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Lewis 1-kW Preprototype System The nom1nal design specificat10ns for the l-kW Redox system assembled at NASA Lewis are pres2nted in table 1. The 120-V dc voltage level was selected to minim1ze I R losses. The decision to group the single cells 1nto four stacks was based partly on a consideration of shunt-current losses (intrastack losses tend to increase with the square of the number of cells 1n a stack) and partly on the fact-that 39-cell stacks represent a reasonable step up from the previous maximum number of cells in a stack, 14. The system capacity of 13 kWh (700 liters of each reactant) was an arbitrary cho1ce based on convenience. For tests actually run in 1980 the volume of reactants used was only about one-fourth of this design capac1ty to m1nim1ze cycle time. The depth of discharge range and the no~inal current density were select2d to avoid excessive pump1ng and cell I R losses. The cell Slze of 320-cm active area was picked because necessary equipment and material were already on hand. The voltage level of 120 V dc presented one system design problem: only 28-V dc pumps, controls, and instruments were readily available, and the lead times necessary to acquire 120-V equipment were not acceptable. It was therefore declded to use the 28-V dc equipment and to hard wire the 28-V pumps across a 24-cell section at the high-voltage end of the Redox system (fig. 6). Because of the variat10n of the voltage across these cells as the state of charge of the system changes, the pumping rates w1ll be greatest when low flow rates are acceptable, and lowest when h1gh rates are needed. Either situation results in system lneff1ciencies when using pumps designed for a slngle operating pOlnt. It was reallzed that the 28-V meter relays, flowmeters, and ampere-hour integrators could not tolerate the voltage changes inherent 1n d1rect connection to the Redox stacks. It was therefore necessary to install a 120-V/28-V dc-dc converter across the system 120-V dc bus. A system redesign would incorporate 120-V equipment. The Redox system is designed so that, of the total of 156 single cells, 60 are separated 1nto 10 six-cell trim packages. Through the use of meter relays, these trim packages are sequent1ally switched into or out of the load circuit to keep the bus voltage at 120 Vdc. The photovoltaic array conSlsts of twenty-two 120 V strings of solar cells, which are also sWltched in and out to keep the array bus at 120 Vdc, if poss1ble, under existing load and insolatlon conditions. Trim cell sW1tching is inhlbited untll all array strings are active, thus avoiding interactions between the two sets of controls. The array can be d1rectly connected across the f1rst 96 cells of the Redox system as shown 1n figure 7 (conf1gurat10n 1) or directly across the system load in parallel with the act1ve Redox cells (conf1guration 2). As the four 39-cell stacks were assembled and tested individually, it was found that they all generated exceSS1ve amounts of hydrogen from their respective chromium electrodes. Since this gassing occurred even during discharge and at relatlvely low states of charge, 1t was evident that the problem was related to the chromium electrode catalyst. Analytical tech- n1ques revealed that the new batch of carbon felt from Wh1Ch these electrodes were produced did not accept the catalyst as had the previous batch. It was decided to proceed wlth the installation of these stacks in 7PDF Image | NASA Redox Storage System Development Project 1980
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