IEC_Elctrical Energy Storage.pdf

Electric vehicles and also high energy versions for

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electric vehicles, and also high-energy versions for storing renewable energy for load-levelling and industrial applications [esp11]. 2.3.2 Flow batteries In conventional secondary batteries, the energy is charged and discharged in the active masses of the electrodes. A flow battery is also a rechargeable battery, but the energy is stored in one or more electroactive species which are dissolved in liquid electrolytes. The electrolytes are stored externally in tanks and pumped through the electrochemical cell that converts chemical energy directly to electricity and vice versa. The power is defined by the size and design of the electrochemical cell whereas the energy depends on the size of the tanks. With this characteristic flow batteries can be fitted to a wide range of stationary applications. Originally developed by NASA in the early 70s as EES for long-term space flights, flow batteries are now receiving attention for storing energy for durations of hours or days with a power of up to several MW. Flow batteries are classified into redox flow batteries and hybrid flow batteries. Redox flow battery (RFB) In redox flow batteries (RFB) two liquid electrolyte dissolutions containing dissolved metal ions as active masses are pumped to the opposite sides of the electrochemical cell. The electrolytes at the negative and positive electrodes are called anolyte and catholyte respectively. During charging and discharging the metal ions stay dissolved in the fluid electrolyte as liquid; no phase change of these active masses takes place. Anolyte and catholyte flow through porous electrodes, separated by a membrane which allows protons to pass through it for the electron transfer process. During the exchange of charge a current flows over the electrodes, which can be used by a battery-powered device. During discharge the electrodes are continually supplied with the dissolved active masses from the tanks; once they are converted the resulting product is removed to the tank. Theoretically a RFB can be “recharged” within a few minutes by pumping out the discharged electrolyte and replacing it with recharged electrolyte. That is why redox flow batteries are under discussion for mobile applications.
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29 However, up to now the energy density of the electrolytes has been too low for electric vehicles. Today various redox couples have been investigated and tested in RFBs, such as a Fe-Ti system, a Fe-Cr system and a polyS-Br system (Regenesys installation in UK with 15 MW and 120 MWh, but never commissioned) [jos09]. The vanadium redox flow battery (VRFB, Figure 2-7) has been developed the furthest; it has been piloted since around 2000 by companies such as Prudent Energy (CN) and Cellstrom (AU). The VRFB uses a V 2+ /V 3+ redox couple as oxidizing agent and a V 5+ /V 4+ redox couple in mild sulphuric acid solution as reducing agent. The main advantage of this battery is the use of ions of the same metal on both sides. Although crossing of metal ions over the membrane cannot be prevented
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