Energy of batteries

System Voltage (V) Specific Energy (Wh 1 - ) Energy Density (Wh L ) Power Density Specific Energy of Cell (Wh kg- ) Specific Energy of Battery (100% SOC) (Wh kg- )... 

FIGURE 1.4 Theoretical and actual specific energy of battery systems. 

In a typical setup, a flow battery consists of two electrolyte reservoirs from which the electrolytes are circulated by pumps through an electrochemical cell stack [4]. The electrolytes in VFB serve as the medium to store and release energy, whereas the volume and the concentration of electrolyte determine the energy of battery. 

Batteries are miniatuie chemical leactois that convert chemical energy into electrical energy on demand. The thermodynamics of battery systems foUow direcdy from that for bulk chemical reactions (10). For the general reaction... 

The detailed mechanism of battery electrode reactions often involves a series of chemical and electrochemical or charge-transfer steps. Electrode reaction sequences can also include diffusion steps on the electrode surface. Because of the high activation energy required to transfer two electrons at one time, the charge-transfer reactions are beheved to occur by a series of one electron-transfer steps illustrated by the reactions of the 2inc electrode in strongly alkaline medium (41). 

In the 1990s, the use of batteries in electric vehicles and for load leveling is being revived partly for environmental reasons and partly because of scarce energy resources. Improvements in battery performance and life, fewer maintenance requirements, and automatic control systems are making these appHcations feasible. Research and development is ongoing all over the world to develop improved lead—acid batteries as weU as other systems to meet these needs. 

Li—Al/FeS cells have demonstrated good performance under EV driving profiles and have deUvered a specific energy of 115 Wh/kg for advanced cell designs. Cycle life expectancy for these cells is projected to be about 400 deep discharge cycles (63). This system shows considerable promise for use as a practical EV battery. 

Electrochemical systems are found in a number of industrial processes. In addition to the subsequent discussions of electrosynthesis, electrochemical techniques are used to measure transport and kinetic properties of systems (see Electroanalyticaltechniques) to provide energy (see Batteries Euel cells) and to produce materials (see Electroplating). Electrochemistry can also play a destmctive role (see Corrosion and corrosion control). The fundamentals necessary to analyze most electrochemical systems have been presented. More details of the fundamentals of electrochemistry are contained in the general references. 

To hasten development of batteries for electric vehicles, Chiysler, Ford, and General Motors formed the U.S. Advanced Batteiy Consortium (USABC). In 1991 USABC, battery manufacturers, the Electric Power Research Institute (EPRI), and the U.S. Department of Energy (DOE) launched a joint research effort to identify, develop and license promising batteiy technology for electric vehicles—vehicles with the range, performance and similar costs of gasoline-powered vehicles. 

The energy costs of building vehicles must also be considered. For ICE vehicles, more energy is usually used in construction of the vehicle than -will be consumed in fuel for driving 100,000 miles. For the EV, the dynamics are even worse since the material and energy costs of batteries are considerable. Batteries are expensive since they entail a substantial amount of material (added weight) and often involve multiple complex construction. For example, the thirty-two advanced lead-acid batteries for the 1995 GM Impact weighed over 850 pounds. 

A battery stores electric energy. Although the concentration of energy is small compared, for example to gasoline, we see a myriad of uses of batteries in radios, cellular phones, flashlights, computers, watches, and so on. The public s demand for these portable products is ever increasing, and scientists strive to develop lighter and better batteries. 

The discharge curve (Fig. 8) is another important feature of battery systems therefore the terminal voltage is plotted against the discharge capacity. For an ideal battery the terminal voltage drops to zero in a single step when the stored energy is completely consumed. 

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