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Section 6.4 Comparison of two sample designs This is because a higher current density improves the ratio between desired charging and discharging currents and parasitic self-discharging currents due to vanadium crossover and shunt currents. However, it is noteworthy that for a given current density, the Coulomb efficiency of design 4.6 is substantially higher than the Coulomb efficiency of design 1.1. This is plausible for two reasons. First, design 4.6 has an approximately threefold larger channel geometry factor, which significantly reduces shunt currents, whereas vanadium crossover is not affected. Secondly, for a given current density, design 4.6 carries a fourfold increased charging/discharging current, because of its larger electrode area. The efficiency variation between the two designs is most significant for the lowest studied operation current density. The voltage efficiency increases for both designs with an increasing flow factor, as shown in Figure 6-4 c) and d). The larger applied flow rate reduces the difference between cell and tank SoC and also reduces the concentration overpotential. The voltage efficiency significantly drops with an increasing current density due to the rising overpotentials. The higher Coulomb and voltage efficiency of design 4.6 also increases the energy efficiency, as shown in Figure 6-4 f). As the Coulomb efficiency does not vary significantly with the flow factor, the energy efficiency follows the trend of voltage efficiency and increases with an increasing flow factor, as shown in Figure 6-4 e) and f). An increasing flow factor first increases both energy and system efficiency. This is mainly because concentration overpotential is lowered by the higher flow rate, while the additional pump power does not yet counterbalance the prevented overpotential losses. If the flow factor is increased beyond a certain value, pump power demand increases so strongly that the reduced concentration overpotential is overcompensated and the efficiency starts to decrease, as shown in Figure 6-4 g) and h). For design 1.1, the efficiency peak is reached for a moderate current density of 50 mAcm-2. For a lower current density, Coulomb losses lower the efficiency. For a higher current density, overpotentials and pump power increases strongly. For design 4.6, the efficiency peak is already reached for 25 mAcm-2, because of increased Coulomb efficiency due to reduced shunt currents. In general, the required flow factors to obtain the efficiency peaks for all current densities are larger for the design 1.1 than for the design 4.6. This is further studied in Section 6.5.2 on page 96. 6.4.3 Discharge capacity of the sample designs The discharge capacity mainly depends on two operational parameters, namely the applied current density and the applied flow rate or flow factor. If cell voltage limits are used to determine the end of the charging and discharging process, the discharge capacity declines with an increasing current density, as shown in Figure 6-5. Herein, the discharge capacity is referred to the total electrolyte volume in the stack and the tanks, to simplify the comparison between the two designs, whose stack and tank 93PDF Image | Model-based Design Vanadium Redox Flow Batteries
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