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Study of redox flow battery systems for residential applications 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627282930 Cycle no Charge - Negative tank Discharge - Negative tank Charge - Positive tank Discharge - Positive tank Figure 5.7 - State of charge of negative side and positive side electrolytes after VisBlue 8 charging and discharging steps at cycles 1, 2, 6 and 30 – lines were added for readability. For long-term operation, with flow rates from 8.5 L min-1 to 9.0 L min-1 for the positive half- cell and from 9.0 L min-1 to 9.5 L min-1 for the negative half-cell, it was observed that the SoC at the end of charging step of the negative tank increased while the SoC of the positive tank decreased. Thus, the negative tank was the charge limiting tank and the positive tank was the discharge limiting tank at cycle 30. As Tomazic and Skyllas-Kazacos and Doan et al. [29, 38] stated, the net volume from water osmosis on a CEM is from the negative to the positive tank, so it should be expected capacity loss on the positive tank. V2+ and V3+ ions should be dragged with the water [87] and react with VO2+, forming VO2+ ions and discharging the positive tank [88]: 2VO2+ + V2+ + 2H+ 3VO2+ + H2O In fact, Figure 5.7 indicates that the state of charge on the positive tank decreased from 76 % state of charge (cycle 6) to 54 % (cycle 30) which is in agreement with the presence of V2+ ions on the positive tank and with the self-discharge side-reaction. Also, the state of charge on the negative tank is increasing with the cycle number. This is an evidence that, since the negative tank is the charge limiting tank for latter cycles, the vanadium total concentration on negative tank decreases due to V2+ ions crossing to the positive tank. Thus, the negative tank has to be further charged to achieve the operation potential [89], lowering the concentration of V3+ ions in negative tank and increasing the state of charge. For further analysis, the vanadium concentration on each tank after each step are plotted against the cycle number on Figure 5.8. Chapter 5: Results and discussion 34 State of chargePDF Image | Tubular Vanadium Air Redox‐flow battery
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