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Charge-dlscharg~ cycles were performed at nomlna1 current densities of 30, 38, and 45 rnA/cmf. The system, initially fully discharged, was charged at the deslred rate until the charging voltage reached 187 V (1.20 V/ce11); then the current was allowed to taper off until a quasl-steady state of charge was attained. Discharge followed immediately and continued unt11 complete. Typical data are shown in f1gure 11 in which state of charge, as indicated by the voltage of the open-circuit cell, is plotted aga1nst watt- hours of capacity for both charge and discharge. For a cycle between the reasonable open-circult voltage lim1ts of 0.94 and 1.15 V, the energy effi- ciency was 56.7 percent. Slml1ar data for ampere-hours of capacity show a coulombic efficiency of 83.9 percent. The contributors to these relatively low efficiencies were the greater-than-expected average cell resistance and the h1gher-than-ca1cu1ated shunt-current losses. As suggested in the pre- ceding, both of these loss mechanisms can be easily and sign1f1cant1y reduced. Flow mapping tests were also performed. In these tests the reactant flow rates were varied durlng a charge-discharge cycle to determine, at various states of charge, what flows were necessary to sustain system per- formance. It was found that flow rates three times greater than the stoi- chiometric requirement (i.e., the ideal, minimum flow for a given current and state of charge) were adequate to prevent any cells from falling to below zero voltage durlng discharge or rising unacceptably during charge. Tests on slngle cells have shown that 1.5 times the sto1chiometr1c flow rate 1S sufficient for good performance, so the fact that the 156-ce11 system re- quires tWlce th1S rate 1nd1cates that some cells are gett1ng only about one- half their share of the total flow. This problem relates to the difficulty in ma1ntain1ng assembly tolerances when producing many cells by hand. Table 3 presents the nominal loss rates for the var10US loss mechanisms for the 1-kW system. The present pumping loss rate relates to the worst- case sltuation: three t1mes the stoich10metric flow rate called for by the des1gn gross current density and the maximum design depth of d1scharge (80 percent). Better control over cell assembly tolerances would assure that all cells rece1ve the1r proport1onate share of the total flow and would thus perm1t operat1on closer to the 1.5 sto1ch1ometrlc flow multiple shown to be suff1c1ent for slngle cells. Pumps hav1ng an e1ectrlc-to-hydrau11c effi- ciency greater than the present 15 percent also would greatly reduce pumping losses. However, for very small systems such as thlS, low pump effic1ency 1S a fact of life unless the pumps are spec1al1y deslgned for the specif1c appl1catlon. Finally, use of an eff1clent pump speed control and solid- state logic devices would perm1t varying the pump speed as a function of the system state of charge instead of operating at the flxed, high speed requ1red for the worst-case situation. Over a complete cycle this capability would greatly reduce the total energy requ1red for pumping. As mentioned earl1er, a simple cell design change can be expected to reduce the 1ntrastack shunt losses by about 50 percent. In addltion, in 11ght of the preced1ng discussion, operation at lower flow rates will permit the narrow1ng of the flow port slots 1n each cell at the expense of only modest pump1ng losses. Within certain 11mits the intrastack shunt losses are directly proport10nal to these flow port w1dths (res1stances). In a like manner, interstack losses can be reduced by increas1ng the length or 9PDF Image | NASA Redox Storage System Development Project 1980
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