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Section 7.2 Comparison of two sample designs for the three-stack string system The first and second stack is connected to the common circuitry via T-junctions. In the main flow direction, they introduce a loss coefficient of 0.2 which is distributed equally on input and output of the T-junction. In branch flow direction, an additional loss coefficient of one is considered. The last stack is connected to the common circuitry via a single 90°-bend with a loss coefficient of 0.3. All stacks are connected to the piping using flexible tubes with a length of 1 m and a diameter which corresponds to the manifold diameter of the particular design. The sudden contraction from pipe to tube diameter causes a loss coefficient of 0.13, calculated with Eq. (2-92) on page 54, and is placed at the stack electrolyte inlet. The sudden expansion of tube to pipe causes a loss coefficient of 0.31, calculated with Eq. (2-91) on page 54, and is placed at the stack electrolyte outlet. Figure 7-1 shows the complete hydraulic circuit. Compared to the single-stack study, the nominal pump capacity is scaled by a factor of three. The relative pump efficiency remains the same. To obtain a comparable energy to power ratio, tank volume is also scaled by a factor of three. Further, it is adapted according to the electrode area, deployed by the design. Hence, the three stack-strings using the cells with an electrode area of 1,000 cm2, 2,000 cm2, 3,000 cm2 and 4,000 cm2 are assigned to tank volumes of 750 L, 1,500 L, 2,250 L and 3,000 L per half-side, respectively. 7.2 Comparison of two sample designs for the three-stack string system 7.2.1 Dynamic simulation results For illustration purposes, dynamic simulation results of a cycle with the lowest studied and the nominal current density are shown in Figure 7-2 and Figure 7-3 for a system with a three-stack string using cell designs 1.1 and 4.6. The applied flow factor in each case is selected according to the process, described in Section 6.3.1 on page 86. Operation with the lowest studied current density If the battery is operated with a current density of 25 mAcm-2, the SoC limits of 5 % and 90 % limit the charging and discharging process. Hence, the cells do not reach the voltage limits, as shown in Figure 7-2 a) and b). Referred to the string voltage, the lower voltage limit is 132 V, whereas the upper voltage limit is 198 V (gray dashed lines). Naturally, pump capacity is not completely exploited for operation with a low current density, as shown in Figure 7-2 c). For the better part of the operation time, the lower flow rate limitation of the pump determines the flow rate. Although the channel geometry factor of design 4.6 is more than three times larger than the channel geometry of design 1.1, the equivalent shunt currents of both designs hardly differ from each other, as shown in Figure 7-2 d). This can be explained by the impact of the external piping network. For design 1.1, the tube diameter is 30 mm, while the pipe diameter is 45 mm. For design 4.6, the tube diameter is 60 mm, while the pipe diameter is 90 mm. Hence, for design 1.1, the external hydraulic network represents a 106PDF Image | Model-based Design Vanadium Redox Flow Batteries
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