h o u t electrolyte stirring, 120% charge w i t h o u t external agitation but w i t h 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 s t i r r i n g o n cycle l i f e [17].
As can be seen from the figure, when lead-acid batteries are
subjected t o charge-discharge cycling w i t h 100% state-of-charge and
electrolyte stirring t o 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, n o t only is the life
of the battery extended, but i t s charge-discharge energy efficiency is
improved as well.
56
4.7. Methods for reducing water losses
Maintenance o f lead-acid batteries consists primarily in periodical
refilling the cells w i t h water. Water is lost f r o m lead-acid batteries
through the following processes:
e Electrolysis of water during overcharge o f t h e battery
o Self-discharge under open-circuit conditions
o Evaporation
of water
Manual filling up o f the cells t o a constant electrolyte level is very
laborious and time-consuming, especially in a b a t t e r y energy storage
plant. Therefore, m a j o r challenges in battery servicing are t o find
a way t o reduce water losses and t o replace manual refill methods.
T h e following methods have been proposed:
a) Single-point (common point) watering. T h i s system i s 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 t h a t can automatically adjust the electrolyte
level in each cell o f the battery. This watering system comprises:
devices t o monitor the level of the electrolyte in the cells and t o
stop the flow of water when t h e electrolyte reaches a previously
adjusted level;
a device for escape of evolved oxygen and hydrogen gases f r o m
the ce ll t o eliminate explosion hazards.
Finally, the design should avoid electrical shorting between the cells
through the common filling system.
These systems have found wide application in stand-by energy facilities for power plants, post offices, cultural and commercial centers
using stand-by lead-acid batteries. A common p o i n t r e f i l l system is
utilized in many battery energy storage plants. Operating experience
of batteries w i t h the above r e f i l l system in various modifications has
shown that it is n o t always sufficiently reliable and safe.
57
b) Catalytic plug recombination of hydrogen and oxygen. Designed t o recombine hydrogen and oxygen t o water that is brought
back i n t o the cell. Instead of a cell valve, a catalytic plug i s included
that enables the following reactions t o proceed:
2H2
+ O2
+
2H2OVapOUr
H~Ovapour+ HZOiiquià
+114 kcal
+9.7 kcal
T h e first reaction requires a stoichiometric ratio between the
evolved H2 and 0 2 , and is accompanied by the release of a great
amount o f heat. T h e 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 f r o m the platinum group are used as catalysts for the recombination. Carbon, alumina or asbestos wool are usually used
as catalyst carriers. T h e m a j o r disadvantage o f this water recycling
m e t h o d is the high price o f the catalyst materials, which has restricted large-scale application of the method.
c) Closed oxygen and hydrogen cycles. During charge-discharge operation, hydrogen and oxygen are often evolved in nonstoichiometric amounts. That is w h y the efficiency of the catalytic
plug i s reduced. A method has been proposed suggesting that t w o
auxiliary catalytic electrodes are fitted in the cells. T h e y are presented in Figure 29.
On one auxiliary electrode, the reaction of oxygen reduction t o
OH- ions proceeds. For this purpose, the oxygen electrode is connected through an appropriate diode t o the lead electrode of the cell
(Fig. 29a). On the other catalytic electrode, hydrogen is oxidized t o
hydrogen ions (Fig. 29b). T o ensure the right potential for the electrochemical reaction of hydrogen oxidation, the hydrogen electrode is
connected through a proper electronic device (diode and resistor) to
58
I
I
pm2
Pb
L
J
Pb02
Fig. 29. Schematic o f cells with auxiliary electrodes
Pb
[is].
the lead dioxide electrode of the cell. T h e reactions proceeding at the
t w o auxiliary electrodes are catalyzed by metals f r o m the platinum
group. T h i s makes the m e t h o d very expensive and hence i t s applicat i o n is strongly restricted. It has recently been found that tungsten
carbide displays similar catalytic properties t o those o f platinum w i t h
respect t o hydrogen and oxygen reactions [19]. Consequently, interest
in this method has increased of late.
d) Valve-regulated recombinant lead-acid batteries.
charge, the following reactions take place in the cell:
positive
plates
negative
plates
-
During
+ 2H20 +PbO2 + 2H+ + H2S04 + 2e;O2 + 2H+ + 2ePbS04 + 2H+ + 2e- +Pb + H2S04
2H+ + 2eH2
PbS04
HzO
-
Figure 30 shows the relationship between the charge acceptance
o f positive and negative plates and the time of charge.
20
2.6
2.4
2
c
I
22
O
1
2
3
2 .o
Chorge tlrne, h
Fig. 30.
C h a r g e acceptance of p o s i t i v e and negative p l a t e s at 40°C
vus. t i m e [2O].
When the positive plate reaches a 60--70% state-of-charge, a reaction o f oxygen evolution starts and i t s rate is increased w i t h time.
T h i s leads t o an equivalent decrease in positive plate charge acceptance. T h e negative plate is charged w i t h 100% charge acceptance
until about 95% state-of-charge. After t h a t , hydrogen evolution
starts. T h i s difference in the behaviour of the H2 and 0 2 reactions has
been exploited in the oxygen cycle. Oxygen evolved first, is brought
t o the negative plate, where it oxidizes the lead in the grid. Thus,
on t h e one hand, the negative plate is kept discharged and hence
evolution of hydrogen is prevented, and o n the other hand, oxygen
itself reacts in the cell. T h e basic problem w i t h this t y p e of oxygen
cycle battery was t o prevent oxygen f r o m being lost f r o m the active
block. A solution was found in immobilizing the H2S04 electrolyte
between the plates. T h i s was achieved by:
60
o Using gelled electrolyte. T h e cracks formed in t h e gel act as
channels for the transport of oxygen. T h i s technology was devised
by the German company Sonnenschein for small lead-acid batteries
(several Ah), and then further developed for large stationary batteries [21]. T h e US company .Johnson Controls Inc. lias enhanced this
technology [22,23] and is now manufacturing gelled-electrolyte batteries for b o t h traction and stationary applications, and for energy
storage systems as well.
o
Using absorptive glass m a t separators. T h e electrolyte in
t h i s case i s absorbed by the glass fibre mat that has t h e property
o f absorbing H2S04and water, and adsorbing oxygen. Oxygen is
retained in the glass mat separator and flows f r o m the positive t o 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. N o w 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 l o w rate, which accumulate above the
electrode stacks in the cells. T o avoid explosion, a valve is fitted t h a t
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 ( B E S S )
5.1.
Functions of lead-acid battery energy storage
systems
After the revival of interest in energy storage systems, the first
batteries t o be used for that purpose were flooded lead-acid batteries
of the traction/industrial type. Early battery storage plants were
installed for demonstration purposes t o validate the feasibility of design, investment , operational and maintenance costs of the system;
t o demonstrate operational and economic benefits; t o establish BESS
applications w i t h higher efficiency; and t o test feasibility of BES systems 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 operational 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 discharging, which leads first t o improvement o f the load factor of baseload generating units, and second t o reduction of energy costs by
storing cheap energy at night and selling it at higher cost during the
daytime peak demand hours.
b) Peak-shaving: Often m a j o r utility customers need instantaneous delivery of peak electric power for meeting technological needs.
T h e lead-acid battery, owing t o i t s low internal resistance and very
short response time, i s capable of dispatching considerable power
w i t h i n several milliseconds. Through charging of the battery f r o m
the energy utility, electric power is concentrated in the battery. T h i s
power can be delivered through high-current discharge of the battery (Fig. 23) when a high peak demand appears. T h i s method of
energy storage allows electric utilities t o 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 local electric system power level, the voltage o f the system begins t o
decline. T h e battery storage unit takes over part of the load by discharging electricity, thus enhancing the stability of the power supply.
d) Frequency control and spinning reserve: To ensure n o r m a l
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 fluctuate. 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 w i t h i n the
desired limits.
'
Utility
lines
charging
(off p e a k )
r--,---
Battery
dixhorgi ng
(on p k )
-- 1
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 t h r o u g h o u t t h e world [27].
Company
Size
In service
Application
Utility operated
1 7 MW
14 MWh
1986
frequency regul.
spinning reserve
Kansai E l e c t r i c Power Co. Ltd
Tatsumi, Japan
1 MW
4 MWh
1986
S o u t h e r n C a l i f o r n i a E d i s o n Co.
Chino, CA, U S A
10 MW
40 MWh
1988
demonstration
multi-purpose
t e s t program
demonstration
multi-purpose
test program
B e r l i n e r Kraft und L i c h t
(BEWAG), Berlin, FRG
Customer operated
Elektrizitätswerk
Hammermiihle
Selters, FRG
400 kW
400 kWh
1980
load-levelling
peak-shaving
H a g e n Battcric AG
Socst, FRG
500 kW
7 MWh
1986
load-levelling
peak-shaving
Crescent El ect r i c
Membership C o r p o r a t i o n
Statesville, NC, U S A
500 kW
500 kWh
1987
load-levelling
peak-shaving
D e l c o Remy. Division
of General M o t o r s
M u n c i e , IN, USA
300 kW
1987
peak-shaving
1989
peak-shaving
Vaal Reefs Exploration
and M i n i n g Co.
South Africa
600 kWh
4 MW
7 MWh
Johnson Controls, I n c .
300 kW
H u m b o l d t Foundry
M i l w a u k e e , WI, USA
600 kWh
64
emergency power
1989
peak-shaving
load-levelling
T h e 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. T h e power of lead-acid b a t t c r y
storage facilities (LABESF) at the customer side is of the order of
300 t o 500 kW. T h e only exception is the L A B E S facility in South
A f r i c a which n o t only serves for peak-shaving purposes, but also for
emergency power supply.
Customer operated lead-acid battery storage facilities are utilized
primarily for peak-shaving and load-levelling. All thcse applications
bring immediate financial profits t o the customers, and make theni
less dependent o n the power supply utilities in peak deniand periods.
T h u s sufficient stability of the technological processes i s guaranteed.
T h e use of lead-acid battery storage plants by electric power generating utilities is aimed n o t only at levelling the electric loads, but
also at improving the quality of t h e 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 and other system instabilities.
W h i c h option t o choose for BEC plant location - before or after
the meter?
I f the power demand is n o t very h i g h and a number of customers
could be grouped o n a territorial principle, siting LABES plants o n
the utility side of the meter is advisable. T h e storage unit should be
installed close t o the substation at i t s low-voltage stage. LABESP
capacity should be properly rated t o meet the local peak power demand.
For m a j o r energy consumers having high peak power demands,
e.g. commuter railroads and stations, metropolitan t r a i n or subway
systems, foundries, large administrative or commercial centers, etc.,
it would be more cost-effective and associated w i t h smaller power
losses t o install the LABEC facility o n the customer side of t h e 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 o f the utilization of
LABEC plants by m a j o r electric power consumers in the USA. T h e
investment pay back periods have also been estimated. T h e obtained
results are summarized in Table 6.
T a b l e 6. E c o n o m i c analysis of selected cases of LABESS a p p l i c a t i o n o n t h e
customer side [il.'
Customer /
Application
D e m a n d Converter B a t t e r y
size
size
charge
MW/ac
$ kW-'
MWh/ac
Commuter railroad
Steel m a n u f a c t u r e
C o p p e r alloys plant
T r u c k part plant
Chemical manufacture
~
~~~
~
~
16.24
11.88
13.50
13.82
9.87
5.8
3.5
0.9
0.5
0.4
5.6
2.3
0.7
0.7
0.5
Capital Payback
cost
$M
2.48
1.46
0.53
0.56
0.32
period
years
2.2
3.5
3.5
4.3
6.2
~~
Based o n a b a t t e r y cost o f 260 $ kWh-', converters at 130 $ kW-', and
balance of plant, 140 $ kWh-I, o p e r a t i n g 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 b o t h cases, applied before or after the meter, battery energy
storage brings significant profits t o the electric utility company.
66
6 . . Lead-acid battery energy storage systems
for load-levelling
6.1. System structure
T h i s consists o f the following basic components:
0
lead-acid battery
o ac/dc power conversion system
(converter, transformer,
dc and ac switchgear)
o facility monitoring and control system.
T h e electric energy i s supplied by a utility distribution network
t h r o u g h an ac switchgear t o the high-to-low-voltage transformer.
T h e n an ac-dc converter follows. T h r o u g h a dc switchgear, the current is fed i n t o 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 i t s voltage in a transformer,
is fed back i n t o the utility distribution line.
Figure 32 gives a schematic o f a lead-acid battery energy storage
system.
Facility monitoring
dc
-I-
1-
1
oc
Charge
Discharge
F i g . 32. Schematic of electric utility b a t t e r y energy storage s y s t e m
[il.
67
T h e battery energy storage plant is located near a substation
o f t h e power supply system, after the transformer, and serves the
substation local area. ‘This is the case of LABES plant before the
meter.
T h e basic components of the system are discussed below. The
system design, properties and parameters can be best described if
based o n a real operating energy storage system. L e t us take for
example the world’s largest, up t o now, lead-acid battery energy
storage system in Chino, California, U S A .
6.2. Chino 10 M W / 4 0 MWh lead-acid battery energy
storage system
6.2.1. Plant layout
Figure 33 presents a floor plan of t h e battery energy storage plant.
Demineralized
Fig. 33. General arrangement of LABES plant at Southern California
Edison Chino Substation [28].
68
T h e Chino LABES plant is sited 50 miles east of Los Angcles,
in the vicinity of the Chino 220 kV Substation of the Southern
California Edison Company. The Electric Power Research I n s t i t u t e
(EPRI) and the International Lead-Zinc Research Organization I n c .
(ILZRO) are project participants.
T h e plant consists of two large parallel buildings housing the
batteries. Between them is the common converter/control building
and related facilities. In the immediate proximity, there is a 12 kV ac
switch rack. T h e system supplies 10 MW of power in four hours or
40 MWh of energy, enough t o meet t h e needs of 5000 customers.
6.2.2. The battery
T h e Exide Corporation has supplied the b a t t e r y which comprises
8256 individual cells, specially designed for deep-discharge capability.
Exide has warranted the life of the cells for a minimum of 2000
cycles, or eight years before battery replacement becomes necessary.
Argonne National Laboratory has tested these batteries and found
that they could endure more t h a n 4000 cycles [29].
T h e battery unit operates at nominal voltage o f 2000 V, generated by strings of 1032 cells connected in series. During charge
and discharge, the battery and the strings change their voltage in
the range f r o m 1750 t o 2800 V. The charge rate’s first step is usually
4000 A, followed by a constant voltage period and finally by the third
step overcharge rate of 1000 A. The nominal daily voltage cut-off is
1800 V based o n 1.75 V per cell. T h e battery delivers 50 MWh eriergy at a maximum 10.5 MW power for 5 h during daily discharge
at 80% DOD, and 40 MWh at 10 MW power for 4 h at 80% DOD.
Six t o 10 hours nightly recharge is performed. Turn-around battery
dc-to-dc energy efficiency is 78%.
69
T h e specified battery characteristics are presented in Table 7.
T a b l e 7. C h i n o L A B E S plant
~
B a t t e r y specification requirements [29].
R a t e d discharge requirement
- energy o u t p u t (MWh)
- power (MW)
- t i m e t o 1.75 V (h)
N o m i n a l voltage
O p e r a t i n g range
N o r m a l daily c u t - o f f voltage
100% DOD
80% DOD
65.8
52.7
10.5
10.5
6.5
5.0
2000 v
f r o m 1750 t o 2800 V
1800 V
B a t t e r y has 1032 cells in series s t r i n g
B a t t e r y l i f e (80% DOD)
Daily recharge
O v e r a l l energy efficiency
- ceii dc t o dc
-battery
O p e r a t i n g t e m p e r a t u r e range
8 years = 2000 cycles
6-10 h
78%
75%
32-1 17°F
S t i b i n e & arsine gas emissions:
P e a k flow
Daily (8 h)
SCFM
SCFM
T o t a l p e r day
T o t a l annual
g
lb
SbH3
5~10-~
3~10-~
34
19
ASH^
2x104
1.1x 10-~
0.77
0.42
T h e cell is an Exide special design hybrid construction w i t h
PbSbAs positive grids and PbCa negative ones. T h e positive active material has a low density and contains anti-shedding additives,
yielding a high active mass utilization coefficient. T h e negative active
mass contains a long-life expander composition and has h i g h density.
T h e cell design features are presented in Table 8.
Figure 34 shows the changes in cell voltage during battery discharge with various discharge currents. Three cut-off voltages are
given in the figure.
70
T a b l e 8. C e l l design features [29].
R a t e d capacity (I= 650 A, 5 h t o 1.75 V)
Energy output
P o s i t i v e plates (17.3” x 13.7” x 0.33”)
grid a l l o y
Negative plates (17.3” x 13.7” x 0.21’’)
grid a l l o y
C e l l cover m a t e r i a l
Cell jar material
Sediment space
Separator s y s t e m ( o n positives)
- absorber mat
- retainer
- separator
A c i d specific g r a v i t y (top-of-charge)
Terminal posts
- design: l o w t o r q u e maintenance
Post-to-cover seal
Cover-to-jar seal
3250 Ah
6.2 kWh
17
Pb-Sb
18
Pb-Ca
PVC
SAN
1.3”
non-woven glass
p e r f o r a t e d PVC
niicroporous r u b b e r
1.28-1.29
l e a d p l a t e d copper
slide-lockTM
s t r u c t u r a l adhesive
1.75 V
\
l.m-
1.67 V
1.600
1.60 v
2000A M O A
1.500
O
I
I
2
4
520A
650 A
-
380 A
I
I
I
6
8
10
220A
I
I
12
14
lime, h
I
I
I
16
Is
20
Fig. 34. I n i t i a l celi voltage transients at discharge [29].
,
2 2 :
,
On the basis of these curves the dependences of mean voltage,
capacity, energy and power o n the rate of discharge were plotted.
T h e y are presented in Fig. 35.
i
1.0
l
'
l
2 4
6
8 10 12 14 16 18 2 0 î . h
I
I
795590
C.
l
390
l
l
l
1.A
'
l
l
210
E.
P.
Ah
4400 -
4000
-
3600
3200 -
8
7
,
2800 I
1
4
(
6
75590
6
ti0 I , A
5
8 10 12 14 16 18 20 t . h
,
,jkW1
390
Fig. 35. M e a n voltage, capacity, energy and power
?IS.
t h e r a t e of discharge.
Increasing the discharge current, the average voltage i s only
slightly changed until the 4-hour rate of discharge, and decreases
rapidly thereafter. T h i s has allowed successful employment of the
b a t t e r y for 4-hour discharge applications. Up t o the sixth hour, the
slope of curves P, E and C is roughly constant and it changes after
that.
T h e internal ohmic resistance of the battery has been determined
(using the A V / A I method) t o be 0.025 ohms during the 10 MW
72
discharge. Average resistance of the string has been measured as
0.20 ohms at full state-of-charge, which means that t h e internal resistance o f the cells is 194 micrq-ohms [30].
A certain number of cells have been subjected t o tests o f 2 cycles
per day at 40 f 5°C. Over 4 years, the cells have undergone more
than 2300 cycles at 80% DOD and are s t i l l “in good health”.
T h e cells are organized in a battery as follows: 6 cells are assembled in a 12 V module with an energy of 36 kWh. A schematic of
this module is given in Fig. 36.
,Hold
&un b r k î
Fig. 36. Drawing of a battery module [29].
44 modules (264 cells) form a row. Four rows separated by foot
aisles f o r m a string. The battery has 8 strings and 1376 modules or
8256 cells housed in t w o battery rooms [29].
73
The large number of cells in the battery create non-homogeneous
conditions in the parallel strings. Depending o n the ohmic resistance of each string, different currents will flow through the different
strings. W i t h o u t equal sharing of current, over a large number of
cycles, the state-of-charge of the cells in some of the strings may become considerably lower t h a n that of the other cells. T h i s could lead
t o “reversal” of the cells during discharge and early cell failure. To
prevent such processes, the difference in state-of-charge o f the cells
must be monitored and controlled. Duringthe initial operation of the
Chino BES system, a difference of about 12% was found between the
individual strings, while after 350 cycles this difference diminished t o
about 4% [31]. Table 9 lists the accessories of each cell.
T a b l e 9. Load-levelling b a t t e r y accessories [29].
Location
1. A u t o m a t i c w a t e r i n g valve
s t i b i n e l a n i n e trap
flame arrester
2.
3.
4.
5.
6.
74
Air lift pump
each c e l l
each cell
Electrolyte withdrawal tube
each cell
Thermocouple well
0.5% of cells
A c i d level i n d i c a t o r
each cell
I n t e r c e l l connectors
l e a d p l a t e d copper
t w o per post
7. H i g h voltage protectors
expanded p l a s t i c t u b e
a r o u n d each t w o i n t e r c e l l
connectors
8. I n t e r m o d u l e cables
9. Lugs o n cables
minimum 2 each 4/0 2000 V
410 l o n g b a r r e l compression
t y p e 2000 V r a t i n g
T h e positive plate grid alloy contains As and Sb. T h e grids corrode, and As and Sb are oxidized and dissolved in the HzS04 electrolyte. These ions are then deposited o n the negative plates, and
when the potential becomes more negative than a certain value, As
and Sb f o r m AsH3 (arsine) and SbH3 (stibine) gases, respectively.
Since these gases are toxic, they should be retained in the cell. T h i s
is accomplished by means o f an arsine and stibine trap. T h e latter
comprises activated chemically treated carbon and absorbs 98% of
the stibine and arsine emissions f r o m the cell for 2-3 years of service
life. T h e trap can be replaced after that. Since oxygen and hydrogen are accumulated above the active block in t h e cell, t o avoid
explosion, a ceramic flame arrestor i s used, sized for a maximum
overcharge current of 200 A per cell [29].
A l i m i t e d number of battery modules are watered simultaneously
when the battery is o n open circuit after the full charge. Water purity
is controlled t o a specific resistivity n o t less than 2 x
Each cell is fitted with an acid level indicator.
l o 6 o h m cm.
Table 10 presents a summary of the materials needed for battery
manufact ure.
Table 10. Materials for 10 MW/40 MWh Chino LABESP [30].
1. Lead
2. Copper (intercells and parts)
3. Steel (racks and trays)
4. Plastic (SAN, PVC, PP)
5. Sulfuric.acid (1.285 sg)
6. Microporous separator
7. Bolts, intercells, racks, trays
8. Cable (4/0)
tons
metric tons
metric tons
m e t r i c tons
metric tons
EA
EA
FT
metric
1560
30
153
90
480
280,704
139,600
6,048
75
Maintenance and monitoring of the battery is the most labourconsuming i t e m o f the LABESP operation and maintenance. T w o
full-time battery electricians w i t h part-time support personnel areneeded t o provide proper maintenance of t h e battery. T h e m a j o r
time-consuming procedures are watering of the battery, cell cleaning
and performance monitoring. For watering of the cells, approximately 6500 gallons of demineralized water are needed every 30 cycles. Cell cleaning takes about 4.5 man-weeks and i s done every six
months. D a i l y monitoring of the voltage, current and capacity o n a
string-by-string basis is performed f r o m the control room. T h e most
i m p o r t a n t performance parameter is the end-of-discharge voltage of
each cell. By means of an infrared camera, w a r m cells are detected
during the end of discharge that denote possible weak cells. The
electrolyte temperature is determined o n the basis of a number of
selected cells.
During i t s operation for over t w o years now, the battery system
has displayed stable and reliable performance parameters close t o the
rated values [30].
6.2.3. Power conditioning system
T h e power conditioning system has been designed by EPRI and
nanufactured by the General Electric Company. It is the interface
between the Edison grid and the battery. T h e power conditioning
system converts the 12 kV/60 Hz ac f r o m the grid t o 2000 V direct
current required for battery charge and vice versa when the battery
discharges electricity t o the Edison grid. Figure 37 shows a block
diagram of the power conditioning system and associated equipment.
T h e converter can function independently as a synchronous generator to maintain unity power factor. T h e converter is a seifcommutated, Gate-Turn-off voltage source, stepped-wave design
w i t h a response time of 16 milliseconds. It consists of three sixpulse three-phase converter bridges connected in series o n the ac side
76
X
utility BrE!!w
~
411
GTC
fOrmW
Fllter
cap
I
n
I
Breaker?
T
1 -
-
T
GTC
-3
Inverter
control
I
LI
I
1
1
5 battery
strings
-&k*
,
I I
Table 11. Major power conditioning system characteristics [32].
Power - real
reactive
Efficiency
Harmonic voltage
Ripple voltage
dc t o battery
Response
Voltage input range
- nominal
~
- max
- min
10 MW, charge/discharge
10 MVA, leading/lagging
97% one way
3% total, 1.5% any single frequency
1.5% R M S voltage
16 ms
2.112 VdC
2.860 Vdc
1.750 V d c
77
T h e dc entrance equipment consists of t w o 3000 A, 3000 V dc
high-speed breakers, dc no-load break battery string connectors,
fuses, and disconnect switches. The ac entrance equipment includes
12 kV vacuum switchgear, single-phase transformers, three-phase
neutral transformer, filter capacitors, surge arresters and ac protect i o n relays. T h e control system o f the converter is a microprocessorbased interface w i t h pre-programmed algorithms for charging and
discharging operations. T h e control system communicates w i t h the
facility monitoring and facility control system [32-341.
6.2.4. Facility m o n i t o r i n g and control system
The microprocessor-based control system provides complete facility supervisory control and data acquisition. It is pre-programmed
w i t h a typical load curve for automatic control of the battery discharge rate, but can be modified readily t o allow for different discharge patterns. Reactive power output is also programmed i n t o
the system. T h e data acquisition system records data o n facility
operations t o allow analysis of the performance and economics of
the system. Analog inputs are scanned at one-second intervals and
stored in the data acquisition system in predefined intervals of one,
ten or sixty seconds. A microcomputer is also connected t o the data
highway access f r o m a remote computer located at the Edison m a i n
office [32-341.
6.2.5. Equipment energy losses
T h e distribution of plant losses f r o m all sources for an average
cycle is presented in Fig. 38.
The energy losses in the battery represent about 10 MWh per
cycle or 61% of the t o t a l losses. T h e inefficiency of the Power Conditioning System accounts for 12% (2 MWh per cycle). Auxiliary
power consumption accounts for the remaining 27% (4 MWh per
78
Total Losses
L16.SMWh
1
Fig. 38. Distribution of equipment losses in Chino plant [31]
cycle). T h e t o t a l energy input, o n a daily basis, is 58.6 MWh and
the useful energy o u t p u t i s 42.1 MWh o n average. B a t t e r y systeni
efficiency is a function of operating parameters such as overcharge,
rate of discharge, etc. [31].
6.2.6. Economics of Chino LABES Plant
The t o t a l cost of the Chino LABES Plant i s about $ 13,560,000
or 1,350 $ kW-'. T h e distribution of these costs in various items i s
presented in Table 12.
Table 12. Chino LABES s y s t e m costs [35].
Item
10 MW lead-acid battery
Power conditioning s y s t e m
Balance of plant
Plant site and tie-ins
Engineering and management
Tax, escalation, and contingency
Cost, $ kW-'
600
190
325
52
76
103
Total: 1,356
79
I t is expected that w i t h mature technologies, the cost per unit kW
generated by subsequent plants of that kind will fall t o $ 600-700.
Chino battery energy storage system is operating at 75% overall
efficiency (ac-ac). T h i s storage system has been designed for demonstration purposes and has been initially intended for load-levelling
applications, as a large active store of energy. That is w h y it has
been designed for a h i g h Ah capacity, 40150 MWh at a power of
10 MW. Later on, other applications o f the system, requiring higher
power, have also been tested and demonstrated.
7 . LABESP for instantaneous (spinning) reserve and frequency control applications
7.1. Island networks
Small energy supply systems, also called island networks, have
serious problems w i t h the quality of the delivered energy and i t s cost.
Energy generating facilities willing t o reduce t h e price of the electric
energy must use large generator plants. A sudden outage o f such
a big generating unit, however, would cause great problems t o the
energy network operator. Besides, when a large industrial consumer
switches o n or off his load (which might comprise for example 5% of
the current system load), this would induce considerable frequency
deviations in the whole power supply system. That is why an island
network must maintain a significant amount of reserve power which
can b e activated at h i g h speed for load-frequency control.
Why frequency control? When a debalance occurs between the
generated power level and the load, the system frequency is changed.
Hence the quality of the delivered electric energy is determined by
the frequency deviations. T h e latter should n o t exceed f 0 . 2 Hz. For
Europe, 5 0 f 0 . 2 Hz.
80
7.2. The BEWAG 8.5/17 M W lead-acid battery energy
storage plant
7.2.1. System frequency response having given rise t o t h e construction of the BEWAG LABES plant
Figure 39 represents the system frequency response of a West
European grid system w i t h a load of about 150,000 MW.
In case of outage of the 1200 MW or 2500 MW units, the system
frequency decreases. In the latter case, a frequency deviation of
more than -0.2 Hz appears, and the system immediately activates
i t s spinning reserve. T h e generated power is increased and the
frequency is brought back w i t h i n t h e required limits.
-Outage of o 1200 MW unit
Outage 01 2500
Mw
P
F
BEWAG island system
O
Fig. 39.
5
10
K
t, s
25
S y s t e m frequency response after unit outages in different grid
systems [33].
That was n o t the case w i t h the West Berlin power supply system,
which was a typical example of an island network. At a n outage of
146 MW, frequency deviations reached up to IM H z . In order t o h o l d
the energy system frequency within the range of 5 0 f 0 . 2 Hz, i t was
81
n o t enough t o have considerable reserve power generators available,
but they also had t o respond very quickly t o the demand changes.
T h i s could b e achieved if the power gradient o f the generating systems
was higher t h a n 5-10 MW s-'. That meant that t h e power system
should comprise generating units with a response t i m e of the order
of milliseconds. Lead-acid batteries display such a response time.
With t h e purpose of improving the spinning reserve and frequency
control of i t s energy network, the Berliner Kraft und Licht Company
(BEWAG) built up a battery energy storage plant based o n highpower lead-acid batteries.
T h i s BES plant was first intended for demonstration purposes.
In 1981 it started operation as a 24 kW test facility, and after a
3-year test period during which encouraging positive results were
obtained, BEWAG decided in 1984 t o install a full-scale 8.5/17 MW
demonstration plant in the power station Steglitz in West Berlin.
Construction of the plant was completed for 18 months, and in 1986
actual operation started.
T h e BES plant was used t o replace a turbo-generator set. Power
of the BES plant was 3 3 . 5 MW against f 7 . 5 MW o f the turbogenerator. However, BESP power gradient (minimum 5 MW s-l)
was much higher than that of the turbo-generator (which could reach
maximiim 4.5 MW s-l).
7.2.2. System design and characteristics
A block diagram of the BEWAG BES plant is given in Fig. 40.
T h e battery energy storage system is connected t o a 30 kV electrical network. T h e plant has two identical six-pulse inverters w i t h
8.5 MW each. The second inverter path operates as reserve for the
first in the frequency control function, and provides additionally an
instant reserve of a t o t a l of 17 MW power [33]. In order t o obtain a
construction easy t o handle and maintain, six-pulse converter bridges
82
3 0 k V Steglitz Gr.B
8
Central
dispatching
room
Fig. 40. Block diagram of BEWAG 8.5/17 MW lead-acid battery energy
storage system [33].
are used w i t h o n l y one thyristor per string. Thyristors have maximum power (12-pulse unit) of 8.5 MW and a rated dc voltage of
1200 V. T h i s design meets the safety regulations well [36].
The battery i s designed t o exhibit a very l o w internal resistance
and a short response time, t o be able t o serve as instant power reserve.
I t must deliver, under the worst operating conditions, i t s maximum
power of 8.5 MW for a period of at least 30 minutes.
83
T h e battery consists of 12 parallel strings of 590 cells each or a
t o t a l number of 7080 cells. T h e battery comprises 1416 modules of 5
cells each housed in a common polypropylene container
- monoblock
module. T h e container and t h e polypropylene lid are heat sealed in
place. In order t o reduce the resistance o f the connectors, the cells
are connected in series in the monoblock through the cell partitions.
Low-antimony tubular positive plates are used. To improve the negative grid conductivity, it i s made of expanded lead-plated copper
mesh after Hagen’s technology. Air lift pumps are provided in each
cell t o stir the electrolyte and prevent it f r o m starving during operation. Refilling of the battery is accomplished automatically by a
common watering system. Each cell is provided w i t h a water-filling
valve. Since high currents flow through the cells and a considerable
amount of heat is released, each cell is equipped w i t h a heat exchanger which is accessible through the central cell vent. In some of
the cells, there are thermometers and specific density probes.
Figure 41 presents a two-day load-frequency response curve of the
BEWAG LABES plant.
O
5
x)
15 2 0 2 5 3 0 3 5 4 0 4 5
Time, h
F i g . 41. T y p i c a l o p e r a t i o n o f test facility, c o m p r i s i n g t w o days o f loadfrequency-control o p e r a t i o n w i t h a subsequent constant-current constantp o t e n t i a l charge
84
[il.
T h e battery i s permanently connected t o the system and monitors
load fluctuations. W h e n t h e frequency begins t o decrease, t h e b a t t e r y
discharges power t o t h e systeni, and vice versa, when t h e frequency
exceeds t h e upper limit, t h e battery charges. ‘ T h e fluctuations in
charge and discharge currents are given in t h e figure. These changes
affect t h e state-of-charge of t h e battery w h i c h is presented in Fig.
41a. T h e battery i s r a t e d in such a way that load fluctuations would
not cause i t s state-of-charge to fall below 50% of t h e noniinal capacity.
Normally, the b a t t e r y operates at 70% state-of-charge.
With t h e introduction of t h e BEWAG LABES plant, previous
frequency deviations of more than f 0 . 2 Hz have been halved owing
t o t h e excellent dynamic response of the lead-acid b a t t e r y energy
storage system. T h e frequency control function of t l i e b a t t e r y is
accomplished in a severe operating mode. On calculating t h e quantity of electricity passing through t h e cells during t h e intermittent
charge-discharge cycles, it tiirns out that t h e capacity turnover for a
24-hour period i s as high as three times t h e battery capacity [37].
T h e energy efficiency of t h e battery is about ô7%,, but nevertheless
there i s t h e tendency t o b a t t e r y overheating in warm weather w h i c h
necessitates restriction of t h e battery inaximum power [37].
T h e battery has also been used as an instantaneous reserve in
reference t o a steam storage system. A comparison of results has
shown clearly t h e superiority of t h e LABES system.
B a t t e r y maintenance comprises automatic fill 111’ of tlie cells
weekly. T h e LABES plant operates unmanned under remote ~ 0 x 1 trol from the power station control room.
85
8 . Lead-acid battery energy storage systems
for peak-shaving
8.1. W h a t i s peak-shaving?
At railway stations, there are usually one or t w o short intervals
during t h e day when several trains depart simultaneously. T h i s creates an extremely high peak power demand. Furthermore, in alloy
production plants employing electric furnaces t o melt t h e alloy components, high power needs may also occur for short periods of time.
Since alloy production is performed in batch mode, t h e above h i g h
energy demand usually appears only two or three times a day. There
are indeed many manufacturing technologies displaying similar power
demand curves w i t h several daily or weekly peaks, w i t h energy needs
fluctuating w i t h i n their normal l i m i t s during the rest of the time.
T h e electric utility customer pays t o the power supply company
two basic types of energy rates:
o for claimed peak demand
o for actually consumed energy.
Despite the short duration of peak load periods, the customer
must pay for the claimed peak power demand, because the latter is
related t o the size and t h e power of the generating plant, and the
capacity of the transniission and distribution lines. These, for their
part, are associated w i t h the amount of capital invested. Often peak
demand charges are close t o the charges paid for actually consumed
energy. In this case, a lead-acid battery energy storage system can
b e installed t o take over the supply of peak power, and hence the
customer w i l l claim only his baseload power needs t o the energy
utility company. T h e battery of this system, charged with electricity
from the utility, accumulates electric power which w i l l then be used
for shaving of peak loads. LABES facilities respond instantaneously
t o load fluctuations. That is why they are most appropriate for
86
tliesc purposcs. Besidcs, when installed in immediate p r o x i m i t y t o
the consunicr, power losses are minimized.
Figure 32 shows t h e peak-shaving profile for a commuter railroad
substation w i t h integrated LABES system
6OC
Commuter rai Iroad
peaking
&OC
200
z
C
O
6
a
- 20c
Charge
- coo
I
1
12
16
Clock lime. hours
I
I
I
O
L
0
.
1
N
Fig. 42. Peak-shaving profile f o r a c o m m u t e r railroad substation.
T h e LABES facility manages the peak loads occurring when
trains pass along the railroad segment served by the substation] and
is then charged t o replenish t h e amount of energy w i t h d r a w n irrespective of the t i m e of day.
As clearly shown by the data in Table 5, LABES systems are used
increasingly for peak-shaving applications throughout the world. The
test results of LABES facilities are so encouraging, and t h e interest
in t h e m so great, that commercial production o f such facilities can
b e expected t o begin in the near future.
87
8.2. Johnson Controls 300 kW/SOO kWh LABES
facility
Johnson Controls I n c . (USA) is a large producer of industrial
energy management and lead-acid battery systems. T h e company
has designed and const,ructed a 300 kW LABES facility based o n
valve-regulated lead-acid (VRLA) cells used for the first t i m e in this
application. T h e system was intended for peak-shaving and installed
at their Humboldt brass foundry for purely demonstration purposes.
T h e design and layout of the facility are presented in Fig. 43.
aL disconnect
22 feet
Fig. 43. Schematic of LABES facility [38].
T h e battery consists of 6 V/180 Ah building blocks. Fifteen
inoiiobloclts are connected in parallel t o give 6 V/2700 Ah modules. Lead-coated copper busbars are used t o connect the lead-coated
copper cell terminals. T h i s results in low battery resistance. Each
niodule is fitted w i t h an internal air manifold t o provide cooling or
heating. An energy management system monitors and controls the
state of each module. T h e battery i s assembled f r o m 64 modules
ensuring 384 V and 2700 Ah at &hour discharge [38,39]. Table 13
gives a summary o f the basic characteristics of the battery.
T a b l e 13. VRLA b a t t e r y at Johnson C o n t r o l s H u m b o l d t brass
f o u n d r y [38, 391.
B a t t e r y rating
2 h at 300 kW
Depth-of-discharge
60%
360 t o 320 V
460 V
Discharge range
End o f charge
GC6-1500D gel cells
192
M o d u l e s (64)
G V, 1500 Ah
4 x 1.5 HP
F o r c e d air cooling
T h e power conditioning systcm is of a diial-bridge six-pulse l i n e
commutated design. The system has a noniinal 384 V dc bus voltage
and a 480 V ac three-phase inpiit. It operates in a constant-power
mode under the management of the control systcni. I n i t i a l battery charge is performed at constant power, and a constant-volt,age
mode is applied for final charging. During peak-shaving (battery
discharge), the power converter operates in a controlled-power mode
under direction of the control system [38].
T h e static power converter i s housed in one cabinet w i t h talie
bridge and firing circuitry, power factor correction capacitors and
the harmonic indicator. The entire LABES facility is under the coilt r o l of a JCI DCC 8500 energy management systcni. Only monthly
inspections of t h e system are needed, because i t operates fully automatically [38].
89
Figure 44 presents the daily power profile with and without t h e
LABES facility.
200
O
I
I
I
O
I I
I
I
6
12
Real time. h
I
18
I
2
Wiih
IABES iocility
12
I8
24
R e a l lime. h
Fig. 44. Daily power consumption of t h e foundry without (a) and with (b)
t h e LABES facility [38].
The good peak-shaving capabilities of the LABES system are obvious. Normal foundry baseload is 1100 kW. At alloy melting, 4-5
discrete power peaks occur of about 1600 kW. The power utility
rate structure includes a monthly peak power demand charge. In-
90
stalling the LABES facility, the m o n t h l y peak demand was reduced
by 300 kW. T h e utility demand charge is about 8 $ kW-'. (In most
industrial regions in the USA, the rate is 10 $ kW-'). A few years
of service o n l y are enough for the facility to become profitable [40].
Of course, the assessment of the economical benefits o f t h i s system
can be performed based o n the peak power rates which differ for the
different countries and regions of the world.
8.3. Lead-acid battery energy storage systems in the
railway transport network
In San Diego (USA), an Exide 200 kW traction type gelledelectrolyte battery i s under construction in a light r a i l transit system
t o meet peak power demand during morning and evening commuter
r u s h hours. T h e battery is produced using Aat positive plates w i t h
low-antimony grids. I t w i l l operate in a 2-hour discharge mode at
80% DOD and w i l l have a capacity of 1620 Ah (8 h rate). T h i s
facility i s designed for purely demonstration purposes: t o determine
technical parameters and economical benefits. I f these prove good
enough, the remaining railway substations w i l l also be equipped w i t h
similar LABES facilities. T h e latter w i l l be installed o n the customer
side of the meter at the 600 V rectifier interface [27].
T h e public transportation system (PTS) in a given c i t y is always
in competition w i t h individual transportation. I t has been established t h a t the attractiveness of a PTS is affected by two m a j o r
system characteristics: travelling t i m e and frequency of trains run-
ning [41]. B o t h are acsociated w i t h high power demands. Figure 45
presents a typical train current-velocity-time diagram of a subway
car.
During acceleration periods, a h i g h power rate is needed. The
running train represents a storage system for kinetic energy. A considerable part of this kinetic energy can be recuperated.
91
O F
- 0.50
energy
Fig. 45. Current-velocity-time diagram [41].
T h e B e r l i n subway system (100 km) has a t o t a l power demand of
about 60 MW. Since 1982, BVG has been using t w i n cars w i t h onboard power converters t o transform the dc current t o ac energy and
feed three-phase asynchronous motors. On braking of the r a i l cars,
the ac energy is converted i n t o dc one and is fed back t o the network
again. It has been established that one rail car only regenerates
about 50% of the acceleration energy. Since there are many rail cars
o n the line, at a certain moment they may regenerate such a great
aniount of braking energy, which niay create very high voltage in the
conductors. To avoid this, much of the braking energy has t o be
damped in braking resistors.
I f however BES facilities are installed at the substations, they can
store about 20% of the braking energy, which could then be utilized
for meeting p a r t of the accelerating energy needs [42]. A model block
diagram of such a substation is presented in Fig. 46.
For o p t i m u m energy recuperation, German experts recommend
that storage should be as close as possible t o the energy source [35].
92
O.
Fig. 46. A m o d e l of t h e BES facility at t h e subway s u b s t a t i o n [42].
T h i s can b e achieved i f bat.tery storage facilities are dispersed to
a great number of stops and are connected t o t h e d c busbar line.
T h e use of braking energy recuperation may lead first' to reduction
of t h e unit size of t h e rectifier stations since t h e acceleration power
can partially b e supplied by battery storage systems. Secondly, BES
facilities serve also as emergency power sources. In t h e case of failure
of t h e public supply network, t h e dispersed LABES systems on t h e
dc busbar lines provide emergency power and trains can leave t h e
t u n n e l and stop at t h e n e x t subway station.
T h e b a t t e r y i s connected in parallel t o t h e rectifier and acts as
a voltage buffer. T h e share of current provided by t h e b a t t e r y depends on t h e ratio between t h e internal resistances of t h e rectifier and
t h e battery. Since battery resistance varies depending on t h e stateof-charge and t h e cycle life, sophisticated management and control
i s necessary t o guarantee reliable operation of t h e battery for this
specific electric transport application.
93
9.
Valve-regulated lead-acid batteries for
b a t t e r y energy storage systems
Valve-regulated lead-acid batteries (VRLA) are maintenancefree, which makes them especially suitable for battery energy storage. T h e y w i l l reduce significantly operation and maintenance costs
of BES systems, and thus improve their competitiveness w i t h other
energy storage options. On the other hand, they w i l l consolidate the
positions of lead-acid batteries in this business field as compared t o
other electrochemical power sources.
The first VRLA batteries were employed by Johnson Controls
I n c . in their 300 k W / 6 0 0 kWh BES facility. These batteries used
gelled electrolyte and displayed very good performance during their
three years of service. T h i s stimulated the wider application of these
batteries. At present, two types of VRLA batteries are under test,
batteries w i t h gelled electrolyte and those w i t h adsorbed glass mat.
Since convection o f the electrolyte in VRLA batteries is strongly
inhibited, and n o forced circulation is possible, two m a j o r problems
arise during battery operation which are inter-related:
a) Electrolyte immobilization creates, even though slowly, a nonhomogeneous and uncontrolled concentration distribution of electrolyte w i t h i n the active block. Since H2S04 is an active material, a
non-uniform distribution of reaction rates in this block occurs. T h i s
affects battery capacity and cycle life. In order t o reduce or totally
eliminate this adverse effect, a special charging mode has been introduced, and an equalizing long charge is carried o u t at the end of
each week.
b) T h e non-uniform reaction rate distribution and the immobilization of the electrolyte in the active block b r i n g about a thermal
non-homogeneity in the cells, and impede the dissipation o f heat
during high-power battery operation.
Possible solutions t o the above defects are sought in cell construction modifications. Smaller size and height o f the cells are recommended for high-current operating modes. Special requirements
are claimed t o the charging mode and the thermal management of
the battery.
In addition t o the irreversible processes typical for flooded lead-
acid cells (grid corrosion, active mass shedding, sulfatization, shortcircuits, etc.), another process also occurs in VRLA batteries, d r y i n g
o u t of the cell. What is more, this process can proceed first, especially
on valve failure or excessive over-charge, and limit battery life.
Assembling of VRLA cells in a battery i s often accomplished
by vertical stacking, which saves floor space and facilitates battery
thermal management blowing air i n t o the cells.
All these problems are in the process of being solved, but even at
this stage, VRLA batteries are preferred by a number of energy producers and consumers. In San Diego, an Exide traction type gelled
electrolyte battery has been installed in a light rail transit system.
At the Plasma Physics Laboratory in Princeton, a 15 MW/5 MWh
LABES plant is t o use VRLA batteries. Sonnenschein has requested
the introduction of i t s VRLA batteries in traction service. Highperformance Hagen VRLA batteries are in stand-by service. YUASA
describes i t s 3000 Ah VRLA battery for stationary service. The
Japanese Storage B a t t e r y Company is also very active in this field.
Rand and Baldsing [43] have reported encouraging performance results o f gelled-electrolyte cells employed for remote-area power suppiy duties. An ever-more pronounced tendency is observed in the
w o r l d battery industry for utilization of VRLA batteries for deep cycling in BES systems. T h i s would allow construction of unmanned
maintenance-free BES facilities managed by remote control systems.
95
10. Strategic advantages o f B E S systems
T h e implementation of BES systems in the power industry offers
the following m a j o r benefits and advantages:
Load-levelling (energy storage).
Considerable cost-effective
benefits are provided by utilizing low-cost energy f r o m the baseload
generating utilities for meeting expensive peak energy demands.
D y n a m i c power response. T h i s is due t o the high operating
flexibility and very short response t i m e provided by lead-acid battery plants. These plants are capable of supplying electric power for
frequency control and instantaneous power reserve, and can be used
for peak-shavirig as well. All these functions provide dynamic power
benefits resulting in improved quality of the energy delivered.
Strategic advantages. These are more general in nature and
have their impact o n environmental protection, efficiency of power
transmission and distribution networks, and economy as a whole.
These benefits cannot be directly measured but can be estimated
by mathematical calculation and modelling. T h e i r importance is
becoming increasingly evident [44]. Some o f these advantages w i l l be
discussed below.
It is general practice t o locate baseload power plants burning
coal or nuclear fuel away f r o m populated centers. In this way the
pollutants, dust, CO1, CO? and NO, emitted as a result o f the combustion process reach the populated areas in strongly reduced concentrations. I t is economically effective t o build peak power plants
w i t h oil/gas-fired combustion turbines w i t h i n the boundaries of populated centers. In this case, however, harmful emissions f r o m these
power plants w i l l cause considerable environmental pollution of the
neighboring towns. LABES plants themselves are clean and quiet.
The o n l y environmental impacts of these plants are related t o the
baseload generating units that supply the charging power. LABES
plants provide pronounced environmental benefits as compared t o
combustion turbine peak plants.
96
W h e n BES plants are installed at t h e utility distribution substation, or BES facilities are located in the vicinity o f the consumer,
transmission and distribution losses are significantly reduced and the
stability of the network is improved. As is known, energy losses are
proportional t o the current squared, and hence by decreasing and
equalizing the current, these losses will be reduced. Additionally, by
shaving t h e peak load transmitted, BESP allow deferral of capital
costs for the construction o f new transmission and distribution capacities. Finally, power stability in t h e transition and distribution
network is improved considerably owing t o the short response time
o f BEC plants. In general, greater benefits will be achieved if BES
plants are located close t o the loads, charging generators are more
distant f o r m t h e load than BES plants, the difference between the
on-peak and off-peak loads is high, and the transmission system is
heavily loaded [44]. BES plants or facilities are m u c h easier t o site at
any p o i n t in the populated centers than combustion turbine facilities.
T h e costs of o i l and gas fuels used by peak power generation
plants depend o n political, economical and other factors, a n d are very
unstable. When combustion turbines are replaced by BES plants this
instability of energy costs i s strongly reduced or eliminated, which i s
a strong financial benefit.
Another iniportant cost benefit is provided by the modular design
of BES plants. B a t t e r y units are comprised of small modules allowing
any load increase t o be met quickly and fully by installing additional
modules w i t h i n a very short time.
So far, only a small number of BES systems have been constructed
throughout the world and these are mostly in the demonstration
stage. T h e full range of benefits provided by BEC systems in the energetics have n o t yet been completely studied. But even at this i n i t i a l
stage of demonstration tests and experimental operation, BES systems have shown promising performance and w i l l obviously become
an important element of the modern power supply system.
97
Acknowledgements
T h e author would like t o acknowledge the kind assistance o f Dr.
W.C. Spindler, consultant t o ILZRO, USA, and Dr. S. Takahashi
from the Government Industrial Research Institute in Osaka, Japan,
in providing the necessary literature, and also for helpful discussions
o n the problems of battery energy storage systems.
References
1. W.C. Spindler, Why Lead-Acid Batteries for Energy Storage, International Lead Zinc Research Organization (ILZRO) Project LE-363,
1989.
2. EPRI
1989.
~
Energy Storage: How t h e New Options Stack up. Brochure,
3.
Japan Industrial Technology Association. The Moonlight Project DeNew
Technologies
for
Energy
Conservation.
veloping
Brochure. 1989.
4.
M. Tada, T. Sakamoto, T. Tanaka, H. Yamamoto, Y. Sera,
T. Nakayama, Outline of the New Facilities of Tatsumi Electric Energy Storage System Test Plant, 2nd Int. Conf. “Batteries for Utility
Energy Storage”, New Port Beach, California, 1989.
5.
M. Tada, E. Furune, T. Tanaka, T. Sakamoto, T. Oota, Y. Sera, T.
Nakayama, Monitor and Control System to the New Facilities of Tats u m i Electric Energy Storage System Test Plant, 2nd Int. Conf. “Batteries for Utility Energy Storage”, New Port Beach, California, 1989.
98
6.
J.L. Sudworth, T h e Sodium Sulfur Battery (J.L. Sudworth, A.R.
Tiller, eds.), Chapman and Hall, London, 1985, p.9.
7.
N. Itoh, T. Nakayama, T.Hiramatsu, S.Takahashi, R&D Evaluation
on Energy Storage Batteries for Power Systems in Japan, R e p o r t
NEDO-OS-8801, Government Industrial Research Institute, Osaka,
Japan.
8. S. Fucuta, T. Hirabayaski, K. Satoh, H. Satoh, Proc. 3rd Int. Conf.
“Batteries for Utility Energy Storage” NEDO, EPHI, BEWAG, Kobe,
Japan, 1991, p. 49.
9.
R.J. Bellows, P.G. Grimes, Zinc/Halogen Battery, in Power Sources
for Electric Vehicles (B.D. McNicol, D.A.J. Rand, eds.), Elsevier, Amsterdam, 1984, p. 621.
10. D.A.J. Rand, Overview of Performances of Candidate Battery Systems, in Power Sources for E l e c t r i c Vehicles (B.D. McNicol, D.A.J.
Rand, eds.), Elsevier, Amsterdam, 1984, p. 721.
11. P. Symons, C. Warde, P. Carr, Development of t h e Zinc/Chlorine Battery for Utility Applications, Part 1-4, EPRI EM-1051 (Project
226-3), 1979.
12. M. Barak, Electrochemical Power Sources, Peter Peregrinius, Stevenage, 1980, p. 251.
13. D. Pavlov, in Power Sources for Electric Vehicles (B.D. McNicol,
D.A.J. Rand, eds.), Elsevier, Amsterdam, 1984, p. 111.
14. P. Montalenti,
Report 6, 1982.
15. G. Smith,
1964, p. 252.
in Power
Sources 9
Storage Batteries,
2nd Ed.,
(J. Thomson,
ed.),
Pitman, London,
16. W.G. Sunu, B.W. Burrows, Power Sources 8 (J. Thomson, ed.), Academic Press, 1981, p. 601.
99
17. I<.
Takahashi, M. Shiomi, T. Funato, M. Tsubota, Proc. 3rd Int.
Conf. “Batteries for Utility Energy Storage” NEDO, EPRI, BEWAG,
Kobe, Japan, 1991, p. 187.
Ruetchi,
18. P.
4 (1966) 383.
J.B.
Ockermann,
Electrochem. Technol.,
19. D. Pavlov, T . Vitanov, I . Nikolov, V. Nikolova, G. Papazov,
K. Petrov, US Patent 4, 925, 746 (1989).
20.
K. Peters, A.I. Harrison, W.H. Durant,
Power Sources 2
(D.H.Collins, ed.), Pergamon Press, Oxford, 1968, p. 1.
21.
H. Tuphorn in Advances in Lead-Acid Ba tte ri e s (K.R. Bullock and
D.Pavlov, eds.), T h e Electrochem. Soc. Inc., Pennington, NJ, USA,
1984, vol. 84-14, p. 441.
22.
K.R. Bullock, B.K. Mahato, G.H. Brilmyer, G.L. Wierschem, ibid,
p. 451.
23.
B.IC Mahato, K.R. Bullock, Progress in Batteries and Solar Cells, 6
(1987) 136.
24.
“Energy
Denver,
cells”, Technical Leaflet
20 M3/77, Gates Energy Products,
USA, 1977.
25. R.F. Nelson, Proc. 4th Int. ILZRO Lead-Acid Battery Seminar, San
Francisco, CA, USA, 1990, p. 31.
26.
K. Okada, in Rechargeable Batteries in Japan (Y. Miyake and A.
Kozawa, eds.), JEC Press Inc., 1977, p. 291.
27. G.M. Cook, W.C. Spindler, J. Power Sources, 33 (1991) 145.
28.
D. Morris, EPRI Journal, March issue, 1988, p. 1.
29.
A.M. Chreitzberg, F.L. Tarantino, I.C. Baeringen, Design of a
10 MW/50 MWh Lead-Acid Load-Levelling Battery, Int. Conf. “Batteries for Utility Energy Storage”, Berlin, 1987.
30.
G.D. Rodriguez, W.C. Spindler, D.S. Carr,
Operating t h e
World’s Largest Lead-Acid Battery Energy Storage System, Proc. 4th
Int. ILZRO Lead-Acid Battery Seminar, San Francisco, CA, USA,
1990.
31. S. Eckroad, Proc. 3rd Int. Conf. Batteries for Utility Energy Storage
NEDO, EPRI, BEWAG, Kobe, Japan, 1991, p. 269.
32. G.D. Rodriguez, W.C. Spindler, Operating t h e World’s Largest LeadLong Beach
Acid Battery Energy Storage System, 4th ABC CSU
Conf., 1989.
~
33. H. Dominik, B. Voigt, Applications and Economics of Battery Energy
Storage Systems, Abstract 14, Lead Battery Power for t h e 90s Conf.,
Paris, 1988.
34. G.D. Rodriguez, Edison Embarks on Another F i r s t : The Chino 10
MW Battery Energy Storage Project, Research Newsletter, 16 (3)
(1987) 1.
35. D.S. Carr, Lead-Acid Battery in U S E l e c t r i c a l Load-Levelling Applications, Abstract 16, Lead Battery Power for the 90s Conf., Paris,
1988.
36. P. Berger, K.G. Kramer, W.Naser, R.Saupe, T h e BEWAG 8.5/17 MW
Battery Energy Storage Demonstration Plant, Int.. Corif. “Batteries
for Utility Energy Storage”, Berlin, 1987 (BEWAG-EPRI-NEDO).
37.
K.G. Kramer, BEWAG Plan Berlin’s New System, Batteries International, l(4) (1990) 32.
38. R. Hamann, R. Scarvaci, G. Brilmyer, ILZRO Lead-Acid Battery Seminar, Orlando, Florida, LEA, 1989.
38. G. H. Brilmyer, Batteries International, 1 (4) (1990) 36.
39. W.C. Spindler, Industrial Applications of Large Lead-Acid Batteries
for Emergency and Peak Power Demand. World Lead-Zinc-Tin Syniposium ’90, Anaheim, CA, 1990.
101
41.
K.G. Kramer, B.Voight, Proc. 3rd 1nt.Conf. Batteries for Utility
Energy Storage NEDO, EPRI, BEWAG, Kobe, Japan, 1991, p. 519.
42.
B. Voight, T. Mierke, H.J. Hinrichs, Batteries International, 1 (4)
(1990) 18.
43.
D.A.J. Rand, W.G.A. Baldsing, J. Power Sources,
23 (1988) 233.
44.
102
R.B. Shainker, Proc. 3rd Int. Conf. “Batteries for Utility Energy
Storage” (BEWAG-EPRI-NEDO), Kobe, Japan,
1991, p. 519.
Chapter 2
ENERGY STORAGE S Y S T E M S FOR
ELECTRIC VEHICLES
G. PAPAZOV
1. M o t o r vehicles and environmental pollution
O w i n g t o the continuous development of h u m a n society and rapid
technological progress, m a n k i n d is now faced w i t h a crucial problem
whose solution w i l l determine i t s future. T h i s problem is the environmental p o l l u t i o n caused by human activities. E r i c h Sauer
[il
illustrates the impact of industry and transport on the environment
in terms of harmful gas emissions measured in Germany in 1980
(Table 1).
Table 1. Harmful gas emissions in Germany
Type of
pollutant
C,H,
CO
NO2
Pb
so2
dust
Industry
min. taons
1.02
5.7
1.56
0.0032
4.04
0.478
%
56
44
68
52
98.5
95.5
[il
Transport
min. tons
0.8
7.3
1.4
0.003
0.06
0.022
%
44
56
32
48
Total
mln. tons
1.82
13.00
2.96
1.5
0.0062
4.1
4.5
0.5
A number of modern internal combustion engined (ICE) vehicles
use appropriate catalytic cartridges that improve fuel combustion, or
103
employ lead-free petrol fuels. Table 2 presents quantitative comparative data on t h e adverse emissions released p e r kilometer both with
and without catalysts [l].
T a b l e 2. Specific gas emissions of ICE vehicles
[il.
~
Type
of p o l l u t a n t
CnHm
CO
NOz
Pb
so2
dust
W i t h o u t catalyst
g km-'
With catalyst
g km-'
2.253
22.999
3.264
0.28
2.2
0.66
0.010
0.0
0.039
0.035
0.039
0.042
Despite the ten-fold decrease in the quantity o f some pollutants,
the amount of harmful exhaust gas emissions f r o m cars o n a worldwide basis i s measured in millions of tons.
T h e above tables d o not include data o n carbon dioxide which
i s t h e end-product o f fuel conibiistion. T h i s COL is accumulated in
t h e atmosphere contributing t o the so-called "greenhouse effect", i.e.
temperature increase.
T h e data presented in Table 2 are average values for the various
gas emissions. These emissions differ in magnitude at different hours,
b o t h daytime and nighttime (Fig. l ) , being largest during the day
w h e n urban traffic is most intense [l].
T h e exhaust gas emission depends also o n the travel speed o f the
ICE vehicle and this relationship is illustrated in Fig. 2 [ l ] .
Evolution of harmful gases (except for KOz) is reduced increasing
t h e travel speed. M o s t of t h e t i m e a car is used in urban conditions:
for driving t o w o r k and b x k home, for shopping, during leisure time,
etc. City driving means travelling at a low speed with frequent halts
104
Fig. 1. 24-hour dist,ribution of gas emissions [l].
Fig. 2. Dependence of gas emission o n vehicle speed [l]
and starts and low operational engine speed, which, according t o t h e
above-mentioned data, is associated w i t h a several-fold increase of the
harmful gas emissions in populated regions. In other words, environmental pollution caused by the transport i s n o t uniformly distributed
over the earth’s surface. I t is greatest in areas with considerable con-
105
centrations of human masses. Thus, the ecological problem related
t o the noxious gas emission f r o m transport vehicles is n o t so much a
problem of the environment, but rather a problem of people's health
and of the genetic stock of mankind. Bearing in mind that motor
vehicles release millions of tons of harmful gases daily, t h i s problem
acquires crucial importance and i t s solution becomes a question of
the survival of our civilization.
At the present moment, large-scale introduction of electric vehicles i s one of the basic ways of reducing the environmental impact of
noxious gases and noise. I f 7,000,000 ICE automobiles are replaced
by electric vehicles, p e t r o l consumption w i l l decrease by about 3.5%,
the amount of toxic gas emissions w i l l be reduced by 20-30%, and
the noise level in the cities w i l l decline significantly [2].
2. Specification o f energy storage systems for
electric vehicles
T h e t e r m electric vehicle indicates any rail-less, autonomous vehicle, independent of external systems, and driven by the energy
generated by an electrochemical power source.
We can regard as ancestor of the modern electric vehicle the transp o r t vehicle devised by the Americans Devenpator and Page in 1837
[3]. Literature data evidence the invention of another electric vehicle
by Robert Davidson f r o m Scotland in the same year [4]. In 1897, a
number of electric-driven taxi cabs were o n the road in London. A
record r u n n i n g speed of 63.3 km h-' was achieved by an electric car
in 1898, and only a year later, in France, a battery-driven car reached
the then unthinkable speed of 106 km h-l. T h e first Russian elect r i c vehicle appeared also in 1899. 6000 electric vehicles were built
in the USA in 1912. T h e basic characteristics, given in averaged
values, of the battery-driven cars produced in 1923 are as follows:
payload 200-250 kg, range between battery charges 50-80 km, speed
106
20-35 kin h-’, battery energy 10-18 kWh at a b a t t e r y weight of
about 750-780 kg [3].
After the 1920s, electric vehicles began t o lose their positions in
the competition w i t h ICE automobiles. In the mid-60s, however,
owing t o the Oil Crisis, interest in electric driven transport vehicles
was revived. In the course of t h e numerous development and demonstration progranis that followed, the electric car exhibited i t s basic
disadvantages associated niairily w i t h the chemical power source used
for energy generation. T h e reduced petrol costs at the beginning of
the 1980s once again caused interest in the electric vehicles t o decline,
and a number of developnient and improvement projects in this field
t o be interrupted. By t h e end of the decade, however, the attent i o n of automobile users and producers towards the electric-driven
car marked a certain rise again. Accounting for the fact that o i l resources are finite, environmental pollution is largely caused by the
exhaust emissions of internal combustion engines, and electric energy
generated by thermal, water and nuclear power stations cannot be
used directly for automobile propulsion, it becomes evident that the
future belongs t o electric vehicles driven by electrochemical power
sources.
What are motor vehicles utilized for o n a worldwide basis, and
how? According t o E. Sauer’s investigations [il,the average distance
run by a motor car varies w i t h mission. T h i s is illustrated in Table
3 which gives d a t a (measured for 1975) about the diverse-task range
distribution of automobiles.
T h e data show that, except for the holiday journeys, the average
range of a single travel is less than 20 km. 94.5% of the yearly
distance run is comprised of such short-range travels. T h e longrange journey (650 km) for holidays accounts for o n l y 5.5% o f the
t o t a l travelling distance during the year. K n o w i n g the frequency of
the various range travels, it turns o u t that 90% of al1 travels are at
a distance below 25 km. On the other hand, all above-mentioned
107
T a b l e 3. T r a v e l mission and range d i s t r i b u t i o n
Travel
mission
% of t h e y e a r l y
professional
educational
business
shopping
leisure time
holidays
23.9
3.4
24.9
11.3
31.0
5.5
distance range
[il.
Single t r a v e l
range, km
11.1
13.7
18.0
7.2
15.7
685.4
trips, w i t h the exception of the holiday journey, are accomplished in
urban conditions, i.e. at a limited speed of up t o 60 km h-'.
T h e above data refer t o Germany. For other countries, there
might be some specific features in the utilization of motor vehicles,
but these deviations f r o m the above average values are unlikely t o be
so significant as t o impose substantial changes o n the overall picture.
It can thus be assumed that a car performs t w o trips daily at an
average distance of 16.7 km each and an average speed of up t o
60 km h-l. T h e da.ily distance run is 33.4 km or 12,200 krn per year,
which may be considered as o p t i m u m utilization of the vehicle
What is the energy needed t o ensure this optimal usage of the
automobile?
Table 4 presents data o n the basic power characteristics o f some
types o f motor cars [1,5]. T h e energy consumption of a V W - G o l f
CitySTROMers electric vehicle for a range of 100 km is presented in
Table 5 [l].
For other types of electric vehicles, the energy demand
varies f r o m 30 t o 50 k W h / 1 0 0 km [3,6].
T h e first question is which type of power source is capable of
supplying this energy? A summary of the basic performance characteristics of various electrochemical power sources i s presented in
Table 6.
[il.
108
T a b l e 4. Power characteristics o f several a u t o m o b i l e types.
Characteristics
Peugeot 205
Automobile type
C i t r o e n C15
18.5
12.7
Maximum power, kW
R a t e d power, kW
VW-Golf
23.0
16.0
24.0
12.0
T a b l e 5. E n e r g y c o n s u m p t i o n p e r 100 km of t r a v e l l i n g range.
Speed, km h-'
Energy, kWh/100 km
70
27.6
50
21.5
c i t y driving
30.0
T a b l e 6. B a s i c performance characteristics of various types o f batteries.
Electrochemical
p o w e r source
Improved Pb/acid
Gelled P b / a c i d
Golf t r a c t i o n P b / a c i d
Tubular P b / a c i d
EV-5T
HED-88
Ni/Zn
Ni/Zn
Ni/Fe
Ni/Fe
Ni/Cd
Allair
Zn/Br
Na/S
Li/MoSz
Specific
energy
Wh kg-'
Specific
power
W kg-'
Cycle
life
References
37
26
30
105
95
96
176
318
323
161
161
161
34.3
37.7
55
70
40
55
52
106
72
175
60
108
> 850
> 100
161
161
I61
161
161
171
112
218
~
163
-
195
-
> 650
~
200
6.5
20
1200
-
-
[71
208
500
1111
~
250
181
191
[lo1
109
Figure 3 illustrates the basic power and energy parameters of
some o f t h e above-mentioned electrochemical power sources [3]. I t
can be seen that non-aqueous-electrolyte batteries ( l i t h i u m and
sodium/sulfur cells) show considerably higher energy performances
than aqueous-electrolyte power sources. As t o the latter, it is evident
that alkaline nickel cells have better energy capacity than lead-acid
storage batteries.
10
I
20
3 0 4 0 5 0 7 0 i o o
10
Specific pciwer, W kg-'
Fig. 3. Power and energy characteristics of several t y p e s of batteries.
T h e energy needed for electric vehicle propulsion depends o n the
car weight and the travelling range, and it can be determined from
the following equation:
110
(1)
E=KGL
where E is the energy delivered by the power source measured in
Wh, K i s the specific energy needed for transportation of 1 kg o f
weight at a distance of 1 km measured in Wh kg-' km-'. When
all side factors of travelling, e d friction, aerodynamic drag, etc.,
are taken i n t o consideration, t h e value of K i s estimated at 0.120.15 Wh kg-' km-' [3]. G is the weight of the vehicle in kg, and
L i s the distance range in km. Thus, for a 1600 kg electric vehicle
t o cover a distance of 100 km, 24 kWh of energy will b e required
supplied by an electrochemical power source.
The t o t a l power delivered by the storage b a t t e r y is utilized for
vehicle acceleration, and for overcoming friction and aerodynamic
drag. Besides, additional power i s also needed t o compensate for the
energy losses during conversion o f the electrical t o mechanical energy
(efficiency ca. 0.7), and also during transmission of this mechanical
energy t o the car wheels (efficiency ca. 0.95). W h e n operated in
urban driving conditions, the maximum power of an electric vehicle
is about 17-19 W kg-', and the average power at constant speed i s
about 12-13 W kg-' [5].
Let us assume that a 4-seat electric car needs an average power
of 20 kW and an energy of 30 kWh for covering a road distance
o f 100 km. Using the data presented in Table 6 we could calculate t h e weight of t h e various batteries that could supply t h e power
needed for this 100 kni range of the vehicle. The results obtained
are summarized in Table 7. I s is evident f r o m the d a t a in the table, t h a t battery weight cornprises a considerable p a r t (25 t o 50%)
of the t o t a l car weight. For comparison, i t can be pointed o u t that
the energy needed for covering t h e same travelling range of 100 krn
could be supplied by aboiit 3 k g of gasoline w i t h specific energy of
11,600 Wh kg-' [3].
111
Table 7. Calculated battery weight, needed for 100 km travelling range.
T y p e of power
Battery weight
kg
source
810
Improved Pb/acid
Gelled Pb/acid
1154
Golf traction Pb/acid
1000
Tubular Pb/acid
875
796
EV-5T
HED-88
Ni/Zn
545
Ni/Fe
750
Ni/Cd
577
Al/air
283
Zn/Br
417
Li/MoSz
500
T h e range of the electric vehicle, according t o eqn (l),
depends
o n the energy capacity of the power source, and the car weight.
K n o w i n g that the energy of the power source can be expressed by
the specific energy W (Wh kg-') and the battery weight M (kg),
the range of the electric vehicle w i t h o u t charging of the battery can
be determined f r o m eyn (2):
L,-
WM
KG
T h e range is directly proportional t o the specific energy and the
weight of the battery, and inversely dependent o n the car weight.
A graphic representation of this dependence for a 1200 kg electric
vehicle (excluding battery weight and including 320 kg payload) using
112
lead-acid batteries (W = 30 Wh kg-') of various b a t t e r y weight, is
given in Fig. 4.
3oom
700
900
il00
Battery weight, kg
Fig. 4. Dependence of electric vehicle r a n g e on b a t t e r y weight.
It i s evident that t h e greater the battery weight, the longer the
range between battery charges. With increasing the weight of t h e
battery, however, the t o t a l car weight is also increased, which, according t o eqn (2), w i l l shorten the travelling range. I t should also
be noted, here, that increasing the t o t a l car weight leads t o reducing
the relative share of the payload transported by the vehicle. Using
this formula, we can determine the maximum theoretical range of a
b a t t e r y driven electric vehicle. That is, the distance covered by the
vehicle when M = G,i.e. when only the battery i s being transported,
the remaining car weight being negligible. T h e obtained values for
the maximum theoretical range for various types of electrochemical power sources w i t h K = 0.15 Wh kg-' km-' are presented in
Table 8.
As evident f r o m the table, the m a x i m u m theoretical range of an
electric vehicle driven by various types of power sources varies w i t h i n
the l i m i t s 200-400 km, except .for the Al/air and Na/C batteries
which provide for longer ranges. The actual range, of course, is twice
113
Table 8. Calculated maximum range for various types of batteries.
Type of power
source
Improved Pb/acid
Gelled Pblacid
Golf traction Pb/acid
Tubular Pb/acid
EV-5T
Specific
energy
Maximum
range
Wh kg-'
km
37
26
30
247
173
200
34.3
37.7
55
70
Ni/Fe
55
Ni/Cd
52
Allair
106
Zn/Br
Na/S
Li/MoSz
72
175
229
251
367
467
267
367
347
707
480
1167
60
400
HED-88
Ni/Zn
Ni/Zn
Ni/Fe
40
t o four times smaller than t h e theoretical one, because the vehicle has
a considerable gross weight, o n the one hand, and should transport
maximum payload, o n the other hand. Hence, the problem of the
o p t i m u m ratio between the battery and vehicle weight arises. I f this
relation is determined o n the basis of the existing operational electric
vehicles, it w i l l show t h a t the power source weight accounts for 25-
40% of the gross car weight. In this case, the actual travelling range
of the vehicle is respectively 25 t o 40% o f the theoretical one.
When discussing the energy performance of electric vehicles, the
problem of o p t i m u m energy utilization should also be given consideration. Thus, for example, the efficiency of internal combustion en-
114
gines is about 25--30'% [ 3 ] . wliich means that from t l i c 11,600 Wli kg-'
specific energy of gasoline only about 3500 Wli kg-' arc u t i l i z e d for
vehicle propulsion. T h e operating efficiency of electric trwctiori systems is much higher rcaching up t,o 85% [ 3 ] . Beside tlicw iicgligiblc
energy losses, electric traction systems allow easy and snioot,li regulation and control of t h e travelling speed. Sonic dynamic parameters
of the electric vehicles are also w o r t h discussing hcrc, c.g. maximum
speed and acceleration rate (Table 9), that are determined by the
energy characteristics of the battery.
T a b l e 9. Some d y n a m i c parameters of electric vehicles.
Electric
vehicle
Peugeot 205
Citroen C15
VW-Golf
ErAZ-3732
VA Z -2801
M a x . speed
km h-'
T i m e for accclcrating
References
f r o m O t o 50 km lip',
c
100
80
100
60
-
11.6
11.6
13.0
151
-
[31
lLO*
131
Pl
I11
* (0-60 km h-I)
By their dynamic parameters, electric vehicles are considerably
inferior t o ICEVs. On the other hand, being designed for predominant c i t y driving, electric vehicles show adequate speed and acceleration performances t h a t meet the requirements set by the conditions
of operation. These facts are of utmost importance in assessing the
prospects for future EV development.
T h e second crucial problem t h a t needs adequate solution i s the
problem of battery charging. The fuel in the tank of an ICE vehicle supplies useful power for a travelling range o f about 500-600 krn
and refuelling at t h e petrol station takes only a couple of minutes.
115
Charging of the electric vehicle battery usually requires several hours,
and t l i c duration of charging is often twice or three times longer than
t h e discharge period, i.e. the t i n i e of utilization o f the car. T h i s results in a cyclic profile of operation o f the electric vehicle, the time of
rcst being much longcr than the driving time. It has been suggested
that, similar to petrol stations, battery charging stations should be
built along the motor roads, where discharged electric vehicle batteries would be l r f t for charging and replaced by ready-for-use charged
ones. This idea is hardly feasible, because, o n the one hand, it presupposes unification of all EV power sources used, i.e. employment
of t h e same type of battery by all electric vehicles, and, o n the other
liaiid, the nuniber of batteries needed w i l l be at least three times the
iiumber of electric vehicles o n the road.
On the basis of the &ta given in Table 3, the average daily
range [l],
the distance covered without battery charge, and the cyclic
mode of EV utilization, it can be concluded that electric vehicles can
meet all travelling requirements, except the long range of the holiday
travel. The elcctric vehicle is used for the purposes outlined in Table 3
usually during the day, so charging of i t s battery could be performed
at night, when the car i s at rest. As regards l o n g distance trips,
public transport such as trains, buses or airplanes, could be used in
these cases. T h e problem of the restricted freedom and m o b i l i t y asso-
ciated w i t h the use of public transport could be easily solved first by
employing petrol-engined automobiles for long-distance travel, and
later by combining the use o f public transport for long-range trips
and rent-a-car electric vehicles for local transportation.
T h e third problem w i t h electric vehicle efficiency is related t o the
life of the battery used for propulsion. I f the day-night cycle mode
of the electric vehicle is assumed, then the b a t t e r y should endure
365 cycles per year. T h e data f r o m Table 6 show that only Ni/Cd,
116
Ni/Fe and tubular Pb/acid batteries can ensure 2-3 years of vehicle
operation without battery change.
And last but not least in importance i s t h e problem of battery
cost. There are two aspects of this problem. T h e first refers t o
t h e cost of t h e battery as related t o t h e cost of t h e electric vehicle
without battery. T h u s for example, t h e ,4g/zi1 battery has a liigli
specific energy (120 Wh kg-') ensuring a travelling range of 300 krri,
but i t s price would b e about 100,000 roubles, while t l i c vehicle's price
without battery would be about 10,000 roubles [3]. That is the I(~asoii
why t h e Ag/Zn battery, in spite of i t s liigli energy perforrriaiice, i s
excluded f r o m t h e list of candida.tes for EV powcr sources. T h e
second aspect of t h e bat tcry cost proi>leiii i s rclatcd to t,lic total
lifetime energy cost, of t l i c battery, in other words t,lic> cricrgy cost 1)cr
1Wh or per 1 lim. Unfortunately, it i s \-cry difficult to nialie a precise
energy cost assessment since a great part of t l i c elcctroclieniical p o w e r
sources are still in t h e prototype phase of devclopmciit a i i c i have no
specified market price. On t h e other hand, prices of coiiiiiiercially
available batteries vary coiisiderahly for thc various coiiiitrics. T h i s
i s well illustrated by t l i c data presentccl in Talilcs 10 and 11 bc~low,
giving the costs per 1 kWh of energy arid 1 kni of range for severai
types of electrochemical powcr sources.
T a b l e 10. Specific energy costs for sornc t y p e s of b a t t e r i e s .
Type of
battery
c o s t , 11s $
per 1 kW1i
per 1 lai
Rcfcrciicrs
Pb/acid
Ni/Cd
0.387'
0.116
Pl
0.263*
Molicel
1.220*
0.079
0.366
[ill
[SI
* Data calculated for energy c o n s i i m p t i o i i of 0.30 k W 1 i km-'.
llÏ
T h e costs (in roubles) per 1 kWh of energy delivered by batteries
produced in t h e USSR are given in Table 11 [3].
T a b l e 11. Specific energy costs of some R u s s i a n batteries.
Type
Pb/acid
Ni/Cd
Ni/Fe
N i /Zi i
Ni/Hi
Pb02/Hz
Zn/Br
Na/S
C o s t p e r 1 kWh,
roubles
Versus
P b / a c i d cost
O. 02-0.04
1.o
0.2-0.4
0.04-0.06
10.0
0.1-0.2
2.0
5.0
0. 1550.30
0.02-0.04
1.o
o. 1
o. 1
5.0
5.0
In spite of t h e incomplete and sometimes contradictory data available, it i s generally accepted that lead-acid and t o some extent Ni/Fe
batteries are m o s t cost-effective for electric vehicle propulsion.
Though there is a certain bias for one or t h e other t y p e of power
source among experts in the field, t h e most appropriate battery t y p e
for electric vehicle application has not been found yet. Despite t h e
numerous design improvements, lead-acid batteries s t i l l remain heavy
systems with ratlier low energy performance; alkaline batteries are
ratlier expensive; N i / F e batteries need improvement of cell cooling
(by electrolyte circulation, for example); Ni/Zn batteries have very
short cycle life; there are a number of unsolved design and technological problenis with high-temperature cells (Na/S); fuel-cells employing platiiiuni metals as catalysts are characterized by high weight
and large volume, short cycle life and complicated operation; Zn/Br
cells require a coniplex system for operation monitoring and control,
and morcovcr bromine is toxic and hence special safety measures
are needed; the exotic metal/hydrogen batteries face t h e problems
of self-discharge and safety [3]. T h i s short overview o f deficiencies
of the various types of electrochemical power sources does n o t mean
that investigators have given up the idea o f finding at least t h e best,
if n o t the ideal, suitable power source for electric vehicle propulsion.
Research activities are being carried o u t with increasing intensity, every research team having i t s own favourite t y p e o f battery. Thus, for
example, investigations in France are concentrated o n Ni/Cd, Ni/Zn
and Ni/Fe cells; Japanese researchers pay greater attention t o Na/S
and Zn/Br; R&D efforts in the USA are devoted m a i n l y t o fuel cells,
A l / a i r and Na/S batteries; lithium batteries are the basic subject
o f investigations in Canada; the basic target o f R&D activities in
Germany and England are Na/S batteries, etc. There is one comm o n feature uniting all research centers, however, and that i s t h a t
all of t h e m show a marked preference t o t h e lead-acid battery as
their favourite electrochemical power source or as a reference stan-
dard. China [12] and I n d i a [13] have also initiated programs for the
development o f lead-acid battery driven electric vehicles.
Consultants f r o m the US Department of Energy [14] have evaluated 24 types of batteries
lead-acid; alkaline; ambient-temperature
Li; flow and metal/air; high-temperature batteries, etc. T h e assessment was based o n designed power packs for an Improved Dual~
Shaft Electric Propulsion (IDSEP) Van. The technical requirements of the battery were as follows:
o max. weight
o max. volume
min. power
min. energy
o min. system voltage
o min. life
0
0
700 kg
600 1
55 1iW
21 1;Wh
120 V
500 cycles
119
T h e criteria for battery evaluation included:
O
cost
o performance - energy vus. power, power 'us.
0
DOD
cycle life
o safety
~
effects of normal and abnormal battery operation on
passengers and public safety
o abuse resistance
effects of overcharge, overdischarge, singlecell failure, shock and vibration
o dc-dc efficiency
~
0 environmental impact - issues in battery manufacture, operat i o n and control
o use of critical resources - use of i m p o r t makerials
0 user issues - type, frequency, and complexity of routine maint eriance
The results f r o m the performed analysis are illustrated in Fig. 5
showing the suitability/techiiical risk relationship for 12 battery
types applicable t o the IDSEP van [14].
As suitable "low-risk? battery types are assessed technologically mature power sources in iarge-scale production. Examples
are the Ni/Fc batteries produced by Eagle-Picher Inc.; sealed leadacid batteries (Electrosource Inc., Concorde) and Na/S batteries
(BBC/Powerplex Technologies Inc., Chloride Silent Power Limited).
T h e group of "medium-risk" technologies include Zn/Br and flowthrough lead-acid batteries developed by Johnson Controls Inc.,
Li/FeS technology pursued by G o u l d I n c . and Electrofuel M a n ufacturing Co., Fe/air of Westinghouse Electric Corporation, and
Na/metal chloride systems developed j o i n t l y by Beta K&D Ltd. and
Harwcll Laboratorics.
T h e highest technical r i s k i s associated w i t h prototype batteries
developed at universities and/or national laboratories, but s t i l l lacki20
Lorv risk
I Medium risk I High risk
Technical risk
Fig. 5. Risk/suitahiiity relationship for battery technologies [i-i].
i n g a mature manufacturing technology. Examples of such battery
technologies are the Li/FeSz battery developed by the Argonne National Laboratory, Lawrence Berkeley Laboratory’s Zn/air project,
and the bipolar lead-acid batteries designed by Ensi Inc. and the Jet
Propulsion Laboratory.
T h e general conclusion from this assessment is that there is n o
ideal battery technology for electric vehicle applications, and further
research activities should be directed mainly at iniprovernent o f the
above-mentioned battery types.
Other problems concerning electric vehicle utilization are general
issues relevant for all types o f automobiles, i.e. t h e issue of energy
(fuel) economy, which can be reduced to considerable decrease in car
weight and improvement of i t s aerodynamic shape. T h e problem
of ICE t o electric vehicle conversion should also b e mentioned here.
T h e simple conversion of a petrol-driven car t o an electric vehicle
using the same basic construction would give a transport vehicle
121
w i t h poor user performance. Thus, for example, a 4 t o 5 seat car
w i t h high speed and practically u n l i m i t e d range would be converted
i n t o a two-seat electric vehicle w i t h l i m i t e d driving speed and l i m i t e d
range. It has been suggested [3] t h a t the design o f new constructions
for electric vehicles is imperative. This, however, would slow down
and complicate the process of transition f r o m ICE t o battery driven
vehicles.
3. C h a r g e and capacity o f batteries for elect r i c vehicles
Charging of batteries is accomplished by l e t t i n g electric current
f r o m an external power source flow t o each battery cell. T h e charge
current should guarantee complete charging of the individual battery
plates, equal state-of-charge of all battery cells or modules, and adequate gas evolution at the end of charge ensuring proper stirring of
the electrolyte.
T h e charge current is the most i m p o r t a n t parameter in the process o f battery charging. It is well k n o w n that i f lead-acid batteries
are charged at a h i g h rate ( w i t h a high charge current), their cycle
life is shortened. At the battery Technology Center of the M e l l o n I n stitute, USA [15], the effect o f charging current o n the performance
parameters o f the Golf-car batteries has been investigated. T h e results obtained are presented in Fig. 6.
W h e n high-rate charge is performed (50 A), battery capacity increases rapidly during the first 20 cycles, but the service life of these
batteries is t h e shortest. The end of battery cycle life in this case
is determined by corrosion of the positive battery plates. At very
low charge currents (12 A), the battery does n o t reach i t s maximum
capacity and has a short cycle life owing t o sulfatization of the plates.
It turned o u t that t h e o p t i m u m charging current ensuring maximum
cycle life of the battery is the current corresponding t o the 5-hour
122
O
M
loo
150
Life, cycles
200
no
Fig. 6. Dependence of cycle life on c h a r g i n g r a t e (151
charge cycle. In this case, shedding o f the positive active mass is the
life limiting factor.
A p p l y i n g constant-current charging modes does n o t seem t o be
the most appropriate method for charging of batteries. Battery
charge should be performed at maximum charge acceptance, avoiding unacceptable temperature rise and minimizing gas evolution. For
electric vehicle applications, particularly, the following t w o charging
modes are recommended [4]:
a) Controlled I / V charging method. In this method, charging
is carried o u t under constant-current conditions during the efficient
stage, followed by a cross-over t o constant-voltage conditions when
a defined voltage is reached (Fig. 7).
123
$:rq-J
Current
e
a
o
o
1
2
3
Hours
I
1
I
I
2
3
Hours
4
5
u 2.4
= 2.2
2.01
O
I
4
I
5
Fig. 7 . B a t te r y charge u n d e r controlled I / V c o n d i t i o n s [4]
The value of the cell voltage i s selected t o give slight gas evolution
and t o enable the battery t o be charged w i t h i n a short time period.
b) Tapered charging method. In this method, the ma'timum
charging current is l i m i t e d by the cell voltage (Fig. 8).
A linear relationship is used provided t h e following conditions
are satisfied: (a) the charging unit must be able t o provide the
maximum current at a c e l l voltage 2.1 V, and to reduce this t o a
defined value on reaching 2.6 V per cell at the end of charge; and
(b) at a voltage of 2.5 V, t h e current must n o t exceed 8.33% Cg.
Under these conditions, the rate of gas evolution w i l l n o t surpass the
permissible limit, and n o damage w i l l be caused t o the battery.
Batteries for electric vehicle propulsion consist of a large number
of battery cells or modules connected in series. Each cell or mod-
ule has i t s o w n internal impedance different f r o m that of other cells
124
7.0
k
5
10
o
I
Chorger oulpul
Amperes per 1OOAh CCIpOC¡ly
2.0
z
-20.:
x
-15
-10
-5
2 .o
O
Fig. 8.
I
I
7
4
I
6
Hours
I
8
I
u
f
8
:
s
$
s
0 5
1 0 1 7
(a) híaximum current at the output of t h e taper charger us. c e l l
voltage; (b) Cell voltage and charging current 71s. t i m e [A].
and modules, which determines the nori-uriifurm distribution of the
charge voltage among the cells. This i s of n o importance at the beginning of charge, but near t h e end of charge, some of the cells will
reach t,he water decomposition voltage more quickly and hence gas
evolution will start in them, while the process of charging will continue in the remaining cells. In this case, a sniall increase in voltage
will cause a considerable increase in gassing and, accordingly, t h e energy consumption will grow significantly. Scientists at the University
of Alabama at Huntsville [15] have devised an average voltage based
control system. A microprocessor measures the voltage differences
between the individual cells (modules) and controls t h e charge current so as to prevent reaching the gas evolution voltage in any of
125
the cells. In this way a better balance of charge is achieved between
the cells and hence better performance characteristics of the battery
are yielded. T h i s voltage control allows reduction o f the energy consumption and this in turn reduces the cost per km distance range of
the electric vehicle.
F r o m the above, i t becomes clear that charging of batteries for
electric vehicles cannot be carried o u t w i t h ordinary charging devices.
M o d e r n chargers should be programmable, w i t h built-in m i c r o p r e
cessors and various sensors t o allow o p t i m u m charging depending
o n the instant state o f the battery. T h i s is especially important
for maintenance-free batteries. The charger should possibly be i n stalled in the electric vehicle itself, which would allow recharging of
the battery at any point of i t s route, provided there is power supply
available. T h i s battery charger is n o t t o o heavy, about 20 kg against
1600 kg vehicle weight [l],
and would n o t degrade i t s performance
parameters.
One way of shortening the charging t i m e may be the so-called
fast battery-to-battery charge [16]. In this case, a stationary battery
pack, which has been previously recharged at a l o w rate, is used as
the source of electrical energy for a rapid charge of the vehicle battery.
Charging takes about 20 min when the source battery has a voltage
20% higher than the rated value. For this type of fast battery-tobattery charge, 25530% more energy is needed than for the normal
fast battery charge.
In the West German M a n n B u s program [15], the so-called bibberonnage charging cycles have been demonstrated. W h i l e waiting
at the bus stop for passengers t o get on, a quick high-current charging
pulse is passed t o the battery of the electric driven bus. In this way,
partial charging of the battery is achieved, and i t s distance range is
extended. According t o the test results obtained, the batteries of the
electric bus have performed well even after a hundred thousand bibberonnage charging cycles. T h e effect o f these strong current pulses
o n battery life has n o t yet been fully elucidated.
126
Bozek et al. [17] have determined the effect of pulsed discharge
o n the specific energy and power of a battery (Fig. 9). It was found
that pulsed discharge l i m i t s the energy provided by the battery at
l o w specific power.
Ï
m
40
ir
I
L
Ha
U
._
._
c
5
rl)
20
IO
specific e n e r g y , Wh kg-’
Fig. 9. Energy and power delivered by a lead-acid battery during pulseand constant-current discharge [4].
Analyses of the performance of lead-acid batteries as energy
source for electric vehicles have shown that the primary factor responsible for t h e decline of battery parameters and for t h e end of
b a t t e r y service life are the heavy, though short-term, peak power
loads it is subjected to during high acceleration driving (on starting
and overtaking). It is considered t h a t charging methods, duration
of rest periods, etc., do not have such an impact o n battery cycle
life
[is].
There are contradictory data about the effect of temperature on
battery cycle life [19, 201. i t i s generally accepted however that bat-
127
k r y failure depends on t h e temperature conditions of service. So
b a t t e r y failure at l o w temperature operation i s due both to irreversible sulfatization of a few positive plates and t o morphological
changes in t h e positive active mass. At high temperature, t h e failu r e i s associated with negative plate deterioration and positive grid
corrosion [ X I ] .
A s t o the decline in negative plate capacity during cycling, it
i s assumed that t h e basic reason for t h i s is passivation of t h e plate
[21]. Continuous refinement of t h e grain structure of t h e PbS04
cryst,als with cycling, worsening of the contact between t h e particles
of t h e negative active mass, and impeded electrolyte diffusion are
also factors causing the plate capacity to decrease.
4.Types o f cycles o f electric vehicle batteries
T h e energy performance of an electrochemical power source depends on a number of factors among which discharge current, temperature and riiimber of charge-discharge cycles are of primary importaiice. Figure 10 shows t h e capacity curve of a lead-acid battery
as a function of discharge current and temperature
[il.
It i s evident f r o m t h e figure that t h e higher t h e discharge current
and t h e lower t h e t,emperature, t h e smaller t h e amount, of energy
delivered by t h e battery.
In electric vehicle applications, batteries operate under complex
temperature conditions. Owing to the high discharge and charge currents, considerable amounts o f Joule heat are released. Conventional
b a t t e r y constructions, as a rule, hamper emission of t h i s heat i n t o
t h e atmosphere, and hence t h e temperature of t h e cells may rise t o
unacceptable levels. On the other hand, low b a t t e r y temperatures
lead t o a sharp decrease in energy performance.
T h i s calls for t h e need for an adequate heating system that would
provide p r e h e a t i n g of t h e battery a,t low surrounding temperatures,
128
C
Fig. 10. Dependence of b a t t e r y c a p a c i t y o11 discharge c u r r e n t and ternperature
[il.
and cooling of t h e cells t o prevent overheating as well as t o allow
utilization o f t h e released heat (during charge and discharge) for
warming of t h e car interior during winter. A fairly sophisticated syst e m of t h i s kind has been dcveloped for t h e VW-Golf CityCTROMers
electric car
[il.
T h e dynamic characteristics of t h e electric vehicle are determined
by t h e power performance of t h e battery used for propulsion. Power
output depends on t h e state of discharge of t h e battery arid usually
degrades rapidly near t h e end of battery cycle life.
Figure 11 illustrates t h e dependence o f t h e battery specific power
on t h e d e p t h of discharge (DOD) [22].
As seen in t h e figure, an abrupt decline in b a t t e r y specific power
i s observed beyond 50% DOD.
T h i s complex multi-factor dependence of b a t t e r y energy and
power performance on t h e one hand, and t h e diverse road and c i t y
driving conditions of t h e electric vehicle on t h e other hand, make
129
M10
O
I
I
I
I
0 2 0 4 0 6 0 8 0 1 0 0
Depth of discharge, */.
F i g . 11. Specific power vs. d e p t h of discharge
characterization o f the system electric vehicle/battery extremely difficult.
Worldwide there is n o unified standard for testing o f batteries for
electric vehicle applications. A wide spectrum o f test profiles are in
use, from simple constant current discharge, through velocity profile,
up t o the fairly complicated real-world driving profiles. Some of these
test procedures are aimed at characterizing the battery as a power
source, arid other tests attempt t o determine battery behaviour under
conditions as close as possible t o real traffic. T h e former test profiles, oriented t o specifying battery parameters] are similar t o traction
battery test procedures and hence will n o t be discussed here.
As t o the second category of test profiles, targeted at determining
the battery behaviour during EV operation] four testing standards
w i l l be described here - the US standard SAE J 227a1 C [3,23],
the European test cycle ECE [1,23], the Simplified Federal Urban
Driving Schedule (SFUDS 79) [24], and t h e Electric Vehicle Battery
Test Cycle (EVBTC) [23].
130
T h e first t w o test procedures, SAE J 227a, C (Fig. 12) and ECE
(Fig. 13) have been developed o n the basis of the so-called velocity
profile.
Time, s
Fig. 12. SAE J 227a, C v e l o c i t y p r o f i l e [3].
Time. s
Fig. 13. ECE v e l o c i t y p r o f i l e
[il.
T h e above test procedure is based o n the dynamics of electric vehicle velocity including starting, accelerating up t o a definite speed,
cruise-driving at t h i s speed for a certain period of time, decelerat-
ing, at a definite rate, t o zero velocity, and standstill period. In
131
SAE J 227a, C, one such test cycle is adopted with acceleration
rate of 0.74 m s - ~ , speed of 50 km h-', deceleration rate at braking 1.23 m s - ~and overall duration of the test cycle 80 s [3]. T h e
ECE test standard envisages three test cycles w i t h more complex
variations of speed and acceieration. For the sake o f simplicity, only
t h e maximum speeds of the three test cycles w i l l b e mentioned here.
These are resp. 15, 32 and 50 km h-' [23].
Test procedures based o n velocity profiles are relatively simple
and easy t o implement, because they are reduced t o driving the vehicle at a definite speed. These tests can be regarded as oriented t o
t h e vehicle as a system in m o t i o n rather than t o the system vehicle/battery. On the basis of t h e results obtained f r o m these tests,
it is very difficult t o compare various types of electrochemical power
sources for EV applications, especially so i f the tests are performed
A summary of the specific power characteristics
determined v i a these tests is presented in Fig. 14 for SAE J 227a, C,
and Fig. 15 for ECE [23].
on different cars.
O
Time.s
Fig. 14. SAE J 227a, C battery power profile [23]
132
3
Tlme. s
Fig. 15. ECE battery power profile [23].
It can be seen that these characteristics are rather complex, especially in Fig. 15, which makes interpretation and analysis of results
w i t h respect t o battery performance very difficult.
W h e n in operation, t h e electric vehicle requires a definite power
level irrespective o f the instantaneous state of the power source. B a t tery power can b e determined by measuring t h e current at constant
battery voltage. Voltage Characteristics, however, depend o n a number of factors, e.g. battery type, depth of discharge, number of cycles,
temperature, etc., and hence, current profiles cannot characterize
fully the dynamic performance of the electric vehicle. Thus, for example, at constant power battery discharge, near the end of discharge
when the voltage is low, the discharge current grows considerably,
i.e. the battery is subjected t o more severe operating conditions as
compared t o those at constant-current discharge. Under these more
severe conditions, the battery capacity is reduced and hence also
i t s energy output. However, these conditions are closer t o the real
driving situation.
133
An example of a power profile test of an electric vehicle i s the
Simplified Federal Urban Driving Schedule (SFUDS 79) illustrated
in Fig. 16 [23,24]. Figure 17 gives the driving speed according t o
SFUDS 79.
-780
rn
2
? 60
[
40
< 20
U
& O
- 20
o
60
120
180uom360
Time, s
Fig. 16. SFUDS 79 battery power p r o f i l e [24].
lime, s
Fig. 17. SFUDS 79 velocity p r o f i l e [24].
In most general terms, t h e above test profile consists of five power
pulses distributed in time so as t o ensure four starts and four stops
of the vehicle. Overall duration of the cycle is 360 s and the distance
covered i s 3100 m.
134
All the above outlined test procedures (Figs. 14, 15 and 16) include the so-called regenerative braking effect consisting essentially
in the ability of the electric motor t o act as a generator of electricity o n vehicle braking (movement w i t h negative acceleration), and
the utilization of t h e generated energy for charging of the battery.
In the above-mentioned figures, the energy used for battery charge
is indicated w i t h a negative sign. Including regenerative braking in
the test cycle w i l l increase battery capacity or, t o b e more precise,
recover part of the energy delivered by the battery. On the other
hand, however, strict standardization o f this t y p e of charge is n o t
possible, because the features of regenerative braking differ considerably for the different types of vehicles. For these reasons, regenerative
braking should b e excluded f r o m EV test schedules [23], but it m a y
and should be utilized in actual vehicle operation, since electric regeneration increases the mileage of the electric vehicle between t w o
full charges of the battery w i t h 18-25% [24]. W h e n regenerative
braking is used, however, the charging pulses should be restricted t o
current values that allow efficient charging w i t h o u t posing a threat
t o battery life.
Urban driving of an electric vehicle can be characterized w i t h
respect t o energy demands as follows: o n starting, when rapid acceleration should be achieved, t h e power demand is very high; then a
period of constant speed cruise follows and the respective power demand is low; this speed is then maintained until the next stop. Based
o n this simplified driving profile representation, t h e Electric Vehicle
B a t t e r y Test Cycle (EVBTC) has been developed [23], presented in
Fig. 18.
T h i s test cycle comprises two power pulse steps w i t h o u t regenerative braking. Overall duration of t h e cycle i s 100 seconds. T h e
spectrum of speed characteristics covered by the EVBTC is presented
in Fig. 19. It is seen that this test procedure is performed w i t h i n a
fairly wide bandwidth of speeds including starting and braking, as
well as driving at varying speed w i t h o u t stopping.
135
Time, s
Fig. 18. EVBTC battery power p r o f i l e (231
Tim. s
Fig. 19. EVBTC vehicle speed characteristics [23].
T h e degree of compliance between the above test cycles and
the real urban driving conditions has been examined using a CityS T R O M e r (VW-Golf) electric vehicle as a reference [23]. T h e results
obtained are presented in Fig. 20 and Table 12.
136
-
Urban driving (recorded)
S E J n?a. C
_..,.........
SFUDS
_____ECE
--- EVBTC
O
20
40
60
Cumulative time, ' / e
Fig. 20. Specific battery power
8s.
1 I
cumulative time for various test profiles
in comparison t o urban driving [23].
Table 12. Characteristics of various t,est cycles 1231.
Test
Urban driving,
recorded
SAE J 227a, C
ECE
SFUDS 79
EVBTC
Average
power
W kg-'
Required
max. power
W kg-'
Standstill
period
Degree of
compliance
%
%
15.5
53
60
30
53
43
44
79
50
42
30
100
90
85
85
95
11.1
9.0
11.8
15.6
137
T h e above comparative results show that, in all characteristics,
the EVBTC test procedure reflects real EV operation in urban driving conditions w i t h a highest degree of accuracy. A m a j o r advantage
of EVBTC is t h a t it yields reproducible and comparable informat i o n o n energy utilization and lifetime, irrespective o f the type and
size of battery system used. The fairly complex and diverse conditions o f real EV operation require constant battery monitoring and
control. The latter should perform the following functions: provide
information about t h e state-of-charge, respectively o f discharge, of
the battery and about the remaining driving range until full battery
discharge; indicate near end of discharge; signal and prevent battery
f r o m operation under abnormal conditions; prevent overpolarization
of individual cells; indicate excessive temperature rise; ensure proper
and complete battery charging; monitor gas evolution; prevent dangerous pressure rise in the cells, etc. These complex monitoring and
control functions cannot be accomplished only by voltage, current
and temperature measurements. A number of sensors are needed as
well as a microprocessor t o collect and process the acquired information, compare the obtained results w i t h the normal EV operation
data, signal or interrupt vehicle operation at noted discrepancy between the specified and the measured values of the parameters under
test. Such a sophisticated multifunction control system is n o t available yet, but would probably be developed when large-scale product i o n of electric vehicles commences.
Since electric vehicles operate under considerable current loads,
reliability of cabling and terminal connections is of utmost importance. Mis-rated and over-long electric conductors, and poor contacts w i t h the power source and the consumer, may lead t o considerable power losses and hence impair electric vehicle performance.
These electrical contacts should, therefore, be periodically checked
or included in the control system.
138
R e q u i r e m e n t s of t h e construction and
m a n u f a c t u r i n g technology o f b a t t e r i e s for EV
energy storage systems
5.
A s a rule, lead-acid batteries used for electric vehicle propulsion
have a modular construction. These modules are usually 6 V tract i o n batteries consisting of three storage cells housed in a plastic
case w i t h a common cover, and intercell through-the-wall connectors. T h i s battery design yields a relatively h i g h specific power out-
put (30-35 Wh kg-'). Chloride Silent Power [6] has developed an
advanced tubular plate lead-acid battery w i t h h i g h specific energy
(37.7 Wh kg-') and specific power (112 W kg-').. T h e capacity of
the modular battery is 167 Ah at unit module weight of 32 kg.
Utilization of non-hermetic electrochemical power sources with
aqueous electrolytes (e.g. lead-acid and alkaline batteries) requires
constant monitoring of the electrolyte level in each battery cell and
replenishment w i t h distilled water when necessary. Filling up is one
of the maintenance procedures most often underestimated and neglected by drivers, and leads t o drying of individual battery cells and
hence shortening of battery cycle life. T o prevent this, single point
watering systems have been developed [6] for monitoring electrolyte
levels and refilling w i t h water when needed.
On.charging of aqueous electrolyte batteries, evolution of hydrogen and oxygen occurs. These two gases f o r m an explosive mixture
which requires special safety measures t o prevent explosion of the
vehicle. There are two possible solutions t o this problem. First,
by designing a common gas vent system t o collect all gases released
from the individual cells, and l e t t h e m o u t i n t o the atmosphere. T h i s
gas vent system can be combined w i t h the refill and cooling battery
systems, an approach adopted by U n i o n Globe Co., in the USA [3].
Second, by the introduction of maintenance-free batteries w h i c h are
of special interest for electric vehicle propulsion applications. These
139
batteries utilize lead-calcium alloys and fumed silica gelled electrolyte
or glass mat separators immobilized electrolyte. An oxygen cycle is
employed w i t h these batteries t o stop the electrolysis of water and
the evolution of oxygen and hydrogen. A s evidenced by a number of
investigations, hydrogen gassing cannot b e completely eliminated in
maintenance-free lead-acid batteries, i.e. these batteries cannot be
sealed. They are usually equipped w i t h a safety valve that controls
the pressure in the cells, and when the pressure rises, the valve is
opened and the accumulated gases are released f r o m the cell. The
use o f a restricted amount of electrolyte, however, decreases battery
energy performance slightly. Thus, a gelled-electrolyte battery developed for the VW Rabbit Sedan [6] has a specific energy of 26 Wh kg-'
and specific power of 95 W kg-'.
T h e m a j o r advantages of the maintenance-free batteries are [6]:
0 n o battery watering or checking of electrolyte level required
(this saves 1 hour per 100 km for each vehicle opèrated);
o greatly reduced hazard of hydrogen explosion;
o reduced energy consumption because of less requirement for
overcharge;
a reduced maintenance of t h e battery ventilation system;
o improved cold weather performance displayed by sealed batteries.
During battery operation, electrolyte density undergoes cyclic
changes resulting in i t s stratification, the denser electrolyte flowing
d o w n t o the b o t t o m part of the plates and the lighter one floating
over it. T h e electrochemical activity of the cells and their capacity
t o accumulate energy are higher in their upper parts than in the
lower ones. At the b o t t o m of the cells, where the acid concentrat i o n i s highest, full charging o f the plates is impeded, they undergo
sulfatization which in turn decreases battery capacity. T h i s capacity
loss i s about 1% for each 0.01 g cmP3 difference between t h e acid
concentration at the t o p and at the b o t t o m o f the cell [25).
140
Various methods for destratification of the electrolyte have been
developed. T h e simplest consists in provoking intense gassing at the
end of battery charge t o stir the electrolyte. T h i s can b e achieved by
applying an appropriate current or by adding high-current pulses t o
the charging current. T h i s method, however, turned out t o lack efficiency. I t cannot be applied t o maintenance-free batteries. Mechanical stirring o f the electrolyte, n o t related t o the charging current, has
proved t o be most effective. T h i s method has been implemented successfully by Johnson Controls Inc., in the USA, where an electrolyk
circulation system has been developed [26]. T h i s system uses a small,
b l o w molded, insert pump w i t h n o moving parts, which i s designed
t o maximize reliability and heat-sealed i n t o each ce11 of the battery
module as part of the final assembly operation. During cycling, a
low-pressure air pulse o f predetermined volume i s forced i n t o the insert pump, collecting higher density electrolyte f r o m the b o t t o m of
the cell stack and dispersing it across the top of the cell. On the basis
of the above electrolyte circulation system, a lightweight container
and a single point watering system, an Improved State of the Art
(ISOA) lead-acid battery has been developed [27]. The results f r o m
testing of the battery are presented in Table 13.
T a b l e 13. Performance of lead-acid batteries for EV
Performance
parameter
Specific energy (Wh kg-')
P o s i t i v e active mass
u t i l i z a t i o n - (Ah kg-')
-
Cycle life
Golf car
b a t t e r y (1978)
ISOA
EV-3000
FFLA
23
42
52.4
63.9
28.5
250'
72.7
32.4
508'
126.7
56.5
130"
* based o n SFUDS 79
** based on 80% DOD c y c l i n g
141
T h e ISOA battery showed a 46% higher specific energy and 80%
longer cycle life t h a n the conventional lead-acid battery. Besides, the
ISOA battery offered the advantages of reduced overcharge and water
addition requirements, and improved system thermal management.
In the ISOA battery, simple circulation o f the electrolyte between
the plates and the separators is achieved w i t h o u t affecting the active
mass. By forcing the electrolyte i n t o the electrodes, more active material sites w i l l be accessed, which w i l l result in enhanced active material utilization and improved system specific energy. Johnson Controls Inc. have developed such a battery system, called the FFLA
(Forced F l o w Lead-Acid) battery [Zí']. The demonstrated performance results of this battery are given in Table 13. I t can be seen
that the specific energy and the active mass utilization of the FFLA
battery are considerably higher than those of the ISOA system. From
the data presented in Table 13, it is difficult t o predict the life span
of these batteries. It can be expected however that their cycle life
w i l l be significantly reduced through mechanical deterioration of the
active mass caused by forcing the electrolyte i n t o the electrodes. T h e
relative complexity of the system including battery, pumps and electrolyte circulation tubes should also be given consideration.
6 . Specification o f operating energy storage
systems for electric vehicles
Classification of electric vehicles is very difficult, because on the
road electric vehicles are mostly demonstration prototypes employing
various power sources w i t h different design and dynamic performance
parameters. T h i s is well illustrated in Table 14 presenting comparative d a t a for several EV types.
Despite the fact that many of the problems facing electric vehicles
are n o t solved yet, a l o t of countries are planning t o start large-scde
142
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rn
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143
production o f these vehicles. In the USA, design and development
o f an electric G - V a n is planned, driven by Chloride tubular lead-acid
batteries [25]. Peugeot intends t o manufacture an electric car com-
mercially using Ni/Cd batteries. In Switzerland, a number of manufacturers and importers have capitalized o n high-profile EV races
and exhibits. A purpose-designed postal van has been brought up t o
the production stage in Finland. In the UK, “W and E Electric Vehicles” has developed and implemented a standard E V conversion
package which can convert any internal combustion engined vehicle
t o electric power.
Fifteen electric vehicles t o o k p a r t in the demonstration “The
Twelve Electric Hours of Bruges”, Belgium, t o show that electric
vehicles could be a solution for urban traffic related problems [29].
7. Lyrical epilogue
L o o k around, dear reader! You w i l l see many, many cars that
have occupied the streets, and even the sidewalks, of all big cities in
the world. Their powerful engines of 40-60-100 and more kilowatts
allow them t o whizz past at a reckless speed. But, instead, most
often y o u w i l l see them creeping one after the other in hundreds,
entrapped in the next traffic jam. If you look inside the car, y o u w i l l
see a SINGLE self-satisfied or more often extremely nervous person.
W h a t a wasteful and irrational world we are living in! A world
conscious only o f i t s present self, w i t h n o thought for i t s “tomorrow”.
And this “tomorrow” is in our hands: we are obliged t o think about
the future and t o safeguard it for our children and for generations t o
come. We must give up this madness of driving super-powerful cars,
and our only salvation is the small, lightweight, two-seater, noiseless,
environmentally safe and cheap ELECTRIC VEHICLE.
144
References:
1.
E. Sauer, Elektroauto, Verlag TU\.’ Rheinland, Köln, 1985.
2.
Anon. Electric Vehicle Prog. 10 (1988) 3
3.
V.A. Shtetin, U.J. Mortowskij, B.I. Tsenter, W.A. Bogomazov, Elektromobil: Technika i Ekonomika, (V.V. Burkov, ed.) Leningrad 1987
(in Russian).
4.
A. Aldous, in Power Sources for Electric Vehicles (B.D. M c N i c o l and
D.A.J. Rand, Eds.) Elsevier, Amsterdam, 1984, p.47.
5
C.P. Peyriere, Proc. 9th Int. Electric Vehicle Symp., EVS88- P16,
Toronto, Canada, 1988.
6.
K.F. Barber, S.K. Takagishi, ibid. EVS88-073.
7.
A. de Guilbert, ibid. EVS88-Pl5.
8.
C. Madery, J.L. Liska, ibid. EVS88-051.
9.
N.P. Fitzpatrick, D.S. Strong, ibid. EVS88-001.
10.
H. Nakao, Y. Suzuki, M. Okawa, ibid. EVS88-042.
11.
L.B. Taylor, D.T. Fouchard, ibid. EVS88-023.
12.
F.E. Zhao, Y.Y. Hu, Electric Vehicle Develop., 8 (1989) 21
13.
K.R. Ramachandran, S.C. Chopra, Proc. I L Z I C Silver Jubilee Conf.
Pb, Zn and Cd into t h e go’s, New Delhi, 1988, p.2.
14.
E.Z. Ratner, G.L. Henriksen, C.J. Warde, Proc. 9th Intl. Electric
Vehicle Symp., EVS88-011, Toronto, Canada, 1988.
15.
W.J. Dippold, ibid. EVS88-022.
16.
H.P. Schoner, L.L. Ogborn, J. Power Sources, 21 (1987) 91.
145
17.
J. M. Bozek, J.J. Smithrick, R.L. Cataldo, J.G. Ewashinka, E V
80,
Report No. 8014,
Electric Vehicle Council,
Washington, 1980.
Expo
18.
J.
Lee,
J.F.
Miller,
C.C. Christianson,
J. Power Sources,
24 (1988) 215.
19.
M. Peohler, H.A. Kiehne, Die. Antrzebsbatterie, VDI-Verlag, VARTA,
Hannover,
1980, p.7.
20.
D.C. Constable, J.R. Gardner, E;. Harris, R.J. Hill,
D.A.J. Rand, L.B. Zalcman, J. Electroanal. C'hem.,
168 (1984) 395.
21.
C.P. Wales, S.M. Caulder, A.C. Simon, J. Electrochem. Soc., 128
(1981) 236.
22.
W.C. Spindler, R.L. Driggans, C.C. Christianson, Proc. 9th Int. Elect r i c Vehicle Symp.. EVS88-032, Toronto, Canada, 1988.
23.
F.H. Klein, U. Wagner, ibid. EVS88-077.
24.
G.H. Cole, ibid. EVS88-078.
25.
W.G. Sunu, B.W. Burrows, in Power Sources 8 (J. Thompson, ed.),
Academic Press, 1981, p.601.
26.
US Patent 4 221 847.
27.
M.G. Andrew, P.A. Budney, J.L. Heder, Proc. 9th Int. Electric Vehicle Symp., EVS88-002, Toronto, Canada, 1988.
28.
Anon. Electric Vehicle
PTOgT.,
12 (1990) 1.
29.
Anon. Electric Vehicle
PTOgT.,
11 (1989) 1.
30.
J. Angelis, D. Sedgwick, Proc. 9th Int. Electric Vehicle Symp.,
EVS88-009, Toronto, Canada, 1988.
ISSUED WITHIN THE SAME SERIES:
Technical Report No. 1: &tate of Research and Future
Trends in Energy Storage Materials: Solid State
lonics and Related Devices..
Technical Report No. 2: <<Aperçudes résultats et des perspectives des recherches scientifiques et médicales
en vue de la prévention et du traitement du SIDAV.
TeXHLlW?cKLlfi AOKJlaA N 2: «0630p pe3yilbTaTOB M
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Technical Report No. 3: «Report of the Working Party on
“Brain Drain Issues in Europe”,,.
Technical Report No. 4: <<Reportof the Advanced Seminar
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Technical Report No. 5: «Conference on Clean Coal Technologies)).
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