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The major cause of imbalance in Redox systems is coevolution of hydrogen at the chromium electrode on charge. This may account for a 2 to 5% loss in amperehourefficiency. Onerebalancecellcanthus continuously balance a system having 20 to 50 Redox cells. Other reactions with minor significance that are also corrected for in this way are air intrusions into the iron solution and the slow liberation of hydrogen from the chemical reduction of hydrogen ion by chromous ion. A more detailed explanation of these reactions in the rebalance aspects of Redox technology are to be found elsewhere (8). To illustrate, a rebalance cell was added to one of the small lab sized cells (14.5 cm active area). Figure 8 shows the result of the internal rebalance modeofoperation. Therateoflossofcapacityis reduced by rebalancing most of the coevolved hydrogen gasduringrecharge. Thefurthereffectofexternal rebalance is seen in Figure 9. Here, external hydrogen is consumed in the rebalance cell to counter otherlossmechanisms. Itshouldbenotedthatthe Redox cell in the system does not need to be with- drawn from service while the rebalance process is being carried out. TRIM CELLS - The operating voltage of a stack of Redox cells varies over a wider span than does a battery system where the electrochemical reactants are solids in both states of charge. This is caused primarily by the rapid change in the Nernst correction atshallowanddeepdepthsofdischarge. Froma voltage regulation standpoint, this condition is un- desirable. However, due to the use of a common set of circulating fluid reactants, extra cells may be switched into or out of the stack circuit to adjust the output voltage to within desired limits. This practice when used in the traditional battery context is referred to as end cell switching; but the end cell(s) would not be at the same state of charge as theothercellsinthebattery. InRedoxsystems, the use of trim cells would not result in different parts of the system being at different states of charge. The output potential of the open circuit voltage cell described earlier or the stack output voltage could be used to provide the signal to auto- matically control this switching. FULL-FUNCTION SYSTEMS - A full-function Redox systemcontainstheabovementionedfeatures. Figure 2 depicts a stack that incorporates these special function cells. Also a blow-up of the simple re- peating units within a stack are shown. All of tested in 0.33 ft sized hardware. A small but com- plete system built for demonstration purposes is shown in Figure 10. The 100-watt average, 200-watt peak stack was used with five gallon storage tanks. These full-function systems have the following charac- teristics: 1. They are fully capable of all rebalancing requirements being performed at the system level on a continuous basis without removal of the stack from service. 2. They can provide an accurate, continuous non- mechanical state-of-charge indication to drive any associated system control logic. 3. They can stay within any reasonable voltage tolerance band during the charge or discharge portion of the cycle. 4. Permanent capacity loss will only result from membrane crossover. Present membranes are now avail- / able that would result in only a 25 percent loss of ' original capacity over a 20 to 30 year period of time. 4 SYSTEM PROJECTIONS The recent advances in the electrode performance andthemembraneselectivityandresistivitycharac- teristics have resulted in the ability to generate near-term system designs with a considerable degree of confidence. For a solar storage application, a performance of 45 watts/ft and 0.9 volts per cell at 50% DOD was assumed. This sizes the stack. The tank sizes assume a greater than stoichiometric amount of reactants: A 20 percent excess is assumed to account for the cycling between 10% and 90% depth of discharge; A further 25% excess of reactants is also added to account for capacity losses over some projected system life requirement. This allows the systemtobeabletomeetits"nameplate"capacity after 25 to 30 years of life. For a small Redox systemwith10klv'outputovera50-hourstorage duration (500 klVh), the component sizes are approxi- mately as depicted in Figure 11. The right circular cylindertanksareabout10.75ft.when1Molar solutions are used. The Redox stack itself would be about 2 ft. x 2 ft. x 4 ft. and would contain about 225 ft2 of cell area. The current cellper- 2 formance is about 25 watts/ft at 0.9 volts. Further improvements in cell performance are thus required. The preliminary cost projections for these systems is S24/kKh for a 50-hour storage system. This is based on projections based on current production techniques for similar-type hardware built for water electrodialysis and fuel cell applications. Also, a more direct and less costly production technique for chromium chloride is assumed. This production does appear to be feasible and preliminary pricing studies have already been completed. More refined hardware cost studies are currently being performed undercon- tract by the Power System Division of United Tech- nologies. By comparison, lead acid batteries for this type application cost about $100 per klVh and yetlackmanyofthesystemfeaturesthatareinherent with Redox energy storage systems. CONCLUDING REMARKS Redox energy storage systems based on iron and chromium chloride solutions have been built and tested in very small sizes (100-watt, 400-watt hour). These systems have demonstrated all the attractive featuresthatareinherenttotheRedoxtechnology. Electrode performance and cycle life based on accelerated tests are sufficient to meet the currently perceived requirements for both the solar photo- voltaic/wind and the electric utility application. The membrane selectivity is more than sufficient for solar applications but further improvements are needed for the electric utility application. Mem- brane area resistivity in the Redox environment is the present pacing item. Slight improvements in this area are needed for the solar application (from 5.8 fl- cm to 3.3 f2-cm ) and significant improve- ments for the electric utility application (5.8 ft-cm to 1.5 fl-cm ). These are not viewed as monumental advances in the state-of-the-art since the major factor involved in the membrane area resistivity is an interaction between the chloro complex of iron in the ferric state. Typical membrane area resis- tivities in hydrochloric acid solutions are 2.0 Q-cm . The main thrust of further membrane development will be directed at reducing this interaction. REFERENCES 1. L. H. Thaller, "Electrically Rechargeable Redox Flow Cells", NASA TM X-71540, 1974. these features have been built into and successfully 2 2PDF Image | RECENT ADVANCES IN REDOX FLOW CELL STORAGE SYSTEMS
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