2 UNESCO REGIONAL OFFICE FOR SCIENCE AND TECHNOLOGY FOR EUROPE (ROSTE) 1262iA DORSODURO - VENICE, ITALY TEL FAX
3 BATTERY ENERGY STORAGE SYSTEMS D. PAVLOV, G. PAPAZOV and M. GERGANSKA UNESCO Regional Office for Science and Technology for Europe (ROSTE)
4 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..
5 Preface In an attempt to make the power industry more effective, a new trend in electric power production has witnessed intense development during recent years, that of energy storage. Several options have been considered for this purpose, one of them being the battery energy storage system. Both classical lead-acid batteries, as well as new advanced types of batteries are being used. A number of demonstration battery energy storage plants and facilities have been designed and built, and are now subjected to testing. It has become general practice for experts in the power industry, and battery researchers and manufacturers to meet at joint conferences to exchange information and opinions on the problems of energy storage. It is now opportune to siirrimarize the results and experiences so far acquired in tlie design arid utilization of battery energy storage systems. In 1954, Elsevier in Amsterdam issued the book entitled Power Sources for Electric Vehicles edited by B.D. McNicol and D.A..J. Rand, which presented a comprehensive survey of the current knowledge in the field. Motor car transport is being increasingly adopted, since it is an important and indispensable element of the normal functioning of every modern social community. It has, however, a serious environmental impact in that it causes considerable air pollution in large cities and densely populated areas. Development and large-scale commercialization of electric vehicles has become oiie of the greatest challenges of the late 20th century. However, the electrocliemica1 power sources used for propulsion of these vehicles cannot yet meet the challenge. Annual international i
6 conferences on the problems of electrochemical power sources show that more effort is being placed on broad-spectrum investigations in the field. Accumulated theoretical knowledge and practical experi- ence on battery energy storage systems for electric vehicle applica- tions should now be analyzed and evaluated. The Regional Office for Science and Technology for Europe (ROSSE) at the United Nations Educational, Scientific and Cultural Organization (UNESCO) entrusted us with 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, with the purpose of attracting wider public attention to the problems of these systems. The current status and the problems confronting battery energy storage systems for the power industry are presented by Prof. DrSci. D. Pavlov, and for electric vehicle applications, by Dr. G. Papazov. The English version of the text was provided by Mrs. M. Gerganska. All three of us work at the Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia, Bulgaria. If we have achieved, even in part, the aims envisioned by UNESCO for this book, and if our efforts contribute, though modestly, to the development of battery energy storage systems, we will be most satisfied. D. Pavlov, G. Papazov, M. Gerganska May 1991, Sofia, Bulgaria 11
7 Contents Preface... i Chapter 1 BATTERY ENERGY STORAGE SYSTEMS FOR THE POWER INDUSTRY 1. Introduction The four basic elements of every national electric power system Power industry and its problems Energy, power and response time Quality of energy supply systems Ecological problems and the development of power industry Electric energy storage Pumped Hydroelectric Energy Storage Systems (PHESS) Compressed-Air Energy Storage Systems (CAESS) Superconducting Magnetic Energy Storage Systems (SMESS) Battery Energy Storage Systems (BECS) The revival of battery energy storage systems Basic principles of battery operation Some advantages of battery energy storage systems Choosing the right option for electric energy storage
8 Basic principles of battery operation Some advantages of battery energy storage systems Choosing the right option for electric energy storage Batteries for energy storage. in operation and under development Development projects for battery energy storage systems Sodium/Sulfur Batteries Principles of cell operation Design of sodium/sulfur cells Specification and test results for battery modules and pilot plant of the Japanese Moonlight Project Zinc/Bromine Batteries Reactions and principles of cell design arid operation Chemistry and electrochemistry of the zinc/bromine cell Battery system design Characteristics of zinc/bromirie batteries Zinc/Chlorine batteries Fundamentals of zinc/chlorine batteries Battery design Battery characteristics iv
9 5.2. Lead-acid battery energy storage systems (LABESS) in operation by 1990 throughout the world Lead-acid battery energy storage systems for load levelling System structure Chino 10 hgw/40 MWh lead-acid battery energy storage systein Plant layout The battery Power conditioning system Facility monitoring and control system Equipment energy losses Economics of Chino LABES Plant LABESS for instantaneous (spinning) reserve and frequency control applications Island networks The BEWAG 8.5/17MW Lead-Acid Battery Energy Storage Plant System frequency response having given rise to the construction of the BEWAG LABES plant System design and characteristics Lead-acid battery energy storage systems for peak shaving What is peak shaving? Johnson Controls 300 1<W/600 1ïWh LABES Facility Lead-acid battery energy storage systems in the railway transport network V
10 9. Valve-regulated lead-acid batteries for battery energy storage systems Strategic advantages of BES systems References Chapter 2 ENERGY STORAGE SYSTEMS FOR ELECTRIC VEHICLES Motor vehicles and environmental pollution Specification of energy storage systems for electric vehicles Charge and capacity of batteries for electric vehicles Types of cycles of electric vehicle batteries Requirements to the construction and manufacturing technology of batteries for EV energy storage systems Specification of operating energy storage systems for electric vehicles..., Lyrical epilogue References.., vi
11 Chapter 1 BATTERY ENERGY STORAGE SYSTEMS FOR THE POWER INDUSTRY D. PAVLOV 1. Introduction 1.1. The four basic elements of every national electric power system Production of electric energy is the basic pillar for normal functioning of every modern social community and a guarantee for its progress. It is organized in an electric power system comprising three basic elements: a) Electric power and energy generating utilities, i.e. electric power plants: thermal power plants fired by coal or nuclear fuel, gas- fired steam plants, oil- or gas-fired combustion turbines, hydroelectric plants, etc. b) Electric power distributing systems including transformer fa- cilities, transmission trunk lines and distribution lines to every cus- tomer. c) Consumers of electric power and energy. These are users in industrial, transport, agricultural and telecommunication contexts, and people in their day-to-day life, administrative buildings, etc. The electric power produced by the generating utilities is deliv- ered through the transmission/distribution system to the consumers for utilization. The consumers demand for electric power varies cyclically during day and night, as well as within the week and the seasons.
12 Demand. This is the rate at which electric energy is delivered to the consumer, measured in kw (kilowatts) integrated over a specific time interval (15 min) [l]. Figure 1 shows an example of a daily customer demand profile. A baseload level of demand is introduced. The power capacity for meeting this demand level is generated and maintairied by thermal power plants fired by low-cost fuels such as coal or nuclear fuel. To be economically effective, baseload generating units should operate at a minimum capacity of MW and under constant load. I I..,, I 3, 5'7'9'11'i3'1'' 17' Hour of day DCoal mgas turbine Bottery Fig. 1. Example of customer energy demand curve for a working day [il. During the night (hours O to 6), the demand decreases to about 15-30% below the baseload level. The daytime demand is signifi- cantly higher than the baseload level. It is served by gas-fired steam plants. They burn natural gas or oil which are more expensive fu- els than coal. There are two peaks in the daytime demand profile related to the increased energy consumption for the transportation of people from home to the working place and back, as well as for increased household needs. Peak power is generated by gas-fired turbines utilizing relatively expensive fuel, and also by hydroelectric 2
13 power plants. The ratio of actual to peak power demand over a given period is called load factor. There is an intrinsic contradiction in each power supply system between producers and consumers of electricity. To be efficient, power plants should operate at constant load. The customers de- mand, on the other hand, undergoes cyclic fluctuations. This leads to inefficient utilization of the generating capacities. A possible so- lution to this problem is the involvement of a new element in the energy system. Electric energy storage. At night, when energy demand is low, generated electric energy is stored in appropriate facilities, and is delivered to meet peak-hour demands during the day. Thus, low-cost fuel power plants work at maximum load during the night and store the generated energy to sell it at increased cost during peak demand periods. The introduction of this fourth element in the electric power system makes its operation more efficient. This not only brings about considerable savings of expensive fuels such as gas and oil, but also improves the load factor of the power generating facilities Power industry and its problems Energy, power and response time It has been established that the different forms of motion (me- chanical, thermal, electromagnetic, gravitational, chemical, etc.) are converted into one another following definite quantitative ratios. To allow measuring of the various forms of motion by a unified measuring unit, the term energy has been introduced. The electrical energy is determined from the product of the voltage and the quantity of charge that passes through an electrical device (load). The work done per unit time is called power. The electrical power is determined from the product of voltage and current. 3
14 In thermal power stations, the chemical energy accumulated in coal, crude oil or natural gas is transformed by burning (oxida- tion of the hydrocarbons) into heat (high-temperature, high-pressure steam), which sets in motion a turbine, whereby the thermal energy is transformed into mechanical. The turbine shaft is connected to the shaft of an electric generator. On rotation, this common shaft drives the rotor of the generator as a result of which the mechanical energy is converted into electrical energy. It is evident that, to obtain elec- trical energy from coal, several processes of energy conversion have to occur. In an electrochemical power source, a battery in particular, this energy transformation path is much shorter. In this case, through electrochemical reactions of oxidation and reduction proceeding on the surface of the two electrodes, the chemical energy is directly transformed into electrical power. Conversion of one type of energy into another requires a certain time period. The time needed for an energy-generating system to change its power from a value (A) to another value (B) is called response (transition) time (Fig. 2). / i ition time _J Time Fig. 2. Power curve showing the change of power from level A to level B. 4
15 A thermal power plant needs tens of minutes to change from one power level to another, while for a battery, the response time is of the order of millionths of a second. For an energy utility to meet all load fluctuations, it should dispose of a system of power plants with various response times ranging from milliseconds to hours Quality of energy supply systems The quality of an electric power supply is determined by the available reserve capacity at the energy utility. Figure 3 illustrates the distribution of the electric-system capacity expressed by a typical weekly load curve of an electric utility Generation for load í No storage) Mon Tue Wed! Thul Fri 1 Sat Sun 1 Baseload 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 SiOMgt Peaking energy from storage Fig. 3. Typical weekly load curve for an electric utility [il. The energy system should have 15-20% of reserve power available to be able to meet any customer demand. If there is no or insufficient reserve capacity and the load level exceeds the power generation level, 5
16 a decline in voltage at the consumer side will appear which would upset the normal operation of the users machines and electrical de- vices or even cause them to fail. For this reason it is essential for the normal functioning and development of each social community to have reliable national and local electric power systems with capacities exceeding the actual energy demand by at least 15%. Unfortunately, however, only rich and advanced modern countries possess such high-grade energy systems. The power systems of most countries in the world have capacities that only just meet their energy demands, and in some cases are simply inadequate. This hunger for electricity is very often a limiting factor for the economical and social development of a country indus try Ecological problems and the development of power The electric energy needs of the population, industry, agriculture, transport, etc. increase every year, and the claims for high-quality electric power become ever more demanding in relation to the in- creasing automation and computerization of the national economy. Previously, these needs were met by expanding the capacities of all types of electric power generating facilities. Operation of these facil- ities, however, is based on the combustion of coal, oil and gas, which is accompanied by harmful gas emissions of COZ, SO2 and others. The increased content of SO2 in the atmosphere has led to the forma- tion of acid rains causing enormous damage to the agricultural crops and the forests. The accumulated CO2 in the air might bring about considerable climatic changes both on a regional and global scale (the so-called greenhouse effect ). Thus the rapid development of power industry has added comfort to society and its individuals, but it has posed very serious ecological problems of a national, regional and global nature. In response to these processes, various organi- 6
17 zations and social movements are being founded whose activities of environmental control and protest actions begin to have a significant impact on the policy of state governments and of companies engaged in electric power production. The efforts of these movements, combined with the wisdom of a number of state governments, have led to the adoption of dead-line terms for decreasing the harmful gm con- tents in the atmosphere, especially those of SO:! and COL, in order to restrict possible environmental damage. Solutions are being sought in several directions: First, in reducing SO2 emissions by building up special facilities at electric power plants for purification of exhaust gases. This method has an undoubtedly beneficial effect on the environmental aspect of electric energy production, but it involves rather expensive, complicated and not fully efficient procedures leading to increase in energy and power costs. Second, in building up a system of energy storage plants which haw a considerable impact on the efficiency of energy utilities as well as significant cost benefits. Third, in thc sphere of electric power consumption, all techno- logical processes of major energy consumers have been revised with regard to power consumption, and the most energy-consuming pro- cedures replaced by new technologies with lower power demands. The basic problems and the development trends of energy storage will be discussed in the chapters to follow. 2. Electric energy storage During the last few decades, several options for electric energy storage have been devised. Many countries have started programs aimed at development of energy storage technologies. The basic prin- ciples of some of these options, that have found successful application, will be outlined below. 7
18 2.1. Pumped Hydroelectric Energy Storage Systems (PHESS) Figure 4 presents the schematic of such a system. Fig. 4. Schematic of a Pumped Hydroelectric Energy Storage System. These energy storage units require two large water reservoirs lo- cated at different heights, so that water fall is possible. During pe- riods of low demand, the excess power is utilized to pump water from the lower reservoir and transfer it to the upper one. At peak demand periods, the pumped storage plant acts as a hydroelectric power plant thus adding capacity to the energy system. This storing option is cost-effective if used only 5 to 8 hours in the peaking range. Its response time is of the order of 5 to 10 minutes. The above energy storage technology has been in use for over 50 years now. At present, there are about 35 pumped storage plants in operation all over the world with a total capacity of 25,000 MW. This energy storage option is most appropriate for countries with mountainous relief. Construction of these plants requires from 8 to 10 years and is often associated with considerable environmental im- 8
19 pact. Pumped hydroelectric energy storage systems are cost-effective if they are designed for power units of over 1000 MW. In Italy, for example, pumped storage plants supply 14% of the net power capacity. In Japan, they amount to about 10% of the national net capacity, while for France, Germany and the UK, this figure is 6%, and 3% for the USA. At the moment, more than 200 pumped storage plants are under construction worldwide. Conse- quently, by the beginning of the next century, this energy storage option will become an important element of many national electric power systems. 2.2.Compressed-Air Energy Storage Systems (CAESS) A compressed-air storage plant uses inexpensive off-peak energy to drive the motor of a compressor for compressing air that is stored in a salt cavern located deep underground or in large hard rock cav- erns. During peak demand periods, gradual release of pressure is performed and the air coming up to the surface is heated by burn- ing oil or gas, and is then expanded through expansion turbines that drive the rotor of an electric current generator. Compressor motor and generator are combined in one machine. During air compression, the rnotor/generator is connected to the compressor and decoupled from the turbine. During electric current generation, the motor/generator is disconnected from the compressor and coupled to the turbine. Compressed-air storage units burn only one third of the fuel used by conventional combustion turbines to produce the same amount of electricity. This leads to a two-third reduction in the en- vironmental pollution caused by the combustion process of turbines which are usually located in urban areas. Ways have been sought for optimization of the system operation, such as return of the heat re- leased during air compression back to the energy system. This energy storage option is cost-effective if operated at a power above 25 MW.
20 For every hour of electric current generation, 1.7 ki of air compression are needed. The response time is about 10 minutes. Efficiency of air compression is 65-75%. The starting period is 20 to 30 minutes. As rcgards the security aspects, measures should be provided against leakage of compressed air. The service life of air-compressed storage plants is about 30 years. A block diagram of such a system is presented in Fig. 5. Fig. 5. Block diagram of a Compressed-Air Energy Storage System . Compressed-air storage technology was first devised in Germany, and since 1978 a 290 MW, four-hour capacity unit has been in op- eration in Huntorf. The plant uses two salt caverns, and storage efficiency of over 80% is reported. The cost of unit power is about 425 $ kw-. A 30-year operational life of the plant is expected. Commercial operation of the German CAES plant has shown that this type of energy storage option is sufficiently reliable. 10
21 Compressed-air storage plants have a negligible environmental impact, and can be built within 2 to 5 years. They arc: fit,ted with modified combustion turbines of routine production. This technology can find application only in countries with natural deep underground, hard rock or salt caverns. At present, several demonstration compressed-air energy storage plants are being built: in the USA, Alabama (110 MW, 26-hour capacity), in the USSR (1050 MW, 10-hour capacity, thrce-unit plant with salt, cavern storage), in Israel (300 h'lw, lo-hour capacity, three- unit plant), etc. The Italian conipany ENEL has st,arted construction of modular mini-units of 25 and 50 MW, arid 10-hour capacity, using aquifer storage Superconducting Magnetic Energy Storage System (SMESS) There is a theoretical and a technical option to store electrical en- ergy as such, without converting it into other forms. This is possible owing to the ability of some substances to become superconducting at extremely low temperatures. Because of the conductor's electrical resistance at anibient temperature, part of the electrical energy is lost in the form of heat emission (joule losses). These losses can be compensated by adding new quantities of electricity to the power supply network. At extremely low temperatures, some alloys and ceramic materi- als achieve superconducting properties, i.e. they lose their electrical resistance. When direct current is fed into an electric circuit of su- perconductors, the current will circulate endlessly along the closed ring without energy losses. When an energy demand appears, the requested electrical power can be drawn from that closed ring. Large-scale investigations are presently being performed aimed at devising a technology for the production of superconducting magnetic 11
22 Fig. 6. Schematic of a Superconducting Magnetic Energy Storage System . energy storage plants. A block diagram of such a plant is presented in Fig. 6. The heart of this storage system is the electromagnetic superconducting coil. The latter operates on direct current. Charging of the electromagnetic coil with electricity from the ac generating utility is accomplished via a two-way converter. A refrigeration system main- tains the temperature of the electromagnetic coil at a very low fixed value. Operation of the electromagnetic coil, converter and refrig- erator is monitored and controlled by a controller. Such an energy storage plant should be sited near a substation where the transformer converts the high voltage energy from the utility network to appro- priate low voltage power. The response time of this type of storage system for switching between charging and discharging is about 20 milliseconds. The ac-ac efficiency is 90% or more. The experimental SMES systems so far set up operate at extremely low temperatures -269 C (4 K), the temperature of liquid helium) and the coil-wire used is made of NbTi and NbSn alloys. 12
23 With the discovery of ceramic high-temperature semiconductors, it can be expected that superconducting magnetic storage plants will be constructed that are capable of operating at the temperature of liquid nitrogen (-196 C). Since the technology for liquid nitrogen production is well advanced and cost-effective, the expenses for construction and maintena.nce of the refrigeration system will be reduced significantly. The current density in superconductive wires may reach ex- tremely high values as the conductor exerts no electrical resistance leading to joule losses. This allows the wire cross-section to be decreased more than five times with respect to copper wires used at ambient temperature. This will change substantially the existing classical electric power system. In Japan, an energy storage project is being developed known as the Moonlight Project. The power of the Japanese super- conducting magnetic storage system is 1000 MW, energy density is 12 Wh kg-, storage efficiency SO-SO %, storage utilization rate approx. 75%. The system will be used for daily and weekly en- ergy storage. Underground bed rocks are required for construction of this system. Location possibilities are restrictcd, because anti- magnetic measures are needed for environmental protection. Protec- tion against superconductive material degradation is also necessary. A 10 MW, two-hour capacity SMESC pilot plant has been developed in the USA. The refrigeration system is based on liquid helium Battery Energy Storage Systems (BESS) The revival of battery energy storage systems At the beginning of this century, electric power supply for industrial and domestic needs was provided by dc generators arid battery facilities operating under floating charge conditions. During this pe- 13
24 riod, batteries proved to be a diverse and flexible means of solving the load factor problem. During the 1930s, an expansion of ac technologies for electric power generation, transmission and distribution applications was noted and, very soon, the dc battery system was abandoned and hence also the storage of energy as an element of the power system. In the 1960s, a powerful reliable and cost-effective static recti- fier was devised. Nuclear power plants were equipped with large stand-by lead-acid battery storage facilities ensuring their reliability by supplying reserve power and energy. New and innovative elec- tric power applications in industry and every-day life brought about radical changes in the profile of the daily, weekly and seasonal de- mand ciirves. To enhance the operational efficiency of electric power utilities, energy storage units were introduced. At first, pumped hydroelectric energy storage plants were used for that purpose, and later, the old lead-acid battery storage systems were revived. They were based on totally new conversion, management and control tech- nologies. At the end of the 1970s, for the first time, BEWAG-AG decided to install the Battery Storage Facility in West Berlin under a test program in order to collect the necessary operational and technical information. It started operation in July Within the Moonlight energy storage project, a 1 MW/4 MWh load-levelling battery plant started operation in 1986 in Tatsumi, Japan. In July 1988, the largest battery plant for load-levelling (10 MW/ 40 MWh) was set in operation in the USA, at Chino, California. Since then, many technologically advanced countries throughout tlie world have started large-scale research and test programs aimed at the introduction of battery energy storage systems in their national ccoiioinies and public services. 14
25 Basic principles of battery operation When two appropriately chosen electrodes are immersed in the respective electrolyte and a direct electric current from an external source flows between them, electrochemical reactions proceed on the electrode surfaces during which inactive substances are transformed into electrochemically active ones. This process is called charging of the electrochemical power source or the battery. As a result of these reactions, electrical energy is converted to chemical and an electromotive force is created between the two electrodes. When the electrodes are interconnected via a load, under the action of this electromotive force, electrocheniical reactions proceed on the electrode surfaces in an opposite direction to the reactions during charge. This process of current generation is called discharge. The battery can endure thousands of charge-discharge cycles. However, parallel to the reversible processes of charge and discharge, certain low rate irreversible processes also take place that limit batt,ery cycle life. During off-peak periods, the battery is charged from the electric power utility via a converter. The latter converts the alternating current into dc. During discharge, the direct current generated in the battery is transformed by the converter iiit,o alternating current and the latter is delivered through the tramfornier to the utility for meeting energy demands. Operation of the converter arid the battery are monitored and controlled by a controller, 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 the latter to exhibit a number of useful advantages leading to significant cost beneíits. The following ecoiioniical features of lead-acid battery storage systems were demonstrated. 15