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that reduces system efflciency. When designing a particular system, one goal is to minimize the sum of the pumping and shunt-current parasitic energy losses. Both of these loss mechanisms are directly influenced by the slngle- cell design - speciflcally, the dimensions of the flow slots through which the reactant fluids enter and leave the cell's active area. Approach The design of a Redox cell for a particular application requires a combination of experlmental data and computations. Cell performance is measured as a function of flow rate, current, and state of charge to arrive at the minimum multiple of the ideal stolchiometric flow rate that wlll sup- port performance. The deslgn flow rate then can be either a constant value set by the extremes ln operating current and state of charge, or lt can be varled continuously throughout each cycle to maintain the flow at the minimum acceptable multiple of the instantaneous stoichiometric flow. Next, for either a flxed flow rate or a range of flow rates, the cell pressure drop is measured for various cell geometries to arrlve at specific pumplng power requlrements for cells and stacks of cells. Shunt losses are calculated for the same cell geometries and various stack sizes and are then added dlrect1y to the pumping power requlrements. The mlnimum algebraic sum for a speciflc stack size corresponds to the most desirable cell geometry. Status The current, total solution concentration, and system state of charge combine to determine the stolchlometrlc flow requirements of a partic~lar system. Figure 34 shows the stoichiometric requirements of a 1000-cm slng1e cell for two dlfferent current values. Figure 35 is termed a "flow map" and is used to find the mlnimum multiple of stoichiometrlc flow that sustains adequate cell performance. This plot shows the actual performance of a cell during discharge at four current values. Also shown are horizontal llnes depicting the IR voltage loss correctlon for each current and a serles of dashed lines that depict the Nernstlan "droop" correctlon accounting for the concentration change of the reactant species across the cell from inlet to outlet. The actual data show an abrupt voltage drop at each current for flow rates just below 1.5 times the stoichiometrlc requirement. This is therefore the mlnimum stoichiometric flow multlple acceptable for adequate cell and stack performance durlng both the charge and discharge portions of a cycle. Figure 36 plots thlS deslgn flow rate for varlOUS currents and system states of charge. Pressure drop data for three flow port widths are shown in figures 37 to 39. Each figure includes two flow port (and electrode cavity) depths for cells with and without electrodes. Figure 40 groups together the pressure drop data for cells without electrodes; figure 41, the data for cells with electrodes. The pressure drop due to the presence of the electrode was generally about 50 percent of the total for the cell. These pressure drop and flow rate values for the various cell geometries can be used to calculate speclfic ldeal pumplng power values, as shown in table 16. Each cell geometry also determlnes a manifold-to-manifold ohmic reslstance, which, when substituted into a NASA shunt-current model, predicts the shunt loss 26PDF Image | NASA Redox Storage System Development Project 1980
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