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Section 6.4 Comparison of two sample designs Finally, from the four RTSE values in the gray boxes, a current-weighted average RTSE of 74.8 % can be derived for design 4.6. 6.4 Comparison of two sample designs 6.4.1 Dynamic simulations To address the characteristics of different designs, two charging/discharging cycles are simulated with a current density of 25 mAcm-2 and 100 mAcm-2, as shown in Figure 6-1 and Figure 6-2. Both cycles are conducted with cell design 1.1 (smallest electrode area, smallest channel geometry factor) and cell design 4.6 (largest electrode area, largest geometry factor). The flow factor according to the previously described selection process is applied to each case. Lowest studied current density During the cycles with the lowest current density, upper and lower cell voltage limits are not reached. Both, charging and discharging process are instead limited by tank SoC limits of 5 % and 90 %, as shown in Figure 6-1 a) and b). The pumps of both systems supply their minimum flow rate for the predominant part of operation time, as shown in Figure 6-1 c). Nominal pump capacity, corresponding to the system’s nominal flow rate according to Table 5-2 on page 80 is not exploited. The absolute value of the equivalent shunt current of design 1.1 is approximately threefold larger than of design 4.6, as shown in Figure 6-1 d). This is because of the short and wide channel of design 1.1. In addition, design 4.6 also has a fourfold increased electrode area and thus a fourfold higher current carrying capability. Hence, while up to 6.4 % of the externally applied current is lost for design 1.1, the highest share of lost current is only 0.6 % for design 4.6, as shown in Figure 6-1 e). The absolute value of the equivalent shunt currents follows the trend of the stack voltage. The threefold larger geometry factor and the fourfold larger electrode area results in a more than tenfold reduced shunt current sensitivity of cell design 4.6. Nominal current density During the cycles with nominal current density, charging and discharging process are governed exclusively by cell voltage limits, as shown in Figure 6-2 a) and b). The target tank SoC of 80 % at the end of the charging process, as postulated in Section 5.2.2, is reached by both designs. Consequently, pump capacity is fully utilized, as shown in Figure 6-2 c). While it is only fully exploited at the very end of the charging process, it is fully utilized for a longer period of time during the discharging process. This is because the upper and lower voltage limits are not equidistant to the equilibrium voltage of 1.39 V. This results in a steeper voltage decrease at the end of the discharging process. Unsymmetrical cell voltage limits are used in practice because the VRFB is more tolerable to low cell voltages than to high cell voltages. 89PDF Image | Model-based Design Vanadium Redox Flow Batteries
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