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The flow of each reactant solution i s totally uithin the respective electrode structure, which provides an excellent mass-transfer environment. The carbon or graphite felt used for the chromium electrode is lightly catalyzed to prc- mote the chromium redox reactions and to provide an overpotential for the hydrogen evolution reaction, which can otherwise occur at appreciable rates. Both porous electrodes serve as electron conductors and provide the sites at which the electrochemical redox reactions occur. The electrodes themselves, however, are inert both chemically and electrochemically in the working cell environment and undergo no physical or chemical changes. Characteristic Advantages of Flow Batteries Because the reactants of a flow battery are not confined within the lndi- vtdual single cells but are stored as solutions i n external tanks, i t i s pos- sible to independently size the energy storage portion (tanks plus reactants) and the power-producing portion (cell stacks) of the system. This in turn allows more efficient use of the stored reactants and also permits simple sys- tem modification and growth, both of which result i n economic benefits. Another advantage to the external storage of the reactants i s that certain operational functions can be carried out at the system level instead of at the single-cell level. For example, since a l l cells are being supplied i n parallel with reactants from comnon sources, insofar as their inlet reactant concentra-- tions are concerned, all cells iire at the same state of charge. This has sev- eral ramifications. First of all, one need not be concerned about individual cells getting out of balance with one another. Thus i t i s not necessary to monitor or manipulate individual cells. Instead any imbalance that does occur can be countered by dealing directly with the stored bulk of the reactants. Another aspect of a l l cells always being at the same state of ct5rge (with respect to Weir Inlet reactants) i s that different cells can be used indif- ferent ways, concurrently. For example, some can be accepting charge from an external source while others are being discharged. This makes it possible to use flow batteries as dc-dc transformers (ref. I ) , as multivoltage output devices, and as storage for sources of intermittent and variable voltage such as wind turbines or solar photovoltaic arrays. I n a l l cases the actual state of charge of the system i s measured simply by determining the open-circuit potential between the stored reactants. Another advantage inherent to this flow battery i s that a l l of the elec- trode reactions occur in the solution phase. Since nc phase changes take place at the electrodes, life-1lmlting processes such as slumping, shedding, or den- drite growth do not occur. Finally, the continuous flow of liquids through the cells of a flow battery serves to equalize their temperatures, eliminate hot spots, and provide a medium for system heat rejection. The Full-Function Redox System The advantageous characteristics of the flow battery concept can be incor- porated into the design of cell stacks and systems. Figure 2 represents a stack that includes a bipolar series of uorking cells plus an open-circuit cell and a rebalance cell. The open-circuit cell is identical to the working cellsPDF Image | NASA Redox Storage System Development Project
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