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voltage until the charglng current tapered dcwn to a steady-state value. Thls steady charge-acceptance rate just balanced the self-discharge rate caused by shunt currents. Discrepancies between the measured and calculated shunt losses Indicated the degree to which the actual stack geometry differed from the model. Tllese studies led to a series of design changes which, as shown In table I,brought the multicell stack configuration into close conformance with the model and minimized the self-discharge rate. After these five-cell stack studies, the next level of scaleup was a 14-cell stack of the 320-cm2 cells. This stack, used as a demonstration devlce, was a full-function unit containing t r i m cells, a charge indicator cell, and a rebalance cell In addition to the standard working cells. A photograph and a stack schematic are shown in figures 2 and 10 (ref. 29). The largest stacks assembled during the course of the Redox project were the 39-cell stacks (320-cm2 cellsj for the 1-kW system. Eight of these stacks were assembled and evaluated. The ftrst set of four stacks was subject to unexpectedly hlgh hydrogen egolu?ion rates. Analysis of the problem led to an improved method of catalyzing the chromium electrodes, and an electrochemical analytical technlque for screening the electrodes before stack assembly (ref. 16). These procedures, applied durlng the preparation of the second set of four stacks, reduced hydrogen evolution rates t o f u l l y acceptable levels. These latter four stacks subsequently became the power section for the 1-kW system. A l l of these scaleups, whether In cell size or In the number of cells per stack, were successful: there was no loss in intrlnslc cell performance (i.e., no development of unexpected polartzations) attributable to the effects of scaling up. The design specifjcations for the 1-kW Redox storage system are given i n table 11. Thls system (fig. 11) and i t s evaluation are discussed i n detafl i n references 12 and 30. The system was des1l;;led to serve as the storage devlce for a photovoltaic array, thus simulating a remote, stand-alone application. The main focus of the evaluation of this systea was to i~vestiqatethe inter- actions at the interfaces between the array, the storage system, and the load. The intention was that the array controls and the Redox system controls never be allowed to work at cross-purposes. The respective control concepks were simple, being based on meter-relay devices, and no conflic--s occurred. The evaluation of the various inefficiencies and loss mechanisms during system operation gave considerable insight regarding possible efficiency Improvements for subsequent, larger s9stems. These improvments would result frav design changes, concept modifications, and altered operating modes and would require no tecnnological breakthroughs. The ability of a flow battery to use trim cells was quite strikingly dem- onstrated. The Redox system was able to discharge to a load at 120 V while simultaneously accepting charge at a lower voltage from the array durlng per- iods of low tnqolation. It thus acted as a dc-dc (ref. 1) transformer as well as a storage device. The system also recovered from inductive load surges within 50 ms. Several different modes of connecting the array to the Redox system were examined I n terms of advantages and disadvantages. The mode to be chosen would depend on the particular application usder consideratton.PDF Image | NASA Redox Storage System Development Project
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