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i n a l l respects but i s isolated electrically so that i t does not carry any cur- rent. The voltage of the open-circuit cell can be related directly to the sys- tem state of charge. I n the Iron-chromium system there are two causes of chemlcal imbalance that can be treated electrochemically. The first cause is the evolution of hydrogen i n the chromium portion of the system. This can occur electrocheml- cally because of imperfect catalysis of the chromium electrode or chemically because of impurities that promote the reduction of water by chromous ions. The second cause of imbalance Is the Intrusion of air (oxygen) into either reactant system. In any case the result is always that the iron reactant becomes more highly charged than the chromium reactant. Thus the purpose of rebalancing !s to discharge the iron reactant until it is at the same state of charge as the chromium reactant. This Is easily accomplished by discharging an electrochemical cell (ref. 2) supplied with a stream of the iron solution as the positive reactant and a stream of hydrogen, from either the .hrom+um tdnk ullage or an external source, as the negative reactant. A schematic cf such a rebalance cell is presented in figure 3 (ref. 3). Since it is possible to charge or discharge various cells in a flow bat- tery at different rates without causing cell-to-cell imbalance, the use of trim cells becomes a feasible means for controlling the Redox system bus voltage. Trim cells are physically the same as the other working cells of a system, and reactants flow through them continuously, as with a l l other cells. However, these cells can be switched sequentially into or out of the electrical circuit as required, to maintain a relatively constant total system voltage. In this way changes in individual cell voltages can be accommodated as the system load or state of charge varies. Design Considerations Flow batteries such as the iron-chromium Redox system are subject to cer- tain inherent fnefficiencles. It i s the responsibility of the designer of such a system to minimize the combined effect of these inefficiencies oa system performance. Shunt currents. - Figure 4 presents a schematic of a four-cell, bipolar stack of Redox cells. Included are the reactant inlet and outlet manifolds and the ports connecting the I mifolds to the ineividual cell reactant cavi- ties. The signlflcant aspect of such a stack configuration l s that electron- conducting bipolar plates at various potentials are interconnected through ion-conducting fluid paths. Provided that electrochemical reactions can pro- ceed at the bipolar plate - electrolyte interfaces, self-discharge will occur through the conductive f l u i d paths. These self--"$charge currents are referred to as "shunt8', or ''circulating", currents. A planform of a typical Redox flow plate (fig. 5) shows flow ports and manifolds. Mathematical analysis (ref. 4) shows that, for a given stack of Redox cells, the shunt currents are best reduced by increasing the ionic resistance of the flow ports. This can be done by increasing the length or reducing the cross-sectional area of the ports (ref. 5). Pump requirement:. - Unfortunately, Increasing the ionic resistance of the cell ports also increases their flow resistance. Therefore a l l else being equal, as shunt-current losses are reduced, the parasltic pump power requiredPDF Image | NASA Redox Storage System Development Project
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