battery energy storage system

UNESCO REGIONAL OFFICE FOR SCIENCE AND 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 rend in electric power product ion has witnessed intense development dur ing recent years, that of energy storage. Several options have been considered for this purpose, one of them 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 jo in t conferences t o exchange informat ion and opinions on the problems of energy storage. It is now opportune to siirrimarize the results and experiences so far acquired in t l i e design arid uti l izat ion of bat tery energy storage systems.

In 1954, Elsevier in Amsterdam issued the book entit led “Power Sources for Electric Vehicles” edited by B.D. McNicol and D.A..J. Rand, which presented a comprehensive survey o f the current knowledge in the field.

Mo to r car transport i s being increasingly adopted, since it is an

important and indispensable element of the normal functioning of every modern social community. I t has, however, a serious environmental impact in that it causes considerable air pol lut ion 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 20th century. However, the electrocliemica1 power sources used for propulsion of these vehicles cannot yet meet the challenge. Annual international

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conferences o n the problems of electrochemical power sources show that more effort is being placed on broad-spectrum investigations in the field. Accumulated theoretical knowledge and practical experience o n battery energy storage systems for electric vehicle applica- t ions should now be analyzed and evaluated.

T h e Regional Office for Science and Technology for Europe (ROSSE) at the Uni ted 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 at t ract ing wider publ ic attention t o the 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 mod- estly, t o the development of battery energy storage systems, we wi l l be most satisfied.

D. Pavlov, G. Papazov, M. Gerganska

May 1991, Sofia, Bulgar ia

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Contents

Preface ................................................ i

Chapter 1

BATTERY ENERGY S T O R A G E S Y S T E M S FOR THE POWER I N D U S T R Y

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . Introduction 1 1.1. The four basic elements of every nat ional electric

power system ......................................... 1

1.2. Power industry and i t s problems ........................ 1.2.1. Energy, power and response t ime . . . . . . . . . . . . . . . . . . 1.2.2. Quality of energy supply systems . . . . . . . . . . . . . . . . .

3 3 5

1.2.3. Ecological problems and the development of power industry ................................ 6

2 . Electric energy storage ........................... 7 2.1. Pumped Hydroelectric Energy Storage Systems (PHESS) . . 8 2.2. Compressed-Air Energy Storage Systems (CAESS) . . . . . . . 9 2.3. Superconducting Magnetic Energy Storage Systems

(SMESS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.4. Battery Energy Storage Systems (BECS) . . . . . . . . . . . . . . . . 13

2.4.2. Basic principles of battery operation . . . . . . . . . . . . . . . 15 2.4.1. The revival of battery energy storage systems . . . . . . 13

2.4.3. Some advantages of battery energy storage systems . 15 2.5. Choosing the right opt ion for electric energy storage . . . . . . 17

... 111

2.4.2. Basic principles of battery operation . . . . . . . . . . . . . . . 15

2.4.3. Some advantages of battery energy storage systems . 15

2.5. Choosing the right option for electric energy storage . . . . . . 17

3 . Batteries for energy storage . in operation and under development ................................ 19

3.2. Sodium/Sulfur Batteries ............................... 20

3.2.1. Principles of cel l operation ....................... 20

3.2.2. Design of sodium/sulfur cells ..................... 23

3.1. Development projects for battery energy storage systems . . 19

3.2.3. Specification and test results for battery modules and pilot plant of the Japanese “Moonlight Project” 24

3.3. Zinc/Bromine Batteries ................................ 28

3.3.1. Reactions and principles of cell design arid operation 28

3.3.2. Chemistry and electrochemistry of the zinc/bromine cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.3.3. Battery system design ........................... 33

3.3.4. Characteristics of zinc/bromirie batteries . . . . . . . . . . . 33

3.4. Zinc/Chlorine batteries ................................ 37

3.4.1. Fundamentals of zinc/chlorine batteries . . . . . . . . . . . . 37

3.4.2. Battery design .................................. 39

3.4.3. Battery characteristics ........................... 40

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5.2. Lead-acid bat tery energy storage systems (LABESS) in operation by 1990 throughout the wor ld . . . . . . . . . . . . . . . 63

6 . Lead-acid battery energy storage systems for load levelling ...................................... 67

6.1. System structure ...................................... 67

systein ............................................... 68 6.2.1. Plant layout .................................... 68 6.2.2. T h e bat tery ..................................... 69 6.2.3. Power conditioning system ....................... 76

6.2.5. Equipment energy losses ......................... 78

6.2. Chino 10 hgW/40 MWh lead-acid battery energy storage

6.2.4. Facil ity monitor ing and control system . . . . . . . . . . . . 78

6.2.6. Economics of Chino LABES Plant . . . . . . . . . . . . . . . . 79

7 . LABESS for instantaneous (spinning) reserve and frequency control applications . . . . . . . . . . . . . . 80

7.1. Island networks ....................................... 80

7.2. The BEWAG 8.5/17MW Lead-Acid Bat tery Energy Storage Plant ......................................... 81

7.2.1. System frequency response having given rise t o the construction of the BEWAG LABES plant . . . . . 81

7.2.2. System design and characteristics . . . . . . . . . . . . . . . . . 82

8 . Lead-acid battery energy storage systems for peak shaving ....................................... 86

8.1. What i s peak shaving? ................................. 86

transport network ..................................... 91

8.2. Johnson Controls 300 1<W/600 1ïWh LABES Facil i ty . . . . . . 88 8.3. Lead-acid battery energy storage systems in the railway

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9. Valve-regulated lead-acid bat ter ies fo r b a t t e r y energy storage systems . . . . . . . .. . . . . . . . . . . . . . . . .. . 94

10. Strateg ic advantages of BES systems .. . . . . . . . . . . 96

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

Chapter 2

ENERGY S T O R A G E S Y S T E M S F O R ELECTRIC V E H I C L E S

1.

2.

3.

4.

5.

6.

7 .

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M o t o r vehicles and env i ronmenta l p o l l u t i o n . . . 103

Specif icat ion of energy storage systems for electr ic vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

Charge a n d capacity o f ba t te r ies for e lectr ic vehicles .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 122

Types o f cycles o f e lectr ic vehicle bat ter ies . . . . 128

Requi rements t o t h e cons t ruc t ion and manu- fac tu r i ng technology o f ba t te r ies for EV energy storage systems ................................... 139

Specif icat ion o f ope ra t i ng energy storage systems for electr ic vehicles . . . , . . . . . . . . . . . . . . . . . 1.12

L y r i c a l epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

References . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Chapter 1

BATTERY ENERGY STORAGE S Y S T E M S FOR THE POWER INDUSTRY

D. PAVLOV

1. Introduction

1.1. T h e four basic elements of every national electric power system

Product ion of electric energy is the basic pillar for normal func- t ioning 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:

electric power plants: thermal power plants fired by coal or nuclear fuel, gas- fired steam plants, oil- or gas-fired combustion turbines, hydroelectric plants, etc.

b) Electric power distributing systems including transformer facilities, transmission trunk lines and distr ibut ion 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.

The electr ic power produced by the generating ut i l i t ies is delivered through t h e transmission/distribution system t o the consumers for uti l ization. The consumers’ demand for electric power varies cyclically dur ing day and night, as well as w i th in the week and the seasons.

a) Electric power and energy generating uti l i t ies, i.e.

Demand. Th is is the rate at which electric energy is delivered t o the consumer, measured in kW (kilowatts) integrated over a specific t ime 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 uni ts should operate at a minimum capacity of 500-1000 MW and under constant load.

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. . , , , I 3 5 ' 7 ' 9 ' 1 1 ' i 3 ' 1 ' ' 1 7 ' 1 9 21 23

DCoal mGas steam @Gas turbine Bottery

Fig. 1. Example of customer energy demand curve for a working day [il.

Hour of day

Dur ing 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 ut i l iz ing relatively expensive fuel, and also by hydroelectric

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power plants. T h e rat io of actual t o peak power demand over a given per iod is called load factor.

There is an intrinsic contradiction in each power supply system between producers and consumers of electricity. T o be efficient, power plants should operate at constant load. T h e customers’ demand, o n the other hand, undergoes cyclic fluctuations. Th is leads t o inefficient ut i l izat ion of t he generating capacities. A possible so-

l u t ion 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 dur ing the night and store the generated energy t o sell it at increased cost during peak demand periods. The introduct ion of this four th element in the electric power system makes i t s operation more efficient. Th i s no t on ly brings about considerable savings of expensive fuels such as gas and oil, but also improves the load factor of the power generating facilities.

1.2. Power i n d u s t r y and i t s problems

1.2.1. Energy, power and response time

I t has been established tha t the different forms o f mo t ion (mechanical , thermal , electromagnetic , gravitational, chemical, etc.) are converted in to one another following definite quantitative ratios. T o allow measuring of the various forms of mot ion by a unified measuring unit, the term energy has been introduced. T h e electrical energy is determined f rom the product of the voltage and the quantity of charge that passes through an electrical device (load).

T h e electrical power is determined f rom the product of voltage and current.

T h e work done per unit t ime is called power.

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In thermal power stations, the chemical energy accumulated in coal, crude o i l or natural gas is transformed by burning (oxida- t ion o f the hydrocarbons) in to heat (high-temperature, high-pressure steam), which sets in mot ion a turbine, whereby the thermal energy is transformed in to mechanical. T h e turbine shaft is connected t o the shaft of an electric generator. On rotation, this common shaft drives the rotor of the generator as a result of which the mechanical energy is converted in to electrical energy. It is evident that, t o obtain elec- t r ica l energy f rom 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 on the surface of the two electrodes, the chemical energy is directly transformed in to electrical power.

Conversion o f one type of energy in to another requires a certain t ime period. T h e t ime needed for an energy-generating system to change i t s power f rom a value (A) t o another value (B) is called response (transit ion) t ime (Fig. 2) .

/ i

ition time _J

Time

Fig. 2. Power curve showing the change of power from level A t o level B.

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A thermal power plant needs tens of minutes t o change f rom one power level t o another, while for a battery, the response time is of the order of mi l l ionths of a second. For an 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 rom 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 distr ibut ion of the electric-system capacity expressed by a typical weekly load curve of an electric utility.

Generation for load í No storage) 100

2.

Generation for load í No storage)

Baseload

Mon Tue Wed! Thul Fri 1 Sat Sun 1 Generation for load íWHh storage)

Mon I Tue I Wed I Thu I Fri I Sat I Sun I I I IReserve

Baseiood energy to S i O M g t

Peaking energy from storage

Fig. 3. T y p i c a l weekly load curve for an electr ic utility [il.

T h e energy system should have 15-20% of reserve power available t o be able to meet any customer demand. I f there is no or insufficient reserve capacity and the load level exceeds the power generation level,

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a decline in voltage at the consumer side w i l l appear which would upset the normal operation of the users’ machines and electrical devices or even cause them t o fail. For this reason it is essential for the normal functioning and development of each social community t o have reliable nat ional and local electric power systems w i t h capacities exceeding the actual energy demand by at least 15%.

Unfortunately, however, on ly r ich and advanced modern countries possess such high-grade energy systems. T h e power systems of most countries in the world have capacities that only just meet their energy demands, and in some cases are s imply inadequate. This hunger for electricity is very often a l imi t ing factor for the economical and social development o f a country.

1.2.3. indus t r y

Ecological problems and the development of power

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 increasing automation and computerization of the nat ional 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 on the combustion of coal, o i l and gas, which is 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 forma- t ion of acid rains causing enormous damage t o the agricultural crops and the forests. The accumulated CO2 in the air might br ing about considerable climatic changes b o t h on a regional and global scale (the so-called “greenhouse effect”). Thus the rapid development of power industry has added comfort t o society and i ts individuals, but it has posed very serious ecological problems of a national, regional and global nature. In response t o these processes, various organi-

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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 pol icy of state governments and of companies engaged in electric power production. The efforts of these movements, combined 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 purif icat ion of exhaust gases. Th is method has an undoubtedly beneficial effect o n the environmental aspect of electric energy production, but it involves rather expensive, complicated and not fully efficient procedures leading t o increase in energy and power costs.

Second, in building up a system of energy storage plants which haw a considerable impact on the efficiency of energy uti l i t ies as well as significant cost benefits.

Third, in thc sphere of electric power consumption, a l l technological processes of major 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 th lower power demands.

The basic problems and the development trends o f energy storage w i l l be discussed in the chapters to follow.

2. Electric energy storage

During the last few decades, several options for electric energy storage have been devised. Many countries have started programs aimed at development of energy storage technologies. T h e basic principles of some of these options, that have found successful application, w i l l be outl ined below.

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2.1. (PHESS)

Pumped Hydroelectric Energy Storage Systems

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. Dur ing periods of low demand, the excess power is ut i l ized t o pump water f rom the lower reservoir and transfer it t o the upper one. At peak demand periods, the pumped storage plant acts as a hydroelectric power plant thus adding capacity to the energy system. Th is storing opt ion is cost-effective i f used only 5 t o 8 hours in the peaking range. I t s response t ime 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 wor ld w i t h a to ta l capacity of 25,000 MW. Th is energy storage opt ion i s most appropriate for countries w i t h mountainous relief. Construction of these plants requires f rom 8 t o 10 years and is often associated w i t h considerable environmental im-

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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% of the nat ional net capacity, whi le for France, Germany and the UK, this figure is 6%, and 3% for the USA. At the moment, more than 200 pumped storage plants are under construction worldwide. Conse- quently, by the beginning o f the next century, this energy storage opt ion w i l l become an impor tant element of many nat ional 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 rock caverns. Dur ing 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 rotor of an electric current generator. Compressor motor and generator are combined in one machine. During air compression, t h e rnotor/generator i s connected to the compressor and decoupled f rom the turbine. During electric current generation, the motor/generator is disconnected f rom the compressor and coupled to the turbine. Compressed-air storage uni ts burn only one third of the fuel used by conventional combustion turbines t o produce the same amount o f electricity. Th i s leads t o a two-third reduction in the en-

vironmental pol lut ion caused by the combustion process of turbines which are usually located in urban areas. Ways have been sought for opt imizat ion of the system operation, such as return of the heat re-

leased dur ing air compression back t o the energy system. Th is energy storage opt ion 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 ime is about 10 minutes. Efficiency of air compression is 65-75%. The starting per iod is 20 to 30 minutes. As rcgards the security aspects, measures should be provided against leakage of compressed air. The service l ife 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 l i fe of the plant is expected. Commercial operation of the German CAES plant has shown that this type of energy storage opt ion i s sufficiently reliable.

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Compressed-air storage plants have a negligible environmental impact, and can be built w i th in 2 t o 5 years. They arc: fit,ted w i t h modified combustion turbines of routine production. Th is technology can find application only in countries w i t h natura l deep underground, ha rd 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, lo-hour capacity, three- unit plant), etc. T h e Italian conipany ENEL has st,arted construction of modular mini-uni ts 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 opt ion t o store electrical energy as such, wi thout converting it in to other forms. Th is 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, par t of the electrical energy is lost in t h e form 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 the i r electrical resistance. When direct current is fed in to an electric circuit o f su- perconductors, the current w i l l circulate endlessly along the closed r ing wi thout energy losses. When an energy demand appears, t he requested electrical power can be drawn f rom that closed ring.

Large-scale investigations are presently being performed aimed at devising a technology for the production of superconducting magnetic

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Fig. 6. Schematic of a Superconducting Magnetic Energy Storage System [2].

energy storage plants. A block diagram of such a plant is presented in Fig. 6.

T h e heart of this storage system is the electromagnetic supercon- duct ing coil. T h e latter operates on direct current. Charging o f the electromagnetic coil w i t h electricity f rom the ac generating utility is accomplished via a two-way converter. A refrigeration system main- tains the temperature of the electromagnetic coil at a very low fixed value. Operation of the electromagnetic coil, converter and refrig- erator is monitored and controlled by a controller. Such an energy storage plant should be sited near a substation where the transformer converts the high voltage energy f rom the utility network t o appro- pr iate low voltage power. The response time of this type of storage system for switching between charging and discharging is about 20 milliseconds. The ac-ac efficiency is 90% or more.

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.

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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 product ion is well advanced and cost-effective, t h e expenses for con- struct ion and maintena.nce o f the refrigeration system will be reduced significantly.

The current density in superconductive wires may reach extremely high values as t h e conductor exerts no electrical resistance leading t o joule losses. Th is allows the wire cross-section t o be decreased more than five times w i t h respect to copper wires used at ambient temperature. Th is w i l l change substantially the existing classical electric power system.

In Japan, an energy storage project is being developed known as the “Moonlight Project”. The power of the Japanese superconducting magnetic storage system is 1000 MW, energy density is 12 Wh kg-’, storage efficiency SO-SO’%, storage uti l izat ion rate approx. 75%. The system wi l l be used for dai ly and weekly en-

ergy storage. Underground bed rocks are required for construction of this system. Locat ion possibilities are restrictcd, because anti- magnetic measures are needed for environmental protection. Protec- t ion against superconductive material degradation is also necessary.

A 10 MW, two-hour capacity SMESC pilot p lant has been developed in the USA. T h e refrigeration system is based on liquid helium.

2.4. Battery Energy Storage Systems (BESS)

2.4.1. The revival of battery energy storage systems

At the 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. Dur ing this pe-

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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 distr ibut ion applications was noted and, very soon, the dc battery system was abandoned and hence also the storage of energy as an element of the power system.

In the 1960s, a powerful reliable and cost-effective static rectifier was devised. Nuclear power plants were equipped w i t h large stand-by lead-acid battery storage facilities ensuring their reliabil i ty by supplying reserve power and energy. New and innovative elec- t r i c power applications in industry and every-day l ife brought about radical changes in the profile of the daily, weekly and seasonal demand ciirves. To enhance the operational efficiency of electric power uti l i t ies, energy storage uni ts were introduced. At first, pumped hydroelectric energy storage plants were used for that purpose, and later, the old lead-acid battery storage systems were revived. They were based on to ta l ly new conversion, management and control technologies.

A t t h e end of the 1970s, for the first time, BEWAG-AG decided t o instal l the Bat tery Storage Facil ity in West Ber l in 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, the largest battery plant for load-levelling (10 MW/ 40 MWh) was set in operation in the USA, at Chino, California.

Since then, many technologically advanced countries throughout tlie wor ld have started large-scale research and test programs aimed at the introduct ion of bat tery energy storage systems in their national ccoiioinies and publ ic services.

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2.4.2. Basic principles of battery operation

When two appropriately chosen electrodes are immersed in the respective electrolyte and a direct electric current f rom an external source flows between them, electrochemical reactions proceed on the electrode surfaces dur ing which inactive substances are transformed in to electrochemically active ones. This process is called charging of the electrochemical power source or the battery. As a result of these reactions, electrical energy is converted to chemical and an electromotive force is created between the two electrodes. When the electrodes are interconnected v ia a load, under the action of this electromotive force, electrocheniical reactions proceed on the electrode surfaces in an opposite direction t o the reactions during charge. This process of current generation is called discharge. The battery can endure thousands of charge-discharge cycles. However, parallel t o the reversible processes of charge and discharge, certain low rate irreversible processes also take place that limit batt,ery cycle life.

Du r ing off-peak periods, the battery is charged f rom the electric power utility via a converter. The latter converts the alternating current in to 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 the process of developnient of the new generation of BEC systems, lead-a,cid batteries were widely used, which allowed t h e latter to exhibi t a number of useful advantages leading to significant cost beneíits. The following ecoiioniical features of lead-acid battery storage systems were demonstrated.

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a) Modular design. Construction of BES plants is realized on a modular basis, i.e. through connecting of the indiv idual bat tery 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 be accomplished in stages according t o demand needs.

b) Short construction terms. All BESS elements are produced at the factories w i th in a few months only and then the actual bui ld ing of the 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 pol lut ion is caused. They are quiet and can be located near, and even in, housing c i ty areas.

d) High level of recycling of the materials employed an the batteries. At the end of a battery's service life, many of the materials used for i t s manufacture can be regenerated.

F rom the above, it follows tha t the revival of BES systems during the 1980s i s a normal process based on the rapid progress of electric power industry.

To b e economically effective, BES systems should meet the following challenging performance requirements:

o 30 years of service l i fe o 75% ac-ac efficiency o Unit power cost about 400-700 $ kW-' e 5 hour discharge.

Many projects are being implemented at present worldwide aimed at achieving the above parameters.

16

2.5. Choosing the right option for electric energy storage

When deciding o n the type of off-peak energy storage system t o adopt, the following considerations should be taken in to account:

o profile o f t h e 24-hour demand cyclogram o available budget and i ts possibilities 0 nat ional topographical peculiarities o environmental aspects and limitations.

According t o an EPRI investigation, there are a great number of hard rock caverns in the USA, which suggests dominat ing 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 cost- effective, 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 out an analysis of the economics of the various options for energy storage. T h e results of these evalu- ations are summarized in Table 1.

An analysis of the results indicates that battery storage systems and superconducting magnetic storage systems are more appropriate for use when there are peak demands w i t h a durat ion of 2 t o 3 hours. Pumped hydroelectric storage plants and compressed-air storage systems may cover efficiently peak power demand needs of up to 10 hours. They can also funct ion as intermediate load power sys terns.

17

Table 1. Estimated costs for energy storage technologies [2] ~ ~

Technology Power Energy Hours of Total related related storage cost $kW- ' $kWh-' $ kW-'

Compressed-air Small module

Large module

Pumped hydro Conventional

Underground (2000 M W )

Battery Lead-acid (10 M W ) Advanced (10 M W )

(25-50 M W )

(110-220 MW)

electric (500-1500 M W )

Supereconducting (Target) magnetic (1000 MW)

575 5 10 625

415 1 10 425

1000 10 10 1100

1040 45 10 1490

125 170 3 635

125 1 O0 3 425

150 275 3 975

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 opt ion is considerably cost-effective. Total costs ($ kW-') for compressed-air and battery storage uni ts show similar values. T h e above rat ing of the various energy storage options wil l, o f course, vary for the various countries, depending on their specific economic and technological conditions and requirements.

18

3. Batteries for energy storage ~ in opera- t ion and under development

3.1. Development projects for battery energy storage systems

T h e “Moonlight Project” is the nickname of an RSrD prograin for energy storage in Japan. T h e nanie “nioonlight” was selected t o imply the analogy between the nioon that does no t sliine with i t s own light but reflects the sun’s light, arid the energy storage batteries that do no 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 Bat te ry Electric Power Storage Systeni. Sonie o f the basic functions o f this system will be discussed below.

T h e Electric Power Research Inst i tute in the USA lias beeii carrying out research and developirieiit activities within the Energy Stor- age Program since 1972. T h e profirani incliirlcs dcvclopIricrit o f lead- acid and advanced batteries.

Both programs are haced o n almost the 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 rom a constructional arid technological po in t o f view, this 100-year o ld battery t o the requirements o f energy storage. Sec- ond, new electrochemical power sources aie being investigated and developed, such as: sodium/sulfur, zinc/chloride, zinc/broniide and redox/Aow batteries.

Japan’s project is targeted at devising a demonstrational model of a battery energy storage system with the following parameters:

o power output ~ 1 MW o charge t ime - 8 h o discharge t ime ~ 8 h

19

o overall energy efficiency - min. 70% (at ac input/output) o service l ife ~ min. 10 years (2000 cycles)

Batteries should conform t o all environmental standards [4]. Analogous specifications have been adopted by the American

10 MW demonstration battery storage plant. Several demonstration and testing battery storage facilities using

lead-acid batteries have been built and are in operation in other countries in the world. T h e testing results of these uni ts will be discussed later. The basic properties and characteristics o f the so- called advanced batteries w i l l be described first.

Sodium

NO

3.2. Sodium/Sulfur ba t te r ies

3.2.1. Principles of cell operation

The sodium/sulfur cell consists of a negative electrode (cathode) of molten sodium (Na) and a positive electrode (anode) of molten sulfur separatkd by a beta-alumina (P-Al20,) ceramic ion-conductive membrane. Through this membrarie, only sodium ions can pass, but not electrons or sulfur ions. A block diagram of a sodium/sulfur cell is given in Fig. 7.

Ij-oiumino Sodium polysulfide

S.NOS, . ( X . 3 i 5 )

Fig.

20

7. Block diagram of a sodium/siilfur celi [5].

The reactions that proceed in the ce l l can be expressed by the following ecpiation:

Discharge

2Na+2S + Na& ( x = 3-5) Charge

Dur ing discharge, metall ic sodium of the negative electrode is ion- ized to positive sodium ions (Nat) and electrons are released. The sodium ions pass through the beta-alumina membrane and reach the positive sulfur electrode. The electrons released o n the negative electrode flow through an external circuit, pass through a load whereby certain useful electric work i s done, and reach the sulfur electrode. There, they are bonded t o the sulfur atoms and form sulfur ions (S2-). These react, w i t h the sodium ions giv ing 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 rom the sodium t o the sulfur electrode doing some work.

During charge, reverse processes take place. In this case, electric energy shoiild he introduced in to the cell t o enable proceeding of the reverse processcs. Under the action of an external \-oltage applied t o the cell, electrons f rom 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 in to 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 funct ion 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 t l ic changcs in the open circuit voltage as a function of anodc coinpositioii.

300'C A 330T 0 360.C

390'C

0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1 Anode composition (mole ratio of sulfur)

Fig. 9. Open circuit voltage as a f i inr t io i i of molar rat io of s i i l f u r iii sodium

polysulfide [5].

22

When the molar ra t io 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 no 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.

A r

s s * N .l

1

3.2.2. Design of sodium/sulfur cells

Schematic of a sodium/sulfur cell is given in Fig. 10.

Insulolion ring (olumina ceramics)

Anode cose í iron 1

Anode (sulfur. graphite (el t ) i (cilumino ceramics)

Solid electrolyte tube

-Colhode metollic

( sod ium , l iber)

L C o t h o d e tube (copper)

Fig. 10. Principal design of sodiurn/sulfur unit cell [5]

T h e beta-alumina membrane is a cylindrical tube w i t h a bo t tom at one end. Sodium is filled inside this tube, and sulfur and sodium

23

polysulfide in a metal cylindrical case outside t l ic t i ihc. At the upper opening o f the beta-alumina tube, the anodc, the cathode and the metal l ic case are connected arid welded t,ogct,lic,r with insiilat- ing ceramics in between. A graphite niat is i nse rkd in t l ic anodic space aimed at improving sulfur and polysulfide clcctric conduct iw ity, whilst in the cathodic space, a stainless steel fibcr is placed t o conduct the current. Th i s fiber retains sodium and is also i i i knded t o prevent release o f sodium in the everit of brcakagc of the beta- alumina tube, thus having an i inportant safety role. T l i e nieta1 case surface is plated with chroni iuni t o prevent corrosion i i i idcr t l ie act ion o f sulfur and sodium polysulfide.

Na/S cells are arranged in a tlierrnoinsulatcd c a e with a iiiaiii-

ta ined constant temperature o f 350°C. Before operation, the batt,ery is pre-heated gradually t o 350°C, arid only after that does t l ie 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 lo t plant o f the Japanese “Moonlight Project”

A 50 kW/400 kWh sodium/sulfiir bat tery module has been pro-

T h e ou tpu t capacity of the plant is 8000 kW1i (1000 kWh x 8 duced by the YUASA company in Japan.

hours). Output voltage is 1000 V dc, ou tpu t current 1011 A dc.

T h e technical results f rom testing o f various configurations of the above battery type are summarized in Table 2.

T h e sodium/sulfur bat,tery plant is located at the Tatsumi substat ion 77 close to the Tatsi imi lead-acid battery energy storage test plant. By the end o f 1990, the 1 M W / 8 MWh sodium/sulfur pi- l o t plant was half completed and operation of 500 1iW output was

started in November 1990.

24

Table 2. Technical results from testing of a sodium/sulfur battery produced by YUASA Battery Co. Ltd.

10 kW class pilot modules [7]

Energy density - per footprint - p e r volume - per weight

Starting time Stopping t i m e

Response to load change Discharge ~ 6 h overall eff.

~ 4 h overall efT. Voltage variation - on charging

Energy consumption in standing ~ hot standing - cold standing

~ on discharging

50 kW class battery modules [SI

Voltage Current output Capacity Current density Electrode area Module composition External dimensions

Energy efficiency ac-ac Energy density ~ per weight

- per volume

- per footprint

52.4 kWh m-2 26.8 kWh m-3 42.6 Wh kg-' 1 s 1 s 10% 1.2 ins-'

76.S% 73.6% 11.8% 4.2%

19.4% day-' 0%: day-'

200 v 250 A 50 kW 400 kW11 50.5 m.4 an-'

495 cm2 (7s x lop) x 1Gs = 1120 cells Width = 2.5 m Length = 2.3 m Height = 2.8 m Weight = 12.8 t 76.6% 31.1 Wli kg-' 17.5 1tWh mP3

48.5 kWh in-'

25

Table 2. (Continued)

1 MW class pilot plant [8]

Beta-alumina tubes: outer diameter length weight

specific resistivity tube resistivity fracture strength

Capacity of the cell Battery output capacity Output voltage Current ac-dc converter Conversion efficiency Output transformer Number of cells Building area Charge/discharge efficiencies dc-dc efficiency (including aux. power consumption) ac-ac efficiency

beta-alumina doped wi th Li20

68 mm 450 mm 3.6 kg

< 4.5 R cm 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 to 96% ac reactor 240 kVA 26,880 cells 800 m2 (total for 2 floors)

approx. 87%

86% approx. 76%

The batteries for this project were made by YUACA Battery Co. L td . in collaboration w i th NGK Spark P lug Co. L td . 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 th no loss of energy during storage. The batteries are compact w i th high storage energy density. Indiv idual Na/C batteries subjected to 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 opti- mizat ion of specific energy and efficiency, and at preventing reactant leakage f rom the cells. Safety is the key t o practical use of these batteries and of course t o extending their life, too.

Whether or not this type of bat tery w i l l ho ld an impor tant posi- t ion in the energy storage system, w i l l depend on i ts service life, o n the competit ive power of i ts price t o that of the lead-acid battery, o n the simplicity and cost-effectiveness of i t s product ion technology yielding rel iabi l i ty 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

Dur ing the Franco-Prussian war, French balloonists f lying over the Prussian lines i l luminated their maps by means of strange prim- i t ive static batteries containing zinc and bromine. Th is i s the first known 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 be 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 Arab o i l embargo, interest in electrochemical power sources increased greatly worldwide, and the question of devising new advanced battery systems gained momentum. Zito, Magnetti-Marell i and E x ~ o r i Research and Engineering (ER&E) were attracted by the high electromotive force provided by Zn/Br batteries and started design and development work o n this type of battery. Magnetti- Marel l i developed an electrolyte circulating battery design, whereby the performance of bo th 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, on testing, these showed encouraging performance parameters. Later on, zinc/bromine batteries were also developed for load-levelling applications.

Operation of Zn/Br systems is based o n the following reactions:

ne Zn2++ 2e- L--T - external circuit

Brz+ 2e- + 2Br-

28

W h e n the reactions proceed in the direction from lef t t o right, bat tery discharge occurs, whi lst in the reverse direction, charging of the bat tery i s accomplished.

T h e theoretical electromotive force of th is battery i s 1.83 V, but since complexing agents are involved, t he open circuit ce11 voltage is lower, namely 1.76 V. T h e theoretical energy density i s 436 Wh kg-', whi le the practical one is only 65 Wh kg-'. Other characteristics are: peak power 95 W kg-', depth of discharge loo%, and energy efficiency 60-65% [9]. The zinc/bromine bat tery 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 to the rapid self- discharge caused by the proper ty of bromine to dissolve readily in zinc bromide electrolytes and to diffuse to the zinc electrode osidizing it. To avoid th is process, it was necessary to remove bromine from the zinc electrode, and to divide the anodic and cathodic sections of the cell by a separator. Thus, a cell design \vas developed in which the bromine compartment of the cell was connected by nieans o f a

tube to a storage compartment for collecting the evolved bromine. T h e electrolyte, forced by a pump, circulated between the electrodes and the storage compartment. Th i s design principle lias led to a

significant decrease in bat tery self-discharge, but lias n o t el ini i i iated it completely. A schematic of the zinc/bromine system i s 1)reseIited in Fig. 12. Atomic bromine formed on the electrode i s indicated by the dots in the figure.

T h i s type of bat tery construction has solved anotlicr problem of zinc/bromine batteries as well. that of the zinc electrode. In the early static battery design, zinc deposited on t l ie clectrode iii the form of non-uniform dendrit ic plating. Dendrites s o n i d m e s grew across to the bromine electrode aiid caused short circuits in the cells. By introducing electrolyte circulat ion also in t l ie zinc half-cell, t l ie zinc deposit, formed over the electrode surface duriiig charge, becaine

29

more uni form and shortages were eliminated. Complete discharge is required for every charge-discharge cycle t o equalize the zinc distri- bu t i on over the 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 the introduct ion of electrolyte circulation, the temperature of the elec- t ro lyte becomes controllable and thermal homogeneity of the whole electrochemical system can be achieved.

Circulating electrolyte batteries require various auxiliaries t o con- t ro l bat tery 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, propor- t ional 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. Through the adoption of the bromine circulation loop and the storage compartment, the quantity of this active mass com-

30

ponent grows significantly. The zinc circulation loop also increases the volume of the electrolyte and hence the quantity of zinc ions. Nevertheless, zinc 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 la t ing deposited o n their surface. When high battery capacity is needed, the thickness of the Zn electrodes is increased.

T h e cathodic half-cell is divided f rom the anodic one by means of a microporous polyethylene separator with pore radius smaller than 1 pm. Such separators are commercially available and are widely used in lead-acid battery manufacture. T h e separator displays barrier properties w i t h respect to bromine diffusion, but it is permeable for the solution ions that carry the electric charges between the two electrode sections of the cell.

Consequently, the problem of mater ia l 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 product ion of these batteries feasible and cost-effective f rom an engineering point of view. However, improvement of the overall battery construction reliabil i ty is necessary t o eliminate a l l hazards of explosion or bromine leakage in the atmosphere.

Bromine is a strong metal-corrosive agent.

3.3.2. Chemistry and electrochemistry of the zinc/bromine cell

Dur ing charge of the zinc/bromine cell, bromine evolved at the electrode associates w i t h the bromide ions and dissolves easily in the solution of zinc bromide:

nBr2 + Br- -3 BrGn+l) (where n = 1,2 or 3)

31

Trihromitle ions (Br:) are formed first, then pentabromide (Br;) and eventually heptabromide (Br;). When the concentration of bromine rises significantly, it is evolved as a separate liquid phase collected in a special bromine storage compartment. In spite of this, however, bromine concentration in the solution remains relatively h igh 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, that 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 elec- t ro lyte composition of a discharged zinc/bromine cell is: zinc bromide 3 M, quaternary ammonium bromide 1 M, KC1 4 M as supporting electrolyte [g].

Dur ing 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 pumped 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 dur ing 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. Dur ing 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 ce l l is about 80% (i.e. 20% is inefficient). The inefficiency is mainly due t o electrode self-discharge. Other sources o f inefficiency are the energy losses for dr iv ing the

32

electrolyte pumps, as well as for supplying power t o t h e bat tery con- t r o l and management system and t o i t s microprocessor.

Discharge

L

O 60 120 I80 U0 Tlme. min

Fig. 13. Charge/discharge curves for a zinc/broniine ce 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 [ l o ]

33

In the monopolar version, there are separate positive and negative electrodes, and all monopolar electrodes are connected in parallel. Iii this way, high battery power and low voltage are achieved.

T h e bipolar battery design uses electrodes which have one zinc and one bromine side. These bipolar electrodes are connected in series, only the end electrodes being monopolar. Cell-to-cell current flows f rom the entire electrode surface and is carried through i t s thickness. T h e output is h igh 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. Th is is a reduced-size model of the 50 kW/400 kWh battery module for the 1 MW pi lo 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 rom November 1989 through September 1990. Operation tests started in December 1990 and will continue till March 1992. Th is is the largest Zn/Br battery in the world. Interesting and useful testing results are expected.

34

Table 3. Resul ts from testing of Zn/Br battery modules produced by M e i -

densha Electric Manufacturing Co., Ltd, Japan. T h e tests were performed

at the Government Industrial Research Ins t i tu te , Osaka [7].

10 kW battery [7]

Configuration (24 cells in series x 3 series in parallel) x 4 in parallel = 360 1600 cm2 x 13 mA cm-' 1.67 V x 166 Ah18 h

Unit cells

Open circui t voltage 43.8 V Maximum charging voltage 50.0 V Charging power 12.7 kW Discharging power 10.0 kW Energy density - per footprint

~ per volume - p e r weight

Dimensions - width 1.37 m - depth 1.59 m - height 1.67 m

33.6 kWh m-' 14.9 kWh m-3 29.1 Wh kg-'

Starting time 1 s Self-discharge rate 7.4% Stopping t i m e 1 s Change time charge-discharge 1 s Change t i m e discharge-charge 1 s Response to load change 10% 0.9 m s Discharge - 6 h capable eff.

- 4 h capable eff. Voltage variation - on charging

Energy consumption in standing 0% Test cycle life

68.3% 66.5% 19.9%

- on discharge 15.5%

appr. 550 cycles End of battery life was due t o carbon plastic electrode degradation

35


50 kW class battery module [8]

Voltage Current Output power Capacity Module configuration

External dimensions ~ width - length ~ height

Weight Energy efficiency ac-ac Energy density - per footprint

~ per volume - per weight

100 v 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-'

Imajuku Energy Storage Test Zn/Br Plant [SI Output capacity Output voltage

current ac-dc converter Output transformer Number of batteries Number of ceils

Submodule battery output power weight dimensions - width

- depth - height

Building area

4.4 MWh (1MW ~ 4 h) ac 6.6 kV; dc 1.1 kV

self-commutated 1000 kVA self-cooling 1200 kVA 24 submodules (series) 30 cel ls (scrics) x 24 stacks (parallel) x 24 submodules (series) = 17,280 cells

dc 1 k A

23 kW 8 tons 1630 mm 1520 mm 3150 mm 735 m2

36

3.4. Ziric/Chlorine batteries

3.4.1. Fundamentals of zinc/chlorine batteries

T h e electrochemical reactions o n which operation of this type of battery is based are as follows:

Z n e Zn2++ 2e- L-T - external circuit

C12+ 2e-F=+ 2C1-

Dur ing battery discharge, the above reactions proceed f r o m left t o right, and in the opposite direction dur ing 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 no 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 c i rcu i t depending on cell design. T h e depth 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 two halo- gens in the cell reservoirs should be used. Th is leads t o substantial differences in design of the two types of zinc/halogen cells.

Chlorine is slightly soluble in zinc chloride solutions. For this reason, dur ing charge, chlorine is evolved in the form o f bubbles that leave the electrolyte forming a gaseous phase. Th is gas should be collected and stored in an appropriate manner, t o be fed back in to the solution and reduced t o chlorine ions at the electrode, when elec- t r i c current is delivered by the battery. So far, there have been two methods for chlorine storage in use. In the first method, chlorine is compressed until l iquefication and is stored as liquid chlorine at pres- sures of 70-80 psig. When electric current is to be generated by the

37

1.75v cut aff

7 40 cm-2

coulûmblc

N

1 voltoge - 'u 2.18 v E 1 -

10- Charge coubmk+ Discharge -

jg- - -~~LcE2--~ - FUII pwer ----7? .- 5 40

\ 20- Usable

2 0 ( ( 1 1 1 1 I l I 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 elec- t ro lyte conductivity between the electrodes. Coulombic efficiency is about 87%, capacity losses being due primarily t o the self-discharge caused by chlorine diffusion towards the zinc electrode. As a rule, n o separator is used in this type of battery. The ne t electrochemical efficiency is decreased, because part of the energy is uti l ized for supplying power t o the auxiliaries: gas and electrolyte pumps, the inert rejection system, the hydrogen recombination system and the cooling system

2.4 2.0

-1.6 u -

-1.2 k --cl8 %

0.4

38

3.4.2. B a t t e r y design

EDA’s (Energy Development Association, USA) Zn/Clp battery design [5] is based on the use of graphite electrodes, a single chlorine circulating loop and cooling of the electrolyte and the gaseous phase t o form C12(H20),. A schematic of EDA ’s Zn/C11 battery is shown in Fig. 16.

e I

heat exchanger

Fig. 16. Schematic of t l i e c i r cu la t i ng 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 potas- s ium chloride ( to improve electrolyte conductivity). Using tubes and pumps, an electrolyte circulation loop is formed. Zinc is deposited o n the cathode. For a uni form plating t o be formed, the current density should be in t h e range f rom 20 t o 45 mA crnp2, and zinc loading between 90-300 mAh crnv2. Chlorine evolved on the anode is removed f rom the stack and pumped in to the hydrate storage reservoir. To allow formation of chlorine hydrate, pr ior t o mixing with chlorine the electrolyte is cooled by a refrigeration system. In the hydrate reservoir, an ice-li le s lurry o f chlorine hydrate is stored.

39

Dur ing battery discharge, the cooled hydrate s lurry froiri t l i e storc is passed through a heat exchanger, where chlorine l iydrntc i s tlcconi- posed. 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 os- idized to zinc ions that react w i t h chloride ions giving zinc cliloride. In the course of these processesi the electrical energy stored dur ing charge i s liberated, i.e. doing work.

T h e chlorine electrode is made of a porous graphite material. I t s surface is activated. T h e chlorine-containing solution passes through the pores of the graphite electrode, whereby chlorine is transformed to chloride ions ( “flow-through electrode”). (In t l i e zinc/bromine cell, the bromine electrode is of the “flow-by” type.) T h e rate of the chlorine-rich electrolyte stream deterniines the rate of bat tery discharge.

Self-discharge of the zinc/chlorine battery i s caused 1)y t h e reac- t i o n between zinc and chlorine. Electrolyte circulatioii aims at faster removal of chlorine f rom the electrode st,acli and hencc rcdiici i ig the self-discharge.

Zinc/chlorine batteries tend to 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 ia ultraviolet irradiation, for example. In this way, potent ia l hazards of explosions in the chlorine storage reservoir are avoided.

Zinc/chlorine battery design is usually based on bipolar electrode stacks.

3.4.3. Bat te ry characteristics

Design and development of a zinc/chlorine battery associated with the Energy Storage Project in Japan was carried out by the Furukawa Electric Co. Ltd. Th is company has developed 1, 10 and

40

50 kW battery configurations. The test results for 1 kW and 10 kW bat tery modules, before completion of t he cycle l ife tests, are presented in Table 4.

Af ter a crit ical analysis of the technological, performance and economical parameters of the above bat tery modules, fur ther development of zinc/chlorine batteries was interrupted. Th is type of bat tery poses serious environmental hazards since chlorine is a toxic

gas.

Table 4. Some resul ts f r o m test ing o f 1 kW and 10 kW Zn/Clz b a t t e r y modules produced by Furukawa Elec t r i c Co. Ltd. T h e tests were per formed at t h e Government Indus t r i a l Research Ins t i tu te , Osaka [7].

1 kW battery

Conf igura t ion

Unit ce l l - voltage

- capaci ty

O p e n c i r cu i t vol tage

Charging power

Discharg ing power (8 h rate)

Coulombic efficiency

Vo l ta ic efficiency

Energy efficiency

Self-discharge r a t e

~ initial energy efficiency

a f te r t w o weeks

self-discharge ra te

- after four weeks

self-discharge ra te

30 cells in series x

2 series in para l le l

2.0 v 75 Ah16 h r a t e

63.0 V 1.41 kW 1.01 kW 84.0% 83.0% 70.5-76%

71.1% 68.7% 3.4% 67.9% 4.5%

41


10 kW battery

Module

Unit ceiis

Open circuit voltage Maximum charging voltage Charging power

Discharging power (8 h) Energy efficiency - overall efficiency

- coulombic efficiency - voltage efficiency - aux. power efficiency

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.

Voltage variation - on charging

Energy consumption on standing

- 4 h capable eff.

- on discharging

- hot standing - cold standing

(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%

42

4. Lead-acid batteries

4.1. Some history

In 1860, Gaston Plante presented t o the French Academy o f Sci- ences a 9-cell bat tery (composed of lead and lead dioxide electrodes immersed in H2S04 solution and separated by rubber tapes) and a re- p o r t entit led “Nouvelle pile secondaire d ’une grande puissance”. Th is 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 mi l l ion cars worldwide have engines driven by high-power lead-acid batteries.

Dur ing t h e per iod 1880--1900, lead-acid batteries found their first practical application in the early power stations. They were used as a stand-by source of energy and power. With the progress of indust,ry and of dc-electroenergetics, product ion and usage of lead- acid ba tk r i cs as energy storage facilities gained increasing popular i ty to reach, in 1930, large-scale commercialization. In most towns in Germany, such as Berlin, Munich, Hamburg, Leipzig, Stuttgard and Bremen, large lead-acid battery storage facilities were in operation. 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 th in 30 minutes. T h e c i ty 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 distr ibut ion in t h e 1930s, the dc battery system was abandoned, to b e revived again dur ing 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. We wi l l discuss the technical and economical aspects of lead-acid battery energy storage

43

technologies in the next sections o n the basis of knowledge, experience and informat ion 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 ts electromotive force (emf) are:

+

- Pb+H,SO, P b S 0 4 + 2 H t + 2 e -

PbOz + 2H+ + HzS04 + 2e- + PbS04 + 2 H z 0 T P L +- external circuit

These reactions together w i t h the corresponding charge/discharge curves of the cell are presented in Fig. 17.

DISCHARGE

O io 20 Time. h

CHARGE

+j- e- . + Z

Recti1 ier f - '

Positive plate Negalive plate

7

I = 3.2 A

2.m io 20

Time, h

Fig. 17. A scheme of t h e charge and discharge react ions proceeding in the

lead-acid cel l and t h e corresponding vol tage transients.

44

Calci i latcd t l icrrr iod~namical ly, the voltage between the lead sulfate and t l i e lead dioxide electrodes in the cell is 2.040 V, but the open circuit voltage is iisually taken as 2.0 V (rated voltage). T h e tlieoretical specific energy of the cell is 170.2 Wh kg-’. To trans- fo rm the lead-acid cell i n to a practical power source, several design requirements must be 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 to 55%) participates in the reactions leading t o generation and accii i i i i i latioii of cncrgy. T h e active materials are f ixed 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 i ises approximately 36% H2S04 solut ion as electrolyte. T h e positive aiitl iiegative plates are interconnected in s e m i - blocks with terni inal posts protr i idir ig f rom the cell. T h e plates of the lead and the lead dioxitle seini-bloclis together with the separators and the electrolyte hetween thein form the active block. In it, all processes occur, eiicrgy is accumulated and electric ci irrei i t is generated during discharge. Ahove the active block, there is a space containing a

certain amoii i i t of HLSO4 soliition (upper reservoir). Below t l ie active block, another free space is available where the shedded active niass is collected t o avoid short-circuits between the plates.

At t l ie end o f charging, decomposition of water takes place and H2 and 0 2 gases are evolved. T h e cell is provided with a valve as an outlet for these gases. Since gas evolut ion is associated with water consurnption, an equivalent amount of water must be added periodically t o the cell iii order t o maintain the required electrolyte conccritratjioii. Water is added through the outlet. T h e cells are mounted in a plastic container f i t ted with a cover. T h e cells are jo ined in series with lead connectors that may b e situated over the cover, or pass through the cell part i t ions ( “through-the-wall’’ arrangement).

45

T h e construction of a coiiventional present-day SLI lmt tcry 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, the close circuit voltage depends on 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 th in a given per iod of time, after which t h e voltage begins t o decrease rapidly. Since deep discharges have an adverse effect upon battery performance, a limit is set for t h e end- of-discharge voltage (U, ~ f inal or cut-off voltage). When the t ime 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 l ine. Th 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 tar ter

battery [12].

Capacity. T h e capacity ( C d ) of a bat tery 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 ime ( t ) needed for reaching the final discharge voltage i s marked o n the abscissa and is known as the rate of discharge. In- ternational and domestic standards require the capacity t o be determined by a discharge w i t h a current at which the battery reaches U, = 1.75 V at 20°C after 20 or 5 h (C20 or CS). Th is capacity is known as the rated capacity. Under normal operating conditions, the battery should no t be discharged beyond 80% of the rated capacity. Th i s capacity is known 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, the HzS04 concentration and the design of the battery.

47

12V11OOAh SLI battery

I I I I I h 20minlOrnin 5 min lmin 1 I I I I l I I

O ~ l O O 2 O O 300 400 6 0 0 1 . A Rate oí discharge (1) or current (I 1

Fig. 20. Capac i ty 'us.

12 V/100 Ah star ter b a t t e r y [13].

discharge cu r ren t (I) or discharge r a t e (t) of a

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 mult ip l ied by i ts capacity. Figure 21 presents the dependence of energy and mean voltage on discharge current. When the discharge current increases, the energy delivered by the battery is decreased. Therefore, in battery energy storage plants, discharge of batteries should be carried out w i t h moderate currents, i.e. 3-10 h discharge rate. T h e delivered energy under rout ine 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 known as specific power of the battery. Figure 22 presents the power us. current dependence. When the current increases, power is also augmented. Therefore, in order t o deliver h igh power, batteries are designed t o be discharged at heavy currents. As the capacity decreases w i t h increase of current, the discharge t ime will decrease rapidly.

48

@ 12VI Kx) Ah SLI battery

20h 20minX)min Smin Imin t I I I I I I

Current, A Fig. 21. (a) Average discharge vol tage vs. discharge cur ren t ; (b) energy vs.

discharge cur ren t for a 12 V/100 Ah b a t t e r y [13].

100 200 300 400 Kx) c Current, A

O

Fig. 22. Power vs. discharge cur ren t of a 12 V/100 Ah s tar te r b a t t e r y [13].

49

Cycle life. T h e service l ife o f a battery is the number of charge/discharge cycles obtained during laboratory bench tests. The 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 l i fe o f a battery may be longer or shorter t han that experienced under laboratory conditions. During practical use, the battery is subjected t o other l i fe-l imiting factors that are no t taken in to consideration in the laboratory tests. Current test procedures are aimed at maximum simulation of real operating conditions.

4.4. Charging characteristics

That par t of the current uti l ized for the format ion o f lead and lead dioxide dur ing battery charge i s called charge acceptance. The 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

o 20 40 60 80 loo120 5 0

Charge ;,, Rated capacity

Fig. 23. Charge acceptance of a t rac t i on ba t te ry ws. i t s state-of-charge

at 30°C [14].

The data show that almost the entire amount o f charging elec- t r ic i ty 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 bat tery charg-

50

ing with a current of 0.1 A per Ah, the cell voltage reaches 2.35 V and gas evolution starts. Af ter that, water decomposition proceeds simultaneously w i t h the charging reactions. T h e charge acceptance is gradually and continuously reduced. Cel l voltage is increased f rom 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 ma in processes that take place. The battery is overcharged.

These stages can be clearly identified by galvanostatically charging a bat tery which has previously been subjected t o three different depths of discharge (50, 75 and 100% DOD). After these discharges, the efficient charge stage acquires three different durations. Figure 24 shows the changes in cell voltage dur ing charge. T h e charging current was 0.1 A per Ah.

2.8 - 50 per cent 75 per cent

dischorqed Time

20

Fig. 24. Changes in celi voltage du r ing charge, fo l lowing three discharge

runs to dif ferent depths [15].

T h e gas evolution voltage is 2.35-2.40 V per cell at 75% state-of- charge.

T h e following parameters are used t o define charging regimes: o In i t ia l and final charging voltage (2.1 t o 2.4-2.7 V per cell). o In i t ia l gas evolution voltage (2.35 t o 2.40 V per cell). o Charging current dur ing the efficient charge stage (0.3 t o

0.1 A Ah-' or I c h = 30-10% C, A).

51

o Current at the beginning of gas evolution (I:h = 0.07 A Ah-'

o Final charging current (ICh = 0.01-0.03 A Ah-' or I!h = 1-3%

o Upper charge temperature limit (45-50'C).

or I,h = 7% C5 A).

c5 A).

T h e durat ion 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 no t be enhanced, thus ensuring long service l ife of the battery.

There are several charging regimes in use for energy storage plant batteries which meet the above requirements: a) controlled current- voltage charging method, b) tapered charging method, and c) pulsed charging method [13].

T h e specific charging method for each battery is usually pre- scribed by the battery manufacturer.

4.5. Effect of electrolyte stratification

During discharge, the concentration of the acid in the cell 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. The formation of concentration gradients in the active block of a 400 Ah battery was studied during cycling w i t h 169 mA Ah-' at 100% DOD until a cut-off voltage of 1.7 V/cel l was reached [16]. Figure 25 shows the concentration changes dur ing four consecutive cycles at a charge/discharge rat io of 1.02.

Stratif ication of the acid is enhanced as the number of cycles is increased. This indicates that the concentration changes accumulate during cycling. The difference between the acid concentration at the top and the bo t ton 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 lec t ro ly te s t ra t i f icat ion measured during b a t t e r y charging t o 2%

overcharge and af ter 100% DOD [16].

As HzS04 is an active material, stratif ication w i l l affect ce l l ca-

pacity. It has been established that the capacity decreases by l% for each 0.01 sg unit of stratification. This capacity loss depends on the DOD and the charge/discharge ratio. With increase in overcharge, the extent of stratif ication (and hence the capacity loss) i s diminished. To eliminate fully the capacity losses due t o strat'ification, the battery should be subjected t o 15% overcharge. Since the ac- t ive 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 the equal- izat ion 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 wi l l increase the corrosion of the positive grids, o n the other hand. That is why the use o f devices for forced electrolyte circulation is recommended. These are usually smal l air-lift pumps tha t let an air flow in to the cell to st ir 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 is about 15% greater than that o f the discharge.

I l

Charge or discharge S

F i g . 26. discharge [17].

Dependence of t h e cel l po ten t i a l o n t h e state-of-charge or

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. Th is can be achieved by reducing the 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 durat ion of overcharge.

T h e highest energy losses are related t o battery overcharge. If the latter is eliminated, however, the process of conversion of lead sulfate t o lead and lead dioxide w i l l no t proceed fully and the battery will be gradually sulfated. Besides, a stratif ication 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 st ir the electrolyte. Th 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 in to the inner tube of a double-wall cylinder f rom the top. Th is air l i f ts the electrolyte, while ascending as bubbles, through the gap between the inner and outer tubes. T h e electrolyte is sucked f rom a hole at the lower part of the outer tube and ejected f rom a hole at the upper part o f the same tube.

T h e obtained results have shown that: first, bat tery charge wi thou t overcharge is possible whereby no plate sulfatization proceeds when the electrolyte is stirred thoroughly t o prevent stratif ication; 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 wi thout electrolyte stir- ring, 120% charge wi thout external agitation but w i t h intense gas evolution st irr ing 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. Inf luence of overcharge and electrolyte s t i r r ing o n cycle l i fe [17].

As can be seen from the figure, when lead-acid batteries are subjected to charge-discharge cycling w i t h 100% state-of-charge and electrolyte st irr ing 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, no t only is the l ife 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 refi l l ing the cells w i t h water. Water is lost f rom lead-acid batteries through the following processes:

e Electrolysis of water during overcharge o f the battery o Self-discharge under open-circuit conditions o Evaporation of water

Manual f i l l ing up of the cells t o a constant electrolyte level is very laborious and time-consuming, especially in a bat tery energy storage plant. Therefore, major challenges in battery servicing are t o find a way t o reduce water losses and t o replace manual ref i l l methods. T h e following methods have been proposed:

a) Single-point (common point) watering. Th is 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 addi t ion system is f i t ted that can automatically adjust the electrolyte level in each cell o f the battery. This watering system comprises:

devices to 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 rom the ce l l t o eliminate explosion hazards.

Finally, the design should avoid electrical shorting between the cells through the common f i l l ing system.

These systems have found wide application in stand-by energy facilities for power plants, post offices, cul tural and commercial centers using stand-by lead-acid batteries. A common point re f i l l system is utilized in many battery energy storage plants. Operat ing experience of batteries w i t h the above re f i l l system in various modifications has shown that it is not always sufficiently reliable and safe.

57

b) Catalyt ic plug recombination of hydrogen and oxygen. De- signed t o recombine hydrogen and oxygen t o water that is brought back in to the cell. Instead of a cell valve, a catalytic plug is included that enables the following reactions t o proceed:

2H2 + O2 + 2H2OVapOUr +114 kcal H~Ovapour + HZOiiquià +9.7 kcal

T h e first reaction requires a stoichiometric rat io between the evolved H2 and 0 2 , and is accompanied by the release of a great amount o f heat. The latter causes the cell temperature t o rise as a result of which the reaction rate is increased. If left uncontrolled, this could lead t o an explosion. Various designs of catalytic plugs have been used t o avoid this hazard.

Metals f rom the platinum group are used as catalysts for the recombination. Carbon, alumina or asbestos wool are usually used as catalyst carriers. T h e major disadvantage o f this water recycling method is the high price o f the catalyst materials, which has restricted large-scale application of the method.

During charge-discharge operation, hydrogen and oxygen are often evolved in non- stoichiometric amounts. That is why the efficiency of the catalytic plug is reduced. A method has been proposed suggesting that two auxiliary catalytic electrodes are f i t ted in the cells. They 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 r ight potential for the electrochemical reaction of hydrogen oxidation, the hydrogen electrode is connected through a proper electronic device (diode and resistor) to

58

c) Closed oxygen and hydrogen cycles.

I I L J pm2 Pb Pb02 Pb

Fig. 29. Schematic of cells with auxiliary electrodes [is].

the lead dioxide electrode of the cell. T h e reactions proceeding at the two auxiliary electrodes are catalyzed by metals f rom the platinum group. Th is makes the method very expensive and hence i t s applica- t ion 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. During charge, the following reactions take place in the cell:

positive plates

PbS04 + 2H20 + PbO2 + 2H+ + H2S04 + 2e-

HzO - ;O2 + 2H+ + 2e-

negative plates

PbS04 + 2H+ + 2e- + Pb + H2S04 2H+ + 2e- - H2

Figure 30 shows the relationship between the charge acceptance o f positive and negative plates and the t ime of charge.

20

2.6

2.4 2 c I

22

2 .o O 1 2 3

Chorge tlrne, h

Fig. 30.

vus. t i m e [2O].

Charge acceptance of pos i t ive and negative p la tes at 40°C

When the positive plate reaches a 60--70% state-of-charge, a reaction o f oxygen evolution starts and i ts rate is increased w i t h time. Th is leads t o an equivalent decrease in positive plate charge accep-

tance. T h e negative plate is charged w i t h 100% charge acceptance until about 95% state-of-charge. After that , hydrogen evolution starts. Th 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 the one hand, the negative plate is kept discharged and hence evolution of hydrogen is prevented, and on the other hand, oxygen itself reacts in the cell. T h e basic problem w i t h this type of oxygen

cycle battery was t o prevent oxygen f rom being lost f rom the active block. A solution was found in immobilizing the H2S04 electrolyte between the plates. Th is was achieved by:

60

o Using gelled electrolyte. The cracks formed in the gel act as channels for the transport of oxygen. Th is 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. l ias enhanced this technology [22,23] and is now manufacturing gelled-electrolyte batteries for bo th t ract ion and stationary applications, and for energy storage systems as well.

o Using absorptive glass mat separators. T h e electrolyte in th is case is absorbed by the glass fibre mat that has the property o f absorbing H2S04 and water, and adsorbing oxygen. Oxygen is retained in the glass mat separator and flows f rom the positive t o the negative plate, while H2S04 and water participate in the reactions in the cell. Gates in the USA [24,25] and I’UASA in Japan [25] were the f i rs t t o employ this technology in small batteries, and later in stationary batteries. Now these batteries are under large-scale tests for energy storage applications.

The problem of oxygen retention between the plates l ias found an adequate technical solution, but even so, over time, ccrtain amounts of hydrogen are evolved at a low rate, which accumulate above the electrode stacks in the cells. T o avoid explosion, a valve is f i t ted that lets the gas out and controls the pressure in the cell. These batteries are maintenance-free and are called valve-regulated batteries.

61

5. Lead-acid batteries in the battery energy storage system (BESS)

5.1. systems

Functions of lead-acid battery energy storage

After the revival of interest in energy storage systems, the first batteries 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 uti l i t ies. Technical, technological and economical informat ion has been gathered over 2-3 years.

Analysis of the testing results of these demonstration BES plants shows that battery energy storage systems can improve the opera- t ional efficiency and the cost-effectiveness of the electric power system by providing the following functions.

a) Load-level l ing: 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 to 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 major 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 th in several milliseconds. Through charging of the battery f rom the energy utility, electric power is concentrated in the battery. Th is power can be delivered through high-current discharge of the battery (Fig. 23) when a high peak demand appears. Th is method of energy storage allows electric uti l i t ies 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. The battery storage unit takes over part of the load by discharging electricity, thus enhancing the stabi l i ty of the power supply.

d) Frequency control and spinning reserve: To ensure normal operation of the customers' electrical devices and machines, stable ac frequency should be provided. Overloading of the electric power system may cause the frequency of the supplied current t o fluctu- ate. Storage batteries may play the role of spinning reserve and, through discharging, compensate for such frequency distortions and thus maintain the frequency of the local power system w i t h i n the desired l imits.

Utility

lines charging (off p e a k )

' Battery dixhorgi ng (on p k )

1 r--,--- --

Fig. 31. Schematic of a customer-owned LABESS [il. 63

Some of the operational LABES systems are listed in Table 5.

Table 5. L A B E S systems in operation by 1990 throughout t h e world [27].

Company Size In service App l i ca t i on

Utility operated

Ber l iner Kraft und L ich t (BEWAG), Berlin, FRG 14 MWh

Kansai E lec t r i c Power Co. Ltd Tatsumi, Japan 4 MWh

17 MW

1 MW

Southern Ca l i fo rn ia Ed ison Co. Chino, CA, U S A 40 MWh

10 MW

Customer operated

Elek t r i z i tä tswerk 400 kW Hammermi ih le 400 kWh Selters, FRG

Hagen Battcric AG 500 kW Socst, FRG 7 MWh

Crescent Electr ic 500 kW Membership Corpo ra t i on 500 kWh Statesvil le, NC, U S A

Delco Remy. Division 300 kW of General M o t o r s 600 kWh Munc ie , IN, USA

Vaal Reefs Exploration 4 MW

Sou th A f r i c a

Johnson Controls, Inc. 300 kW H u m b o l d t Foundry 600 kWh Mi lwaukee, WI, USA

and M i n i n g Co. 7 MWh

1986

1986

1988

1980

1986

1987

1987

1989

1989

frequency regul. spinning reserve

demonstrat ion mul t i -purpose tes t program demonstrat ion mul t i -purpose test program

load-levell ing peak-shaving

load- level l ing peak-shaving

load-levelling peak-shaving

peak-shaving

peak-shaving emergency power

peak-shaving load- level l ing

64

T h e table shows that when battery storage plants (LABESP) are installed at the side of the electric power generating uti l i t ies, they have a power of over 1 MW. T h e power of lead-acid bat tcry 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 facil i ty in South Afr ica which no t only serves for peak-shaving purposes, but also for emergency power supply.

Customer operated lead-acid battery storage facilities are uti l ized primarily for peak-shaving and load-levelling. Al l thcse applications bring immediate financial profits t o the customers, and make theni less dependent on the power supply ut i l i t ies in peak deniand periods. Thus sufficient stabil ity of the technological processes i s guaranteed.

T h e use of lead-acid battery storage plants by electric power generating uti l i t ies is aimed no t only at levelling the electric loads, but also at improving the qual i ty of t h e energy delivered by the ut i l i t ies t o the customers. LABES systems can funct ion a.? spinning reserves; provide instantaneous fast power reserve; regulate frequency, voltage damp-out, subsynchronous oscillations and other system instabilities.

Wh ich opt ion t o choose for BEC plant location - before or after the meter?

I f the power demand is no t very h igh and a number of customers could be grouped on a terr i tor ial principle, si t ing LABES plants on 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 ma jo r energy consumers having high peak power demands, e.g. commuter railroads and stations, metropol i tan t ra in 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 instal l the LABEC facil ity o n the customer side of the meter.

65

Battery charging could be accomplished at any off-peak intervals during the day and the night.

Spindler [ l ] has performed a cost analysis o f the ut i l izat ion of LABEC plants by major electric power consumers in the USA. The investment pay back periods have also been estimated. T h e obtained results are summarized in Table 6.

Table 6. Economic analysis of selected cases of LABESS appl icat ion o n the customer side [il.'

Customer / D e m a n d Converter B a t t e r y Capital Payback App l i ca t i on charge size size cost per iod

$ kW-' M W / a c M W h / a c $M years

C o m m u t e r ra i l r oad 16.24 5.8 5.6 2.48 2.2 Steel manufacture 11.88 3.5 2.3 1.46 3.5 Copper al loys plant 13.50 0.9 0.7 0.53 3.5 T ruck part plant 13.82 0.5 0.7 0.56 4.3 Chemical manufacture 9.87 0.4 0.5 0.32 6.2 ~ ~~~ ~ ~ ~~

Based 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, operat ing 250 days yr-l.

It can be seen that with commuter and metropol i tan railroads, as well as steel and alloys product ion using electric furnaces, customer LABEC plant construction is economically effective.

In b o t h cases, applied before or after the meter, bat tery 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

Th is consists o f the following basic components:

0 lead-acid battery o ac/dc power conversion system

o facil i ty monitor ing and control system.

(converter, transformer, dc and ac switchgear)

T h e electric energy is supplied by a utility distr ibut ion network through an ac switchgear t o the high-to-low-voltage transformer. T h e n an ac-dc converter follows. Through a dc switchgear, the current is fed in to the battery t o charge it. Operation of all these LABES plant components is managed by a monitor ing 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 in to the utility distr ibut ion line.

Figure 32 gives a schematic o f a lead-acid battery energy storage system.

Facility monitoring

dc 1 oc -I- Charge -1 - Discharge

F i g . 32. Schematic of electr ic utility b a t t e r y energy storage system [il. 67

T h e battery energy storage plant is located near a substation o f the 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. Let us take for example the world’s largest, up t o now, lead-acid battery energy storage system in Chino, California, USA.

6.2. Chino 10 MW/40 MWh lead-acid battery energy storage system

6.2.1. Plant layout

Figure 33 presents a floor plan of the battery energy storage plant.

Demineralized

Fig. 33. Edison Chino Substation [28].

68

General arrangement of LABES plant at Southern California

T h e Chino LABES plant is sited 50 miles east of Los Angcles, in the vic in i ty of the Chino 220 kV Substation of the Southern California Edison Company. The Electric Power Research Ins t i t u te (EPRI) and the International Lead-Zinc Research Organization Inc . (ILZRO) are project participants.

T h e plant consists of two large parallel buildings housing the batteries. Between them is the common converter/control bui ld ing 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 the needs of 5000 customers.

6.2.2. The battery

T h e Exide Corporation has supplied the bat tery 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 Nat ional Laboratory has tested these batteries and found that they could endure more than 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 rom 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. The battery delivers 50 MWh eri- ergy at a maximum 10.5 MW power for 5 h during dai ly 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.

Table 7. Ch ino L A B E S plant ~ B a t t e r y specif icat ion requirements [29].

R a t e d discharge requirement - energy ou tpu t (MWh) - power (MW) - t i m e t o 1.75 V (h)

N o m i n a l voltage Opera t i ng range N o r m a l daily cut -of f voltage B a t t e r y has 1032 cells in series s t r ing B a t t e r y l i fe (80% DOD) Daily recharge Overa l l energy efficiency - ceii dc t o dc - bat te ry

Opera t i ng temperature range St ib ine & arsine gas emissions:

Peak flow SCFM Daily (8 h) SCFM T o t a l pe r day g T o t a l annual lb

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

8 years = 2000 cycles

6-10 h

78% 75% 32-1 17°F

SbH3 ASH^ 5 ~ 1 0 - ~ 2 x 1 0 4 3 ~ 1 0 - ~ 1.1 x 10-~ 34 0.77 19 0.42

T h e cell is an Exide special design hybrid construction w i t h PbSbAs positive grids and PbCa negative ones. The positive ac- t ive mater ia l has a low density and contains anti-shedding additives, yielding a high active mass ut i l izat ion coefficient. T h e negative active mass contains a long-life expander composition and has high density.

T h e cell design features are presented in Table 8. Figure 34 shows the changes in cell voltage during battery dis-

charge with various discharge currents. Three cut-off voltages are given in the figure.

70

Table 8. Ce l l design features [29].

,

R a t e d capaci ty (I = 650 A, 5 h t o 1.75 V) Energy o u t p u t Posi t ive plates (17.3” x 13.7” x 0.33”)

grid a l loy Negat ive plates (17.3” x 13.7” x 0.21’’)

grid a l loy Ce l l cover ma te r ia l Ce l l j a r ma te r ia l Sediment space Separator system (on positives)

- absorber mat - reta iner - separator

A c i d specific g rav i t y (top-of-charge) Te rm ina l posts

Post-to-cover seal Cover- to- jar seal

- design: l o w torque maintenance

3250 Ah 6.2 kWh 17

1 8 Pb-Ca PVC SAN 1.3”

Pb-Sb

non-woven glass per forated PVC ni icroporous r u b b e r 1.28-1.29

l ead p l a t e d copper slide-lockTM s t ruc tu ra l adhesive

l.m -

1.600 2000A MOA 520A

1.75 V

1.67 V \ 1.60 v

380 A 220 - A

I I I I I I I I I I ,

O 2 4 6 8 10 12 14 16 Is 20 2 2 :

650 A 1.500

l ime , h

Fig. 34. I n i t i a l cel i vol tage transients at discharge [29].

On the basis of these curves the dependences of mean voltage, capacity, energy and power on the rate of discharge were plotted. They are presented in Fig. 35.

1.0

C . Ah

4400

4000

3600

3200

2800

2 4 6 8 10 12 14 16 18 2 0 î . h i l ' l l l l l ' l l

I I 795590 390 1.A 210 E . P.

-

-

- , ( , ti0 , j k W 1 I,A

-

- - -

-

I 1

7 5 5 9 0 390

8

7

6

5 4 6 8 10 12 14 16 18 20 t . h

Fig. 35. M e a n voltage, capacity, energy and power ?IS. t h e ra te 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. Th i s has allowed successful employment of the bat tery for 4-hour discharge applications. Up t o the s ixth 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 AV/A I method) t o be 0.025 ohms during the 10 MW

72

discharge. Average resistance of the str ing has been measured as 0.20 ohms at full state-of-charge, which means that the 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 st 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 brk î

Fig. 36. Drawing of a battery module [29].

44 modules (264 cells) form a row. Four rows separated by foot aisles form a string. The battery has 8 strings and 1376 modules or 8256 cells housed in two 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 thou 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 than that of the other cells. Th is 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. During the 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.

Table 9. Load-level l ing ba t te ry accessories [29].

Loca t ion

1. A u t o m a t i c water ing valve s t i b i n e l a n i n e trap flame arrester

2. Air lift pump

3. E lec t ro l y te w i thd rawa l t ube 4. Thermocouple we l l

5. A c i d level ind icator 6. In te rce l l connectors

7. H i g h voltage protectors

8. In te rmodu le cables 9. Lugs o n cables

each ce l l

each cel l

each cel l

0.5% of cells each cel l

l ead p la ted copper t w o per pos t expanded p last ic t ube a round each t w o in terce l l connectors

minimum 2 each 4/0 2000 V 410 long ba r re l compression t y p e 2000 V r a t i n g

74

T h e positive plate grid alloy contains As and Sb. T h e grids cor- rode, 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 potent ia l becomes more negative than a certain value, As and Sb form AsH3 (arsine) and SbH3 (stibine) gases, respectively. Since these gases are toxic, they should be retained in the cell. Th is 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 rom 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 the 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 imi ted number of bat tery 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 no t less than 2 x l o 6 o h m cm. Each cell is f i t ted with an acid level indicator.

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)

metr ic tons metric tons metric tons

metr ic tons metric tons

EA EA FT

1560 30

153 90

480 280,704 139,600

6,048

75

Maintenance and monitor ing of the battery is the most labour- consuming i t e m o f the LABESP operation and maintenance. T w o ful l - t ime battery electricians w i t h part- t ime support personnel are- needed t o provide proper maintenance of the battery. T h e major 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. Cel l cleaning takes about 4.5 man-weeks and is done every six months. Da i l y monitor ing of the voltage, current and capacity on a string-by-string basis is performed f rom the control room. T h e most impor tant performance parameter is the end-of-discharge voltage of each cell. By means of an infrared camera, warm cells are detected dur ing the end of discharge that denote possible weak cells. The electrolyte temperature is determined on the basis of a number of selected cells.

During i ts operation for over two 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 rom 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. The converter is a seif- commutated, Gate-Turn-of f voltage source, stepped-wave design w i t h a response t ime of 16 milliseconds. It consists of three six- pulse three-phase converter bridges connected in series o n the ac side

76

X f O r m W GTC I T 1 -

utility BrE!!w ~ 411 - T

Fllter cap

I GTC

- 3 Inverter control ,

Table 11. Major power conditioning system characteristics [32].

n

B r e a k e r ?

II L I 1 1

5 battery strings

-&k*

I I

Power - real ~ reactive

Efficiency Harmonic voltage Ripple voltage

Response Voltage input range

- nominal - max - min

dc to battery

10 MW, charge/discharge 10 MVA, leading/lagging 97% one way 3% total, 1.5% any single frequency

1.5% RMS voltage 16 ms

2.112 VdC 2.860 Vdc

1.750 V d c

77

T h e dc entrance equipment consists of two 3000 A, 3000 V dc high-speed breakers, dc no-load break battery str ing connectors, fuses, and disconnect switches. The ac entrance equipment includes 12 kV vacuum switchgear, single-phase transformers, three-phase neutral transformer, f i l ter capacitors, surge arresters and ac protec- t ion relays. T h e control system o f the converter is a microprocessor- based interface w i t h pre-programmed algorithms for charging and discharging operations. T h e control system communicates w i t h the facil i ty monitor ing and facil i ty control system [32-341.

6.2.4. Faci l i ty monitor ing and control system

The microprocessor-based control system provides complete faci l i ty 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 in to the system. T h e data acquisition system records data o n facil i ty operations to 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 s ixty seconds. A microcomputer is also connected t o the data highway access f rom a remote computer located at the Edison main office [32-341.

6.2.5. Equipment energy losses

T h e distr ibut ion of p lant losses f rom 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 to ta l losses. T h e inefficiency of the Power Con- di t ioning 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 to ta l energy input, on a dai ly basis, is 58.6 MWh and the useful energy output is 42.1 MWh o n average. Bat tery systeni efficiency is a funct ion of operating parameters such as overcharge, rate of discharge, etc. [31].

6.2.6. Economics of Chino LABES Plant

The tota l cost of the Chino LABES Plant i s about $ 13,560,000 or 1,350 $ kW-'. T h e distr ibut ion of these costs in various items i s presented in Table 12.

Table 12. Chino LABES system costs [35].

I t e m Cost, $ kW-'

10 MW lead-acid battery 600

Balance of plant 325 Plant site and tie-ins 52 Engineering and management 76 Tax, escalation, and contingency 103

Total: 1,356

Power conditioning system 190

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). Th i s storage system has been designed for demonstrat ion purposes and has been initially intended for load-levelling applications, as a large active store of energy. That is why it has been designed for a h igh 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 ts cost. Energy generating facilities wi l l ing t o reduce the 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 on 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 be activated at h igh 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. The latter should no t exceed f 0 . 2 Hz. For Europe, 50 f0 .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 to the con- struct ion 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 th in the required l imits.

-Outage of o 1200 MW unit

Outage 01 2500 Mw

P F

BEWAG island system

O 5 10 K 25 t , s

Fig. 39. systems [33].

System frequency response af ter unit outages in di f ferent grid

That was no t the case w i t h the West Ber l in power supply system, which was a typical example of an island network. At an outage of 146 MW, frequency deviations reached up to I MHz. In order to hold the energy system frequency within the range of 50f0.2 Hz, it was

81

no t enough t o have considerable reserve power generators available, but they also had t o respond very quickly t o the demand changes. Th is could be achieved if the power gradient o f the generating systems was higher than 5-10 MW s-'. That meant that the power system should comprise generating uni ts with a response t ime of the order of milliseconds. Lead-acid batteries display such a response time. With the 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 on high- power lead-acid batteries.

Th is 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 per iod during which encouraging positive results were obtained, BEWAG decided in 1984 t o instal l 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 of the turbo- generator. 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 elec-

tr ical 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 to ta 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 on ly one thyristor per string. Thyr istors have maximum power (12-pulse unit) of 8.5 MW and a rated dc voltage of 1200 V. This design meets the safety regulations well [36].

The battery i s designed t o exhibit a very l ow 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 per iod of at least 30 minutes.

83

T h e battery consists of 12 parallel strings of 590 cells each or a to ta 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 the polypropylene lid are heat sealed in place. In order to reduce the resistance o f the connectors, the cells are connected in series in the monoblock through the cell partit ions. Low-antimony tubular positive plates are used. To improve the negative grid conductivity, it is made of expanded lead-plated copper mesh after Hagen’s technology. Air lift pumps are provided in each cell t o st ir the electrolyte and prevent it f rom starving during operation. Refil l ing of the battery is accomplished automatically by a common watering system. Each cell is provided w i t h a water-fi l l ing 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

Fig. 41. T y p i c a l operat ion o f test faci l i ty, compr is ing two days o f load-

frequency-control operat ion w i t h a subsequent constant-current constant-

p o t e n t i a l charge [il. 84

T h e battery i s permanently connected to the system and monitors load fluctuations. W h e n the frequency begins to decrease, t he bat tery discharges power to the systeni, and vice versa, when the frequency exceeds the upper limit, the battery charges. ‘ T h e fluctuations in charge and discharge currents are given in the figure. These changes affect t he state-of-charge of the battery which is presented in Fig. 41a. T h e battery i s ra ted in such a way that load fluctuations would not cause i t s state-of-charge to fall below 50% of the noniinal capacity. Normally, the bat tery operates at 70% state-of-charge.

With the introduction of the BEWAG LABES plant, previous frequency deviations of more than f 0 . 2 Hz have been halved owing to the excellent dynamic response of the lead-acid bat tery energy storage system. T h e frequency control function of t l ie bat tery is accomplished in a severe operating mode. On calculating the quantity of electricity passing through the cells during the intermit tent charge-discharge cycles, it t i i rns out that the capacity turnover for a 24-hour period is as high as three times the battery capacity [37] .

T h e energy efficiency of the battery is about ô7%,, but nevertheless there is the tendency to bat tery overheating in warm weather which necessitates restriction of the battery inaximum power [37] .

T h e battery has also been used as an instantaneous reserve in reference to a steam storage system. A comparison of results has shown clearly the superiority of the LABES system.

Bat tery maintenance comprises automatic fill 111’ of t l ie cells weekly. T h e LABES plant operates unmanned under remote ~ 0 x 1 -

trol from the power stat ion control room.

85

8. Lead-acid battery energy storage systems for peak-shaving

8.1. What i s peak-shaving?

At railway stations, there are usually one or two short intervals during the day when several trains depart simultaneously. Th i s creates an extremely high peak power demand. Furthermore, in alloy product ion plants employing electric furnaces t o melt the alloy components, high power needs may also occur for short periods of time. Since alloy product ion is performed in batch mode, the above h igh 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 th in their normal l imi ts dur ing 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 durat ion of peak load periods, the customer must pay for the claimed peak power demand, because the latter is related to the size and the power of the generating plant, and the capacity of the transniission and distr ibut ion 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 be installed t o take over the supply of peak power, and hence the customer w i l l c la im 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 prox imi ty 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

&OC

200

z C O

6 a

- 20c

- coo

Commuter rai Iroad peaking

Charge

I I I I 1 . 1 O L 0 12 16 N

Clock lime. hours

Fig. 42. Peak-shaving profile for a commuter railroad substation.

T h e LABES facil ity manages the peak loads occurring when trains pass along the railroad segment served by the substation] and is then charged t o replenish the amount of energy wi thdrawn irrespective of the t ime 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 the interest in them so great, that commercial product ion o f such facilities can be expected to begin in the near future.

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8.2. Johnson Controls 300 kW/SOO kWh LABES facility

Johnson Controls Inc . (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 facil ity based on valve-regulated lead-acid (VRLA) cells used for the first t ime 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 facil i ty 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 to give 6 V/2700 Ah modules. Lead-coated copper busbars are used t o connect the lead-coated copper cel l terminals. Th i s results in low battery resistance. Each niodule is f i t ted 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 rom 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.

Table 13. VRLA b a t t e r y at Johnson Cont ro ls H u m b o l d t brass foundry [38, 391.

B a t t e r y rating Depth-of-discharge Discharge range End o f charge GC6-1500D gel cells Modu les (64) Forced air cool ing

2 h at 300 kW 60% 360 t o 320 V 460 V 192 G V, 1500 Ah 4 x 1.5 HP

T h e power conditioning systcm is of a diial-bridge six-pulse l i n e commutated design. The system has a noni inal 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. In i t ia l battery charge is performed at constant power, and a constant-volt,age mode is applied for f inal 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 tal ie bridge and firing circuitry, power factor correction capacitors and the harmonic indicator. The entire LABES facil ity is under the coil- t ro l of a JCI DCC 8500 energy management systcni. Only monthly inspections of t h e system are needed, because i t operates fully auto- mat ical ly [38].

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Figure 44 presents the daily power profile with and without t h e LABES facility.

I 200 I I

O I I I I I I O 6 12 18 2

Real time. h

Wiih IABES iocility

24 I8 12 Real lime. h

Fig. 44. Daily power consumption of the foundry without (a) and with (b)

the LABES facility [38].

The good peak-shaving capabilities of the LABES system are ob- vious. 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-

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stall ing the LABES facility, the month ly peak demand was reduced by 300 kW. T h e utility demand charge is about 8 $ kW-'. (In most industr ia l regions in the USA, the rate is 10 $ kW-'). A few years of service only are enough for the facil ity to become profitable [40]. Of course, the assessment of the economical benefits o f th is 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 tract ion type gelled- electrolyte battery i s under construction in a light ra i l transit system t o meet peak power demand dur ing morning and evening commuter rush hours. The 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 wi l l have a capacity of 1620 Ah (8 h rate). Th is facil i ty i s designed for purely demonstration purposes: to 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 publ ic transportation system (PTS) in a given c i ty is always in competit ion w i t h individual transportation. I t has been estab- l ished tha t the attractiveness of a PTS is affected by two major system characteristics: travell ing t i m e and frequency of trains running [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 igh power rate is needed. The running train represents a storage system for k inet ic energy. A considerable part of this kinetic energy can be recuperated.

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Fig. 45.

O F

energy - 0.50

Current-velocity-time diagram [41].

T h e Ber l in subway system (100 km) has a to ta l power demand of about 60 MW. Since 1982, BVG has been using t w i n cars w i t h on- board power converters t o transform the dc current t o ac energy and feed three-phase asynchronous motors. On braking of the ra i l cars, the ac energy is converted in to 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 uti l ized for meeting par t of the accelerating energy needs [42]. A model block diagram of such a substation is presented in Fig. 46.

For opt imum energy recuperation, German experts recommend that storage should be as close as possible t o the energy source [35].

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O.

Fig. 46. A model of t h e BES facility at t h e subway substat ion [42].

Th is can b e achieved i f bat.tery storage facilities are dispersed to a great number of stops and are connected t o the dc busbar line. T h e use of braking energy recuperation may lead first' to reduction of the unit size of the rectifier stations since the acceleration power can partially b e supplied by battery storage systems. Secondly, BES facilities serve also as emergency power sources. In the case of failure of the publ ic supply network, the dispersed LABES systems on the dc busbar lines provide emergency power and trains can leave the tunnel and stop at the next subway station.

T h e bat tery is connected in parallel to the rectifier and acts as a voltage buffer. T h e share of current provided by the bat tery depends on the ratio between the internal resistances of the rectifier and the battery. Since battery resistance varies depending on the state- of-charge and the cycle life, sophisticated management and control i s necessary to guarantee reliable operation of the battery for th is specific electric transport application.

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9. Valve-regulated lead-acid batteries for bat tery energy storage systems

Valve-regulated lead-acid batteries (VRLA) are maintenance- free, which makes them especially suitable for battery energy storage. They 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 Inc . in their 300 kW/600 kWh BES facility. These batteries used gelled electrolyte and displayed very good performance dur ing their three years of service. Th is 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 no forced circulation is possible, two major problems arise during battery operation which are inter-related:

a) Electrolyte immobilization creates, even though slowly, a non- homogeneous and uncontrolled concentration distr ibut ion of elec- t ro lyte w i th in the active block. Since H2S04 is an active material, a non-uniform distr ibut ion of reaction rates in this block occurs. This affects battery capacity and cycle l ife. In order t o reduce or total ly eliminate this adverse effect, a special charging mode has been introduced, and an equalizing long charge is carried out at the end of each week.

b) T h e non-uniform reaction rate distr ibut ion and the immobi- l ization of the electrolyte in the active block br ing 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 con- struct ion 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 addi t ion t o the irreversible processes typical for flooded lead- acid cells (grid corrosion, active mass shedding, sulfatization, short- circuits, etc.), another process also occurs in VRLA batteries, dry ing ou 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 in to 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 tract ion type gelled electrolyte battery has been installed in a l ight 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 introduct ion of i t s VRLA batteries in tract ion service. High- performance Hagen VRLA batteries are in stand-by service. YUASA describes i t s 3000 Ah VRLA battery for stationary service. The Japanese Storage Bat tery 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 sup- piy duties. An ever-more pronounced tendency is observed in the wor ld battery industry for ut i l izat ion of VRLA batteries for deep cy- c l ing in BES systems. Th is would allow construction of unmanned maintenance-free BES facilities managed by remote control systems.

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10. Strategic advantages of B E S systems

T h e implementation of BES systems in the power industry offers the following major benefits and advantages:

Load-levelling (energy storage). Considerable cost-effective benefits are provided by uti l iz ing low-cost energy f rom the baseload generating uti l i t ies for meeting expensive peak energy demands.

Dynamic power response. Th is is due t o the high operating flexibil i ty and very short response t ime 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 distr ibut ion networks, and economy as a whole. These benefits cannot be directly measured but can be estimated by mathematical calculation and modelling. Thei 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 rom populated centers. In this way the pollutants, dust, CO1, CO? and NO, emit ted as a result o f the com- bust ion process reach the populated areas in strongly reduced con- centrations. I t is economically effective t o build peak power plants w i t h oil/gas-fired combustion turbines w i th in the boundaries of populated centers. In this case, however, harmful emissions f rom these power plants w i l l cause considerable environmental pol lut ion of the neighboring towns. LABES plants themselves are clean and quiet. The only environmental impacts of these plants are related t o the baseload generating uni ts that supply the charging power. LABES plants provide pronounced environmental benefits as compared t o combustion turbine peak plants.

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W h e n BES plants are installed at the utility distr ibut ion substation, or BES facilities are located in the vic in i ty o f the consumer, transmission and distr ibut ion losses are significantly reduced and the stabi l i ty of the network is improved. As is known, energy losses are proport ional t o the current squared, and hence by decreasing and equalizing the current, these losses will be reduced. Additionally, by shaving the peak load transmitted, BESP allow deferral of capital costs for the construction o f new transmission and distr ibut ion capacities. Finally, power stabi l i ty in the transition and distr ibut ion 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 form the 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 much easier t o site at any point 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 polit ical, economical and other factors, and are very unstable. When combustion turbines are replaced by BES plants this instabi l i ty of energy costs i s strongly reduced or eliminated, which is a strong financial benefit.

Another in iportant cost benefit is provided by the modular design of BES plants. Bat tery uni ts are comprised of small modules allowing any load increase t o be met quickly and fully by instal l ing additional modules w i th in a very short t ime.

So far, only a small number of BES systems have been constructed throughout the world and these are most ly in the demonstration stage. T h e full range of benefits provided by BEC systems in the en- ergetics have no t yet been completely studied. But even at this in i t ia 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.

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Acknowledgements

T h e author would l ike t o acknowledge the kind assistance o f Dr. W.C. Spindler, consultant t o ILZRO, USA, and Dr. S. Takahashi from the Government Industr ia l Research Inst i tute 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, Inter- national Lead Zinc Research Organization (ILZRO) Project LE-363, 1989.

2. EPRI ~ Energy Storage: How t h e New Options Stack up. Brochure, 1989.

3. Japan Industrial Technology Association. The Moonlight Project De- veloping New Technologies for Energy Conservation. Brochure. 1989.

4. M. Tada, T. Sakamoto, T. Tanaka, H. Yamamoto, Y. Sera, T. Nakayama, Outline of the New Facilities of Tatsumi Electric En- ergy Storage System Test Plant, 2nd Int. Conf. “Batteries for Uti l i ty Energy Storage”, New Port Beach, California, 1989.

M. Tada, E. Furune, T. Tanaka, T. Sakamoto, T. Oota, Y. Sera, T. Nakayama, Monitor and Control System to the New Facilities of Tat- s u m i Electr ic Energy Storage System Test Plant, 2nd Int. Conf. “Bat- teries for Util i ty Energy Storage”, New Port Beach, California, 1989.

5.

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6. J.L. Sudworth, T h e Sodium Sulfur Battery (J.L. Sudworth, A.R. Ti l ler , eds.), Chapman and Hall, London, 1985, p.9.

7. N. Itoh, T. Nakayama, T.Hiramatsu, S.Takahashi, R&D Evaluation on Energy Storage Batter ies for Power Systems in Japan, Report NEDO-OS-8801, Government Industrial Research Inst i tute, 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.

R.J. Bellows, P.G. Grimes, Zinc/Halogen Battery, in Power Sources for Electr ic Vehicles (B.D. McNicol, D.A.J. Rand, eds.), Elsevier, Am- sterdam, 1984, p. 621.

9.

10. D.A.J. Rand, Overview of Performances of Candidate Battery Sys- tems, in Power Sources for Electr ic Vehicles (B.D. McNicol, D.A.J. Rand, eds.), Elsevier, Amsterdam, 1984, p. 721.

11. P. Symons, C. Warde, P. Carr, Development of the Zinc/Chlorine Bat- tery for Utility Applications, Part 1-4, EPRI EM-1051 (Project 226-3), 1979.

12. M. Barak, Electrochemical Power Sources, Peter Peregrinius, Steve- nage, 1980, p. 251.

13. D. Pavlov, in Power Sources for Electric Vehicles (B.D. McNicol, D.A.J. Rand, eds.), Elsevier, Amsterdam, 1984, p. 11 1.

14. P. Montalenti, in Power Sources 9 (J. Thomson, ed.), Report 6, 1982.

15. G. Smith, Storage Batteries, 2nd Ed., Pitman, London, 1964, p. 252.

16. W.G. Sunu, B.W. Burrows, Power Sources 8 (J. Thomson, ed.), Aca- demic Press, 1981, p. 601.

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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.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

P. Ruetchi, J.B. Ockermann, Electrochem. Technol., 4 (1966) 383.

D. Pavlov, T . Vitanov, I . Nikolov, V. Nikolova, G. Papazov, K. Petrov, US Patent 4, 925, 746 (1989).

K. Peters, A.I. Harrison, W.H. Durant, Power Sources 2 (D.H. Collins, ed.), Pergamon Press, Oxford, 1968, p. 1.

H. Tuphorn in Advances in Lead-Acid Batter ies (K.R. Bullock and D.Pavlov, eds.), The Electrochem. Soc. Inc., Pennington, NJ, USA, 1984, vol. 84-14, p. 441.

K.R. Bullock, B.K. Mahato, G.H. Brilmyer, G.L. Wierschem, ibid, p. 451.

B.IC Mahato, K.R. Bullock, Progress in Batteries and Solar Cells, 6 (1987) 136.

“Energy cells”, Technical Leaflet 20 M3/77, Gates Energy Products, Denver, USA, 1977.

R.F. Nelson, Proc. 4th Int. ILZRO Lead-Acid Battery Seminar, San Francisco, CA, USA, 1990, p. 31.

K. Okada, in Rechargeable Batter ies in Japan (Y. Miyake and A. Kozawa, eds.), JEC Press Inc., 1977, p. 291.

G.M. Cook, W.C. Spindler, J. Power Sources, 33 (1991) 145.

D. Morris, EPRI Journal, March issue, 1988, p. 1.

A.M. Chreitzberg, F.L. Tarantino, I.C. Baeringen, Design of a 10 MW/50 MWh Lead-Acid Load-Levelling Battery, Int. Conf. “Bat- teries 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 Uti l i ty Energy Storage NEDO, EPRI, BEWAG, Kobe, Japan, 1991, p. 269.

32. G.D. Rodriguez, W.C. Spindler, Operating t h e World’s Largest Lead- Acid Battery Energy Storage System, 4th ABC CSU ~ Long Beach Conf., 1989.

33. H. Dominik, B. Voigt, Applications and Economics of Battery Energy Storage Systems, Abstract 14, Lead Battery Power for the 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 US E lec t r i ca l Load-Levelling Appli- cations, 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 Uti l i ty Energy Storage”, Berlin, 1987 (BEWAG-EPRI-NEDO).

37. K.G. Kramer, BEWAG Plan Berlin’s New System, Batteries Interna- tional, l (4) (1990) 32.

R. Hamann, R. Scarvaci, G. Brilmyer, ILZRO Lead-Acid Battery Sem- inar, Orlando, Florida, LEA, 1989.

38.

38.

39.

G. H. Brilmyer, Batteries International, 1 (4) (1990) 36.

W.C. Spindler, Industrial Applications of Large Lead-Acid Batteries for Emergency and Peak Power Demand. World Lead-Zinc-Tin Syni- posium ’90, Anaheim, CA, 1990.

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41. K.G. Kramer, B.Voight, Proc. 3rd 1nt.Conf. Batteries for Util i ty Energy Storage NEDO, EPRI, BEWAG, Kobe, Japan, 1991, p. 519.

B. Voight, T. Mierke, H.J. Hinrichs, Batteries International, 1 (4) (1990) 18.

42.

43. D.A.J. Rand, W.G.A. Baldsing, J. Power Sources, 23 (1988) 233.

44. R.B. Shainker, Proc. 3rd Int. Conf. “Batteries for Util i ty Energy Storage” (BEWAG-EPRI-NEDO), Kobe, Japan, 1991, p. 519.

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Chapter 2

ENERGY STORAGE S Y S T E M S FOR ELECTRIC VEHICLES

G. PAPAZOV

1. t ion

M o t o r vehicles and environmental pollu-

Owing to the continuous development of human society and rapid technological progress, mankind is now faced w i t h a crucial problem whose solution w i l l determine i t s future. Th is problem is the environmental po l lu t ion caused by human activities. Er ich 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 [il

Type of Industry Transport Total pollutant min. taons % min. tons % mln. tons

C,,H, 1.02 56 0.8 44 1.82 CO 5.7 44 7.3 56 13.00 NO2 1.56 68 1.4 32 2.96 Pb 0.0032 52 0.003 48 0.0062 so2 4.04 98.5 0.06 1.5 4.1 dust 0.478 95.5 0.022 4.5 0.5

A number of modern internal combustion engined (ICE) vehicles use appropriate catalytic cartridges that improve fuel combustion, or

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employ lead-free petrol fuels. Table 2 presents quanti tat ive compar- at ive data on the adverse emissions released per ki lometer both with and without catalysts [l].

Table 2. Specific gas emissions of ICE vehicles [il. ~

T y p e W i t h o u t cata lyst With catalyst of po l lu tan t g km-' g km-'

CnHm 2.253 0.28 CO 22.999 2.2 NOz 3.264 0.66 Pb 0.010 0.0 so2 0.039 0.039 dust 0.035 0.042

Despite the ten-fold decrease in the quantity o f some pollutants, the amount of harmful exhaust gas emissions f rom cars o n a worldwide basis is measured in mil l ions of tons.

T h e above tables do not include data o n carbon dioxide which is the end-product o f fuel conibiistion. Th is COL is accumulated in the atmosphere contr ibuting 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 nightt ime (Fig. l ) , being largest during the day when 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 i l lustrated in Fig. 2 [ l ] .

Evolut ion of harmful gases (except for KOz) is reduced increasing the travel speed. Most of the t ime a car is used in urban conditions: for driving t o work and b x k home, for shopping, during leisure time, etc. City driving means travel l ing at a low speed with frequent halts

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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 to the 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 pol lut ion caused by the transport i s n o t uni formly distr ibuted over the earth’s surface. I t is greatest in areas with considerable con-

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centrations of human masses. Thus, the ecological problem related t o the noxious gas emission f rom transport vehicles is no 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 mill ions of tons of harmful gases daily, th is problem acquires crucial importance and i ts solution becomes a question of the survival of our civilization.

At the present moment, large-scale introduct ion of electric vehicles is 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, pet ro l consumption w i l l decrease by about 3.5%, the amount of toxic gas emissions wi l l be reduced by 20-30%, and the noise level in the cities w i l l decline significantly [2].

2. Specification of energy storage systems for electric vehicles

T h e te rm 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 trans- p 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 rom Scotland in the same year [4]. In 1897, a number of electric-driven taxi cabs were on the road in London. A record running 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 elec- t r ic 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

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20-35 k in h-’, battery energy 10-18 kWh at a bat tery weight of about 750-780 kg [3].

Af ter the 1920s, electric vehicles began t o lose their positions in the competit ion 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 niairi ly w i t h the chemical power source used for energy generation. The 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 to be interrupted. By t h e end of the decade, however, the atten- t ion 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 pol lut ion 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 uti l ized 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. Th is is i l lustrated in Table 3 which gives data (measured for 1975) about the diverse-task range distr ibut ion 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 long- range journey (650 km) for holidays accounts for on ly 5.5% of the to ta l travell ing distance dur ing the year. Knowing the frequency of the various range travels, it turns out that 90% of al1 travels are at a distance below 25 km. On the other hand, all above-mentioned

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Table 3. Travel mission and range d i s t r i bu t i on [il.

Travel % of the year ly Single t ravel mission distance range range, km

professional 23.9 educat ional 3.4 business 24.9 shopping 11.3 leisure time 31.0 hol idays 5.5

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-'.

For other countries, there might be some specific features in the ut i l izat ion of motor vehicles, but these deviations f rom the above average values are unlikely t o be so significant as to impose substantial changes on the overall picture. It can thus be assumed that a car performs two tr ips daily at an average distance of 16.7 km each and an average speed of up to 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 opt imum uti l izat ion of the vehicle [il.

What is the energy needed t o ensure this opt imal 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 VW-Go l f Ci tySTROMers 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 rom 30 to 50 kWh/100 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 is presented in Table 6.

T h e above data refer t o Germany.

108

Table 4. Power characteristics o f several automobi le types.

Character ist ics Au tomob i le t y p e Peugeot 205 C i t r o e n C15 V W - G o l f

Maximum power, kW 18.5 23.0 24.0 R a t e d power, kW 12.7 16.0 12.0

Table 5. Energy consumpt ion p e r 100 km of t rave l l ing range.

Speed, km h-' 50 70 c i t y driving Energy, kWh/100 km 21.5 27.6 30.0

Table 6. Bas ic performance characteristics of var ious types o f bat ter ies.

Electrochemical power source

Improved Pb /ac id Gel led Pb /ac id Golf t rac t i on Pb /ac id Tubular Pb /ac id

HED-88 EV-5T

Ni/Zn Ni/Zn N i /Fe N i /Fe Ni/Cd Allair Zn/Br Na/S L i /MoSz

Specific Specific Cyc le energy power l i f e References

Wh kg-' W kg-'

37 105 176 161 26 95 318 161 30 96 323 161

34.3 108 > 850 161 37.7 112 > 100 161 55 218 195 I61

161 40 163 > 650 161

52 200 1200 181

72 20 250 [lo1

60 208 500 1111

~ - 70

55 ~ 171

106 6.5 191

[71

-

~

- - 175

109

Figure 3 illustrates the basic power and energy parameters of some o f the above-mentioned electrochemical power sources [3]. I t can be seen that non-aqueous-electrolyte batteries ( l i th ium 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 10 20 3 0 4 0 5 0 7 0 i o o

Specific pciwer, W kg-'

Fig. 3. Power and energy characteristics of several types of batteries.

T h e energy needed for electric vehicle propulsion depends on the car weight and the travell ing range, and it can be determined from the following equation:

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(1 ) E = K G L

where E is the energy delivered by the power source measured in Wh, K i s the specific energy needed for t ransportat ion of 1 kg o f weight at a distance of 1 km measured in Wh kg-' km-'. When all side factors of travelling, ed friction, aerodynamic drag, etc., are taken in to consideration, the value of K i s estimated at 0.12- 0.15 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 to cover a distance of 100 km, 24 kWh of energy will b e required supplied by an electrochemical power source.

The tota l power delivered by the storage bat tery is ut i l ized for vehicle acceleration, and for overcoming fr ict ion 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 dur ing transmission of this mechanical energy t o the car wheels (efficiency ca. 0.95). When operated in urban dr iv ing 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 calcu- late the weight of t h e various batteries that could supply the power needed for this 100 kni range of the vehicle. The results obtained are summarized in Table 7. I s is evident f rom the data in the table, that battery weight cornprises a considerable par t (25 t o 50%) of the to ta l car weight. For comparison, i t can be pointed out that the energy needed for covering the same travell ing range of 100 krn could be supplied by aboi i t 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

source kg

Improved Pb/acid

Gelled Pb/acid

Golf traction Pb/acid

Tubular Pb/acid EV-5T HED-88

Ni/Zn

Ni/Fe

Ni/Cd

Al/air

Zn/Br

Li/MoSz

810

1154

1000

875 796

545

750

577

283

417

500

T h e range of the electric vehicle, according t o eqn (l), depends on the energy capacity of the power source, and the car weight. Knowing 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 wi thout charging of the battery can be determined f rom eyn (2):

W M L , - 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 bat tery weight, is given in Fig. 4.

3 o o m 700 900 il00 Battery weight, kg

Fig. 4. Dependence of electr ic vehicle range on b a t t e r y weight.

It is evident that the greater the battery weight, the longer the range between battery charges. With increasing the weight of the battery, however, the tota 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 tota 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 bat tery 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 rom the table, the max imum theoretical range of an electric vehicle driven by various types of power sources varies w i th in the l im i ts 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 Specific Maximum source energy range

Wh kg-' km

Improved Pb/acid

Gelled Pblacid

Golf traction Pb/acid

Tubular Pb/acid EV-5T HED-88

Ni/Zn Ni/Zn Ni/Fe N i / F e Ni/Cd

Allair

Zn/Br Na/S Li/MoSz

37 26 30

34.3 37.7 55 70 40 55 52

106 72

175 60

247 173 200

229 251 367 467 267 367 347 707 480 1167 400

t o four times smaller than the 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 opt imum ratio between the battery and vehicle weight arises. I f this relation is determined on the basis of the existing operational electric vehicles, it w i l l show that the power source weight accounts for 25- 40% of the gross car weight. In this case, the actual travell ing range of the vehicle is respectively 25 t o 40% of the theoretical one.

When discussing the energy performance of electric vehicles, the problem of opt imum energy ut i l izat ion should also be given consideration. Thus, for example, the efficiency of internal combustion en-

114

gines is about 25--30'% [ 3 ] . wli ich means that f rom t l ic 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 t ract ion systems allow easy and snioot,li regu- lat ion and control of t h e travelling speed. Sonic dynamic parameters of the electric vehicles are also wor th discussing hcrc, c.g. maximum speed and acceleration rate (Table 9), that are determined by the energy characteristics of the battery.

Table 9. Some dynamic parameters of electric vehicles.

E lec t r i c M a x . speed T i m e for accclcrat ing References vehic le km h-' f rom O t o 50 km lip', c

Peugeot 205 100 11.6 151

[31 l L O * 131

Citroen C15 80 11.6 Pl VW-Golf 100 13.0 I11

- ErAZ-3732 60 VA Z -280 1 -

* (0-60 km h-I)

By their dynamic parameters, electric vehicles are considerably inferior to ICEVs. On the other hand, being designed for predomi- nant c i ty driving, electric vehicles show adequate speed and acceler- at ion performances that 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 that needs adequate solution i s the problem of battery charging. The fuel in the tank of an ICE vehicle supplies useful power for a travell ing range o f about 500-600 krn and refuelling at the petrol station takes only a couple of minutes.

115

Charging of the electric vehicle battery usually requires several hours, and t l ic durat ion of charging is often twice or three times longer than t h e discharge period, i.e. the t in ie of uti l ization o f the car. Th is re-

sults in a cyclic profile of operation o f the electric vehicle, the time of

rcst being much longcr than the dr iv ing 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, on the one hand, it pre- supposes 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 util ization, it can be concluded that electric vehicles can meet all travell ing requirements, except the long range of the holiday travel. The elcctric vehicle is used for the purposes outl ined in Table 3 usually dur ing the day, so charging of i t s battery could be performed

at n ight, when the car i s at rest. As regards long 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 mobi l i ty associated w i t h the use of publ ic 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 tr ips

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 bat tery should endure 365 cycles per year. T h e data f rom Table 6 show that only Ni/Cd,

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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 is t he problem of battery cost. There are two aspects of th is problem. T h e first refers t o the cost of the battery as related t o the cost of the electric vehicle without battery. Thus for example, the ,4g/zi1 battery has a liigli specific energy (120 Wh kg-') ensuring a travell ing 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 the Ag/Zn battery, in spite of i t s li igli energy perforrriaiice, is

excluded f rom the l is t of candida.tes for EV powcr sources. T h e second aspect of the bat tcry cost proi>leiii i s rc latcd to t,lic total

l i fetime energy cost, of t l i c battery, in other words t,l ic> cricrgy cost 1)cr 1 Wh or per 1 lim. Unfortunately, it i s \-cry diff icult to nialie a precise

energy cost assessment since a great part of t l i c elcctroclieniical p o w e r

sources are st i l l in the prototype phase of devclopmciit a i i c i have no

specified market price. On t h e other hand, prices of coiiii i iercially available batteries vary coiisiderahly for thc various coiiiitrics. Th is i s well i l lustrated 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.

Table 10. Specific energy costs for sornc t ypes of bat ter ies.

Type of cost , 11s $ Rc fc rc i i c r s battery per 1 kW1i per 1 lai

Pb/acid 0.387' 0.116 Pl Ni/Cd 0.263* 0.079 [SI Mo l ice l 1.220* 0.366 [ill

* Data calculated for energy cons i impt io i i of 0.30 k W 1 i km-'.

l l Ï

T h e costs (in roubles) per 1 kWh of energy delivered by batteries produced in the USSR are given in Table 11 [3].

Table 11. Specific energy costs of some Russian batteries.

Type Cost per 1 kWh, Versus roubles P b / a c i d cost

Pb/acid Ni/Cd N i / F e

Ni /Z i i Ni/Hi

Pb02/Hz Zn/Br Na/S

O. 02-0.04 0.2-0.4

0.04-0.06 0.1-0.2

0. 1550.30 0.02-0.04

o. 1

o. 1

1 .o 10.0 2.0 5.0

1 .o 5.0

5.0

In spite of the incomplete and sometimes contradictory data available, it i s generally accepted that lead-acid and to some extent Ni/Fe batteries are most cost-effective for electric vehicle propulsion.

Though there is a certain bias for one or the other t ype of power source among experts in the field, the most appropriate battery type for electric vehicle application has not been found yet. Despite the numerous design improvements, lead-acid batteries s t i l l remain heavy systems with rat l ier low energy performance; alkaline batteries are rat l ier expensive; Ni /Fe 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 l ife 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 the problems

of self-discharge and safety [3]. Th is 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 the best, if no t the ideal, suitable power source for electric vehicle propulsion. Research activities are being carried out with increasing intensity, every research team having i ts own favourite type o f battery. Thus, for

example, investigations in France are concentrated on Ni/Cd, Ni/Zn and Ni/Fe cells; Japanese researchers pay greater at tent ion t o Na/S and Zn/Br; R&D efforts in the USA are devoted main ly 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 com- m o n feature uniting all research centers, however, and that is that all of t hem show a marked preference t o the lead-acid battery as their favourite electrochemical power source or as a reference standard. China [12] and Ind ia [13] have also init iated programs for the development o f lead-acid battery driven electric vehicles.

Consultants f rom 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 on designed power packs for an Improved Dual- Shaft Electric Propulsion (IDSEP) Van. The technical requirements of the battery were as follows:

o max. weight 700 kg o max. volume 600 1

0 min. energy 21 1;Wh o min. system voltage o min. l i fe 500 cycles

0 min. power 55 1iW

120 V

119

T h e criteria for battery evaluation included:

O cost o performance - energy vus. power, power 'us. DOD 0 cycle l ife o safety ~ effects of normal and abnormal battery operation on

o abuse resistance ~ effects of overcharge, overdischarge, single-

o dc-dc efficiency 0 environmental impact - issues in battery manufacture, opera-

o use of crit ical resources - use of impor t makerials 0 user issues - type, frequency, and complexity of routine main-

passengers and public safety

cell failure, shock and vibrat ion

t ion and control

t eriance

The results f rom the performed analysis are il lustrated in Fig. 5 showing the suitabil ity/techii ical 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 lead- acid 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 flow- through lead-acid batteries developed by Johnson Controls Inc., Li/FeS technology pursued by Gould Inc . and Electrofuel Man- ufacturing Co., Fe/air of Westinghouse Electric Corporation, and Na/metal chloride systems developed jo in t ly by Beta K&D Ltd. and Harwcl l Laboratorics.

T h e highest technical r i sk i s associated w i t h prototype batteries developed at universities and/or nat ional laboratories, but s t i l l lack-

i 20

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 Na- t ional 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 no

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 ut i l izat ion are general issues relevant for all types o f automobiles, i.e. the 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 be mentioned here. The 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 un l imi ted range would be converted in to a two-seat electric vehicle w i t h l imi ted driving speed and l imi ted range. It has been suggested [3] that the design o f new constructions for electric vehicles is imperative. This, however, would slow down and complicate the process of transition f rom ICE t o battery driven vehicles.

3. Charge and capacity o f batteries for elec- t r ic vehicles

Charging of batteries is accomplished by le t t ing electric current f rom 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 bat tery cells or modules, and adequate gas evolution at the end of charge ensuring proper st irr ing of the electrolyte.

T h e charge current is the most impor tant parameter in the process o f bat tery charging. It is well known that i f lead-acid batteries are charged at a h igh rate ( w i t h a high charge current), their cycle l i fe is shortened. At the battery Technology Center of the Mel lon I n - stitute, USA [15], the effect o f charging current on 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 dur ing the first 20 cycles, but the service l ife of these batteries is the shortest. The end of battery cycle l i fe in this case is determined by corrosion of the positive battery plates. At very low charge currents (12 A), the battery does no t reach i t s maximum capacity and has a short cycle l ife owing t o sulfatization of the plates. It turned out that the opt imum charging current ensuring maximum cycle life of the battery is the current corresponding t o the 5-hour

122

O M loo 150 200 no Life, cycles

Fig. 6. Dependence of cycle l i fe on charg ing r a t e (151

charge cycle. In this case, shedding o f the positive active mass is the l i fe limiting factor.

Apply ing constant-current charging modes does no t seem t o be the most appropriate method for charging of batteries. Battery charge should be performed at maximum charge acceptance, avoid- ing unacceptable temperature rise and minimizing gas evolution. For electric vehicle applications, particularly, the following two charging modes are recommended [4]:

a) Controlled I / V charging method. In this method, charging is carried out 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

Current $:rq-J e a o

o 1 2 3 4 5 Hours

u 2.4

= 2.2

2.01 I I I I I O 1 2 3 4 5

Fig. 7 . Battery charge under control led I /V cond i t ions [4]

Hours

The value of the cell voltage i s selected t o give slight gas evolution

b) Tapered charging method. In this method, the ma'timum

and to enable the battery t o be charged w i th in a short t ime period.

charging current is l imi ted 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 ce 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, the current must no t exceed 8.33% Cg. Under these conditions, the rate of gas evolution w i l l not 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 module has i ts own internal impedance different f rom that of other cells

124

7.0 k o 5 10 Chorger oulpul

I

Amperes per 1OOAh CCIpOC¡ly z 2.0 -20.:

x f 8

-10 : s s

-15 u

- 5 $

2 .o I I I I I 0 5 O 7 4 6 8 1 0 1 7

Hours

Fig. 8. (a) híaximum current at the output of the taper charger us. ce 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 is of no 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 the 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. Th is voltage control allows reduction o f the energy consumpt ion and this in turn reduces the cost per km distance range of the electric vehicle.

F rom the above, i t becomes clear that charging of batteries for electric vehicles cannot be carried out w i t h ordinary charging devices. Modern chargers should be programmable, w i t h built-in m ic rop re cessors and various sensors to allow opt imum charging depending o n the instant state o f the battery. Th i s is especially important for maintenance-free batteries. The charger should possibly be installed 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. Th is battery charger is not too heavy, about 20 kg against 1600 kg vehicle weight [l], and would no t degrade i t s performance parameters.

One way of shortening the charging t ime 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 low 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-to- battery charge, 25530% more energy is needed than for the normal fast bat tery charge.

In the West German M a n n Bus program [15], the so-called bib- beronnage charging cycles have been demonstrated. Whi le 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 ts 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 bib- beronnage charging cycles. T h e effect o f these strong current pulses o n battery l ife has no 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 imi ts the energy provided by the battery at low specific power.

Ï 40 m

I ir

L

Ha U ._ c ._ 5

20 rl)

IO

specific energy , Wh kg-’

Fig. 9. Energy and power delivered by a lead-acid battery during pulse-

and 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 the decline of battery parameters and for t h e end of bat tery service l ife are the heavy, though short-term, peak power loads it is subjected to during high acceleration driving (on start ing and overtaking). It is considered that charging methods, durat ion of rest periods, etc., do not have such an impact o n battery cycle l ife [is].

There are contradictory data about the effect of temperature on battery cycle l i fe [19, 201. i t i s generally accepted however that bat-

127

k r y failure depends on the temperature conditions of service. So bat tery failure at low temperature operation is due both to irreversible sulfatization of a few positive plates and to morphological changes in the positive active mass. At high temperature, the failure i s associated with negative plate deterioration and positive grid corrosion [X I ] .

A s to the decline in negative plate capacity during cycling, it i s assumed that the basic reason for th is is passivation of the plate

[21]. Continuous refinement of the grain structure of the PbS04 cryst,als with cycling, worsening of the contact between the particles of the negative active mass, and impeded electrolyte diffusion are also factors causing the plate capacity to decrease.

4.Types of cycles of 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 im-

portaiice. Figure 10 shows the capacity curve of a lead-acid battery as a function of discharge current and temperature [il.

It is evident f rom the figure that the higher the discharge current and the lower the t,emperature, the smaller the amount, of energy delivered by the battery.

In electric vehicle applications, batteries operate under complex temperature conditions. Owing to the high discharge and charge currents, considerable amounts of Joule heat are released. Conventional bat tery constructions, as a rule, hamper emission of th is heat i n to the atmosphere, and hence the temperature of the cells may rise to unacceptable levels. On the other hand, low bat tery temperatures lead to a sharp decrease in energy performance.

Th is calls for the need for an adequate heating system that would provide preheat ing of the battery a,t low surrounding temperatures,

128

C

Fig. 10. Dependence of b a t t e r y capaci ty o11 discharge current and ternper-

a tu re [il.

and cooling of the cells to prevent overheating as well as to allow ut i l izat ion of t he released heat (during charge and discharge) for warming of the car interior during winter. A fairly sophisticated sys- t e m of th is kind has been dcveloped for the VW-Golf Ci tyCTROMers electric car [il.

T h e dynamic characteristics of the electric vehicle are determined by the power performance of the battery used for propulsion. Power output depends on the state of discharge of the battery arid usually degrades rapidly near the end of battery cycle life.

Figure 11 illustrates the dependence of the battery specific power on the depth of discharge (DOD) [22].

As seen in the figure, an abrupt decline in bat tery specific power i s observed beyond 50% DOD.

T h i s complex multi-factor dependence of bat tery energy and power performance on the one hand, and the diverse road and c i t y driving conditions of the electric vehicle on the other hand, make

129

M -

10 O I I I I

Depth of discharge, */.

-

0 2 0 4 0 6 0 8 0 1 0 0

F i g . 11. Specific power vs. d e p t h of discharge

characterization o f the system electric vehicle/battery extremely dif- f icult .

Worldwide there is no unified standard for testing o f batteries for electric vehicle applications. A wide spectrum o f test profiles are in use, f rom 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 at tempt 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 t ract ion battery test procedures and hence will not be discussed here.

As t o the second category of test profiles, targeted at determining the battery behaviour dur ing 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 the Electric Vehicle Battery Test Cycle (EVBTC) [23].

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T h e first two 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 veloc i ty prof i le [3].

Time. s

Fig. 13. ECE veloc i ty prof i le [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 th is speed for a certain period of time, decelerat- ing, at a definite rate, t o zero velocity, and standsti l l 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 durat ion of the test cycle 80 s [3]. The 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 the maximum speeds of the three test cycles w i l l be 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 dr iv ing the vehicle at a definite speed. These tests can be regarded as oriented t o the vehicle as a system in mot ion rather than t o the system vehicle/battery. On the basis of the results obtained f rom these tests, it is very diff icult t o compare various types of electrochemical power sources for EV applications, especially so i f the tests are performed on different cars. A summary of the specific power characteristics determined v ia these tests is presented in Fig. 14 for SAE J 227a, C, and Fig. 15 for ECE [23].

O Time.s

Fig. 14. SAE J 227a, C battery power profi le [23]

132

3

Fig. 15. ECE

Tlme. s

battery power profi le [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 diff icult.

When in operation, t h e electric vehicle requires a definite power level irrespective o f the instantaneous state of the power source. Bat- tery power can be 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 Simplif ied Federal Urban Driving Schedule (SFUDS 79) i l lustrated in Fig. 16 [23,24]. Figure 17 gives the driving speed according t o SFUDS 79.

- 7 8 0 rn 2

? 60

[ 40

& O

U < 20

- 20 o 60 120 180uom360

Time, s

Fig. 16. SFUDS 79 battery power prof i le [24].

lime, s

Fig. 17. SFUDS 79 velocity prof i le [24].

In most general terms, the above test profile consists of five power pulses distributed in t ime 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 is 3100 m.

134

All the above outl ined 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 uti l izat ion of t he 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 be more precise, recover part of the energy delivered by the battery. On the other hand, however, strict standardization o f this type of charge is not possible, because the features of regenerative braking differ considerably for the different types of vehicles. For these reasons, regenerative braking should be excluded f rom EV test schedules [23], but it may and should be utilized in actual vehicle operation, since electric re- generation increases the mileage of the electric vehicle between two full charges of the battery w i t h 18-25% [24]. When regenerative braking is used, however, the charging pulses should be restricted t o current values that allow efficient charging wi thout 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, the power demand is very high; then a per iod of constant speed cruise follows and the respective power demand is low; this speed is then maintained until the next stop. Based on this simplified driving profile representation, the Electric Vehicle Bat tery Test Cycle (EVBTC) has been developed [23], presented in Fig. 18.

Th is test cycle comprises two power pulse steps wi thout regenerative braking. Overall durat ion of t he 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 th in a fairly wide bandwidth of speeds including start ing and braking, as well as driving at varying speed wi thout stopping.

135

Time, s

Fig. 18. EVBTC battery power prof i le (231

T i m . 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 City- STROMer (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 1 Cumulative time, ' / e

I

Fig. 20. Specific battery power 8s. cumulative time for various test profiles in comparison to urban driving [23].

Table 12. Characteristics of various t,est cycles 1231.

Test Average Required Standstill Degree of power max. power period compliance

W kg-' W kg-' % %

Urban driving, recorded 15.5 53 30 100 SAE J 227a, C 11.1 60 53 90 ECE 9.0 43 44 85 SFUDS 79 11.8 79 42 85 EVBTC 15.6 50 30 95

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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 majo r advantage of EVBTC is that it yields reproducible and comparable informa- t ion on energy ut i l izat ion and lifetime, irrespective o f the type and size of bat tery system used. The fairly complex and diverse condi- t ions o f real EV operation require constant battery monitor ing and control. The latter should perform the following functions: provide informat ion about the 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 rom operation under abnormal conditions; prevent overpolarization of indiv idual cells; indicate excessive temperature rise; ensure proper and complete battery charging; monitor gas evolution; prevent dan- gerous pressure rise in the cells, etc. These complex monitor ing 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 mult i funct ion control system is no t available yet, but would probably be developed when large-scale produc- t ion of electric vehicles commences.

Since electric vehicles operate under considerable current loads, rel iabi l i ty 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

5. Requirements of the construction and manufacturing technology o f batteries for EV energy storage systems

As a rule, lead-acid batteries used for electric vehicle propulsion have a modular construction. These modules are usually 6 V trac- t ion batteries consisting of three storage cells housed in a plastic case w i t h a common cover, and intercell through-the-wall connectors. Th i s battery design yields a relatively h igh specific power output (30-35 Wh kg-'). Chloride Silent Power [6] has developed an advanced tubular plate lead-acid battery w i t h h igh 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.

Ut i l izat ion of non-hermetic electrochemical power sources with aqueous electrolytes (e.g. lead-acid and alkaline batteries) requires constant monitor ing of the electrolyte level in each battery cell and replenishment w i t h disti l led water when necessary. Filling up is one of the maintenance procedures most often underestimated and ne-

glected by drivers, and leads t o drying of individual battery cells and hence shortening of bat tery cycle life. T o prevent this, single point watering systems have been developed [6] for monitor ing 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 form 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 them out in to the atmosphere. Th is gas vent system can be combined w i t h the ref i l l and cooling battery systems, an approach adopted by Union Globe Co., in the USA [3]. Second, by the introduct ion of maintenance-free batteries which are of special interest for electric vehicle propulsion applications. These

139

batteries uti l ize 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. As evidenced by a number of investigations, hydrogen gassing cannot be 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 rom 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 major advantages of the maintenance-free batteries are [6]: 0 n o battery watering or checking of electrolyte level required

o greatly reduced hazard of hydrogen explosion; o reduced energy consumption because of less requirement for

a reduced maintenance of the battery ventilation system; o improved cold weather performance displayed by sealed bat-

teries. During battery operation, electrolyte density undergoes cyclic

changes resulting in i ts stratification, the denser electrolyte flowing down t o the b o t t o m part of the plates and the l ighter one floating over it. T h e electrochemical act iv i ty of the cells and their capacity t o accumulate energy are higher in their upper parts than in the lower ones. At the bo t tom of the cells, where the acid concentra- t ion is highest, full charging o f the plates is impeded, they undergo sulfatization which in turn decreases battery capacity. Th i s capacity loss is about 1% for each 0.01 g cmP3 difference between the acid concentration at the top and at the bo t tom o f the cell [25).

140

(this saves 1 hour per 100 km for each vehicle opèrated);

overcharge;

Various methods for destratification of the electrolyte have been developed. T h e simplest consists in provoking intense gassing at the end of bat tery charge t o st ir the electrolyte. Th i s can be achieved by applying an appropriate current or by adding high-current pulses t o the charging current. Th i s method, however, turned out t o lack efficiency. I t cannot be applied t o maintenance-free batteries. Mechani- cal st irr ing o f the electrolyte, no t related t o the charging current, has proved t o be most effective. Th is method has been implemented suc- cessfully by Johnson Controls Inc., in the USA, where an electrolyk circulation system has been developed [26]. Th i s system uses a small, b low molded, insert pump w i t h n o moving parts, which is designed t o maximize rel iabi l i ty and heat-sealed in to each ce11 of the battery module as part of the final assembly operation. During cycling, a low-pressure air pulse o f predetermined volume is forced in to the insert pump, collecting higher density electrolyte f rom 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 rom testing of the battery are presented in Table 13.

Table 13. Performance of lead-acid batteries for EV

Performance Golf car ISOA FFLA parameter ba t te ry (1978) EV-3000

Specific energy (Wh kg-') 23 42 52.4 Posi t ive active mass

u t i l i za t ion - (Ah kg-') 63.9 72.7 126.7 - 28.5 32.4 56.5

Cycle l i fe 250' 508' 130"

* based o n SFUDS 79 ** based on 80% DOD cycl ing

141

T h e ISOA battery showed a 46% higher specific energy and 80% longer cycle l ife than the conventional lead-acid battery. Besides, the ISOA battery offered the advantages of reduced overcharge and water addi t ion requirements, and improved system thermal management.

In the ISOA battery, simple circulation o f the electrolyte between the plates and the separators is achieved wi thout affecting the active mass. By forcing the electrolyte in to the electrodes, more active material sites w i l l be accessed, which w i l l result in enhanced active material ut i l izat ion and improved system specific energy. Johnson Con- trols Inc. have developed such a battery system, called the FFLA (Forced F low 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 ut i l izat ion of the FFLA battery are considerably higher than those of the ISOA system. From the data presented in Table 13, it is diff icult t o predict the l ife span of these batteries. It can be expected however that their cycle l ife w i l l be significantly reduced through mechanical deterioration of the active mass caused by forcing the electrolyte in to the electrodes. The relative complexity of the system including battery, pumps and elec- t ro lyte circulation tubes should also be given consideration.

6 . Specification o f operating energy storage systems for electric vehicles

Classification of electric vehicles is very diff icult, because on the road electric vehicles are most ly demonstration prototypes employing various power sources w i t h different design and dynamic performance parameters. Th is is well i l lustrated in Table 14 presenting compara- t ive data for several EV types.

Despite the fact that many of the problems facing electric vehicles are n o t solved yet, a lo t of countries are planning t o start large-scde

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product ion o f these vehicles. In the USA, design and development o f an electric G-Van is planned, driven by Chloride tubular lead-acid batteries [25]. Peugeot intends to manufacture an electric car commercially 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 product ion stage in Finland. In the UK, “W and E Electric Ve- hicles” 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 took par 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

Look 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 you 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, you w i l l see a SINGLE self-satisfied or more often extremely nervous person. W h a t a wasteful and irrat ional wor ld 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 to 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.

2.

3.

4.

5

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

E. Sauer, Elektroauto, Verlag TU\.’ Rheinland, Köln, 1985.

Anon. Electric Vehicle Prog. 10 (1988) 3

V.A. Shtetin, U.J. Mortowskij, B.I. Tsenter, W.A. Bogomazov, Elek- tromobil: Technika i Ekonomika, (V.V. Burkov, ed.) Leningrad 1987 (in Russian).

A. Aldous, in Power Sources for Electric Vehicles (B.D. McNico l and D.A.J. Rand, Eds.) Elsevier, Amsterdam, 1984, p.47.

C.P. Peyriere, Proc. 9th Int. Electric Vehicle Symp., EVS88- P16, Toronto, Canada, 1988.

K.F. Barber, S.K. Takagishi, ibid. EVS88-073.

A. de Guilbert, ibid. EVS88-Pl5.

C. Madery, J.L. Liska, ibid. EVS88-051.

N.P. Fitzpatrick, D.S. Strong, ibid. EVS88-001.

H. Nakao, Y. Suzuki, M. Okawa, ibid. EVS88-042.

L.B. Taylor, D.T. Fouchard, ibid. EVS88-023.

F.E. Zhao, Y.Y. Hu, Electric Vehicle Develop., 8 (1989) 21

K.R. Ramachandran, S.C. Chopra, Proc. ILZIC Silver Jubilee Conf. Pb, Zn and Cd into the go’s, New Delhi, 1988, p.2.

E.Z. Ratner, G.L. Henriksen, C.J. Warde, Proc. 9th Intl. Electric Vehicle Symp., EVS88-011, Toronto, Canada, 1988.

W.J. Dippold, ibid. EVS88-022.

H.P. Schoner, L.L. Ogborn, J. Power Sources, 21 (1987) 91.

145

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

J. M. Bozek, J.J. Smithrick, R.L. Cataldo, J.G. Ewashinka, E V Expo 80, Report No. 8014, Electr ic Vehicle Council, Washington, 1980.

J. Lee, J.F. Mi l ler , C.C. Christianson, J. Power Sources, 24 (1988) 215.

M. Peohler, H.A. Kiehne, Die. Antrzebsbatterie, VDI-Verlag, VARTA, Hannover, 1980, p.7.

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.

C.P. Wales, S.M. Caulder, A.C. Simon, J. Electrochem. Soc., 128 (1981) 236.

W.C. Spindler, R.L. Driggans, C.C. Christianson, Proc. 9th Int. Elec- t r i c Vehicle Symp.. EVS88-032, Toronto, Canada, 1988.

F.H. Klein, U. Wagner, ibid. EVS88-077.

G.H. Cole, ibid. EVS88-078.

W.G. Sunu, B.W. Burrows, in Power Sources 8 (J. Thompson, ed.), Academic Press, 1981, p.601.

US Patent 4 221 847.

M.G. Andrew, P.A. Budney, J.L. Heder, Proc. 9th Int. Electric Ve- hic le Symp., EVS88-002, Toronto, Canada, 1988.

Anon. Electric Vehicle PTOgT., 12 (1990) 1.

Anon. Electric Vehicle PTOgT., 11 (1989) 1.

J. Angelis, D. Sedgwick, Proc. 9th Int. Electric Vehicle Symp., EVS88-009, Toronto, Canada, 1988.

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