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to keep the reactants flowing is Increased (ref. 6). Thus a tradeoff must be made between the two loss mechanisms represented by shunt currents and fluid pumping in order to minimize the sum of the losses. The critical design param eters are the single-cell flow port geometries and the reactant flow rates (ref. 7). There i s some degree of latitude I n selecting the design flow rates: the ideal lower limit, of course, would be the stoichiometric flow rate, but at this flow rate Nernstian corrections and concentration polarizations would impair working cell performance and efficiency (ref. 8). Therefore another tradeoff occurs, between the reactant flow rates (1.e.. pump power) and the electrochemical performance of the working cells in the system. It thus can be appreciated that designing a flow battery to maximize system energy efficiency i s a somewhat complex process requ?r i n g ant l y t i c a l models, experimental data, and iterative solutions. Intrastack flow maldistributlon. - I n a typical stack of Redox flow cells the flow geometry i s such that each reactant enters the stack a t r.ie end, flows through the cells i n parallel from the inlet manifold to the exit manifold, and leaves the stack at tne other end. If laminar flaw Is assumed throughout, analysis indicates that the reactant flow through the center cells of the stack will be less than that at the ends (ref. 7). Unl 5s care is taken in cell and stack design, the central cells can be starved fol reactant and suffer a per- formance loss. The critical design parameter is the ratio of the flow resist- ance of the flow port to the flow resistance of the manifold segment connecting adjacent cells: the greater the ratio, the more uniform the f!ow distribution. There are several wcys to deal with intrastack flow maldistribution. One would be to use a great enough flow rate so that even the central cel3s of a stack would receive adequate flow. Another would be to increase the flow-port flow resistance of the cells. Both of these approaches increase the pu~nppower requirement of the battery system and thus add even more complexity to the process of system design. -Reactant backmixing. - Another efficiency penalty i s associated wlth t b classical flow battery (fig. 1) because of i t s two-tank configuration. Ir discharge mode, for example, partially depleted reactants leaving the cell rc continuously returned to their respective tanks, where they can mix with less- depleted reactants (assuming that significant mixing does occur i n the tanks). Therefore the reactants leaving the tanks and entering the cells at any time are at a lower concentration and activity than would be the case had this mix- Ing not occurred, and the cell voltages are correspondingly less. One way to circumvent the loss of performance and efficiency associated with this contin- uous reactant dilution would be to use two tanks for each reactant (ref. 2), with one tank for each reactant servlng as the receiver for that reactant as It exits the cells. For a system operating at constant current i n thls four- tank mode, voltage would remain constant u n t i l a l l of the reacZants had passed through the cells and been collected in their respective receivers. Flow could then be reversed and the constant-current discharge would proceed a t ,I lower constant voltage. A t a l l tlmes, however, the system voltage would be greater than would be the case for the two-tartk operating mode (fig. 6). Analysis Indicates that a benefit of 3 to 5 percent in energy efficiency can be obtalned i n t h l s way, a t the expense of increased system complexity and cost. The preceding discussion of the classic flow battery, as exemplified by the Redox Iron-chromium system, shows the desJ1rablecharacteristics and capa- bilities that are unique to flow batteries. However, to reap the full beneflt of these inherent advantages, great attention must be given to cell, stack,PDF Image | NASA Redox Storage System Development Project
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