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Section 7.3 Evaluation of the twenty-four cell designs in a three-stack string a) Design 1.1 b) Design 4.6 25 mAcm-2 50 mAcm-2 75 mAcm-2 100 mAcm-2 20 15 10 20 15 10 1.5 2 2.5 3 3.5 41.5 2 2.5 3 3.5 4 Flow factor Flow factor Figure 7-6: Specific discharge capacity over flow factor and current density for design 1.1 and design 4.6 in a three-stack string 7.3 Evaluation of the twenty-four cell designs in a three-stack string 7.3.1 Coulomb efficiency The impact of channel geometry factor and electrode area on Coulomb efficiency is stronger for the three-stack string than for the single stack, as shown in Figure 6-6 on page 95 and in Figure 7-7. Both, a larger channel geometry factor and a larger electrode area significantly increase the Coulomb efficiency. Unfortunately, both mechanisms face a saturation effect. Hence, it is hardly possible to boost the Coulomb efficiency of a 1,000-cm2 electrode to the level of a 4,000-cm2 electrode only by means of an increased channel geometry factor. 7.3.2 Voltage efficiency For the sake of completeness, the voltage efficiency of the three-stack string for a cycle with the nominal current density is studied as well. However, if we compare the derived results of the three-stack string, shown in Figure 7-8, with the corresponding results of the single-stack system, shown in Figure 6-7 on page 95, we see that the electric series connection does not affect the voltage efficiency. 7.3.3 Optimal flow factor Compared to the hydraulic resistance of the stack, the hydraulic resistance of the external hydraulic circuit is significantly smaller. Further, we just learned that the voltage efficiency is not affected by serially connecting several stacks electrically. Compared to the single-stack system, the first fact implies a similar pump power demand of the three-stack string. The second fact implies a similar concentration overpotential. 111PDF Image | Model-based Design Vanadium Redox Flow Batteries
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