Batteries, and fuel cells, essentially comprise two electrodes immersed in a chemical solution (usually) termed an electrolyte, while externally the electrodes are connected to an electrical circuit. An electrolyte is any substance containing free ions (an atom or molecule having lost or gained one or more electrons relative to its normal complement) and thus behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. The most common manifestation is as solutions of acids, bases or salts. Electrolyte solutions are normally formed when a salt is placed into a solvent such as water and the individual components dissociate due to the thermodynamic interactions between solvent and solute molecules, in a process called solvation. For example, when table salt, NaCl, is placed in water, positively charged sodium ions and negatively charged chlorine ions are formed . In general terms, an electrolyte is a material that dissolves in water to yield a solution that conducts an electric current. It may be described as concentrated if, in solution, it has a high concentration of ions, or dilute if it has a low concentration. If a high proportion of the solute (e.g., salt) dissociates to form free ions, the electrolyte is strong. On the other hand, if most of the solute does not dissociate, the electrolyte is weak.
The process of charge separation and energy accumulation in a battery can probably best be explained by reference to a class with which most people will be familiar - namely the lead-acid battery which provides starting power in road vehicles. This chemical storage format has been around a very long time , in electrical engineering terms, having been invented by Gaston Plante in 1859. In the lead-acid storage cell the cathode is formed from spongy lead (Pb), while the anode is also made of lead but coated with lead dioxide (PbO2). The two electrodes are usually interleaved to expose maximum surface with alternating anode and cathode surfaces. These plates are immersed in an electrolyte, which comprises a solution of sulphuric acid (H2SO4) diluted with water (H2O). In a fully charged battery the proportions are 25% acid to 75% water. Lead reacts quite strongly with sulphuric acid to form lead sulphate (PbSO4) and water . During the reaction, which occurs because of the dictates of the second law of thermodynamics (Sect. 2.2), free electrons are formed at the cathode and work is performed on these electrons moving them from the cathode to the anode, which becomes negatively charged. In so doing the cathode becomes deficient of electrons and hence positively charged. In a battery unconnected to an outside electrical circuit (open-circuited) the reaction will continue until the potential between each pair of plates (2.014 V) matches the chemical potentials driving the reaction. This voltage is usually referred to as the electromotive force (emf). Note that in relation to the outside circuit the cathode plates are connected to the positive terminal of the battery, while the anode plates are connected to the negative terminal. With six anodes and six cathodes (six cells) the battery will deliver 12.084 V. Furthermore, if it is connected to an electrical load, such as a car starter motor, current can be drawn until both the anode and cathode are fully coated with lead sulphate at which point the electrolyte has also become highly diluted with water. The process can be reversed and the battery recharged by passing a DC current through the battery such that electrons are made to flow from the anode to the cathode .
The electrical energy provided by a battery during discharge is derived from the electrochemical reactions taking place between the electrolyte and the active materials in the anode and the cathode. In the case of a lead-acid battery the reaction is between the sulphuric acid and the lead in the cathode and the lead dioxide in the anode. The greater the amount of active material the greater is the storage capacity of the battery. The electrochemical laws of Faraday  provide the method of calculating these amounts, and when applied to the lead-acid battery yield the result that for 1 A-h of electrical capacity, 4.46 g of lead dioxide and 3.87 g of lead is required . In practice, from three to five times these theoretical amounts is needed, depending on the type of cell and the thickness and number of plates. Given that 1 A-h from a 12 V battery represents 12 W-h or 43.2 kJ, then we can conclude that a lead-acid battery has a storage capacity of the order of 0.61 MJ/kg. This is similar in level to thermal storage based on phase change techniques (see Sect. 4.4), but is more than twenty times greater than is currently available from flywheel storage (28 kJ/kg), and greatly exceeds the per kilogram values associated with hydro-electric pump storage. On the other hand, petrol has an energy storage capacity of 47 MJ/kg, while hydrogen provides 143 MJ/kg. It is hardly surprising, therefore, that mankind has largely ignored renewable energy sources in favour of fossil fuels.
While weight for weight, or volume for volume, batteries tend to be the most compact of electrical energy storage media, transference of energy into and out of a battery generates rather significant levels of power loss, which can represent a major problem for some storage applications. If we consider, for the sake of illustration, the energy required to recharge a typical 40 A-h, 12 V car battery, and if we further consider that the process is lossless, then the energy input is simply 40 x 12 = 480 W-h. If you have ever trickle charged a battery you will know that the charging process generates heat, which typically absorbs about 15% of the input power. Therefore to achieve the same level of charge we will require
480 x 1.15 = 552 W-h. A battery charger connected into the main electrical supply contains transformers and rectifiers, which also generate thermal losses. It is estimated that a typical battery charger is about 60% efficient . Therefore, the energy required from the 'mains' supply is 552/0.60 = 920 W-h, that is 920 watts for an hour. But from Chap. 3 we know that almost 50% of the prime power supplied to the generation station turbines (whether fossil fuel, nuclear, hydro, solar, etc.) is lost in the electricity generation, transmission and distribution systems. Consequently prime power input at the power station in order to recharge our 12 V battery is of the order of 2 kW for an hour, or 7 MJ! This means that employing batteries for small scale storage purposes, such as to power vehicles, exerts a very expensive level of demand on primary energy sources, and as we shall see later this potentially very significant drain on renewable supplies could have a major impact on the extent to which road vehicles and in particular private cars can form part of a sustainable future even if these vehicles are electrically driven.
At the scale of storage required by the electrical power industry the unavoidable inefficiencies of battery charging and discharging are not really a problem, since the power that will be employed to recharge a battery storage plant attached to a power station would otherwise be wasted. Recharging will generally be performed when demand is low and when the wind still blows and the waves still batter the shore. In the early days of electric power generation, very large storage battery arrays were commonly installed near power stations as an essential back up for controlling demand fluctuations and for emergency systems. At that time all of the electrical power being generated and distributed was DC, which meant that the battery bank could be connected directly to the power lines. They were used to assist in ensuring economic operation of power stations and in the maintenance of supply. In so doing they were subjected to regular cycles of discharge and charge, and lead-acid batteries were harnessed for this role. Towards the beginning of the twentieth century the electrical supply industry was developing rapidly, and the advantages of very high voltage AC transmission became apparent. As a result many of the original DC stations were scrapped. However, the wholesale adoption of high voltage AC electrical power generation and transmission introduced new problems requiring the presence, at power stations, of back-up battery storage systems. Batteries were, and are, considered to be the best source of electrical supply for operating remote control switch gear, circuit-breakers, remote control equipment, and many safety and protective devices required by modern generation and distribution plants. Battery types and sizes vary considerably from station to station. For example, at the Sizewell nuclear power station in the UK the following batteries are employed . Two 440 V batteries each with 224 cells are connected in parallel and are used to power emergency systems for the reactor. Each battery can supply 1300 A-h, which is equivalent to 1.144 MW-h. One 240 V battery (120 cells, 210 A-h) with an energy capacity of 50 kW-h, powers the emergency lighting, and supplies emergency power for the oil pumps. A third battery operating at 110 V (55 cells, 1200 A-h) has a capacity of 132 kW-h and is used for switching operations, while a fourth (50 V, 24 cells, 200 A-h) has a storage capacity of 10 kW-h, which is enough to power an auto matic telephone exchange and station alarms. All of these battery banks are constructed from enclosed lead-acid type cells.
Clearly battery banks of moderate power have been in operation in power stations for a very long time and the technology is mature at this level of power. Renewable power stations, however, will require storage capacities that are at least an order higher than is currently the norm. Battery banks capable of storing more than 10 MW-h will be required to provide back-up storage for renewable power stations and considerable research effort is being directed towards this aim [22, 23]. Theoretically, high energy density batteries would use anodes composed of alkali metals such as sodium, lithium and potassium, which are the most reactive of metallic materials. Nickel-cadmium and nickel-zinc batteries are also being re-examined and have been shown to be potentially capable of high storage densities. Calculations, and prototype testing, suggest that alkali metal batteries are really the only source of electric power propulsion which can compete with the internal combustion engine for power delivery and range. Many combinations of reactive chemicals have been researched in the pursuit of battery solutions offering higher energy densities than the staple lead-acid version. Four chemical combinations give considerable hope that a major advance is close. These are sodium-sulphur, lithium-sulphur, lithium-chlorine and zinc-chlorine. These advanced batteries, and in particular the sodium-sulphur couple using a solid electrolyte and lithium-sulphur couple using a fused salt electrolyte, are at the prototype stage of development.
The sodium-sulphur (Na-S) battery is representative of what are termed high temperature advanced concept developments. For example, a 1 MJ capacity battery for electric vehicle applications is at an advanced stage of development at Chloride Silent Power in the UK with the collaboration of General Electric in the USA . Similar battery concepts are being researched by Ford (USA), Brown Boveri (Germany) and British Rail (UK). All use a test-tube shaped ceramic container, made of beta-alumina, which is conducting to sodium ions. The tube contains molten Na in its interior (anode) and is surrounded by a sulphur melt (cathode) housed in a case, which collects the current. The operating temperature of the system is between 300 and 400°C, and the cell voltage, derived from the chemical reaction between the sodium and the sulphur to produce sodium polysulphide , is 2.08 V. The theoretical energy density of these batteries is about 2.7 MJ/kg, more than four times the level of the lead-acid battery. These batteries also have the additional advantage over lead-acid of better depth of discharge (~ 80%), no maintenance such as adding distilled water, and a plentiful supply of the raw materials from which they are constructed. Over 200 MW of sodium-sulphur capacity have been deployed in Japan. Generally, this has been in installations exhibiting power outputs up to 12 MW, with energy storage times of 7 hours at the rated power. In June 2006 the American Electric Power Corporation began operating the first 1 MW sodium-sulphur storage system in the USA. The acquisition of a further 6 MW of storage capacity of this type is planned.
A good example of the progress that is being made in the development of large electrochemical storage systems for electrical supply back-up, is the new battery energy storage system (BESS) at Fairbanks, in Alaska. This battery system is designed to stabilise the local grid and reduce its vulnerability to events like the blackout that occurred five years ago, on 14th August 2003, in the north eastern USA and Canada. A consortium led by the Swiss company ABB, the leading power and automation technology group, supplied and installed the BESS. At the heart of this powerful electrochemical storage system are two core components. First are the nickel-cadmium (NiCad) batteries, developed by the French company, SAFT. The 1500 ton battery bank comprises nearly 13,760 rechargeable cells in four parallel strings. Second is the convertor, designed and supplied by ABB. The convertor changes the battery's DC power into AC power ready for use in the local grid transmission system. The system is configured to operate in several distinct modes, each of them aimed at stabilising the generators if power supply problems occur. During commissioning tests in 2003 the SAFT battery and the ABB power conversion system surpassed the highest previously recorded output from a battery system by achieving a peak discharge of 26.7 MW with just two of the four battery strings operational. This makes the Alaskan BESS over 27% more powerful than the previously most powerful example, namely a 21 MW BESS commissioned by the Puerto Rico Power Authority at Sabana Llana, Puerto Rico in 1994. Although the Fairbanks plant is initially configured with four battery strings, reports  suggest that it can readily be expanded to six strings to provide a full 40 MW for 15 min. Recharging would take between 5 and 8 hours. The facility which occupies an area about the size of a soccer pitch, can ultimately accommodate up to eight battery strings, giving considerable flexibility to boost output or to prolong the useful life of the system beyond the planned operation span of 20 years.
Batteries are also being developed in which the electrolyte, instead of being sealed within the battery, is continually being replenished and returned to external storage tanks. These batteries are a form of fuel cell and generally exist in one of three types: zinc-bromine, vanadium redox and sodium-bromide. For example, with the zinc-bromine flow battery  a solution of zinc bromide is stored in two tanks. When the battery is charged or discharged the solutions (electrolytes) are pumped through a reaction vessel (battery) and back into the tanks. One tank is used to store the electrolyte for the positive electrode reactions and the other for the negative. Zinc-bromine (ZnBr) batteries display energy densities comparable with that of lead-acid types, generally of the order of 0.27 to 0.31 MJ/kg. The primary features of the zinc-bromine battery are superior depth of discharge; long life cycle; large capacity range (50 kWh), stackable to 500 kWh systems; and independence of power delivery capability from the stored energy rating.
In each cell of a ZnBr battery, two different electrolytes flow past carbon-plastic composite electrodes in two compartments separated by a microporous polyolefin membrane. During discharge, zinc (Zn) and bromine (Br) combine into zinc bromide, generating 1.8 V across each cell. This will increase the Zn ion density (each with a positive charge equal to twice the electron charge) and Br ion density (negatively charge equivalent to the electronic charge) in both electrolyte tanks . During charge, metallic zinc will be deposited (plated) as a thin film on one side of the carbon-plastic composite electrode. Meanwhile, bromine evolves as a dilute solution on the other side of the membrane, reacting with other agents (organic amines) to make thick bromine oil that sinks down to the bottom of the electrolytic tank. It is allowed to mix with the rest of the electrolyte during discharge. The net efficiency of this type of battery cell is about 75%.
The development of the zinc-bromide battery is attributed to Exxon and the first examples appeared at the beginning of the 1970s. Over the years, many multi-kWh batteries of this type have been built and tested. Meidisha demonstrated a 1 MW/4 MW-h ZnBr battery in 1991 at Kyushu Electric Power company in Japan. Some multi-kWh units are now available pre-assembled, complete with plumbing and power electronics. ZBB, a company which specialises in zinc-bromine flow technology, in partnership with Sandia National Laboratories, is installing a 400 kW-h advanced BESS near Michigan in the USA.
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